Trace elements at the intersection of marine biological and geochemical evolution

The Co/Ti trend shows an expansion of authigenic Co enrichments in iron formations between ~2.8 and 1.8 Ga.

“The Co/Ti trend shows an expansion of authigenic Co enrichments in iron formations between ~2.8-1.8Ga.”

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In several instances, fundamental differences exist between thermodynamic solubility models and the sedimentary proxy record.

“In several instances, fundamental differences between thermodynamic based solubility models and the sedimentary proxy record highlight the need for a multi-disciplinary approach when evaluating paleomarine trace element abundances.”

A unidirectional decline in molar Ni/Fe is stark and robust.

“A unidirectional decline in molar Ni/Fe is stark and robust.”

A direct relationship between concentrations in organic-rich sediments and dissolved concentrations in overlying anoxic and sulfidic bottom waters has been demonstrated for some trace elements such as Mo and Zn.

“(4) a direct relationship between concentrations in organic-rich sediments and dissolved concentrations in overlying anoxic and sulfidic bottom waters has been demonstrated for some trace elements (e.g., Mo, Zn; Algeo and Lyons, 2006;Scott et al., 2013).”

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• cites Scott et al., 2013 sourceNot checked yet

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Iron would have been much more abundant in surface waters on a more reducing early Earth when basic microbial metabolic machinery was being established.

“By contrast, iron would have been much more abundant in surface waters on a more reducing Earth when basic microbial metabolic machinery was being established (e.g., Poulton and Canfield, 2011;David and Alm, 2011).”

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Despite challenges to individual indicators, there is a growing consensus favoring an origin of life on Earth before ~3.5 Ga, and perhaps much earlier.

“Despite these challenges, however, there is a growing consensus that favors an origin of life on Earth before ~3.5 Ga, and perhaps much earlier.”

Complex animal life did not evolve until almost three billion years later in the Neoproterozoic.

“For reasons still debated, complex animal life did not evolve until almost three billion years later in the Neoproterozoic (e.g., Pecoits et al., 2012).”

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Several trace elements such as Mo and U are known to scale with organic carbon during deposition and burial in euxinic water columns.

“(3) several trace elements (e.g., Mo, U) are known to scale with organic carbon during deposition and burial in euxinic water columns (Algeo and Lyons, 2006)”

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The biogeochemical cycling of many bioessential elements can vary dramatically under different aqueous conditions.

“The idea that an organism's trace element requirements are dependent on the environment in which it evolved stems from the simple observation that the biogeochemical cycling of many bioessential elements can vary dramatically under different aqueous conditions (see also Williams and Rickaby, 2012).”

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The chemical composition of iron formations directly reflects trace element availability in the water column at the time of mineral precipitation and deposition.

“Therefore, it is thought that their chemical composition directly reflects availability in the water column at the time of mineral precipitation and deposition.”

The IF and shale trace element records published to date generally support the idea that redox chemistry played a central role in the evolution of marine elemental cycling, with the GOE and NOE standing out.

“The IF and shale trace element records published to date generally support the idea that redox chemistry played a central role in the evolution of marine elemental cycling, with two particular events standing out: the Great Oxidation Event (GOE) ~2.4 billion years ago (Ga), and the Neoproterozoic oxygenation event (NOE) ~0.7 Ga.”

Iron formations have proven useful as paleomarine proxies because trace metal content in least metamorphosed units is almost entirely derived from seawater.

“As iron-and silica-rich chemical sedimentary deposits, IF have proven useful because the trace metal content in the least metamorphosed units are almost entirely derived from seawater (see Bjerrum and Canfield, 2002;Konhauser et al., 2007;2009;2015;Robbins et al., 2013).”

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• cites Konhauser et al., 2007 sourceNot checked yet

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Following the evolution of stem group eukaryotes in the early mid-Proterozoic, eukaryotic diversity was relatively static from 1.8 to 0.8 Ga.

“Following the evolution of stem group eukaryotes in the early mid-Proterozoic (Butterfield, 2000;Knoll, 2014), there is a relatively static period in eukaryotic diversity lasting from 1.8 to 0.8 Ga.”

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• cites Knoll, 2014 sourceNot checked yet

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S-MIF effectively disappeared from the sedimentary record between 2.45 and 2.32 Ga.

“that effectively disappeared from the sedimentary record between 2.45 and 2.32 Ga (e.g., Bekker et al., 2004;Farquhar et al., 2000;2011).”

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• cites Farquhar et al., 2000 sourceNot checked yet

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Some trace element limitations in the modern ocean may be an evolutionary legacy of earlier life adapted to more replete conditions.

“In this light, some trace element limitations observed in the modern ocean might be thought of as an evolutionary legacy of earlier life adapted to more replete conditions.”

Modern iron demand can be viewed as an evolutionary relict from lineages that emerged under ancient conditions of relative iron abundance.

“Thus, modern iron demand can be thought of as an evolutionary relict, stemming from the emergence of lineages under ancient environmental conditions of relative Fe abundance.”

Shales provide a more continuous temporal record because shale is relatively common in the geologic record.

“(2) they provide a more continuous temporal record since shale is relatively common in the geologic record”

Organic-rich, fine-grained siliciclastic shales with >0.5 wt% TOC are attractive as paleomarine proxies because they have modern analogues in anoxic basins.

“Another powerful source of information is the shale record, especially organic matter-rich, fine-grained, siliciclastics (with >0.5 wt% total organic carbon, TOC), which are attractive because (1) they have modern analogues in anoxic basins (e.g., Black Sea, Cariaco Basin)”

Recent Cr and U trace element proxy studies pushed the onset of the GOE back to 2.48-2.47 Ga.

“Recent studies based on the trace element proxies of Cr and U, as indicators for continental oxidative weathering have pushed the onset of the GOE back to 2.48-2.47Ga (Konhauser et al. (2011) and Partin et al. (2013a), respectively.”

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Moore et al. (2013) highlighted spatial heterogeneity on a global scale in surface waters for major limiting nutrients such as nitrate or phosphate, as well as trace metals.

“As well, Moore et al. (2013) have highlighted spatial heterogeneity on a global scale in surface waters with regards to both major limiting nutrients such as nitrate or phosphate, as well as trace metals.”

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Much current work on trace element proxy records has been inspired by the idea that trace metal biolimitation may have been significantly different in the deep past.

“Much of the current work focused on trace element proxy records (Fig. 3) has been inspired by the idea that trace metal biolimitation may have been significantly different in the deep past (Javaux et al., 2001;Anbar and Knoll, 2002).”

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• cites Anbar and Knoll, 2002 sourceNot checked yet

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In Anbar and Knoll's bio-inorganic bridge, changes in early ocean redox chemistry directly affected trace element bioavailability and the evolutionary trajectory of life.

“In Anbar and Knoll's (2002) 'bio-inorganic bridge', changes in early ocean redox chemistry directly affected the bioavailability of trace elements and, in turn, the evolutionary trajectory of life.”

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This work suggests that utilization of trace metals in biology generally follows the pattern Fe >> Zn > Mn >> Mo, Co, Cu >> Ni > W, V.

“This work suggests that the utilization of trace metals in biology generally follows the pattern of Fe >> Zn > Mn >> Mo, Co, Cu >> Ni > W, V.”

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Inverse modeling based on utilization of trace elements in metalloenzymes further supports a strong linkage between evolving ocean chemistry and biological trace element dependency.

“Inverse modeling based on the utilization of trace elements in metalloenzymes (Zerkle et al., 2005) further supports a strong linkage between evolving ocean chemistry and biological trace element dependency.”

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The genomic record for several trace metals has been linked to evolving marine geochemistry.

“Indeed, the genomic record for several trace metals has been linked to evolving marine geochemistry.”

Zerkle et al. (2006) and Glass et al. (2009) have shown that very low Mo concentrations can support near modern levels of nitrogen fixation.

“although Zerkle et al. (2006) and Glass et al. (2009) have both shown that very low Mo concentrations are capable of supporting near modern levels of nitrogen fixation.”

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The models of Saito et al. support the hypothesis that modern cyanobacterial trace element nutrient requirements may be viewed as the direct result of early evolution in ancient oceans limited in certain trace elements (Cu, Cd, Zn) and enriched in others (Fe, Mn, Co, Ni).

“The models of Saito et al. (2003) support the hypothesis that modern cyanobacterial trace element nutrient requirements may be viewed as the direct result of their early evolution in ancient oceans limited in certain trace elements (Cu, Cd, Zn) and enriched in others (Fe, Mn, Co, Ni).”

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If Proterozoic oceans were characterized by expanded euxinia, many trace elements such as Cd, Cu, and Zn could have been drawn down to biolimiting concentrations.

“These authors also found that if Proterozoic oceans were characterized by expanded euxinia (e.g., Canfield, 1998), many of these trace elements (e.g., Cd, Cu, Zn) could have been drawn down to concentrations that were biolimiting.”

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Because nitrogenases with a Mo-Fe cofactor are more efficient than Fe-Fe and V-Fe versions, low Mo concentrations would have negatively impacted primary producers' ability to fix N2.

“As nitrogenases containing a Mo-Fe metal cofactor are more efficient than alternative Fe-Fe and V-Fe versions (Eady, 1996), low Mo concentrations would have negatively impacted the ability of primary producers to fix N2.”

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These limitations would have attenuated carbon and oxygen fluxes from the biosphere prior to extensive oxygenation of the deep oceans in the late Neoproterozoic.

“These limitations, in turn, would have effectively attenuated the carbon and oxygen fluxes from the biosphere prior to the extensive oxygenation of the deep oceans in the late Neoproterozoic.”

Recent studies suggest the link between trace element availability, utilization, and metallome requirements may have been more complex.

“However, recent studies have suggested that the link between trace element availability, utilization, and metallome requirements may have been more complex.”

Under more oxic conditions first observed in the Cryogenian and continuing into the Cambrian-Ordovician, models indicated Fe and Mn concentrations in seawater would have been drawn down to modern levels by oxide mineral precipitation.

“Under more oxic conditions first observed in the Cryogenian and continuing to develop into the Cambrian-Ordovician (Large et al., 2014;Sperling et al., 2015;Sahoo et al., 2016), the models of Saito et al. (2003) indicated that Fe and Mn concentrations in seawater would have been drawn down to modern levels as the result of oxide mineral precipitation.”

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• cites Sperling et al., 2015 sourceNot checked yet

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• cites Sahoo et al., 2016 sourceNot checked yet

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In the case of Zn, enrichments in black shales and IFs suggest the oceanic Zn reservoir has been relatively constant since the Archean.

“In the case of Zn, the records of enrichments in both black shales and IFs suggest that the size of the oceanic Zn reservoir has been relatively constant since the Archean (Scott et al., 2013;Robbins et al., 2013).”

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Following increasing oxygenation in the late Neoproterozoic, sulfide complexes became less abundant, resulting in greater availability of Cd, Cu, and Zn.

“Following increasing oxygenation in late Neoproterozoic (Canfield et al., 2007;Scott et al., 2008;Sahoo et al., 2012;Lyons et al., 2014a, Planavsky et al., 2014a;Sahoo et al., 2016), these sulfide complexes would have become less abundant, resulting in greater availability of Cd, Cu, and Zn.”

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Links between chemical evolution of early oceans and trace element complement of organisms are also informed by emergence or disappearance of metal-binding protein fold families or fold super families.

“Links between the chemical evolution of the early oceans and the trace element complement of organisms are also informed by the emergence or disappearance of metalbinding protein fold families (FF) or fold super families (FSF).”

Loss or gain of FF or FSF can be viewed as key evolutionary events that in many cases appear to track fundamental shifts in paleomarine redox chemistry.

“The loss or gain of these FF or FSF can be viewed as key evolutionary events that in many cases appear to track fundamental shifts in paleomarine redox chemistry (Dupont et al., 2006;2010).”

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For many elements, seawater abundances are subject to kinetic controls where the dissolved reservoir scales with input/output fluxes without approaching solubility limits.

“For many elements, seawater abundances are subject to kinetic controls (e.g., Broecker, 1971) where the dissolved reservoir scales with input/output fluxes without approaching solubility limits.”

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Trace metal concentrations have an upper limit, after which they become toxic rather than beneficial for the organism.

“Third, trace metal concentrations do have an upper limit, after which they become toxic as opposed to being beneficial for the organism.”

Given knowledge of spatial variation in modern oceans and the sedimentary record, the assumption of a homogenous ocean is certainly incorrect.

“Given our knowledge regarding spatial variation both in modern oceans and as recorded by the sedimentary record, the assumption of a homogenous ocean is certainly incorrect.”

Capsule experiments simulating high-temperature, high-pressure diagenesis of ferrihydrite to hematite found little Ni or Zn mobility.

“recent experimental work using diagenetic capsule experiments (Robbins et al., 2015) demonstrated limited mobility for Ni and Zn during simulated diagenetic treatments at high temperature and pressure that capture the transformation from ferrihydrite to hematite.”

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Glass and Orphan (2012) discuss how methane production can be stimulated by addition of 0.2-2 μM of trace metals such as Fe, Cu, or Mo to some methanogen communities.

“Indeed, Glass and Orphan (2012) discuss how the production of methane can be stimulated by the addition of 0.2-2 µM of trace metals such as Fe, Cu, or Mo to some methanogen communities.”

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A recent assessment of a compilation of Fe speciation data suggests bottom waters may have remained ferruginous well into the Paleozoic.

“A recent assessment of a compilation of Fe speciation data suggests that bottom waters may have remained ferruginous well into the Paleozoic (Sperling et al., 2015).”

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Disappearance of S-MIF is perhaps the most accepted evidence for onset of widespread and permanent oxygenation of Earth's surface to levels above 10^-5 PAL.

“The disappearance of S-MIF is perhaps the most accepted evidence for the onset of widespread and permanent oxygenation of the Earth's surface to levels above 10 -5 times present atmospheric levels (PAL) (see Pavlov and Kasting, 2002;Bekker et al., 2004;Farquhar et al., 2000;2011).”

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• cites Farquhar et al., 2000 sourceNot checked yet

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Studies of ancient environments have shown basin-scale differences in water column chemistry due to stratification with depth or proximity to shoreline.

“Further, studies of ancient environments have already shown that there can be basin scale differences in water column chemistry that as the result of stratification with depth or proximity to shoreline (e.g., Poulton et al., 2010).”

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In modern marine environments, plankton can utilize very low levels of trace metals via complexing ligands that likely evolved to be highly efficient at extracting low levels of these micronutrients from seawater.

“In modern marine environments, Morel and Price (2003) have demonstrated how plankton are able to utilize very low levels of trace metals via complexing ligands that likely evolved to be highly efficient at extracting low levels of these micronutrients from seawater.”

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The ratio of Fe in highly reactive phases to total Fe effectively diagnoses sediment deposition from anoxic water column settings.

“The ratio of Fe in highly reactive phases (the sum of the four aforementioned phases) to total Fe effectively diagnoses sediment deposition from anoxic water column settings.”

Combining this with the ratio of pyrite extractable Fe to highly reactive Fe allows samples deemed anoxic to be categorized as ferruginous or euxinic.

“Combining this further with the ratio of pyrite extractable Fe to highly reactive Fe allows samples deemed anoxic to be further categorized as ferruginous or euxinic.”

SCM now successfully models trace metal adsorption onto microbial surfaces, such as marine cyanobacteria and anoxygenic photosynthesizers, not just minerals.

“the surface complexation modeling (SCM) approach, grounded in equilibrium thermodynamics through mass action expressions, has been extended beyond minerals to successfully model trace metal adsorption onto microbial surfaces (Fein et al., 1997(Fein et al., , 2005)), including marine cyanobacteria (Liu et al., 2015) and anoxygenic photosynthesizers (Martinez et al., 2016).”

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Below this threshold, photolytic reactions between ultraviolet rays and sulfur gases in the atmosphere produce isotopic anomalies that deviate from mass-dependent behavior and are recorded in marine sedimentary rocks.

“Below this threshold, photolytic reactions between ultraviolet rays and sulfur gases in the atmosphere produce isotopic anomalies that deviate from mass dependent behavior and are subsequently recorded in marine sedimentary rocks.”

Most prokaryotes and single-celled eukaryotes seem to favor trace metal concentrations on the order of nM to μM levels.

“Second, most prokaryotes and single celled eukaryotes seem to favor trace metal concentrations on the order of nM to µM levels.”

Given low levels of trace metals in modern oceans, organisms have had to develop strategies for dealing with possible micronutrient limitation.

“Finally, given the low levels of trace metals in modern oceans, organisms have had to develop strategies for dealing with the possibility of micronutrient limitation (e.g., Morel and Price, 2003;Morel, 2008).”

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Even if metals such as Cr do not have a direct biological role, their isotopes shed light on production and trajectory of oxygen in the water column.

“Second, and perhaps more important for the purposes of this review, even if the metals themselves do not have a direct biological role (i.e., Cr as opposed to Mo), they shed light on the production and trajectory of oxygen in the water column.”

Deposition of IF appears tied to periods of enhanced magmatic and hydrothermal activity associated with large igneous province emplacement that delivered large amounts of ferrous iron to anoxic deep oceans.

“Deposition of IF appears tied to periods of enhanced magmatic and hydrothermal activity (associated with large igneous province emplacement; Isley and Abbot, 1999) that delivered large amounts of ferrous iron to anoxic deep oceans (Bekker et al., 2010;2014).”

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On geologic timescales, P is generally viewed as the nutrient that limits marine productivity.

“Phosphorus is typically considered to be the limiting nutrient for marine productivity on geologic time scales (Tyrell, 1999).”

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Overall, these arguments support using modern hydrothermal exhalites to extend the iron formation record.

“Collectively, these considerations suggest that the use of modern hydrothermal exhalites to extend the IF record is justified.”

Diagenesis can change carbonate δ13C, δ18O, and trace element contents, which restricts their reliability as paleoenvironmental proxies.

“These processes can alter carbonate δ 13 C and δ 18 O, as well as the trace element compositions (e.g., Schrag et al., 2013;Swart and Kennedy, 2012), limiting their use for paleoenvironmental proxies.”

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In sum, Phanerozoic shale studies of P show the black shale record's promise for tracing changes in P availability over time.

“Overall, the Phanerozoic shale studies of P highlight the potential power of the black shale record in elucidating temporal trends in P availability.”

The tight P-Fe coupling during burial implies that ancient black shales can distinguish anoxic/nonsulfidic from euxinic deposition.

“This strong coupling of P and Fe during burial suggests that the black shale record in deep time can be used to distinguish anoxic/nonsulfidic versus euxinic deposition.”

Because of intense biological uptake and adsorption onto Fe oxyhydroxides, P usually occurs at low levels in seawater (0.001-3.5 μM).

“Due to strong biological scavenging and sorption to Fe oxyhydroxides, P is typically present at fairly low concentrations in seawater (0.001-3.5 µM; Bruland and Lohan (2003)).”

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Carbonates are capable of recording and retaining ancient seawater chemistry.

“Carbonate rocks can also capture and preserve records of ancient marine chemistry.”

When Fe speciation independently establishes a black shale's redox setting, its Mo, U, Re, V, and Cr enrichment can trace first-order changes in metal levels and global marine redox.

“If black shales can independently be determined to have been deposited under specific redox conditions (by Fe speciation), the degree of metal enrichment (Mo, U, Re, V, Cr) can then be used to track first order shifts in metal concentrations and the global marine redox state (Emerson and Huested, 1991;Lyons et al., 2009).”

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The pyrite record is probably most useful for generating broad, first-order trends.

“As with the black shale and IF records, the greatest value of the pyrite record likely lies in its ability to produce broad first-order trends.”

Because partitioning scenarios depend on matrix effects like varying Si levels and extra cations, they cannot be directly compared.

“As these partitioning scenarios are sensitive to matrix effects, such as different Si concentrations (e.g., Konhauser et al. (2007Konhauser et al. ( , 2009) ) and the presence of additional cations, (Jones et al. (2015), see also section 4.1), they cannot be directly equated.”

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The prevailing marine redox state chiefly determines the dissolved inventory of redox-sensitive elements in the ocean.

“the dominant marine redox condition is the primary control on the size of the dissolved ocean inventory of redox sensitive elements (Emerson and Huested, 1991)”

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For carbonate-specific redox proxies, the main obstacle is fully understanding how the proxy responds to marine, meteoric, and burial diagenesis.

“In general, the most important hurdle for any carbonate-specific redox proxy is a thorough understanding of the behavior of the proxy during marine, meteoric, and burial diagenesis.”

For elements like Zn, black shales yield independent data on seawater trace element reservoir evolution that supplements the iron formation record.

“For some trace elements, the black shale record provides valuable independent information about the evolution of seawater trace element reservoirs, such as Zn, complementing the IF record (c.f.Scott et al., 2013 andRobbins et al., 2013).”

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Trace element contents might also help separate primary from secondary signals in cherts.

“Trace element abundances themselves may also help resolve primary versus secondary signals preserved in cherts (Baldwin et al. 2011;Marin-Carbonne et al., 2014).”

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Enrichments in euxinic and anoxic sediments are primarily controlled by the marine metal reservoir.

“the marine metal reservoir exerts a first order control on enrichments in euxinic and anoxic sediments (Algeo and Lyons, 2006;Lyons et al., 2009;Scott et al., 2008;Reinhard et al., 2013b).”

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Compiling metal enrichments over time from black shales in basins well connected to the open ocean should track how seawater metal reservoirs evolved.

“A temporal compilation of metal enrichments from black shales deposited in sedimentary basins with relatively unrestricted connection to the open ocean should therefore reveal the evolution of seawater metal reservoirs through time.”

If no oxidizing agents enter black shales after deposition, sulfide-associated trace elements such as Mo, Zn, Ni, and Co should stay largely immobile during diagenesis.

“Tribovillard et al. (2006) indicated that unless black shales are exposed to an influx of oxidizing agents after deposition, a number of trace elements commonly associated with sulfides should remain relatively immobile during diagenesis; these include Mo, Zn, Ni, and Co, amongst others.”

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One last constraint on the iron formation record is that no direct modern analogue exists.

“A final limitation on the use of the IF record is the lack of direct modern analogues.”

Like hydrothermal pyrite, which records ore-fluid chemistry, syngenetic to early diagenetic pyrite can record shifts in seawater composition.

“Those authors suggested that in the same way that hydrothermal pyrite can track the chemistry of ore-forming fluids (Large et al., 2009), syngenetic to early diagenetic pyrite can track changes in seawater composition.”

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Black shales rich in organic matter and deposited beneath anoxic, sulfidic pore waters can accumulate redox-sensitive metals, particularly Mo.

“By contrast, organic-rich black shales deposited under anoxic and sulfidic pore waters may become enriched in redox-sensitive trace metals, especially Mo (Algeo and Lyons, 2006;März et al., 2008;Scott and Lyons, 2012).”

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Hydrothermal deposits allow limited testing of experimental and hypothesized trace element partitioning onto iron formations.

“Such hydrothermal deposits provide an opportunity to test experimental and hypothesized partitioning scenarios for trace elements onto IF-but only to a limited extent.”

Assessments of pyrite versus matrix incorporation, sulfide recrystallization, and formation site, plus further studies, indicate that the pyrite record can preserve seawater signatures.

“Large et al. (2014) also provided an assessment of the importance of pyrite versus matrix trace element incorporation, sulfide recrystallization, and the location in the water column or sediment where pyrite forms, which suggest the record is favorable to recording seawater signatures; a result further supported by several additional studies (Gregory et al., 2014;Large et al., 2015;Mukherjee and”

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Algeo and Lyons (2006) found that dissolved Mo in euxinic bottom waters scales with Mo/TOC ratios in underlying organic-rich sediments.

“Algeo and Lyons (2006) showed that the dissolved Mo concentration in euxinic bottom waters is proportional to the observed Mo/TOC ratios in the underlying organic-rich sediments.”

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Fe speciation in sediments reveals local seafloor paleoredox conditions-oxic, anoxic ferruginous, or euxinic.

“sedimentary Fe speciation analysis (Poulton and Canfield, 2005) serves as an indicator of local paleoredox conditions on the ocean floor (i.e., oxic; anoxic, ferruginous; or euxinic) (e.g., Poulton et al., 2004;Canfield et al., 2007;2008;Lyons et al., 2009;Reinhard et al., 2009;Poulton et al., 2010;Planavsky et al., 2011;Asael et al., 2013).”

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A compilation of Mo isotope data for euxinic black shales reveals significant oscillations in ocean redox conditions and the seawater Mo inventory across the Precambrian-Phanerozoic transition before permanent widespread ocean oxygenation.

“A recent compilation of Mo isotope data for euxinic black shales also reveals that significant oscillations in ocean redox conditions, and hence the seawater Mo inventory, may have occurred across the Precambrian-Phanerozoic transition before the advent of permanent widespread ocean oxygenation”

• cites Dahl et al., 2010 sourceNot checked yet

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• cites Kendall et al., 2015aNot checked yet

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• cites Sahoo et al., 2016 sourceNot checked yet

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Similar Mo isotope signatures for expanded ocean euxinia have been documented from Neoproterozoic black shales, including the Walcott Member and the Black River Dolomite.

“Similar Mo isotope signatures for expanded ocean euxinia have also been documented from Neoproterozoic black shales, including the ~750 Ma Walcott Member of the Chuar Group, Grand Canyon (Dahl et al., 2011) and the ca.640 Ma Black River Dolomite (Kendall et al., 2015a).”

• cites Dahl et al., 2011 sourceNot checked yet

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• cites Kendall et al., 2015aNot checked yet

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A critical level of at least 11 μmol L-1 H2S(aq) is required for sediments to record a seawater Mo isotope signature.

“Essentially, in order to record a seawater signature, a critical level of at least 11 µmol L -1 H2S(aq) is required.”

Changes in the paleomarine Ni reservoir likely had profound impacts on the Precambrian biosphere.

“Regardless, changes in the paleomarine Ni reservoir would likely have had profound impacts on the Precambrian biosphere.”

High Ni and Co values in Late Permian pyrites support the assertion that Ni and Co may be associated with Siberian traps volcanism and the Permian mass extinction.

“An observation that supports this assertion is the high Ni and Co values observed in pyrites in the Late Permian (Large et al., 2014), in which case the Ni and Co may be associated with the Siberian traps volcanism and Permian mass extinction”

• cites Large et al., 2014 sourceNot checked yet

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• cites Rothman et al., 2014 sourceNot checked yet

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Seawater Mo isotope compositions similar to modern seawater are first inferred from the sedimentary record at about 560 Ma.

“It is not until ca.560 Ma that seawater Mo isotope compositions similar to modern seawater are first inferred from the sedimentary rock record”

• cites Sahoo et al., 2016 sourceNot checked yet

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To track early ocean Mo isotope composition, black shales need to be deposited under conditions with persistent and appreciable dissolved sulfide.

“Accordingly, in order to track the Mo isotope composition of the early oceans, black shales need to be deposited under conditions with persistent and appreciable levels of dissolved sulfide”

• cites Neubert et al., 2008 sourceNot checked yet

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• cites Gordon et al., 2009 sourceNot checked yet

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• cites Arnold et al., 2012 sourceNot checked yet

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This Ni decline would have strongly impacted methanogenic bacteria because they are highly dependent on dissolved Ni availability over this concentration range.

“This decline would have strongly impacted methanogenic bacteria, since they are highly dependent on dissolved Ni availability over this range.”

A similar conclusion of euxinia for the late Paleoproterozoic and early Mesoproterozoic ocean was drawn from Fe speciation and Mo concentration and isotope data from the Rove and Velkerri formations.

“A similar conclusion was drawn for the late Paleoproterozoic and early Mesoproterozoic ocean based on sedimentary Fe speciation and Mo concentration and isotope data from the ca.1.8 Ga Rove Formation (Canada) and ca.1.4 Ga Velkerri Formation (Australia)”

• cites Arnold et al., 2004 sourceNot checked yet

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• cites Kendall et al., 2009 sourceNot checked yet

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• cites [2011]Not checked yet

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Using experimentally derived Ni partitioning coefficients to Si-rich ferrihydrite and assumed elevated Precambrian silica concentrations, paleomarine Ni concentrations were estimated to have dropped by more than half between 2.7 and 2.5 Ga, from close to 400 nM to less than 200 nM.

“Using experimentally derived Ni partitioning coefficients to Si-rich ferrihydrite (in light of assumed elevated Precambrian silica concentrations), it was estimated that paleomarine Ni concentrations dropped by more than half between 2.7 and 2.5 Ga (Fig. 6), from close to 400 nM to less than 200 nM.”

• AssumptionThe authors state this as an assumption: Precambrian seawater had elevated silica concentrations.

When free hydrogen sulfide is abundant (10−3-10−4 M), Mo becomes particle-reactive thiomolybdates and is rapidly removed from seawater-the Mo geochemical switch.

“in the presence of abundant (10 -3 -10 -4 M) free hydrogen sulfide, Mo is converted to a series of particle-reactive thiomolybdates and is efficiently removed from seawater (the so-called Mo”

• cites Helz et al., 1996 sourceNot checked yet

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Initial estimates from geochemical modeling and microbial genomics suggested that seawater Ni concentrations were fairly uniform from the Archean through to the modern.

“Initial estimates of Ni concentrations in the early ocean were derived from geochemical modeling (Saito et al., 2003) and microbial genomics (Zerkle et al., 2005). These studies suggested that seawater Ni concentrations were fairly uniform from the Archean through to the modern”

• cites Saito et al., 2003 sourceNot checked yet

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• cites Zerkle et al., 2005 sourceNot checked yet

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Mo isotope data combined with U and Fe isotope data and Fe speciation indicate a dramatic decline in ocean oxygenation after the Great Oxidation Event.

“Molybdenum isotope data has also been used in conjunction with U and Fe isotope data and sedimentary Fe speciation to demonstrate that a dramatic decline in ocean oxygenation occurred following the GOE”

• cites Asael et al., 2013 sourceNot checked yet

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• cites Partin et al., 2013bNot checked yet

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Large et al. presented Ni analyses in sedimentary to early diagenetic pyrite that mirrored the temporal trend in Ni/Fe ratios observed in iron formations.

“Large et al. (2014) presented a suite of Ni analyses in sedimentary to early diagenetic pyrite that mirrored the observed temporal trend in Ni/Fe ratios observed in IF.”

• cites Large et al., 2014 sourceNot checked yet

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Stüeken et al. report 3.2 Ga fluvial to marine sedimentary rocks with an average nitrogen isotopic composition of 0.0 ± 1.2‰.

“Stüeken et al. (2015a) report a suite of fluvial to marine sedimentary rocks at 3.2 Ga that have an average nitrogen isotopic composition (δ 15 N) of 0.0 ± 1.2 ‰.”

• cites Stüeken et al., 2015aNot checked yet

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The decline in Ni in iron formations remained clear after a near-doubling of available data, suggesting the observed decline is robust.

“This trend remained clear after a near-doubling of the available IF data (Konhauser et al., 2015), suggesting that the observed decline in Ni in IF is robust.”

• cites Konhauser et al., 2015 sourceNot checked yet

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A compilation of Ni contents in iron formations through time revealed a unidirectional and rapid decrease in Ni just prior to the Great Oxidation Event.

“However, a compilation of Ni contents in IF through time (Konhauser et al., 2009) revealed a unidirectional and rapid decrease in Ni just prior to the GOE”

• cites Konhauser et al., 2009 sourceNot checked yet

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Consequently, Mo concentrations in sediments deposited under well-constrained water-column redox conditions can serve as a tracker of ocean oxygenation over time.

“For these reasons, the concentration of Mo in sediments deposited under well-constrained water column redox conditions can be used to track the oxygenation of the oceans through time.”

The presumed consistency in seawater Ni concentrations largely stems from Ni behaving conservatively in waters under various redox conditions.

“The presumed consistency largely stems from Ni behaving conservatively in waters under various redox conditions”

• cites Saito et al., 2003 sourceNot checked yet

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Iron speciation analysis supported the interpretation of euxinia in the samples studied by Asael et al.

“The interpretation of euxinia was further supported by the iron speciation analysis conducted on their samples.”

• cites Asael et al., 2013 sourceNot checked yet

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Mo-nitrogenase activity at very low Mo levels is physiologically consistent with culture studies.

“This would be physiologically consistent with observations of Mo-nitrogenase activity at very low Mo levels in culture studies”

• cites Zerkle et al., 2006 sourceNot checked yet

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• cites Glass et al., 2009 sourceNot checked yet

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First-order trends in Ni abundances in the iron formation record are also recorded in sedimentary pyrite.

“Interestingly, the first-order trends in Ni abundances observed in the IF record (Fig. 6) are also recorded in sedimentary pyrite.”

• cites Large et al., 2014 sourceNot checked yet

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Agreement between the iron formation and pyrite proxy records supports the interpretation that they reflect first-order trends in paleomarine bioavailability.

“Such agreement between two distinct proxy records is encouraging and supports the interpretation that they reflect first-order trends in paleomarine bioavailability.”

An examination of Co concentrations in iron formations and pyrite through time suggests an expansion of the paleomarine Co reservoir between 2.8 and 1.8 Ga.

“An examination of Co concentrations in IF and pyrite through time suggests an expansion of the paleomarine Co reservoir between 2.8 and 1.8 Ga”

• cites Swanner et al., 2014 sourceNot checked yet

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This disparity can be linked to an overestimation by early geochemical models of the effects of Zn aqueous complexation by sulfides during the Precambrian.

“This disparity can be linked to an overestimation of the effects of Zn aqueous complexation by sulfides during the Precambrian by early geochemical models”

• cites Saito et al., 2003 sourceNot checked yet

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In modern oceans, the majority of Zn in surface waters is complexed with organic ligands (90-98%).

“In the modern oceans, the majority of Zn in surface waters is complexed with organic ligands (90-98%).”

• cites Bruland, 1989 sourceNot checked yet

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Recent culture work demonstrated that weakly binding organic ligands can increase Zn uptake rates in phytoplankton.

“Consistent with this idea is recent culture work after Xu et al. (2012) and Aristilde et al. (2012), where it was demonstrated that weakly binding organic ligands can in fact increase Zn uptake rates in phytoplankton.”

• cites Xu et al., 2012Not checked yet

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• cites Aristilde et al., 2012 sourceNot checked yet

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For Cr, no significant authigenic enrichments in middle Proterozoic black shales were taken to indicate pervasive anoxia.

“For Cr, there are no significant authigenic enrichments in middle Proterozoic black shales and this was taken to indicate pervasive anoxia”

• cites Reinhard et al., 2013bNot checked yet

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Cr is subject to strong stable isotope fractionation during redox reactions, especially reduction.

“Furthermore, Cr is subject to strong stable isotope fractionation during redox reactions, especially reduction”

• cites Ellis et al., 2002 sourceNot checked yet

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Production of trace hydrogen sulfide through microbial sulfate reduction may strongly govern aqueous Ni concentrations even under ferruginous conditions.

“Further, production of trace hydrogen sulfide through microbial sulfate reduction may play a strong role in governing aqueous Ni concentrations, even under ferruginous conditions”

• cites Crowe et al. 2008 sourceNot checked yet

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• cites Zegeye et al. 2012 sourceNot checked yet

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This conclusion is indicated by a large increase in Co/Ti ratios in iron formations, euxinic shales, and pyrite relative to the evolving continental crust, with a peak at about 2.4 Ga.

“This conclusion is indicated by a large increase in Co/Ti ratios in IF (Fig. 8), euxinic shales, and pyrite relative to the evolving continental crust, with a peak in Co/Ti values observed at ~2.4 Ga.”

• cites Swanner et al., 2014 sourceNot checked yet

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Studies in Lake Matano show that green rust may play an important role in scavenging Ni from ferruginous water columns.

“Studies in the ferruginous ocean analogue Lake Matano, Indonesia, have shown that green rust may play an important role in scavenging Ni from ferruginous water columns”

• cites Zegeye et al. 2012 sourceNot checked yet

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The updated view of a relatively static paleomarine Zn reservoir contradicts earlier geochemical models that linked low Zn requirements of cyanobacteria to low Precambrian total Zn and linked eukaryotic proliferation to a Phanerozoic Zn increase.

“The updated view of a relatively static paleomarine Zn reservoir (Fig. 7) contradicts the findings of earlier geochemical models that linked (1) the low Zn requirements of cyanobacteria to the predicted low levels of total Zn in the Precambrian oceans, and also (2) the proliferation of eukaryotes to an increase in total Zn during the Phanerozoic”

• cites Saito et al., 2003 sourceNot checked yet

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• cites Dupont et al., 2010 sourceNot checked yet

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The expansion of the Co reservoir between 2.8 and 1.8 Ga may be coincident with increased mantle plume activity and associated hydrothermal inputs.

“The expansion of the Co reservoir between 2.8 and 1.8 Ga (Fig. 8) may be coincident with increased mantle plume activity and associated hydrothermal inputs”

• cites Swanner et al., 2014 sourceNot checked yet

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Prior to compilations of Zn data from the sedimentary proxy record, estimates of Zn concentrations and bioavailability came from geochemical modeling.

“Prior to compilations of Zn data from the sedimentary proxy record, the only estimates for Zn concentrations and bioavailability came from geochemical modeling”

• cites Saito et al., 2003 sourceNot checked yet

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Pervasive anoxic conditions would help keep Co in solution, so the residence time of Co would be higher in more anoxic oceans compared to modern well-oxygenated oceans.

“Pervasive anoxic conditions would help keep Co in solution, and as such the residence time of Co would be higher in more anoxic oceans compared to modern, welloxygenated oceans”

• cites Swanner et al., 2014 sourceNot checked yet

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• cites Saito and Moffett, 2002Not checked yet

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A single study on Zn isotopes in iron formations found some of the most enriched δ66Zn values between 2.7 and 1.8 Ga.

“A single study on Zn isotopes in IF (Pons et al., 2013) found some of the most enriched δ 66 Zn values between 2.7 Ga and 1.8 Ga.”

• cites Pons et al., 2013 sourceNot checked yet

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This is consistent with relatively high Co concentrations in the anoxic ferruginous waters of Lake Matano compared to its oxic surface waters.

“This is consistent with the relatively high Co concentrations in the anoxic ferruginous waters of Lake Matano compared to its oxic surface waters”

• cites Crowe et al. 2008 sourceNot checked yet

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The syngenetic and early diagenetic pyrite record for Co indicates a steadier decline in Co levels from 3 Ga to present.

“However, the syngenetic and early diagenetic pyrite record for Co presented by Large et al. (2014) indicates a steadier decline in Co levels from 3 Ga to present.”

• cites Large et al., 2014 sourceNot checked yet

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Efficient burial of Cr can occur in sediments deposited under anoxic and ferruginous waters.

“Efficient burial of Cr can occur in sediments deposited under anoxic and ferruginous waters.”

This expansion is broadly consistent with an increase in genes that bind Co around this time.

“This expansion is also broadly consistent with an increase in genes that bind Co around this time”

• cites David and Alm, 2011Not checked yet

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In some modern anoxic aqueous systems with a strong redoxcline, such as Jellyfish Lake, total dissolved Zn concentrations increase with depth due to formation of aqueous sulfide complexes.

“For instance, in some conditions where a strong redoxcline exists, such as in Jellyfish Lake, Palau, total dissolved Zn concentrations increase with depth due to the formation of aqueous sulfide complexes”

• cites Landing et al., 1991 sourceNot checked yet

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• cites Dierssen et al., 2001 sourceNot checked yet

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The presence of iodine in carbonate indicates O2 > 1 μM at the site of carbonate precipitation, i.e., in surface waters.

“This relationship suggests that the simple presence of iodine in carbonate indicates the presence of O2 > 1 µM at the site of carbonate precipitation, i.e., in surface waters”

• cites Hardisty et al., 2014 sourceNot checked yet

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This level of dissolved O2 in surface waters is generally consistent with the 0.1-1% PAL proposed as necessary for early animals.

“This level of dissolved O2 in surface waters is generally consistent with the 0.1-1% PAL proposed as necessary for early animals”

• cites Mills et al., 2014 sourceNot checked yet

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• cites Sperling et al., 2013 sourceNot checked yet

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Mo burial is most efficient in sediments deposited from euxinic waters.

“By contrast, Mo burial, as discussed above, is most efficient in sediments deposited from euxinic waters.”

Konhauser et al. examined temporal trends in the degree of Cr enrichment in iron formations and found a peak at 2.48-2.32 Ga.

“Subsequently, Konhauser et al. (2011) examined temporal trends in the degree of Cr enrichment in IF and found a peak at 2.48-2.32Ga”

• cites Konhauser et al., 2011 sourceNot checked yet

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Most original organic-bound iodine is lost through diagenetic carbon remineralization.

“However, most of the original organic-bound iodine is lost through diagenetic carbon remineralization”

• cites Kennedy and Elderfield, 1987Not checked yet

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Together, Cr and Mo can constrain the relative extent of ferruginous and euxinic conditions.

“Together, these two elements can constrain the relative extent of ferruginous and euxinic conditions.”

Such a record indicates the potential for habitable marine environments, at least in terms of minimum O2 thresholds, hundreds of millions of years before the first evidence for animal life or eukaryotic diversification.

“Such a record indicates the potential for habitable marine environments, at least in terms of minimum O2 thresholds, hundreds of millions of years before the first evidence for animal life or eukaryotic diversification”

• cites Gingras et al., 2011 sourceNot checked yet

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• cites Knoll, 2014 sourceNot checked yet

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I/(Ca+Mg) ratios may indicate a shift to low oxygen marine conditions prior to the Oceanic Anoxic Event, as defined by δ13C and black shale deposition.

“Instead, the I/(Ca+Mg) ratios may indicate a shift to low oxygen marine conditions prior to the OAE, as defined by δ 13 C and black shale deposition.”

The idea of early O2 production is supported by the Mo isotope composition of iron formations from the Pongola Supergroup, where scaling between Mo isotope composition and Mn/Fe ratio indicates a Mn(IV) oxide exit channel for Mo at that time.

“The idea of early O2 production is supported by the Mo isotope composition of IF also obtained from within the Pongola Supergroup, where scaling between Mo isotope composition and Mn/Fe ratio indicate the presence of a Mn(IV) oxide exit channel for Mo at that time”

• cites Planavsky et al., 2014bNot checked yet

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This conclusion would suggest the presence and activity of oxygenic photosynthesis about 600 million years prior to the disappearance of mass-independent sulfur isotope fractionation during the Great Oxidation Event.

“This conclusion would suggest the presence and activity of oxygenic photosynthesis about ~600 million years prior to S-MIF disappearance during the GOE.”

If changes in the total marine iodine reservoir coincide with anoxia, either locally or globally, this reservoir change may be unobservable because I/(Ca+Mg) exclusively tracks iodate.

“In other words, if changes in the total marine iodine reservoir coincide with anoxia, either locally or globally, this reservoir change may be unobservable because as I/(Ca+Mg) exclusively tracks iodate”

• cites Lu et al., 2010Not checked yet

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• cites Zhou et al., 2015 sourceNot checked yet

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Normalized seawater is depleted in iodine by a factor of about 40 compared to Br and Cl.

“However, normalized seawater is depleted in iodine by a factor of ~40 when compared to these other halogens”

• cites Sharp and Draper, 2013Not checked yet

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Coupled U, Mo, and Fe isotope analyses in the ~2.05 Ga Zaonega Formation suggest euxinic depositional conditions similar to modern euxinic settings.

“In the late Paleoproterozoic, coupled U, Mo, and Fe isotope analyses in the ~2.05Ga Zaonega Formation suggest euxinic depositional conditions-showing values similar to those found in modern euxinic settings (Asael et al. 2103).”

• cites Asael et al. 2103Not checked yet

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After the initial peak, U and U/Fe ratios in the IF record return to low levels during the late Paleoproterozoic (post 2.05 Ga), until an increase in Neoproterozoic IF and Phanerozoic ironstones.

“Following this initial peak, U and U/Fe ratios in the IF record return to low levels during the late Paleoproterozoic (post 2.05 Ga), until an increase is observed in Neoproterozoic IF and Phanerozoic ironstones (Partin et al., 2013a).”

• cites Partin et al., 2013aNot checked yet

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U isotope fractionations are linked to changes in the oxidation state of U.

“since U isotope fractionations are linked to changes in the oxidation state of U.”

Microbial U(VI) reduction, along with abiotic processes such as sorption to organic matter and co-precipitation with iron oxides, sequesters marine U into sediments.

“Microbial U(VI) reduction, in addition to abiotic mechanisms of reduction such as sorption to organic matter or co-precipitation with iron oxides, sequesters marine U into sediments.”

This low redox state is consistent with a decrease in oxygenation of the atmosphere-ocean system following the GOE, as inferred from the black shale U abundance record.

“This low redox state is consistent with a decrease in oxygenation of the atmosphere-ocean system following the GOE, as inferred for the late Paleo-to Mesoproterozoic from the black shale U abundance record (Partin et al., 2013b).”

• cites Partin et al., 2013bNot checked yet

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Given these examples, Se likely plays a key role as a catalyst in intercellular processes.

“Given these examples of Se utilization in biological systems, it is likely that Se plays a key role as a catalyst in intercellular processes.”

In biological systems, Se is found predominately in selenocysteine, a cysteine analogue where sulfur is replaced by selenium.

“In biological systems, selenium (Se) is found predominately in the alternative amino-acid selenocysteine, a cysteine analogue where the sulfur is replaced by selenium”

• cites Hatfield and Gladyshev, 2002Not checked yet

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An increase in U concentration and U/Fe ratios in the IF record around 2.47-2.43 Ga appears to mark the onset of the GOE.

“An increase in U concentration and U/Fe ratios is observed in the IF record around 2.47-2.43Ga and appears to mark the onset of the GOE”

• cites Partin et al., 2013a,bNot checked yet

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Several Phanerozoic mass extinctions-end-Ordovician, end-Devonian, and end-Triassic-correlate with severe Se depletion in the pyrite record.

“Several global mass extinctions in the Phanerozoic have been correlated to severe Se depletion in the pyrite record, including the end-Ordovician, -Devonian and -Triassic events (Long et al., 2016).”

• cites Long et al., 2016 sourceNot checked yet

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There is no evidence for mass-independent fractionation of Se before the Great Oxidation Event.

“there is no evidence for mass-independent fractionation (MIF) of Se prior to the GOE.”

A peak in both BIF and black shale U concentrations by 2.32 Ga corroborates the onset of the GOE.

“corroborated by a peak in both BIF and black shale U concentrations by 2.32 Ga (Fig. 11).”

• cites Partin et al., 2013a,bNot checked yet

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Se stress may have contributed to these mass extinctions.

“and that this may have been a contributing factor in the extinctions.”

The pyrite record would indicate low levels of O2 in the Proterozoic, although with some variability, before the onset of widespread ocean oxygenation in the Neoproterozoic.

“Accordingly, the pyrite record would then indicate low levels of O2 in the Proterozoic, although with some variability, before the onset of widespread ocean oxygenation in the Neoproterozoic.”

Laser ablation analyses of black shale matrices show an increase in U during the Mesoproterozoic, with a peak between 1.4 and 1.7 Ga.

“laser ablation analyses of black shale matrices by Large et al. (2014) seem to show an increase in U during the Mesoproterozoic, with a peak between 1.4 to 1.7 Ga”

• cites Large et al., 2014 sourceNot checked yet

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Organisms that rely on Se for biological functions may have become stressed during these extinctions.

“suggesting that organisms relying on Se for biological functions may have become stressed”

U isotopes in the Mt. McRae shale suggest U(IV) oxidation and mobilization as early as 2.5 Ga.

“Evidence from U isotopes (δ 238 U) in the Mt.McRae shale suggest U(IV) oxidation and mobilization as early as 2.5 Ga (Kendall et al., 2013).”

• cites Kendall et al., 2013 sourceNot checked yet

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These processes allow U concentration in the sedimentary record to be used as a proxy for changes in oxygen content of the atmosphere-ocean system.

“These processes provide a means to use U concentration in the sedimentary record as a proxy for changes in the oxygen content of the atmosphere-ocean system.”

Large et al. also show a peak with a similar maximum at 3.1 Ga, which is difficult to reconcile with either interpretation of Proterozoic U abundances.

“Although, they also show a peak with a similar maximum at 3.1 Ga which is difficult to reconcile with either interpretation of Proterozoic U abundances.”

The negative Se isotope fractionation in those non-marine samples is attributed to oxidative weathering in terrestrial settings.

“this negative fractionation was attributed to oxidative weathering in terrestrial settings.”

• cites Stüeken et al. (2015b)Not checked yet

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Johnson (2004) and Johnson and Bullen (2004) calculated that significant stable isotope fractionation should occur during redox transformations between Se's four common valence states.

“The potential of Se and its stable isotopes as a paleo-redox tracer was first assessed by Johnson (2004) and Johnson and Bullen (2004), who calculated that significant stable isotope fractionation should occur during redox transformations between its four common valence states (VI, IV, 0, -II).”

• cites Johnson, 2004Not checked yet

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• cites Johnson and Bullen, 2004Not checked yet

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This possibility was shown true for a thioredoxin, with the selenocysteine version of the protein being 10-fold more catalytically active.

“Recently, this possibility was shown to be true for a thioredoxin, with the selenocysteine version of the protein being 10-fold more catalytically active”

• cites Kim et al., 2015 sourceNot checked yet

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U-Th-Pb systematics and δ56Fe values from the 3.2 Ga Manzimnyama IF are consistent with ambient dissolved oxygen concentrations between 0.4 and 10 μM.

“Recent work using U-Th-Pb systematics (corrected U concentrations) and d 56 Fe values from the 3.2 Ga Manzimnyama IF in South Africa are consistent with ambient dissolved oxygen concentrations between 0.4 and 10 µM (Satkoski et al., 2015).”

• cites Satkoski et al., 2015Not checked yet

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The pyrite record shows a decrease in Se of almost two orders of magnitude coincident with these extinctions.

“Coincident with these extinctions the pyrite record shows a dramatic decrease in Se, almost two orders of magnitude”

• cites Long et al., 2016 sourceNot checked yet

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Some bacteria catalyze reduction of U(VI) to U(IV) to obtain energy, including some dissimilatory Fe(III)- and sulfate-reducing bacteria.

“Some bacteria catalyze the reduction of U(VI) to U(IV) in order to obtain energy, including a few dissimilatory Fe(III)-and sulfate-reducing bacteria”

• cites Lovley et al., 1991 sourceNot checked yet

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• cites Behrends and Van Cappellen, 2005Not checked yet

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Temporal trends of Mo, V, and potentially Cu suggest that N-fixing cyanobacteria in early oceans may have been affected by biolimiting levels of multiple trace metals, especially before the GOE.

“Collectively, the temporal trends of Mo, V, and potentially Cu, suggest a scenario where N-fixing cyanobacteria in the early oceans may have been affected by biolimiting levels of multiple trace metals, especially prior to the GOE.”

Despite declining Fe concentrations after the final Paleoproterozoic widespread IF deposition at 1.88 Ga, Fe utilization in biological systems remains high.

“However, despite declining Fe concentrations following the final Paleoproterozoic episode of widespread IF deposition at 1.88 Ga (Rasmussen et al., 2012), Fe utilization in biological systems remains high.”

• cites Rasmussen et al., 2012 sourceNot checked yet

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Cd-sulfides have recently been identified as an important sink from oxygen-deficient water columns.

“Cd-sulfides have recently been identified as an important sink from oxygen-deficient water columns (Janssen et al., 2014).”

• cites Janssen et al., 2014 sourceNot checked yet

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In modern extant lineages, W may fill biological roles by substituting for Mo.

“In modern extant lineages, it may also fill biological roles by substituting for Mo (e.g., Frausto da Silva and Williams, 2001;L'vov et al., 2002).”

• cites Frausto da Silva and Williams, 2001Not checked yet

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• cites L'vov et al., 2002 sourceNot checked yet

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Thermodynamic modeling indicates that the seawater Cu reservoir should have reached a low in Proterozoic oceans due to expanded water column euxinia.

“Similar to Zn, thermodynamic modeling indicates that the seawater Cu reservoir should have reached a low in Proterozoic oceans as the result of expanded water column euxinia (e.g., Frausto da Silva and Williams, 2001;Saito et al., 2003).”

• cites Frausto da Silva and Williams, 2001Not checked yet

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• cites Saito et al., 2003 sourceNot checked yet

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Studies have demonstrated the utility of U, a non-bioessential element, for tracking redox evolution of Earth's atmosphere and oceans.

“Collectively, studies such as these have demonstrated the utility in using U, a non-bioessential element, for tracking the redox evolution of the Earth's atmosphere and oceans.”

The results of Chi-Fru et al. (2016) seem to indicate that such a scenario was unlikely, considering the relatively static sedimentary Cu abundance record.

“The results of Chi-Fru et al. (2016) would seem to indicate that such a scenario was unlikely, considering the relatively static sedimentary Cu abundance record they present.”

• cites Chi-Fru et al. (2016) sourceNot checked yet

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W may play a key role in biogeochemical cycling of N prior to the Neoproterozoic.

“Hence, W may also play a key role in the biogeochemical cycling of N prior to the Neoproterozoic.”

Cadmium stimulates Thalassiosira weissflogii under Zn-limiting conditions because it is used in an alternative carbonic anhydrase.

“Cadmium has been shown to stimulate Thalassiosira weissflogii under Zn limiting conditions because it is used in an alternative form of carbonic anhydrase”

• cites Lane and Morel, 2000Not checked yet

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Dissolved Fe concentrations in Archean and Paleoproterozoic oceans must have been high to permit formation of extensive IF via Fe(II) oxidation by anoxygenic and oxygenic photosynthesis.

“Dissolved Fe concentrations in the Archean and Paleoproterozoic oceans would have had to have been high in order to permit the formation of extensive IF via Fe(II) oxidation by anoxygenic and oxygenic photosynthesis (Bekker et al., 2010).”

• cites Bekker et al., 2010 sourceNot checked yet

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Several recent molecular innovations are employed by modern organisms to reduce Fe requirements, but always with a tradeoff.

“Several recent molecular innovations are employed by organisms in the modern environment to reduce Fe requirements-but always with a tradeoff.”

Vanadium is an important bioessential trace element for the early nitrogen cycle.

“Vanadium is also an important bioessential trace element for the early nitrogen cycle.”

High dissolved Fe concentrations in Archean seawater may have directly impacted cyanobacterial activity.

“High dissolved concentrations of Fe in Archean seawater may have also directly impacted the activity of cyanobacteria.”

High δ238U values indicate pervasive ocean oxygenation between 560 and 551 Ma.

“High δ 238 U values indicate pervasive ocean oxygenation between 560-551 Ma.”

• cites Kendall et al., 2015aNot checked yet

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Authigenic Cu enrichments in BIF and black shales, normalized to detrital input using Ti, are remarkably static throughout the Precambrian.

“recent examination of authigenic Cu enrichments in BIF and black shales, normalized to detrital input using Ti, are remarkably static throughout the Precambrian (Chi-Fru et al., 2016).”

• cites Chi-Fru et al., 2016 sourceNot checked yet

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Only a handful of bioessential trace elements have been investigated in the sedimentary records IF, black shale, and pyrite.

“only a handful of bioessential trace elements have been investigated in the sedimentary records -IF, black shale, and pyrite (Figs. 3 and 12).”

• cites Figs. 3 and 12Not checked yet

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In cyanobacteria cultured under high-Fe conditions, destructive reactive oxygen species increase: two-fold at 100 μM Fe(II) and five-fold at 1000 μM Fe(II).

“Swanner et al. (2015) showed that for cyanobacteria cultured under high-Fe conditions, there is an increase in destructive reactive oxygen species (ROS) produced in the cell; indeed, they reported a two-fold increase in ROS at 100 µM Fe(II) and a five-fold increase at 1000 µM Fe(II).”

• cites Swanner et al. (2015) sourceNot checked yet

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These ROS would have negatively affected survival of early cyanobacteria, potentially delaying proliferation of oxygenic photosynthesis before the GOE.

“These ROS would have negatively affected the survival of early cyanobacteria, potentially delaying the proliferation of oxygenic photosynthesis prior to the GOE.”

Uranium enrichments increased drastically corresponding to Neoproterozoic oxygenation.

“before the drastic increase in U enrichments corresponding to Neoproterozoic oxygenation.”

• cites Partin et al. (2013b)Not checked yet

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A slight increase in δ53Cr coupled with elevated Cr enrichments is coincident with the onset of deep-water euxinic conditions in the Cariaco Basin.

“A slight increase in δ 53 Cr coupled with elevated Cr enrichments is coincident with the onset of deep-water euxinic conditions in the basin.”

• cites Reinhard et al. (2014) sourceNot checked yet

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Studies on the proxy record indicate a complex and dynamic interplay between biological and geochemical processes that have affected both Earth's surface composition through time and the evolution of life.

“Collectively, studies on the proxy record thus far indicate that there is a complex and dynamic interplay between biological and geochemical processes that have not only affected the composition of Earth's surface through time, but the evolution of life as well.”

Pogge von Strandmann et al. found declining δ82/76Se values in Neoproterozoic black shales, attributed to protracted oxygenation between ~770 Ma and early Cambrian.

“Pogge von Strandmann et al. (2015) also examined the Se isotope record in the Neoproterozoic and found declining d 82/76 Se values in black shales that they attributed to a protracted oxygenation of the ocean-atmosphere system between ~770 Ma and the early Cambrian.”

• cites Pogge von Strandmann et al. (2015) sourceNot checked yet

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The chromium abundance record in black shales, normalized to titanium, shows a decline from the Archean to the Proterozoic.

“there is a decline in the Archean to Proterozoic”

• cites Reinhard et al. (2013b)Not checked yet

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Clear differences exist between predicted primitive ocean values based on sulfide mineral solubility and those indicated by the proxy record.

“Clear differences exist between the predicted primitive ocean values of Williams and Frausto da Silva (2003), who based their estimates off of sulfide mineral solubility, and those indicated by the proxy record.”

• cites Williams and Frausto da Silva (2003)Not checked yet

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Ni and Zn patterns in the rock record diverge greatly from modeling and genomic suggestions.

“Highlighted are nickel (red) and zinc (blue) whose patterns in the rock record diverge greatly from these modeling and genomic suggestions and are discussed in detail below.”

After that increase, oxygen levels fell to low values in the Paleoproterozoic.

“a subsequent fall to low oxygen levels in the Paleoproterozoic”

• cites Partin et al. (2013b)Not checked yet

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The Cr record links biological and geochemical evolution indirectly via the effect of O2 on both.

“Nevertheless, the Cr record links biological and geochemical evolution indirectly via the effect that O2 has on both.”

Se isotope analyses have been used to infer free O2 in the Neoarchean.

“Se isotope analyses have also been used to infer the presence of free O2 in the Neoarchean (Stüeken et al., 2015c).”

• cites Stüeken et al., 2015cNot checked yet

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δ53Cr values in Precambrian black shales are similar to crustal Cr until the Neoproterozoic.

“Collectively, this record shows that δ 53 Cr values in Precambrian black shales are similar in composition to crustal Cr until the Neoproterozoic.”

• cites Planavsky et al. (2014a)Not checked yet

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Beginning approximately 0.8 Ga, relatively large positive δ53Cr values are observed in black shales and mudstones, and in some Phanerozoic ironstones.

“Beginning approximately 0.8 Ga, relatively large positive δ 53 Cr values are observed in black shales and mudstones, as well as in some Phanerozoic ironstones.”

• cites Planavsky et al. (2014a)Not checked yet

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Significant progress has been made in examining sedimentary proxy records (black shales, IF, pyrite) for paleomarine availability of biologically critical trace elements including P, Mo, Ni, Zn, Cr, and Co.

“There has been significant progress made in recent years in the examination of sedimentary proxy records (black shales, IF, pyrite) for the paleomarine availability of biologically critical trace elements, including P, Mo, Ni, Zn, Cr, and Co (Fig. 12).”

• cites Fig. 12Not checked yet

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Declining Fe concentrations may have stimulated evolutionary development of organic ligand complexes such as siderophores for facilitated Fe acquisition.

“Declining Fe concentrations may also have stimulated the evolutionary development of organic ligand complexes, such as siderophores, that target elements such as Fe for facilitated acquisition (Hider, 1984;Neilands, 1989;Kendall et al., 2012).”

• cites Hider, 1984 sourceNot checked yet

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• cites Neilands, 1989Not checked yet

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• cites Kendall et al., 2012Not checked yet

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The capacity of δ53Cr from shales and ironstones to resolve changes in oceanic oxygenation depends on discriminating authigenic from detrital Cr.

“The capacity of δ 53 Cr from shales and ironstones to resolve changes in oceanic oxygenation state depends on ability to discriminate authigenic from detrital Cr, and this stands as a key challenge and opportunity.”

Robbins et al. found generally constant Zn enrichments through time in IF.

“This finding was bolstered by Robbins et al. (2013) who examined Zn concentrations and Zn/Fe ratios in IF (Fig. 7) and found generally constant Zn enrichments through time.”

• cites Robbins et al. (2013) sourceNot checked yet

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In all cases, the exchange necessitates acquisition of a different micronutrient.

“In all cases, the exchange necessitates the acquisition of a different micronutrient.”

Some proxy records exhibit values more typical of hydrothermal fluids, possibly indicating a greater relative contribution from hydrothermal sources to early oceans.

“Some proxy records exhibit values more typical of hydrothermal fluids, possibly indicating a greater relative contribution from hydrothermal sources to the early oceans (e.g., Robbins et al., 2013).”

• cites Robbins et al., 2013 sourceNot checked yet

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Uranium delivered by oxidative weathering increased between 2.4 and 2.0 Ga, corresponding to the Great Oxidation Event.

“an increase in U, delivered by oxidative weathering, between 2.4 and 2.0 Ga corresponding to the GOE”

• cites Partin et al. (2013b)Not checked yet

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Similar evidence for reliability of black shales in tracking δ53Cr signatures comes from the Peru Margin upwelling zone, comparable to modern deepwater values.

“Similar evidence of the reliability of black shales in tracking δ 53 Cr signatures is presented by Gueguen et al. (2016) from the upwelling zone of the Peru Margin, comparable to modern deepwater values.”

• cites Gueguen et al. (2016) sourceNot checked yet

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Muted chromium enrichments characterize the Mesoproterozoic.

“muted enrichments characterize the Mesoproterozoic”

• cites Reinhard et al. (2013b)Not checked yet

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