The History of Ocean Oxygenation

The O2-rich ocean-atmosphere system of the modern Earth supports high levels of standing biomass, energy flow, and ecological complexity.

“The amount of metabolic energy available to a biosphere powered by O 2 is vast, such that the O 2 -rich ocean-atmosphere system of the modern Earth supports staggering levels of standing biomass (Bar-On et al. 2018), energy flow (Falkowski et al. 2008), and ecological complexity (Steele 1974).”

• cites Bar-On et al. 2018 sourceSupported
• cites Falkowski et al. 2008 sourceNot checked — no full text of the source
• cites Steele 1974 sourceNot checked — no full text of the source

An extremely low-oxygen atmosphere does not necessarily imply a pervasively anoxic surface ocean.

“However, an extremely low-oxygen atmosphere does not necessarily imply a pervasively anoxic surface ocean (Kasting 1991, Olson et al. 2013, Reinhard et al. 2016).”

• cites Kasting 1991 sourceNot checked — no full text of the source
• cites Olson et al. 2013 sourceNot checked — no full text of the source
• cites Reinhard et al. 2016 sourceSupported

Stable nitrogen isotope data indicate the onset of a pervasive aerobic nitrogen cycle in the surface ocean during this interval.

“Stable nitrogen isotope data indicate the onset of a pervasive aerobic nitrogen cycle in the surface ocean during this interval (Luo et al. 2018, Zerkle et al. 2017), but it is not straightforward to tie this quantitatively to an atmospheric O 2 value or global ocean oxygenation landscape.”

• cites Luo et al. 2018 sourcePartly supported

  The Discussion (page not provided) links nitrogen-isotope signals to aerobic nitrogen cycling and increased surface-water oxygen availability. But it also says positive isotope values occur before the Great Oxygenation Event, while the Introduction describes the timing of a modern nitrogen cycle as contested. The paper therefore does not establish the claimed onset of a pervasive surface-ocean aerobic nitrogen cycle during the interval. (LLM, conf=0.86)

  Experimental — accuracy not yet measured
• cites Zerkle et al. 2017 sourceNot checked — no full text of the source

Atmospheric O2 during the mid-Proterozoic was well below that of the modern Earth.

“Although the precise range of atmospheric pO 2 values during the mid-Proterozoic is not yet clear (Gregory et al. 2021, Liu et al. 2021, Planavsky et al. 2018a), it is widely accepted that atmospheric O 2 was well below that of the modern Earth.”

• cites Gregory et al. 2021 sourceNot checked — no full text of the source
• cites Liu et al. 2021 sourceSupported
• cites Planavsky et al. 2018aNot matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

There are geochemical signals for anoxic conditions in very-shallow-water environments, including in the wave-mixed portions of the oceans.

“In addition, there are geochemical signals for anoxic conditions in very-shallow-water environments, including in the wave-mixed portions of the oceans (Bellefroid et al. 2019, Doyle et al. 2018, Liu et al. 2021, Tang et al. 2016).”

• cites Bellefroid et al. 2019 sourceNot checked — no full text of the source
• cites Doyle et al. 2018 sourceNot checked — no full text of the source
• cites Liu et al. 2021 sourcePartly supported

  The Results (Sample selection and data filtering; page unavailable) describe geochemical measurements from relatively shallow-marine carbonates deposited beneath well-mixed surface waters. The Discussion (page unavailable) interprets the measurements as evidence for low mid-Proterozoic atmospheric oxygen, but does not establish anoxic conditions in very shallow or wave-mixed water. (LLM, conf=0.84)

  Experimental — accuracy not yet measured

Speciation of iron-bearing minerals in marginal siliciclastic sedimentary rocks suggests that anoxic bottom waters were common on shelf and slope environments but were buffered by dissolved Fe2+ rather than hydrogen sulfide, termed ferruginous conditions.

“The speciation of iron-bearing minerals in marginal siliciclastic sedimentary rocks (Guilbaud et al. 2015, Planavsky et al. 2011, Poulton et al. 2010, Sperling et al. 2015) suggests that anoxic bottom waters were common on shelf and slope environments but were buffered by dissolved Fe 2+ rather than hydrogen sulfide (termed ferruginous conditions).”

• cites Guilbaud et al. 2015 sourcePartly supported

  Split into 2 atoms — 0 confirmed, 2 partially supported, 0 disputed, 0 misattributed, 0 unverifiable. Expand the claim to see per-atom verdicts.

  Experimental — accuracy not yet measured
• cites Planavsky et al. 2011 sourceNot checked — no full text of the source
• cites Poulton et al. 2010 sourceNot checked — no full text of the source

When coupled to mass balance modeling of oceanic trace element inventories, these data suggest that euxinia covered only a very small fraction of the seafloor.

“and, when coupled to mass balance modeling of oceanic trace element inventories, suggest that the most common form of anoxia in modern oceans-euxinia (anoxic but containing dissolved H 2 S)-covered only a very small fraction of the seafloor (Reinhard et al. 2013).”

• cites Reinhard et al. 2013 sourceSupported

Contrasting behaviors of molybdenum, chromium, uranium, and rhenium in organic-rich marginal sedimentary rocks deposited throughout the mid-Proterozoic qualitatively indicate widespread marine anoxia.

“The contrasting behaviors of molybdenum, chromium, uranium, and rhenium in organic-rich marginal sedimentary rocks deposited throughout the mid-Proterozoic qualitatively indicate widespread marine anoxia (Partin et al. 2013, Reinhard et al. 2013, Scott et al. 2008, Sheen et al. 2018)”

• cites Partin et al. 2013 sourceNot checked — no full text of the source
• cites Reinhard et al. 2013 sourcePartly supported

  The Mid-Proterozoic Geochemical Record and Conclusions link contrasting molybdenum and chromium enrichments in marine shales to pervasive mid-Proterozoic anoxia. The paper’s analysis does not establish the claimed contrasting behavior of uranium and rhenium, so it supports only the molybdenum-and-chromium portion of the attribution. (LLM, conf=0.88)

  Experimental — accuracy not yet measured
• cites Scott et al. 2008 sourceNot checked — no full text of the source

Sulfur and uranium isotope systematics indicate multiple intense spikes in the pervasiveness of marine anoxia during the early (∼530 Mya) and late (∼500 Mya) Cambrian.

“For instance, sulfur and uranium isotope systematics indicate multiple intense spikes in the pervasiveness of marine anoxia during the early (∼530 Mya) and late (∼500 Mya) Cambrian (Dahl et al. 2019, Gill et al. 2011)”

• cites Dahl et al. 2019 sourcePartly supported

  The Results (Uranium Isotopes) and Discussion (Massive Expansions in the Extent of Seafloor Anoxia; page unavailable) link repeated early Cambrian uranium-isotope excursions to major expansions of marine anoxia. The paper also measures sulfur isotopes, but does not present them as evidence for repeated anoxia spikes, and its reported spikes are in the early Cambrian rather than around 500 Ma. It mentions a Cambrian anoxic event elsewhere without establishing the claimed late-Cambrian pattern. (LLM, conf=0.86)

  Experimental — accuracy not yet measured
• cites Gill et al. 2011 sourceNot checked — no full text of the source

The mid-Proterozoic PO4 3- inventory was on the order of 5% of the present oceanic level.

“In parallel, large-ensemble biogeochemical modeling (Ozaki et al. 2019) suggests a mid-Proterozoic PO 4 3-inventory on the order of 5% of the present oceanic level (Figure 6), consistent with results from low-order Earth system models (e.g., Laakso & Schrag 2017).”

• cites Ozaki et al. 2019 sourceSupported
• cites Laakso & Schrag 2017 sourceNot checked — no full text of the source

There is a striking coincidence between the emergence of deep rooting systems during the Silurian-Devonian transition (∼410-390 Mya) and an increase in the proportion of marine sedimentary rock samples deposited under oxygenated waters.

“In particular, there is a striking coincidence between the emergence of deep rooting systems during the Silurian-Devonian transition (∼410-390 Mya) and an increase in the proportion of marine sedimentary rock samples deposited under oxygenated waters (Dahl et al. 2010, Sperling et al. 2015, Stolper & Keller 2018).”

• cites Dahl et al. 2010 sourcePartly supported

  The Conclusions (page unavailable) report that ocean oxygenation around 400 Ma coincided with the initial diversification of vascular plants. The paper does not identify the emergence of deep rooting systems as the coincident event or report an increase in the proportion of marine sedimentary rock samples deposited under oxygenated waters; it infers ocean oxygenation from molybdenum proxies. (LLM, conf=0.87)

  Experimental — accuracy not yet measured
• cites Sperling et al. 2015 sourceNot checked — no full text of the source
• cites Stolper & Keller 2018 sourceNot checked — no full text of the source

Metal isotope systematics limit the extent of benthic anoxia to less than ∼1% of global seafloor area over the last ∼50 million years.

“For example, metal isotope systematics limit the extent of benthic anoxia to less than ∼1% of global seafloor area over the same interval (Wang et al. 2016).”

• cites Wang et al. 2016 sourceSupported

Over the last ∼50 million years, global average surface temperatures steadily decreased from nearly 30°C during the early Eocene to less than 10°C during the most recent Pleistocene glacial epoch.

“For example, over the last ∼50 million years, global average surface temperatures have steadily decreased from nearly 30°C during the early Eocene (Burke et al. 2018, Hansen et al. 2013) to less than 10°C during the most recent Pleistocene glacial epoch (e.g., Tierney et al. 2020).”

• cites Burke et al. 2018 sourceNot checked — no full text of the source
• cites Hansen et al. 2013 sourceSupported
• cites Tierney et al. 2020 sourcePartly supported

  The paper’s LGM temperature-results section reports that global mean surface temperature was 5.9°C lower than in the Late Holocene. It reports a temperature change, not an absolute LGM temperature, so it does not establish the cited claim of less than 10°C. The early-Eocene figure is attributed to other citations in the sentence. (LLM, conf=0.82)

  Experimental — accuracy not yet measured

There was a shift in samples deposited under anoxic bottom waters from being dominated by ferruginous conditions to being dominated by euxinic conditions.

“There also appear to be changes in samples deposited under anoxic bottom waters, with a shift from samples being dominated by ferruginous conditions to being dominated by euxinic conditions (Sperling et al. 2021).”

• cites Sperling et al. 2021 sourceSupported

There appear to have been transient intervals of ocean oxygenation intensity during the Cambrian that may have been similar to that of the modern Earth.

“while there appear to have been transient intervals of ocean oxygenation intensity that may have been similar to that of the modern Earth (Dahl et al. 2019).”

• cites Dahl et al. 2019 sourceSupported

At modern biological pump strength, a pervasively suboxic deep ocean with dissolved O2 on the order of 1 μmol kg-1 occurs when atmospheric pO2 drops below approximately 30% that of the modern Earth.

“For instance, at a modern biological pump strength, a pervasively suboxic deep ocean-with dissolved [O 2 ] on the order of 1 μmol kg -1 -occurs when atmospheric pO 2 drops below ∼30% that of the modern Earth”

• AssumptionThe authors state this as an assumption: Bioavailable phosphorus (as PO4^3-) governs the steady-state organic carbon export flux from the surface ocean.
• AssumptionThe authors state this as an assumption: In the simple model, rates of O2 supply to the deep ocean are controlled by atmospheric O2 abundance and rates of ocean mixing.

In the simple model, an average deep-ocean O2 concentration comparable to that of the modern deep ocean requires an atmospheric pO2 of at least approximately 50% that of the modern Earth regardless of the strength of the biological pump.

“while an average deep-ocean [O 2 ] comparable to that of the modern deep ocean (∼170 μmol kg -1 ) requires an atmospheric pO 2 of at least ∼50% that of the modern Earth regardless of the strength of the biological pump.”

• AssumptionThe authors state this as an assumption: Bioavailable phosphorus (as PO4^3-) governs the steady-state organic carbon export flux from the surface ocean.
• AssumptionThe authors state this as an assumption: In the simple model, rates of O2 supply to the deep ocean are controlled by atmospheric O2 abundance and rates of ocean mixing.

Large regions of the ocean interior can be strongly deficient in dissolved oxygen.

“By contrast, large regions of the ocean interior can be strongly deficient in dissolved oxygen (Figure 1b,d).”

The pervasively suboxic deep-ocean condition can be maintained at atmospheric pO2 values more than an order of magnitude below approximately 30% of the modern Earth if the marine nutrient inventory is sufficiently decreased.

“but this condition can be maintained at atmospheric pO 2 values more than an order of magnitude below this if the marine nutrient inventory is sufficiently decreased (Figure 2b).”

• AssumptionThe authors state this as an assumption: Bioavailable phosphorus (as PO4^3-) governs the steady-state organic carbon export flux from the surface ocean.
• AssumptionThe authors state this as an assumption: In the simple model, rates of O2 supply to the deep ocean are controlled by atmospheric O2 abundance and rates of ocean mixing.

In the simple model, on the order of 30% of the present atmospheric O2 level is required to maintain a deep-ocean O2 concentration above approximately 10 μmol kg-1.

“For example, in this model, on the order of 30% of the present atmospheric O 2 level is required to maintain a deep-ocean [O 2 ] above ∼10 μmol kg -1 (Figure 2b)”

• AssumptionThe authors state this as an assumption: Bioavailable phosphorus (as PO4^3-) governs the steady-state organic carbon export flux from the surface ocean.
• AssumptionThe authors state this as an assumption: In the simple model, rates of O2 supply to the deep ocean are controlled by atmospheric O2 abundance and rates of ocean mixing.

Surface O2 tends to be locally elevated in coastal environments and regions of intense vertical nutrient exchange, which in a roughly modern ocean circulation regime include the major eastern boundary systems, the zone of equatorial divergence, and the Southern Ocean.

“Surface [O 2 ] tends to be locally elevated in coastal environments and regions of intense vertical nutrient exchange, which in a roughly modern ocean circulation regime include the major eastern boundary systems, the zone of equatorial divergence, and the Southern Ocean (Figure 4c).”

Combined estimates of global ocean oxygenation from uranium isotope records and carbon release rates from carbon isotope records and volcanic province eruption rates indicate a broad scaling between carbon release rate and the intensity of transient ocean deoxygenation.

“Combined estimates of global ocean oxygenation (from uranium isotope records) and carbon release rates (from carbon isotope records and volcanic province eruption rates) indicate a broad scaling between carbon release rate and the intensity of transient ocean deoxygenation (Figure 9).”

• Also stated in: § DISCLOSURE STATEMENT · plain

  “Relationship between the magnitude of carbon cycle perturbation and the transient increase in deep-ocean anoxia for a series of warming events in Earth's Mesozoic/Cenozoic history.”

A weakly oxygenated state can also be maintained at progressively higher marine nutrient abundances provided that atmospheric pO2 also rises.

“Such a weakly oxygenated state can also be maintained at progressively higher marine nutrient abundances provided that atmospheric pO 2 also rises (Figure 2b).”

There is very little evidence for major changes in ocean oxygenation over the last ∼50 million years.

“Interestingly, however, there is very little evidence for major changes in ocean oxygenation over the last ∼50 million years.”

At such low atmospheric pO2 values, the ocean interior would have remained functionally anoxic, even at minuscule ocean nutrient abundances.

“At such low atmospheric pO 2 values, the ocean interior would have remained functionally anoxic, even at minuscule ocean nutrient abundances (Figure 2b).”

The abundance of O2 in Earth's ocean-atmosphere system has changed dramatically as the Earth system has evolved.

“However, the abundance of O 2 in Earth's ocean-atmosphere system has changed dramatically as the Earth system has evolved (Canfield 2005, Kump 2008, Lyons et al. 2014).”

• cites Canfield 2005 sourceNot checked — no full text of the source
• cites Kump 2008 sourceNot checked — no full text of the source
• cites Lyons et al. 2014 sourceNot checked — no full text of the source

Photolytic and lightning-induced breakdown of CO2, along with disproportionation of H2O2 produced from photolysis of water vapor, would have yielded an average ground-level O2 abundance on the order of 10^-11 times the present atmospheric level.

“Photolytic and lightning-induced breakdown of CO 2 , along with a disproportionation of H 2 O 2 produced from photolysis of water vapor, would have yielded an average ground-level O 2 abundance on the order of 10 -11 times the present atmospheric level (PAL) (Haqq-Misra et al. 2011).”

• cites Haqq-Misra et al. 2011 sourceNot checked — no full text of the source

Prior to the evolution of oxygenic photosynthesis, the abundance of O2 near Earth's surface would have been extremely low.

“Prior to the evolution of oxygenic photosynthesis, the abundance of O 2 near Earth's surface would have been extremely low (Kasting et al. 1979).”

• cites Kasting et al. 1979 sourceNot checked — no full text of the source

Production and preservation of non-mass-dependent sulfur isotope anomalies in Archean and earliest Paleoproterozoic sedimentary rocks indicate that atmospheric pO2 was maintained at extremely low values, likely below approximately 10^-7 PAL, for most of Archean time.

“However, the production and preservation of non-mass-dependent sulfur isotope anomalies (NMD-S) in Archean and earliest Paleoproterozoic sedimentary rocks indicate that atmospheric pO 2 was maintained at extremely low values, likely below ∼10 -7 PAL, for most of Archean time (Farquhar et al. 2000, Pavlov & Kasting 2002, Zahnle et al. 2006) (Figure 3a).”

• cites Farquhar et al. 2000 sourceNot checked — no full text of the source
• cites Pavlov & Kasting 2002 sourceNot checked — no full text of the source
• cites Zahnle et al. 2006 sourceNot checked — no full text of the source

A drop in the marine nutrient inventory can counteract the impacts of lower atmospheric pO2 on deep-ocean oxygenation.

“Another key principle this model illustrates is that a drop in the marine nutrient inventory can counteract the impacts of lower atmospheric pO 2 on deep-ocean oxygenation.”

• AssumptionThe authors state this as an assumption: Bioavailable phosphorus (as PO4^3-) governs the steady-state organic carbon export flux from the surface ocean.
• AssumptionThe authors state this as an assumption: In the simple model, rates of O2 supply to the deep ocean are controlled by atmospheric O2 abundance and rates of ocean mixing.

Large-scale dynamics of ocean oxygenation have changed dramatically throughout Earth's history, in step with major changes in atmospheric O2 abundance and marine nutrient availability.

“The large-scale dynamics of ocean oxygenation have changed dramatically throughout Earth's history, in step with major changes in the abundance of O 2 in the atmosphere and changes to marine nutrient availability.”

Throughout Earth's history, the oxygenation landscape of the oceans has been controlled by the balance between supply of O2 to the ocean interior and the distribution of respiratory demands within the ocean water column.

“Throughout Earth's history, as is the case today, the oxygenation landscape of the oceans has been controlled by the balance between the supply of O 2 to the ocean interior and the distribution of respiratory demands within the ocean water column.”

In modern oceans, the distribution of dissolved O2 is principally regulated by the interaction between supply from the oxygen-rich atmosphere and consumption via aerobic respiration or reduced species generated by anaerobic microbial metabolism.

“In the modern oceans, the distribution of dissolved O 2 is principally regulated by the interaction between the supply of O 2 from the oxygen-rich atmosphere and the consumption of O 2 via aerobic respiration or the reduced species generated by anaerobic microbial metabolism.”

The abundance of atmospheric O2 and marine nutrient inventory are not independent variables, because the global biogeochemical cycles of major nutrients fueling the biological pump are mechanistically linked to the oxygenation state of the ocean-atmosphere system.

“An important fact that can be obscured by this style of analysis is that the abundance of atmospheric O 2 and marine nutrient inventory are not independent variables, because the global biogeochemical cycles of major nutrients fueling the biological pump (nitrogen, phosphorus, and a range of trace elements) are mechanistically linked to the oxygenation state of the oceanatmosphere system.”

Even today many regions of Earth's oceans are poised on the brink of anoxic conditions.

“Indeed, even today many regions of Earth's oceans are poised on the brink of anoxic conditions (Keeling et al. 2010), and such regions are often critically important for the large-scale biogeochemical cycles of many linked major and trace element species.”

• cites Keeling et al. 2010 sourceNot checked — no full text of the source

In deeper oceans, limited respiratory demand and deep-water renewal result in relatively high deep-ocean dissolved O2, particularly in regions of intense deep convection such as the North Atlantic.

“In the deeper oceans, limited respiratory demand and deep-water renewal result in relatively high deep-ocean dissolved O 2 , particularly in regions of intense deep convection (e.g., the North Atlantic; Figure 1b,d).”

Not all combinations of ocean nutrient inventory and atmospheric O2 level are permissible at steady state, because many will be incapable of balancing long-term redox balance within the ocean-atmosphere-crust system.

“As a result, not all combinations of ocean nutrient inventory and atmospheric O 2 level are permissible at steady state, because many will be incapable of balancing long-term redox balance within the ocean-atmosphere-crust system.”

Photochemical models indicate that pO2 must be above approximately 10^-5 PAL to prevent production and delivery of NMD-S anomalies near Earth's surface.

“Photochemical models indicate that pO 2 must be above ∼10 -5 PAL to prevent the production and delivery of NMD-S anomalies near Earth's surface (Pavlov & Kasting 2002, Zahnle et al. 2006).”

• cites Pavlov & Kasting 2002 sourceNot checked — no full text of the source
• cites Zahnle et al. 2006 sourceNot checked — no full text of the source

The oldest bedded sulfate evaporite deposits in Earth's rock record appear during this interval, in the Lucknow Formation in South Africa and the Tulomozero Formation in northwest Russia.

“The oldest bedded sulfate evaporite deposits in Earth's rock record also appear during this interval, in the Lucknow Formation in South Africa (Schröder et al. 2008) and the Tulomozero Formation in northwest Russia (Melezhik et al. 2005).”

• cites Schröder et al. 2008 sourceNot checked — no full text of the source
• cites Melezhik et al. 2005 sourceNot checked — no full text of the source

Formation of gypsum before halite in these systems implies seawater sulfate concentrations above approximately 1-2 mmol kg-1.

“The formation of gypsum (a hydrated sulfate mineral, CaSO 4 •2H 2 O) before halite (NaCl) in these systems implies seawater sulfate concentrations above ∼1-2 mmol kg -1 (compared with 28 mmol kg -1 in modern seawater).”

The interpretation of sediment manganese enrichments is robust regardless of the possibility of oxygen-free manganese oxidation through UV photooxidation, photosynthetic oxidation, or oxidation with elemental sulfur, given rapid manganese reduction under anoxic conditions.

“This interpretation of sediment manganese enrichments is robust regardless of the possibility of oxygen-free manganese oxidation-through UV photooxidation (Anbar & Holland 1992), photosynthetic oxidation ( Johnson et al. 2013), or oxidation with elemental sulfur (Daye et al. 2019)-given rapid manganese reduction under anoxic conditions (e.g., Busigny et al. 2014).”

• cites Anbar & Holland 1992 sourceNot checked — no full text of the source
• cites Johnson et al. 2013 sourceNot checked — no full text of the source
• cites Daye et al. 2019 sourceNot checked — no full text of the source

Petrographic, geochemical, sedimentological, and isotopic data suggest rapid and effective production, stabilization, and delivery of manganese mineral phases to sediments during deposition of the Sinqeni Formation.

“Petrographic, geochemical, sedimentological, and isotopic data all suggest rapid and effective production, stabilization, and delivery of manganese mineral phases to sediments during the deposition of the Sinqeni Formation (Ossa Ossa et al. 2016, 2018;Planavsky et al. 2014).”

• cites Ossa Ossa et al. 2016, 2018Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

• cites Planavsky et al. 2014 sourceNot checked — no full text of the source

Similar geochemical signatures are observed in slightly younger Archean iron formations.

“Similar geochemical signatures are also observed in slightly younger Archean iron formations (Kurzweil et al. 2016).”

• cites Kurzweil et al. 2016 sourceNot checked — no full text of the source

In the Nauga Formation, observations co-occur with evidence from stable iron isotopes for partial iron oxidation.

“In the Nauga Formation, these observations co-occur with evidence from stable iron isotopes for partial iron oxidation (Czaja et al. 2012).”

• cites Czaja et al. 2012 sourceNot checked — no full text of the source

There is evidence for local accumulation of dissolved O2 in surface-ocean environments prior to the initial oxygenation of Earth's atmosphere.

“At the same time, however, there is evidence for the local accumulation of dissolved O 2 in surface-ocean environments prior to the initial oxygenation of Earth's atmosphere.”

Anomalously positive carbon isotope records during this time period likely point to high rates of primary productivity.

“Anomalously positive carbon isotope records during this time period-whether they track a local or global signal (Geyman & Maloof 2019, Karhu & Holland 1996)-likely point to high rates of primary productivity.”

• cites Geyman & Maloof 2019 sourceNot checked — no full text of the source
• cites Karhu & Holland 1996Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

Genomic evidence indicates a relatively rapid and comprehensive expansion of electron transfer pathways and oxygen-utilizing enzymes during the Archean.

“This is consistent with genomic evidence for a relatively rapid and comprehensive expansion of electron transfer pathways (David & Alm 2011) and oxygenutilizing enzymes ( Jabło ńska & Tawfik 2021) during the Archean.”

• cites David & Alm 2011Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

• cites Jabło ńska & Tawfik 2021Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

The oasis regime implies that oxygenic photosynthesis, along with an inclusive range of aerobic and high-potential electron transfer metabolisms, could exist in shallow-ocean environments while potentially remaining cryptic to geochemical tracers of atmospheric O2 abundance.

“Second, this regime of ocean oxygenation implies that oxygenic photosynthesis, along with an inclusive range of aerobic and high-potential electron transfer metabolisms, could exist in shallow-ocean environments while potentially remaining cryptic to geochemical tracers of atmospheric O 2 abundance.”

Significant enrichments in solid-phase manganese, along with co-occurring enrichments in 95Mo, occur in iron-rich sedimentary rocks from the Sinqeni Formation deposited at approximately 3.0 Gya.

“Particularly striking are significant enrichments in solid-phase manganese, along with co-occurring enrichments in 95 Mo, in iron-rich sedimentary rocks from the Sinqeni Formation (Mozaan Group, South Africa) deposited at ∼3.0 Gya (Figure 5).”

All of these observations are consistent with an atmospheric pO2 perhaps orders of magnitude below what should be required to ventilate large regions of the ocean interior.

“However, all of these observations are consistent with an atmospheric pO 2 perhaps orders of magnitude below what should be required to ventilate large regions of the ocean interior (Figure 2b).”

Massive iron formations, many of which appear to have formed under a water column in which iron was always more abundant than oxygen, indicate essentially oxygen-free oceans during some periods of Archean time.

“Massive iron formations-many of which appear to have formed under a water column in which iron was always more abundant than oxygen (Busigny et al. 2014, Planavsky et al. 2012)-also indicate essentially oxygen-free oceans during some periods of Archean time.”

• cites Busigny et al. 2014 sourceNot checked — no full text of the source
• cites Planavsky et al. 2012Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

As the ocean nutrient abundance drops, the oasis/equilibrium transition should shift to lower atmospheric pO2.

“Specifically, as the ocean nutrient abundance drops, the oasis/equilibrium transition should shift to lower atmospheric pO 2 .”

A wide range of observations from Earth's rock record indicate a transition to a more oxidizing Earth surface environment after approximately 2.3 Gya, the Great Oxidation Event.

“A wide range of observations from Earth's rock record indicate a transition to a more oxidizing Earth surface environment after ∼2.3 Gya-the Great Oxidation Event (Bekker et al. 2005 and references therein).”

• cites Bekker et al. 2005 sourceNot checked — no full text of the source

Stable calcium isotope composition of Tulomozero evaporites, together with presence of magnesium sulfate minerals, constrains contemporaneous seawater to contain at least approximately 10 mmol kg-1.

“The stable calcium isotope composition of Tulomozero evaporites, together with the presence of magnesium sulfate minerals, further constrains contemporaneous seawater to contain at least ∼10 mmol kg -1 (Blättler et al. 2018).”

• cites Blättler et al. 2018 sourceNot checked — no full text of the source

Accumulating sedimentological and geochemical observations suggest a transient period of elevated atmospheric O2 between approximately 2.2 and 2.0 Gya in association with a very large perturbation to Earth's carbon cycle.

“Accumulating sedimentological and geochemical observations suggest a transient period of elevated atmospheric O 2 between approximately 2.2 and 2.0 Gya (Figure 3a) in association with a very large perturbation to Earth's carbon cycle (Karhu & Holland 1996).”

• cites Karhu & Holland 1996Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

The emergence and ecological expansion of oxygenic photosynthesis would have given rise to two broad regimes of surface-ocean oxygenation.

“The emergence and ecological expansion of oxygenic photosynthesis would thus have given rise to two broad regimes of surface-ocean oxygenation (Figure 4).”

In the equilibrium regime, surface dissolved O2 is governed largely by gas exchange equilibrium with the atmosphere.

“In the equilibrium regime-characteristic of the modern oceans-surface dissolved [O 2 ] is governed largely by gas exchange equilibrium with the atmosphere.”

Authigenic iodine enrichments in shallow marine carbonates and early diagenetic carbonate concretions suggest widespread oxygenation of the upper oceans during some intervals of the mid-Proterozoic.

“Authigenic iodine enrichments in shallow marine carbonates and early diagenetic carbonate concretions also suggest widespread oxygenation of the upper oceans during some intervals of the mid-Proterozoic (Liu et al. 2020, Shang et al. 2019).”

• cites Liu et al. 2020 sourceNot checked — no full text of the source
• cites Shang et al. 2019 sourceNot checked — no full text of the source

Redox-sensitive trace elements, sedimentary iron speciation, and stable molybdenum isotope systematics in the Francevillian Group in western Africa suggest local ocean oxygenation beneath storm wave base at approximately 2.1-2.0 Gya.

“Redox-sensitive trace elements, sedimentary iron speciation, and stable molybdenum isotope systematics in the Francevillian Group in western Africa suggest local ocean oxygenation beneath storm wave base (e.g., on the order of 10-100 m) at ∼2.1-2.0Gya (Canfield et al. 2015).”

• cites Canfield et al. 2015Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

The copper-rich composition of volcanogenic massive sulfide deposits requires hydrostatic pressure equivalent to a depth of at least ∼1 km to prevent loss of copper prior to mineralization at the seafloor.

“Importantly, the copper-rich composition of these deposits requires hydrostatic pressure equivalent to a depth of at least ∼1 km to prevent loss of copper prior to mineralization at the seafloor (Slack et al. 2007(Slack et al. , 2009)).”

• cites Slack et al. 2007 sourceNot checked — no full text of the source
• cites Slack et al. 2009 sourceNot checked — no full text of the source

Multiproxy investigation of the approximately 1.8-Gya Stambaugh Formation indicates episodic deep-water oxygenation against a background of predominantly anoxic conditions.

“For example, multiproxy investigation of the ∼1.8-GyaStambaugh Formation in the United States indicates episodic deep-water oxygenation against a background of predominantly anoxic conditions (Planavsky et al. 2018b).”

• cites Planavsky et al. 2018bNot matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

Limited reactive iron enrichment and muted enrichment of redox-sensitive trace elements in basinal shales from the Kaltasy Formation are consistent with at least weakly oxygenated conditions in some deep-shelf environments contemporaneous with euxinic and ferruginous conditions in similar depositional environments elsewhere.

“Limited reactive iron enrichment and muted enrichment of redox-sensitive trace elements in basinal shales from the Kaltasy Formation in central Russia (∼1.4 Gya) are consistent with at least weakly oxygenated conditions in some deep-shelf environments contemporaneous with euxinic and ferruginous conditions in similar depositional environments elsewhere (Sperling et al. 2014).”

• cites Sperling et al. 2014 sourceNot checked — no full text of the source

Ocean-sediment models of global sulfur cycling imply that these sulfate levels require largely oxygenated oceans.

“For instance, ocean-sediment models of global sulfur cycling imply that these sulfate levels require largely oxygenated oceans (Fakhraee et al. 2019).”

• cites Fakhraee et al. 2019 sourceNot checked — no full text of the source

Volcanogenic massive sulfide deposits provide one of the few windows into true deep-ocean chemistry during mid-Proterozoic time.

“Volcanogenic massive sulfide deposits, often containing iron-rich chemical sediments and associated sulfide ores rich in copper, provide one of the few windows into true deep-ocean chemistry during mid-Proterozoic time (Slack et al. 2007).”

• cites Slack et al. 2007 sourceNot checked — no full text of the source

• AssumptionRests on an assumption: True archives of deep-ocean oxygenation prior to the Cenozoic are extremely rare.

Existing constraints on ocean oxygenation during the mid-Proterozoic yield complex and variable signals.

“Existing constraints on ocean oxygenation during the mid-Proterozoic (∼1.8-0.8Gya) yield complex and variable signals.”

Marked redox-sensitive trace element and 238U enrichments in siliciclastic sedimentary rocks from the Zaonega Formation provide additional evidence of widespread ocean oxygenation at approximately 2.0 Gya.

“Marked redox-sensitive trace element and 238 U enrichments in siliciclastic sedimentary rocks from the slightly younger Zaonega Formation in northwest Russia provide additional evidence of widespread ocean oxygenation at ∼2.0 Gya (Mänd et al. 2020).”

• cites Mänd et al. 2020Not matched

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Taken together with quantitative estimates of elevated seawater sulfate, these observations point toward pervasive ocean oxygenation, perhaps on a scale comparable to that of the recent/modern Earth.

“However, taken together with quantitative estimates of elevated seawater SO 4 2-, these observations point toward pervasive ocean oxygenation, perhaps on a scale comparable to that of the recent/modern Earth.”

Taken together, these observations indicate pervasively anoxic, but sulfur-poor, ocean chemistry for much of mid-Proterozoic time.

“Taken together, these observations indicate pervasively anoxic, but sulfur-poor, ocean chemistry for much of mid-Proterozoic time.”

These observations are consistent with recent oxygen isotopic evidence for a dramatic drop in oceanic primary production after approximately 2 Gya.

“These observations are consistent with recent oxygen isotopic evidence for a dramatic drop in oceanic primary production after ∼2 Gya (Hodgskiss et al. 2019).”

• cites Hodgskiss et al. 2019 sourceNot checked — no full text of the source

There is evidence for a relatively rapid drop in ocean-atmosphere oxygen levels and marine primary production following this apparent period of transient ocean oxygenation.

“There is evidence for a relatively rapid drop in ocean-atmosphere oxygen levels and marine primary production following this apparent period of transient ocean oxygenation.”

Trace and rare-earth element systematics of similar volcanogenic massive sulfide deposits formed at approximately 1.8, 1.7, 1.4, and 1.2 Gya reflect weakly oxygenated (suboxic) conditions in the deep ocean for at least some periods of the mid-Proterozoic.

“The trace and rare-earth element systematics of similar volcanogenic massive sulfide deposits formed at approximately 1.8 Gya, 1.7 Gya, 1.4 Gya, and 1.2 Gya have been interpreted to reflect weakly oxygenated (suboxic) conditions in the deep ocean for at least some periods of the mid-Proterozoic (Little et al. 2021;Slack et al. 2007Slack et al. , 2009)).”

• cites Little et al. 2021 sourceNot checked — no full text of the source
• cites Slack et al. 2007 sourceNot checked — no full text of the source
• cites Slack et al. 2009 sourceNot checked — no full text of the source

There is evidence from rare-earth element systematics for anoxic conditions in very shallow water, likely greater than 50 m.

“There is also evidence from rare-earth element systematics for anoxic conditions in very shallow water (likely >50 m) (Bellefroid et al. 2018).”

• cites Bellefroid et al. 2018 sourceNot checked — no full text of the source

Stable oxygen isotope compositions of marine and lacustrine evaporite minerals suggest reduced biospheric productivity during the mid-Proterozoic.

“A lowproductivity marine biosphere is also consistent with the stable oxygen isotope compositions of marine and lacustrine evaporite minerals, which have been interpreted to suggest reduced biospheric productivity during the mid-Proterozoic (Crockford et al. 2018).”

• cites Crockford et al. 2018 sourceNot checked — no full text of the source

The apparent expansion of well-oxygenated ocean conditions correlates temporally with the rise of vascular land plants and with putative increases in the depth and intensity of rooting in terrestrial ecosystems.

“The apparent expansion of well-oxygenated ocean conditions correlates temporally with the rise of vascular land plants, and specifically with putative increases in the depth and intensity of rooting in terrestrial ecosystems (e.g., Wallace et al. 2017).”

• cites Wallace et al. 2017 sourceNot checked — no full text of the source

A weakly oxygenated state is fully consistent with constraints based on stable uranium isotope systematics and the geochemistry of deep-sea volcanogenic massive sulfide deposits.

“and is fully consistent with constraints based on stable uranium isotope systematics (Yang et al. 2017) and the geochemistry of deep-sea volcanogenic massive sulfide deposits (Slack et al. 2007(Slack et al. , 2009)).”

• cites Yang et al. 2017 sourceNot checked — no full text of the source
• cites Slack et al. 2007 sourceNot checked — no full text of the source
• cites Slack et al. 2009 sourceNot checked — no full text of the source

A weakly oxygenated redox state would be difficult to distinguish from oxic depositional conditions using iron speciation or trace element enrichments.

“This redox state would be difficult to distinguish from oxic depositional conditions using iron speciation or trace element enrichments (Scholz 2018, Sperling et al. 2014)”

• cites Scholz 2018 sourceNot checked — no full text of the source
• cites Sperling et al. 2014 sourceNot checked — no full text of the source

Earth's mid-Proterozoic rock record provides evidence for an oligotrophic ocean in which the ocean redox structure is maintained in a mixed anoxic/weakly oxygenated state at relatively low atmospheric pO2 and low marine nutrient abundance.

“Altogether, Earth's mid-Proterozoic rock record provides evidence for an oligotrophic ocean in which the ocean redox structure is maintained in a mixed anoxic/weakly oxygenated state at relatively low atmospheric pO 2 and low marine nutrient abundance.”

Rapid temperature changes during transient carbon cycle perturbations appear to cause significant increases in the extent of marine anoxia.

“In marked contrast to protracted secular climate cooling over the last ∼50 million years, rapid temperature changes during transient carbon cycle perturbations appear to cause significant increases in the extent of marine anoxia (Figure 9).”

The oceans did not transition into a modern, well-oxygenated ocean until late in Earth's history (<400 Ma), after the initial emergence of land plants.

“it seems increasingly likely that the oceans did not transition into a modern, well-oxygenated ocean until late in Earth's history (<400 Ma), after the initial emergence of land plants.”

The extent of marine anoxia appears to have waxed and waned periodically, particularly during the Cambrian.

“Similar to the Neoproterozoic, there are intervals-particularly during the Cambrian-during which the extent of marine anoxia appears to have waxed and waned periodically.”

Combined evidence from local, regional, and global redox proxies suggests extensive anoxia during the Cambrian, Ordovician, and Silurian.

“Specifically, combined evidence from local (Sperling et al. 2015), regional (Lu et al. 2018, Wallace et al. 2017), and global (Dahl et al. 2019, Wei et al. 2020) redox proxies suggests extensive anoxia during the Cambrian (∼541-485 Mya), Ordovician (∼485-444 Mya), and Silurian (∼444-420 Mya).”

• cites Sperling et al. 2015 sourceNot checked — no full text of the source
• cites Lu et al. 2018Not matched

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• cites Wallace et al. 2017 sourceNot checked — no full text of the source

Current state-of-the-art Earth system models forced by temperature changes alone do a poor job of predicting the spatial pattern of anoxic conditions.

“However, although lower oxygen solubility and intensified ocean stratification can be important drivers of ocean deoxygenation (see above), current state-of-the-art Earth system models forced by temperature changes alone (Penn et al. 2018) do a poor job of predicting the spatial pattern of anoxic conditions (Eltom et al. 2017;Huang et al. 2017Huang et al. , 2019;;Isozaki 1997;Lei et al. 2017).”

• cites Penn et al. 2018 sourceNot checked — no full text of the source
• cites Eltom et al. 2017 sourceNot checked — no full text of the source
• cites Huang et al. 2017Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

The Mesozoic and Cenozoic Earth system has been characterized by relatively well-oxygenated oceans overall.

“It is useful to distinguish between two different modes of climate shift observed in the Mesozoic (∼251-65 Mya) and Cenozoic (∼65 Mya to the present) Earth system, which has been characterized by relatively well-oxygenated oceans overall.”

Regions of the ocean interior could become weakly oxygenated despite relatively low atmospheric pO2.

“One result is the possibility that regions of the ocean interior could become weakly oxygenated despite relatively low atmospheric pO 2 .”

The oxygenation of Earth's oceans has followed largely in step with secular increases in atmospheric pO2 and co-occurring changes to marine nutrient abundance.

“The oxygenation of Earth's oceans has thus followed largely in step with secular increases in atmospheric pO 2 and co-occurring changes to marine nutrient abundance.”

Despite evidence for step changes in the background state of the marine redox landscape, there have been significant transient ocean oxygenation and deoxygenation events through the Proterozoic and Phanerozoic.

“Despite evidence for step changes in the background state of the marine redox landscape, there have been significant transient ocean oxygenation and deoxygenation events through the Proterozoic and Phanerozoic.”

The overall magnitude of disequilibrium between local dissolved [O2] and the global atmosphere is governed by patterns of ocean circulation and marine nutrient availability.

“However, the overall magnitude of disequilibrium between local dissolved [O 2 ] and the global atmosphere is governed by patterns of ocean circulation and marine nutrient availability.”

Large-scale patterns of ocean oxygenation are driven fundamentally by the combined effects of atmospheric pO2 and marine nutrient availability.

“Large-scale patterns of ocean oxygenation are driven fundamentally by the combined effects of the partial pressure of O 2 (pO 2 ) in the atmosphere and marine nutrient availability.”

Atmospheric pO2 and marine nutrient availability are mechanistically linked, such that low-O2, high-productivity Earth system states are unlikely to be stable unless geophysical reductant fluxes are large.

“Atmospheric pO 2 and marine nutrient availability are mechanistically linked, such that low-O 2 , high-productivity Earth system states are unlikely to be stable unless geophysical reductant fluxes are large.”

Co-occurring changes to global nutrient cycling, such as increased chemical weathering and phosphorus release to the oceans at elevated temperature and circulation-driven changes to nutrient recycling within marine ecosystems, also play key roles in driving ocean deoxygenation during transient climate warming.

“This suggests that co-occurring changes to global nutrient cycling, such as increased chemical weathering and phosphorus release to the oceans at elevated temperature and circulation-driven changes to the recycling of nutrients within marine ecosystems, also play key roles in driving ocean deoxygenation during transient climate warming.”