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Cited by 106Paleontology and Stratigraphy of FossilsGeochemistry and Elemental AnalysisGeological and Geochemical Analysis

The chromium isotope composition of reducing and oxic marine sediments

Open access
Gueguen, Bleuenn, Reinhard, Christopher T., Algeo, Thomas J., Peterson, Larry C., Nielsen, Sune G., Wang, Xiangli, Rowe, Harry, Planavsky, Noah J.·2016·Geochimica et Cosmochimica Acta
DOI 10.1016/j.gca.2016.04.004
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Written from the paper’s full text.

TL;DR

The paper reports Cr isotope compositions of reducing Peru Margin and Cariaco Basin sediments and globally distributed oxic marine sediments to test Cr isotopes as a paleoredox proxy. It finds oxic sediments near bulk silicate Earth, Cariaco authigenic Cr near deepwater values, and Peru Margin authigenic Cr averaging +0.61‰ with δ53Cr correlating with δ15N across glacial-interglacial cycles.

Key findings

  • Oxic marine sediments have an average bulk δ53Cr of −0.05 ± 0.10‰ (2σ, n = 25), within the range of bulk silicate Earth; leach fractions are indistinguishable from bulk values, suggesting detrital Cr isotope composition is fairly constant and that authigenic δ53Cr estimates depend mainly on detrital/authigenic Cr proportions.
  • Cariaco Basin core-top leach fractions have an average δ53Cr of +0.38 ± 0.10‰ (2σ, n = 7), with no clear dependence on sample location; bulk sediments average +0.25 ± 0.12‰ (2σ, n = 7), and the authigenic value is close to Atlantic deepwater Cr (~+0.5‰).
  • Peru Margin samples have average δ53Cr of +0.59 ± 0.06‰ (2σ) for bulk sediment and +0.61 ± 0.06‰ (2σ) for leach fractions; bulk values range from +0.40‰ to +0.91‰ and leach values from +0.44‰ to +0.95‰.
  • Peru Margin δ53Cr correlates positively with δ15N (R² = 0.66; R² = 0.80 without two outliers), with higher values during interglacial intervals and lower values during glacial intervals, suggesting Cr isotopes may track short-term ocean oxygenation changes.

Methods

  • Samples include Peru Margin ODP Site 680 Hole A Unit 1 sediments from 0.23–13.00 m core depth spanning since ~160 kyr B.P., seven Cariaco Basin box-core surface samples, and globally distributed oxic marine sediments (n = 25) from the WHOI repository, mostly from below 3000 m water depth.
  • Chromium isotopes were measured on bulk sediment and weak-acid leach fractions (cold 3N HNO3 for ~24 h) by double-spike MC-ICP-MS; elemental concentrations were measured by XRF and ICP-MS, and nitrogen isotopes by EA-IRMS.
  • Authigenic δ53Cr was estimated by mass balance using Cr/Ti ratios and assumed detrital Cr/Ti and δ53Cr values, and compared with directly measured leach fractions.

Limitations

  • The paper states that the local seawater δ53Cr source for Peru Margin sediments is not known, so the effect of partial Cr reduction during Cr removal cannot be accurately determined; it also states that more work is needed on modern seawater Cr isotope composition, especially open-ocean deep water.
  • The paper states that Cariaco Basin variability explanations remain speculative and require dissolved δ53Cr measurements in Cariaco Basin waters; quantifying burial-related Cr isotope fractionation will require measuring the overlying water column along the Peru Margin.
  • Evident from the text: Cariaco surface samples may have lost the surface fluff layer and are probably only a few hundred years old; no surface sediments were measured for ODP Site 1002; detrital Cr/Ti and δ53Cr assumptions may not always hold; and some leach fractions may include minor detrital or clay-bound Cr.

Model-written summary. Check it against the paper before relying on it.

Integrity signals

Registry

Crossref · 10.1016/j.gca.2016.04.004

Cites

72 of 72 references verified0 citation errors · 0 not checkable

Statistics

Nothing to recompute.

Claims the paper makes

0 of 75 cited sources checked · 0 supported · 0 flagged

  • Findingown data

    In Peru Margin samples, δ53Cr correlates with δ15N over glacial-interglacial timescales.

    “we found that the δ 53 Cr of Peru Margin samples correlates with δ 15 N on glacial-interglacial timescales”
    Quote in the paperSubstantial claim
  • Findingown data

    The Cariaco leach average is close to the δ53Cr of Atlantic deepwater Cr, about +0.5‰.

    “and is close to that of Atlantic deepwater Cr (~+0.5‰).”
    Quote in the paperSubstantial claim
  • Findingown data

    The oxic marine sediments analyzed in this study have an average δ53Cr of -0.05 ± 0.10‰ (2σ, n = 25).

    “The average δ 53 Cr value of the oxic sediments examined here is -0.05 ± 0.10‰ (2σ, n = 25), which is within the range of δ 53 Cr values characteristic of the bulk silicate Earth.”
    Quote in the paperSubstantial claim
    • Rests on an assumption: The oxic marine sediments examined contain minimal authigenic Cr.
  • Findingown data

    That average δ53Cr for oxic sediments lies within the range typical of bulk silicate Earth.

    “The average δ 53 Cr value of the oxic sediments examined here is -0.05 ± 0.10‰ (2σ, n = 25), which is within the range of δ 53 Cr values characteristic of the bulk silicate Earth.”
    Quote in the paperSubstantial claim
    • Rests on an assumption: The oxic marine sediments examined contain minimal authigenic Cr.
  • Findingown data

    Peru Margin leach fractions have an average δ53Cr of +0.61 ± 0.06‰ (2σ).

    “and leach fractions (+0.61 ± 0.06‰; 2σ)”
    Quote in the paperSubstantial claim
  • May combine two claims. 36
  • Classified as probably not a substantial claim. 30
  • The quote only partly states this claim. 2

  • May combine two claims.

    Findingcitation

    Cariaco Basin core-top samples exhibit a range of bulk sediment δ53Cr values, with some as heavy as Atlantic seawater values and others comparatively lighter.

    “The Cariaco Basin core-top samples exhibit a range of bulk sediment δ 53 Cr values, with some as heavy as Atlantic seawater values (Bonnand et al., 2013) and others comparatively lighter (e.g., samples 4 and 7 in Fig. 5).”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Bonnand et al., 2013 source

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Findingnone given

    The proportion of clay minerals in the Peru Margin sediment can reach about 40%.

    “The proportion of clay minerals in the sediment can be as high as ~40%.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Findingown data

    Cariaco Basin bulk sediment has a lighter chromium isotope composition than Peru Margin sediment, with an average δ53Cr of +0.25 ± 0.12‰ (2σ; n = 7).

    “In contrast to Peru Margin sediments, the Cr isotope composition of Cariaco Basin bulk sediment is lighter with an average of +0.25 ± 0.12‰ (2σ; n = 7).”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingown data

    Peru Margin total organic carbon values span roughly 18 to 0.7 weight percent, with most samples above 5 weight percent.

    “Total organic content (TOC) displays a large range of values from ~18 wt% to ~0.7 wt%, though most TOC values are >5 wt%.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Findingown data

    Cariaco Basin Sites 4 and 7, both along the northwestern margin, can be distinguished by their bulk sediment δ53Cr values, and Site 4 has the lightest authigenic δ53Cr value.

    “Sites 4 and 7, which are both located along the northwestern margin of the basin, can be distinguished on the basis of their bulk sediment δ 53 Cr values, and Site 4 has the lightest authigenic δ 53 Cr value.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Findingown data

    Peru Margin Cr/Ti ratios range from 0.02 to 0.11, with most values around 0.05.

    “Cr/Ti ratios vary from 0.02 to 0.11 but most values are ~0.05”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Findingown data

    Peru Margin sediments are significantly more enriched in 53Cr than Cariaco Basin sediments and are generally isotopically heavier than published Atlantic and Pacific deepwater values.

    “First, the Peru Margin sediments are significantly more 53 Cr-enriched than the Cariaco Basin sediments and generally isotopically heavier than published Atlantic and Pacific ocean deepwater values.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingown data

    The δ53Cr of oxic sediments is within uncertainty of high-temperature rocks and similar to estimated bulk mantle Cr.

    “and that of high-temperature rocks (-0.12 ± 0.13‰;Wang et al., 2016)”
    • Also stated in: § Bulk Leach · boosted
      “but it is still well within the uncertainty range of the latter and similar to the estimated value for the bulk mantle Cr (-0.079 ± 0.129‰;Farkaš et al., 2013).”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites Wang et al., 2016 source

      Not checked in the free run.

    • cites Farkaš et al., 2013 source

      Not checked in the free run.

  • May combine two claims. The quote only partly states this claim.

    Findingcitation

    The average Cr/Ti ratio of oxic deep-ocean sediments in this study is 0.017, similar to the UCC and to Cariaco Basin oxic sediments.

    “which is also similar to the average Cr/Ti ratio calculated for the oxic deep-ocean sediments from this study (0.017).”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Reinhard et al. (2014) source

      Not checked in the free run.

  • May combine two claims.

    Findingown data

    Cariaco Basin surface samples average +0.38 ± 0.10‰ δ53Cr, at the low end of published values for Atlantic seawater and for authigenic Cr at Cariaco Site 1002.

    “Cariaco Basin core-top samples yielded an average δ 53 Cr value (+0.38 ± 0.10 ‰) that is in the lower part of the range of previously published Cr isotope values for Atlantic seawater and the authigenic Cr fraction of Cariaco Basin Site 1002 sediments.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Findingown data

    A key feature of the Peru Margin data is the strong positive correlation between the δ53Cr and δ15N profiles, which appear to track interglacial/glacial cycles over the last 160 thousand years.

    “Perhaps the most notable feature of the Peru Margin data is the striking positive correlation between the δ 53 Cr and δ 15 N profiles (Fig. 4), which appear to track interglacial/glacial cycles over the last 160 kyr B.P. (Fig. 2).”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Premisecitation

    The marine Cr cycle is mainly governed by inputs from oxidative weathering on land and by removal through authigenic burial in reducing margin sediments and anoxic basins.

    “The marine Cr cycle is primarily controlled by dissolved/particulate inputs derived from oxidative weathering on the continents and removal of Cr via authigenic burial in reducing continental margin sediments and anoxic marine basins”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites Van der Weijden and Reith, 1982 source

      Not checked in the free run.

    • cites Reinhard et al., 2013 source

      Not checked in the free run.

  • May combine two claims.

    Premisereasoning

    Oxidation of Cr(III) to Cr(VI) produces soluble chromate oxyanions that rivers carry to the oceans.

    “Oxidation of Cr(III) to Cr(VI) results in the formation of very soluble chromate oxyanion species (HCrO4 -, CrO4 2-) that are subsequently transported by rivers to the oceans.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Interpretationown data

    The Peru Margin bulk and leach δ53Cr values are comparable to Cariaco Basin samples and modern deepwater values.

    “are also comparable with those of Cariaco Basin samples and modern deepwater values.”
    Quote in the paperSubstantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Study design and conductown data

    XRF raw intensities were calibrated with USGS and internal black shale standards.

    “Raw intensities were calibrated with both USGS (SDO-1, SCo-1, SGR-1) and internal black shale standards (analyzed by the commercial company XRAL, X-Ray Assay Laboratories Ltd., Canada).”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    Scheiderich et al. (2015) reported seawater δ53Cr values from +0.61 to +1.55‰ for Arctic, Pacific, North Atlantic, and Paraná estuary samples.

    “Scheiderich et al. (2015) measured the δ 53 Cr of seawater bodies from the Arctic (Beaufort Sea), Pacific, and North Atlantic oceans, as well as seawater samples from the Paraná estuary, and obtained δ 53 Cr values ranging from +0.61 to +1.55 ‰.”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Scheiderich et al. (2015) source

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    Cariaco Basin anoxic sediments have roughly constant Cr isotope compositions for most of the last ~14.5 kyr, and detrital-corrected δ53Cr values fall within published Atlantic deepwater values.

    “Anoxic sediments of the Cariaco Basin (northern Venezuela, Caribbean Sea) show roughly constant Cr isotope compositions for most of the last ~14.5 kyr (Reinhard et al., 2014), and δ 53 Cr values corrected for detrital Cr inputs fall within the range of published Atlantic deepwater values.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Reinhard et al., 2014 source

      Not checked in the free run.

  • May combine two claims.

    Study design and conductcitation

    The O isotope profile for the study section matches the standard marine O-isotope curve, allowing accurate dating.

    “Comparison of the O isotope profile for the study section (Wefer et al., 1990) with the standard marine O-isotope curve from Imbrie et al. (1984) shows consistent results and allows the study section to be accurately dated on this basis.”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites Wefer et al., 1990

      Not checked in the free run.

    • cites Imbrie et al., 1984

      Not checked in the free run.

  • The quote only partly states this claim.

    Interpretationown data

    This Cariaco observation supports the hypothesis that quantitative CrO4 2- reduction can reliably record ambient seawater Cr isotope composition.

    “This observation is consistent with the hypothesis that, in some cases, quantitative CrO4 2-reduction can provide a reliable record of the Cr isotope composition of ambient seawater.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Premisereasoning

    After delivery to the oceans, Cr is removed by burial in reducing environments, where reduction of Cr(VI) to Cr(III) partitions Cr into the authigenic sediment fraction.

    “Upon delivery to the oceans, Cr is removed from the oceanic pool through burial in reducing environments where reduction of soluble Cr(VI)O4 2-to Cr(III) by a range of reductants results in the partitioning of Cr into the authigenic fraction of marine sediments.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Study design and conductreasoning

    The Cariaco surface sediment age is probably a few hundred years because the surface fluff layer may have been lost during sampling.

    “Because the surface fluff layer may have been lost during sampling, the age of the surface sediments is probably a few hundred years.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Interpretationreasoning

    Authigenic burial fluxes and Cr isotope fractionation during authigenic enrichment depend strongly on the thickness of the oxygenated sediment layer, which may be important for regulating oceanic Cr isotope mass balance.

    “Thus, both authigenic burial fluxes and Cr isotope fractionation during authigenic enrichment depend strongly on the thickness of the oxygenated sediment layer, a relationship that may be of primary importance in regulating oceanic Cr isotope mass balance (e.g., Reinhard et al., 2014).”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Reinhard et al., 2014 source

      Not checked in the free run.

  • May combine two claims.

    Interpretationreasoning

    When the diffusive boundary layer is large relative to the reaction-diffusion length scale, reduced Cr is mostly retained and Cr isotope fractionation relative to seawater is minimal.

    “On the other hand, if the diffusive boundary layer is large relative to the reaction-diffusion length scale, reduced Cr is mostly retained within the sediment, and Cr isotope fractionation relative to seawater will be minimal.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Study design and conductown data

    XRF analytical precision was better than ±2% for major elements and ±5% for trace elements, with detection limits of about 1-2 ppm for most trace elements.

    “Analytical precision was better than ±2% for major elements and ±5% for trace elements, and detection limits were ~1-2 ppm for most trace elements.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Interpretationcitation

    Because Cr(III)-Cr(VI) isotopic exchange is slow at Earth-surface conditions, low-temperature Cr isotope behavior is likely dominated by kinetic fractionations from partial Cr(VI) reduction.

    “due to the long timescales of isotopic exchange between Cr(III) and Cr(VI) under Earth-surface conditions (e.g., Wang et al., 2015), the isotope systematics of low-temperature Cr cycling are likely to be dominated by kinetic fractionations associated with partial Cr(VI) reduction.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Wang et al., 2015 source

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    The isotopic composition of natural Cr species is mainly controlled by redox state changes, with large fractionations expected for equilibrium Cr(III)-Cr(VI) exchange and incomplete Cr(VI) reduction.

    “the isotopic composition of natural Cr species is governed principally by shifts in redox state -with significant isotopic fractionations expected for both equilibrium exchange between Cr(III) and Cr(VI) species and incomplete reduction of Cr(VI) to Cr(III)”
    Quote in the paperSubstantial claim0 of 3 sources checked
    • cites Schauble et al., 2004

      Not checked in the free run.

    • cites Zink et al., 2010 source

      Not checked in the free run.

    • cites Wang et al., 2015 source

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    Cr(VI) reduction has been observed in open-ocean oxygen-minimum zones, but its effect on water-column Cr isotope composition has not been shown in natural samples.

    “although the reduction of Cr(VI) in open-ocean OMZ (oxygen-minimum zone) regions has been observed (Cranston and Murray, 1978;Cranston, 1983;Murray et al., 1983;Rue et al., 1997), the actual effect of this process on the Cr isotope composition in the water column has not been demonstrated in natural samples.”
    Quote in the paperSubstantial claim0 of 3 sources checked
    • cites Cranston and Murray, 1978

      Not checked in the free run.

    • cites Cranston, 1983 source

      Not checked in the free run.

    • cites Murray et al., 1983

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Study design and conductnone given

    TOC was measured with an Eltra 2000 C-S analyzer calibrated to USGS and internal laboratory standards.

    “Total organic carbon (TOC) was determined using an Eltra 2000 C-S analyzer with results calibrated to USGS and internal laboratory standards.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Premisecitation

    Hydrothermal Cr fluxes are small relative to the riverine flux, about 0.60% and 0.02% of it, respectively.

    “but these fluxes are small compared to the riverine flux (i.e., ~0.60% and ~0.02% of the riverine flux respectively, Reinhard et al., 2013)”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Reinhard et al., 2013 source

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    Cr has a shorter residence time, about 9 kyr, than U and Mo, about 800 kyr and 400 kyr.

    “Cr appears to have a shorter residence time (~9 kyr; Reinhard et al., 2013) than other redox-sensitive trace elements with tractable isotopic systems such as U and Mo (~800 kyr and ~400 kyr; Morford and Emerson, 1999;Dunk et al., 2002).”
    Quote in the paperSubstantial claim0 of 3 sources checked
    • cites Reinhard et al., 2013 source

      Not checked in the free run.

    • cites Morford and Emerson, 1999 source

      Not checked in the free run.

    • cites Dunk et al., 2002 source

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Study design and conductown data

    TOC analytical precision was better than ±2.5% of reported values.

    “Analytical precision was better than ±2.5% of reported TOC values.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Interpretationcitation

    When sediment oxygen penetration greatly exceeds the reaction-diffusion length scale, reduced Cr is mostly retained but authigenic burial fluxes are very low.

    “If sediment oxygen penetration dramatically exceeds the reaction-diffusion length scale, reduced Cr will be mostly retained but authigenic burial fluxes will be very low (e.g., Clark and Johnson, 2008).”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Clark and Johnson, 2008

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Study design and conductcitation

    Recent radiometrically calibrated SPECMAP O-isotope stacks do not appreciably change the timescale used in this study.

    “Note that recent radiometrically calibrated SPECMAP O-isotope stacks have not appreciably changed the timescale used in this study (Lisiecki and Raymo, 2005;Thompson and Goldstein, 2006).”
    Quote in the paperNot a substantial claim0 of 2 sources checked
    • cites Lisiecki and Raymo, 2005

      Not checked in the free run.

    • cites Thompson and Goldstein, 2006

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    Carbonates and oxic siliciclastic sediments make up a relatively small part of the oceanic Cr sink, and their Cr isotope compositions are not expected to be strongly fractionated relative to the oceanic pool.

    “carbonates and oxic siliciclastic sediments are thought to account for a relatively small fraction of the Cr sink flux in the oceanic Cr budget, and the Cr isotope compositions of these facies are not expected to be strongly fractionated relative to the oceanic pool”
    Quote in the paperSubstantial claim0 of 3 sources checked
    • cites Schoenberg et al., 2008 source

      Not checked in the free run.

    • cites Frei et al., 2011 source

      Not checked in the free run.

    • cites Bonnand et al., 2013 source

      Not checked in the free run.

  • May combine two claims.

    Premisenone given

    Earth-surface Cr isotope variation mainly comes from fractionation during subaerial weathering and transport, and from partial reduction in reducing marine facies and the integrated mass flux into those sediments.

    “Isotopic variation in the Earth-surface Cr cycle is mainly due to (1) the net effect of processes leading to isotopic fractionation during subaerial weathering and transport of Cr, and (2) partial reduction in reducing marine facies and the integrated mass flux into such sediments, the latter of which scales with their spatial distribution in the marine realm.”
    Quote in the paperSubstantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Study design and conductown data

    The 2se from 50 cycles per sample was generally about 0.04‰.

    “In addition, a two-standard deviation of the mean (2se) is systematically calculated using the 50 cycles of measurement obtained for each sample during MC-ICP-MS analysis and is generally about 0.04 ‰ on the reported delta values.”
    Quote in the paperNot a substantial claim

  • Classified as probably not a substantial claim.

    Study design and conductreasoning

    Complete oxidation of Cr(III) to Cr(VI) was ensured before column loading by adding ammonium persulfate and heating.

    “because only Cr(VI) is retained on the column, complete oxidation of Cr(III) to Cr(VI) was ensured through addition of ammonium persulfate and heating of the samples for 2 hours at 110°C.”
    Quote in the paperSubstantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Interpretationreasoning

    Lower Cr/Ti ratios of about 0.03 in the Cariaco Basin are also consistent with potentially higher detrital Cr/Ti ratios in Cariaco Basin sediments.

    “and also with potentially higher detrital Cr/Ti ratios in the Cariaco Basin sediments (see discussion).”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Study design and conductnone given

    The third separation step removed residual Ti using cationic resin AG50W-X8.

    “In the third step, traces of Ti were removed using a cationic resin AG50W-X8 (200-400 mesh).”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Study design and conductnone given

    The second separation step removed residual Fe after the first elution.

    “The second step removed traces of Fe that may remain after the first elution.”
    Quote in the paperNot a substantial claim

  • Classified as probably not a substantial claim.

    Study design and conductown data

    Samples were run in high-resolution mode to resolve polyatomic interferences.

    “Samples were run in high-resolution mode to resolve polyatomic interferences such as 40 Ar 12 C + , 40 Ar 14 N + and 40 Ar 16 O + .”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Interpretationown data

    This consistency suggests either that chromium isotope fractionation during authigenic uptake in sediments is roughly constant across the Cariaco Basin, or that uptake during reduction is effectively quantitative and fractionations from partial reduction are minimized.

    “This suggests that either Cr isotope fractionation during authigenic uptake in sediments is roughly constant across the basin, or that uptake during reduction is effectively quantitative and fractionations due to partial reduction are minimized (e.g., Reinhard et al., 2014).”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Reinhard et al., 2014 source

      Not checked in the free run.

    • The authors state this as an assumption: In calculating authigenic chromium isotope composition, the detrital fraction was taken to have a constant Cr/Ti ratio and a δ53Cr equal to bulk silicate Earth.

  • May combine two claims.

    Interpretationreasoning

    The chromium isotope composition of the leach fraction of Peru Margin samples is remarkably similar to that of the bulk sediment in most samples, likely mainly because of large authigenic chromium enrichments.

    “Second, the Cr isotope composition of the leach fraction of Peru Margin samples is remarkably similar to that of the bulk sediment in most samples, which is likely mainly a result of the large authigenic Cr enrichments.”
    Quote in the paperSubstantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Study design and conductown data

    Analytical precision for δ15N was better than ±0.05‰.

    “Analytical precision is better than ±0.05‰ for reported δ 15 N values.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Study design and conductcitation

    The double-spike data reduction model follows the iterative method of Siebert et al. (2001).

    “The double-spike data reduction model is based on the iterative method described by (Siebert et al., 2001).”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Siebert et al., 2001 source

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Study design and conductown data

    External precision is reported as 2σ uncertainty based on duplicate analyses of geological reference materials processed through columns with samples.

    “External precision is reported as two sigma (2σ) uncertainty, calculated based on duplicate analysis of geological reference materials (GRMs) processed through ion-exchange chromatography columns along with samples (BHVO-2 and Nod-A-1 were systematically processed with 18 samples).”
    Quote in the paperNot a substantial claim

  • Classified as probably not a substantial claim.

    Study design and conductnone given

    Cr isotopes were measured by MC-ICP-MS with a double-spike correction for instrumental mass bias.

    “Chromium isotopes were measured by MC-ICP-MS using a double-spike correction for instrumental mass bias.”
    Quote in the paperSubstantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    The average upper continental crust has a Cr/Ti ratio of about 0.02.

    “which is higher than the Cr/Ti of ~0.02 for average UCC (Rudnick and Gao, 2014).”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Rudnick and Gao, 2014

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Study design and conductown data

    Nod-A-1 gave δ53Cr 0.07 ± 0.09‰ (n = 16).

    “and Nod-A-1 yielded a δ 53 Cr value of 0.07 ± 0.09 ‰ (n=16).”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Study design and conductnone given

    δ15N values were calibrated with three in-house glycine standards previously calibrated against international standards.

    “The δ 15 N of samples was calibrated using three in-house glycine standards that were previously calibrated against international standards.”
    Quote in the paperNot a substantial claim

  • Classified as probably not a substantial claim.

    Study design and conductnone given

    Standard-sample-bracketing used spiked NIST SRM 979 measured before and after each sample.

    “Standard-sample-bracketing was employed during the analytical session by measuring a spiked Cr isotope standard NIST SRM 979 before and after each natural sample.”
    Quote in the paperSubstantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Study design and conductcitation

    The Cr isotope leaching approach followed a method developed by Reinhard et al. (2014) and previously used for ODP Site 1002.

    “Our approach was based on a leaching method developed by Reinhard et al. (2014) and previously employed in constructing a downcore Cr isotope record from ODP Site 1002 (Cariaco Basin).”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Reinhard et al. (2014) source

      Not checked in the free run.

  • May combine two claims.

    Study design and conductcitation

    BHVO-2 gave δ53Cr -0.12 ± 0.09‰ (n = 25), similar to published BHVO-1 values.

    “The δ 53 Cr value for BHVO-2 is -0.12 ±0.09 ‰ (n=25), which is similar to published values in the literature for BHVO-1 (a geostandard collected at the same location as BHVO-2; Schoenberg et al., 2008)”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Schoenberg et al., 2008 source

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

    Study design and conductown data

    ICP-MS measurement precision on raw intensities was generally better than ±4%, and USGS geostandards BHVO-2 and NOD-A-1 processed with samples were within ±10% of reported values.

    “Measurement precision on raw intensities for major and trace elements was generally better than ±4% and USGS geostandards BHVO-2 and NOD-A-1 that were processed with samples during each run are within a ±10% range of reported values.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Interpretationreasoning

    This variability in Cariaco Basin bulk sediment δ53Cr is somewhat difficult to explain because all sites are located below the chemocline and have similar sediment chemistry, including Cr/Ti ratios.

    “This variability is somewhat difficult to explain given that all sites are located below the chemocline and exhibit similar sediment chemistry (including Cr/Ti ratios).”
    Quote in the paperSubstantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Study design and conductnone given

    Chemical yields were generally 75% to 85%.

    “Chemical yields are generally between 75% and 85%.”
    Quote in the paperNot a substantial claim

  • Classified as probably not a substantial claim.

    Study design and conductcitation

    Cr isotopes were measured on a Neptune Plus MC-ICP-MS at Yale following the analytical protocol of Schoenberg et al. (2008) and Reinhard et al. (2014).

    “Chromium isotopes were measured on a Neptune Plus (Thermo-Finnigan) MC-ICP-MS at Yale University according to the analytical protocol of (Schoenberg et al., 2008;Reinhard et al., 2014).”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites Schoenberg et al., 2008 source

      Not checked in the free run.

    • cites Reinhard et al., 2014 source

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Study design and conductown data

    Fe, Ti and V were monitored and used to correct for potential isobaric interferences on Cr isotopes.

    “Although our chemical procedure should ensure complete removal of Fe, Ti and V, the presence of these three elements was monitored by measuring 56 Fe, 49 Ti and 51 V, which were used to correct our samples for potential interferences of 54 Fe on 54 Cr, and 50 Ti and 50 V on 50 Cr.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

    Interpretationreasoning

    Some detrital chromium fractions may be liberated in weak acid leaches, which would lower the δ53Cr of the leach.

    “Alternatively, this might suggest that there are detrital Cr fractions that are liberated in weak acid leaches, which would decrease the δ 53 Cr in the leach.”
    Quote in the paperNot a substantial claim

  • Classified as probably not a substantial claim. May combine two claims.

    Premisecitation

    Cariaco Basin oxic sediments have an average Cr/Ti ratio of about 0.021, indistinguishable from UCC.

    “Reinhard et al. (2014) argued that sediments deposited under oxic conditions in the Cariaco Basin have an average Cr/Ti ratio (~0.021) indistinguishable from that of the UCC”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Reinhard et al. (2014) source

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Study design and conductown data

    The 2σ error for chromium isotope measurements was calculated from replicated measurements of the isotopic standard NIST SRM 979.

    “The 2σ error for Cr isotope measurements was calculated on replicated measurements of the isotopic standard NIST SRM 979 (see text for more details).”
    Quote in the paperNot a substantial claim

  • Classified as probably not a substantial claim.

    Interpretationreasoning

    This pattern is conventionally interpreted as indicating more widespread thermocline suboxia during interglacial periods.

    “the conventional interpretation of this pattern is that thermocline suboxia was more widespread during interglacial periods.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Premisereasoning

    The isotopically heavy nitrate produced by water-column denitrification is transferred to surface waters by upwelling and assimilated by phytoplankton, and sinking organic matter transfers this heavy nitrogen isotope signature to the sediment.

    “The isotopically heavy nitrate produced by this process is then transferred to surface waters by upwelling and assimilated by phytoplankton.Ultimately, sinking organic matter will transfer this heavy N isotope signature to the sediment.”
    Quote in the paperSubstantial claim

  • Classified as probably not a substantial claim.

    Interpretationreasoning

    If limited chromium isotope fractionation between seawater and the oxic and anoxic sinks holds, the oxic sink should barely affect temporal changes in seawater chromium isotope composition.

    “If correct, then the oxic sink should have a negligible influence on temporal fluctuations in seawater Cr isotope composition.”
    Quote in the paperSubstantial claim
    • The authors state this as an assumption: There is limited chromium isotope fractionation between seawater and the oxic and anoxic sinks of chromium.
64 of 64

  • The authors state this as an assumption.

    The detrital Cr/Ti ratio can be taken as the average upper continental crust ratio of 0.024 µg.µg-1 or estimated from a local detrital baseline.

    “The detrital Cr/Ti can be set equal to the average upper continental crust (UCC) ratio of 0.024 µg.µg -1 (Rudnick and Gao, 2014) or, if possible, estimated from a local detrital baseline”

  • The authors state this as an assumption.

    The detrital δ53Cr value can be assumed similar to that of the Bulk Silicate Earth, -0.124 ± 0.101‰.

    “whereas the δ 53 Cr value of the detrital component can be assumed similar to that of the Bulk Silicate Earth (BSE) (δ 53 Cr = -0.124± 0.101 ‰; Schoenber at al., 2008).”

  • The authors state this as an assumption.

    The globally integrated riverine Cr flux to the oceans has limited isotope variability.

    “assuming limited isotope variability in the globally integrated riverine Cr flux to the oceans”

  • The paper relies on this without support.

    The oxic marine sediments examined contain minimal authigenic Cr.

    “we also measured δ 53 Cr on a variety of oxic marine sediments that contain minimal authigenic Cr.”

  • The paper relies on this without support.

    Cr isotope variation in deep open-ocean seawater is not well established.

    “Lastly, the Cr isotope variation in deep open-ocean seawater is also not well established.”

  • The paper relies on this without support.

    Temporal fluctuations of redox conditions and the impact of imbalances between the riverine Cr source flux and the marine reducing sink flux on the oceanic Cr budget remain poorly constrained.

    “Both temporal fluctuations of redox conditions in modern and ancient environments and the impact of imbalances between the riverine Cr source flux (i.e., continental oxidative weathering) and the size of the marine reducing sink flux on the oceanic Cr budget remain poorly constrained at present.”

  • The authors state this as an assumption.

    In calculating authigenic chromium isotope composition, the detrital fraction was taken to have a constant Cr/Ti ratio and a δ53Cr equal to bulk silicate Earth.

    “For purposes of calculating the isotope composition of the authigenic Cr component in this study, the detrital component was assumed to have an invariant Cr/Ti ratio and δ 53 Cr value equal to those of BSE (Schoenberg et al., 2008).”

  • The authors state this as an assumption.

    There is limited chromium isotope fractionation between seawater and the oxic and anoxic sinks of chromium.

    “It has been assumed that there is limited Cr isotope fractionation between seawater and the oxic and anoxic sinks of chromium (Bonnand et al., 2013;Reinhard et al., 2014).”

  • The authors state this as an assumption.

    Oxic marine sediments contain negligible amounts of authigenic chromium, so the δ53Cr value of the detrital chromium component of marine sediments can be assumed to be fairly constant.

    “assuming that these sediments contain negligible amounts of authigenic Cr, the δ 53 Cr value of the detrital Cr component of marine sediments can be assumed to be fairly constant.”

  • The paper relies on this without support.

    Chromium has a marine residence time of approximately 9 thousand years.

    “the short marine residence time of Cr (~9 kyr) relative to the tempo of glacial-interglacial transitions”
10 of 10

Claims other papers make about this one

Each is checked against this paper’s own text: does it say what the citing paper says it does?

  • 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.”

    From Trace elements at the intersection of marine biological and geochemical evolution (2016) · cites this paper as Gueguen et al. (2016)

Self-check

Percentages, sums and reported differences re-computed from the paper’s own numbers.

Each contradiction between two passages listed here was confirmed by an independent check.

  • Oxic-sediment Cr/Ti percentages conflict with tabulated ratios

    Minor

    The quoted eight-sample block alone contains three ratios outside 0.01-0.02. With 25 samples in total, at least three outside the interval rules out the claim that over 90% fall within it.

  • Oxic-sediment samples exceed the stated sediment-depth limit

    Minor

    The methods state that samples came from 0-9 cm core depth, whereas Table 3 includes samples from 10-11, 12-13, and 23-24 cm.

  • Oxic-sediment leach value exceeds the stated range

    Minor

    The stated leach range ends at +0.03‰, but Table 3 reports a leach value of +0.10‰.

3 of 3

Cites

Flagged first, then by citations

8 of the 80 references couldn’t be matched to a known paper.

Research works printed but not linked: 2

  1. Dean, 1990, Scientific results – Peru Continental Margin, vol. 112
  2. Pereira, 2015, Chromium-isotope signatures in scleractinian corals from the Rocas Atoll, Tropical South Atlantic, Geobiology

Cited by

Sorted by citations

Related

Authors
Gueguen, B., Reinhard, C. T., Algeo, T. J., Peterson, L. C., Nielsen, S. G., Wang, X., Rowe, H., & Planavsky, N. J.
Affiliations
  • Georgia Institute of Technology (US)
  • University of Cincinnati (US)
  • University of Miami (US)
  • Woods Hole Oceanographic Institution (US)
  • Yale University (US)
  • Bureau of Economic Geology (US)
ORCID
Venue
Geochimica et Cosmochimica Acta
Publisher
Elsevier BV
Published
2016-07
Volume
184
Pages
1-19
ISSN
0016-7037 (ISSN-L)
DOI
10.1016/j.gca.2016.04.004
OpenAlex
W2312569948
Semantic Scholar
4bd209399fb15d13b32f7be68f1b801017970b8e
Cited by
106
References
80
Citations by year
2026: 5 · 2025: 8 · 2024: 10 · 2023: 9 · 2022: 9 · 2021: 13 · 2020: 6 · 2019: 14 · 2018: 18 · 2017: 8 · 2016: 5 · 2015: 1
FWCI
12.47
Type
journal article
Language
en
Open access
Licence
Locations
Geochimica et Cosmochimica Acta (acceptedVersion)
Funders
  • China University of Geosciences, Wuhan (GPMR201301)
  • Institute for Biospheric Studies, Yale University
  • National Aeronautics and Space Administration
Topics
  • Physical Sciences › Earth and Planetary Sciences › Paleontology › Paleontology and Stratigraphy of Fossils
  • Physical Sciences › Earth and Planetary Sciences › Geochemistry and Petrology › Geochemistry and Elemental Analysis
  • Physical Sciences › Earth and Planetary Sciences › Geophysics › Geological and Geochemical Analysis
Keywords
chromium isotopes, marine sediment, redox conditions, paleoredox proxies, Peru continental margin, Cariaco Basin, upwelling zones, nitrogen isotopes, redox-sensitive trace metals, authigenic phases, leaching

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Registry, retraction, Cites and statistics results are computed, not judged, with one exception: a printed reference that no index lists is read once by a model before it counts as a citation error. Claims are extracted by a model and kept only when their quote appears verbatim in the text; each cited source is then checked, with a model’s help, for whether it says what the claim relies on. Those verdicts can be wrong — read the claim’s quote and the cited source before relying on one.

Claims read by deepseek-v4.1-flash under definition version 3 (extract-v3) on 2026-10-08.

Classified by jev-1.13-20260917 (gold-guideline-v0.2) on 2026-10-08.

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