Anoxic development of sapropel S1 in the Nile Fan inferred from redox sensitive proxies, Fe speciation, Fe and Mo isotopes

Matthews, Alan; Azrieli-Tal, Irit; Benkovitz, Ayelet; Bar‐Matthews, Miryam; Vance, Derek; Poulton, Simon W.; Teutsch, Nadya; Almogi‐Labin, Ahuva; Archer, Corey

doi:10.1016/j.chemgeo.2017.10.028

Cited by 27 publications

(45 citation statements)

References 115 publications

(150 reference statements)

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“…1; Supplementary Information) are similar to those previously published values for sapropel S1, suggesting that these sediments were deposited under mildly euxinic conditions ([H 2 S] < 11 μM) ( Fig. 2; Azrieli-Tall et al, 2014;Matthews et al, 2017;Andersen et al, 2020). Alternative models for low δ 98 Mo values for these sediments, such as deposition associated with Fe or Mn (oxyhydr)oxides (e.g., Reitz et al, 2007) are not supported by the Mn, Mo, and U concentration data as these data do not show evidence for a strong Fe-Mn particulate shuttle (Supplementary Information; Figs.…”

Section: Discussionsupporting

confidence: 89%

“…Interpretative Framework for Mo Concentrations and Isotopes. Combining Black Sea surface sediment data and a mechanistic understanding of Mo isotope systematics derived from kinetic models (Matthews et al, 2017) may facilitate a more detailed interpretation of sedimentary Mo signals. Surface sediment data from different Black Sea water depths highlights systematic patterns that reflect variations in dissolved Mo and H 2 S concentrations ( Fig.…”

Section: Discussionmentioning

confidence: 99%

“…High δ 98 Mo (up to ∼2.3 ‰) values have been found for S5, reflecting strongly euxinic conditions with long deep water renewal times (Andersen et al, 2018). By contrast, low δ 98 Mo values (mostly <1 ‰) have been found for sapropel S1, indicating on average lower bottom water H 2 S concentrations (Reitz et al, 2007;Azrieli-Tal et al, 2014;Matthews et al, 2017;Andersen et al, 2020).…”

Section: Introductionmentioning

confidence: 96%

“…Low δ 98 Mo values in mildly euxinic environments can be attributed to isotope fractionation associated with the incomplete conversion of molybdate to thiomolybdate species, with the more sulfidised species becoming progressively isotopically lighter (Tossell, 2005;Kerl et al, 2017). Large isotopic differences between sediments and seawater for such sediments can be modelled by the kinetically controlled preferential scavenging of the more sulfidised thiomolybdate species (Dahl et al, 2010;Matthews et al, 2017). Sedimentary Mo isotope values only approach the original seawater composition in strongly restricted and euxinic basins (e.g., Neubert et al, 2008;Brüske et al, 2020).…”

Section: Introductionmentioning

confidence: 99%

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Molybdenum isotope constraints on the temporal development of sulfidic conditions during Mediterranean sapropel intervals

Sweere¹,

Hennekam²,

Vance³

et al. 2021

Geochem. Persp. Let.

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Mediterranean sapropels represent some of the largest scale deoxygenation events in recent Earth history. Here, we use high resolution Mo isotope data for seven such events (sapropels S3 to S9) to semi-quantitatively constrain past H 2 S concentrations using a new interpretive framework. Bottom water H 2 S was present for all studied sapropels, but the extent of redox changes varied considerably between them, the ultimate driver likely being variations in monsoon strength. Nearquantitative removal of Mo (δ 98 Mo > 2 ‰) during deposition of sapropels S5 and S7 suggests predominantly highly sulfidic conditions with long deep water residence times, comparable to the modern Black Sea, whereas considerably lower δ 98 Mo values for sapropels S3, S4, S8, and S9 (−0.4 to þ0.9 ‰) imply mildly euxinic conditions only (0 < H 2 S < 11 μmol/L). The high resolution data reveal consistent temporal patterns that track the development of basin restriction and euxinia over several kyr. These observations illustrate how Mo isotopes can provide quantitative constraints on basin wide redox changes on relatively short time scales.

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Section: Discussionsupporting

confidence: 89%

Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 96%

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

Molybdenum isotope constraints on the temporal development of sulfidic conditions during Mediterranean sapropel intervals

Sweere¹,

Hennekam²,

Vance³

et al. 2021

Geochem. Persp. Let.

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“…Iron speciation measurements were made at the Weizmann Institute of Science using the calibrated extraction of ( 63 ), as modified for application to recent sediments [e.g., ( 64 , 65 )]. The extracted fractions are defined operationally, after ( 66 , 67 ).…”

Section: Methodsmentioning

confidence: 99%

Strong local, not global, controls on marine pyrite sulfur isotopes

et al. 2021

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Understanding variation in the sulfur isotopic composition of sedimentary pyrite (δ34Spyr) is motivated by the key role of sulfur biogeochemistry in regulating Earth’s surface oxidation state. Until recently, the impact of local depositional conditions on δ34Spyr has remained underappreciated, and stratigraphic variations in δ34Spyr were interpreted mostly to reflect global changes in biogeochemical cycling. We present two coeval δ34Spyr records from shelf and basin settings in a single sedimentary system. Despite their proximity and contemporaneous deposition, these two records preserve radically different geochemical signals. Swings of ~65‰ in shelf δ34Spyr track short-term variations in local sedimentation and are completely absent from the abyssal record. In contrast, a long-term ~30‰ decrease in abyssal δ34Spyr reflects regional changes in ocean circulation and/or sustained pyrite formation. These results highlight strong local controls on δ34Spyr, calling for reevaluation of the current practice of using δ34Spyr stratigraphic variations to infer global changes in Earth’s surface environment.

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Tracing water column euxinia in Eastern Mediterranean Sapropels S5 and S7

et al. 2020

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Sapropels S5 and S7 formed in the semi-enclosed Eastern Mediterranean Sea (EMS) during peak interglacial periods MIS5e and MIS7a, respectively, are considered among the most strongly developed Quaternary sapropels. This study investigates the redox dynamics of the water column during their formation, via Fe isotope and Fe speciation studies of cores taken at 2550 m depth at site ODP-967. Both sapropels show an inverse correlation between d 56 Fe and Fe T /Al, with slopes mostly matching that found for the Black Sea, pointing to a benthic shelf to basin shuttle of Fe and subsequent precipitation of Fe sulphides in euxinic bottom waters. An exception to these Black Sea-type trends occurs during the later, peak, stages of S7, where the negative d 56 Fe -Fe T /Al slope shallows. Fe speciation studies reveal that the dominant highly reactive Fe phase (Fe HR ) in the sapropels is pyrite, with Fe (oxyhydr)oxides forming the second major mineral component. Correspondingly, Fe HR /Fe T plots show increased strengthening of anoxic water conditions during the passage from presapropel sediment into the sapropel. Nevertheless, despite the evidence for euxinic conditions from both Fe isotopes and high Mo concentrations in the sapropel, Fe py /Fe HR ratios remain below values commonly used to identify water column euxinia. This apparent contradiction is ascribed to the sedimentary preservation of a high flux of crystalline Fe (oxyhydr)oxide minerals to the basin, which resulted in a relatively low degree of sulphidation, despite the presence of euxinic bottom waters.Thus, the operationally defined ferruginous/euxinic boundary for EMS sapropels is better placed at Fe py /Fe HR = 0.6, which is somewhat below the usually ascribed lower limit of 0.7. Consistent with the significant presence of crystalline Fe (oxyhydr)oxides, the change in the d 56 Fe -Fe T /Al slope during peak S7 is ascribed to an enhanced monsoon-driven flux of detrital Fe(III) oxides from the River Nile into the EMS basin and comcomitant diagenetic sulphidation. Euxinic water column conditions in sapropel S5 and S7 are interpreted here to reflect the positive balance between dissolved sulphide formation and rates of reductive dissolution of Fe (oxyhydr)oxide minerals. Both of these parameters in turn depend on the extent to which water overturn times are reduced and export productivity increased during sapropel formation.

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Anoxic development of sapropel S1 in the Nile Fan inferred from redox sensitive proxies, Fe speciation, Fe and Mo isotopes

Cited by 27 publications

References 115 publications

Molybdenum isotope constraints on the temporal development of sulfidic conditions during Mediterranean sapropel intervals

Molybdenum isotope constraints on the temporal development of sulfidic conditions during Mediterranean sapropel intervals

Strong local, not global, controls on marine pyrite sulfur isotopes

Tracing water column euxinia in Eastern Mediterranean Sapropels S5 and S7

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