TY - JOUR

T1 - Ocean redox conditions between the snowballs

T2 - geochemical constraints from Arena Formation, East Greenland

AU - Scheller, Eva L.

AU - Dickson, Alexander J.

AU - Canfield, Donald E.

AU - Korte, Christoph

AU - Kristiansen, Kasper K.

AU - Dahl, Tais W.

PY - 2018/12/1

Y1 - 2018/12/1

N2 - The emergence of animal ecosystems is largely believed to have occurred in increasingly oxygenated oceans after the termination of the Sturtian and Marinoan glaciations. This transition has led to several hypotheses for the mechanism driving ocean oxygenation and animal evolution. One hypothesis is that enhanced weathering increased oceanic nutrient levels, primary productivity and organic carbon burial, and ultimately oxygenated the atmosphere and oceans. Another hypothesis suggests that an animal-driven reorganization of the marine biogeochemical cycles might have oxygenated the oceans. Through molybdenum (Mo), carbon (C), sulfur (S) isotopes and iron (Fe) speciation results from the Arena Fm, East Greenland, this study constrains ocean redox conditions during the Cryogenian, after the Sturtian deglaciation and before the major radiation of animals. Carbon and sulfur isotope stratigraphy is used to correlate the Arena Fm with other formations worldwide between the Sturtian and Marinoan glaciations (~720-635 Ma). The lower part of the Arena Fm (~25 m) consists of black shales deposited under locally euxinic conditions as evidenced by high proportions of highly reactive iron (FeHR/FeT > 0.38) and pyrite (FePY/FeHR > 0.7). These black shales display small Mo enrichments (<3 ppm) and low Mo/TOC compared to overlying shales and Phanerozoic euxinic sediments. The maximum δ98Mo value is observed in the basal Arena Fm (1.5‰). Many samples display lower δ98Mo than typical oceanic input fluxes, which can be explained by Mo isotope fractionation from a marine Mo pool with δ98Mo ~1.3‰, similar to that inferred from other Cryogenic euxinic basins. The combination of low [Mo] and δ98Mo suggests that widespread anoxia prevailed in the oceans at this time. Our data are consistent with most other studies from this time suggesting that ocean oxygenation was not linked to Snowball Earth deglaciation, but was delayed until animals effectively entered the scene.

AB - The emergence of animal ecosystems is largely believed to have occurred in increasingly oxygenated oceans after the termination of the Sturtian and Marinoan glaciations. This transition has led to several hypotheses for the mechanism driving ocean oxygenation and animal evolution. One hypothesis is that enhanced weathering increased oceanic nutrient levels, primary productivity and organic carbon burial, and ultimately oxygenated the atmosphere and oceans. Another hypothesis suggests that an animal-driven reorganization of the marine biogeochemical cycles might have oxygenated the oceans. Through molybdenum (Mo), carbon (C), sulfur (S) isotopes and iron (Fe) speciation results from the Arena Fm, East Greenland, this study constrains ocean redox conditions during the Cryogenian, after the Sturtian deglaciation and before the major radiation of animals. Carbon and sulfur isotope stratigraphy is used to correlate the Arena Fm with other formations worldwide between the Sturtian and Marinoan glaciations (~720-635 Ma). The lower part of the Arena Fm (~25 m) consists of black shales deposited under locally euxinic conditions as evidenced by high proportions of highly reactive iron (FeHR/FeT > 0.38) and pyrite (FePY/FeHR > 0.7). These black shales display small Mo enrichments (<3 ppm) and low Mo/TOC compared to overlying shales and Phanerozoic euxinic sediments. The maximum δ98Mo value is observed in the basal Arena Fm (1.5‰). Many samples display lower δ98Mo than typical oceanic input fluxes, which can be explained by Mo isotope fractionation from a marine Mo pool with δ98Mo ~1.3‰, similar to that inferred from other Cryogenic euxinic basins. The combination of low [Mo] and δ98Mo suggests that widespread anoxia prevailed in the oceans at this time. Our data are consistent with most other studies from this time suggesting that ocean oxygenation was not linked to Snowball Earth deglaciation, but was delayed until animals effectively entered the scene.

KW - Cryogenian

KW - Molybdenum

KW - Molybdenum isotopes

KW - Neoproterozoic

KW - Ocean anoxia

KW - Ocean oxygenation

KW - Snowball Earth

KW - Stable isotope fractionation

U2 - 10.1016/j.precamres.2017.12.009

DO - 10.1016/j.precamres.2017.12.009

M3 - Journal article

AN - SCOPUS:85038821348

VL - 319

SP - 173

EP - 186

JO - Precambrian Research

JF - Precambrian Research

SN - 0301-9268

ER -