Title: Iron isotope fractionation during pyrite formation in a sulfidic Precambrian ocean analogue

Abstract

We present that the chemical response of the Precambrian oceans to rising atmospheric O₂ levels remains controversial. The iron isotope signature of sedimentary pyrite is widely used to trace the microbial and redox states of the ocean, yet the iron isotope fractionation accompanying pyrite formation in nature is difficult to constrain due to the complexity of the pyrite formation process, difficulties in translating the iron isotope systematics of experimental studies to natural settings, and insufficient iron isotope datasets for natural euxinic (i.e. anoxic and sulfidic) marine basins where pyrite formation occurs. Herein we demonstrate, that a large, permil-level shift in the isotope composition of dissolved iron occurs in the Black Sea euxinic water column during syngenetic pyrite formation. Specifically, iron removal to syngenetic pyrite gives rise to an iron isotope fractionation factor between Fe(II) and FeS₂ of 2.75 permil (‰), the largest yet reported for reactions under natural conditions that do not involve iron redox chemistry. These iron isotope systematics offer the potential to generate permil-level shifts in the sedimentary pyrite iron isotope record due to partial drawdown of the oceanic iron inventory. The implication is that the iron stable isotope signatures of sedimentary pyrites may record fundamental regime shiftsmore » between pyrite formation under sulfur-limited conditions and pyrite formation under iron-limited conditions. To this end, the iron isotope signatures of sedimentary pyrite may best represent the extent of euxinia in the past global ocean, rather than its oxygenation state. On this basis, the reinterpreted sedimentary pyrite Fe isotope record suggests a fundamental shift towards more sulfidic oceanic conditions coincident with the ‘Great Oxidation Event’ around 2.3 billion years ago. Importantly, this does not require the chemical state of the ocean to shift from mainly de-oxygenated to predominantly oxygenated in parallel with the permanent rise in atmospheric oxygen, contrary to other interpretations based on iron isotope systematics.« less

Authors:

Rolison, John M. ^[1]; Stirling, Claudine H. ^[1]; Middag, Rob  ^[2];

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Gault-Ringold, Melanie ^[3]; George, Ejin ^[1]; Rijkenberg, Micha J. A. ^[4]

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+ Show Author Affiliations

1. University of Otago, Dunedin (New Zealand). Department of Chemistry and Centre for Trace Element Analysis
2. University of Otago, Dunedin (New Zealand). Department of Chemistry ; Royal Netherlands Institute for Sea Research, Texel (The Netherlands). Department of Ocean Systems (OCS); Utrecht Univeristy, Texel (The Netherlands)
3. University of Otago, Dunedin (New Zealand). Department of Chemistry and Centre for Trace Element Analysis; Univ. of Tasmania, Hobart, TAS (Australia). Antarctic Climate and Ecosystems CRC
4. Royal Netherlands Institute for Sea Research, Texel (The Netherlands). Department of Ocean Systems (OCS); Utrecht Univeristy, Texel (The Netherlands)

Publication Date:

2018-02-19

Research Org.:

Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)

Sponsoring Org.:

USDOE National Nuclear Security Administration (NNSA)

OSTI Identifier:

1424114

Report Number(s):

LLNL-JRNL-725981
Journal ID: ISSN 0012-821X; TRN: US1801909

Grant/Contract Number:  

AC52-07NA27344

Resource Type:

Accepted Manuscript

Journal Name:

Earth and Planetary Science Letters

Additional Journal Information:

Journal Volume: 488; Journal Issue: C; Journal ID: ISSN 0012-821X

Publisher:

Elsevier

Country of Publication:

United States

Language:

English

Subject:

54 ENVIRONMENTAL SCIENCES; iron isotope fractionation; paleo-redox proxy; pyrite formation; redox-sensitive trace metal; Black Sea; Great Oxidation Event