Title: Shifting modes of iron sulfidization at the onset of OAE-2 drive regional shifts in pyrite δ³⁴S records

Abstract

We report total reduced inorganic S isotope ratios (δ³⁴S_CRS) shift toward more negative values across much of the southern North Atlantic just before the onset of the Cenomanian-Turonian Ocean Anoxic Event (OAE-2). At the same time, there is no parallel isotopic change in the significantly larger pool of kerogen (organic) S, which indicates that the distribution and S-isotope composition of sulfide in the environment likely did not drive the change in δ³⁴S_CRS. Here, we investigate possible explanations for the negative shift in δ³⁴S_CRSvalues and their divergence from organic S by isolating iron sulfides for morphological identification and grain-specific isotopic analysis using secondary ion mass spectrometry (SIMS). In pre- and syn-OAE-2 sedimentary rocks from Demerara Rise, we find four distinct morphologies of iron sulfides: pyrite framboids (1–20 μm diameter), irregular pyrite aggregates (1–38 μm diameter), large cemented pyrite aggregates (~60 μm diameter), and irregular and cemented aggregates of the pyrite polymorph marcasite (1–45 μm diameter). These different textural groups have distinct S-isotopic compositions that are largely consistent through the onset of OAE-2. As such, the secular change in bulk δ³⁴S_CRS values likely reflects the changing proportions of these phases stratigraphically across OAE-2. All textural groups feature resolvable intra-grain δ³⁴S variability, suggestingmore » that the environments in which they formed were characterized by dynamic sulfide δ³⁴S values and/or by partial closed-system distillation. We use grain-specific δ³⁴S distributions to rule out shoaling of the chemocline within the sediments as a mechanism for the observed decrease in δ³⁴S_CRS. Instead, we propose that changes in the reactivity of the iron species delivered to Demerara Rise over the ~200 kyr leading up to the onset of OAE-2 impacted the relative contributions of pyrite with S-isotope signatures reflecting the water column, shallow sediments, and deeper sediments to the bulk sedimentary δ³⁴S_CRS value. Specifically, the change in iron reactivity at the onset of OAE-2 favored the production of ³⁴S-depleted large, cemented aggregates and framboids at the expense of more ³⁴S-enriched irregular aggregates. Our results underscore that bulk δ³⁴S_CRS measurements integrate multiple reduced phases that form via distinct reaction mechanisms and potentially in different parts of the depositional environment. Grain-specific SIMS analyses dramatically enrich our ability to interpret pyrite isotopic patterns in the geologic record.« less

Authors:

Bryant, R. N.  ^[1];

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Jones, C. ^[2]; Raven, M. R. ^[3]; Owens, J. D. ^[4]; Fike, D. A. ^[2]

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1. Washington Univ., St. Louis, MO (United States); Univ. of Chicago, IL (United States)
2. Washington Univ., St. Louis, MO (United States)
3. Washington Univ., St. Louis, MO (United States); Univ. of California, Santa Barbara, CA (United States)
4. Florida State Univ., Tallahassee, FL (United States). National High Magnetic Field Lab. (MagLab)

Publication Date:

Thu Aug 06 00:00:00 EDT 2020

Research Org.:

Washington Univ., St. Louis, MO (United States)

Sponsoring Org.:

USDOE Office of Science (SC), Biological and Environmental Research (BER); Washington University; American Chemical Society Petroleum Research; National Science Foundation (NSF); State of Florida

OSTI Identifier:

1851965

Alternate Identifier(s):

OSTI ID: 1810899

Grant/Contract Number:  

SC0014613; 57548-ND2; DMR1644779

Resource Type:

Accepted Manuscript

Journal Name:

Chemical Geology

Additional Journal Information:

Journal Volume: 553; Journal Issue: C; Journal ID: ISSN 0009-2541

Publisher:

Elsevier

Country of Publication:

United States

Language:

English

Subject:

58 GEOSCIENCES; OAE-2; S isotopes; Fe sulfides; SIMS; Raman; Pyrite; Marcasite