Biogeochemical sulfur cycling during Cretaceous oceanic anoxic events: A comparison of OAE1a and OAE2

Abstract

Biogeochemical sulfur cycling has varied widely over geologic time, mainly in response to changes in primary productivity and organic carbon burial, volcanism, weathering, and evaporite deposition. Several of these processes are explicitly linked to discreet (<1.2 Ma) intervals of widespread organic carbon burial, termed oceanic anoxic events (OAEs). During the Cretaceous, there is a highly distinctive ~4‰ negative excursion in the sulfur isotope composition of seawater sulfate (δ34SSO4) that is bracketed by the two most prominent OAEs (OAE1a and OAE2). This excursion lasted for ~25 Ma and has been variously attributed to enhanced volcanism, changes in weathering, evaporite burial, and/or changes in modes of organic carbon remineralization. We present new high‐resolution carbon and sulfur isotope records from carbonate‐associated sulfate and pyrite for OAE1a and OAE2. OAE1a is characterized by a monotonic decrease in δ34SSO4 values. Both negative and positive δ34SSO4 excursions are associated with OAE2. To refine hypotheses for the observed changes in biogeochemical sulfur cycling associated with these events, we use a simple sulfur isotope box model. Both empirical and modeling results indicate that δ34SSO4 variability was dominated by input fluxes during OAE1a, whereas enhanced volcanism, weathering, and pyrite burial controlled δ34SSO4 records during OAE2. Our analysis supports the conclusion that Cretaceous marine sulfate concentrations were much lower than modern concentrations and indicates that increases in marine sulfate occurred at the onset of both events. We conclude that increases in marine sulfate from low background concentrations, in conjunction with other environmental characteristics, contributed to the development of OAEs.

Sulfur isotope analysis of microcrystalline iron sulfides using secondary ion mass spectrometry imaging: Extracting local paleo‐environmental information from modern and ancient sediments

“…The FeS 2 recovery was estimated to be 54.5%, which again closely matches the measured recovery for our synthetic sediment samples. In addition, the physically extracted iron sulfides had a bulk δ 34 S value of −42.5 ± 0.2‰, whereas the bulk untreated sample had a δ 34 S value of −42.1 ± 0.2‰ . The agreement between the isotopic compositions of the chemically and physically extracted iron sulfides indicates that the physical extraction procedure did not impart any isotopic bias on the population of iron sulfides in the sample.…”

“…Optical microscope images of mounted extract from Cismon (Figure S7, supporting information) display the presence of both euhedral marcasite and framboidal pyrite, as supported by laser Raman microprobe spot analyses and SEM (Figure S8, supporting information). Pyrite and marcasite grains from Cismon measured by SIMS had average δ 34 S values of −41.9 ± 5.2‰ (1σ; n = 113) and −48.5 ± 5.1‰ (1σ; n = 161; Figure ), and cemented pyrite aggregates had an average δ 34 S value of −42.2 ± 0.6‰ (1σ; n = 2), compared with the bulk δ 34 S composition of −42.1‰ . There was little intra‐grain δ 34 S variation in marcasite grains (average 1σ = ±2.9‰, compared with an average intra‐grain standard error of ±3.5‰, 1σ; Figures S9A–S9D, supporting information), and very little intra‐grain δ 34 S variation in pyrite grains (average 1σ = ±3.7‰, compared with an average intra‐grain standard error of ±2.9‰, 1σ; Figures S9E–S9H, supporting information).…”

“…Sediments from the Cismon section are thought to represent nearly the opposite end‐member case to Santa Barbara Basin, with very low bulk CRS δ 34 S values (avg. –42.7‰) . These values have been interpreted to represent pyrite formation in a system that was open to the diffusive supply of sulfate, so a narrow intra‐sample range of δ 34 S values is expected.…”

“…23,24,28,29,37 The average SIMS δ 34 S values for iron sulfides from samples from the Cismon section (−45.8 ± 6.1‰, 1σ; n = 274; Figure 5) and the Demerara Rise (−25.3 ± 11.2‰, 1σ; n = 45; Figure 6) are close to the previously reported bulk δ 34 S CRS values (−42.1‰, and − 24.4‰, respectively). 51,52 The average δ 34 S value of the Santa Barbara Basin pyrites (+21.7 ± 10.3‰, 1σ; n = 53; Figure 4) is 5.6‰ higher than the bulk δ 34 S CRS value (+16.1‰), 50 Importantly, the method documents δ 34 S variability within a sample, which is critical for interpreting bulk 'CRS' δ 34 S values.…”

Massive volcanism, evaporite deposition, and the chemical evolution of the Early Cretaceous ocean

“…OAE1a occurred at ca. 125 Ma (Ogg et al, 2012), although its temporal relationship with the negative δ 34 S sulfate shift has not been documented unambiguously (Gomes et al, 2016). Given the strong coupling between the geochemical cycles of carbon and sulfur, improved understanding of how ocean chemistry and sulfate concentrations evolved over this period could provide insight into the factors that conditioned the oceans for carbon cycle instability One way to decipher the importance of increased volcanism and/or weathering rates on the sulfur cycle during this time is through the use of strontium (Sr) isotopes, because the geochemical cycles of S and Sr are linked through shared input fluxes of riverine (weathering) and hydrothermal inputs.…”