Santa Barbara Basin Flood Layers: Impact on Sediment Diagenesis

Abstract

The presence of gray layers, representing flood deposits, has previously been well documented in sediments obtained from Santa Barbara Basin, California. We studied geochemical aspects of these layers, in particular the carbon and sulfur contents and isotopic compositions, and found evidence of rapid pyrite formation. The data are consistent with complete (or nearly complete) sulfate consumption within the pore waters of a gray layer. This requires sulfate reduction rates (SRRs) that exceed the rate of resupply by diffusion, thus on the order of milimolar per day. The apparent rapidity of this diagenetic reaction is addressed. The net result is the precipitation of authigenic pyrite crystals enriched in 34S that become part of the sediment record. Thus, any interpretation of S isotope fluctuations in the marine sedimentary rock record obtained from pyrite must take into account the phenomena we describe and document here.

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

“…The recovery of pyrite for the Santa Barbara Basin sediment sample was estimated to be 41.3% by comparing the extract mass with the previously measured total S abundance . The extract purity for the Cismon section FeS 2 was estimated using EA to be 61%, and by comparing the extract mass with the CRS abundance.…”

“…The sample average δ 34 S pyrite value was +21.7 ± 10.3‰ (1σ; n = 53; Figure D). The δ 34 S pyrite variability in this sample (Figures B and D) overlaps the bulk δ 34 S CRS value for the sample of +16.1‰, and is not normally distributed (Figure D).…”

“…The average SIMS δ 34 S values for iron sulfides from samples from the Cismon section (−45.8 ± 6.1‰, 1σ; n = 274; Figure ) and the Demerara Rise (−25.3 ± 11.2‰, 1σ; n = 45; Figure ) are close to the previously reported bulk δ 34 S CRS values (−42.1‰, and − 24.4‰, respectively) . The average δ 34 S value of the Santa Barbara Basin pyrites (+21.7 ± 10.3‰, 1σ; n = 53; Figure ) is 5.6‰ higher than the bulk δ 34 S CRS value (+16.1‰), but the large range of grain‐specific values (−42.7‰ to +28.9‰) overlaps with the bulk value, and corresponds to a textural dichotomy between isotopically light framboids and isotopically heavy irregular aggregates. Accordingly, the discrepancy between bulk and average grain‐specific δ 34 S values is probably the result of insufficiently representative sampling of the two textural components for SIMS analysis.…”

“…For use as an additional S‐isotopic standard for SIMS experiments, we obtained a single large (~1 cm‐diameter) euhedral marcasite crystal from Ward's Science, sourced from the Jiří open‐pit lignite mine in the Czech Republic. For use in further SIMS S‐isotope experiments, we selected a modern sediment sample from the Santa Barbara Basin (CA, USA), and two mid‐Cretaceous, Cenomanian–Turonian Ocean Anoxia Event (OAE 2) shale samples from the Cismon section in Italy, and the Demerara Rise (Atlantic Ocean) …”

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

“…Recorded δ 34 SCRS values can thus reflect local environmental factors (e.g., the 'openness' of the environment of mineral formation with respect to sulfate, or the abundance and reactivity of iron minerals; Bryant et al, 2019;Fike et al, 2015) and ecological factors [e.g., the magnitude of the biological fractionation associated with microbial sulfate reduction, εmic (Leavitt et al, 2013;Sim et al, 2011bSim et al, , 2011aWing and Halevy, 2014)]. Additionally, late-stage fluids can produce highly 34 S-enriched pyrites, often with irregular textures such as overgrowths and cements (Berelson et al, 2018;Bryant et al, 2019;Cui et al, 2018;Raiswell, 1982). Any single or a combination of these factors could have contributed to the observed δ 34 SCRS decrease toward the onset of OAE-2 at Demerara Rise and cannot be ruled out based on bulk δ 34 SCRS data alone.…”