55-6 Changes in Relative Sea Level Modulate the Sulfur Isotope Record of Marine Pyrite: Case Studies in the Cretaceous Western Interior Seaway

Session: Sequence Stratigraphic, Geochemical, and Geochronologic Correlation of the Cenomanian-Turonian Ocean Anoxic Event 2 (OAE2) in the Cretaceous Western Interior Seaway (KWIS) and the Gulf Coast

Presenting Author:

Authors:

Abstract:

The history of relative sea level (RSL) change in the Cretaceous Western Interior Seaway (WIS) can be evaluated on both longer-term “cyclothemic” timescales, as well as shorter-term orbitally-forced time scales. Paleoshoreline migrations modulated sediment supply to the basin such that RSL transgressions starved distal offshore locations of fine-grained detrital material while regressions enhanced delivery of mud to the basin. Many studies have noted sedimentologic observations in proximal offshore facies that are consistent with this concept, however, constraining the magnitudes of RSL change and determining how far into offshore facies tracts such effects persist has remained a challenge. Building on prior studies that established detailed litho-, bio-, and chemostratigraphic frameworks for the Cenomanian-Turonian Greenhorn Cyclothem, and the Coniacian-Campanian Niobrara Cyclothem, we examine the relationship between changes in RSL and offshore (fine-grained) sediment supply using the isotopic composition of authigenic pyrite sulfur (δ³⁴S_pyr). The basis for this proxy, recently applied to Pleistocene and Holocene sediments, is derived from the influence of sedimentation rate upon the early diagenesis of organic carbon by sulfate reduction, with sulfide ultimately captured in sedimentary pyrite. Intervals of RSL fall are associated with high rates of sedimentation, which isolate pore waters more rapidly, resulting in a reservoir effect that drives progressively heavier δ³⁴S_py values. In contrast, intervals of RSL rise tend to be associated with a decreased supply of fine-grained sediment, allowing fractionation of S-isotopes during microbial sulfate reduction to approach theoretical limits (~70‰), resulting in much more negative d³⁴S_pyr values. Using δ³⁴S_pyr profiles developed for the Cenomanian-Turonian boundary interval in Utah and Colorado, and the Late Turonian to Early Coniacian interval in western Colorado, we test this hypothesis on two different scales, respectively: (1) the short-term (parasequence) scale and (2) the long-term (cyclothemic) scale. In both cases we observe clear evidence for consistent negative d³⁴S_pyr excursions during transgressive episodes, and a less pronounced effect in progressively more distal sites where shifts in the position of the paleoshoreline due to RSL change appear to be muted. We conclude that the sulfur isotopic composition of authigenic to early diagenetic pyrite provides a robust indicator of sedimentation rate changes related to RSL fluctuations only within the proximal offshore facies tract (100-200km)

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

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