160-5 Cold Seeps as Transient Refugia During Oceanic Anoxic Event 2 in the Western Interior Seaway

Abstract:

Cold seeps occur along continental margins worldwide where they support productive chemosynthetic-based communities. Seeps provide key ecosystem services through carbon sequestration and provide essential resources for commercially important species. Yet, their ecological functions change through time, and many are threatened by anthropogenic stressors. The fossil record provides a means to examine the resilience of these ecosystems during past biodiversity crises.

Western Interior Seaway (WIS) strata preserve an expanded record of Oceanic Anoxic Event 2 (OAE2; ~94Ma), a major carbon-cycle perturbation marked by extinctions across the Cenomanian–Turonian boundary. Although OAE2’s impacts on WIS fauna are well studied, seep ecosystems and their potential to buffer organisms from regional stress remain poorly understood. We compared WIS seep assemblages with coeval non-seep faunal datasets (Utah, Arizona) and evaluated how the ecological role of seeps varied through time. Methane-derived authigenic carbonates (MDACs) occur throughout an 11-m section of the Tropic Shale (Utah) and record several hundred thousand years of dynamic seep activity. MDAC morphology, petrography, and isotope geochemistry (δ¹³C_carb, δ¹⁸O_carb) were used to reconstruct changes in methane flux and carbonate formation. Macroinvertebrate abundance and diversity were quantified for comparison.

The lower seep interval records effusive seepage, high rates of methane oxidation, and extensive carbonate precipitation at the sediment-water interface. Large, fossiliferous, micritic MDACs yield δ¹³C values between ~–31 to –38.5‰ and seawater-like δ¹⁸O signatures, consistent with a methane-derived carbon source. Fossils are commonly preserved in three-dimensions and retain original shell material, indicating rapid cementation. These assemblages are substantially more abundant and average more than twice the generic and functional richness of non-seep assemblages. Early cementation likely enhanced fossil preservation, so this enrichment likely reflects both ecological and taphonomic processes. Up-section, MDACs decrease in size and extent, suggesting waning methane flux and a shift to diffusive seepage. MDACs are less ¹³C-depleted, more diagenetically altered, and fossil preservation deteriorates. Assemblages are characterized by fragmentary fossils, impressions, and resistant fish remains as diversity declines.

These patterns suggest that effusive seeps acted as transient refugia during OAE2, maintaining ecologically complex communities during regional environmental stress–though this capacity weakened as methane flux and carbonate formation declined. This deep-time record highlights how changes in seep processes alters their resilience during crises and provides a framework for evaluating modern seeps as they face increasing anthropogenic disturbances.

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