256-11 Shifting ecological and fluid flow regimes at a Cretaceous cold seep field across Oceanic Anoxic Event 2 in the Western Interior Seaway

Session: Life and Environments Through Time and Space: Multi-Record Approaches to Stratigraphic Paleobiology, Part I

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Hydrocarbon seeps fuel complex biogeochemical interactions that support chemosynthetic ecosystems across the global ocean. A central process is anaerobic methane oxidation (AOM) via sulfate-reduction, which generates sulfide that sustains free-living and chemosymbiotic sulfide-oxidizers. Chemosymbiotic macrofauna shape seep environments by increasing AOM rates, detoxifying sediments, and oxygenating porewaters, promoting colonization and survival of other benthic fauna. AOM produces ¹³C-depleted bicarbonate that drives seep-associated carbonate (SAC) formation in the sulfate-methane-transition-zone (SMTZ) with depleted δ¹³C values. However, variable methane flux, AOM rates, and SMTZ depth lead to variable SAC morphologies and isotopic compositions. Fossil SACs thus serve as biogeochemical archives of past seep activity.

We examined a 14-m thick exposure of the Cenomanian-Turonian Tropic Shale (Utah, USA) which preserves an extensive record of dynamic seepage during Oceanic Anoxic Event 2 (OAE2; ~94Ma), a major carbon cycle perturbation associated with marine extinctions. Tropic Shale seeps provide a unique paleontological resource for examining how both OAE2 and shifting seepage regimes influenced benthic communities through time. SAC morphology, mineralogy, and stable isotope geochemistry (δ¹³C_carb, δ¹⁸O_carb) was studied to reconstruct seep dynamics; the abundance, richness, and functional diversity of associated macroinvertebrates were quantified to assess community shifts across deposits.

We document a reduction in the size, distribution, and faunal content of SACs up-section: large, fossiliferous carbonates transition to scattered, centimeter-scale SACs with depauperate fauna and a loss of chemosymbiotic bivalves. δ¹³C of larger SACs are highly depleted (~-32– -40‰) and their δ¹⁸O reflective of ambient bottom-water temperature and seawater composition (~-2– -4‰), suggesting rapid methane transport, high AOM rates, and a SMTZ close to the sediment-water-interface. Smaller SACs show a broader range of δ¹³C–δ¹⁸O, reflecting a mix of DIC sources and influence of isotopically-altered porewaters. These carbonates likely formed slowly under diffusive seepage at depth. Lastly, fossiliferous SACs linked to effusive seepage occur during peak OAE2 conditions when diversity elsewhere in the seaway was periodically suppressed. We interpret seep community changes as partly driven by local environmental shifts rather than basin-wide perturbations. A reduction of methane emissions, AOM rates, and downward displacement of the SMTZ coincides with a loss of chemosymbiotic bivalves and benthic biodiversity. This highlights the ecological importance of chemosymbiotic taxa in seep environments, as their presence or absence exerts strong controls over ecosystem structure and functioning.

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

doi: 10.1130/abs/2025AM-10498

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