DYNAMIC HYDROCARBON SEEPAGE ACROSS THE CENOMANIAN-TURONIAN BOUNDARY IN THE WESTERN INTERIOR SEAWAY AND ITS EFFECTS ON MACROINVERTEBRATE FAUNA DURING OCEANIC ANOXIC EVENT 2

Chemosynthetic production at cold seeps creates nutrient-rich, diverse habitats that sustain dense biotic communities. Though seeps are ubiquitous across the global ocean and have a rich fossil record, their influences on the structure of surrounding communities are understudied. The North American Western Interior Seaway (WIS) was the site of widespread seep activity during the Cretaceous. The Tropic Shale of Utah preserves seep deposits spanning the Cenomanian-Turonian (C-T) boundary and is thus coeval with Oceanic Anoxic Event 2 (OAE2), a major carbon cycle perturbation. Here we investigate the spatio-temporal evolution of this long-lived seep field and the macrofaunal communities that inhabited it. Specifically, this research examines: 1. How do changes in seep dynamics (CH₄ flux, anaerobic methane oxidation (AOM) rate, etc.) govern the distribution of fauna? 2. How did seep and non-seep WIS paleocommunities respond to OAE2? Seep-associated concretions (SACs) were analyzed for macroinvertebrate abundance, richness, and functional diversity. Non-seep macroinvertebrate occurrences were obtained from the Paleobiology Database from correlative WIS sites for comparison. Changes in seep dynamics were interpreted based on SAC morphology and mineralogy. Large SACs often form under high AOM and methane flow rates. Large SACs were highly fossiliferous and contained abundant epifauna and nekton. This suggests a stable seep food web and that SACs extended above the sediment-water interface (SWI). Smaller SACs were less fossiliferous, suggesting precipitation below the SWI around seep margins. During peak OAE2, seeps contained more functionally and taxonomically diverse fauna – often in higher abundances – than non-seeps. However, faunal abundance declines across all localities up to the C-T boundary and seep style changes markedly. Smaller SACs indicate a reduction in methane flow and likely AOM rates. Yet, SACs still contain a richer fauna and abundant ammonites, the latter of which are virtually absent from correlative non-seep sites. These data suggest that chemosynthetic production at seeps remained stable enough to impact the overlying water column and potentially provided a consistent food source for fauna during repeated perturbations to photosynthetic-based food webs in the WIS during OAE2.