COMMUNITY SUCCESSION IN CRETACEOUS COLD-SEEP FAUNAS: USING A GUILD-BASED MODERN-ANALOG APPROACH TO RECOGNIZE ECOLOGICAL PARALLELS WITH MODERN SEEPS

Modern faunal assemblages at cold-seeps vary in composition over the lifetime of the seep as conditions progresses from an early environment with high sulfide and methane and low oxygen to a later environment less influenced by seep fluids with more structural complexity. Here we use a guild-based approach to compare the ecological similarity of Cretaceous cold-seep assemblages preserved in the Pierre Shale surrounding the Black Hills and modern cold-seep assemblages from five previously recognized cold-seep biofacies including Gulf of Mexico seeps where others have quantitatively demonstrated seep succession. Before testing hypothesis of ecological similarity, we modify modern assemblage data to include only those taxa with fossilizable hardparts greater than 5 mm in length to make the modern and fossil datasets more comparable.

Guild-based ordination analyses allow us to apply ecological gradients emergent from the modern faunal samples to the fossil assemblages despite differences in taxonomic composition, the water depth of deposition, and substrate conditions. We find that while the Black Hills assemblages have more compositional similarity to soft-substrate cold-seep biofacies dominated by infaunal chemosymbiotic bivalves, the faunal variation among Black Hills cold-seep assemblages form a gradient parallel in ordination space to the successional gradient from modern mussel-dominated seeps to later-stage tubeworm-dominated seeps. The subset of Black Hills assemblages representing the early stage have low among-sample dissimilarity similar to younger successional stage modern seeps (mussel biofacies), further suggesting these fossil assemblages developed in a more stressed environment than those fossil assemblages analogous to later successional stage modern seeps. Thus, Cretaceous cold-seeps from the Black Hills preserve analogous patterns of succession as observed at modern cold-seeps with changes in seep fluids. Broader application of guilds to community-level questions can help recognize more precise analogs between modern and fossil ecosystems, allowing us to better leverage the fossil record to address long-term ecological and environmental changes.