Are ancient seep environments distinguishable by benthic foraminiferal assemblages? A case study of the Cretaceous Western Interior Seaway

Methane cold seeps are food-rich seafloor habitats that served as oases for macrofaunal communities in the Cretaceous Western Interior Seaway (WIS). Although seep-obligate taxa occur at modern seeps, macrofaunal inhabitants were not endemic to seep environments in the WIS. Here we investigate benthic foraminiferal assemblages across the WIS and test whether benthic foraminifera from seeps are distinct from non-seep environments. We compile abundance data from Campanian to Maastrichtian seep, nearshore, and offshore environments and use constrained multivariate ordination to center our analysis on differences in assemblage composition as it relates to environment setting. Genus-level analysis reveals that offshore settings have more calcareous taxa than nearshore settings, which, in contrast, have representation from various agglutinated genera. Analyses of guilds and morphotypes disregard test-wall type and still find compositional differences among environments that indicate a nearshore–offshore gradient in organic carbon supply. We also find that seep assemblages are generally more similar to offshore assemblages than nearshore ones, reflecting their position near the central axis of the WIS. However, there are no genera, guilds, or morphotypes unique to seeps. We further compare between assemblages from immature, predominantly soft-ground seeps and mature, predominantly hard-ground seeps. We find compositional differences consistent with abated chemical stressors and a well-developed physical substrate associated with seep maturity. Thus, although seep environments draw their benthic foraminiferal faunas from the surrounding metacommunity, assemblages reflect the environmental differences present at seeps and could be used as an additional proxy for seep settings and seep maturity in the WIS.