BENTHIC FORAMINIFERA FROM AN EQUATORIAL PACIFIC SEAMOUNT DURING GREENHOUSE CLIMATE

The interaction between geological, oceanographic and biological factors around seamounts creates unusual settings, with intensified currents as the main driver. These currents may cause food particles to be swept away from and concentrated in other parts of the seamounts, breaking the link between primary productivity and arrival of food at the seafloor. We studied benthic foraminiferal faunas on Allison Guyot (ODP Site 865), an equatorial Pacific seamount, across the Paleocene - middle Eocene, in order to understand the structure of the assemblages in this unusual setting and their response to hyperthermal events, including the Paleocene-Eocene Thermal Maximum (PETM) and Eocene Thermal Maximum-3 (ETM3).

Assemblages are diverse across the studied interval, and dominated by foraminifera with calcareous tests and an infaunal (buried) mode of life, including cylindrical taxa with a complex aperture and buliminids. Cibicidoides spp. and Nuttallides truempyi are the most common epifaunal taxa. The dominance of infaunal foraminifera is unexpected in such a deep (~1300 m paleodepth) setting under highly oligotrophic surface conditions. We suggest this dominance of infaunal taxa is caused by the seamount-setting. An active current regime atop the seamount may have favored the potential infaunal suspension feeders anchored by spines, as well as species living epifaunally attached to hard surfaces (Cibicidoides spp.), both successful life strategies under an active current system.

During the PETM, both cylindrical taxa and Cibicidoides spp. declined in abundance, possibly due to their susceptibility to CaCO₃ corrosive waters. In contrast, buliminids may have proliferated due to their deeper infaunal life style, in pore waters, less undersaturated due to carbonate dissolution. The abundance of buliminids in part may reflect increased food availability at the seafloor during hyperthermals, independently of primary productivity. We suggest that faunas across the PETM may have been affected by CaCO₃ corrosion and increased food supply due to enhanced current activity, whereas assemblages across ETM3 point to increased food supply. Even in a seamount setting, benthic foraminifera suffered global extinction or temporal disappearance of 30% of the species, but no extinctions occurred during ETM3.