Cenozoic bathyal and abyssal ostracods beneath the oligotrophic South Pacific Gyre (IODP Expedition 329 Sites U1367, U1368 and U1370)

Abstract

This study reports on Paleocene through Pleistocene deep sea ostracod assemblages from carbonate sequences recovered at Integrated Ocean Drilling Program (IODP) Sites U1367, U1368, and U1370 during Expedition 329 to the South Pacific Gyre (SPG). The SPG has the lowest surface primary productivity in the world oceans, and the sedimentary system beneath it has the lowest rates of organic matter flux to the seafloor in the ocean. Results show that this oligotrophic oceanic environment supported abundant and diverse ostracod faunas during the Oligocene, and the middle Miocene suggesting that ostracods are capable of exploiting low levels of food availability in the deep ocean. The SPG records revealed ostracod assemblages following major global climatic transitions: the Cretaceous/Paleocene (~65.5Ma) and the Eocene/Oligocene (~33.5Ma) transitions, and the middle Miocene climatic optimum (~14Ma). The Danian ostracod assemblage is scarce and characterized by Paraphysocythere spp. and Anebocythereis hostizea. High faunal turnover by the early Oligocene gives rise to an assemblage predominantly characterized by typical lower bathyal and abyssal genera Krithe, Poseidonamicus, Argilloecia, and Cytheropteron that are indicative of cold, moderately oxygenated conditions and signal the cooling of the deep ocean after the E/O transition. An increase in abundance of robust epifaunal taxa Bradleya, Henryhowella, Dutoitella, Pennyella, and Rugocythereis, and a pronounced decrease Argilloecia and Cytheropteron characterize the middle Miocene and Pleistocene assemblages, and indicate the presence of more corrosive bottom waters and enhanced oligotrophic conditions in the SPG. However, low variability in the ostracod assemblage taxonomic composition and stratigraphic distribution at Sites U1367 and U1368 suggest that mostly stable paleoceanographic conditions existed in the South Pacific Gyre during the early to the late Oligocene and from the middle to late Miocene (U1368). Declines in ostracod abundances toward the top of the sedimentary sequences at these sites are synchronous with the late Oligocene warming and the late Miocene carbonate crash in the equatorial Pacific, but they seem to also overlap the time when the site locations subsided below the calcium carbonate compensation depth (CCD) as they moved away from the East Pacific Rise.