Dwarfing of ostracodes during the Paleocene–Eocene Thermal Maximum at DSDP Site 401 (Bay of Biscay, North Atlantic) and its implication for changes in organic carbon cycle in deep-sea benthic ecosystem

The Paleocene–Eocene Thermal Maximum (PETM) has been held up as a past analog to future environmental change and presents the opportunity to study climate impacts on marine communities. During the PETM, deep-sea benthic foraminifers decreased their body-size and increased their productivity, metabolic rates, and food consumption in response to abruptly increasing temperature and changing surface water productivity. Here we find that three species of marine ostracodes – part of the multicellular benthos – also experienced a reduction in body-size in response to PETM warming in the Northeast Atlantic. Positive correlations between ostracode body-size and benthic foraminiferal oxygen isotope data indicate that reduced body-sizes were caused by high temperature during the PETM. Dwarfing of ostracodes during the PETM interval suggests that their food consumption rates, and lifetimes, were less than those of ostracodes in the pre-PETM interval. This interpretation is based upon previous studies that show that food consumption and ostracode lifetimes both decrease as ostracodes increase their growth rates under higher temperatures. Estimates of body volume, temperature, and individual abundance suggest a decline in lifetime metabolic rate in ostracode individuals during the core of the PETM. In addition there is also a decrease in biomass flux in the whole ostracode community at this time. The decline in biomass flux at the community level suggests major changes in the energy flow and carbon cycling in deep-sea benthic ecosystem and the reduction of organic carbon flux between the ostracode community and sediments during the PETM. The decline in ostracode carbon flux stands in contrast with previous interpretations that carbon cycling by benthic foraminifera increased during the PETM. Hence, our data suggest that the benthic community response to the PETM was not uniform between different taxonomic groups.