Which was the habitat of early Eocene planktic foraminifer Chiloguembelina? Stable isotope paleobiology from the Atlantic Ocean and implication for paleoceanographic reconstructions

The planktic foraminiferal genus Chiloguembelina is characterized by tiny test and biserial disposition of chambers. Largely accepted ecological interpretation ascribes biserial planktic foraminifera as eutrophic and low-oxygen tolerant forms inhabiting the Oxygen Minimum Zone (OMZ). However, this ecological interpretation does not agree with all available data from Cenozoic record. Actually, chiloguembelinid stable isotope paleobiology is highly variable through time and space as it indicates mixed-layer habitat on earliest Eocene, middle Eocene and Late Oligocene but thermocline habitat during middle Eocene from northwest Atlantic Ocean. However, chiloguembelinid stable-isotope data were so far lacking from the early Eocene. This interval includes the early Eocene Climatic Optimum (EECO, ~53–48 Ma) that is the crucial time when Earth's surface temperatures and pCO2 reached their maximum levels in the Cenozoic. We present here new chiloguembelinid oxygen and carbon stable-isotope data from early Eocene Atlantic Ocean Drilling Program (ODP) Sites 1051, 1258, 1263 and 690. We also generate new data on Chiloguembelina abundance from Site 1051 and compare our record with those previously published from sites 1263 and 1258 in order to correlate the early Eocene paleoceanographic distribution of this genus through the Atlantic Ocean. Interestingly, Chiloguembelina became virtually absent at the beginning of the EECO in the subtropical, equatorial and temperate Atlantic sites. Our findings clearly prove that early Eocene Chiloguembelina occupied a deep-water niche throughout the Atlantic Ocean as demonstrated by its stable isotope signature close to the deep-dweller Subbotina and benthic foraminifera. These evidences provide critical paleoceanographic implications suggesting that intermediate water temperatures probably rose significantly during the EECO thus becoming too warm for this genus. Elevated ocean temperatures enhanced the rate of bacterial respiration and remineralization significantly thus resulting in more efficient recycling of nutrients higher in the water column. This would have induced a restricted food supply deeper in the thermocline and cut out the chiloguembelinid niche. The agreement of the observed decline in abundance of chiloguembelinids at the study sites with recently published foraminifera-bound nitrogen isotope data suggests that enhanced oxygenation of the thermocline in the early EECO might have also played a major role in driving the disappearance of chiloguembelinids, which in itself represent a new evidence for the largely accepted view that these forms inhabited the OMZ. Very likely a combination of reduced food supply, increase in thermocline temperature and oxygen content resulted in the decline of chiloguembelinids in the early EECO.