PLANKTONIC FORAMINIFERAL BLEACHING DURING EARLY EOCENE HYPERTHERMAL EVENTS

Stable isotopic values of foraminiferal tests are important tools for understanding Earth’s oceanographic and climatic history, yet the isotopic composition of foraminiferal tests is affected by multiple species-specific factors, including biology, ecology, environment, and—for fossil tests—diagenesis, making it problematic to deconvolve these determinants.

Foraminifera have developed symbiotic relationships with photosynthetic algae to aid growth, nutrition, and reproduction. A significant correlation between foraminiferal test size and stable carbon isotopic ratios (δ¹³C) in foraminiferal tests is commonly used as indicator of photosymbiont activity. A lack of correlation in extant species (known to harbor photosymbionts) has been shown to result from symbiont loss (“bleaching”). However, conclusively identifying bleaching and describing its causes in the geologic record is difficult.

We analyze size-specific carbon and oxygen stable isotopes of three species of upper Paleocene – early Eocene foraminifera from IODP Site 1209 (Pacific Ocean). Acarinina soldadoensis and Morozovella subbotinae are interpreted as photosymbiont-bearing, whereas Subbotina eocaena is interpreted as asymbiotic. By comparing trends across the Paleocene-Eocene Thermal Maximum, Eocene Thermal Maximum 2 and H2, we investigate the sensitivity of isotopic variability to temperature changes of varying amplitude.

Our data confirm a symbiotic lifestyle of Acarinina and Morozovella, and show reduced test size-δ¹³C slope gradient for Morozovella during the PETM, relative to background conditions. However, a similar pattern is observed in the asymbiotic species S. eocaena for the PETM, raising doubts about the symbiont bleaching hypothesis.

Thus, we are also investigating alternative causes for the isotope trends, such as effects of temperature dependence of respiration, changes in depth habitat, irradiance, and carbonate ion effects. Ultimately, our research may offer new insights and invoke additional questions into the influence of sudden and persistent temperature and environmental changes on planktonic foraminiferal ecology.