Photosymbiont associations persisted in planktic foraminifera during early Eocene hyperthermals at Shatsky Rise (Pacific Ocean)

Understanding the sensitivity of species-level responses to long-term warming will become increasingly important as we look towards a warmer future. Here, we examine photosymbiont associations in planktic foraminifera at Shatsky Rise (ODP Site 1209, Pacific Ocean) across periods of global warming of differing magnitude and duration. We compare published data from the Paleocene-Eocene Thermal Maximum (PETM; similar to 55.9 Ma) with data from the less intense Eocene Thermal Maximum 2 (ETM2; similar to 54.0 Ma), and H2 events (similar to 3.9 Ma). We use a positive relationship between test size and carbon isotope value (size-delta C-13) in foraminifera shells as a proxy for photosymbiosis in Morozovella subbotinae and Acarinina soldadoensis, and find no change in photosymbiont associations during the less intense warming events, in contrast with PETM records indicating a shift in symbiosis in A. soldadoensis (but not M. subbotinae). Declines in abundance and differing preservation potential of the asymbiotic species Subbotina roesnaesensis along with sediment mixing likely account for diminished differences in delta C-13 between symbiotic and asymbiotic species from the PETM and ETM2. We therefore conclude that photosymbiont associations were maintained in both A. soldadoensis and M. subbotinae across ETM2 and H2. Our findings support one or both of the hypotheses that 1) changing symbiotic associations in response to warming during the PETM allowed A. soldadoensis and perhaps other acarininids to thrive through subsequent hyperthermals or 2) some critical environmental threshold value was not reached in these less intense hyperthermals.

Automated summary

Understanding the sensitivity of species-level responses to long-term warming is crucial for our future in a warmer climate. This study examines photosymbiont associations in planktic foraminifera at Shatsky Rise, comparing data from different periods of global warming. The results suggest that photosymbiont associations remained stable during less intense warming events, in contrast to the Paleocene-Eocene Thermal Maximum records.