Bleaching of Symbiotic Foraminifera During Extreme Global Warming at the Paleocene- Eocene Boundary

Rapid global warming during the Paleocene-Eocene Thermal Maximum (PETM, 55 Ma) is accompanied by a pronounced decrease in carbon isotopes believed to record the massive input of greenhouse gases into the biosphere. Curiously, the magnitude of this carbon isotope excursion varies between species of foraminifera, reaching as much as -4 per mil in Southern Ocean surface-dwelling species, and being distinctly smaller than this in thermocline and bottom-dwelling species. New data shows that the carbon excursion is magnified in species that bore algal symbionts because the symbiotic relationship was briefly eliminated just before and during the most intense global warming associated with the PETM. The early onset of the loss of photosymbiosis suggests that environmental change substantially preceded the whole ocean change in carbon isotopes and suggests that impact events and mass submarine slope failures reported during the PETM are not the ultimate triggers for this event. The ecological loss of photosymbiosis also explains why the magnitude of the carbon isotope excursion is larger in some species than in others and does not require either that greenhouse gases flooded the atmosphere before being mixed into the deep sea or that the input of carbon into the biosphere was substantially larger than is anticipated for a -2.5 per mil carbon isotope anomaly. The bleaching of photosymbiotic foraminifera suggests that warming was rapid enough, and severe enough, to largely or entirely eliminate symbiosis for 40 kyr.