UC Riverside

There is ample evidence from benthic foraminiferal oxygen isotope records that the Earth’s climate experienced dramatic long-term variations from the late Paleocene to early-middle Eocene. Beginning from ~57 Ma and culminating in the Early Eocene Climate Optimum (EECO) between ~53-51 Ma, the recorded 1‰ decrease in δ18O indicates warming of ~4 to 5 °C, followed by the onset of long-term cooling at the early-middle Eocene boundary (~48 Ma). Benthic foraminifera also record major changes in carbon isotopes during this time period, with a decline in δ13C of ~2 to 2.5‰, indicating that the temperature changes coincided with profound changes in the carbon cycle. These trends are consistent with an overall increase in atmospheric CO2 for the duration of the event, perhaps caused by a protracted increase in volcanic outgassing or decrease in net organic carbon burial. However, there is a temporal offset between the benthic foraminiferal δ13C and δ18O signals, with the long-term minimum in δ13C significantly preceding peak global temperatures indicated by the long-term minimum in δ18O. If a global signal, this offset may suggest changes in the source of carbon fluxes across this interval of long-term global warming, rather than the existence of a single-source carbon forcing and resultant temperature response. To constrain possible causes of the offset, we first analyze the relative timing of isotope signals from each individual site within the Cramer et al. 2009 benthic foraminiferal stable isotope composite. Our analysis shows that trends in δ13C consistently lead trends in δ18O at each site. However, the length of the offset varies significantly between sites. We find that the duration of this offset is greatest at southern sites, potentially indicating that changing patterns of deep-water formation contribute to the apparent decoupling between δ13C and δ18O trends. As δ13C is a gauge of water mass age, spatial gradients in benthic foraminiferal δ13C can indicate regional differences in deep water formation, shedding light on the evolution of global circulation. We next take an additional investigative approach utilizing the cGENIE intermediate complexity Earth system model. We use cGENIE in order to test the impact of enhanced volcanic outgassing on spatial patterns in benthic δ13C and deep ocean temperature, and relate this to changes in the large-scale ocean overturning circulation.