Insights into Ocean Acidification During the Middle Eocene Climatic Optimum from Boron Isotopes at Southern Ocean Site 738

Abstract:

The Middle Eocene Climatic Optimum (MECO) is a ~650-kyr interval of global warming, with a brief ~50 ky long peak warming interval, and an abrupt termination. Deep sea and surface ocean temperature evolution across this interval are fairly well constrained, but thus far we have little understanding of the mechanisms responsible for the gradual warming and rapid recovery. Carbonate mass accumulation rates suggest a shoaling of the carbonate compensation depth, and studies on alkenones indicate increasing atmospheric CO₂ levels during the MECO. This suggests an increase in surface ocean CO₂, and consequently ocean acidification. However, the severity and timing of the proposed ocean acidification with respect to the onset, peak warming and the termination are currently not well resolved.

The boron isotopic composition (δ¹¹B) recorded in planktic foraminifer shells offers an opportunity to infer oceanic pH across this interval. We are working on a boron isotope reconstruction from Southern Ocean IODP site 738 and South Atlantic IODP site 1263, covering 42.0 to 38.5 Ma. These sites are characterized by good carbonate preservation and well-defined age models have been established. Additionally, ecology, nutrient content and bottom-water oxygenation have been shown to change significantly across the event towards a more eutrophic, periodically oxygen-depleted environment supporting different biological communities.

We selected the planktic foraminifera species Acarinina spinuloinflata for this study because it is symbiont-bearing, suggesting a near-surface habitat and little vertical migration in the water column, and because of its abundance in the samples. δ¹¹B data will be translated to surface ocean pH and atmospheric pCO₂ will be approximated to refine knowledge about the carbon cycle during this time. Parallel analysis of two core sites will help to evaluate the tenacity of the data.