Reconstructing seawater carbonate chemistry using foraminiferal B/Ca

Abstract

Planktonic B/Ca holds promise of reconstructing the surface seawater carbonate chemistry, which is linked to the past levels of atmospheric partial pressure of CO2. However, it is still not clear to what extent the carbonate chemistry control on planktonic B/Ca is complicated by physiological activities of foraminifera. In this thesis, the environmental controls on B/Ca in two planktonic foraminiferal species, Globigerinoides ruber (white) and Globigerinoides sacculifer (without final sac-like chamber), are examined using core-top samples from the Atlantic Ocean. For an accurate estimation of the ambient calcification environments, the calcification depths and seasonality of the studied species at these core-top locations are determined by planktonic Mg/Ca. The dissolution effect on B/Ca, which is evident from planktonic B/Ca data from three depth transects, proves to have little impact on the interpretation of planktonic B/Ca data from the Atlantic core-tops. While the carbonate chemistry control on planktonic B/Ca can be detected from these core-top data, a strong calcification rate control on planktonic B/Ca is demonstrated by the correlation between B/Ca and Sr/Ca, where Sr/Ca is employed as an indicator of the calcification rate. This calcification rate control on planktonic B/Ca complicates the direct link between planktonic B/Ca and seawater carbonate chemistry. Nevertheless, based on the different responses of B/Ca in the two studied species to the calcification rate, it is possible that B/Ca in some species are less susceptible to this influence, and thus can be employed for carbonate chemistry reconstructions.

Benthic B/Ca is a recently developed quantitative proxy for deepwater carbonate ion concentration ([CO32-]). In this thesis, deepwater [CO32-] variations during the last 150 thousand years are reconstructed using two cores from the deep Southwestern Pacific, a critical but not fully investigated region to regulate the carbon inventory in the deep ocean. Since Marine Isotope Stage (MIS) 5a, the deep the SW Pacific [CO32-] varied in concert with benthic carbon isotopes (δ13Cb). These covariations, on the timescale of ~10 kyr, are similar to those observed in the deep Atlantic. This suggests that the deep SW Pacific [CO32-] could be affected by changes in the Atlantic Overturning Circulation and the biological pump. However, the deep SW Pacific [CO32-] showed little change during Termination II (T II), despite that variations of δ13Cb, neodymium isotopes (εNd), and foraminifer-bound nitrogen isotopes (δ15N) were similar to those during Termination I (T I). This difference is interpreted as the result of coral reef regrowth on continental shelves, which decreased the whole ocean [CO32-] and counteracted the [CO32-] increase due to the influences from Atlantic Overturning Circulation and the biological pump changes during T II. Show more