The contribution of coccolithophores to the optical and inorganic carbon budgets during the Southern Ocean Gas Exchange Experiment: New evidence in support of the “Great Calcite Belt” hypothesis

Abstract

[1] We describe optical observations from the Southern Ocean (SO) Gas Exchange Experiment (GasEx) during March 2008, designed to test the hypothesis that enhanced reflectance of the Southern Ocean waters was related to a high abundance of coccolithophorids. We present multiple lines of evidence in support of this hypothesis: (1) birefringence microscopy, (2) along-track measurements of acid labile backscattering, (3) analytical measurements of particulate inorganic carbon (PIC), and (4) above-water radiometry measurements made from both ship and satellite. At the SO GasEx study site (∼51°S, 38.5°W) coccolithophores and coccoliths were abundant (∼300 cells mL−1 and 20,000 coccoliths mL−1) and concentrations were stable over time scales of weeks. Enhanced concentrations of PIC were observed between the Subtropical Front (STF) and the northern part of the Subantarctic Front (SAF) and between the SAF region and Polar Front. Biogenic silica was elevated with the highest concentrations south of the STF. These results, along with other recent microscope observations of coccolithophores made by other workers around the Southern Ocean, suggest that the region of enhanced reflectance is, at least partially, due to coccolithophores. However, the PIC estimated from above-water reflectance measurements was 2–3X the PIC actually measured in the water. One possible reason for this is the presence of other sources of scattering, such as microbubbles (at least during stormy conditions). These observations are consistent with the hypothesis that this region of elevated reflectance and PIC is part of a significantly larger region which encircles the entire Southern Ocean each austral summer, a band that we call the “Great Calcite Belt.”