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The Great Calcite Belt (GCB) is a region of elevated surface reflectance in the Southern Ocean (SO) covering~16% of the global ocean and is thought to result from elevated, seasonal concentrations of coccolithophores. Here we describe field observations and experiments from two cruises that crossed the GCB in the Atlantic and Indian sectors of the SO. We confirm the presence of coccolithophores, their coccoliths, and associated optical scattering, located primarily in the region of the subtropical, Agulhas, and Subantarctic frontal regions. Coccolithophore-rich regions were typically associated with high-velocity frontal regions with higher seawater partial pressures of CO 2 (pCO 2 ) than the atmosphere, sufficient to reverse the direction of gas exchange to a CO 2 source. There was no calcium carbonate (CaCO 3 ) enhancement of particulate organic carbon (POC) export, but there were increased POC transfer efficiencies in high-flux particulate inorganic carbon regions. Contemporaneous observations are synthesized with results of trace-metal incubation experiments, 234 Th-based flux estimates, and remotely sensed observations to generate a mandala that summarizes our understanding about the factors that regulate the location of the GCB.

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.”

We report surface observations of a mesoscale coccolithophore bloom at the shelf break of the Patagonian Shelf during December 2008, representing the densest coccolithophore population in the Southern Ocean. The bloom was most intense within the Falklands Current, northeast of the Falkland Islands. Emiliania huxleyi dominated bloom waters, with a mixed E. huxleyi and Prorocentrum sp. dinoflagellate bloom to the west and mixed assemblage of diatoms, dinoflagellates, and flagellates to the east. Optical measurements of coccolith light scattering, analytical measurements of their calcite, and microscopic counts all showed this to be an intense coccolithophore bloom. Average particulate inorganic carbon per coccolith in the bloom was low, typical of the B coccolith morphotype and in agreement with independent measurements made by scanning electron microscopy. Highest particulate inorganic carbon (measured optically and chemically) was observed when residual nitrate (defined as the difference, [NO { 3 ] 2 [Si(OH) 4 ]) was 10-17 mmol L 21 and nitrate to phosphate ratios were close to Redfield values. Elevated particle backscattering was observed in the E. huxleyi bloom, whereas the highest particle scattering occurred in the adjoining Prorocentrum sp. bloom. Backscattering from coccolithophores represented up to 50% of the total backscattering (from organic and inorganic particles) along the main axis of the E. huxleyi bloom. Chlorophyll-specific absorption in the coccolithophore bloom was typical of marine phytoplankton. Residual nitrate plotted vs. temperature showed that the E. huxleyi bloom was associated with waters between 5uC and 15uC, with depleted silicate. Results suggest that previous drawdown of silicate by diatoms occurred prior to the densest E. huxleyi blooms over the Patagonian Shelf. We speculate that such conditions might also be important for annual development of the broader Great Calcite Belt and other coccolithophore blooms.

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