Phytoplankton mineralization in the tropical and subtropical Atlantic Ocean

Organic carbon fluxes to the deep ocean may be enhanced by association with ballast mineral material such as calcite and opal. We made simultaneous measurements of the upper ocean production of calcite (calcification), opal (silicification) and organic carbon (photosynthesis) at 14 stations between 42°S and 49°N in the Atlantic Ocean. These measurements confirm the light-dependency of calcification and photosynthesis, and the substrate dependency of silicification. We estimate that mineralizing phytoplankton represent ~5–20% of organic carbon fixation, with similar contributions from both coccolithophores and diatoms. Estimates of average turnover times for calcite and phytoplankton carbon are ~3 days, indicative of their relatively labile nature. By comparison, average turnover times for opal and particulate organic carbon are ~10 days. Rapid turnover of calcite suggests an important role for the plankton community in removing calcite from the upper ocean. Comparison of our surface production rates to sediment trap data confirms that ~70% of calcite is dissolved in the upper 2–3 km, and only a small proportion of total organic carbon (<2%) reaches the deep ocean.