Sinking of coccolith carbonate and potential contribution to organic carbon ballasting in the deep ocean

There exists a great need to better understand the controls on organic carbon sequestration to the deep sea, by virtue of its role in modulating atmospheric CO 2 concentrations. Recent studies suggest that organic fluxes to the oceanic interior are higher in regions dominated by carbonate sedimentation, thus also concluding that relatively heavy carbonate particles are an effective mineral ballast for organic carbon. CaCO 3 production in the pelagic ocean is mainly mediated by foraminifera and coccolithophores, but the precise role of these carbonate producers as mineral ballast for organic carbon in the ocean has not yet been tested. Here we evaluate sediment-trap flux data from an array of 13 moorings (15 traps) situated in diverse conditions and with global coverage, all for organic carbon flux, calculated coccolith-carbonate flux, and fine fraction flux (when available). Discrepancies are recorded between the amount of carbonate fine fraction and calculated coccolith carbonate, and there is the need to determine the origin of the unknown CaCO 3 in the fine fraction. The coccolith-carbonate flux magnitude is determined not only by the coccolith flux number but also by the carbonate mass of the key species. For example, very abundant coccolithophore species such as Emiliania huxleyi have a very low species-specific carbonate mass, and are therefore of lower carbonate flux significance than expected intuitively. Among the main coccolith carbonate species contributors in our sediment-trap sites are Calcidiscus leptoporus and Helicosphaera carteri, in that these are all relatively massive compared to their numerical abundances. We observed generally positive correlations between calculated coccolith carbonate and organic carbon daily fluxes, suggesting that on a global basis there is a ballasting mechanism at work, seemingly most efficiently with C. leptoporus in the carbonate-dominated North Atlantic. A fairly constant global relationship between annual fluxes of calculated coccolith carbonate and organic carbon implies some uniformity in the PIC/POC “rain ratio”. However, improvements in coccolith carbonate estimation are needed to not only understand this ratio currently, but also to help understand future sequestration of organic carbon to the oceanic interior.