The fate of coccoliths at 48°N 21°W, Northeastern Atlantic

As part of the Joint Global Ocean Flux Study Program (JGOFS) North Atlantic Bloom Experiment (NABE), coccolithophore fluxes were recorded over one-year period, from April 1989 to April 1990. Three vertically moored sediment traps, synchronized on a 14-day interval, were deployed at 48°N 21°W at three water depths, at 1, 2 and 3.7 km (sea floor at 4.4 km). We present the results on the vertical variability of coccolith and coccosphere fluxes through the water column. Calcium carbonate was the largest component of the total mass flux, followed by particulate organic matter and opal. Coccolith and coccosphere seasonal fluxes followed the seasonality of the biogeochemical particle fluxes at all three depths, with maxima occurring during the spring particle “bloom”. The decrease in correlation between coccosphere and coccolith fluxes with depth (from r=0.98 at 1 km depth to 0.60 at 3.7 km depth) documents that disaggregation of the coccospheres occurs during settling in the deeper part of the water column between 1 and 3.7 km water depth. Annual coccosphere and coccolith fluxes were 1.5×108 and 0.9×1011 m−2 year−1 respectively, at 1 km depth (or 3.1×108 and 1.4×1011 m−2 year−1 including the spring 1990 episode). For the 3.7 km trap the corresponding values were 0.5×108 and 1.1×1011 (0.6×108 and 1.1×1011 including the partially recorded spring 1990 episode). The slight increase in coccolith numbers with water depth can be explained by coccosphere disintegration and lateral influx. Nevertheless, selective dissolution of delicate coccolith species from 1 to 3.7 km water depth occurs (e.g., relative abundances of Oolithothus fragilis coccoliths decrease from 3% at 1 km to less than 0.5% at 3.7 km water depth). Emiliania huxleyi, Gephyrocapsa muellerae, Calcidiscus leptoporus and Coccolithus pelagicus constitute from 65% (at 1 km depth) to 95% (at 3.7 km depth) of the total coccolith assemblages. The same four coccolith taxa dominate the fossil assemblages in the underlying surface sediments. Selective dissolution continues in the sediments, most notably the abundance of the solution-resistant species C. leptoporus increases from 10 to 15% in the trap samples to 15–40% in the sediments. Annual flux of calcium carbonate in sediment trap samples accounts for about 60% of the total annual mass flux, with the coccolith fraction (<32 μm) being about 45% of the annual carbonate flux (mean value of 41% at 1 km and 50% at 3.7 km water depth). The correlation between the <32 μm CaCO3 and the calculated coccolithophore and calcisphere CaCO3 fluxes decreases with depth (r=0.92 at 1 km and r=0.86 at 3.7 km), indicating that partial dissolution and fragmentation of the delicate, less calcified, coccolithophore and planktic foraminifera species occurs during settling. This fine fragmentation of biogenic CaCO3 can contribute to the <32 μm fraction causing the decrease of correlation, with the calculated CaCO3 fluxes being based only on the flux of intact coccoliths and calcispheres.