The fate of coccoliths at 48 degrees N 21 degrees 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-yea r period, from April 1989 to April 1990. Three vertically moored sediment t raps, synchronized on a 14-day interval, were deployed at 48 degrees N 21 d egrees W at three water depths, at 1, 2 and 3.7 km (sea floor at 4.4 km). W e present the results on the vertical variability of coccolith and coccosph ere fluxes through the water column. Calcium carbonate was the largest comp onent of the total mass flux, followed by articulate organic matter and opa l. Coccolith and coccosphere seasonal fluxes followed the seasonality of th e biogeochemical particle fluxes at all three depths, with maxima occurring during the spring particle "bloom". The decrease in correlation between co ccosphere 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 occu rs 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 x 10(8) a nd 0.9 x 10(11)m(-2)year(-1) respectively, at 1 km depth (or 3.1 x 10(8) an d 1.4 x 10(11) m(-2) year(-1) including the spring 1990 episode). For the 3 .7 km trap the corresponding values were 0.5 x 10(8) and 1.1 x 10(11) (0.6 x 10(8) and 1.1 x 10(11) including the partially recorded spring 1990 episo de). The slight increase in coccolith numbers with water depth can be expla ined by coccosphere disintegration and lateral influx. Nevertheless, select ive dissolution of delicate coccolith species from 1 to 3,7 km water depth occurs (e.g., relative abundances of Oolithothus fragilis coccoliths decrea se from 3% at 1 km to less than 0.5% at 3.7 km water depth). Emiliania huxl eyi, Gephyrocapsa muellerae, Calcidiscus leptoporus and Coccolithus pelagic us constitute from 65% (at 1 km depth) to 95% (at 3.7 km depth) of the tota l coccolith assemblages. The same four coccolith taxa dominate the fossil a ssemblages in the underlying surface sediments. Selective dissolution conti nues in the sediments, most notably the abundance of the solution-resistant species C. leptoporus increases from 10 to 15% in the trap samples to 15-4 0% in the sediments. Annual Bur of calcium carbonate in sediment trap sampl es accounts for about 60% of the total annual mass flux, with the coccolith fraction ( < 32 mu m) being about 45% of the annual carbonate flux (mean v alue of 41 % at 1 km and 50% at 3.7 km water depth). ro The correlation bet ween the < 32 mu m 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, l ess calcified, coccolithophore and planktic foraminifera species occurs dur ing settling. This fine fragmentation of biogenic CaCO3 can contribute to t he < 32 um fraction causing the decrease of correlation, with the calculate d CaCO3 fluxes being based only on the flux of intact coccoliths and calcis pheres. (C) 2000 Elsevier Science Ltd. All rights reserved.