Calcareous phytoplankton standing stocks, fluxes and accumulation in Holocene sediments off Bermuda (N. Atlantic)
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Standing stocks and taxonomic composition of dominant coccolithophore taxa in the photic zone at Bermuda Hydrostation “S” are compared to their fluxes measured in a sediment trap at 3200 m water depth, located 1200 m above the sea floor, and to their accumulation in Holocene sediments on the sea floor. A pronounced monthly variability of coccolithophore cell densities in the photic zone over the three-year plankton sampling interval (1991-1994) was observed. In the sediment trap only an integrated seasonal signal is reflected in the fluxes of coccoliths and the fine fraction (< 38 mu m) carbonate, which were determined for 12 biweekly sampling intervals during 1992. The average daily flux of all coccoliths during 1992 was 1.4 x 10(9) coccoliths m(-2) d(-1), but it varied by at least a factor of five, with highest fluxes in late winter and late spring and low fluxes in late fall. The fluxes of individual coccoliths of the two dominant species, E. huxleyi and F. profunda, accounted for 85% of the total coccolith fluxes. The contribution of the other biogenic carbonate (mostly calcispheres of thoracosphaerids) to the fine fraction carbonate fluxes was minor compared to that of coccoliths. Fluxes of the fine fraction carbonate to the deep Sargasso Sea, determined over 2-week intervals in 1992, were high in early spring (up to 23 mg CaCO3 m(-2) d(-1)) and low in late fall (minimum of 5 mg CaCO3 m(-2) d(-1)), with an annual mean of 12 mg CaCO3 m(-2) d(-1). The surface sediments accumulating SE of Bermuda at 4300 m water depth consist of 11% (by weight) planktic foraminifera, 56% fine fraction ( < 38 mu m) carbonate and 33% inorganic clay. The C-14 age difference between the planktic foraminifera carbonate (1.5 ka) and the fine fraction carbonate (6.7 ka) at 13 cm core depth, together with the fact that the accumulation rate of fine fraction carbonate in the sediment is about an order of magnitude higher than the corresponding fluxes to the trap, confirms the previous reports of large-scale resuspension of fine-grained sediments in this area. The taxonomic composition of the coccolith assemblages accumulating in Holocene sediments near Bermuda is very similar to the composition of coccoliths sampled in the trap and to that of the average living assemblage, suggesting that transport and dissolution processes in this area affect all taxa in a similar way. (C) 2000 Elsevier Science Ltd. All rights reserved.