Coccolithophorids on the continental slope of the Bay of Biscay―production, transport and contribution to mass fluxes
Résumé
Coccoliths collected by sediment traps deployed on the slope of the Bay of Biscay (northeastern
Atlantic), from June 1990 to August 1991, were examined to determine their contribution
to the transport of carbonate on a mid-latitude continental margin. They also were used as
tracers of particle transfer processes on this slope. Two traps located at 1900 m, respectively at
2300 (Mooring Site 1) and 3000 m (Mooring Site 2) water depths provided high-resolution
(4}7 days) time-series samples covering a 14-month period at MS2 and a 3-month period at
MS1. Coccoliths from 28 species were identi"ed over the course of the experiment, among
which Emiliania huxleyi was always dominant (relative abundance range: 59}93%). Total
coccoliths number #uxes were high but variable, ranging from 390]106 to 1610]106 coccoliths
m~2 day~1 at MS1, and from 58]106 to 1500]106 coccoliths m~2 day~1 at MS2. The
time-weighted mean #ux, calculated for the whole experiment at MS2, was 499]106 coccoliths
m~2 day~1. Estimate of coccoliths minimal contribution to total carbonate #ux at 1900 m
depth averaged 12%, which represented a weighted mean #ux of 7.3 mg m~2 day~1
(2.7 g m~2 yr~1). Lateral transport of coccoliths resuspended from shelf and/or upper slope
sediments seems to be the dominant transfer process to depth on this northeastern Atlantic
slope. Nevertheless, the clear seasonal succession observed in the species composition implies
that the deposition/resuspension/transport sequence is rapid (presumably less than a few
months). Several short and unsmoothed signals directly issued from coccoliths bloom events
also were recorded in our traps, a result that indicates rapid settling rates. The overall coccolith
sedimentation processes appear as being quite diversi"ed, but quantitative and qualitative
analyses of aggregates collected by the traps suggest that they are important carriers of
coccoliths in this margin environment.