At. Haidar et al., Calcareous phytoplankton standing stocks, fluxes and accumulation in Holocene sediments off Bermuda (N. Atlantic), DEEP-SEA II, 47(9-11), 2000, pp. 1907-1938
Citations number
47
Categorie Soggetti
Aquatic Sciences","Earth Sciences
Journal title
DEEP-SEA RESEARCH PART II-TOPICAL STUDIES IN OCEANOGRAPHY
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 th
e sea floor, and to their accumulation in Holocene sediments on the sea flo
or. A pronounced monthly variability of coccolithophore cell densities in t
he photic zone over the three-year plankton sampling interval (1991-1994) w
as observed. In the sediment trap only an integrated seasonal signal is ref
lected in the fluxes of coccoliths and the fine fraction (< 38 mu m) carbon
ate, 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) coccol
iths 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 flux
es 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 coccoli
ths. Fluxes of the fine fraction carbonate to the deep Sargasso Sea, determ
ined 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 acc
umulating SE of Bermuda at 4300 m water depth consist of 11% (by weight) pl
anktic foraminifera, 56% fine fraction ( < 38 mu m) carbonate and 33% inorg
anic clay. The C-14 age difference between the planktic foraminifera carbon
ate (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 carbonat
e in the sediment is about an order of magnitude higher than the correspond
ing fluxes to the trap, confirms the previous reports of large-scale resusp
ension 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 disso
lution processes in this area affect all taxa in a similar way. (C) 2000 El
sevier Science Ltd. All rights reserved.