Alkenone fluxes and anomalous U-37(K)' values during 1989-1990 in the Northeast Atlantic (48 degrees N 21 degrees W)

We report and discuss new data on alkenone fluxes from the North East Atlan tic (48 degrees N 21 degrees W), in a site investigated by the Joint Global Ocean Flux Study and the North Atlantic Bloom Experiment from April 1989-M arch 1990. The investigated sediment trap station is situated within the No rth Atlantic Transition Zone in a region influenced by upwelling events ind uced by mesoscale eddy activity, for which branches of the North Atlantic C urrent act as major sources. The flux of alkenones in the NE Atlantic was c onsistent with the monthly evolution of particulate fluxes, rapidly increas ing from April 1989 to June 1989 (peak bloom), and gradually declining unti l August 1989. The timing of the only bloom observed is not equivalent to t he timing of alkenone flux maxima recorded elsewhere in the world's oceans. Hence, we further confirm that there is not a world-wide regular season of alkenone blooms. The annual alkenone flux profile in our trap resembled th at of the coccospheres, but not so much that of coccoliths. This discrepanc y may indicate that alkenones were mainly transported downwards to the sea- floor by coccospheres, as part of the intact structure of the dead coccolit hophorid. The sedimentary concentration of alkenones, which is related to p roductivity and efficiency of preservation, may then be also partly depende nt on the rate of disaggregation of coccospheres in the water column, and t he factors that control them. Comparison of the ratios of amounts of alkeno nes per coccosphere and coccoliths, with those of amounts of alkenones per call numbers from culture and field data, suggests that there could have be en a contribution of alkenones to the traps by Coccolithus pelagicus, despi te this species considered devoid of alkenones. We suggest, hence, that fur ther laboratory tests of this species are carried out. U-37(K)' in the sedi ment trap material showed a poor correlation with sea surface temperature d uring spring 1989. We propose that the anomalously low U-37(K)' values refl ects an input of alkenones from elsewhere to the trap, not from the overlyi ng surface waters, associated to the presence of several eddies and phytopl anktonic material trapped within. The origin of the eddy-bound alkenones ma y be found in subpolar waters, as indicated by the presence of C. pelagicus tests in the trap, and also by the low U-37(K)' values which are similar t o the values found in surface sediments near the polar front. Our data, thu s, further illustrates the challenge of using short-term studies of annual water-column alkenone fluxes to understand average sedimentation patterns o f alkenones to the deep ocean. (C) 2000 Elsevier Science B.V. All rights re served.