Bioturbation and Holocene sediment accumulation fluxes in the north-east Atlantic Ocean (Benthic Boundary Layer experiment sites)

Bioturbation and Holocene sediment accumulation are quantified in the three experimental areas of the Benthic Boundary Layer (BENBO) programme by mean s of the natural radionuclides C-14 and Pb-210 and the artificial radionucl ides Cs-137 and Am-241. The Holocene accumulation rates, determined by the radiocarbon method, are 4.4 and 6.5 cm kyr(-1) at sites B (Rockall Plateau, 1100 m water depth) and C (Feni Drift, 1925 m water depth), respectively. Accumulation at site A, situated between Feni Drift and Porcupine Bank at 3 570 m water depth, was interrupted by an erosional event in the mid-Holocen e, which removed 0.25 m or more of the uppermost sediment present at that t ime. The estimated accumulation rate since that event is 2.1 cm kyr(-1). Di fferent estimates of surficial bioturbation mixing depths at site B are ret urned by the Pb-210(excess) and C-14 methods, with the former indicating <1 0 cm and the latter unusually deep at 17 cm. At site C, Pb-210(excess) and the fallout radionuclides, Cs-137 and Am-241, are present in two distinctly -separated depth zones, with the deepest mixing down to similar to 15 cm, s imilar to the C-14 mixed layer depth. This is ascribed to deep burrowing by sipunculid or echiuran worms at site C, and similar deep mixing is inferre d to be necessary at site B to produce the differences in mixed layer depth s derived from the longer- and shorter-lived radionuclide profiles, althoug h the deep burrowing episodes must be rare (<1 event per 10(2) yr). The gre ater accumulation rate at site C compared with site B is produced by an enh anced flux of current-driven clay- and silt-sized material. This fine mater ial both dilutes the CaCO3 content of the site C sediment and is responsibl e for the higher C-org content observed at site C compared with site B. Unl ike the site B and C sediments, which are fine-grained, the coarse, 63-125 mu m size fraction is the most abundant size class in the late Holocene sed iments at site A, suggesting that the sediments at this location are winnow ed. (C) 2000 Elsevier Science Ltd All rights reserved.