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 means of the natural radionuclides C-14 and Pb-210 and the artificial radionuclides 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 3570 m water depth, was interrupted by an erosional event in the mid-Holocene, which removed 0.25 m or more of the uppermost sediment present at that time. The estimated accumulation rate since that event is 2.1 cm kyr(-1). Different estimates of surficial bioturbation mixing depths at site B are returned by the Pb-210(excess) and C-14 methods, with the former indicating <10 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, similar 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 inferred to be necessary at site B to produce the differences in mixed layer depths derived from the longer- and shorter-lived radionuclide profiles, although the deep burrowing episodes must be rare (<1 event per 10(2) yr). The greater accumulation rate at site C compared with site B is produced by an enhanced flux of current-driven clay- and silt-sized material. This fine material both dilutes the CaCO3 content of the site C sediment and is responsible for the higher C-org content observed at site C compared with site B. Unlike 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 sediments at site A, suggesting that the sediments at this location are winnowed. (C) 2000 Elsevier Science Ltd All rights reserved.