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

• Published in: Marine Geology
• Main Authors: Thomson, J., Brown, L., Nixon, S., Cook, G.T., MacKenzie, A.B.
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: 2000
• Subjects: QE Geology; Porcupine Bank; Rockall Plateau; North East Atlantic
• Online Access: https://eprints.gla.ac.uk/908/; https://doi.org/10.1016/S0025-3227(00)00077-3

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 lt 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 ( lt 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.