Bioturbation and Holocene sediment accumulation fluxes in the north-east Atlantic Ocean (Benthic Boundary Layer experiment sites)
✍ Scribed by J. Thomson; L. Brown; S. Nixon; G.T. Cook; A.B. MacKenzie
- Publisher
- Elsevier Science
- Year
- 2000
- Tongue
- English
- Weight
- 743 KB
- Volume
- 169
- Category
- Article
- ISSN
- 0025-3227
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✦ Synopsis
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 14 C and 210 Pb and the artificial radionuclides 137 Cs and 241 Am. The Holocene accumulation rates, determined by the radiocarbon method, are 4.4 and 6.5 cm kyr Ϫ1 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 Ϫ1 . Different estimates of surficial bioturbation mixing depths at site B are returned by the 210 Pb excess and 14 C methods, with the former indicating Ͻ10 cm and the latter unusually deep at 17 cm. At site C, 210 Pb excess and the fallout radionuclides, 137 Cs and 241 Am, are present in two distinctly-separated depth zones, with the deepest mixing down to ϳ15 cm, similar to the 14 C 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 (Ͻ1 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 CaCO 3 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 mm 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.