Deposition, Mixing and Storage Timescales at the Benthic Boundary Layer

• Main Author: Brown, Louise
• Format: Thesis
• Language: English
• Published: ProQuest Dissertations & Theses 2001
• Subjects: North East Atlantic; Planktonic foraminifera
• Online Access: http://theses.gla.ac.uk/76093/; http://theses.gla.ac.uk/76093/1/13818890.pdf

With an area of approximately 3.6 x 10e8 km2, the oceans constitute a reservoir of central importance in global biogeochemical cycles and represent a major sink for anthropogenic contaminants. The fluxes of biological, geological and anthropogenic materials to deep ocean sediments and their subsequent burial are key processes in the removal of particulate matter from the zone of bioavailability. However, within the benthic boundary layer, a host of physical, biological and chemical processes modify the material arriving at the sea floor and influence its rate of removal from the biosphere. The NERC Benthic Boundary (BENBO) study, of which this project is a component, was initiated in order to examine these fluxes to the sea floor and their fate in the benthic zone. Radionuclides are an important tool in quantitatively determining a number of key benthic boundary processes; specifically sediment accumulation and the biological mixing processes acting on sediments after deposition. Analyses of natural and man-made radionuclides incorporated in the sediment were used to examine these processes on 103-year (14C), 102-year (210Pbexcess) and decadal (241Am, 137Cs,238Pu, 238,239Pu) timescales at the three Benthic Boundary Layer Experiment (BENBO) sites in the North East Atlantic Ocean. Radiometric dating of bulk carbonate was used to determine both the Holocene sediment accumulation history and depth of sediment homogenisation by biological mixing on the 103-year timescale. At two of the sites, BENBO B and BENBO C, sediment accumulated at a constant rate over the Holocene, at 4.3 cm ky-1 and 6.5 cm ky-1 respectively. At the third site, BENBO A, the irregular radiocarbon-depth profile is considered to be generated by erosion and redeposition of sediments in the mid-Holocene. The two sites demonstrating constant sediment accumulation both show surface mixed layers which are deeper than is typical of open ocean sites. Fluxes of 210Pbexcess to the sediments follow the general pattern of increasing flux with increasing water column depth, and comparison with North-East Atlantic water column 226Ra data indicates that the BENBO sites are not regions of enhanced radionuclide scavenging. The mean 238Pu:239,240Pu activity ratios determined at sites B and C are 0.032 and 0.039 indicates atmospheric weapons testing fallout is the dominant source of man-made radionuclides to these sites. The 210Pbexcess and man-made radionuclide profiles were also used to determine the prevailing short term (<102 y) mixing mechanism. The profile at site A indicated mixing dominated by biodiffusive processes, and a biodiffusion coefficient of 0.088 cm ky-1 was calculated. The profile at site B was also initially interpreted as biodiffusive mixing. The rate determined by 230Pbexcess at this site was 0.045 cm ky-1, with no significant variation between the mixing coefficients calculated for natural (230Pbexces) and man-made (239,240Pu) radionuclides. At site C, subsurface peaks in 230Pbexcess specific activity indicated bioturbation by advective rather than biodiffusive processes. Replicate sub cores from sites B and C were also analysed for 230Pbexcess, 241Am and 137Cs, and indicated strong variability in bioturbation over a small (<1 m2) spatial scale. Measurement of the 210Pbexcess activity of sipunculid burrow material confirmed that bioturbation by large, infaunal organisms is responsible for generating the subsurface peaks in radionuclide activity found in sub cores B(vi), C(ii) and C(iv). Size selective mixing was examined by 14C dating of hand picked planktonic foraminifera samples. The extent and direction of the foraminiferal sample age offset, relative to bulk carbonate are shown to be a function of both sampling method and the ecology of the site.