Developing sediment geochronologies for high-latitude continental shelf deposits: a radiochemical approach

Abstract

During January and February of 1986, 47 box cores and 55 piston cores were collected from the Bransfield Strait, Gerlache Strait and Marguerite Bay, Antarctica. Apparent sediment accumulation rates established for 100-yr time scales with excess 210Pb chronologies range from 0.08 to 0.34 cm/yr in the Bransfield Strait, 0.15 to 0.51 cm/yr in Gerlache Strait and from 0.10 to 0.27 cm/yr in Marguerite Bay. For the Antarctic Peninsula continental shelf, 137Cs appears to have a limited usefulness in assessing the effects of deep sediment mixing by benthic fauna on seabed excess 210Pb profiles because the observed activities for the regions examined are close to counter detection limits. Conventional 14C ages of surface sediments from the peninsula region range from 1370 to 2820 yr. The spatial variations in the conventional 14C surface ages are thought to be influenced predominantly by inputs of older eroded sediment from lateral transport, current winnowing, mass wasting and/or a variable carbon source. On a 1000-yr time scale, 14C measurements of five piston cores from the Bransfield Strait yielded sediment accumulation rates ranging from 0.06 to 0.49 cm/yr. These data support the assertion by Nelson (1988) that the excess 210Pb sediment accumulation rates (range of 0.08 to 0.34 cm/yr) for this region represent true rates. The two sediment accumulation rates of 0.01 and 0.08 cm/yr for the Gerlache Strait are significantly lower than the observed apparent 210Pb accumulation rates (mean = 0.27 ± 0.12 cm/yr) for this region. Likewise, two piston cores from Marguerite Bay yielded 14C sediment accumulation rates of 0.02 and 0.05 cm/yr, which also are considerably lower than the excess 210Pb apparent accumulation rates (mean = 0.17 ± 0.06 cm/yr) for this area. The one-dimensional advection-diffusion equation of Goldberg and Koide (Geochim. Cosmochim. Acta, Vol. 51, 1962) was solved simulatneously with 210Pb and 14C data for BC18 and BC113 to resolved the effects of sediment mixing and accumulation in Bransfield Strait and Marguerite Bay box core samples, respectively. This model indicates that 210Pb and 14C sediment accumulation rates agree within a factor of 2 in the Bransfield Strait, whereas in Marguerite Bay the apparent 210Pb accumulation rates may be as much as an order of magnitude too high. In the southern high-latitude environments of the Antarctic Peninsula, radiocarbon measurements in the seabed are a better approach than 210Pb for establishing accurate rates of sediment accumulation. The 14C chronologies, when coupled with 210Pb data, provide more reliable assessments of biological deep mixing and sediment accumulation rates than do excess 210Pb data coupled with measurements of bomb-produced 137Cs. The sediment accumulation rates for both 100-yr and 1000-yr time scales tend to decrease southward from the Bransfield Strait to Marguerite Bay, which is consistent with glacial-marine sedimentation processes (as modulated by climate) for the western Antarctic Peninsula. Seabed measurements of 210Pb, 137Cs and 14C also provides valuable insights on water circulation and upwelling, atmospheric inputs and sediment depositional processes for the Antarctic Peninsula region.