The cycling and accumulation of biogenic silica and organic carbon in Antarctic deep-sea and continental margin environments

Abstract

Rates of biogenic silica and organic carbon accumulation are reported for Antarctic deep-sea and continental margin deposits. Naturally occurring radionuclides ( 226Ra, 231Pa, and 230Th) were used to establish rates of sediment accumulation in the rapidly accumulating siliceous deposits beneath the Antarctic Polar Front. The rates were as high as 20–180 cm ka −1, and when coupled with biogenic silica and organic carbon measurements, yield accumulation rates as high as 35 mg cm −2 year −1 for silica and 0.3 mg cm −2 year −1 for organic carbon. SiO 2/organic C weight ratios in Polar Front sediments are typically about 100, although values as high as 300 were measured. Considering that the SiO 2/organic C ratio in Polar Front plankton ranges from 0.5 to 2, the high ratios observed in Polar Front sediments indicate that during settling and burial an enrichment of 50–600-fold occurs in biogenic silica relative to organic carbon. Rates of sediment accumulation were determined for the continental margin deposits of the Brans-field Strait and the Ross Sea using 210Pb and 14C chronologies. Accumulation rates ranged from 0.02 to 0.5 cm year −1. Based on these data and measurements of biogenic silica and organic carbon content, typical rates of accumulation on the continental margin are of the order of 3–12 mg cm −2 year −1 for silica and 0.1–0.8 mg cm −2 year −1 for organic carbon. The SiO 2/organic C weight ratios in these continental margin sediments range from 3 to 32. Comparing the accumulation rate data with estimates of annual silica and organic carbon production rates indicates that approximately 25–50% of the gross silica production in surface waters is preserved in the sea-bed, in contrast to less than 5% of the organic carbon. In some of the continental margin environments, lateral transport of biogenic material can create local areas where the rate of silica accumulation is equal to as much as 70% of the production in the overlying water column. In the Southern Ocean environments examined in this study, biogenic silica is preferentially preserved in the sedimentary record relative to organic carbon. This trend is consistent with the greater role of Southern Ocean deposits in the global silica cycle as compared with the organic carbon cycle.