Benthic remineralization and burial of biogenic SiO2, CaCO3, organic carbon, and detrital material in the Southern Ocean along a transect at 170° West

Abstract We investigated the composition, recycling, and mass accumulation rates of sediments along a transect in the Southern Ocean located from 66°S to 57°S at 170°W. This transect also corresponds to the location of a sediment trap mooring line. The sediments at the seven sites studied range from largely terrigenous material to nearly pure (>90%) biogenic silica. CaCO3 is a minor but persistent component at most sites. Mass accumulation rates have been determined on the basis of excess 230Th in the sediments, i.e., 230Th-normalized accumulation rates. The influence of redistribution of sediments on the sea floor has been estimated from 14C analyses. The recycling of material delivered to the sediments has been characterized on the basis of pore water studies that make extensive use of both in situ sampling and shipboard extractions. The influence of the highly variable rates of input of particulate matter that characterize much of the Southern Ocean upon pore water gradients and fluxes across the sediment water interface has been considered. We find only poor correspondence between BSiO2 burial fraction (=burial/particulate flux), a quantifiable measure of preservation efficiency, and BSiO2 particulate rain along the transect. However, preservation does appear to be closely linked to a combination of sedimentation rate and particulate rain. The burial fraction of BSiO2 is small relative to benthic rain (5–19%). Despite the small fraction buried, burial flux normalized to (sedimentation rate)1/2 appears to provide a very consistent means of predicting benthic particulate rain over a large range of rain rates, including data from a number of different studies and environments. At sites with BSiO2 rain ⩾250 mmol m−2 yr−1 the average difference between predicted and observed rain is 25–30%. Such rain rates occur in many marine areas, particularly the Southern Ocean, with the result that this relationship potentially provides a means of estimating BSiO2 benthic rain over prolonged periods in the past on the basis of readily measured sediment parameters. At the southern-most deep ocean station, the particulate flux was characterized by an extremely high Corg/CaCO3 ratio (>10), but this high ratio does not appear to have a substantial influence on CaCO3 burial. CaCO3 is preserved in the sediments at this site despite a particulate flux with a 10-fold excess of Corg above that required for complete dissolution in the sediments. The unexpectedly high preservation of CaCO3 is due largely to the very steep Corg oxidation rate profile at this site. As a result, a large fraction of the organic matter oxidized in the sediments does so in close proximity to the sediment–water interface where most of the metabolic CO2 is neutralized by CO32− from the overlying water, rather than by the dissolution of sedimentary CaCO3. Diagenetic modeling indicates that at several of the stations, the remineralization fluxes of carbonate species across the sediment–water interface may not have been at steady state as a result of the highly pulsed nature of particulate rain in this environment. We estimate that at the time of our sampling it is possible that near-interface fluxes could have been a factor of 1.6–2 times the annual average. At every site on the transect, the burial fluxes of detrital material are substantially greater than the detrital particulate rain measured in the sediment traps, by as much as a factor of 40. Detrital burial is bimodal, being greatest at the southern and northern extremes of the transect. We postulate that the excess of burial over particulate rain in the south reflects the contribution of ice rafted debris at these high latitudes. Increases in the supply at the northern stations must have a different source. We believe that the excess at these stations is material eroded from the sea floor to the west, possibly on the Campbell Plateau, and advected by currents to the northern portion of the transect at depths below the shallow traps.