The sedimentation, bioturbation and organic matter degradation as revealed by excess 230Th and 210Pb in the Cosmonaut Sea

Abstract

Sedimentation and bioturbation (the activity of benthic organisms) play an important role in organic matter degradation. The excess 230Th and 210Pb in a sediment core were used to evaluate the sedimentation, bioturbation and organic degradation processes in the Cosmonaut Sea, East Antarctica. Our results show that the sedimentation rate estimated from excess 230Th through the Constant Flux (CF) and Constant Flux and Constant Sedimentation (CFCS) models has changed in the past 200 ka. The mass accumulation and the linear sedimentation rates vary from 0.36 to 0.76 kg m-2 ka-1 and from 0.6 to 1.3 mm ka-1, which are significantly lower than the rates in other Antarctic seas. The stage III (120–202 ka), which corresponds to Marine Isotope Stage (MIS) 6 (glacial period), had the lowest sedimentation rate, indicating less sediment input by sea ice expansion. The bioturbation coefficient estimated by excess 210Pb is as low as 0.023 ± 0.003 cm2 a-1, showing inactive benthic activities. The average C/N ratio of organic matter is 7.2 ± 0.4, which is close to that of fresh organic matter and lower than the typical value of deep-sea sediments, indicating that organic matter is less degraded. The degradation rates of organic carbon and organic nitrogen estimated by the bio-diffusion model are 0.61 ka-1 and 0.29 ka-1, respectively, which are significantly lower than previously reported values for Southern Ocean sites. Despite the low sedimentation rate, low rates of bioturbation and inefficient organic matter degradation are conducive to the preservation of organic matter in sediments of the Cosmonaut Sea.