Bioturbation into a recently emplaced deep-sea turbidite surface as revealed by 210Pb excess, 230Th excess and planktonic foraminifera distributions

Abstract

This study concerns the surface of a single distal turbidite emplaced on the Madeira Abyssal Plain a few hundred years ago. The vertical and horizontal homogeneity of the unit and the low pelagic sediment accumulation rate enable a ready identification of the post-emplacement signals of near-surface bioturbation. Data have been gathered to define 210Pb excess, 230Th excess and pelagic foraminifera profiles with depth. These are used as constant input flux tracers and treated by models conventionally used to determine bioturbation parameters. The mixed layer depths (7–11 cm), bioturbation coefficients (0.08–0.18 cm 2 y −1) and inventory data (10–23 dpm cm −2) derived from the 210Pb excess data in five out of six cores are similar to those previously observed elsewhere with the same model in the deep-sea under steady-state sediment accumulation conditions. Although the mixed layer depths are similar for all three tracers, the 210Pb excess bioturbation intensities are larger by a factor of > 3 than those estimated from 230Th excess profiles and about an order of magnitude larger than those from the foraminifera profiles. It is suggested that the conventional 210Pb excess model and those for other short-lived tracers probably over-estimate the intensity of biomixing of foraminifera for palaeontological and related geochemical purposes.