SORTABLE SILT AND FINE SEDIMENT SIZE COMPOSITION SLICING - PARAMETERSFOR PALEOCURRENT SPEED AND PALEOCEANOGRAPHY

Fine sediment size (<63 mu m) is best measured by a sedimentation tech nique which records the whole size distribution. Repeated size measure ment with intermediate steps of removal of components by dissolution, allows inference of the size distribution of the removed component as well as the residue. In this way, the size of the biogenic and lithoge nic (noncarbonate) fractions can be determined. Observations of many s ize distributions suggest a minimum in grain size frequency curves at 8 to 10 mu m. The dynamics of sediment erosion, deposition, and aggreg ate breakup suggest that fine sediment behavior is dominantly cohesive below 10-mu m grain size, and noncohesive above that size. Thus silt coarser than 10 mu m displays size sorting in response to hydrodynamic processes and its properties may be used to infer current speed. Silt that is finer than 10 mu m behaves in the same way as clay (<2 mu m). Useful parameters of the distribution are the 10-63 mu m mean size an d the percentage 10-63 mu m in the fine fraction. We cannot use size d istributions to distinguish the nature of the currents. Therefore, to infer water mass advection speeds (i.e., the mean kinetic energy of th e flow, K-M), regions of high eddy kinetic energy (K-E) must be avoide d. At the present, such abyssal regions lie under the high surface K-E of major current systems: Gulf Stream, Kuroshio, Agulhas, Antarctic C ircumpolar Current, and Brazil/Falkland currents in the Argentine Basi n. This is probably a satisfactory guide for the Pleistocene. With reg ard to the, carbonate subfraction of the size spectrum, size modes due to both coccoliths and foraminiferal fragments can be recognized and analyzed, with the boundary between them again at about 10 mu m. The f lux of less than 10 mu m carbonate, at pelagic sites above the lysocli ne, is another candidate for a productivity indicator.