Sortable silt and fine sediment size/composition slicing: Parameters for palaeocurrent speed and palaeoceanography

Abstract

Fine sediment size (<63 µm) is best measured by a sedimentation technique which records the whole size distribution. Repeated size measurement 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 lithogenic (noncarbonate) fractions can be determined. Observations of many size distributions suggest a minimum in grain size frequency curves at 8 to 10 µm. The dynamics of sediment erosion, deposition, and aggregate breakup suggest that fine sediment behavior is dominantly cohesive below 10‐µm grain size, and noncohesive above that size. Thus silt coarser than 10 µm displays size sorting in response to hydrodynamic processes and its properties may be used to infer current speed. Silt that is finer than 10 µm behaves in the same way as clay (<2 µm). Useful parameters of the distribution are the 10–63 µm mean size and the percentage 10–63 µm in the fine fraction. We cannot use size distributions to distinguish the nature of the currents. Therefore, to infer water mass advection speeds (i.e., the mean kinetic energy of the flow, KM), regions of high eddy kinetic energy (KE) must be avoided. At the present, such abyssal regions lie under the high surface KE of major current systems: Gulf Stream, Kuroshio, Agulhas, Antarctic Circumpolar Current, and Brazil/Falkland currents in the Argentine Basin. This is probably a satisfactory guide for the Pleistocene. With regard 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 µm. The flux of less than 10 µm carbonate, at pelagic sites above the lysocline, is another candidate for a productivity indicator.

Environmental changes off North Iceland during the deglaciation and the Holocene: foraminifera, diatoms and stable isotopes

“…Benthic foraminiferal biozones (from Eiríksson et al, 2000a) and diatom biozones (this paper) are indicated, as well as the level of the Vedde and Saksunarvatn tephras (stippled lines). Sorting (standard deviation) was calculated using moment statistics (McBride, 1971), and the sortable silt percentage and mean were calculated as the percentage of the 10-63-m fraction out of the total fines and the mean grain size of that fraction ( McCave et al, 1995).…”

Controls on carbonate and terrigenous deposition in the incipient Benguela upwelling system during the middle to the late Miocene (ODP Sites 1085 and 1087)

“…Interestingly, the mean size of terrigenous sortable silt could also be used as an indicator of relative bottom current strength (McCave et al, 1995b). Following these interpretations, we assume that terrigenous sortable silt variability at the study area may be used as an indicator of paleo bottom-current strength and/or dust supply derived from the Namib Desert.…”

Quaternary sediment dynamics in the Belgica mound province, Porcupine Seabight: ice-rafting events and contour current processes

“…The bulk particle size distributions were statistically analysed to produce mean, sorting and skewness. The sortable silt index (mean 10-63 lm) from mud-dominated sediments was also derived as a proxy for bottom-current strength (McCave et al 1995).…”

“…Various authors consider sediment drifts as ideal targets for palaeoceanographic and palaeoclimatologic studies (McCave et al 1995;Rebesco and Stow 2001). Van Rooij et al (2003) also recognised a close relationship between the positions of the Belgica mounds and the presence and shape of the surrounding sediment drift bodies.…”

“…The mean sortable silt proxy (Fig. 11), which can provide estimates in bottom-current intensity (McCave et al 1995), reveals short-term variations. Since the inferred MIS4, five intervals of peak currents (28-33 lm) and five intervals of lower currents (22-26 lm) are observed.…”