Terrigenous Sedimentation on the Submarine Shirshov Ridge (Bering Sea) during the Last Deglaciation

Abstract

The submarine Shirshov Ridge is an independent system of terrigenous sedimentation, which is geomorphologically isolated from bottom terrigenous influx into the deep-water basin of the Bering Sea. Using the ridge as example, we studied background hemipelagic sedimentation of the finely dispersed terrigenous suspended material from water column and deposition of the coarser grained ice-rafted material in the western part of the deep-water basin. The grain-size and mineral composition of postglacial sediments of the Shirshov Ridge was studied in cores SO201-2-85KL and SO201-2-77KL taken from local basins in the central and southern parts of the ridge, respectively. Statistic treatment of uninterrupted grain-size distributions (GD) of terrigenous component of the postglacial sediments by end-member (EM) modelling revealed that the grain-size distributions of terrigenous sediments from two cores are determined by the mixing of three EMs. EM-1 and EM-2 reflect the hemipelagic sedimentation with and without bottom currents influence respectively, while EM-3 with mode at fine-grained sand characterizes GD of the ice-rafted material. Reconstructed mechanisms of terrigenous influx on the Shirshov Ridge involve advection of the suspended matter with surface and intermediate water masses and ice-rafting. The relative role of both mechanisms of the terrigenous sedimentation under the influence of varying bottom current velocities for intervals of Last Glacial Maximum, early deglaciation, Heinrich event 1, Bolling–Allerod, Younger Dryas, and Early Holocene is estimated. It is ascertained that the grain-size distribution of terrigenous component is defined by climate variations, sea ice coverage, sea ice drift pathways, conditions of fast ice melting, and mobility of bottom waters. High concentrations of drifting ice or seasonal sea ice cover likely existed above the southern part of the ridge during the second half of the Heinrich 1 event. The low mobility of bottom waters facilitated only the subice hemipelagic sedimentation of fine fractions from the background reserve of suspended material. A sharp reduction of ice-rafted flux was reconstructed for the Bolling–Allerod warming interval. Bottom currents affected sedimentation in the central part of the ridge during the entire deglaciation (in addition to the second half of the Heinrich 1 event), and in the southern part during the Bolling–Allerod, Younger Dryas, and Early Holocene.