Particle-size dependent magnetic properties of Scotia Sea sediments since the Last Glacial Maximum: Glacial ice-sheet discharge controlling magnetic proxies

Abstract

The strong glacial–interglacial similarity between the magnetic susceptibility (MS) of Southern Ocean sediments and Antarctic ice core dust records has often been used to reconstruct Southern Hemisphere atmospheric variability. Although evaluation of various magnetic properties is essential for identifying the magnetic carriers linked to sedimentological variation, detailed magnetic studies are not sufficient in the Scotia Sea. Here we investigate the bulk and particle-size dependent magnetic properties of Scotia Sea sediments over the past ~22 kyr, to determine the main sediment transport mechanism driving bulk magnetic proxies including MS. In bulk sediments, MS is highest during the last glacial period and is accompanied by an increase in the concentration and grain size of ferrimagnetic and antiferromagnetic minerals. For magnetic mineral assemblages, coarse detrital magnetite is dominant. Of three particle-size fractions (>63, 16–63, and <16 μm), the coarse silt fraction (16–63 μm) is responsible for the magnetic properties of bulk glacial sediments. Such dominant contribution of coarse silts rules out a major input of dust, which is expected as finer silt and clay. The silt fraction exhibits a co-varying magnetic mineral concentration with that of the sand fraction (>63 μm) throughout the last deglaciation, indicating a close linkage between their input mechanisms. Thus, the sediment particles ranging from sand to coarse silt, which control the bulk glacial magnetic proxies, are most plausibly transported by iceberg-rafted debris (IRD). As hematite is relatively concentrated in the sand fraction, the hematite contribution in the bulk sediment can highlight IRD-related magnetic signals rather than magnetite. The bulk hematite contribution simultaneously varies with the deglacial influx of coarse IRD particles (>1 mm) in Scotia Sea sediments, although their glacial inconsistency possibly suggests a different IRD input mechanism during the advancement and retreat of the ice sheet. Consequently, the glacial increase in the bulk magnetic concentration indicates vigorous iceberg calving activity in the Scotia Sea and further suggests the coupled cryosphere–atmosphere system.