Late Pleistocene sediment accumulation in the lower slope off the Rio Grande terrace, southern Brazilian Continental Margin

Abstract

Here are described the analyses of sediments from the lower slope of the Pelotas Basin in the southern Brazilian continental margin, southwestern Atlantic. The samples were obtained from the core SIS-249, collected adjacent to the Rio Grande Terrace (RGT) at a water depth of 2091 m, an area poorly known in terms of sedimentary characteristics compared to the oil and gas-rich basins to the north. The age model indicates that the sampled sediments were accumulated during the last glacial period, between 112.5 and 34.47 ka (MIS 5 to MIS 3). The fine (<63 μm) fraction is dominated by silt with subordinate clay, whereas the coarse (sandy) fraction is chiefly composed of biogenic carbonate (tests of foraminifers) with subordinate very fine (63–125 μm) lithogenic sand. The composition of the lithogenic sand is essentially uniform downcore, dominated by quartz with glaucony as the dominant accessory mineral, and the presence of augite, derived from Mesozoic volcanic rocks, points to a source from the adjacent coastal plain. The uniform mineral composition indicates continuous input of sediments removed from the RGT by action of the Brazil Current (BC) and transferred to the lower slope by gravity flows. The higher proportion of lithogenic sand from the upper RGT along the intervals corresponding to MIS 5 and MIS 3 points to erosion of the terrace due to a more landward position of the BC, related to sea-level highstand and falling stage tracts. The higher amount of silt along the MIS 4 interval suggests increased transfer of sediments from the flanks of the RGT as the result of a more downslope position of the BC driven by the sea-level lowstand. Winnowing of the sortable silt fraction during MIS 5 and 3 points to influence of the southward-flowing North Atlantic Deep Water (NADW). The higher estimated accumulation rates and less negative peaks of δ13C in benthic foraminifers Uvigerina peregrina during MIS 4 could have been caused by oscillations of water masses with distinct isotopic compositions, but also by increased burial and decomposition of organic matter as a result of high ocean surface primary productivity, indicated by the correspondence between the δ13C oscillations and higher accumulation rates and proportion of biogenic sand in the core. Higher productivity was apparently driven by episodes of increased eolian transport of dust from Patagonia and central-western Argentina in milenial timescales, coeval with oscillations in strength and position of the southern westerly winds (SWW) system.