Modern sedimentary dynamics in the Southwestern Atlantic Contouritic Depositional System: New insights from the Uruguayan margin based on a geochemical approach

摘要

Recent high-resolution, multidisciplinary studies have provided models of modern and ancient sedimentary processes along the Uruguayan slope (34 degrees S to 36 degrees S), a strategic location in the Southwestern Atlantic margin (SAM). According to these models, modern surface sedimentation is controlled by the transition between the Subtropical Shelf Front and the Brazil-Malvinas Confluence (BMC), and presumably by the summer Rio de la Plata (RdIP) buoyant plume. Ancient sedimentary dynamics is reflected as the northernmost distribution of the contouritic terraces present in the Argentinean margin, reflecting the position of the BMC during glacial stages. In this work, the spatial correlation of previous evidence from modern (surface fades) and ancient (geomorphological features) sedimentation provides a sound framework in order to interpret novel information on the geochemical composition of surface sediments. Proxies for the origins of lithogenic (Nd, Al/Si, and Fe/K) and organic matter (C/N, delta O-13, and delta N-15), previously calibrated for the SAM, and the elemental compositions (AI, TI, Fe, K, Mn, Ca, P, Mg, Ba, and Si) of surface sediments were used to test working hypotheses on sediment provenance, surface productivity and the environmental control of elementary composition. The distribution of surface sediments is correlated with regional geomorphology, which includes contouritic terraces in the south and a progradational slope in the north. This supports the idea of a strong morphological control on the modern sedimentary processes. However, the distribution and content of terrigenous, biogenic and erosional elements, together with the dominance of Pampean Patagonian sediments, suggests that 1) the RdIP does not contribute with any lithogenic sediments to the slope and, instead, fertilizes open waters with contributions consisting of mixed marine organic matter; 2) in the north, the strong action of the BC and the re-circulated AAIW below promotes redistribution of sediments from the shelf and upper slope towards the middle slope, including sandy spillover/slump transport and hemipelagic sedimentation re-suspended from the continental shelf; 3) with respect to glacial stages, this across-slope sedimentation was displaced southwards dominating even the northernmost distribution of contouritic terraces and 4) sedimentation related to contouritic features, as modern depositional and erosional sedimentation, was restricted to the south accompanying the distribution of the MC. Thus, although the core hypothesis of the previous modern sedimentary model can be retained, considerable improvements in our knowledge of the sediment dynamics in the SAM have emerged from the present analysis of geochemical proxies. (C) 2016 Elsevier B.V. All rights reserved.