Biological proxies as paleoceanographical tools in Contourite Depositional System of the Argentine Continental Margin: Potential uses and limitations

Abstract

Along the Argentine continental margin, one of the largest Countourite Depositional System (CDS) of the world has developed, which is characterized by large-scale contourite channels, terraces and plastered drift sequences. Due to the predominance of contour currents specially in the upper and middle slope of the Northern Continental Margin, many studies have focused on the morphology and architecture of this CDS in this setting. Recently, some papers have pointed out on the relation between the depths of the terraces and the depths of the water masses interfaces. These studies are based mainly on geophysical, geological and oceanographic data, but micropaleontological studies confirming some hypothesis concerning timing and processes involved are still lacking. The presence of hemipelagic facies with abundant calcareous microfauna, mainly planktonic and benthic foraminifera, in sediments from the upper and middle slope of the Northern Argentine Continental Margin allow to infer that micropaleontological studies have the potential to reconstruct the long-term evolution of some paleoceanographic parameters, i.e. sea surface temperatures (SSTs) by using multivariate techniques such as transfer function. However, micropaleontological studies carried out on lower slope sediment cores (~3400 m) around 38°S indicate that calcareous microfauna is usually confined to the upper 2-3 meters of sediment cores, while deeper levels are barren of calcareous microfauna theoretically as a consequence of the influence of corrosive cold water masses of Antarctic origin, the Antarctic Bottom Water during coldest periods. However, distribution of calcareous fauna along a shallower sediment core LBIV-10 (~900 m, base of the upper slope) indicates that dissolution events are not confined to the lower slope. This core is located within a contouritic channel in the upper part of the Ewing Terrace, under the influence of the Antarctic Intermediate Water (AAIW). Quantitative SSTs estimates from fertile levels indicate fully glacial winter and summer SSTs. If barren levels are associated with coldest events, when influence of corrosive cold water masses of Antarctic origin attains its maximum, therefore fertile levels must be associated with warmer events. Thus, planktonic assemblages of core LBIV-10 are not actually reflecting the properties of superficial waters during glacial times at 38°S as they seem to do. Assemblages from this core are with all probability winnowed taphocoenosis, northward displaced assemblages under the influence of the AAIW and hence SSTs signal is misleading as a consequence of taphonomic processes. Metabolic supralysocline calcite dissolution and physical destruction of test due to enhanced transport along the contouritic channel during coldest periods (as Preu et al., in press proposed) can not be discarded.