Taphonomic processes in the Contouritic Depositional System of Argentina

Abstract

The Argentine Continental Margin (ACM) comprises one of the largest contourite depositional systems worldwide, with evidences of strong gravitational downslope and alongslope sediment transport processes. Strong bottom currents produce contourites characterized by successions of sandy, silty and/or muddy facies plastered drifts, terraces and channels (Hernández-Molina et al., 2009; Krastel et al., 2011). The Ewing terrace, located at ~1000-1400 m depth, represents a wide area between the erosive middle slope and the lower slope. It is comprised by deposits resultant from the turbulent water-masses interfaces during their S-N flow along the ACM (Ewing y Lonardi, 1971; Preu et al., 2013). These alongshore-strong bottom currents could have affected the paleoclimatic/paleoceanographic signal codified in the fossil assemblages preserved in the sediments due to biostratinomic processes. In order to identify operative taphonomic processes occurred on the Ewing Terrace during the late Quaternary, assemblages of planktonic foraminifera from sediment core SHN-T379 (38°39?47.08?S - 54°48?30.05?W; ~1000 m) were analyzed.Studies performed on lower-slope sediment cores evidenced that the sediment below ~3 m is barren of calcareous fauna because of a major influence of corrosive waters of Antarctic origin during glacial periods (Laprida et al., 2011; 2014; García Chapori et al., 2014). However, the analysis of sin- and post- sedimentary taphonomic processes at core SHN-T379 indicates that the middle slope also suffered a major influence of waters of Antarctic origin during glacial periods. In the lower slope, barren intervals tend to be associated with the coldest events (stadials) and fertile intervals with warmer events (interstadials and interglacials). However, specific composition analyses and quantitative sea-surface temperature estimates from fertile intervals of core SHN-T379 indicated extremely cold conditions. Negative anomalies (>-12ºC) suggest that the planktonic assemblages are not actually reflecting properties of superficial waters during glacial times, but rather winnowing and advection patterns associated with along-slope sedimentary transport processes. Core SHN-T379 is located within a contouritic channel in the upper part of the Ewing terrace, under the influence of the Antarctic Intermediate Water. Thus, its fossil content would have suffered an outstanding northward displacement under the influence of this current. This would explain the misleading sea-surface temperature signal. The main forcing of this misleading signal is the preferential preservation of robust subpolar species (i.e., N. pachyderma) and the physical destruction of transitional species (i.e., G. bulloides) due to bedload transport along the contouritic channel during the coldest periods (Preu et al., 2013).