Abstract
Abstract. The continental shelf adjacent to the Río de la Plata (RdlP) exhibits extremely complex hydrographic and ecological characteristics which are of great socioeconomic importance. Since the long-term environmental variations related to the atmospheric (wind fields), hydrologic (freshwater plume), and oceanographic (currents and fronts) regimes are little known, the aim of this study is to reconstruct the changes in the terrigenous input into the inner continental shelf during the late Holocene period (associated with the RdlP sediment discharge) and to unravel the climatic forcing mechanisms behind them. To achieve this, we retrieved a 10 m long sediment core from the RdlP mud depocenter at 57 m water depth (GeoB 13813-4). The radiocarbon age control indicated an extremely high sedimentation rate of 0.8 cm per year, encompassing the past 1200 years (AD 750–2000). We used element ratios (Ti / Ca, Fe / Ca, Ti / Al, Fe / K) as regional proxies for the fluvial input signal and the variations in relative abundance of salinity-indicative diatom groups (freshwater versus marine-brackish) to assess the variability in terrigenous freshwater and sediment discharges. Ti / Ca, Fe / Ca, Ti / Al, Fe / K and the freshwater diatom group showed the lowest values between AD 850 and 1300, while the highest values occurred between AD 1300 and 1850. The variations in the sedimentary record can be attributed to the Medieval Climatic Anomaly (MCA) and the Little Ice Age (LIA), both of which had a significant impact on rainfall and wind patterns over the region. During the MCA, a weakening of the South American summer monsoon system (SAMS) and the South Atlantic Convergence Zone (SACZ), could explain the lowest element ratios (indicative of a lower terrigenous input) and a marine-dominated diatom record, both indicative of a reduced RdlP freshwater plume. In contrast, during the LIA, a strengthening of SAMS and SACZ may have led to an expansion of the RdlP river plume to the far north, as indicated by higher element ratios and a marked freshwater diatom signal. Furthermore, a possible multidecadal oscillation probably associated with Atlantic Multidecadal Oscillation (AMO) since AD 1300 reflects the variability in both the SAMS and SACZ systems.
Highlights
The Río de la Plata (RdlP) estuary is fed by the Paraná and the Uruguay rivers and drains into the southwestern Atlantic Ocean (SWAO), forming the second largest estuary system in South America (Bisbal, 1995; Acha et al, 2003)
The RdlP mud depocenter shows an exceptionally high sedimentation rate (0.8 cm yr−1 on average; Perez et al, 2016) compared with other records from the southern Brazilian continental shelf (Mahiques et al, 2009; Chiessi et al, 2014). This high sedimentation rate is consequence of the enormous amount of sediment transported by the Paraná and Uruguay rivers into the RdlP watershed and further onto the Uruguayan shelf (Lantzsch et al, 2014)
During the Medieval Climatic Anomaly (MCA) (AD 800–1300) a reduction in SAMS and South Atlantic Convergence Zone (SACZ) activities would have caused a decrease in the rainfall rate over the RdlP drainage basin, resulting in more estuarine-marine conditions predominating over a freshwawww.clim-past.net/12/623/2016/
Summary
The Río de la Plata (RdlP) estuary is fed by the Paraná and the Uruguay rivers and drains into the southwestern Atlantic Ocean (SWAO), forming the second largest estuary system in South America (Bisbal, 1995; Acha et al, 2003). The RdlP is the main source of continental freshwater and sediments entering the SWAO (Piola et al, 2008; Krastel et al, 2011, 2012; Razik et al, 2013; Lantzsch et al, 2014; Nagai et al, 2014). In this sense, the RdlP provides an average annual suspended sediment load of 79.8 × 106 t yr−1 (Depetris et al, 2003). The low-salinity waters on the inner part of the continental shelf extend downwards to a depth of approximately 50 m, while the outer part of the continental shelf (from 50 to 200 m) is influenced by the Subtropical Confluence, where the warm, salty southward-flowing Brazil Current collides with the cold and less salty northward-flowing Malvinas Current (Piola et al, 2000)
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