Variability in terrigenous sediment supply offshore of the Rio de la Plata (Uruguay) recording the continental climatic history over the past 1200 years

Abstract

Abstract. The continental shelf adjacent to the Río de la Plata (RdlP) exhibits extremely complex hydrographic and ecological characteristics which are of great socio-economic 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 a depth of 57 m (GeoB 13813-4). The radiocarbon age control indicated an extremely high sedimentation rate of 0.8 cm per year, encompassing the past 1200 years (750–2000 AD). 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 vs. marine-brackish) to assess the variability in terrigenous water and sediment discharge. Ti / Ca, Fe / Ca, Ti / Al, Fe / K and the freshwater diatom group showed the lowest values between 850 and 1300 AD, while the highest values occurred between 1300 and 1850 AD. The variations in the sedimentary record can be attributed to such regional and global climatic episodes as 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 northward migration of the Intertropical Confluence Zone (ITCZ) 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, the southward migration of the ITCZ accompanied by El Niño-like state conditions may have led to an expansion of RdlP river plume far to the north, as indicated by higher element ratios and a marked freshwater diatom signal. During the current warm period (i.e., after 1900 AD), the highest values in the element ratios and a pronounced marine to marine-brackish diatom record was found. This discordance between diatom record and sediment supply during the last century could be the consequence of an anthropogenic impact on the drainage basin and the RdlP system, expressed by the artificial increase in metal concentrations in the offshore sediments.

Coherent South American Monsoon Variability During the Last Millennium Revealed Through High‐Resolution Proxy Records

“…The comparison with a precipitation reconstruction for the La Plata Basin from GeoB13813‐4 (Perez et al, ; Figure c) shows the same excursion toward wet anomalies (higher freshwater diatom percentage) during the LIA period as seen in the first PC δ 18 O , indicative of a stronger monsoon. The La Plata drainage basin is a region that can receive high precipitation amounts during both strong and weak monsoon regimes.…”

“…Precipitation variability over South America during the Last Millennium. (a) First PC δ 18 O mode, (b) second PC δ 18 O mode, (c) fresh water diatomaceous percentage on the La Plata river estuary (Perez et al, ), and (d) Cariaco Basin sediment Titanium ratio (Haug et al, ). The colors represent the different climate regimes found through a change‐point detection test based on the mean, evaluated on the first PC δ 18 O .…”

“…Wavelet and spectral analyses (see the supporting information) were performed on the first and second PC δ 18 O revealing significant periodicities at 256, 128, 70, and 32 years. Previous studies, such as Novello et al (), Apaéstegui et al (), Bernal et al (), and Perez et al (), have argued that the longer cycle periodicities (128 and 256 years) can be attributed to solar variability and multidecadal internal variability such as the AMO (Atlantic Multidecadal Oscillation). Recent studies by Jones and Carvalho () show a possible influence of the AMO on South American hydroclimate over the last 100 years.…”

Weather, Hydrological and Oceanographic Conditions of the Northern Coast of the Río de la Plata Estuary during ENSO 2009–2010

“…Nevertheless, estuarine zones are expected to experience a greater impact of hydrologic effects (fresh water or ocean water inputs) [14]. ENSO impacts in coastal zones include increased rainfall, hydrological changes in basins, modifications in river flow [15], and generation of inter-annual variations in fresh water discharges for estuarine systems [16,17].…”

“…ENSO is considered in the RdlP as a large-scale atmospheric forcing effect on the discharges of the tributary rivers [15,17], changes in salinity and nutrients [18], as well as in the location and location of turbidity or salinity fronts [19,20]. Several studies have described the relationship between ENSO events, RdlP flow, and salinity of the estuarine system [18,19,21].…”