Last millennium intensification of decadal and interannual river discharge cycles into the Southwestern Atlantic Ocean increases shelf productivity

Large rivers represent transitional environments between the coast and the open ocean which discharge is influenced by both climate and anthropogenic impacts. In general, historical information on river discharges does not extend beyond 100-yr data series. This is the case of the Río de la Plata (RdlP) watershed, which represents a very important geographic region sensitive to El Niño Southern Oscillation (ENSO). Thus, this study analyzes a continuous millennial, high-resolution record of river discharge oscillation cycles into the ocean and associated productivity changes. We used two 10-m-long sediment cores retrieved from the high-resolution RdlP mud depocenter from the inner continental shelf off Uruguay, namely GeoB 13813-4 and GeoB 13817-2. The first sediment core yielded a mean linear sedimentation rate of 9.7 mm yr−1, while the second exhibited a lower resolution, with a mean rate of 7.1 mm yr−1. Differences in the sedimentation rate are attributed to the distance to the continental source. We performed 2-mm-step-size XRF scanning and used the Ti/Al, Fe/K, Fe/Ca and Si/Al element ratios as proxies for terrigenous supply into the Southwestern Atlantic Ocean (SWAO), and Fe/Mn as a proxy of the redox conditions. At the same time, Ba/Al ratios recorded productivity changes. We detected significant cycles of 50, 20, 10, 2–7 yr, and less than 1 yr for most of the element ratios of both cores. We inferred that cyclicity was related to the Climatic Modes of Oscillation (CMO): Atlantic Multidecadal Oscillation (AMO), Pacific Decadal Oscillation (PDO), Southern Annular Mode (SAM) and ENSO. The CMO play a crucial role in modulating the geochemical characteristic of the terrigenous fine sediments, building up the RdlP mud depocenter. The process of millennial intensification of river discharge, and the associated increase in productivity, were both modulated by the ENSO mode of variability, particularly after the onset of the Little Ice Age (LIA) especially after 1500 CE. During the LIA, ENSO displayed mostly decadal cycles, whilst after the end of the LIA and onset of the Current Warm Period (CWP), the cyclicity intensified to half-decade cycles. After 1970s, river discharge attained maximum magnitude and frequency as a sign of combined natural and anthropogenic forcing, but we also inferred a concomitant increase in productivity. Herein, we introduce evidence that global warming is causing an intensification of the inter-annual hydro-climatic variability within SESA and land-use practices (mainly intensification of soya crops and deforestation) are producing significant soil erosion after 1970. This intensification represents an early warning signal to anticipate a further increase in continental input and productivity within SWAO, which holds both regional and global implicances.