Tracing last millennium cycles of Río de la Plata Plume Water input into the Southwestern Atlantic Ocean.

Recent studies established a strong connection between instrumental hydroclimatic changes in the Río de la Plata (RdlP) watershed and the influx of continental terrigenous material and productivity changes. This correspondence was further validated for the last millennium. Here, we identified centennial, multi-decadal, and interannual changes in diatom composition, as a proxy for La Plata Plume Water (PPW) discharge into the inner shelf. Core GeoB13813-4 diatom data were revisited, and grouped into freshwater, marine, and brackish categories for multivariate, timeseries, and correlation analyses. Such record exhibits a noteworthy resemblance to paleoclimatic and paleoceanographic variations throughout the past millennium. A Detrended Correspondence Analysis (DCA) on the dominant taxa, and Axis-1 revealed a salinity gradient over the past millennium. DCA1-scores exhibited significant correlation with reconstructions of El Niño Southern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO), and the Atlantic Multidecadal Oscillation (AMO) over the same period. By plotting DCA1-scores, we inferred time intervals with varying levels of PPW influence. During ∼936-1500 CE, the PPW influence was weak, as the record was dominated by marine taxa, indicating lateral transport from the Brazilian upwelled water and a stronger influence of the Subtropical Shelf Water. After 1500 CE, a well-developed plume was inferred, with the highest influence occurring after 1800 CE, as evidenced by an increased presence of freshwater/brackish taxa. This increased PPW influence after 1500 CE can be attributed to the onset of more humid conditions and associated increased river discharge, particularly under El Niño conditions, with further ENSO intensification after 1800 CE. We identified significant centennial (250 years) and multidecadal cycles (25-50 years) in diatom community composition. These cycles are related to solar forcing, PDO, and AMO. During the 20th Century, cycles of 2 to 7 years were associated with interannual ENSO variability, while 40-year cycles were linked to AMO and PDO variability.