Holocene Composition, Origin, and Evolution of Terrigenous Organic Matter in the Southwestern Atlantic Ocean: Insights About Vegetation, Climate and Oceanographic Variations

Two marine sediment cores (MDBT 557 and MDBT 561) from the southern Brazilian shelf were analyzed to reconstruct Holocene variations in terrigenous organic matter (OM) input, vegetation dynamics, and the climatic–oceanographic processes influencing sediment deposition. Using sedimentological, molecular and isotopic proxies we identified site‐specific differences in OM delivery but coherent long‐term regional trends. During the early Holocene, both records indicate low terrigenous input, reflecting a weakened South American Monsoon System (SAMS) over the La Plata River (LPR) basin and reduced continental runoff, which, despite lower relative sea level (RSL), likely limited effective sediment export from the LPR and smaller coastal watersheds. Molecular proxies show that vegetation in southeastern South America was dominated by C 4 ‐rich grasslands, with minimal forest contribution, consistent with drier regional conditions. In the middle Holocene, weakened monsoon precipitation combined with a sustained RSL highstand restricted sediment dispersal across the shelf, maintaining low terrigenous OM input. Nonetheless, subtle lipid biomarker shifts suggest transitional vegetation states and gradual increases in humidity at the basin margins. A marked change occurs around 6,000 cal BP, when hydrodynamic reorganizations expanded the LPR plume over the shelf, altering the provenance of terrigenous OM. This shift marks the late‐Holocene onset conditions, characterized by increased precipitation driven by strengthening summer insolation, intensification of the SAMS, and the establishment of a modern ENSO‐like regime after ∼4,000 cal BP. Consequently, terrigenous OM input and forest‐derived signatures increased substantially, reflecting the expansion of Araucaria and Atlantic Forest taxa across southeastern South America.