Description: Continental and marine paleoclimate archives from northwestern and northeastern South America recorded positive precipitation anomalies during Heinrich Stadials (HS). These anomalies have been classically attributed to enhanced austral summer (monsoon) precipitation. However, the lack of marine paleoclimate records off eastern South America as well as inconsistencies between southeastern South American continental and marine records hamper a comprehensive understanding of the mechanism responsible for (sub-) tropical South American hydroclimate response to HS. Here we investigate piston core M125-95-3 collected off eastern South America (10.94°S) and simulate South American HS conditions with a high-resolution version of an atmosphere-ocean general circulation model. Further, meridional changes in precipitation over (sub-) tropical South America were assessed with a thorough compilation of previously available marine paleorecords. Our ln(Ti/Ca) and ln(Fe/K) data show increases during HS6-Younger Dryas. It is the first core off eastern South America and the southernmost from the Atlantic continental margin of South America that unequivocally records HS-related positive precipitation anomalies. Based on our new data, model results and the compilation of available marine records, we propose a new mechanism for the positive precipitation anomalies over tropical South America during HS. The new mechanism involves austral summer precipitation increases only over eastern South America while the rest of tropical South America experienced precipitation increases during the winter, challenging the widely held assumption of a strengthened monsoon. South American precipitation changes were triggered by dynamic and thermodynamic processes including a stronger moisture supply from the equatorial North Atlantic (tropical South Atlantic) in austral winter (summer).

Description: Despite its great ecological importance, the main factors governing tree cover in tropical savannas as well as savanna-forest boundaries are still largely unknown. Here we address this issue by investigating marine sediment records of long-chain n-alkane stable carbon (δ13Cwax) and hydrogen (δDwax) isotopes from a core collected off eastern tropical South America spanning the last ca. 45 thousand years. While δ13Cwax is a proxy for the main photosynthetic pathway of terrestrial vegetation, tracking the relative proportion of C3 (mainly trees) versus C4 (mainly grasses) plants, δDwax is a proxy for continental precipitation, tracking the intensity of rainfall. The investigated core was collected off the mouth of the São Francisco River drainage basin, a tropical savanna-dominated region with dry austral autumn, winter and spring. On top of millennial-scale changes, driven by anomalies in the amount of precipitation associated with Heinrich Stadials, we identify a marked obliquity control over the expansion and contraction of tree and grass cover. During periods of maximum (minimum) obliquity, trees (grasses) reached maximum coverage. We suggest that maximum (minimum) obliquity decreased (increased) the length of the dry season allowing (hampering) the expansion of tree-dominated vegetation. Periods of maximum obliquity induced an anomalous heating (cooling) of the summer (winter) hemisphere that in combination with a delayed response of the climate system slightly increased autumn precipitation over the São Francisco River drainage basin, through a shift of the Intertropical Convergence Zone towards or further into the anomalously heated hemisphere. We found that atmospheric CO2 concentration has only a secondary effect on tree cover. Our results underline the importance of the dry season length as a governing factor in the long-term control of tree cover in tropical landscapes.