Description: In order to assess the response of vegetation from eastern tropical South America to hydroclimate changes during the last ca. 45 kyr, we analyzed marine sediment core M125-95-3 that archived sediments from the São Francisco River drainage basin. We used the stable carbon (δ13C) and hydrogen (δD) isotopes of long-chain n-alkanes (i.e., C29-C31) to reconstruct regional vegetation composition and precipitation dynamics, respectively. The data set contains marine sedimentary records of long-chain n-alkane δ13C and δD, as well as the isotopic difference between homologues (Δδ13C31-29), and locally estimated scatterplot smoothing (LOESS) of indicated data. The δ13Cwax refers to the weighted averages of δ13C29 and δ13C31 values. The δDwax refers to the weighted averages of δD29 and δD31 values.

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.