Effects of tropical precipitation variability on the composition of fluvial sediments from SE Brazil in glacial and interglacial times (MIS 6-5)

The present-day hydrological cycle in southeastern Brazil depends on the intensity of the South American Summer Monsoon (SASM) with strong monsoonal precipitation during austral summer (DJF) and weak precipitation during austral winter (JJA). On glacial-interglacial timescales, monsoonal intensity was mainly controlled by precession-forced changes in insolation.

Relatively little is known to date about the spatial distribution of precipitation in the hinterland and coastal areas of SE Brazil and the resulting variability of the fluvial discharge on glacial-interglacial timescales. The Doce River basin is situated at the northern boundary of the present-day South Atlantic Convergence Zone (SACZ), wherefore its run-off and suspension load respond sensitive to changes in both summer monsoon and coastal winter precipitation. The soil and rock distribution of the basin allows for the study of the relative proportions of terrigenous up- and lowland sources among the transported fluvial sediments.

We studied the mineralogical composition and crystallinity of the non-carbonate fine fraction from late Marine Isotope Stage (MIS) 6 to MIS 5 (150-70 ka) in a marine sediment core obtained in the proximity of the Doce river mouth (20° S, 38° W, 2 km water depth). The main non-carbonate mineral content comprises quartz, albite, illite, kaolinite and gibbsite. The relative abundances of the mineral assemblage show distinct changes relative to changes in summer insolation as well as across the MIS 6-5 transition. Thereby, the mineral assemblage shows a distinct end-member pattern, with high contents of illite (80 % 2M-polytype) and high illite crystallinity as a proxy for stronger physical erosion of the parent rocks in the steep upland, and high contents of kaolinite and gibbsite as proxy for intense tropical soil erosion in the lowlands.

During MIS 5, the insolation dependent cyclicity seen in the mineral assemblage shows high illite/kaolinite ratios when austral summer insolation is high and low illite/kaolinite ratios in low insolation phases. This pattern is not visible in late MIS 6, when very low illite/kaolinite ratios are present during high austral summer insolation.

We consider the spatial changes in erosion intensity to be caused by variations in the regional precipitation pattern. Thereby, pronounced upland erosion is caused by severe precipitation and discharge events during a strong SASM. A relatively increased lowland erosion indicates both increased austral winter precipitation due to stronger trade wind forcing and a weaker monsoonal system in the upper discharge area. The lack of a strong insolation-control on illite/kaolinite ratios during MIS 6 is interpreted as an overall weakening of the SASM system during glacial periods, when austral winter precipitation exerted a stronger control on the hydrological budget of the Doce River.