Spatiotemporal Discharge Variability of the Doce River in SE Brazil During MIS 6 and 5

Abstract

The modern precipitation balance in southeastern (SE) Brazil is regulated by the South American summer Monsoon and threatened by global climate change. On glacial-interglacial timescales, monsoon intensity was strongly controlled by precession-forced changes in insolation. To date, relatively little is known about the spatiotemporal distribution of tropical precipitation in SE Brazil and the resulting variability of fluvial discharge on glacial-interglacial timescales. Here, we present X-ray diffraction-derived mineralogical data for the 150–70 ka period (marine isotope stage (MIS) 6 to MIS 5) from the Doce River basin. This area was sensitive to changes in monsoonal precipitation intensity due to its proximity to the South Atlantic Convergence Zone. The data, obtained from a marine sediment core (M125-55–7) close to the Doce river mouth (20°S), show pronounced changes in the Doce River suspension load’s mineralogical composition on glacial-interglacial and precessional timescales. While the ratio of silicates to carbonates displays precession-paced changes, the mineralogical composition of the carbonate-free fraction discriminates between two assemblages which strongly vary between glacial and interglacial time scales, with precession-forced variability only visible in MIS 5. The first assemblage, dominated by high contents of kaolinite and gibbsite, indicates intensified lowland erosion of mature tropical soils. The second one, characterized by higher contents of the well-ordered illite, quartz and albite, points to intensified erosion of immature soils in the upper Doce Basin. High kaolinite contents in the silicate fraction prevailed in late MIS 6 and indicate pronounced lowland soil erosion along a steepened topographic gradient. The illite-rich mineral assemblage was more abundant in MIS 5, particularly during times of high austral summer insolation, indicating strong monsoonal rainfall and intense physical erosion in the upper catchment. When the summer monsoon weakened in times of lower insolation, the mineral assemblage was dominated by kaolinite again, indicative of lower precipitation and runoff in the upper catchment and dominant lowland erosion.

A Comparison Study of Mg/Ca‐, Alkenone‐, and TEX86‐Derived Temperatures for the Brazilian Margin

“…The influence of sea level on the BIT index is clearly evident during Termination II, when the abrupt increase in sea level at ∼132 ka corresponds to a strong reduction of BIT values (Figure 4). In addition, clay mineral analyses of Core M125‐55‐7 indicate that precipitation in the densely vegetated low‐land catchment area of the Doce River was enhanced during late MIS 6 (Arndt et al., 2022; Hou et al., 2020a) which would also facilitate soil erosion and thus increase BIT values. Hence, a combination of sea level variations and run‐off changes in the hinterland may have caused the fluctuations in T TEX86_H .…”

“…A particular motivation for this study derives from the fact that previous studies indicated unusually warm SST leading to the last two deglaciations (particularly during late MIS 6) implying an anomalous heat accumulation in the WTSA (Hou et al., 2020b; Santos et al., 2017). This pooling of warm surface waters is interpreted to have strongly impacted continental precipitation on South America by shifting the convection centers and enhancing coastal rainfall (Arndt et al., 2022; Hou et al., 2020a). However, since the available records were based on a single SST proxy, namely Mg/Ca analysis of G. ruber (p), the question remains if these warm temperatures might reflect a warm‐season bias of the Mg/Ca‐derived temperatures during this interval.…”