Extreme wet events in Southern Brazil during the Heinrich Stadials

We are making continuous efforts to improve both the data resolution and chronology of the speleothem d18O record from Botuvera Cave, southern Brazil, one of the very few high-precision and high-resolution records from southern tropical continents covering the last ~ 100 kyr. In this study, we particularly focus on precipitation change in southern Brazil during the extreme cold time intervals in the North Atlantic, the so-called Heinrich (H) Stadials. With precise U/Th dating, high-resolution d18O profiles from multiple Botuvera stalagmites show that the H2, H3 and H4 stadials are about 1,000 years long and centered approximately 24.6 kyr BP, 30.0 kyr BP and 39.0 kyr BP, respectively. Following the earlier studies on Brazilian speleothem d18O records, as well as trace element analysis and modeling simulations, we interpret the calcite d18O as a proxy of monsoonal rainfall in the region. Broadly speaking, the lower calcite d18O, the higher rainfall, and vice versa. The abrupt drops on calcite d18O during these intervals, with an amplitude of ~ 2% VPDB, suggest that climate change in this region is manifested as a dramatic increasing of rainfall. This confirms our previous finding that on millennial timescales, precipitation change in southern Brazil is anti-phased with that in the north, such as eastern Asia, which is probably controlled by the meridional migration of the intertropical convergence zone (ITCZ). The fast growth rates of the samples enable us to characterize detail precipitation changes in southern Brazil during these three Heinrich Stadials. Precipitation increases abruptly in the earlier stage, evidenced by a d18O drop rate of ~ 5%/kyr, whereas during the later stage, precipitation decreases more gradually, with a d18O rising rate of ~ 2.5%/kyr. This asymmetric pattern of change is different from the temperature behavior in the North Atlantic, which warms much more dramatically in the final stage of a Heinrich Stadial than it cools in the initial phase. Such decoupling may suggest that ITCZ meridional migration is preferably controlled by the temperature gradient between the mid and low latitudes in the cold hemisphere. It has been further noticed that calcite d18O values are higher right after the Heinrich Stadials than those immediately preceding the events, hence, an even drier climate after the extreme wet events. These values, in a range between -3.4% and -2.7%, however, are far below the typical value of ~ -1.5% in early Holocene. This observation suggested that, although the Atlantic overturning circulation may "overshoot" directly following its weaken mode during the Heinrich Stadials, the atmospheric response did not react in a proportional manner.