Variations in the South Atlantic Convergence Zone over the mid-to-late Holocene inferred from speleothem δ18O in central Brazil

Abstract

Most precipitation records within the South American Summer Monsoon (SASM) domain bear a trend following the Southern Hemisphere summer insolation (SHSI). However, a growing number of past hydroclimate records have revealed a high degree of spatial variability in rainfall trends across tropical South America. Here we report a new speleothem δ18O record from central Brazil that underlies the South Atlantic Convergence Zone (SACZ). Our record reveals a nearly invariant rainfall trend over the mid-to-late Holocene, suggesting that the SACZ has maintained its latitudinal position and intensity despite the rising SHSI and consequently increasing monsoon strength in the core SASM zone. The record also shows no obvious response in SACZ rainfall to millennial-scale temperature anomalies in the North Atlantic. Instead, the long-term SACZ behaviour is likely dominantly influenced by subtropical South Atlantic sea surface temperatures, which can affect the position and intensity of moisture convergence over the continent. Our central Brazil speleothem record suggests that orbital-scale insolation change is not the dominant control of long-term SACZ rainfall trends. Alternative drivers of hydroclimate change need to be explored to account for regional differences in rainfall patterns within the SASM system.