Calibration of speleothem δ¹⁸O records against hydroclimate instrumental records in Central Brazil

Abstract

δ¹⁸O in speleothems is a powerful proxy for reconstruction of precipitation patterns in tropical and sub-tropical regions. The aim of this study is to calibrate the δ¹⁸O record of speleothems against historical precipitation and river discharge data in central Brazil, a region directly influenced by the Southern Atlantic Convergence Zone (SACZ), a major feature of the South American Monsoon System (SAMS). The present work is based on a sub-annual resolution speleothem record covering the last 141 years (the period between the years 1870 and 2011) from a cave in central Brazil. The comparison of this record with instrumental hydroclimate records since 1921 allows defining a strong relationship between precipitation variability and stable oxygen isotope ratios from speleothems. The results from a monitoring program of climatic parameters and isotopic composition of rainfall and cave seepage waters performed in the same cave, show that the rain δ¹⁸O variability is dominated by the amount effect in this region, while δ¹⁸O drip water remains almost constant over the monitored period (1.5 years). The δ¹⁸O of modern calcite, on the other hand, shows clear seasonal variations, with more negative values observed during the rainy season, which implies that other factors also influence the isotopic composition of carbonate. However, the relationship between δ¹⁸O of carbonate deposits and rainwater is supported by the results from the comparison between speleothem δ18O records and historical hydroclimate records. A significant correlation between speleothem δ¹⁸O and monsoon rainfall variability is observed on sub-decadal time scales, especially for the monsoon period (DJFM and NDJFM), once the rainfall record have been smoothed with a 7–9 years running mean. This study confirms that speleothem δ¹⁸O is directly associated with monsoon rainfall variability in central Brazil. The relationship between speleothem δ¹⁸O records and hydroclimatic historical records allows approximation of the absolute changes in mean annual rainfall during the last millennia in the SACZ/SAMS domain.