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.
Bibliographical noteFunding Information:
This research was supported by the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) grant 2011/12238-6 to Moquet, 2012/1187-4 to Karmann, 2012/50260-6 , 2013/50297 to Cruz, 2011/12013-4 to Stríkis, 2012/03942-4 and 2014/10095-1 to Novello and 590172/2011-5 to PRIMO cooperative project (CNPq-IRD). It also benefited from grants from the US-NSF ( 1103403 to R.L.E and H.C. and 1303828 to M.V.). MD acknowledge funding by the Irish Research Council (IRC) by a Government of Ireland Postdoctoral Fellowship ( GOIPD/2015/789 ). We thank the three anonymous reviewers for their constructive recommendations under the review process. We also thank Ramiro Hilário dos Santos for in-situ water sampling and his help during field work, Dr. Osmar Antunes for his support for carbonate stable isotope analyses in the stable isotope laboratory of the IGc-USP (Sao Paulo — Brazil), Dr. Luis Mancini and Eduardo Carvalho for the water sample stable isotope analyses in the Laboratory of Geochronology in the University of Brasilia (Brazil), Dr. Damien Guillaume for carbonate mineralogical analyses at the GET Laboratory in Toulouse (France), Laia Comas Bru for providing the code for the Monte Carlo tests, Dr. Giuliano Locosselli for help during field work and in laminae counting and Dr. Guillaume Bertrand for his help in improving this manuscript. We thank the IBAMA and ICMBio for permission to collect stalagmite samples.
- Central Brazil
- Climate calibration
- Stable isotopes