ANTHROPOGENIC DROUGHT CAPTURED IN OXYGEN ISOTOPE FROM SOUTHERN AMERICAN SPELEOTHEM

The increase in global temperature and evapotranspiration potential caused by climate change has a strong impact on the water balance of tropical regions. In this sense, reconstituting hydro-climatic parameters beyond the instrumental series is vital to the understanding the full amplitude of natural climate variability and to assess the contribution of anthropogenic forcing to regional climate. A pervasive drying trended recorded in the streamflow from São Francisco drainage basin points to an unprecedented long-term drought stating by the 1980s following the rising of surface temperatures. The warming plays an important role in this regard, exacerbating drought conditions by increasing water loss through higher evapotranspiration demands (Chagas et al., 2022).

In order to reconstruct the hydroclimate variability to beyond the instrumental period we produced an annual-resolved oxygen isotope record using stalagmites from a well-ventilated cave located in middle course of São Francisco Drainage basin, at Januária city Brazil northern of Minas Gerais State. The cave, locally known as Lapa da Onça, features a wide entrance with numerous actively dripping stalagmites in the entrance chamber. The atmosphere in the sampling sites is characterized by large seasonal amplitudes in relative humidity and temperature, reflecting the atmospheric and environmental conditions outside the cave (Strikis et al., 2024).

Cave monitoring results show a consistent coupling between seasonal variation in oxygen isotope composition from speleothem calcite and the cave atmosphere relative humidity and temperature. At intra-annual time-scale season variation in cave temperature rise as a major driver of oxygen isotope variability. Conversely, at interannual time scale, variation in evaporative demand emerges as a primary driver of the isotope variability.

Accordingly, isotope effects associated with periods of reduced rainfall (rainfall isotope enrichment through degree of rainout upstream) are magnified by the kinetic isotope effects associated to increasing in evaporation during water dripping. Such combined isotope effects results in a marked increase in δ¹⁸O values of speleothem calcite.

To assess how the current warming period has effects the local hydrologic balance we performed an high resolution isotope sampling with 0.1 mm on two speleothems with high deposition rate, near 1 mm.yr^-1. Both speleothems exhibit annual-resolved chronologies, established through U-Th dating and layer counting.

Comparison between oxygen and carbon isotope profiles from Onça stalagmites with instrumental records from local meteorological station points to the evaporative demand which ultimately is temperature dependent as a major driver of oxygen and carbon isotope variability. By extending our record back to the year 1298, we observe that the increase in δ¹⁸O and δ¹³C has no parallel over the past 720 years. These results reinforce the premise of a severe long-term drought in the tropics of Southern Hemisphere associated with the anthropogenic warming.

References

Stríkis, N.M.; Buarque, P.F.S.M.; Cruz, F.W.; Bernal, J.P.; Vuille, M.; Tejedor, E.; Santos, M.S.; Shimizu, M.H.; Ampuero, A.; Du, W.; Sampaio, G.; Sales, H.R.; Campos, J.L.; Kayano, M.; Apaèstegui, J.; Fu, R.R.. Cheng, H.; Edwards, R.L.; Chaves, V.; Franscischini, D.S.; Arruda, M.A.Z.; Novello, V.F. Modern anthropogenic drought in Central Brazil unprecedented during last 700 years. Nature Communications, v.15, p. 1728, 2024.

Chagas, V.B.P.; Chaffe, P.L.B.; Blöschl, G. Climate and land management accelerate the Brazilian water cycle. Nature Communications, v. 13, p. 5136, 2022