Holocene Extreme Precipitation Frequency and Paleoclimate Reconstruction Based on High-Resolution Multi-Proxy Records from Speleothems from South-East Brazil

Observational evidence and future projections have shown the increase in frequency and intensity of extreme rainfall events worldwide. However, the data available is scarce and mainly restricted to the last century. In Brazil, these events trigger floods and landslides, accounting for 74% of natural disaster-related deaths with an impact of 6.2 billion dollars between 2010 and 2019. The lack of data going beyond the instrumental record hampers robust assessments of the effects of climate change on those events, precluding the inquiry of whether they are part of natural variability or influenced by human activities. Thus, reconstructing the frequency of extreme rainfall events beyond the period of instrumental data, is essential to understand how the current warming trend affects the frequency and magnitude of such events.

We use detrital layers within stalagmites from flooded galleries as records of cave-flood activity and as archives for past extreme hydrological events. In this context, we investigate stalagmites from three caves in SE Brazil, where modern floods are related to extreme rainfall events as indicated by cave monitoring, along with multi-proxy analyses to reconstruct past changes in paleohydrology. A large set of samples is used to build a robust cave flooding record for the Holocene, and coeval stalagmites are used for record replicability. Extreme rainfall events in southeastern Brazil are mainly associated with the South American Monsoon System between late-autumn and early-spring and extratropical excursions between late-spring and early-autumn. Stalagmite δ18O records are used to reconstruct regional changes in moisture source related to seasonal shifts in atmospheric circulation from a monsoonal regime (more negative δ18O) to a more extratropical regime (less negative δ18O). Correlation between trace element ratios shows that prior-calcite/aragonite precipitation is the main processes controlling the trace element concentrations. Therefore, high-resolution records of trace element ratios along with δ13C are used to reconstruct local hydrological conditions. This multi-proxy approach provides an exceptional opportunity to understand how past climate and environmental changes influenced the occurrence of extreme rainfall events during the Holocene in southeastern Brazil.