Reconstruction of Holocene coupling between the South America Monsoon System and local moisture variability from speleothem δ¹⁸O and ⁸⁷Sr/⁸⁶Sr records

Abstract

Investigating controls on past variability of South American hydroclimate is critical to assessing its response to future warming scenarios. δ¹⁸O records from South America offer insight into past variability of the South American Monsoon System (SAMS). The controls on precipitation δ¹⁸O values, however, can be decoupled from precipitation amount and thereby limit investigations of variability in local moisture conditions. Here we use a principle components analysis to assess the coherence of speleothem and lake core Holocene δ¹⁸O records in tropical South America to evaluate the extent to which δ¹⁸O variability reflects changes in SAMS intensity at different sites across the region. The main mode of variability across Holocene δ¹⁸O records (PC1) closely tracks austral summertime insolation, consistent with existing work. PC1 is strongly expressed at sites towards the periphery of the continent, while variability at interior sites bear little similarity that implicates controls, in addition to monsoon intensity, on these δ¹⁸O records. Further, we develop speleothem ⁸⁷Sr/⁸⁶Sr records spanning the Holocene from Tamboril Cave (Brazilian Highlands), Paraíso Cave (eastern Amazon Basin), Jaraguá Cave (Mato Grosso Plateau), and Botuverá Cave (Atlantic coastal plain) to investigate coupling between reconstructed monsoon variability and local moisture conditions. Speleothem ⁸⁷Sr/⁸⁶Sr variability is interpreted as a proxy of local moisture conditions, reflecting the degree of water-rock interaction with the cave host rock as driven by variations in water residence time. Speleothem ⁸⁷Sr/⁸⁶Sr records from all the sites, except Botuverá cave, do not co-vary with PC1, suggesting that local moisture conditions do not necessary follow variations in monsoon intensity at these interior sites. These speleothem ⁸⁷Sr/⁸⁶Sr records, however, generally suggest dry mid-Holocene conditions, consistent with interpretations of other paleo-moisture records in the region. These results highlight that dynamics, in addition to SAMS variability, might influence δ¹⁸O variability as well as local moisture conditions at interior sites, and highlight the need for δ¹⁸O-independent reconstructions of moisture conditions.