Speleothem records of interannual variability in the tropics during the Holocene

Reconstruction of changes in interannual variability through the Holocene period can provide valuable insight into the sensitivity of internal modes of variability (e.g. ENSO, PDO) to external forcing. Coral and bivalve records are widely used examine past changes in short-term variability, however, far fewer reconstructions exist in the terrestrial realm. Here, we use the SISAL (Speleothem Isotopes Synthesis and AnaLysis) database to examine changes in the amplitude of interannual/short-term variability across the monsoon regions, recorded by speleothem δ¹⁸O standard deviation (δ¹⁸O s.d.). First, we identified and corrected for any confounding factors that may obscure interannual climate signals in the speleothem records, such as variable growth rate. We used a multiple linear regression model to constrain relationships between δ¹⁸O s.d. and growth rate and mean climate (represented by mean δ¹⁸O), then used these relationships to apply a correction. Second, corrected δ¹⁸O s.d. trends were combined into regional monsoon composites, representing changes in short-term variability across the Holocene. Trends are very similar between raw and corrected δ¹⁸O s.d., suggesting that confounding factors have a minimal effect. Trends differ amongst regions: the Indian monsoon shows gradually increasing δ¹⁸O s.d. through the Holocene, whilst the East Asian monsoon shows no significant changes through this period. The Indonesian-Australian monsoon shows higher than present δ¹⁸O s.d. during the mid- to early Holocene and the South American monsoon shows multi-centennial scale fluctuations between higher and lower δ¹⁸O s.d.