Fig. 1: Global distribution of speleothem δ¹⁸O and Mg/Ca records and season-specific rainfall.

The global map is accompanied by insets: a South American monsoon region and b Australasian monsoon region. These areas were selected to illustrate speleothem δ¹⁸O record density and Mg/Ca presence. The yellow star marks the location of the Sulawesi speleothem δ¹⁸O and Mg/Ca records for this study. Global speleothem locations are from the SISAL V2 database¹⁴. The number of speleothem entities associated with each site is indicated by colour and size of circles. Black dots denote sites with Mg/Ca data reported in SISAL V2. Because the northern Borneo sites are in close proximity to each other, resulting in overlapping dots, the number of stalagmite records is indicated with a line to each site (inset, b). In addition, a Mg/Ca marker was added over northern Borneo as the data exists⁸⁵ but is not recorded in the SISAL V2 database. Tropical rainfall for June–August (JJA, green scale) and December–February (blue scale) represents corresponding regional summer monsoon precipitation. Darker shades signify average monthly rainfall exceeding 500 mm. Precipitation data are from WorldClim 2.1 and span 1970–2000 at 5 min resolution⁸⁶. The map was created using QGIS software and country boundaries are based on the geometry from EBM v2020 scale 1:60 million (EuroGeographics and UNFAO⁸⁷).