Fig. 1

Anjokipoty Cave settings. (A) Regional map overlayed by the major ocean currents and the southern position of the ITCZ in austral summer and the geographic location of Anjokipoty Cave and the main city, Mahajanga. The background map is a Google Earth Imagery, Data SIO, NOAA, U.S. Navy, NGA, GEBCO produced using Google Earth Pro 7.3.6.10201 (64-bit). (B) Measured mean rainfall amount from Mahajanga and the corresponding mean water δ¹⁸O values (circle³²) along with the measured Anjokipoty δ¹⁸O drip water values (diamond, this study). (C) Detailed map of the principal chamber of Anjokipoty Cave that was mapped in 2018–2019 and showing the sampling location (red star). The cave was georeferenced in ArcGIS 10.5 using the Oblique Mercator Laborde projection³³. (D) Photo of the bedrock exposure and vegetation cover right above the cave. (E) Close-up photo of the exposed bedrock and less developed soil and grasses taken during the 2018 field investigation (the white square in the center is a 15 × 20 cm notebook). (F) C4 savanna grasses typical of the cave surroundings at higher elevation with endemic satra palms in valleys. Additional cave setting information is available in the Supplementary.