Pronounced drought and wet periods during the Maya Terminal Classic period reconstructed using stalagmite calcium isotopes

The sociopolitical decline of Classic Maya Civilisation and associated depopulation of the Maya Lowlands in the Terminal Classic period (c. 800–1000 CE) are thought to have coincided with frequent regional drought events. Proxy-based palaeoclimate evidence from lake sediment cores and speleothems revealed the frequency, duration and relative severity of regional droughts, but the complex nature of environmental proxies has complicated simple translation to quantitative estimates of rainfall amount. Calcium isotope ratios (δ⁴⁴Ca) are a novel speleothem proxy that responds to changes in epikarst groundwater availability, which varies with rainfall quantity. Thus, δ⁴⁴Ca can be used to infer semi-quantitative palaeorainfall amount. Here we present δ¹⁸O, δ¹³C, and δ⁴⁴Ca time series for the last 1500 years from HOBO-5, a stalagmite from Columnas Cave, in the Puuc Region of the Maya Lowlands, Yucatán, Mexico. Increases in δ¹⁸O and δ¹³C values coincide with previously identified episodes of drought in the region. We complement these traditional proxy records with δ⁴⁴Ca, which enables derivation of semi-quantitative estimates of past rainfall. This new palaeoclimate record implies a reduction in mean rainfall of 43 % during the late Terminal Classic and early Postclassic periods (890–1160 CE), suggesting that soil and vegetation were slow to recover after repeated droughts. This aridity was preceded by a wet period characterised by a >60 % increase in precipitation (865–890 CE), between drought intervals in the Terminal Classic. Our findings support prior evidence for multiple episodes of severe rainfall reduction and high precipitation variability, coinciding with ancient Maya cultural upheaval.