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Maya rainfall became harder to forecast after 500 CE

A Belize speleothem shows seasonal rainfall predictability collapsing after 500 CE and bottoming in the Terminal Classic, alongside drought.

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What cave record is analyzed?

YOK-G speleothem from Yok Balum Cave, southern Belize.

Why use δ13C rather than δ18O for local seasonality?

δ13C tracks local hydroclimate; δ18O mixes distant convection and moisture source.

What is the modern seasonal rainfall contrast?

400–700 mm/month in June–September versus 40–70 mm in February–April.

When does YOK-G show drying?

A drying trend from 500 CE toward dry conditions 600–800 CE.

When are extreme wet/dry years most frequent?

700–900 CE, after a quieter 550–700 CE.

How is seasonal predictability measured?

Recurrence plots of annual δ13C segments; similar years mean higher τpred.

When does τpred fall below significance?

Between 800 and 1000 CE.

When did predictability start to deteriorate?

After 500 CE, with the strongest drop after 800 CE.

Could low sampling resolution fake the collapse?

Only a fraction of the τpred drop is explained by downsampling toy models.

When does predictability recover?

It minima near 1000 CE then is high between 1050–1200 CE.

How are dating errors handled?

2σ ≈ 5 years, propagated through a COPRA age-model ensemble.