Corals Ba/Ca records uncover mid-twentieth century onset of land use change associated with industrial deforestation in Malaysian Borneo.

The increasing demand for wood, pasture, and palm oil drives deforestation and stands as the largest threats to rainforests. Whilst many consequences of deforestation are well understood, the effects on coastal ecosystems remain less clear. This issue is very apparent in Malaysian Borneo where the lack of historical deforestation data makes characterising baseline environmental conditions challenging. Building upon a previous study testing the suitability of coral Ba/Ca records as proxies for riverine sediment, we extend these records to the late nineteenth century, revealing a significant mid-20th-century surge in riverine barium levels, and a gradual lag within records consistent with distance from the river. We argue this increase is associated with the onset of industrial deforestation supported by historical logging records as well as land use data. Ba/Ca records provide unequivocal evidence for the temporal onset and magnitude of the impact of deforestation raising baseline sediment discharge in the nearshore waters.