Decadal-scale climate cycles preserved in megaherbivore dental tissues in tropical Asia

Julien Louys , Justyna Jolanta Miszkiewicz, Pennilyn Higgins, Mathieu Duval, Yahdi Zaim, Mika R. Puspaningrum, Yan Rizal, Aswan Aswan, Joshua Rowe, Pauline Basilia, Loic Martin, Sebastian Breitenbach, Gilbert Price

Rainforests have emerged as key biomes for understanding the initial dispersal of modern humans from Africa. In Southeast Asia, the oldest dated records of humans are associated with rainforests, but recent fossil records suggest these biomes were structurally different from today. Climatic fluctuations associated with insolation changes brought about smooth transitions between rain, dry and montane forests due to impacts of glacial-interglacial and solar cycles on the Asian-Australian monsoon. However, the timing and nature of these changes remain poorly understood due to a lack of fossil archives resolved to seasonal or annual scales from the largely submerged Sunda Shelf. Here, we show that large megaherbivore dental tissues record Schwabe sunspot cycles. We demonstrate that δ13C and δ18O in a modern African elephant tusk are significantly correlated with insolation changes over solar cycles 21 and 23. Based on this relationship, we examine sunspot cycles in fossil elephant teeth from Bangka Island, one of the emergent Sunda islands at southern end of the so-called savannah corridor. Combined U-series and ESR dating indicates the teeth are early Holocene, and bulk δ13C and δ18O values show the elephant lived in wet rainforests. Periodicity analyses suggest that sunspot activity peaked at the beginning and end of tooth enamel formation, associated with higher δ13C and lower δ18O. The presented specimens indicate that a modern rainforest and monsoonal system, whereby a negative Indian Ocean Dipole coupled with La Niña produces higher regional precipitation, was in place by the early Holocene in Island Southeast Asia.