The stable oxygen isotope (δ18O) composition of ancient teak log coffins captures the Asian monsoon 2000 years ago in northwestern Thailand

The past climate in northwestern Thailand remains insufficiently understood because of the limitation of climate proxies. We present a new record of paleoclimate activity during 2050–1551 years BP (before the present), based on the analysis of the oxygen isotope ratios (δ18O) of tree-ring cellulose in ancient teak log coffins excavated from Namjang Cave in Mae Hong Son Province, northwestern Thailand. The ages of the teak log coffin samples were determined using C-14 dating. The average CoffinNJ δ18O value was 23.48 ± 0.77‰, values ranging from 21.23 to 25.42‰. The mean May–October (MO) rainfall reconstructed from the CoffinNJ δ18O data was 274 mm. The mean May–October (MO) rainfall data showed significant negative correlations with stalagmite δ18O data from Laos (r = − 0.254, p < 0.01), highlighting the existence of a weak monsoon from 291 to 294 AD and strong monsoon in 132 AD and 142 AD. Additionally, the MO rainfall data showed a significant positive correlation with reconstructed rainfall in Tibet (r = 0.347, p < 0.01). Spectral analysis of the CoffinNJ δ18O values revealed centennial cycles related to the sunspot number. The CoffinNJ δ18O values have a positive significant correlation with the sunspot number (r = 0.410, p < 0.01) over the entire period. Moreover, we found a highly significant positive correlation (r = 0.644, p < 0.01) between the CoffinNJ δ18O values and stalagmite δ18O values from Wanxiang Cave, China, and this correlation is related variations in the Asian monsoon. We conclude that the CoffinNJ δ18O data reflect the Asian monsoon from 2000 years ago and have the potential to be a paleoclimate proxy in northwestern Thailand.