Stalagmite evidence for East Asian winter monsoon variability and ¹⁸O-depleted surface water in the Japan Sea during the last glacial period

In the East Asian monsoon area, stalagmites generally record lower and higher oxygen isotope (δ¹⁸O) levels during warm humid interglacial and cold dry glacial periods, respectively. Here, we report unusually low stalagmite δ¹⁸O from the last glacial period (ca. 32.2–22.3 ka) in Fukugaguchi Cave, Niigata Prefecture, Japan, where a major moisture source is the East Asian winter monsoon (EAWM) that carries vapor from the warm surface of the Japan Sea. The δ¹⁸O profile of this stalagmite may imply millennial-scale changes, and high δ¹⁸O intervals that are related to Dansgaard–Oeschger (D–O) interstadials. More importantly, the stalagmite exhibits low overall δ¹⁸O values; the mean δ¹⁸O (− 8.87‰) is distinctly lower than the mid-Holocene mean of another stalagmite from the same cave (4.2–8.2 ka, − 7.64‰). An interpretation assuming a more intense EAWM and greater vapor transportation during the last glacial period, compared with the mid-Holocene, contradicts the limited inflow of the Tsushima Warm Current into the Japan Sea because of lowered sea level. Additionally, our model calculation using δ¹⁸O data from meteoric water indicated that the amount effect of winter meteoric water was insignificant (1.2‰/1000 mm). Low stalagmite δ¹⁸O for the last glacial period in Fukugaguchi Cave most likely resulted from ¹⁸O-depleted surface water, which developed in the isolated Japan Sea. The estimated amplitude of the δ¹⁸O decrease in surface water was ~ 3‰ at most, consistent with the abnormally low values for foraminifera (by ~ 2.5‰) in sediment during the last glacial period, shown by samples collected from the Japan Sea. This is the first terrestrial evidence of ¹⁸O depletion in Japan Sea surface water during the last glacial period. [Figure not available: see fulltext.].