Oxygen isotope evidence for a stronger winter monsoon current during the last glaciation

A VARIETY of palaeoclimate data from the northern Indian Ocean has shown that the Asian summer monsoon was weaker during the Last Glacial Maximum (∼18,000 years ago) than it is today 1–5 . It has also been inferred from these data that the winter monsoon was stronger at that time 5 . Here we present evidence that the transport of low-salinity water from the Bay of Bengal to the Arabian Sea, caused by the winter monsoon 6 was higher during the Last Glacial Maximum. This inference is based on a negative excursion, up to about 1‰ in magnitude, in the oxygen isotope ratios of three planktonic foraminiferal species, detected in a sediment core from the Arabian Sea by high-resolution (1–2 cm) sampling. Our results place an upper limit of about 4,000 years on the length of time during which the winter monsoon was stronger than today.