Seasonal wet-dry variability of the Asian monsoon since the middle Pleistocene

Abstract

Wet summers and dry winters are an essential feature of monsoon climates, but quantification of change in wet-dry seasonality through time is very challenging because most geological materials fail to record sub-annual environmental signals, instead integrating years, decades, or centuries. Here we quantify Asian wet-dry seasonality since the middle Pleistocene for monsoon climates using high-resolution stable oxygen isotope analysis of land snail shells from the Chinese Loess Plateau. This approach allows us to collect data on single seasons and years in geologic history. Modern shells show that: 1) the δ18Oshell values are higher in boreal winter (dry) seasons and lower in boreal summer (wet) seasons; 2) winter – summer differences in δ18Oshell (up to 19.2‰) are much larger than that expected from seasonal temperature variation; and 3) the δ18Oshell range (the difference between the most positive and the most negative values) within a shell records the degree of evaporation of snail body water in the dry season, combined with a rainfall δ18O/temperature indicator in the wet season. This range can be used as a wet-dry seasonality proxy index, although clearly more work is required to calibrate this index. δ18Oshell ranges in glacial loess strata (mean = 13.6‰) are systematically larger than those from interglacial paleosol layers (mean = 8.6‰), suggesting that a significant difference in winter aridity was present, with drier winters in glacial periods and wetter winters in interglacial periods since the middle Pleistocene. This pattern may have been modified by the intrusion of the westerlies into the region in glacial dry seasons. Summer minimum δ18Oshell values are similar throughout the record, which contrasts significantly with glacial/interglacial speleothem records, especially if these represent typical monsoon stable isotope patterns in rains. However, if some moisture from the westerlies contribute more negative δ18O values in glacial intervals, these minimum δ18O values may not contradict the patterns seen in the speleothem record. Future work should focus on the seasonal timing of the isotope cycles in land snail shells.