What controls deep tropical long-term hydroclimate change: a view from speleothems

Abstract:

The driving mechanism of long-term monsoon rainfall changes remains debated. Studies on deep sea sediments and on land archives, such as lake sediments and aeolian dusts, have traditionally suggested that monsoon records are characterized by a strong ~100,000-year cycle, similar to those in global ice volume, greenhouse gas concentration in the atmosphere. Hence, it has been postulated that monsoon rainfall should be largely modulated by changes of glacial boundary conditions. However, the relatively poor geochronology constrains in record age models often complicate a correct data evaluation.

The other school of thought proposed that monsoon rainfall changes should directly response to insolation. Oxygen isotopic records from speleothems, aided by precise U/Th age control have often shown a dominant ~20,000-year periodicity, while the ~100,000-year periodicity is rather weak or non-existent in the records. However, it has been argued that calcite speleothem δ¹⁸O may not simply represent monsoonal rainfall change.

Here, we compare our newly reconstructed speleothem records with published data. These records are from the deep tropics, and their δ¹⁸O values are unequivocal indicators of regional rainfall changes. We found that very few speleothem δ¹⁸O records from the deep tropics follow rainy-season insolation changes. Instead, their δ¹⁸O values were mostly low during the last glacial period, and the values were high during the Holocene, which suggest that glacial boundary conditions can indeed play a key role in monsoon changes.