Orbital-scale timing and mechanisms driving Late Pleistocene Indo-Asian summer monsoons: Reinterpreting cave speleothem δ18O

摘要

Southeast China cave δ18O, often interpreted as a pure East Asian summer monsoon proxy, lags maximum northern hemisphere summer insolation by 2.9±0.3 kyrs at the precession cycle. The Arabian Sea summer monsoon stack lags by 8±1 kyr, consistent with 13 other Indian and East Asian summer monsoon proxies from marine, lake, and terrestrial archives. This 5 kyr phase difference cannot be attributed to age control inadequacies in the marine chronology; it requires reconciliation in the context of proxy interpretation. Both of these lags are incompatible with a direct response to northern hemisphere summer insolation, implicating additional forcing mechanisms. Analysis of heterodynes in the cave δ18O spectrum demonstrates that variance contained in the Arabian Sea summer monsoon proxies also resides in the cave δ18O record. This variance is subtracted from the cave δ18O record yielding a residual that is highly coherent and in phase with precession minima, reflecting the impact of winter temperature change on cave δ18O (meteorological precipitation under cold conditions). Thus, we argue that the timing of light cave δ18O peaks cannot be interpreted as reflecting the timing of strong summer monsoons alone. The 2.9 kyr precession band phase lag of cave δ18O reflects the combined influence of summer monsoon forcing with a phase lag of 8 kyrs relative to precession minima and winter temperature forcing that is in phase with precession minima. This interpretation is consistent with modern seasonality in the amount and isotopic composition of rainfall in southeast China.