Warren Beck, Research Scientist, Physics and Geosciences Department, University of Arizona
Abstract for Weekly Colloquium on Thursday, Frbruary 22, 2018 at 4 pm in Harvill 318
Cosmogenic 10Be flux from the atmosphere can be used as a proxy for rainfall. Using this proxy we derive a 550 Ka-long record of East Asian summer monsoon (EASM) rainfall from Chinese Loess. This record is forced at orbital precession frequencies, with higher rainfall observed during northern hemisphere summer insolation maxima, though this correlation weakens during cold interstadials. The 10Be monsoon rainfall proxy is even more strongly correlated with global ice-volume variations, which at first glance seems to support Milankovitch theory of high northern latitude orbital forcing of climate.
Our rainfall proxy is moderately well correlated with Chinese cave d18O records, which are also thought to be a proxy for Asian Monsoon intensity--possibly via rainfall amount effects. Because of their strong correlation with high northern latitude summer solar insolation, the cave records have likewise been argued to support Milankovitch theory, which dictates that high northern latitude solar insolation controls ice volume, and through ice albedo effects controls global temperature, and by corollary also controls Asian monsoon intensity. Nevertheless, we show Chinese cave d18O are only very weakly correlated with global ice volume.
This leaves us with a puzzling conundrum. We have two alleged monsoon intensity proxies, both showing strong precessional forcing, however one is highly correlated with high northern latitude solar insolation, but not ice volume, with the reverse for the other. Taken together they seem to reveal a problem with classical Milankovitch theory.
We explain these findings by suggesting that Chinese cave d18O is not a rainfall amount proxy but rather is controlled by moisture mixing from two isotopically distinct sources: the Indian Summer Monsoon vs the Western North Pacific Summer Monsoon. We argue that it is the low latitude interhemispheric insolation gradient--not high northern latitude insolation--which controls the Asian monsoon intensity and moisture sources, and that it may be the Asian Monsoon which ultimately controls global ice volume via monsoon dynamics, rather than the reverse.