Grand Canyon Paleoclimate and the Deglacial Activation of the North American Monsoon

Abstract:

We reconstruct a 5600-yr long deglacial climate history from a precisely dated Grand Canyon stalagmite, southwestern United States. Based on the absolute values of speleothem calcite δ¹⁸O values, we show that the monsoon was mostly weak during the warm conditions of Greenland Interstadial 1 (GI-1, the Bølling-Allerød) and the cool conditions of Greenland stadial 1 (GS-1, the Younger Dryas), when infiltration was dominated by winter precipitation. An increase in δ¹⁸O values shows an increasing contribution of warm season moisture to cave infiltration over the three millennia following GS-1, which we interpret to represent a gradual activation of the summer monsoon. At its peak, speleothem δ¹⁸O values of around -7‰ VPDB suggest up to an equal mix of summer and winter infiltration, assuming no kinetic effects on soil waters or drip waters. Today, spring and drip water δ¹⁸O values indicate infiltration dominated by winter precipitation. The monsoon activation shows a similar pacing to climate changes in the Great Basin, but contrasts with abrupt climate changes registered in Greenland and elsewhere in the Northern Hemisphere. A gradual activation of the summer monsoon over the Grand Canyon is consistent with a forcing related to mid- to high-latitude feedbacks associated with a delayed response to northern hemisphere summer insolation and ice sheet retreat.