What Is the Forcing Mechanism of Orbital Scale Paleoclimatic Change in Western North America?

Abstract:

The pacing of paleoclimatic change in the Great Basin of western North America is closely linked to the classical Milankovitch forcing of precessional-scale summer insolation variations. Cave stalagmites from Nevada show that oxygen isotope variations (δ¹⁸O) closely track summer insolation, and show a peak-to-peak match with insolation minima and maxima over the last 175,000 years. The strong correspondence between stalagmite δ¹⁸O and summer insolation suggests a fast-response of atmospheric circulation to orbital forcing in western North America. In particular, we observe very low δ¹⁸O values (cold conditions) during Marine Isotope Stage 5d that track a prominent summer insolation minima. The small change in global ice volume during MIS 5d contrasts with the large cooling of Great Basin paleoclimate, and suggests that changes in northern hemisphere ice sheet extent was not a dominant driving factor of Great Basin climate then. Further, records of eastern Pacific Ocean sea surface temperature do not closely track Great Basin δ¹⁸O minima, suggesting that the main forcing does not reside in the ocean. Instead, we suggest that a rapid increase in the areal extent of winter snow and sea-ice extent resulted in exceptionally cold continental interior climate during MIS5d, both through a direct thermal (cooling) effect, and indirect circulation effects (more northerly-sourced air masses). Extrapolating the MIS 5d climate by analogy, we suggest that the driving mechanism for orbital-scale paleoclimate variability relates to changes in the sea-to-land temperature gradient, which drove the δ¹⁸O values of atmospheric precipitation to low values by enhanced air mass distillation during insolation minima. Because of the long transport distance of air masses up and over major orographic barriers such as the Sierra Nevada, and the strong continentality of the interior Great Basin, this mechanism can explain the exceptional sensitivity of Nevada δ¹⁸O values to orbital forcing.