MILANKOVITCH PHASING RELATIONSHIPS AND GREAT BASIN PALEOCLIMATE

The controls on Great Basin paleoclimate have been debated for more than 130 years since the classic monographs on Pluvial Lakes Lahontan and Bonneville, yet it has been difficult to place their climate records into a firm chronological framework, or to provide a conceptual model that allows for future climate projections. The 175,000 yr-long Leviathan chronology from vadose zone caves in Nevada has confirmed that the classic Milankovitch forcing of northern hemisphere summer insolation (NHSI) at 65°N insolation is the prime mover of Great Basin paleoclimate. However, analysis of phasing relationships to Milankovitch forcing, using June 21 insolation at 65°N, shows a persistent delay in Great Basin paleoclimate of 7 to 54° relative to the 23,000 yr precession timescale at key maxima and minima. For example, peak interglacial δ¹⁸O values correlative to Marine Isotope Stage 5e and 1 are delayed by 1050 and 3200 years. The persistent delays suggest that Great Basin climate is indirectly forced by a fast-response component of the climate system. A plausible mechanism is variation in the extent of a sea ice, mountain glaciers, and terrestrial snow cover, which impacted atmospheric circulation on sub-orbital timescales in response to Milankovitch variations. Because anthropogenic global warming is expected to result in reduced northern hemisphere snow and ice cover, the Great Basin is likely to see warming associated with both changing atmospheric circulation and local radiative effects over coming millennia. Finally, we show that several additional millennia are required for the correlative isotopic anomalies to be registered in Devils Hole calcite, and the durations of some isotopic anomalies are extenuated beyond the time scales predicted by Milankovitch forcing. We suggest that the aquifer system feeding Devils Hole delays, dampens, and extenuates surface climate variations, likely due to isotopic dispersion during water parcel transit from the recharge area. The timings and duration of Devils Hole isotopic anomalies should therefore be considered minima and maxima, respectively.