Great Basin Paleoclimate and Aridity Linked to Arctic Warming and Tropical Pacific Sea Surface Temperatures

Abstract The arid southwestern United States is susceptible to sustained droughts that impact water resources and economic activity for millions of residents. Previous work has not systematically investigated the structure, timing, and possible forcings of Holocene Great Basin sub‐orbital hydroclimate changes, impeding our ability to understand the potential future controls on Southwestern aridity. The objective of this paper is to constrain the potential forcings on Holocene aridity and temperature, via comparison of new high‐resolution speleothem data, an Aridity Index synthesizing hydroclimate records, and linkages of Southwestern paleoclimate to other regions. The high‐resolution data from Leviathan Cave provide a paleoclimate record since 13,400 yr ago: A cool Younger Dryas was followed by two pronounced Middle Holocene aridity intervals between 9,850 and 5,310 yr B2k characterized by low growth rates and high δ 18 O and δ 13 C values. Subsequently, stalagmite δ 18 O values show near‐modern levels for the last four millennia during which time growth rates were high and δ 13 C values were low in response to wetter conditions. The regional Aridity Index documents that Middle Holocene drying coincided with a warm Arctic and decreased sea ice extent, a warm western tropical Pacific, and a large sea surface temperature gradient across the tropical Pacific, all of which likely responded to northern hemisphere summer insolation forcing. Our data suggest that extreme Middle Holocene aridity is more severe than the short medieval droughts evident in the tree ring record, and such extreme aridity may represent a worst‐case analog for future climate.