The Southwestern United States Altithermal Interval (9.5–7 ka): Longest dry period of the Holocene

The global Altithermal Interval, the warm Holocene climatic optimum after the Last Glacial Maximum that lasted several millennia (11-5 ka), is a likely analog for future climatic conditions forced by Anthropocene global warming. Regional studies of this period globally are essential for improving the climate models that will help in our understanding of Earth's rapidly changing climate. The southwestern United States (SW USA) experienced a regional Altithermal that remains to be sufficiently characterized. We show evidence for Altithermal dryness from coralloidal stalagmite growth from two New Mexico stalagmites, and a hiatus of speleothem growth defined by multiple stalagmites from 9.5 to 7 ka. Further, we present novel speleothem climate proxies that we qualitatively link to higher aridity and temperature during this ∼2500-year period, which we define as the SW USA Altithermal Interval. We hypothesize that during the global Altithermal the two principal oceanic subtropical high-pressure systems in the North Pacific and North Atlantic were strengthened and contributed to persistent arid conditions throughout the SW USA during peak summer insolation.