DID SUPPRESSION OF EL NIŅO CAUSE MID-HOLOCENE DROUGHT IN SOUTHWESTERN NORTH AMERICA?

Large-scale eolian landforms in Estancia basin, central New Mexico, record two episodes of extreme drought and low groundwater levels during the middle Holocene followed by a rise in the water table through the late Holocene. Blowouts and associated lunettes formed when groundwater levels fell below the desiccated floor of pluvial Lake Estancia, allowing widespread deflation of Pleistocene lake sediments. AMS radiocarbon dates on gastropods found at the base of lunette deposits indicate that deflation initiated at ~7 ¹⁴C kyr B.P., and examination of the architecture of lunettes and blowouts reveals two distinct episodes of deflation. When deflation ceased, playa fill began to accumulate in blowouts, and dates on reproductive cysts of Artemia salina (brine shrimp) found in fill sediments suggest wetter conditions by 5.4 ¹⁴C kyr B.P. Further radiocarbon dates on Artemia cysts reveal a gradual infilling of playas through the late Holocene, which we attribute to a rising water table.

Elevation of the water table in Estancia basin presently is regulated by a balance between evaporation from the playas that occupy the blowouts and recharge of aquifers in the adjacent Manzano Mountains. Isotopic analyses of precipitation and groundwater reveal that recharge originates primarily as winter moisture, which is augmented during El Niņos and significantly diminished during La Niņas. Thus, the late Holocene rise in the water table, as well as the low water table during the middle Holocene (blowouts and lunettes) may reflect the mid-Holocene suppression of El Niņo, followed by its increasing influence during the late Holocene, as suggested by numerical models and recent geologic evidence from South America and Australia.