APPLYING NOBLE GAS MEASUREMENTS TO IDENTIFY MOUNTAIN-BLOCK RECHARGE PRESENT AND PAST IN THE EASTERN ESPANOLA BASIN, NEW MEXICO

Noble gas recharge temperatures (NGTs) and radiocarbon ages were determined for groundwater samples collected in the eastern Española Basin, New Mexico, to identify mountain-block recharge (MBR) in waters less than 10 ka old and to evaluate possible changes in MBR over the past 35 ka. For Holocene samples from the southeastern part of the basin, NGTs are dominantly 2-4 degrees C cooler than the measured water-table temperature near the mountain front and computed minimum MBR fractions are dominantly 0.2-0.5, consistent with previous large MBR estimates. NGTs do not display the distinct low during the last glacial maximum (LGM) observed in other paleorecharge studies. Instead, samples with the coldest NGTs were recharged 25-35 ka ago. A proposed explanation is that higher precipitation rates during the LGM resulted in a lower mean recharge elevation for the basin, essentially buffering the effect of the lower mean annual air temperature and producing NGTs similar to the Holocene. In the period preceding the LGM, precipitation rates more like today’s resulted in Holocene-like MBR fractions, producing a mean NGT about 5 degrees cooler than the Holocene, as expected.