MILLENNIAL-SCALE STREAM DYNAMICS DURING THE PAST 12,000 YEARS IN NORTHEASTERN NEW MEXICO

We studied the alluvial histories of seven small (<40 km2) watersheds drained by intermittently flowing streams in the uplands of northeastern New Mexico. The data comes from radiocarbon-dated stratigraphy exposed in the walls of arroyos and the cut banks of permanent-channel streams. Results show that over the last 12,000 14C years, nine episodes of valley aggradation separated by incision events occurred in rough synchrony in these drainages. Some of these episodes coincide with well-known climatic fluctuations. For instance, aggradation occurred during the Younger Dryas, 11,200-12,900 cal yr B.P., and valley floors persisted in an incised state 10,000-8500 cal yr B.P. during the peak in Milankovitch summer insolation. All the study reaches are now incised after aggrading over the preceding millennium during the Little Ice Age. Incision probably occurs in these drainage basins when a change to more frequent summer floods takes place during or immediately after a period of prolonged winter drought. Aggradation accompanies other combinations of seasonal climatic conditions, particularly when summer floods are smaller and less frequent during times of overall wetter climate when vegetation cover is denser. We suggest that incision-causing, winter-dry, summer-flood episodes correspond to prolonged periods of La Niña-like conditions when sea surface temperatures in the eastern Pacific are cooler than normal for at least several decades, and the North American Monsoon intensifies. Our results suggest the occurrence of a millennium-scale climate oscillation that can amplify or dampen the effects of shorter climatic cycles like the El Niño-Southern Oscillation and the unidirectional trends like anthropogenic warming. Comparisons with water-level records in the Estancia Basin in central New Mexico suggest that the same millennial-scale quasi-cycle also operated during the late Pleistocene. Today we seem to be near the beginning of a winter-dry, summer-flood incision episode.