An enhanced role for the Tropical Pacific on the humid Pleistocene-Holocene transition in southwestern North America

Climate effects on landscape evolution during the Late Pleistocene-Holocene transition (similar to 14.6-8 ka) in southwestern North America traditionally are linked to the activity of the North American Monsoon and to vegetation change related to a decrease in winter precipitation acting in response to orbital cyclicity. We performed an integrated analysis of regional alluvial fan, lacustrine and paleobotanical records for the area comparing them with hemispheric and regional paleoclimate proxies. Our focus was on the potential role the Tropical Pacific has as a synoptic pattern modulator and moisture source for hydro-geomorphic activity in the region. Our analysis indicates that the onset of alluvial fan aggradation in most of the region at similar to 13.5 ka could have been a response to semi-permanent El Nino-like conditions in the Tropical Pacific, which enhanced the frequency of winter frontal storms as well as increased penetration of tropical cyclones in the region. The North American Monsoon was restricted in extent and intensity until similar to 7 ka and probably was not a major factor in alluvial fan aggradation. A second stage of alluvial fan aggradation from 11.5 to similar to 9 ka was dominated by hyper-concentrated flows and sheet-flood sedimentation, along with deposition in fluvial settings. Storms were probably were linked to landfall of enhanced water vapor bands in the leading edge of winter extra-tropical cyclones with moisture advected directly from the Tropical Pacific. At similar to 8 ka, favorable conditions for the occurrence of these storms waned and storm tracks shifted northward. Analysis of modern analogs for storm types described above as prevalent during this period indicates that changes in circulation patterns across the Tropical Pacific can affect storm properties enough to explain the observed geomorphic effects, regardless of other factors traditionally considered of large impact like vegetation change. Our results suggest that the Tropical Pacific plays a larger role than currently thought in landscape evolution of the region. (C) 2013 Elsevier Ltd. All rights reserved.