New Mexico Geological Society Annual Spring Meeting — Abstracts

A long-lived middle Pleistocene lake formed in the Valle Grande when a post-caldera eruption (South Mountain rhyolite) dammed the drainage from the VG to San Diego Canyon. The deposits of this ancient lake were cored in May 2004 (GLAD 5 drilling project) and a total depth of 81 m of lacustrine mud and silts and gravels was recovered. The middle Pleistocene age of the core is constrained by an Ar-Ar date of 552 ± 3 kyr from a tephra at 75.8 m depth and a possible paleomagnetic field event corresponding to the Calabrian Ridge 2 event (~515 ± 3 kyr) at 17.25 m depth.

Initial analyses show considerable down-core variability in a variety of core properties. A major facies change at 27 m depth associated with lake shallowing correlates with changes in magnetic susceptibility and sediment density as well as a sharp increase in organic carbon, and a positive shift in carbon isotopes. The pollen spectra from sediments above 27 m indicate thermal maximum-like conditions. An age model constructed for core VC-3 shows this dramatic warming occurred at ~ 522 kyr, consistent with several published dates for glacial termination VI (OIS 14 to OIS 13). Cool, glacial conditions in the lower section of the core are punctuated by a warming event from ~536 to 532 kyr (49 to 42 m depth) and are followed by a return to glacial conditions from ~532 to 522 kyr. This pattern is reminiscent of the deglacial pattern of warming in the late Pleistocene.