A LONG MIDDLE PLEISTOCENE CLIMATE RECORD (MIS 14 TO 9) FROM LACUSTRINE SEDIMENTS IN THE VALLES CALDERA, NEW MEXICO

A long-lived middle Pleistocene lake formed in the Valle Grande, a large moat valley of the Valles Caldera in northern New Mexico, when a post-caldera rhyolite dome eruption dammed the drainage out of the caldera. The deposits of this lake were cored in May 2004 (GLAD5 project, hole VC-3) and 81 m of mostly lacustrine silty mud was recovered. A chronology has been established for VC-3 with a basal tephra Ar-Ar date of 552 +/- 3 kyr, and a detailed orbitally tuned record of organic carbon and other climate sensitive proxies of climate in the record. In this orbitally tuned record, glacial terminations V and VI correlate in time with other long Pleistocene records (marine SPECMAP, Lake Baikal) and two geomagnetic field polarity events in the core (14α and 11α) correlate with globally recognized events. This record spans a critical interval of the middle Pleistocene from MIS 14 (552 kyr B.P.) to MIS 9 (~310 kyr B.P.), at which time the lacustrine sediments filled the available accommodation space in the caldera moat. Multiple analyses including core sedimentology and stratigraphy, sediment density and rock magnetic properties, organic carbon content and carbon isotopic ratios, C/N ratios, and pollen content reveal at least two glacial/interglacial cycles in the core (MIS 14 to MIS 9). Glacial terminations V and VI and interglacials MIS 13 and MIS 11 are captured at a high resolution. Both of these interglacials are relatively long compared with the intervening glacials, and interglacial MIS 13 is significantly muted in amplitude compared with MIS 11. Large amplitude hydrologic variability is evident in the interglacial MIS 11 section. Here, prominent wet-dry cycles with a ~11 ka duration are shown by correlative changes in sedimentology (laminated vs. mudcracked horizons) and in lake productivity (organic carbon). We hypothesize that this variability arises from a split-precessional cycle that modulates the strength of the southwest North American summer monsoon and hence summer rainfall amounts and lake levels during this extended interglacial period.