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Center for Nonlinear Studies |
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 eruption (South Mountain rhyolite) 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 tentative chronology has been established for VC-3 with a basal tephra Ar-Ar date of 552 +/- 3 kyr, a correlation of glacial terminations V and VI in the core with other long Pleistocene records (SPECMAP) and the recognition of two geomagnetic field polarity events in the core (14a and 11a) which can be correlated with globally recognized events. This record spans a critical interval of the middle Pleistocene from MIS 14 (552 kyr B.P.) to MIS 10 (~350 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 two glacial/interglacial cycles in the core (MIS 14 to MIS 10). Glacial terminations V and VI and complete sections spanning interglacials MIS 13 and MIS 11 are captured at a high resolution. In the VC-3 record, both of these interglacials are relatively long compared with the intervening glacials (MIS 14 and MIS 12), and interglacial MIS 13 is significantly muted in amplitude compared with MIS 11. These features are similar to several other mid-Pleistocene records. Of particular interest is relatively large amplitude hydrologic variability 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, biogenic silica). 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 during this extended interglacial period. Similar periodicities are not found during the glacial periods, as the lake moisture source is dominated by westerly frontal systems and a southerly deflected polar jet stream.
Host: Todd Ringler 7-7744