Two middle Pleistocene glacial-interglacial cycles from the Valle Grande, Jemez Mountains, northern New Mexico

Abstract

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 were recovered. A tentative chronology has been established for VC-3 with a basal tephra Ar-Ar date of 552 +/- 3 ka, a correlation of major climatic changes in the core with other long Pleistocene records (deep sea oxygen isotope records and long Antarctic ice core records), and the recognition of two geomagnetic field polarity events in the core which can be correlated with globally recognized events. This record spans a critical interval of the middle Pleistocene from MIS 14 (552 ka) to MIS 10 (~360 ka), 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 isotope ratios, C/N ratios, and pollen content reveal two glacial/interglacial cycles in the core (MIS 14 to MIS 10). This record includes glacial terminations V and VI and complete sections spanning interglacials MIS 13 and MIS 11. 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. The glacial terminations are quite abrupt in this record with notable changes in sedimentation, organic carbon content, C/N ratios and watershed vegetation type. Termination V is the largest climate change evident in this part of the middle Pleistocene. The glacial inceptions tend to be more gradual, on the order of a few thousand years.