Quaternary climate inferences for the southernmost Rocky Mountains from cosmogenic dating and glacier modeling at Costilla Massif, New Mexico.

Despite a significant body of work utilizing terrestrial cosmogenic nuclide dating to examine the late Quaternary glacial history of the mid to northern Rocky Mountains, we lack an understanding of alpine glacial and ice cap responses to climate change in the southern Rocky Mountains (SRM) and the Southwestern United States (SWUS). While limited work has examined the timing of glacial advance and retreat in the SRM of New Mexico using relative age dating techniques, only one study to date has examined last glacial maximum glacier extents in a mountain valley in the southern Sangre de Cristo mountains (SdCm). The lack of age control for the southernmost Rocky Mountain glacial sequences limits our understanding of the timing for SWUS glacial retreat in response to late Quaternary warming periods. Additionally, while limited work has suggested an absence of Holocene glaciation in valley glacier systems at the far southern extreme of the SdCm near Santa Fe, New Mexico, the southernmost limit to Holocene glaciation within the United States remains uncertain.

Here, we develop preliminary moraine chronology, model glacier and ice cap extents, and produce a paleoclimate record throughout the late Quaternary at Costilla Massif in the SdCm of New Mexico. We aim to use the glacial and paleoclimatic records to examine variations in climate between Costilla Massif and other glaciated regions of the Rocky Mountains and test the hypothesis that latest Pleistocene and Holocene glaciation occurred in the SRM. We use 10Be terrestrial cosmogenic nuclide exposure dating of quartz monzonite boulders to develop a glacial chronology for six moraines in two glaciated valleys at Costilla Massif. We use the updated glacial energy/mass balance of Plummer and Phillips to (1) model the extent of valley glaciation and (2) determine paleoclimatic deviations from modern conditions required to sustain glaciers at each moraine position. We compare ice cap extents at Costilla Massif with similar small ice caps throughout the southern Sdm to determine changes in extent related to fluctuations in local and regional climate. We then compare our moraine-derived paleoclimate record with similar records elsewhere in the Rocky Mountains and other climatic proxy records throughout the SWUS and SRM regions to provide analysis of warming trends during the Late Quaternary.

Preliminary soil relative age dating techniques indicate glacial landforms at Costilla Massif range in age from MIS6 (~195 – 123 ka) to the Holocene. Given their limited extent and relative lack of soil development, we hypothesize that the youngest cirque glaciers at Costilla Massif are of Holocene age. Additionally, we predict the Costilla Ice Cap persisted into the Holocene. We predict that valley glaciers at Costilla massif began to retreat earlier than occurred in the mid- to northern Rocky Mountains at similar rates to elsewhere in the SdCm. However, the presence of a locally extensive ice cap and local variations in topography and precipitation and temperature regime compared to elsewhere in the SdCm permitted stabilization of cirque glaciation during the early Holocene in contrast to previous studies suggesting a deglaciation of the SdCm by about 15 ka.