EVIDENCE OF SUB-STAGE CLIMATIC SHIFTS DURING MIS 11 REFINED FROM DIATOM ASSEMBLAGE RECONSTRUCTION IN THE VALLES CALDERA, NEW MEXICO

Samples from the Valles Caldera sediment core (VC-3) are being analyzed for diatoms to infer lake level change across MIS 11 (Marine Isotope Stage) through assemblage reconstruction. Comparison against previously published pollen, isotopic, and MAT (Mean Annual Temperature) data from Fawcett et al. (2011) will help refine our understanding of the climatic changes on the sub-orbital timescale in MIS 11. MIS 11 was the longest and warmest interglacial period of the past 500 kyr that occurred between ~426 ka and ~370 ka and is widely considered an analog for the Holocene and future climatic variance. Most records from MIS 11 are either marine or ice-core based, with comparatively few terrestrial records, making VC-3 unique among lacustrine sediment records spanning the middle Pleistocene. VC-3 captures climatic responses to insolation variations at a higher resolution and allowed Fawcett et al. (2011) to distinguish five sub-stage events. Other records of a similar caliber, such as the one from Lake Baikal in Siberia, only note three sub-stage events in MIS 11. The five substages - two cool and three warm - were identified from MAT estimates based on variations in pollen taxa and aquatic productivity proxies that were correlated with the three processional cycles within MIS 11. These climatic variations that occur throughout MIS 11 should have measurable impacts on the diatom community assemblages present, with secession from cold to warm / warm to cold taxa tracking rapid shifts in water conditions and by extrapolation, climate. Preliminary analysis of the samples corroborates this hypothesis with assemblage shifts from sections where benthic genera – such as Pseudostaurosira, Epithemia, and Navicula – dominate to sections where planktonic taxa – such as Lindavia, Aulacoseira and Ellerbeckia – dominate. Future work will quantify the relative abundance of different species for the purpose of reconstructing factors associated with the sub-stage events such as lake level change, open vs. closed basin conditions, eutrophication events, and precipitation source (winter snowpack vs. summer monsoon).