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 Marine Isotope Stage (MIS 11) through assemblage reconstruction. 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 climate regimes. 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: two cool and three warm. Other records of a similar caliber, such as the one from Lake Baikal in Siberia, only note three sub-stage events in MIS 11. Preliminary analysis of the diatom community assemblages shows rapid shifts between benthic (bottom-dwelling) and planktonic (water column) taxa, indicating that the diatoms responded to insolation variations at the same resolution as the previously published proxy data from Fawcett et al. (2011). Samples from sediment core sections 14H-1 and 8H-2 show dominance of cold-water taxa – such as Ellerbeckia Teres and Pinnularia Borealis – which corroborates the interpretation that these sections are a part of interglacial periods MIS 12 and MIS 10, respectively. The sections within MIS 11 show rapid shifts between benthic genera – such as Pseudostaurosira, Sellaphora, and Planothidium, and planktonic taxa – such as Fragilaria, Aulacoseira and Stephanodiscus. Planktonic species abundance during the transition between glacial MIS 12 and interglacial MIS 11 in sections 13H-2 and 13H-1 is interpreted to mean the formation of a deep lake immediately after the transition as the region warms. On-going work will continue to quantify the relative abundance of different species for the purpose of associating assemblage changes to lake level change, eutrophication events, open vs. closed basin conditions, and precipitation source.