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

Diatom analysis from the Valles Caldera sediment core (VC-3) are being analyzed to infer climate and 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 has five distinguishable 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. 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. Additionally, species with relative abundances greater than 10% within each sample are being analyzed for their autecological preferences. A sample from sediment core section 11H-2 – which roughly dates to ~400 ka – is dominated by species that prefer shallow, eutrophic conditions, supporting the interpretation that a glade environment occupied the caldera during this core section. Similarly, 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 also show rapid shifts in the species compositions of planktonic and benthic genera in each sample – with species such as Lindavia intermedia and Stephanodiscus niagarae dominating samples the other is absent in. Planktonic species abundance during the transition between glacial MIS 12 and interglacial MIS 11 is interpreted to mean the deepening of the lake 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 fluctuations in lake level, water conditions, and atmospheric climate.