DIATOM EVIDENCE FOR A GROUNDWATER DIVIDE THAT LIMITED THE EXTENT OF LAKE ESTANCIA, NEW MEXICO HIGHSTANDS DURING THE LAST GLACIAL MAXIMUM

A high-resolution time series of diatoms in lacustrine sediments from the Estancia Basin of central New Mexico reveals decadal to millennial changes in water chemistry during the Last Glacial Maximum (LGM) and suggests that groundwater leakage limited the size of pluvial Lake Estancia highstands. Work by Allen and Anderson (2000) showed that the lake expanded from 400-1100 km² surface area and 5 to 45 m water depth within a matter of decades at least five times during the LGM, with each highstand reaching the same elevation. Numerical modeling experiments (Menking et al., 2004) showed that lowstand to highstand transitions required increases in precipitation of 1.5-2 times modern but also demonstrated that these conditions could not have lasted for more than a few decades or the lake would have exceeded the highstand shoreline and overflowed into an adjacent basin. The fact that the lake returned to the same elevation suggested that successive highstands were climatologically similar, and high-resolution work on the LGM stratigraphic section conducted by Menking (2015) showed that solar cycles were a likely driver of brief pulses of moisture.

Here we present an alternative scenario, that once the lake reached a particular threshold volume, groundwater leakage prevented further expansion, and we show that the highstands previously identified by Allen and Anderson (2000) were different from one another ecologically, from which we infer that climate may have been more variable than we previously thought. The LGM diatom sequence is dominated by freshwater planktic species of the Cyclotella ocellata complex, the freshwater to brackish littoral species Pseudostaurosira brevistriata, and the brackish to saline species Diploneis smithii, all of which show decadal to centennial oscillations similar to those revealed by previously described mineralogical time series (Menking, 2015). Also notable is a high abundance of the very salt intolerant Lindavia affinis at the base of the LGM section. Canonical Correspondence Analysis using the mineralogical time series as proxies for wetter and drier conditions shows that diatom assemblages varied with salinity and pH/alkalinity, among other factors, and suggests that groundwater outflow led to variable water chemistry for different highstands of the same magnitude. Juxtaposition of freshwater planktic diatoms (L. affinis) with highly saline benthic species (Campylodiscus clypeus) in the same samples also reveals periods in which the lake was strongly chemically stratified.