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