CONTINUOUS HOLOCENE RECORD OF DIATOM STRATIGRAPHY, PALEOHYDROLOGY, AND ANTHROPOGENIC ACTIVITY IN A SPRING-MOUND IN SOUTHWESTERN UNITED-STATES

This study presents the first continuous record of fossil diatoms taken from an open spring-mound in southwestern United States. Diatoms were analyzed from a radiocarbon-dated core taken from Montezuma Well, a near thermally constant spring in north-central Arizona. Fluctuations in total diatom density, oscillations in the relative abundance of Anomoeoneis sphaerophora, and intermittent deposition of calcite suggest that water levels in Montezuma Well underwent dramatic fluctuations to the degree of being intermittently dry, or at least very shallow, during the middle Holocene (approximately 8000-5000 yr B.P.). The fluctuations in water level probably correspond to oscillations in regional temperature and precipitation, which regulate hydrologic input and evaporation rates. The dramatic fluctuations in water level during the middle Holocene suggest that the endemic biota of Montezuma Well underwent relatively rapid speciation within the past approximately 5000 yr. The appearance of endemic species (Gomphonema montezumense and Cyclotella pseudostelligera f. parva) at approximately 5000-3000 vr B.P. supports this hypothesis. Diatom indicators for organic enrichment (Aulacoseira granulata and A. islandica) closely coincide with the prehistoric native occupation of Montezuma Well. (C) 1994 University of Washington.