PALEOLIMNOLOGY OF LAKE TEXCOCO, MEXICO. EVIDENCE FROM DIATOMS1

Abstract

A 46‐in core from the lacsutrine sediments beneath Mexico City was analyzed to establish a stratigraphic sequence of diatom assemblages for use in interpreting the climatic and limnologic history of ancient Lake Texcoco.Diatoms were found in nearly every 20‐cm sample interval, and several major zones were established. Planktonic and benthonic‐epiphytic assemblages alternate throughout the core, both of fresh‐ and brackish‐water types. The alternations reflect the fact that the coring site is marginal to the main basin of the lake, and limnologic conditions change as water levels rise and fall.A freshwater planktonic assemblage dominated by Stephanodiscus niagarae in deeper parts of the core indicates that a large, cool, and possibly deep lake existed about 100,000 years ago, either because of pluvial or because of tectonic factors. This is replaced (depth 35 to 30 m) by a freshwater benthonic‐epiphytic assemblage characterized by Denticula elegans and other marsh diatoms. The marshes were probably maintained by springs from the shore when the lake was reduced to saline pools in the center of the basin.As water levels rose again (core depths 30 to 5 m), brackish water flooded the marshes, and brackish benthonic diatoms (such as Anomoeoneis costata, Campylodiscus clypeus, and Nitzschia frustulum) replaced the earlier floras. These were periodically replaced by brackish planktonic diatoms (as Cyclotella striata and Cyclotella quillensis) when the lake was deeper, but the earlier freshwater planktonic flora never recurred. The same brackish planktonic and benthonic diatoms that prevailed for several tens of thousands of years are found today confined to the brackish pools of Lake Texcoco that are remnants of the former larger lake. The long interval of fluctuating brackish floras probably represents Wisconsin time.The last 10,000 years of the lake’s history is marked by a return of the marsh flora, suggesting a climate drier than that of Wisconsin time. A marked climatic change, however, is not necessary to explain this last change in the diatom flora, and it seems likely that the pluvial climate inferred for the southwestern United States had less effect at the latitude of Mexico City (19° 30′) than farther north.