Short-term climatic change in lake sediments from lake Alchichica, Oriental, Mexico

Abstract

Reduced precipitation in central Mexico is related to ENSO events. These short-term climatic fluctuations can modify mixing-stratification patterns of lake systems, reflected in the sediments deposited at the bottom year after year. Lake sediments can also record longer climatic trends. We present preliminary results of an evaluation of the potential of Lake Alchichica to record short-term fluctuations as well as middle-term climatic variability. Diatom, loss on ignition (LOI) and total pigment analyses were carried out on a 168 cm core from the NE shore of Alchichica. Two 14-C dates on bulk sediment give an approximate chronological frame for the sequence. Total pigment content was a poor palaeoenvironmental proxy in this lake, as pigments were undetected below 56 cm, suggesting that concentration depends on degradation processes. Diatom preservation in the sediments was poor and restricted to the upper 44 cm. High pH and alkalinity in Alchichica favor diatom dissolution, limiting the potential of this lake for palaeolimnologic studies. Diatom assemblages in the upper 44 cm of the sequence suggest that the lake has remained an alkaline-subsaline system during the recent past, probably over the past 300 yr., with a very recent decrease of lake level. There is no record of short-term climatic events such as ENSO. Sediment stratigraphy and LOI data, however, suggest a past event of unknown age (>300 yr. ?), with lower than present lake levels and higher alkalinity.