Non-marine ostracods as indicators of Holocene environmental change in central Mexico
A late Holocene palaeolimnological record for Central Mexico has been obtained from Lake Pátzcuaro, using recent and fossil ostracods. Lake Pátzcuaro (19[degrees]35' N-19[degrees]45t N,lOI °32' W-lOl [degrees]43' W) is a closed-basin lake which responds rapidly to changes in the ratio of precipitation/evaporation in the region. The waters of Lake Pátzcuaro are of Na-HC03 type and are currently close to the branchpoint for calcite precipitation. The records from four lake-sediment cores, dated by AMS radiocarbon methods, cover the last 3,650 years. Work on the sediment cores involved ostracod faunal palaeoecology coupled with analysis of the trace-element (Mg/Ca and Sr/Ca) and stable-isotope (180/160 and 13C/12C) composition of ostracod valves. The faunal distribution is determined by the presence or absence of aquatic vegetation and, to a lesser extent, salinity. The 180/160 and 13C/12C ratios in ostracod calcite show good agreement with palaeolimnological inferences from the faunal assemblages, primarily recording changing precipitation/evaporation and primary productivity levels, respectively. The trace-element chemistry of ostracod valves shows poor agreement with the faunal and isotope records and appears to be dependent on a number of complex factors, such as the changing solute composition of surface runoff, which is, in tum, dependent on the degree of weathering of the regolith through which the water has passed. This study shows that wetter conditions existed in Central Mexico between approximately 3,650-2,300, 1,300-800 and 500-300 yr BP, characterised by fluctuating lake levels. Catchment stability was associated with the onset of these wetter conditions, which ultimately increased primary productivity in the lake, owing to reduced turbidity. Each wet phase was separated by a prolonged dry period, during which time, the lake level was relatively stable. The good agreement between the findings of this study and those elsewhere within the Neotropics suggests that these abrupt climate changes occurred on at least a regional scale.