A late Holocene palaeolimnological record from central Mexico, based on faunal and stable-isotope analysis of ostracod shells

A late Holocene palaeolimnological record for central Mexico has been obtained from Lake Patzcuaro, using recent and fossil ostracods. Lake Patzcuaro, Michoacan, is a closed-basin lake which responds rapidly to changes in the ratio of precipitation/evaporation in the region. The record from a single lake-sediment core, dated by AMS radiocarbon method, covers the last similar to 3,530 yrs, and is based on ostracod faunal palaeoecology coupled with analysis of the stable-isotope ((18)O/(16)O and (13)C/(12)C) composition of ostracod valves. The faunal distribution is determined by the presence or absence of aquatic vegetation and, to a lesser extent, salinity. The (18)O/(16)O and (13)C/(12)C ratios in ostracod calcite show good agreement with palaeolimnological inferences from the faunal assemblages, principally recording changing precipitation/evaporation and primary-productivity levels, respectively. Wetter conditions existed in central Mexico between approximately similar to 3,600 and similar to 2,390 yr BP, between similar to 1,330 to similar to 1,120 yr BP, and from similar to 220 yr BP to present, characterised by fluctuating lake levels. A dilution of the sediment load in the lake reduced turbidity levels allowing for a marked increase in productivity. During these phases, the combination of a deeper lake and increased macrophyte cover reduced the degree of mixing of the waterbody. In the earliest of these phases there was sufficient stratification of the waterbody for methanogenesis to occur in the sediment interstices. The wet phases were separated by prolonged dry periods, during which time the climatic conditions were relatively stable. Good agreement was found between the findings of this study and others from the central Mexican/Caribbean region suggesting that abrupt climate changes occurred at least at a regional scale.