Quaternary palaeolimnology of a Mexican crater lake
La Piscina de Yuriria is a small (0.75 Km2), hydrologically closed crater lake situated in the Neovolcanic Axis of Central Mexico (20013'N, 100008'W, 1740m a.s.l.). The water level has fluctuated markedly over the last decade and, at present, has a chemical composition of Na-C03-CI type. It represents a highly evolved water body modified by evaporative concentration and dissolution of the salt crust produced during desiccation in 1987. Water reappeared in the basin during 1991 and was very shallow and anoxic. The lake basin is known to have been occupied by man since -3,100 yr BP and so provides an excellent opportunity to examine the nature and relative importance of anthropogenic disturbance and naturally induced climatic change. The record for a single 14.3m core (YC2) from La Piscina de Yuriria extends back -26,600 yr BP with the dating control provided by 16 radiocarbon dates. The project used a variety of techniques to reconstruct the palaeolimnology of the lake including: (i) ostracods; faunal assemblage analyses; trace-element and stable oxygen and carbon isotope analyses; (ii) Sedimentological analyses; mineralogical analyses; grain-size analyses; X-Ray diffraction; stable oxygen and carbonisotope analyses of fine-grained carbonate; Laser ablation lCP-MS; (iii) Sediment geochemistry. A number of moist and dry phases have been identified, through the core, which are controlled primarily by variations in precipitation/evaporation (PIE) ratios over La Piscina de Yuriria. A number of periods of catchment disturbance have also been identified which occured under both moist and dry conditions. The base of the core (-26,600 yr BP) reflects the most arid phase with stable conditions within the catchment. At 25,000 yr BP freshwater flowed into a salt filled basin with a second phase of moist conditions at 22,200 yr BP. The onset of drier conditions at 20,000 yr BP resulted in a pronounced phase of catchment erosion. The dry period continued and intensified between 19,000 and 14,700 yr BP. During this period two wet events were identified at 17,000 and 15,000 yr BP. Dry conditions resumed between -12,100 and 11,500 yr BP and were then replaced by moist conditions. Between -12,000 and -7,000 yr BP conditions appeared to be generally unstable, alternating between dry and wet phases. Between 8,000 and -3,000 yr BP conditions were more moist. A short-lived dry event occurred at -3,300 yr BP, which resulted in renewed catchment disturbance. Dry conditions were identified between-2,500 yr BP and -900 yr BP where a short-lived wet event produced: stable catchment conditions. Catchment disturbance resumed and continued toward the top of the core reflecting slightly drier conditions than during the moist event. The record is dominated by a strong climatic signal which does not appear to have been masked by anthropogenic effects during the last 3,100 yr BP. Comparison of the events identified in the core, with other sites within the Neotropics, suggests that many of the climatic changes occurred on a regional scale.