Sta. Cruz Atizapán: a 22-ka lake level record and climatic implications for the late Holocene human occupation in the Upper Lerma Basin, Central Mexico

Abstract

The Upper Lerma is a high altitude basin with three water bodies linked by the Lerma River. This basin has a long archaeological history, characterised by the establishment of settlements within the lacustrine ecosystem itself (man-made islands) during the late Classic to Epiclassic (AD 550–900), which were abandoned by the end of the Epiclassic. The Upper Lerma is an ideal site to study climatic and environmental conditions during the period of human occupation, as well as during the last full-glacial/interglacial cycle. Two sediment cores (STCRZ: 9.54 m and Almoloya del Rı́o: 5.12 m) were recovered from the highest lake in the system (Chignahuapan). Ten radiocarbon dates provide chronologies for these sequences in which the Tres Cruces Tephra (c. 8500 yr BP) and the Upper Toluca Pumice (c. 11 600 yr BP) serve as stratigraphic markers. Magnetic properties, loss on ignition, and diatom analyses were used to infer lake level fluctuations during the last c. 22 000 yr BP. The Late Pleistocene environment was characterised by a freshwater lake. High sediment input and variable lake levels are recorded during the Last Glacial Maximum (c. 19 000–16 000 yr BP), while slightly higher water levels and reduced sediment input are recorded during the Late Glacial (c. 16 000–11 000 yr BP). A short episode of shallow conditions is inferred by c. 12 400 yr BP. Holocene lake levels were generally shallower, and three episodes of very shallow, slightly alkaline waters are identified. The first dates to the early Holocene (c. 11 000–7000 yr BP). The second is centred at c. 4600/4500 yr BP. The third occurred between c. 2000 (?) and 800 yr BP (c. 200 BC–AD 1100, calibrated ages) with very shallow water after c. 1400 yr BP (AD 550, calibrated age). Lake level increased after c. 800 yr BP. These three shallow water events are also recorded at other sites in Central Mexico indicating regional climatic trends rather than local events. A deeper water phase occurred between 7000 and 6400/6200 yr BP. The last shallow water phase correlates with the Classic and Epiclassic periods (AD 200–900), and shallowest conditions occurred in the late Classic to Epiclassic (c. AD 550–900), when the construction of man-made islands reached a peak. An increase in lake level after c. 800 yr BP (AD 1100 calibrated age) may have led to the abandonment of this life strategy.