Lake Levels since about 40,000 Years Ago at Lake Chalco, near Mexico City

Abstract

Diatoms, magnetic susceptibility, organic content, and14C ages of sediments from a 26-m core suggest that Lake Chalco, in the southern part of the basin of Mexico, went through a series of major fluctuations during the late Pleistocene and the Holocene. Before ca. 39,00014C yr B.P. the lake was very deep (about 8–10 m), alkaline, and saline. It then became shallow (<2 m) for most of the time between ca. 39,000 and 22,500 yr B.P. Chalco deepened to about 4–5 m about the time of a major eruption of nearby Popocatepetl volcano ca. 22,000 yr B.P. The lake remained relatively deep and fresh until ca. 18,500 yr B.P., when lower levels and alternating acidic to freshwater conditions were established. After 14,500 yr B.P. lake level rose slightly, but by ca. 10,000 yr B.P. Chalco became very shallow (<2 m), remaining as a low, alkaline saline marsh until ca. 6000 yr B.P. This period corresponds with the Playa cultural phase, during which the earliest human settlements in the basin were established. After ca. 6000 yr B.P. Chalco became a fresh to slightly alkaline shallow lake a few meters deep.

Orbital pacing and ocean circulation-induced collapses of the Mesoamerican monsoon over the past 22,000 y

“…A synthesis of Mesoamerican climate records (Fig. 3), including a well-dated central Mexico glacial chronology (34) and lacustrine records from the Yucatan Peninsula (19,35) and Central Mexico (36)(37)(38), shows that paleoclimate varied coherently since the LGM (all records are shown on a calendar age scale; see SI Text for methods). Our observation of an active LGM monsoon is supported by wet conditions inferred from high magnetic susceptibility in Lake Petén Itzá sediments (19), during which time pollen-inferred LSTs were lowered by ∼4°C (35).…”

Scientific Drilling

“…Previous studies at this location ( Fig. 2) have exploited geochemical, geological, and ecological proxy records (Bradbury, 1989;Lozano-Garcia et al, 1993;LozanoGarcia and Ortega-Guerrero, 1994;Urrutia-Fucugauchi et al, 1994;Caballero-Miranda, 1997;LozanoGarcia and Xelhuantzi-Lopez, 1997;Caballero and Ortega Guerrero, 1998). This body of research demonstrates that Chalco sediments have the potential to provide unique knowledge of interannual through orbital-scale variations in the North American monsoon and the hydrological balance of the neotropics.…”

Tepexpan Palaeoindian site, Basin of Mexico: multi-proxy evidence for environmental change during the late Pleistocene–late Holocene

“…An alternative explanation for enhanced periods of river inflow is increases in meltwater flow from mountain glaciers in the basin. Lozano-García et al (1993) postulate that Popocatépetl volcanism may have increased meltwater runoff to Lake Chalco in the LGM, however this was most prevalent from 22.5-19 ka BP (Bradbury, 1989;Caballero and Ortega-Guerrero, 1998). Fluctuations between acid and neutral water at Chalco between 19 ka and 15 ka BP may also have been due to volcanism affecting the mountain ice fields above the lake, or the direct consequence of ashfall on lake chemistry (Caballero, 1997).…”

“…There is increasing evidence for dry, possibly very dry, conditions in the basin at this time, but local conditions are variable with arguments both for a relatively moist period (15-10 ka BP) (Lozano-García et al, 1993;Caballero and OrtegaGuerrero, 1998) and increased aridity (Bradbury, 1989;Caballero et al 1999), with Lake Texcoco becoming saline and the marsh areas expanding. In the south of the basin increased freshwater input from the catchment may have been caused by reduced catchment vegetation due to the intense volcanism (Caballero and Ortega-Guerrero, 1998). The combined results of drying and volcanism have been used to explain why many records suggest a hiatus in sedimentation around Texcoco from ca.14 500-6000 BP (Bradbury, 1989;Lozano-García and Ortega-Guerrero, 1998;Sedov et al, 2001).…”

“…Lacustrine studies in the basin have focussed predominantly on Lake Chalco to the south of Texcoco (Sears, 1952;Bradbury, 1971;Watts and Bradbury, 1982;González Quintero, 1986;Lozano-García et al 1993;Lozano-García and Ortega-Guerrero, 1994;Caballero, 1997;Caballero and Ortega-Guerrero, 1998;Ortega-Guerrero et al, 2000) and Tecocomulco in the north of the Basin (Caballero et al, 1999). There has also been a pollen study of a core from the centre of Lake Texcoco (Lozano-García and Ortega-Guerrero, 1998) and diatom and pollen work on sections from across the basin, including several sites around Texcoco (see Bradbury, 1971;.…”