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

Abstract

The Tepexpan Palaeoindian skeleton was discovered in 1947 close to the former Lake Texcoco margin, in the Basin of Mexico. The find has been the object of considerable interest and discussion over the last 60 years regarding its real age and archaeological interpretation.Here we report new AMS radiocarbon dates associated with the sedimentary sequence at Tepexpan with ages between 19 110 ± 90 and 612 ± 22 14 C years BP and a new uraniumseries date for the skeleton with an age of 4700 ± 200 years BP that indicates a Middle Holocene age. The sedimentary succession was studied in detail using: stable isotopes, diatoms, organic geochemistry and tephrochronology. The multi-proxy evidence suggests large changes around the margins of Lake Texcoco in terms of the balance between aquatic 2 and terrestrial plants, C 3 and C 4 plants, saline, alkaline and freshwater conditions, volcanic activity, marginal reworking of lake sediments and input from the drainage basin through the Late Pleistocene-Late Holocene. These changes had large impacts on the prehistoric human populations living by the lake shores since the Late Pleistocene in the Basin of Mexico.

Long-term Holocene groundwater fluctuations in a chalk catchment: evidence from Rock-Eval pyrolysis of riparian peats

“…Similarly, the oxygen index (OI) calculated using the formula S3 × 100 / TOC provides an estimate of the amount of oxygen containing compounds, a parameter that increases upon aerobic biological decay (Slowakiewicz et al , ), whereas thermal maxima (Tmax) values provide a means to estimate changes in organic matter type input because the maximum temperature at which bound hydrocarbons are cracked is related to the proportions of biopolymers (cellulose, xylans and phenolics such as lignin) and bonding between them (Marchand et al , ; Carrie et al , ). Normal alkanes ( n ‐alkanes) are a subclass of wax lipid widely used to provenance organic matter in modern and lake and river sediments (Meyers, ; Lamb et al , ). Aquatic algae and plankton synthesize mainly odd numbered short‐chain homologues (C 13 –C 25 ) maximizing at about C 15 or C 17 .…”

Paleoindian sites from the Basin of Mexico: Evidence from stratigraphy, tephrochronology and dating

“…There appears to be no in situ PWA in this trench as originally described by Mooser and Gonzalez-Rul (1961), See Figure 12, but without detailed tephra geochemistry and radiocarbon dating , which was not available at the time, the most obvious feature in the layer would have been the presence of pumices. The lack of in situ PWA tephra has also been noted at the Tepexpan (Lamb et al 2009) and Tocuila sites (Gonzalez et al 2014), in similar marginal lake locations. The youngest tephra (Sample Santa 16) is a 7cm dark grey/black basaltic ash with abundant ostracods, dominated by tephra with a mean SiO 2 composition of 58%, although there a few shards of rhyolitic composition.…”

“…In the lowest sample A0 there are again two tephra populations, one with a mean of 61% Si0 2 (PWA) and the other with a rhyolitic composition (mean of 73% Si0 2, ) which is considered to be reworked from a tephra from Tlaloc volcano. There is similar reworking of tephra populations from Lake Texcoco marginal areas in the Tepexpan Paleoindian and Tocuila Mammoths sites (Gonzalez and Huddart, 2007;Lamb et al, 2009;Gonzalez et al, 2014) . The 10cm unit of sub-angular andesite gravels in an ash matrix is interpreted as reworked hillside scree, incorporated with ash by hillslope processes, such as insolation weathering and then slopewash, or solifluction and is not a beach as interpreted by Lorenzo and Mirambell (1986a).…”

“…However for the PWA ash there is also evidence for multiple phases of deposition and considerable thicknesses in the basin because there is extensive reworking and redeposition of this ash on the hill slopes and into the margins of the Texcoco and Chalco Lakes. Thus, great care is required when interpreting volcanic ashes in marginal lake sites, where only by using detailed geochemistry studies of single tephra grains, detailed dating and good vertical and horizontal exposure of the stratigraphic layers is it possible to identify those tephras which are truly in situ, or reworked (Lamb et al 2009;Gonzalez et al 2014).…”

“…Climatic change in the last 50,000 ka has driven large fluctuations in vegetation and lacustrine environments in the basin (e.g. Lamb et al, 2009). …”