Abstract

ORTEGA-GUERRERO, Beatriz et al. Paleoambiental reconstruction of the Xochimilco sub-basin, central Mexico, between 18000 and 5000 years cal before present. Rev. mex. cienc. geol [online]. 2018, vol.35, n.3, pp.254-267. ISSN 2007-2902.  https://doi.org/10.22201/cgeo.20072902e.2018.3.779.

We present a record of millennial-scale paleoenvironmental variations from a site in Xochimilco basin, central Mexico, between 18000 and 5000 years cal BP, using the magnetic mineralogy, geochemical elements content and diatom assemblages. The analyzed sedimentary sequence is chronologically constrained by the recognition of well-known tephra layers widely used as stratigraphical markers in the region: the Tutti Frutti Pumice from Popocatépetl stratovolcano (17070 years cal BP), and the Upper Toluca Pumice (12320 years cal BP) from the Nevado de Toluca, as well as a 14C date from an ostracod rich horizon. The temporal variations recorded in magnetic susceptibility (χ), Fe oxides and oxyhydroxides (hematite and goethite), ultra-fine magnetic minerals, titanium, organic and inorganic carbon, and the iron/titanium, calcium/titanium, organic carbon/nitrogen, silicon/titanium ratios and diatom content indicate fluctuations in runoff, origin of magnetic minerals, salinity, productivity and sources of organic matter. These variations respond to millennial-scale climatic oscillations. The deglacial period (18000-11700 years cal BP) was characterized by intense volcanic activity, and the dominance of clastic sedimentation in a fresh water shallow lake. The Heinrich 1 (cold), Bølling/Allerød (warm) and Younger Dryas (cold) events, were recorded in the runoff, organic matter content and carbonate precipitation proxies. The driest conditions in all of the sequence were recorded during early Holocene (11700-8200 years cal BP). The return to more humid conditions and the establishing of a fresh water lake during the late mid Holocene (8200-5000 years cal BP), could promote the settling of communities on the lake shore.

Keywords : millennial climatic oscillations; basin of México; central Mexico; Holocene; paleoenvironments; magnetic properties.