Environmental conditions at the end of the Isotopic Stage 6 (EI 6:> 130000 years) in the center of Mexico: characterization of a section of laminated sediments from Lago de Chalco
Annually laminated sediments (varves) frequently form in lakes with contrasting seasonal environments, and are better preserved in deep water bodies, with low oxygen content at the water/sediment interface. We characterized deposits in a laminated section within the sedimentary record from Lake Chalco, Basin of Mexico. The laminated sediments are intercalated with massive sediments, volcaniclastic material and 13 micritic mud layers. This laminated sequence corresponds to the depth interval 122.5 to 106 m and has an age >130 ka, 4 corresponding with the end of the penultimate glacial stage or Marine Isotopic Stage 6 (MIS 6). Dark and light lamina, as well as the micritic mud components, were identified by microscopic observation of sediment smear slides; some sediment in the dark laminae, light laminae and micritic mud layers was also analyzed using a scanning electron microscope. X-ray fluorescence was used to determine Ti content and Si/Ti and Ca/Ti ratios. These analyses enabled us to determine that the light laminae have high Si/Ti values, are formed mostly by diatom valves (Stephanodiscus niagarae and S. oregonicus), and represent a biogenic diatom ooze. Dark laminae have high Ti values and are formed mainly by clastic sediments including plagioclases and amphiboles. We propose an annual depositional model in which light laminae correspond to winter diatom blooms, whereas dark laminae were deposited by allochthonous inwash during the summer rainy season, thus forming a biogenic-clastic varve. Diatom ecology suggests the laminated sequence corresponds to a relatively deep, eutrophic to hypertrophic lake with cool to cold waters. This inference is consistent with the age of the deposits, which corresponds with the penultimate glacial maximum or MIS 6, for which we infer cooler and moistier conditions than for the last glacial maximum i.e. MIS 2. Sedimentation rates estimated by varve counts suggest that the laminated sequence was deposited during a ~5,000 yr interval. The micritic mud layers have high Ca/Ti and Si/Ti ratios and are formed mainly by authigenic carbonates, sponge spicules, diatoms and ostracode valves. We propose that these chemical facies were deposited during periods of higher evaporation and water concentration and also lower lake level and high biological productivity. The recurrence of these events must be further studied, but based on the mean sedimentation rate of 2.9 mm/yr, calculated from varve counting we suggest that they have a 140-yr to 289-yr recurrence interval.