Data from the Lake Chichancanab core supports the following interpretation that begins at the base of the core. From 9200 to 7800 years BP, there was no lake at the coring site as indicated by
the absence of aquatic microfossils and the presence of land snails. Beginning at about 7800 years BP, the lake began to fill but the salinity was much higher than today. Evidence for this includes high sulfur content indicating gypsum precipitation,
very high 18O and 16O ratios in both ostracods and gastropods, and the occurrence of a benthic foraminifer, Ammonia beccarri. Foraminifers are almost exclusively marine forms but this species can tolerate a wide range of salinity (7 to 67 parts per
thousand); however, it only reproduces between 13 and 40 parts per thousand. The large number of specimens of A. beccarri suggests salinities of at least 13 ppt (the modern lake salinity is only 4 ppt).
The lake basin was filled by 7000 years
BP, and relatively wet conditions prevailed from 7000 to 3000 years BP as evidenced by low sulfur, high CaCO3, and low 18O and 16O ratios of ostracods and gastropods. Beginning about 3000 years BP, a drying trend began that culminated in peak arid
conditions between 1300 and 1100 years BP. Evidence for climatic drying includes an increase in gypsum (S) precipitation and an increase in 18O and 16O ratios. The peak of this arid event is well dated by an AMS-14C date of a seed taken from the height
of the sulfur and oxygen isotope values. The radiocarbon date of the seed is 1140 +/-35 years BP, which translates to a calendar date of 893 A.D. The collapse of the Classic Maya civilization occurred between 800 and 900 A.D.
David A. Hodell
Department of Geology, University of Florida
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