Comparison of Biomarker Hydrogen and Shell-Carbonate Oxygen Isotope Proxies In Lake Chichancanab Sediment (Yucatan Peninsula, Mexico): Implications for the Terminal Classic Maya Collapse

The Terminal Classic collapse of Maya civilization (750 to 1050 C.E) has been associated with a period of sustained drought in the lowland Neotropics. Evidence for hydrologic changes is inferred primarily from isotopic, mineralogical and elemental characteristics of lacustrine and marine sediment records. In particular, increases in shell-carbonate d18O and gypsum deposition at Lake Chichancanab (Yucatan, Mexico), and reduced titanium concentrations in the marine Cariaco Basin (Venezuela), have been proposed as evidence of reduced regional precipitation. There remain questions, however, about the severity, timing and spatial extent of this drought. Furthermore, there is considerable debate about the link between this postulated climate change and societal collapse.

We are conducting a regional evaluation of terrestrial-derived biomarkers and their isotopic compositions, which should be sensitive to changes in evaporation and transpiration, to assess the severity and spatial extent of hydrologic changes during the Classic Maya period. Our preliminary data, derived from hydrogen isotope values (dD) of C29, C31 and C33 terrestrial-derived n-alkanes in Lake Chichancanab sediments, show centennial-scale variability in evapo-transpiration over the past 3000 years, including during the Terminal Classic period. This record, however, does not reveal significant dD increase above the mean value during the Terminal Classic, implying that regional aridity during the collapse was not more severe than at other times. This finding is in conflict with records of shell-carbonate d18O and sulfate deposition within the same core, which were interpreted as indicating increased evaporation/precipitation (E/P) during this period. Inconsistency between these two proxy records suggests that: 1) organic biomarkers fail to record aridity reliably, or 2) one or both sets of climate proxies have been misinterpreted. We will expand our analysis to include other biomarkers at this site and other localities, and we will ground-truth the use of hydrogen isotopes of lipids as recorders of regional E/P.