The main objective of the proposal was to develop high-resolution (~2 to 5 yr), time-overlapping and replicated δ¹⁸O records over the past five millennia from U-series dated stalagmites collected in an ENSO-sensitive region of southwestern Mexico.  Droughts are a recurring feature of Mexican climate, but few high-resolution data are available to test for a climate change forcing of Mesoamerican civilizations. Speleothems provide the possibility of constructing absolutely dated high-resolution records of climate change.  In this project we use uranium-series dating in combination of measurements of oxygen isotopes in speleothem in order to reconstruct the climatic backdrop for the rise and fall of Mesoamerican civilizations.  We published a study in the journal Geology consisting of a quantitative 2400-yr rainfall reconstruction for the Basin of Mexico from a precisely dated and highly resolved speleothem, that documents highly variable rainfall over the past two and a half millennia. Dry conditions peaked during a 150-yr-long late Classic megadrought that culminated at 770 CE, which followed centuries of climatic drying that spanned the fall of the city of Teotihuacán at ca. 550 CE. Wettest conditions in the 1450s CE were associated with flooding in the Basin of Mexico. Our data suggest that rainfall variability was likely forced by the El Niño /Southern Oscillation, and impacts on spring-fed irrigation agriculture may have been a stressor on Mesoamerican civilizations.

As part of the project we also studied the dating complexities that may arise as a result of the alteration of aragonite to calcite.  This is important because the strength of speleothem-based climate reconstruction is related to the absolute chronology provided by the U-Th system.  We analyzed uranium-series concentrations and isotopic ratios in amixed aragonite and calcite stalagmite from Juxtlahuaca Cave, from the Sierra Madre del Sur of Mexico. The U-series data for the aragonite layers return highly precise and stratigraphically correct ages over the past ca. 4300 years. In contrast, age determinations from calcite layers are too old by several hundred years relative to the precise aragonite ages, have analytical uncertainties an order of magnitude larger than aragonite ages, and yield ages that do not overlap the aragonite ages within analytical uncertainties. Based on geochemical and petrographic observations, we interpret the calcite layers to have formed from recrystallization of aragonitesoon after primary aragonite deposition. Calcite occurs asdiscontinuous lenses on and off the growth axis, and laminae can be traced between aragonite and calcite layers, demonstrating that visible growth banding is not effaced in the recrystallization process. Paired aragonite-calcite U-series data from coeval stratigraphic layers demonstrate that uranium concentrations decrease by two orders of magnitude during calcitization, and result in decreased [²³⁴U/²³⁸U] and increased [²³⁰Th/²³⁸U]. Uranium loss during diagenesis mimics a need for an age correction using an initial 230Th/232Th ratio one to two orders of magnitude larger than the Bulk Earth ratio of 4.4 - 2.2 x 10^-6. A need for apparent high initial ²³⁰Th/²³²Th ratios results from ingrowth of ²³⁰Th during ²³⁴U decay.

Aside from the scientific contribution, we pay attention to the human resource development aspect of our project.  Jon Baker, the PhD student working in the LVIS lab, has gained an impressive level of competence in stable isotope analytical techniques. Jon is not financially supported on the project, but he has been involved in drilling of stalagmites for stable isotopes, and analysis on the mass spec. This teaching exper...

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