17,000 Years of Climate Change: The Phytolith Record from Hall's Cave, Texas
Joines, Jason Paul
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We used modern analog technique to develop phytolith-based transfer functions. We applied these transfer functions to phytolith assemblages in sediments from Hall's Cave, Texas to reconstruct mean annual precipitation and temperature for the central Edwards Plateau from 17,550 BP to 730 BP and tested these reconstructions for statistical significance. We also interpreted the phytolith assemblage and applied phytolith indices of woody cover and of C 3 versus C 4 grasses to reconstruct Edwards Plateau vegetation over the same period. Reconstructed mean annual precipitation (RMAP) was less than 450 mm during the last glacial period with the exception of a spike to over 1150 mm at 17,160 BP. As glacial conditions ended RMAP progressively increased with oscillations between modern (800 mm) and higher values until reaching a high of over 1200 mm at 9860 BP. Then RMAP gradually decreased to less than 825 mm at 6890 BP followed by a gradual increase to over 1325 mm at 2560 BP. RMAP then dropped sharply to less than 625 mm at 1640 BP followed by an increase to above modern values by 730 BP. Reconstructed mean annual temperature (RMAT) followed a similar trend. RMAT was much cooler than present with a minimum of less than 10 C during the last glacial period. RMAT also spiked at 17,160 BP approaching 15 C before declining again. After glacial conditions ended RMAT generally increased reaching 17 C by 3620 BP. After 2560 BP RMAT declined sharply to near 12.5 C at 1640 BP before increasing again reaching 14 C by 730 BP. RMAP proved to be statistically significant. We also have confidence in the trend exhibited by RMAT but temperatures may be underestimated. Vegetation on the Edwards Plateau near the end of the last glacial period was open woodland or savanna with mixed C 3 and C 4 grasses changing to closed woodland by 16,740 BP and transitioning to forest by 14,940 BP with grasses nearly absent. Forest with little or no grass was the most common vegetation for the next 12,000 years. Open woodland or savanna with mixed C 3 and C 4 grasses re-appeared at 2560 BP transitioning to a mixed C 3 and C 4 grassland by 2230 BP and to C 4 grassland by 730 BP.
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