SPELEOTHEM PALEOCLIMATOLOGY OF THE YUCATAN PENINSULA

Changes in rainfall amount are often invoked as the major component of past climate shifts in low-latitude regions. Speleothems can serve as archives for interpreting past climate changes in these regions, with the Amount Effect thought to account for shifts in δ¹⁸O of speleothem calcite. We are studying modern rainfall and speleothems from the Yucatan Peninsula, Mexico to: 1) test the validity of the Amount Effect in this region, and 2) understand climate change on the Peninsula during the late Pleistocene. The Amount Effect is expressed by plotting monthly (intra-annual) rainfall amount versus weighted monthly δ¹⁸O values. Months of low rainfall have relatively enriched δ¹⁸O values, while months of high precipitation have depleted δ¹⁸O values. We question, however, if this effect operates across longer time scales, i.e. year-to-year, or decades to centuries. Time resolution of speleothem samples can range over multiple years. Establishing that the Amount Effect does operate on longer timescales is required to determine if changes recorded in speleothem calcite δ¹⁸O are indeed related to rainfall amount. U/Th ages indicate the Yucatan speleothems we studied grew during the late Pleistocene. Samples were analyzed for δ¹⁸O at 0.5mm intervals along the stalagmite growth axis. The records indicate centennial-scale variability superimposed upon a gradual increase through time in δ¹⁸O until growth stopped at the end of the deglacial. Similar centennial variability is expressed in the % reflectance record of marine cores from the Cariaco Basin, north of Venezuela (Peterson et al. 2000), but the variability does not match Greenland ice core data. This suggests a regionally coherent, rather than global climate change pattern.