Dynamic physicochemical influences on speleothem paleoclimate proxy archives: A story of four north Florida caves.

摘要

Stalagmites, or speleothems, are secondary cave deposits that record the isotope and trace element composition of the parent dripwaters within their crystal matrix. Changes in dripwater oxygen and carbon isotopes (delta 18O and delta13C) and trace element to calcium (X/Ca) ratios are controlled by a number of parameters, including changes in rainfall intensity, vegetation regime, and hydrology above the cave. Cave and speleothem studies over the last fifteen years have demonstrated that in order to generate quantitative climate reconstructions, it is necessary to understand the transfer function between changing weather, and changing cave dripwater chemistry above the speleothem being used for reconstruction.;In an effort to develop a deeper understanding of these transfer functions, three caves in Marianna, FL were instrumented for several years to monitor rainfall amount, ventilation, temperature, drip rates, dripwater chemistry, and calcite chemistry farmed atop stalagmites that had potential to yield a Holocene rainfall record. In two of the three caves it was found that dripwater X/Ca ratios increase during dry periods and decrease during wet periods, while drips in the third cave exhibited muted variations in X/Ca ratios. A mathematical model was developed to demonstrate that X/Ca ratios are a function of (1) mixing between dissolved limestone and dissolved dolomite, and (2) subsequent evolution away from the dissolved bedrock signature to higher X/Ca ratios as calcium is removed from solution by prior-calcite-precipitation (PCP) upstream of the speleothem. This model provides a diagnostic framework to judge whether dripwaters at each location are entirely controlled hydrologic saturation, and are thus suitable recorders of wet-versus-dry conditions. Timeseries measurement of rainfall amount and changing calcite chemistry allowed quantitative calibration of the transfer function between weather and the speleothem proxy record, a term dubbed 'speleochemical rainfall response'..;This speleochemical rainfall response was then applied to stalagmite HRC2 to generate the first quantitative record of rainfall amount in the southeastern United States. The record shows that over the last 1,200 years there have been two significant dry periods. Rainfall in Marianna decreased by up to -60 mm month-1 during the Little Ice Age (650-150 years BP). An equivalent decrease in rainfall amount is only observed during the last 60 years. This record suggests that trends in modern climatic influence over Marianna rainfall have the potential to deliver north Florida into severe drought conditions within the next several decades.