The Effects of Cave Ventilation on Stalagmite Deposition and Paleoclimate Records--A Case Study from Cathedral Caverns, Alabama

摘要

Speleothems (e.g., stalagmites) have been established as valuable archives of paleoclimate proxies (δ O_(carbonage) and δ C_(carbonate)). Studying stalagmites along the Gulf Coast has a high potential for producing paleo-rainfall records due to the abundance of carbonate (karst) in the region. In order to properly interpret these proxy records, the conditions within the cave in which the stalagmite was deposited must be well understood. Although the air temperature (-mean annual temperature) and relative humidity (~100%) of the atmosphere within caves are typically quite stable, recent studies have shown that slow density-driven movement of cave air can have profound seasonal changes in the chemistry of the cave air (pCO_2 and δ~(13)C), dripwater ([DIC], δ~(13)C_(DIC)), and the stalagmite (δ C~(13)C_(carbonate)). Furthermore, the primary season (e.g., winter) of stalagmite deposition can be controlled by this seemingly subtle process, which could have a profound influence on both the δ~(18)O and δ~(13)C chemistry preserved in the stalagmite. The measurement of pCO_2 throughout the year is the first step in determining cave ventilation and stagnation, because typically CO_2-rich cave air is diluted by CO_2-depleted atmospheric air during winter (ventilation mode). An on-going study since January 2015 at Cathedral Caverns (34°34'N, 86°13'W), located in northeastern Alabama, consists of the collection of air from the back of the cave (as well as ten other locations within the cave) near our stalagmite sample location. Preliminary results have shown a minimum and a maximum of 550 ppmV in February and 2820 ppmV in August, respectively, which indicates ventilation in winter (low pCO_2 values) and stagnation in summer (high pCO_2 values). These preliminary data indicate that a winter-bias of calcite deposition may be occurring and the interpretation of the paleo-record from Cathedral Caverns could be affected by such bias.