Insights into aragonite diagenesis as evidenced from spelean carbonate

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Cave deposits are increasingly being used as paleoclimate archives. Though a majority of cave deposits are composed of calcite, cave deposits composed of aragonite are quite common. Aragonite, being unstable in the cave environment, will readily alter to stable calcite. Paleoclimatologists are reluctant to sample in areas where aragonite has altered to calcite lest the data derived be a result of diagenesis and not indicative of the original paleoclimate signal. The avoidance of these altered areas comes at the expense of potentially important paleoclimate data. Because aragonite is unstable near the surface of the earth and readily alters to calcite, studying the transformation of aragonite to calcite is difficult as instances where aragonite is adjoined to secondary calcite are extremely rare. DeSoto Caverns allow an opportunity to comment on the aragonite to calcite transformation as primary aragonite is adjoined to secondary calcite. Aragonite adjoined to secondary calcite provides an opportunity to study whether the original paleoclimate signal in aragonite can be preserved in secondary calcite. Two cored stalagmites, DSSG-5 and DSSG-6, were sampled in order to comment on the aragonite to calcite transformation and its chemical implications. Stalagmites from DeSoto Caverns reveal the aragonite to calcite transformation to be variable under similar physicochemical conditions. Two neomorphic calcite fabrics, equant and columnar, were observed. Equant calcite inherits textural and chemical features of primary aragonite, namely the ä13C, ä18O, Mg, Sr, and Ba. Columnar calcite is fabric destructive and gains Mg and but loses Ba and Sr from aragonite. The carbon and oxygen isotope composition of columnar calcite, relative to aragonite, agrees well with experimental stable isotope fractionation studies. Equant calcite and columnar calcite represent replacement in a closed system and open system respectively. Equant calcite is favored where fluid-filled pores and permeability of aragonite are low. Columnar calcite, however, is favored where fluid-filled pores and permeability of aragonite are high. Open system and closed system replacement fabrics observed in such close proximity have important implications with respect to the stabilization of carbonate units with time. The use of altered speleothems in the study of paleoclimate may be possible if the system in which the alteration occurs is well known.

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