Derivation of Environmental Signals from Chemically Altered Speleothems: Initial Study
Abstract
Carbonate speleothems are useful archives of land-based paleoclimate studies. Trace elements (e.g., Mg, Sr, Ba) and stable isotopes of carbon and oxygen that are incorporated into the carbonate minerals are used to infer changes in air temperature, vegetation, rainfall amount and short-lived events such as hurricanes. Most speleothems exhibit stable calcite mineralogy but a growing number of studied archives display aragonite mineralogy that tends to convert to calcite with time. Aragonite is the second most abundant cave mineral after calcite and the number of aragonite speleothems serving as canonic paleoclimate archives has recently expanded. Conventional practice is to avoid areas of aragonite alteration lest the data derived is a product of diagenesis and not reflective of the original paleoclimatic signal. This avoidance comes at the expense of potentially useful data that could represent important time intervals of climate change. Hence an understanding of the diagenetic behavior of aragonite with time is essential to fully retrieve the paleoclimate "signals" being preserved in the primary deposit. Here we present a petrographic and geochemical initial study of the aragonite to calcite transformation as evidenced in stalagmites from DeSoto Caverns, AL. Two types of calcite replacements were identified that vary with respect to the nature of replacement as well as their isotopic compositions: (i) equant/blocky calcite composed of fine to coarse crystals (50 μm to 2 cm size) whose replacement occurs along isolated and small transformation fronts, and (ii) columnar calcite comprised of elongated crystals with a length to width ratio > 6 whose replacement habit occurs on a much larger scale and across a wider area compared to the equant/blocky calcite. Profiles, from the precursor aragonite across the transformation boundary to the calcite replacement fabrics, were run in order to ascertain the effect of the mineral transformation on the δ18O and δ13C compositions. The results yield isotope fractionations for aragonite/equant calcite and aragonite/columnar calcite pairs of 0.2 ± 0.3‰ and 0.4 ± 0.2‰ for δ18O> (n1=88; n2=23) and 0.0 ± 0.2‰ and 1.9 ± 0.4‰ for δ13C (n1=88; n 2=23), respectively. The fractionations between aragonite/columnar calcite agree well with experiments performed under temperatures similar to that of the cave environment (~17°C) and are practically invariant along the growth axis of the stalagmite suggesting diagenesis is taking place within a closed or semi-closed system. Our data suggest that secondary calcite, under favorable conditions, likely inherits the original paleoclimate signal even after mineral replacement.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFMPP11A1423P
- Keywords:
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- 1041 GEOCHEMISTRY / Stable isotope geochemistry;
- 1051 GEOCHEMISTRY / Sedimentary geochemistry;
- 1065 GEOCHEMISTRY / Major and trace element geochemistry;
- 3625 MINERALOGY AND PETROLOGY / Petrography;
- microstructures;
- and textures