Stable Isotope Studies of Water Extracted from Speleothems

Abstract

<p>Calcite speleothems, if shown to be formed under conditions of isotopic equilibrium with their parent seepage waters, can be used to determine relative changes in the past climates from measurement of δ^1 ^8O of the calcite. Furthermore, if δ^1 ^8O of the parent seepage water can be estimated and shown to be equivalent to meteoric precipitation falling at the cave site, then depositional temperatures can be recovered from the termperature dependent oxygen isotope fractionation of the calcitewater pairs. Cave temperatures generally reflect mean annual surface temperatures above the cave. Thus, should the depositional temperature accurately record the cave temperature, then analysis of successive growth layers in speleothems should provide a direct measure of past temperature change at a given site. Estimates of δ^1 ^8O of past seepage water can be made, in principle, because speleothems incorporate seepage water within inclusions as they grow. However, since oxygen in the fluid inclusion water may exchange with that in the carbonate phase, hydrogen isotopes, which cannot exchange, are measured instead. δ^1 ^8O of the original seepage water can then be inferred from δD, if seepage water can be shown to be equivalent to included water, because meteoric relationships exist which link them. Work prior to this study suggested that, following the method outlined above, isotopic temperatures could be recovered from spleothems.</p> <p>The early part of this study involved the construction and operation of a mass spectrometer and associated extraction line for δD analyses and some modification to existing apparatus for δ^1 ^8O analysis.</p> <p>Water samples were then analysed from a number of cave sites, mainly in the Eastern U.S., to check the isotopic equivalence of seepage water to meteoric precipitation and observe seasonal isotopic variations in seepage and soil water with respect to meteoric precipitation. The general conclusion reached was that seepage water was equivalent to meteoric precipitation and that no seasonal variation occurred in those caves studied in detail. Study of an Antarctic Ice Core showed that the δ^1 ^8O-δD relation of meteoric precipitation during the Wisconsin Glacial Maximum was different from that at present.</p> <p>The latter part of this study was concerned with the systematics of water extraction and the isotopic measurement of water-calcite pairs from speleothems. A previously developed technique, involving crushing of the sample in vacuum prior to freezing over the liberated water, was found not to be as reproducible as had been stated in the literature, and, as a result, an extraction method by heating was developed to replace it. Although great difficulty was encountered in getting this technique to function, it was found, eventually, to save time in operation, to liberate up to 2.4 times more water and give greater reproducibility than the crushing method. Attempts to demonstrate the isotopic equality of the two methods was only partially successful however, but both methods seemed to yield water from speleothems that was isotopically depleted with respect to their parent seepage waters.</p> <p>For the latter reason, experiments were undertaken to characterize the water within the calcite because of the possible presence of structural, isotopically modified water. Petrographic observations left no doubt as to the presence of fluid inclusions from 50μm down to unresolvable dimensions but it was questionable as to whether or not all the water lay in these sites. IR, polarized IR and neutron diffraction spectroscopy of powders in the 100μm range failed to reveal the presence of oriented water in the calcite lattice although 'liquid' water was still observed.</p> <p>The isotopic depletion of calcite-bound water with respect to parent seepage water in modern' samples was found to be 22.1 ± 3.9% in δD. This value, when applied to δD profiles of six fossil speleothems to retrieve original seepage water δD gives rise to calculated calcite-water paleotemperatures that are above zero. Comparison of these modified δD and temperature profiles with other records, such as deep sea and ice cores suggested that paleoclimatic information can be obtained from isotopic studies of speleothems. The mechanism(s) responsible for the isotopic fractionation of calcite-bound water as it is incorporated into the speleothems during growth is speculated upon as an absorption-like phenomenon but no firm conclusions are reached.<p>