Isotopic Geochemistry of Speleothems and Its Application to the Study of Past Climates

Abstract

The isotopic geochemistry of the precipitation of calcite on speleothems e.g. stalactite, stalagmites, has been investigated with the aim of finding whether variations in the isotopic ratios in speleothems can be used to obtain palaeoclimatic information. The expected δ13C values of carbon species in the solutions entering limestone caves have been calculated For two different geochemical models of the limestone weathering process. These were: (1) The solution of calcium carbonate from the limestone in the absence of a reservoir of gaseous carbon dioxide. (2) The solution of calcium carbonate from the limestone in the presence of a reservoir of gaseous carbon dioxide. The expected δ13C and δ18O values of calcite precipitated from the ground waters entering caves have been calculated for several models of the precipitation system. These models were: (1) Transport of carbon dioxide from the cave atmosphere to the bulk atmosphere being the limiting step. (2) Transport of carbon dioxide from aqueous carbon dioxide to gaseous carbon dioxide being the rate limiting step. (3) Decarboxylation of bicarbonate ions or carbonic acid molecules being the rate limiting step. (4) Exchange with carbon dioxide of atmospheric origin with carbon species in the solution. (5) Precipitation of calcium carbonate through the evaporation of water. The effect that a change in climate would have on the δ13C and δ18O values of the calcite precipitated on speleothems has also been discussed. Measurements have been made of the δ13C and δ18O profiles through twelve speleothems collected from seven caves in different parts of New Zealand. A study has been made of the δ13C values of carbon species dissolved in the ground water flowing into caves and of the carbon dioxide in cave atmospheres. These measurements have shown that speleothems may be classified into three groups by the variations in the 13C/12C and 18O/16O ratios of the calcite deposited on them. These three groups were: (1) Speleothems in which calcite has been deposited through the evaporation of water from the solutions flowing over them. These are close to being in carbon isotopic equilibrium with the carbon dioxide in the bulk atmosphere, and their oxygen isotopic ratios ate more positive than they would be if the calcite was in isotopic equilibrium with the mean annual rain water. They are all composed of poorly cemented, randomly orientated microcrystalline calcite. (2) Speleothems in which calcite has been deposited through rapid loss of carbon dioxide from solution. These show a kinetic fractionation of carbon and oxygen isotopes, calcite precipitated showing a simultaneous enrichment in 13C and 18O, the enrichment in 13C being approximately four times as great as the enrichment in 18O. (3) Speleothems in which calcite has been deposited such that the loss of carbon dioxide was sufficiently slow to enable isotopic equilibrium to be maintained between aqueous carbon dioxide and bicarbonate ions. The calcite deposited on these speleothems is in oxygen isotopic equilibrium with the mean annual rain water of the regions in which they are growing, and consequently variations in δ13C and δ18O are generally unrelated. samples from four of the speleothems, on which calcite has been deposited in isotopic equilibrium with the mean annual rain water, have been carbon dated. Since this dating has shown that the variations in δ18O values in these speleothems were not only of the same magnitude, but also occurred simultaneously, New Zealand-wide variations in mean temperature or in the composition of the mean rain water must have occurred. This implies that the climate of New Zealand must have altered. It has been shown that there is a close relationship between the δ18O values of calcite deposited within the last 1,000 years on one of the speleothems, with the mean temperature deduced for central England over the same period of time. If it is assumed that the composition of the mean annual rain water had not changed, then temperature changes of the same magnitude as those deduced for central England Would have altered the magnitude of the oxygen isotopic fractionation between water and calcite sufficiently to give the observed variation in the δ18O in New Zealand speleothems. Durinq the last 100,000 years, a sequence similar to the glacial and interglacial, stadial and interstadial periods of the Northern Hemisphere has been recorded in New Zealand speleothems, with the δ18O values of the epeleothem calcite becoming more positive at times of colder temperatures. Some minor discrepancies exist however between the timing of major changes in the δ18O of the speleothem calcite and the major climatic events recorded elsewhere. These discrepancies may have been a result of difficulties in placing variations in δ18O values on a time scale, or may have been caused by a change in the temperature gradient between New Zealand and the regions from which New Zealand's rain is evaporated. Estimates have been made of the possible changes in the temperature gradient between New Zealand and the regions of evaporation. This was achieved by comparing the changes in the δ18O values of the calcite precipitated on New Zealand speleothems with changes in the δ18O values of calcite precipitated by Planktonic foraminifera from tropical Pacific Ocean surface waters (Emiliani (1966)). The resulting curve suggested that a glacial phase is preceded by an increased thermal gradient between equatorial and mid-latitudinal zones, and that an interglacial phase is preceded by a thermal gradient greater than at present. Thus variations in the δ18O values of calcite precipitated on suitable speleothems may yield data on changes in the thermal gradient between tropical and temperate zones, which in turn may yield information on the mechanics of major or global climatic events. However, the accuracy of this curve depends on the accuracy of the dating of events recorded in deep sea sediments.