Title

Pleistocene Climates Determined From Stable Isotope and Geochronologic Studies of Speleothem

Creator

Melvyn Gascoyne

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Publication Date

January 1979

Abstract

Speleothems are calcium carbonate deposits, such as stalagmites, stalactites and flowstones, formed in a cave environment by loss of carbon dioxide from saturated groundwaters. Variations in the 18o/16o ratio of calcite are related to changes in depositional temperature provided that the speleothem formed in isotopic equilibrium with its seepage water (conditions characterised by slow coloss and no evaporation of seepage water). Variations in 18o/16o of the seepage water will also be reproduced in the calcite but allowance for this effect can be made if 18o/16o ratio of the source (ocean water) can be estimated (from deep sea sediment cores) and if the influence of temperature on 18o/16o ratio of precipitation at the site can be determined. Because cave temperatures closely approximate mean annual surface temperature, the axial 18o/16o record of a speleothem is therefore an indication of paleoclimate and of temperature change over the period of its growth.

The frequency distribution of age measurements for several speleothems from an area may also be used as a paleoclimatic indicator because cold or glacial conditions above the cave will inhibit speleothem growth by freezing water at the surface and removing vegetation and soil cover, the main source of CO2 for the limestone dissolution-reprecipitation process.

Pure, non-porous calcite speleothems from several limestone regions have been dated in this study by the 230Th/234U method, and in cases of uranium-rich speleothems, by the 231Pa/230Th method also. For ten such cases, examined, good agreement of ages determined by both dating methods was found.

Relatively few deposits however, have shown deposition under isotopic equilibriwn conditions, due probably to the well-ventilated nature of the caves studied.

The age distribution for speleothem from Cascade Cave on Vancouver Island, B.C., indicates growth during the mid-Wisconsin interstadial dated as 65 -30,000 yrs. B.P. Stable isotope profiles for two speleothems which grew

Keywords

Temperature Dependen, Trace Elements, Calcite, Magnesium, Strontium, 23U/238U, 18O/16O, Speleothems, Paleoclimatic Indicator, Cold,, Or Glacial Conditions, Calcite Speleothems, Limestone, 230Th/234U, 231Pa/230Th, Groundwaters, Carbon Dixoide, Stalagmites, Carbonate Deposits, Stalactites, Flowstones

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Subject: topical

Temperature Dependen; Trace Elements; Calcite; Magnesium; Strontium; 23U/238U; 18O/16O; Speleothems; Paleoclimatic Indicator; Cold,; Or Glacial Conditions; Calcite Speleothems; Limestone; 230Th/234U; 231Pa/230Th; Groundwaters; Carbon Dixoide; Stalagmites; Carbonate Deposits; Stalactites; Flowstones

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Article

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serial

Identifier

SFS0072082_00001

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