Speleothems as proxy for the carbon isotope composition of atmospheric CO2

We have measured the stable isotope ratios of carbon in a suite of recent cave deposits (< 200 years) from the San Saba County, Texas, USA. The methodology for dating these deposits using excess 210pb was recently established [Baskaran and Iliffe, 1993]. The carbon isotope ratios of these samples, spanning the thne period -• 1800-1990 AD, reflect the carbon isotope ratio of atmospheric CO2 for the same period. The pathways by which the fi13C of atmospheric CO2 is imprinted on these speleothems can be explained using a model developed by Ceding (1984). The results suggest hat the carbon isotope ratios of speleothems can be used to develop long-term, high-resolution chronologies of the fi13C of atmospheric CO2 and, by implication, the concentration of the atmospheric CO2. Introduction relatively young cave deposit system (<200 yr) from the San Saba County in Texas, USA. Materials and Methods Three types of speleothems, two normal icicle-shaped stalactites, a tubular soda straw stalactite and a stalagmite were dated using the method of Baskaran and Iliffe (1993). The 210pb concentrations and the ages of these samples are given in Baskaran and Iliffe (1993). The altitude of the cave is -• 570 m and the relative humidity in the cave is near 100%. Details on this cave system are given elsewhere [Smith, 1973]. The calcite samples were reacted with 100% H3PO4 at 25 ø C and the CO2 gas was analyzed in a VG OPTIMA Isotope Ratio Mass spectrometer. The isotopic ratios are reported in the usual b-notation where: Speleothems have been widely used as a proxy to retrieve continental paleoenvironmental information. It has been observed that the optimum clhnatic conditions for speleothem growth are wet and warm environments as opposed to cold and dry weather conditions [Harmon et at., 1975; Ayliffe and Veeh, 1989]. For example, higher speleothem growth rates during interglacial periods has been observed [Harmon et al., 1975]. All stable isotope studies of speleothems published so far have dealt with glacial-interglacial time scales, 10500 ka (kilo annum). Oxygen isotope ratios together with radiometric dating of speleothems have provided significant palaeoclimatic information [Hendy, 1971; Harmon et al., 1975; Atkinson et al., 1978; Goede and Harmon, 1983; Ayliffe and Veeh, 1989; Brook et al., 1990; Dorale et at.,' 1992]. Carbon isotope ratios of cave deposits, on the other hand, have received much less attention. Recently, we have shown that a large amount of excess 210pb (half-life = 22.1 yr) is produced within the cryslats of speleothems and that this excess can be utilized to date them [Baskaran and Iliffe, 1993]. Lead-210 is produced from its precursor, 222Rn (half-life = 3.8 d) at a constant rate. As 222Rn-containing water in caves drips from stalactites to stalagmites, 210pb, the relatively longer lived daughter of 222Rn, is produced according to the equation [21øpb] = [222Rn] x (1 -exp(-•pb At)) where •Lpb is the decay constant of 210pb ' and At is the th'ne for which the water drop is in contact with the calcium carbonate. We report here the stable isotope ratios of carbon fi'om a Copyright 1993 by the American Geophysical Union. Paper number 93GL02690 0094-8534/93/93 GL-02690503.00 15 %o = [(RSample / Rstandard)-I ] X 1000