Like speleothems, travertine/tufa deposited at the surface in karst areas can also be an important geological archive for terrestrial paleoclimatic studies. In particular, because of the high depositional rates (mm to cm per year), high-resolution (up to seasonal, monthly or even daily) paleoclimatic information can be obtained.

However, one must understand the cause of variations in the geochemical proxies (chiefly stable oxygen and carbon isotope compositions), and the quantitative relationships between these proxies and climatic/environmental factors before using them to reconstruct climate changes. Now, most researches concentrate on the record of oxygen isotope, few studies have attempted to investigate the suitability of the commonly accepted oxygen isotope equilibrium factor and the factors affecting oxygen isotope equilibrium in travertine-depositing environments.

The travertine research group led by Prof. LIU Zaihua from the State Key Laboratory of Environmental Geochemistry (SKLEG), Institute of Geochemistry, Chinese Academy of Sciences (IGCAS) has made a new progress in revealing controlling mechanism of oxygen isotopic fractionation in travertine.

The research finds that the oxygen isotope fractionation may be controlled by the deposition rate. The equilibrium between the travertine and water is attained under the lower deposition rate conditions. When the calcite deposition rate is lower than 0.38 mg cm^-2d^-1, oxygen isotopic equilibrium between calcite and water is attained. The result supports the results of Coplen (2007) that the equilibrium fractionation factor may be greater than the commonly accepted one derived by Kim and O’Neil (1997). The relationship between oxygen isotope fractionation factor and calcite deposition rate is built and agrees with the results of Dietzel et al. (2009) who has found that the kinetic-isotope effect favors preferential incorporation of ¹⁶O in solid calcite as the calcite deposition rate increases.

The research also points out, in order to exclude the effect of varying deposition rates, the deposition rate of fossil travertine/tufa must be considered when using travertine/tufa δ¹⁸O records to reconstruct paleo-temperature. For epigenic tufas (where the CO2 is sourced from soil and atmosphere) where the calcite deposition rates are higher in summer-autumn (June-October) and lower in winter-spring (November-May), the effect of deposition rate will amplify the negative drift in tufa δ¹⁸O in summer-autumn due to the higher deposition rates, and also amplify the positive drift in tufa δ¹⁸O in winter-spring due to the lower deposition rates. Therefore, the range of variation in tufa δ¹⁸O might be increased by the effect of deposition rate. In this case, the range of paleo-temperatures calculated from tufa δ¹⁸O values is larger than the actual temperature change (about 8 ^oC). For endogenic travertine (where the CO₂ is sourced from a range of situations including hydrolysis and oxidation of reduced carbon, decarbonation of limestone or directly from the upper mantle), the calcite deposition rates are lower in warm rainy season and higher in cold dry season, the lower (higher) rate may partially counteract the negative (positive) drift in travertine δ¹⁸O values in warm wet (cold dry) seasonal conditions. Therefore, the range of variation in endogenic travertine δ¹⁸O may be reduced, and the range of paleo-temperatures calculated by using travertine δ¹⁸O values becomes smaller than the actual temperature change.

This work was supported by the National Natural Science Foundation of China (Grant Nos. 41172232 and 41372263), the National Basic Research Program of China (Grant No. 2013CB956703) and the Open Fund of State Key Laboratory of Environmental Geochemistry and Karst Dynamic Laboratory (Grant No. KDL2011-05).

Websites of the paper: http://www.sciencedirect.com/science/article/pii/S0016703714001604 

(By SUN Hailong)