TRIPLE OXYGEN ISOTOPES IN SOILS AND LAKES

Variations of ¹⁸O/¹⁶O ratios (denoted as δ¹⁸O) of minerals and natural waters have been widely used to infer paleoenvironmental parameters. However, interpretations of δ¹⁸O are nonunique because δ¹⁸O is determined by temperature, topography and hydrological processes such as evaporation and precipitation. Deviations in ¹⁷O/¹⁶O relative to ¹⁸O/¹⁶O [Δ¹⁷O = ln(δ¹⁷O+1) ‑ λ*ln(δ¹⁸O+1)] have been shown to be sensitive to evaporation due to the difference in isotopic fractionation exponent [θ = ln(δ¹⁷O+1) / ln(δ¹⁸O+1)] between water-vapor equilibrium and vapor diffusion in the air. Thus, the triple oxygen isotope systems of minerals and natural waters may provide additional information on hydrologic processes that are not discernable from analysis of ¹⁸O/¹⁶O ratios alone.

We present Δ¹⁷O data on soil carbonates from North America, East Africa and East Asia and build a dynamic model to simulate triple oxygen isotopic compositions in soil water. Our model shows that (1) persistent evaporation will result in lowered Δ¹⁷O values of soil water and (2) these ¹⁷O-depleted signals in soil water diminish sharply as depth increases. The model results are supported by our new data. For example, the triple oxygen isotopic compositions of deep soil carbonates (>50 cm) are similar to or yield slightly lower Δ¹⁷O values than meteoric water.

We also develop a model to predict triple isotope compositions of lake waters for different hydrological scenarios. Our model predicts that lake waters with the same δ¹⁸O value may have different Δ¹⁷O values due to different relative influences of isotopic compositions in precipitation, lake water mixing and evaporation. Through-flowing lakes with minimal evaporation should have Δ¹⁷O values similar to local precipitation, whereas highly evaporated lakes should have distinctly negative Δ¹⁷O values.

In summary, our models and preliminary data suggest that Δ¹⁷O will be a valuable addition to the terrestrial paleoclimate toolkit, most notably for its ability to record the effects of aridity.