MECHANISMS OF OXYGEN ISOTOPIC FRACTIONATION BETWEEN DRIP WATER AND SPELEOTHEM CALCITE: EVIDENCE FROM A 10 YEAR MONITORING STUDY, CENTRAL TEXAS, USA

Oxygen isotope values (δ¹⁸O) were measured for calcite precipitated on artificial substrates and associated drip water at a long-term monitoring site in a central Texas cave to evaluate isotopic fractionation and relationships with climatic parameters. Substrates were renewed and water samples collected every 4-8 weeks from 1998 to 2008.

δ¹⁸O values of calcite from the substrates range from -5.9‰ to -4.1‰. Temporal trends show no significant correlation with environmental variables such as CO₂ level and drip rate inside the cave, or temperature and rainfall outside the cave.

Using measured drip-water temperature (T_w), drip-water δ¹⁸O, and equilibrium fractionation factors from the literature, we calculated predicted equilibrium calcite δ¹⁸O values. We then compared them with the measured calcite δ¹⁸O, and defined the “departure from equilibrium fractionation” as Δ¹⁸O_m-e. There is a linear relationship between T_w and Δ¹⁸O_m-e: T_w = 1.6 Δ¹⁸O_m-e+18.8; r² = 0.5. Because lower T_w at the studied site has been shown to correspond to faster calcite growth, the observed relationship contradicts conceptual models that predict faster calcite growth leads to larger positive Δ¹⁸O_m-e.

If slower calcite growth in fact facilitates equilibrium fractionation (smaller Δ¹⁸O_m-e), our data suggests a larger than commonly accepted equilibrium fractionation factor. Our data also suggests that faster growth leads to a negative shift of calcite δ¹⁸O from the equilibrium value predicted by this larger fractionation factor. Such a shift can be explained by calcite precipitation via an entrapment process, whereby CO₃^2- with more negative δ¹⁸O than HCO₃^- is deposited onto the crystal surface with negligible isotopic fractionation. Faster growth would reduce further isotopic exchange and result in calcite with more negative Δ¹⁸O _m-e values.

If correct, growth rate may be an important factor controlling oxygen isotopic fractionations. Growth rate shifts within a speleothem time series may affect speleothem δ¹⁸O variations. This study also suggests limitations to the Hendy test for equilibrium precipitation, which does not consider growth rate factor.