Climatic influences on the offset between d18O of cave drip waters and precipitation inferred from global monitoring data

Abstract

We present a meta-analysis of data from 22 caves and 96 drip sites from 4 continents where both the cave drip water d18O and the weighted mean d18O of precipitation have been measured. Drip water d18O is similar to the weighted mean d18O of precipitation (within ± 0.3 ‰) for sites where mean annual temperature (MAT) is less than 15 °C (85% of drips where MAT < 15 °C) and an aridity index (annual precipitation (P) / annual potential evapotranspiration (PET)) is greater than 0.65 (74% of drips at sites where P/PET > 0.65). In contrast, at warmer locations with increased water deficit, drip water d18O deviates from the weighed mean precipitation d18O by +3 ‰ and -1.5 ‰. We argue that this is due to evaporation in the soil and shallow vadose zone (thereby increasing drip water d18O) and lower water storage in the vadose zone, leading to relatively less mixing (thereby increasing the range in drip water d18O to more closely reflect recharge water d18O). Speleothems that have formed close to isotopic equilibrium are likely to have an oxygen isotope composition that contains a mixed signal of cave air temperature and precipitation d18O only in cool and temperate regions (T < 15 °C), or very wet climates where P/PET >> 0.65. In contrast, in warmer and drier environments, speleothems which have formed close to equilibrium will have d18O that reflects cave air temperature and a seasonal bias toward the d18O composition of rain in periods of high recharge, as well as the extent of evaporative fractionation of stored karst water.

Global Isotope Hydrogeology―Review

“…Further intercomparison of groundwater and precipitation isotope compositions with cave drip waters may help to address some of the aforementioned limitations (e.g., as cave drip waters are less likely to contain, for example, paleowaters or high‐elevation recharge; e.g., Partin et al, ). Recent global syntheses are beginning to quantify the ways that hydroclimatology leads to isotopic offsets between cave waters and annual amount‐weighted precipitation (Baker et al, ). Comparing such syntheses against other nearby comparisons of precipitation and groundwater isotopic compositions may help to test the ways that precipitation infiltrates karst systems (represented by most studied cave systems) versus nonkarst aquifer systems (represented by most of the studies compiled here; see Hartmann et al, ).…”

Investigating the 8.2 ka event in northwestern Madagascar: Insight from data–model comparisons

“…In summary, the modern cave temperature (~24.5Ð26 ¡C), the relative humidity (72Ð87%), the PCO2 (~400Ð500 ppm), and the results from both layer-specific and along the growth axis Hendy tests suggest that kinetic fractionation occurs today, and it could also happen in the past in our study cave, except at the onset of the sub-event when relative humidity in the cave was predicted to be ~100%. Kinetic isotopic effects are a very common phenomenon in caves, where mean annual cave temperature exceeds 10¡C (e.g., Baker et al, 2018).…”

20,000 years of societal vulnerability and adaptation to climate change in southwest Asia

“…Stalagmite δ 18 O values are primarily influenced by the δ 18 O of cave drip water, usually a function of surface precipitation, where δ 18 O of precipitation is influenced by multiple climate parameters such as temperature, and the origin and amount of rainfall (see below). The interpretation of δ 18 O stalagmite calcite values is therefore not straightforward, and may be further complicated by other factors, as shown by recent work in other semiarid environments, that demonstrate that speleothems respond to complex recharge, as well as in‐cave, processes (e.g., Baker et al, ; Cuthbert, Baker, et al, ; Cuthbert, Rau, et al, ; Markowska et al, ). The majority of stalagmite records from the region have been interpreted as reflecting changes in the amount of precipitation (e.g., Bar‐Matthews, Ayalon, Gilmour, Matthews, & Hawkesworth, ; Cheng et al, ; Fleitmann et al, ; Flohr et al, ) and changes in the source of moisture (e.g., Fleitmann et al, ; Ünal‐İmer et al, ).…”