Abstract
The oxygen isotope composition of speleothems is a widely used proxy for past climate change. Robust use of this proxy depends on understanding the relationship between precipitation and cave drip water δ18O. Here, we present the first global analysis, based on data from 163 drip sites, from 39 caves on five continents, showing that drip water δ18O is most similar to the amount-weighted precipitation δ18O where mean annual temperature (MAT) is < 10 °C. By contrast, for seasonal climates with MAT > 10 °C and < 16 °C, drip water δ18O records the recharge-weighted δ18O. This implies that the δ18O of speleothems (formed in near isotopic equilibrium) are most likely to directly reflect meteoric precipitation in cool climates only. In warmer and drier environments, speleothems will have a seasonal bias toward the precipitation δ18O of recharge periods and, in some cases, the extent of evaporative fractionation of stored karst water.
Highlights
The oxygen isotope composition of speleothems is a widely used proxy for past climate change
In water-limited environments, potential mechanisms by which δ18Ospeleo can be modified during transit from the source, include evaporative fractionation of water δ18O in the soil; a shallow vadose zone or cave; selective recharge, whereby rainfall events with high amount or intensity have a distinct isotopic composition, typically low δ18O; non-equilibrium deposition during speleothem formation[23,24,25,26,27]
The analyses show that drip water δ18O is most similar to the amount-weighted precipitation δ18O, when mean annual temperature is < 10 °C
Summary
The oxygen isotope composition of speleothems is a widely used proxy for past climate change. For seasonal climates with MAT > 10 °C and < 16 °C, drip water δ18O records the recharge-weighted δ18O This implies that the δ18O of speleothems (formed in near isotopic equilibrium) are most likely to directly reflect meteoric precipitation in cool climates only. Multiple processes determine the oxygen isotope composition of speleothems (δ18Ospeleo), with the potential climate signal reflecting the source water (meteoric precipitation) δ18O (δ18Oprecip) and its relationship to local and regional climate. This signal is transferred to the cave through the vadose zone, where it may be mixed with existing waters and fractionated by evaporation. The implications for speleothem palaeoclimatology are that speleothems (if formed near isotopic equilibrium) are most likely to directly reflect meteoric precipitation δ18O only in cooler climates
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