Contemporaneously growing speleothems and their value to decipher in-cave processes – A modelling approach

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In recent decades speleothems have become valuable climate archives and are of crucial importance for the investigation of past climate conditions. Albeit much progress has been made to understand speleothem proxies, it remains difficult to differentiate between a direct climate signal and variations, which occurred due to in-cave processes like prior carbonate precipitation (PCP), CO2 degassing or exchange between dissolved inorganic carbon and cave air CO2. Here, we investigate a large quantity of previously published data sets of stable C and O isotopic composition as well as growth rate of contemporaneously growing speleothems from the same cave site. We argue that differences in those paleoclimate proxies arise mainly by differences of drip site specific conditions as climate conditions for pairs of contemporaneously growing speleothems can be considered to be similar. To better understand the observed differences in the isotopic composition and growth rate of contemporaneously growing speleothems, we investigate the impact of in-cave processes using a speleothem isotope and growth model. Evolution of the isotope composition is modelled as a Rayleigh process, which includes CO2 degassing and carbonate precipitation, oxygen and carbon exchange between the bicarbonate and the drip water and between bicarbonate and cave air CO2. The model is able to calculate growth rates and accounts for carbonate deposition in form of prior carbonate precipitation as well as carbonate deposition at the speleothem top. We find that CO2 exchange and PCP are necessary to explain the observed isotopic and growth rate differences in previously published contemporaneously growing speleothems. Thus, both processes need to be accounted for in speleothem-based paleoclimate reconstructions, which highlight the importance of recent efforts to correct speleothem isotope records for cave-internal effects. Furthermore, this study demonstrates the advantage of investigating pairs of contemporaneously growing speleothems from one cave to evaluate the importance of cave-internal processes in individual caves and if they potentially changed over time.