Main controls on the stable carbon isotope composition of speleothems

Abstract

The climatic controls on the stable carbon isotopic composition (δ 13 C) of speleothem carbonate are less often discussed in the scientific literature in contrast to the frequently used stable oxygen isotopes. Various local processes influence speleothem δ 13 C values and confident and detailed interpretations of this proxy are often complex. A better understanding of speleothem δ 13 C values is critical to improving the amount of information that can be gained from existing and future records.This contribution aims to disentangle the various processes governing speleothem δ 13 C values and assess their relative importance. Using a large data set of previously published records we examine the spatial imprint of climate-related processes in speleothem δ 13 C values deposited post-1900 CE, a period during which global temperature and climate data is readily available. Additionally, we investigate the causes for differences in average δ 13 C values and growth rate under identical climatic conditions by analysing pairs of contemporaneously deposited speleothems from the same caves.This approach allows to focus on carbonate dissolution and fractionation processes during carbonate precipitation, which we evaluate using existing geochemical models. Our analysis of a large global data set of records reveals evidence for a temperature control, likely driven by vegetation and soil processes, on δ 13 C values in recently deposited speleothems. Moreover, datamodel intercomparison shows that calcite precipitation occurring along water flow paths prior to reaching the top of the speleothem can explain the wide δ 13 C range observed for concurrently deposited samples from the same cave. We demonstrate that using the combined information of contemporaneously growing speleothems is a powerful tool to decipher controls on δ 13 C values, which facilitates a more detailed discussion of speleothem δ 13 C values as a proxy for climate conditions and local soil-karst processes.

Response to the Reviewer 2

“…As we state above: "Our paper is the first to discuss both isotopes in a large set of data from South America, which makes our results novel and relevant to the speleothem community. Similar studies have been carried out in other regions of the world with differing results, mainly because d18O is not considered a proxy for the hydrological conditions outside the tropics, and because the strong temperature seasonality at high latitudes strongly affects the d13C in stalagmites (see Fohlmeister et al, 2020)". In this way our results are unique.…”

Late Quaternary Climate Variability and Change from Aotearoa New Zealand Speleothems: Progress in Age Modelling, Oxygen Isotope Master Record Construction and Proxy-Model Comparisons

“…This decreases the potential for inorganic δ 13 C enrichment of δ 13 C calcite , suggesting wetter periods should have more negative δ 13 C values. With this in mind, this interpretive framework could be validated using a combined δ 13 C, 14 C approach [151].…”

Stalagmite carbon isotopes suggest deglacial increase in soil respiration in Western Europe driven by temperature change

“…Changes in the amount of PCP the dripwater experiences en route to the speleothem can lead to significant variability in δ 13 Cspel records (Fohlmeister et al, 2020), and are tightly coupled to changes in cave air pCO2 and cave ventilation dynamics. Higher cave air pCO2 and a reduced CO2-gradient between the supersaturated dripwater solution and the cave air result in less PCP, and vice-versa for lower cave air pCO2.…”

“…However, formal testing of this mechanism has so far not been attempted, mainly because of the numerous and complex processes that influence δ 13 Cspel (Fohlmeister et al, 2020).…”

“…Secondly, δ 13 Cspel can be modulated by changes in karst hydrology, i.e., carbonate bedrock dissolution regime (Hendy, 1971). Thirdly, compounded changes in hydrology and cave atmospheric pCO2 can lead to prior calcite precipitation (PCP) during carbonate precipitation (Fohlmeister et al, 2020). Altitudinal transects in caves in the European Alps have shown that changes in soil respiration, vegetation, and temperature have a tractable effect on speleothem fabrics, stable oxygen isotope ratios, and δ 13 Cspel (Borsato et al, 2015).…”