Embracing the karst hydrological control on speleothem oxygen isotope variability
Authors: Treble Pauline , Baker Andy , Priestley Stacey , Griffiths Alan 
Published: February 2023

The influence of karst hydrology or &#8216;flowpaths&#8217; on speleothem oxygen isotopic (&#948;18O) values has been simulated using karst forward models. &#160;Cave monitoring studies have also shown that variability in dripwater &#948;18O can be directly related to whether flowpaths are dominated by preferential/quick flow or diffuse/slow flow which challenges the paradigm of speleothems as archives of past variability in mean rainfall &#948;18O.&#160; Yet it is not known how common this flowpath effect is and whether it should be considered in the interpretation of speleothem &#948;18O records. &#160;Recently, Treble et al. (2022) analysed two global databases: SISAL v2 (Comas-Bru et al., 2020) and an extended compilation of dripwater from Baker et al. (2019).&#160; It was demonstrated that within-cave variability in mean &#948;18O values were common worldwide in both datasets. &#160;An analysis of cave meta-data demonstrated that the flowpath effect is unrelated to climate, cave depth or lithology; further supporting the ubiquitous nature of flowpaths, i.e., there is (1) a mixture of preferential and diffuse flow for all karstified carbonate rocks due to its triple-porosity nature (primary=matrix, secondary=fracture, tertiary=pipes and conduit); and (2) differences in soil/epikarst water storage and drainage characteristics.We demonstrate how a mechanistic understanding of flowpaths can lead to a more robust interpretation using a case study that is also relevant for managing water resources in the Mediterranean-type climate of south-west Australia.&#160; Using seven modern stalagmite records from four caves, plus dripwater data, we demonstrate that the cave &#948;18O record shows a common response to a sustained decrease in rainfall that impacted the region in the 1970s, characterised by a rise or &#8216;uptick&#8217; in &#948;18O (Priestley et al., 2022).&#160; Mean annual rainfall &#948;18O values over the same period were quantified using observed and modelled data to have varied by &#8722;0.4 to +0.1 &#8240; whereas the speleothem uptick is +1.5 &#8240;.&#160; The much larger magnitude of the uptick is consistent with a reduction in the preferential-flow component to these caves driven by reduced rainfall recharge. &#160;Preferential flow is an important contribution to groundwater. &#160;The &#8216;uptick&#8217; or reduction in preferential flow implies that rainfall recharge to groundwater across the study region may no longer be reliably occurring.&#160; The longer paleo-record for south-west Australia confirms that no replicated upticks are seen in the last 800 years in stalagmites from the region and highlights the impact of climate change to water security in a region heavily dependent on groundwater.&#160;Comas-Bru, L. et al. SISALv2 (2020). A comprehensive speleothem isotope database with multiple age-depth models. Earth Syst. Sci. Data 12, 2579&#8211;2606 (2020).Priestley, S., et al. (2022). Caves provide early warning of unprecedented decrease in rainfall recharge of groundwater. Research Square. doi:https://doi.org/10.21203/rs.3.rs-1556439/v1Treble, P. C., et al. (2022). Ubiquitous karst hydrological control on speleothem oxygen isotope variability in a global study. Communications Earth & Environment, 3(1), 29. doi:10.1038/s43247-022-00347-3.