Insights into recharge processes and speleothem proxy archives from long-term monitoring networks of cave drip water hydrology
- 1UNSW Sydney, Australia
- 2ANSTO, Lucas Heights, Australia
- 3University of Freiburg, Germany School of Agriculture and Environment, University of Western Australia, Perth, Australia
- 4Earth and Ocean Sciences, Cardiff University, UK
- 5Max Planck Institute for Chemistry, Mainz, Germany
- 6School of Agriculture and Environment, University of Western Australia, Perth, Australia
Since 2010 we have established cave drip water hydrological monitoring networks in four contrasting climate zones (Mediterranean, montane, semi-arid and sub-tropical) across continental Australia. Deploying over one hundred automated drip loggers, we combine these long-term monitoring datasets with climate and water isotope data, lidar mapping, electrical resistivity imaging and karst hydrological modelling to provide insights into recharge processes and the impact of hydrological variability on speleothem proxy archives.
We identify increases in drip discharge and compare the timing of those events to antecedent climate conditions (rainfall, evapotranspiration). We find rainfall recharge thresholds vary with climate. At our montane site, recharge occurs after 13 to 31 mm rainfall events, depending on antecedent conditions. At the semi-arid site, recharge occurs after 40 mm rainfall events, and at our sub-tropical sites, recharge occurs following all instances where > 93 mm / week of precipitation occurs, with lower precipitation thresholds (down to 33 mm / week) possible depending on antecedent conditions and at sites with limited vegetation cover. We use these recharge thresholds to constrain simple soil moisture balance models to better understand soil and karst storage volumes. Combined with electrical resistivity imaging, we can relate recharge to the caves to subsurface water flow paths and karst water stores.
At our montane and Mediterranean climate sites, relatively consistent drip water isotopic composition confirms the presence of well-mixed water stores. This allows us to quantify the extent of speleothem oxygen isotope variability due to fractionation associated with changes in drip rate. We identify significant differences in long-term mean drip rates between different drip sites within a cave, and significant differences in event-based drip rate responses within a cave. Drip hydrological variability helps explain the within-cave variability of speleothem oxygen isotope composition observed at both sites, and helps identify the primary drip water oxygen isotope signal.
At our semi-arid site, drip water isotopic composition is dominated by epikarst evaporation and our drip water monitoring demonstrates that recharge events are infrequent (~1.6 per year). Using both observational and modelling data, we quantify the relative importance of evaporative fractionation in the epikarst and fractionation during calcite precipitation. Using modern speleothem samples, we demonstrate that the oxygen isotope signal in this water limited environment reflects the balance between the oxygen isotope composition of recharge and its subsequent fractionation in the soil, epikarst and cave.
How to cite: Baker, A., Treble, P., Hartmann, A., Cuthbert, M., Markowska, M., Berthelin, R., Tadros, C., Leopold, M., and Hankin, S.: Insights into recharge processes and speleothem proxy archives from long-term monitoring networks of cave drip water hydrology , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1686, https://doi.org/10.5194/egusphere-egu2020-1686, 2019
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For such a semi arid condition, if the rainfall is irregular, how does such conditoin affect the growth of speleothem?
In other words, if the Laminae not formed regularly, is this helpful or a conditoin to avoid for the later paleoclimate study?
Thank you for your comment. The three examples in the presentation aren't from semi-arid environments, but they can be water limited. The first example is a subtropical climate and over the monitoring period there has been a long-term positive water balance. However, recharge is highly episodic, with very high rainfall totals in late summer of some years as this is when the monsoon trough is closest to the region. In contrast, in Western Australia, the final example, the climate is highly seasonal and Mediterranean, and we see a seasonal water cycle of surplus and deficit.
Back to your question, the implaictions for speleothem deposition will be varied. In the subtropical site, the episodic recharge can lead to continuous dripping at sites with good storage, and event based recharge at sites with low storage. Whereas in the Western Australian sites, we would expect to see a strong seasonality in dripping and annual laminae (which we do see in trace element profiles in the region).
The montane site in the Snowy Mountains (the second example) has recharge every winter, typically starting in April or May, occuring when climate cools and cold and wet frontal systems pentrate to the region. Additional recharge can occur in some, but not all, summer seasons. We have yet to determine the seasonal variation in recharge thresholds at this site, but it is planned to do so.
It is really nice work, which you presented here. A lot of data with much to think about.
I am espeaccialy interested in the d18O model-measurement comparison of your work.
eg. on page 7:
I strongly agree that the modelled WC data fit nicely to the measured data. The variability of d18O and even the values of the maxima are well captured. But I wonder why there is an offset for the minimum values between data and measurements. Do you have any idea, about the reason for that?
Many thanks for your comment.
We can't give a definite answer, but can state that the model only considers karst hydrological processes on water isotopes, , including epikarst evaporation of water, and as no in-cave fractionation component. We also use the Kim and O'Neil equation to convert to calcite oxygen isotope values, rather than an alternative equation that might include in-cave fractionation.
Stalagmite WC has a correlation between O and C isotopes (R-sq=77%) whoch suggestst there is additional in-cave fractionation processes and this could explain the offse between model and observation.
There's a longer set of evidence in Monika's paper (section 5.2.2.). It is paywalled here at Geochimica et Cosmochmica Aca, we can provide it on request.
I hope that answers your question? The model code is published with the paper if you are interested in applying it elsewhere (see )
Apologies for the typos.
Also, the links didn't appear, so if you are interested, the paper is recently published
Markowska, M., Cuthbert, M.O., Baker, A., Treble, P.C., Andersen, M.S., Adler, L., Griffiths, A., and Frisia, S., 2020. Modern speleothem oxygen isotope hydroclimate records in water-limited SE Australia. Geochimica et Cosmochimica Acta, 270, 431-448
and the matlab model code is on Figshare, search for Mark Cuthbert 3847054