Water Oxygen Isotope Systematics from Source to Stalagmites. Andy Baker , Ian Acworth, Martin Andersen, Mark Cuthbert, Peter Graham, Cath Jex, Gregoire Mariethoz, Chris Marjo, Monika Markowska , Gabriel Rau, Hamid Roshan, Helen Rutlidge and Pauline Treble [email protected] Overview.
Water Oxygen Isotope Systematics from Source to Stalagmites
Andy Baker, Ian Acworth, Martin Andersen, Mark Cuthbert, Peter Graham, Cath Jex, Gregoire Mariethoz, Chris Marjo, Monika Markowska, Gabriel Rau, Hamid Roshan, Helen Rutlidge and Pauline Treble
d18O of precipitation is well understood
But what do we know about the processes affecting d18O between the surface and the stalagmite?
Changes in d18O during speleothem formation are well understood
Middle figure: Baker A. and Fairchild IJ (2012) Nature Education Knowledge 3(10): 16. Lower figure: Dreybrodt W and Deininger M (2014) Geochimica et CosmochimicaActa 125 433-439
Here we report the first evidence for:
1. Epikarst Evaporation
2. Evaporative Cooling
Figures from Osborne RAL 2010 Geol. Soc. Pub. London Spec Pub 346, 289-308 and Jex CN et al., 2012. Int. J. Speleology, 41 285-298.
Evaporative fractionation of d18O is well known and understood.
Soil water d18O during evaporation is well understood and studied.
The epikarst is well known to delay water movement.
Does evaporation occur here? To what extent is epikarst evaporation important in the fractionation of infiltration water d18O?
Monthly mean drip water and rainfall d18O
Figure from Cuthbert MO et al.2014a Earth and Planetary Science Letters, 395, 194-204
River and Groundwater
All drip waters are:
Heavier than the local river and ground water samples.
Fall on or close to the LMWL, heavier than the weighted mean of precipitation, indicative of evaporation in a high humidity atmosphere (Gonfiantini,R.,1986).
Gonfiantini (1986). Environmental isotopes in lake studies. In: Fritz,P., Fontes, J.Ch. (Eds.), Handbook of Environmental Isotope Geochemistry, vol.2: The Terrestrial Environment. p.113–168.)
Figure from Cuthbert MO et al.2014a op. cit.
Recharge from epikarst stores,
Evaporative enrichment occurs with time
Recharge through fractures,
d18O = ~d18O rainfall
Figure from Cuthbert MO et al.2014aop.cit.
We modelled epikarst evaporation, to demonstrate that this process was necessary to explain the drip water isotope composition. See Cuthbert et al (2014).
Epikarst evaporation summary. For our cave (with a given morphology, recharge frequency, and climate of MAT 17 °C), epikarst evaporation 0.5‰ enriches drip water d18O by ~0.5‰ per month and is the dominant process affecting drip water d18O.
Model cartoon from Cuthbert MO et al.2014aop. cit.
We know that evaporation will cool the water which is being evaporated. However, this process has never been quantified in the cave environment.
This is surprising given that cave relative humidity is known to often be <100%, and that ventilation effects are known and can also cause evaporation.
So we designed an artificial irrigation experiment to look at evaporative cooling.
Figure from Cuthbert MO et al.2014b ScientificReports, DOI: 10.1038/srep05162
Figure from Cuthbert MO et al.2014b,op. cit.
Dreybrodt W and
Deininger M 2014
Evaporative cooling is a function of both wind speed, relative humidity and temperature. In caves, drip rate is also important.
In our experiment, we observed cooling of 0.5 °C (91% RH) and up to 1.5 °C (80-93% RH). Corresponding speleothems will be precipitated out of thermal equilibrium with the surrounding cave atmosphere.
Evaporation is easy to measure in a cave, but rarely measured. Our cave isn’t unusual compared to other studies.
Evaporative cooling summary. Ventilated caves, and caves with RH<100%, are likely to have drip waters which are evaporatively cooled. Speleothem d18Oc will record evaporatively cooled drip water temperature.