- Title
- Carbon mass-balance modelling and carbon isotope exchange processes in dynamic caves
- Creator
- Frisia, Silvia; Fairchild, Ian J.; Fohlmeister, Jens; Miorandi, Renza; Spötl, Christoph; Borsato, Andrea
- Relation
- Geochimica et Cosmochimica Acta Vol. 75, Issue 2, p. 380-400
- Publisher Link
- http://dx.doi.org/10.1016/j.gca.2010.10.021
- Publisher
- Elsevier
- Resource Type
- journal article
- Date
- 2011
- Description
- Diverse interpretations have been made of carbon isotope time series in speleothems, reflecting multiple potential controls. Here we study the dynamics of ¹³C and ¹²C cycling in a particularly well-constrained site to improve our understanding of processes affecting speleothem δ¹³C values. The small, tubular Grotta di Ernesto cave (NE Italy) hosts annually-laminated speleothem archives of climatic and environmental changes. Temperature, air pressure, pCO₂, dissolved inorganic carbon (DIC) and their C isotopic compositions were monitored for up to five years in soil water and gas, cave dripwater and cave air. Mass-balance models were constructed for CO₂ concentrations and tested against the carbon isotope data. Air advection forces winter pCO₂ to drop in the cave air to ca. 500 ppm from a summer peak of ca. 1500 ppm, with a rate of air exchange between cave and free atmosphere of approximately 0.4 days. The process of cave ventilation forces degassing of CO₂ from the dripwater, prior to any calcite precipitation onto the stalagmites. This phase of degassing causes kinetic isotope fractionation, i.e. ¹³C-enrichment of dripwater whose δ¹³CDIC values are already higher (by about 1‰) than those of soil water due to dissolution of the carbonate rock. A subsequent systematic shift to even higher δ¹³C values, from −11.5‰ in the cave drips to about −8‰ calculated for the solution film on top of stalagmites, is related to degassing on the stalagmite top and equilibration with the cave air. Mass-balance modelling of C fluxes reveals that a very small percentage of isotopically depleted cave air CO₂ evolves from the first phase of dripwater degassing, and shifts the winter cave air composition toward slightly more depleted values than those calculated for equilibrium. The systematic ¹³C-enrichment from the soil to the stalagmites at Grotta di Ernesto is independent of drip rate, and forced by the difference in pCO₂ between cave water and cave air. This implies that speleothem δ¹³C values may not be simply interpreted either in terms of hydrology or soil processes.
- Subject
- caves; carbon; isotopes
- Identifier
- http://hdl.handle.net/1959.13/937170
- Identifier
- uon:12517
- Identifier
- ISSN:0016-7037
- Language
- eng
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