Uni-Bayreuth

Sprungmarken

 

Outgasssing of carbon dioxide from a karst river – insights into the carbon cycle from stable istopes of dissolved inorganic carbon

Robert van Geldern1, Peter Schulte1, Michael Mader1, Johannes A.C. Barth1
1 GeoZentrum Nordbayern, Universität Erlangen-Nürnberg

O 6.12 in Groundwater, soil and surface water interactions

30.05.2014, 15:40-16:00, H19, NW II

The Franconian Alp in Northern Bavaria is a karst terrain formed by upper Jurassic lime­stone underlain by sandstones and shale. The area is drained by several small to medium size streams. To examine the carbon turnover and carbon flux of such river systems the Wiesent river catchment was investigated by major ion analysis and standard hydrological field para­meters together with water stable isotope anal­ysis (δ2H and δ18O) and carbon stable isotopes of the dissolved in­organic carbon (δ13CDIC). Sampling was per­formed regularly from February to November to cover all season.

Except temperature, the river source shows constant values for all parameters that indicate feeding by ground­water from a large karst water body. The δ18O and δ2H values of the river water are almost constant over the entire 65 km course of the river. They show almost no seasonal response with δ18O values between ‑9.0 and ‑9.5‰. The influx of surface water either via small tribu­taries, surface run-off or soil water seems to play a minor role. Compa­rison with the local meteoric water line (LMWL) clearly indicates that the river is fed by modern meteoric recharge.

The total DIC concentration declines by ~2.3mmol/L from the source to the confluence with the Regnitz river. The pH increases from 7.2 to 8.4 at which the largest rise of ~1 pH-unit occurs during the first 5 km downstream. Calculation of the pCO2 from field-determined pH and tempera­ture show large excess in CO2 with respect to the atmospheric concentration of 380 ppmV (Fig. 1). This indicates that loss of carbon is caused by degassing of CO2 from the river and mainly occurs in the headwaters of the stream where pCO2 quickly drops from around 20.000 to 3000 ppmV. This change in DIC concentration is accompanied by an increase of +2‰ of d13CDIC values that is caused by a shift in the relative distribution of the carbon species within the river water.

Chemical and isotope analyses of the river revealed a ground water dominated system with constant gaining conditions (i.e. water from river bank flows into the river) where tributaries and surface water influences are only of minor importance. High excess con­centrations of pCO2 at the source cause a rapid degassing within the first few km of the river course. The observed δ13CDIC changes are con­sequently related to CO2 loss from the river to the atmosphere and not caused by atmospheric CO2 exchange.

 

Figure 1: Downstream evolution of pCO2 calculated from Henry’s law, c(DIC), T and pH. Dashed line at 380 ppmV is atmospheric pCO2. Different symbols correspond to different sampling dates.
Figure 1: Downstream evolution of pCO2 calculated from Henry’s law, c(DIC), T and pH. Dashed line at 380 ppmV is atmospheric pCO2. Different symbols correspond to different sampling dates.



Export as iCal: Export iCal

last modified 2013-11-01