Based on carbon isotope ratio measurements in some European rivers (Pawellek and Veizer, 1994) a model of the dissolved inorganic carbon (DIC) acquisition of rivers in general is developed which assigns different DIC-sources depending on geology of the river catchment and the prevailing climate: Rivers draining carbonate-rich catchments yield d13CDIC values around -11, typical for carbonate dissolution involving soil CO2 from decayed organic matter, a process mostly independent of ambient temperature as long as liquid water is available in the soils. Rivers draining carbonate-poor catchments yield low d13CDIC values around -20 at high temperatures as a result of organic matter decay in the soils and show d13CDIC typical of atmospheric CO2 (ca. 0) at low temperature, the latter entering interflow and groundwater by diffusion. These features predict a seasonality of d13CDIC in temperate climates, which can be demonstrated for two Central European rivers (Inn and Ilz, Germany). The carbonate-poor Ilz shows drastic negative d13CDIC excursions in summer, whereas the carbonate-buffered Inn exhibits a stable d13CDIC signal.
The model is applied to previously published d13CDIC data for some of the world's major rivers (Rhine (Buhl et al., 1991), Amazon (Longinelli and Edmond, 1993), Orinoco (Tan and Edmond, 1993), St Lawrence (Yang et al., 1993), MacKenzie (Hitchon and Krouse, 1972)), all of which agree with its predictions. The Rhine (d13CDIC around -11) is dominated by carbonate dissolution in the catchment, involving biogenic soil CO2. The Amazon (d13CDIC around -20), draining mostly carbonate-free tropical forest soils, is controlled by the decay of organic matter in the catchment, imposing the low d13CDIC values. Tributaries of the Orinoco show distinct differences in d13CDIC depending on the occurrence (-10 ) or absence (ca. -18) of carbonates in their catchments. The St Lawrence is decoupled from the catchment control over the DIC cycle by extensive exchange with atmospheric CO2 in the Great Lakes (d13CDIC values near 0) due to the long residence time of the water. The St Lawrence reflects this equilibrium without major changes - a feature, which is also shown by the uppermost sampling sites of the Rhine, being close to Lake Constance. The MacKenzie river basin data point to carbonate dissolution triggered by biogenic soils CO2 being the dominant DIC source, as
in most of the Rhine. The DIC acquisition model may
have effects on global estimations of carbon export from the continents, which have to account for the carbon cycling through the various continental system compartments.
Buhl, D., Neuser, R. D., Richter, D. K., Riedel, D., Roberts, B., Strauss, H. & Veizer, J., Naturwissenschaften 78, 337-346 (1991).
Hitchon, B. & Krouse, H. R., Geochim. Cosmochim. Acta 36, 1337-1357 (1972).
Longinelli, A. & Edmond, L. M., J. Geophys. Res. 88, 3703-3717 (1983).
Pawellek, F. & Veizer, J., Isr. J. Earth Sci. 43, 187-194 (1994).
Tan, F. C. & Edmond, J. M., Estuarine, Coastal and Shelf Science 36, 541-547 (1993).
Yang, C., Telmer, K. & Veizer, J., Joint CSCE-ASCE Nat. Conf. on Environm. Engin., 12.-14.07 1993, Montréal / Québec, Canada, 673-680 (1993).
Fig. 1: left: schematic representation of *13C values of the carbon-species involved in riverine DIC acquisition. right: *13CDIC values previously published by other worker, summarized.