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Abstract

A 3-year systematic investigation (2007–2010) of water chemistry using conventional and stable isotope tracers helped us delineate hydro-biogeochemical processes in the karstic Krka River watershed. By mineralogical and stable isotopic examination of tufa deposits and combining the results with water chemistry, we were able to evaluate the suitability of tufa deposits as recorders of environmental conditions in the time of their precipitation in a complex karstic environment with diffusive groundwater recharge. This is particularly crucial in the palaeoenvironmental studies. The main aims of the study were (1) to quantitatively determine calcite versus dolomite dissolution, carbonate weathering rates and associated CO2 consumption, (2) to provide a detailed insight into the CO2 dynamics in the watershed and (3) to evaluate the potential use of tufa precipitates as natural recorders of environmental conditions.
The Krka water geochemistry is influenced by enhanced carbonate weathering caused by dissolution of calcite and dolomite via carbonic acid within the watershed. The Mg2+/Ca2+ molar ratio in the waters is a useful tool for reconstructing sources of water dissolving different lithological types. A downstream decreasing trend of Mg2+/Ca2+ molar ratios revealed that the headwaters of the Krka are recharged by groundwaters dissolving predominantly dolomite bedrock, whereas the tributaries and groundwater discharging into the main channel in the lower reaches dissolve predominantly calcite/limestone lithology. Dolomite dissolution contributes over 70 % of the total carbonate dissolved load in the headwaters while in downstream sections, dissolution of dolomite contributes only ~ 50 % of the total dissolved carbonate load. Due to high solubility of carbonate bedrock that covers over 80 % of the Krka watershed area and intensive water-rock interaction in the aquifer(s), the carbonate weathering rates (CWR), ranging from 186 t/km2/yr to 293 t/km2/yr and associated CO2 consumption of 16 x 10[SUP]5 mol of C/km2/yr are amongst the highest on a global scale.
As found in numerous studies (named throughout the Discussion section) stable isotopes of oxygen and carbon were found useful for tracing water and dissolved carbon fluxes in the watershed. Mean residence times (MRT) of Krka waters were estimated to range from 1.3 to 4.7 years; the highest MRT were in the upper reaches (average ~ 3.0 years) showing that waters reflect isotopic composition of groundwaters that were in longer contact with the aquifer bedrock compared to those recharging the lower reaches (average MRT 1.6 years). The dissolved inorganic carbon (DIC) in Krka is a mixture of carbon derived from soil CO2 and from carbonate dissolution, which reflects in high DIC (3.61–5.94 mM) and pCO2 content (up to 10-1.2 bars). Based on the δ13CDIC values of the headwaters (average -13.6 ‰) we found that carbonate mineral dissolution primarily occurs under open system conditions. In comparison with the δ13CDIC values in the headwaters, the DIC in the main stream and the tributaries exhibited higher δ (average -12.6 ‰). The enrichment of DIC with 13C downstream ranged from 0.1 ‰ to 2.6 ‰ resulting from CO2 outgassing caused by equilibration between dissolved and atmospheric CO2 rather than photosynthesis or in-stream calcium carbonate precipitation.
The mass balance calculations showed that carbonate dissolution prevails in spring and summer, whereas degradation of organic matter in the soil horizon was found to be more
expressed in autumn and winter. The lowest DIC fluxes were calculated for summer (average 2.0×10[SUP]8 mol/month), while the highest were usually observed in spring and autumn due to increased discharge (average 5.4×10[SUP]8 mol/month and 5.5×10[SUP]8 mol/month, respectively). The estimated total diffusive loss of CO2 ranged from 2.0×10[SUP]8 to 3.0×10[SUP]8 mol of C/yr; the CO2 fluxes were in general the highest in autumn and the lowest in summer. Based on the diffusion boundary layer model, we estimated that ~ 1.3 × 10[SUP]8 mol of C/yr is deposited as calcite in the main stream of Krka.
Tufa in Krka is dominantly low Mg-calcite with significant amount (up to 19 %) of detrital component (calcite, dolomite and quartz). The most prominent macroscopic feature of Krka tufa is its porous structure with cavities of 5 mm to over 1.5 cm in size, whereas the internal structure of presently forming tufa deposits in Krka is strongly related to organic particles recognized as vacuolar facies (after Manzo et al., 2012). Two main types of calcite precipitates were recognized; first is sparry cement calcite filling intergranular space that is clearly of inorganic origin and the second type is fine-crystalline calcite (micrite, microsparite) with dendrolitic or laminated texture that resembles organic growth. Besides sparitic cements, voids contain detrital material, among which quartz and carbonate grains were recognized.
The precipitation of tufa in the Krka River is conditioned by turbulent conditions and high water temperatures rather than elevated Ca2+ and HCO3 concentrations. The presence of biota acts as a substrate for calcite precipitation, whereas detrital component fills the pores lowering the porosity of tufa deposits. The precipitation rates of calcite deposition in Krka River were calculated assuming diffusion boundary layer conditions and considering the density of tufa to be 2.2 g/cm3. The rate of precipitation in Krka River amounts to 0.7 to 2.1 mm/yr, which is consistent with the findings from thin section examination, where we observed that growth rate of tufa in the Krka is from 1 to ~2 mm/yr when assuming a combination of dense and porous layer in the laminated fabric presents a one year cycle of calcite precipitated.
Evaluation of the extent to which tufa deposits reflect environmental conditions using isotope fractionation equations published in the literature highly depends on the temperature constants developed for particular equations. Moreover, δ18O values and Mg/Ca molar ratios in Krka waters are not related to water temperature changes, thus leading to discrepancies in the interpretation of (palaeo)environmental conditions. We found that none of the published fractionation equations fit our situation, which means that Krka tufas are unsuitable for the interpretation/reconstruction of environmental conditions during calcite precipitation based on theoretically postulated facts. This could be mainly due to complex processes associated with isotopic fractionation during tufa precipitation, diffusive groundwater input in the Krka watershed that constantly changes biogeochemical conditions in the water and/or significant fraction of detrital component in tufa.