Experimental determination of natural carbonate rock dissolution rates with a focus on temperature dependency

• We obtained low-temperature dissolution rates of natural carbonate lithologies.
• The activation energies are shown to be lithology dependent.
• Heterogeneities in crystal size may lead to flow pathways.
• Crystal size heterogeneities may affect the long-term denudation rates.

The denudation of carbonate rocks at landscape scale is controlled by factors like mineral composition, temperature, precipitation, runoff, fracture spacing and vegetation cover. Knowledge on carbonate denudation is important in order to understand landscape development and long-term terrestrial/marine carbon transport, but there are few laboratory studies done on weathering rates of natural carbonate rocks under the low temperatures relevant for glacial-interglacial periods. To enhance the understanding of carbonate dissolution kinetics we studied low-temperature dissolution reactions of various natural Triassic carbonate rocks belonging to the Lower Muschelkalk in Germany. We conducted batch and flow-through experiments investigating the direct correlation of dissolution rates with temperature, and to establish whether the fine-grained carbonate rocks (micrite) are more reactive than the coarser-grained sparitic limestones. By increasing the temperature from 5 to 26 °C far-from-equilibrium dissolution rates of micritic and sparitic limestone samples increased from 2.42 × 10− 07 to 10.88 × 10− 07 and 4.19 × 10− 07 to 7.74 × 10− 07 mol m− 2 s− 1, respectively (Specific Surface Areas (SSA) of about 0.006–0.01 m2/g). The dissolution rates of dolomite rock samples varied only slightly from 1.06 × 10− 07 to 2.02 × 10− 07 mol m− 2 s− 1 (SSA approximately 0.002 m2/g) in the temperature range 5–25 °C at circum-neutral pH. The obtained apparent activation energies are in the range of earlier experiments done at higher temperatures, but there is a distinct difference between the calcite in the micrite (~ 51 kJ/mol) and sparitic (~ 20–22 kJ/mol) lithologies, indicating that the dissolution mechanisms are not the same. Using these activation energies in modelling of natural carbonate denudation we see that there is a clear effect of changing temperature, but this is mostly through the increased solubility at lower temperatures and not through the increasing far-from-equilibrium dissolution rates at higher temperatures. Formation of fluid pathways by preferential dissolution of framework calcite crystals is suggested to form infiltration pathways and affect denudation rates. The difference in crystal size between the micritic and sparitic limestones will affect the formation of such pathways (larger crystals may create fewer and larger conduits) and this is expected to be more important for the long-term denudation than the differences in activation energies.