14C in speleothems
Final Report Abstract
The difference between the 14C activity of both cave drip water and speleothem calcite and the atmospheric 14C level is mainly controlled by processes in the unsaturated soil zone, i.e., production of CO2 from soil organic matter and carbonate dissolution. The process of calcite dissolution depends on the partial pressure of CO2 (pCO2) of the soil air and the dissolution mode, i.e., in an open, closed or intermediate system. To estimate the ages of the different soil reservoirs and to calculate the recent soil air 14C content, annual cycles of cave drip water as well as a stalagmite top grown during the last 100 years were analysed. The annual cycle of 14C in cave drip water mirrors the processes in the soil above the cave. The investigation of the present-day situation was complemented with the investigation of the 14C content of the calcite deposited within the last century on stalagmite ER-77. Application of the model of Genty and Massault (1999), yielded three soil reservoirs with life times of 5, 10 and 140 years. The dead carbon fraction (dcf) was estimated to 11.1% from the prebomb interval. In Holocene stalagmites ER-76 and BU-1 the dcf shows variability between 0 and 15%, suggesting calcite dissolution in open and intermediate systems. We developed a drip water model, which calculates both the concentrations of the different carbon species during limestone dissolution and the 14C and δ13C isotope composition of the solution. The model includes the open and closed calcite dissolution system as well as intermediate modes. We tested the model by comparing the modelled carbon isotopes with the measurements. The comparison shows that the model can reproduce the observed annual cycle in the 14C and is in agreement (despite a small offset) with the range of the δ13C measurements.
