Dead carbon in stalagmites: Carbonate bedrock paleodissolution vs. ageing of soil organic matter. Implications for C-13 variations in speleotherms

Twenty-one C-14 accelerator mass spectrometric analyses were obtained for three Holocene stalagmites from the Uamh-an-Tartair cave (Sutherland, Scotland) in order to estimate the past dead carbon proportion (dcp). Results show that the dcp increases from 22 to 38% from 3780 years ago to the present. Because delta C-13 variation is small within each sample, it is concluded that this dcp increase is the product of the ageing of soil organic matter related to peat bog development above the cave that produced older soil CO2 and not from a more intense dissolution of the surrounding carbonates, which would have led to a delta C-13 increase. Comparison with samples from other sites in Europe shows no intersite correlation between dcp and delta C-13, but a relatively good intersite correlation is observed between dcp and average site temperature. Thus, temperature may be a major factor controlling the production of old soil organic matter CO2 and, therefore, the dead carbon content of seepage water, In contrast to the Scotland stalagmites, two other Holocene samples from sites in southern France and Belgium exhibit a good correlation between delta C-13 and dcp, which can be explained in terms of variations in the intensity of limestone dissolution. Consequently, delta C-13 variations observed in stalagmites are not always due to changes in the vegetation type (C3/C4) as has been commonly assumed; C-13/C-12 variations in speleothem calcite may also be controlled by the soil organic matter age and, in some cases, by the intensity of the limestone dissolution. Conversely, a largely constant speleothem delta C-13 signal, as observed for the Scotland stalagmites, does not necessarily imply that surface climate and vegetation conditions were stable since the dcp variations. in this case, are clearly related to the peat bog development during stalagmite growth