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

Twenty-one C-14 accelerator mass spectrometric analyses were obtained for t hree Holocene stalagmites from the Uamh-an-Tartair cave (Sutherland, Scotla nd) in order to estimate the past dead carbon proportion (dcp). Results sho w 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, whi ch would have led to a delta C-13 increase. Comparison with samples from other sites in Europe shows no intersite corre lation between dcp and delta C-13, but a relatively good intersite correlat ion is observed between dcp and average site temperature. Thus, temperature may be a major factor controlling the production of old soil organic matte r CO2 and, therefore, the dead carbon content of seepage water, In contrast to the Scotland stalagmites, two other Holocene samples from sites in sout hern France and Belgium exhibit a good correlation between delta C-13 and d cp, which can be explained in terms of variations in the intensity of limes tone dissolution. Consequently, delta C-13 variations observed in stalagmit es 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 c ontrolled by the soil organic matter age and, in some cases, by the intensi ty of the limestone dissolution. Conversely, a largely constant speleothem delta C-13 signal, as observed for the Scotland stalagmites, does not neces sarily imply that surface climate and vegetation conditions were stable sin ce the dcp variations. in this case, are clearly related to the peat bog de velopment during stalagmite growth. Copyright (C) 2001 Elsevier Science Ltd .