Soil cation exchange capacity: main factor of shell carbonate diagenesis

Shell carbonate diagenesis occurs in interaction with soil solution, where the concentration of Ca2+ is in equilibrium with exchangeable Ca2+ and weathering of Ca-bearing minerals. While the exchange process takes place within seconds, the dissolution equilibrium with Ca-bearing minerals achieves after months. We hypothesized that shell carbonate diagenesis proceeds slower in soils with high cation exchange capacity (CEC) than those with low CEC. The goal of this study was to determine the effects of soil CEC and exchangeable cations on shell carbonate diagenesis. Shells of Protothaca staminea were mixed in glass bottles with 1) carbonate-free sand (CEC = 0.37 cmol+kg-1) (S), 2) a native loamy soil (CEC = 16 cmol+kg-1) (LS) and 3) the same loamy soil saturated with KCl (replacing exchangeable cations with K+) (LSK). Samples were incubated at room temperature for 5, 20, 60 and 120 days. Bottles air was labeled with 14CO2 at the beginning and day 55. 14C was measured at sampling dates in bottles air, soil solutions, bulk soils and shells. Dissolved and exchangeable cations were measured. Shell carbonate diagenesis in LS and LSK (0.016 and 0.024 mg CaCO3, respectively) was one order of magnitude lower than in S (0.13 mg CaCO3). Shell carbonate dissolution and consequently recrystallization decreased at high amounts of exchangeable Ca2+ because exchange is faster than dissolution. Therefore, soil CEC and composition of exchangeable cations is a determinant factor of shell carbonate diagenesis and it should be considered by radiocarbon dating. Because shells in soils with lower CEC undergo more intensive diagenesis, they need further pretreatments before dating. Soil CEC should be also included in shell carbonate diagenesis models. Furthermore, 14C labeling can be used to investigate the rates of minerals weathering - at least for Ca-bearing minerals - and soil acidification.