Cation exchange retards shell carbonate recrystallization: consequences for dating and paleoenvironmental reconstructions

Abstract

The radiocarbon method has been frequently used to date mollusk shell carbonate. The accuracy of estimated ages, however, depends on the degree and completeness of shell carbonate recrystallization. Although the effect of contamination of the shell CaCO3 with environmental carbon (C) is well known, the role of Ca2+ in diagenetic processes remains unclear. Addition of young C to shells during diagenesis occurs in soil solution, where the Ca2+ concentration is in equilibrium with exchangeable Ca2+ and/or weathering of Ca-bearing minerals. While the exchange process takes place within seconds, the dissolution equilibrium requires longer timescales (on the order of months). It has therefore been hypothesized that the dissolution and recrystallization of shell carbonate in soils with higher cation exchange capacity (CEC) should proceed slower compared to those with low CEC. The objective was to determine the effects of soil CEC and exchangeable cations on shell carbonate recrystallization using the 14C labeling approach. Shell particles of the bivalve Protothaca staminea were mixed with carbonate-free sand (CEC=0.37cmol+kg−1) (Sand), a loamy soil (CEC=16cmol+kg−1) (Loam) or the same loamy soil saturated with KCl, where exchangeable cations were replaced with K+ (Exchanged). The high-sensitivity 14C labeling/tracing approach was used to determine carbonate recrystallization rates. Shell carbonate recrystallization after 120days in Loam and Exchanged (0.016 and 0.024mg CaCO3, respectively) showed one order of magnitude lower recrystallization than in Sand (0.13mg CaCO3). A high level of soil exchangeable Ca2+ decreased the solubility of shell carbonate and consequently its recrystallization because the exchange is faster than dissolution. Therefore, soil CEC and cation composition are determinant factors of shell carbonate recrystallization. Shells in soils with low CEC may undergo more intensive recrystallization; hence they may need further pretreatments before the dating procedure.