Recrystallization of shell carbonate in soil: 14C labeling, modeling and relevance for dating and paleo-reconstructions

Abstract

Mollusk shells are commonly present in a broad array of geological and archaeological contexts. The shell carbonate can serve for numerical age determination (Δ14C) and as a paleoenvironmental indicator (δ18O, δ13C). Shell carbonate recrystallization in soils, however, may re-equilibrate the carbon (C) isotopic signature with soil CO2. The equilibration dynamics remain poorly understood because of the absence of suitable experimental approaches. Here we used the artificial 14C-labeling technique to study the process of shell carbonate recrystallization as a function of time.Organic-free and organic-containing shell particles of Protothaca staminea were mixed with loess or a carbonate-free loamy soil. The mixtures were placed in air-tight bottles, where the bottle air containing 14CO2 (pCO2=2%). The 14C activity of shells was measured over time and related to the recrystallization of shell carbonate.Recrystallization of shell carbonate already began after one day. The recrystallization rates were 10−3%day−1 in organic-containing shell embedded in soil and 1.6·10−2%day−1 in organic-free shells in loess. Removal of organic compounds increased shell porosity, and so, increased the contact surface for exchange with soil solution. Organic-free shells recrystallized much faster in loess (0.56% in 56days) than in other treatments. Recrystallization was 2 to 7 times higher in loess (in the presence and absence of organic compounds, respectively) than in carbonate-free soil. Loess carbonate itself can recrystallize and accumulate on shells, leading to overestimation of shell carbonate recrystallization. A model for shell carbonate recrystallization as a function of time was developed. The model considers the presence or absence of organic compounds in shell structure and geogenic carbonates in the embedding matrix. The model enabled all results to be fitted with R2=0.98.The modelled time necessary for nearly full recrystallization (95% of shell carbonate) was 88years for organic-free shells in loess and up to 770years for organic-containing shells in carbonate-free soil. After this period, the original isotopic signature will vanish completely and will be replaced by a new δ13C and Δ14C signature in the shell structure. Thus, shell carbonate recrystallization may proceed relatively rapidly in terms of geologic time. This is necessary to consider in the interpretation of dating results and paleoenvironmental reconstructions.