Geochemical gradients within modern and fossil shells of Concholepas concholepas from Northern Chile: An insight into U-Th systematics and diagenetic/authigenic isotopic imprints in mollusk shells

Seriate geochemical measurements through shells of one modern, one Holocene , and two Sangamonian Concholepas concholepas, from marine terraces of Nort hern Chile, were performed to document diagenetic vs. authigenic geochemica l signatures, and to better interpret U-series ages on such material. Subsa mples were recovered by drilling from the outer calcitic layer to the inner aragonitic layer of each of the studied shells. Unfortunately, this sampli ng procedure induces artifacts, notably the convertion of up to similar to 20% of calcite into aragonite, and of up to similar to 6% of aragonite into calcite, as well as in the epimerization of a few percent of isoleucine in to D-alloisoleucine/L-isoleucine, Negligible sampling artifacts were notice d for stable isotope and total amino acid contents. Diagenetic effects on t he geochemical properties of the shells are particularly pronounced in the inner aragonitic layer and more discrete in the outer calcitic layer. The t ime-dependent decay of the organic matrix of the shell is illustrated by a one order of magnitude lower total amino acid content in the Sangamonian sp ecimens by comparison with the modern shell. Conversely, the Sangamonian sh ells U contents increase by a similar factor and C-13-O-18 enrichments as h igh as 2 to 3 parts per thousand seem also to occur through the same time i nterval possibly due to partial replacement of aragonite by gypsum. The dec ay of the organic matrix of the aragonitic layer of the shell is thought to play a major role with respect to U-uptake processes and stable isotope sh ifts. Nevertheless, asymptotic Th-230-ages (similar to 100 ka) in the inner U-rich layers of the Sangamonian shells, and U-234/U-238 ratios compatible with a marine origin for U, suggest U-uptake within a short diagenetic int erval, when marine waters were still bathing the embedding sediment. Thus, U-series ages on fossil mollusks from such a hyper arid environment should not differ much from the age of the corresponding marine unit deposition. H owever, the diagenetic enrichments in stable isotopes raise concerns about their use for paleoenvironmental reconstructions under such climate conditi ons. Copyright (C) 2000 Elsevier Science Ltd.