Abstract: Stable isotope ratios of mollusk shell carbonates have long been used to reconstruct past environmental conditions. Although shells also contain organics, they are seldom used in (paleo)climatic studies. Here, we extract the acid soluble and insoluble fractions of the organic matrix of modern Mytilus galloprovincialis shells from three sites along a coast?to-upper estuary environmental gradient, to measure their hydrogen (?2H) and oxygen (?18O) isotope compositions. Both organic fractions showed isotopic signatures significantly different from those of carbonate and water at each site, indicating the involvement of different fractionation mechanisms. The soluble fraction showed gradual differences in isotope values along the gradient, while the insoluble fraction showed ?2H - ?18O correlation regressions subparallel to the Global and Local Meteoric Water Lines. These results showed great potential of the shell organic matrix stable isotopes as possible (paleo)environmental proxies, stimulating further research to better define the fractionation mechanisms involved.

Table of hydrogen and oxygen stable isotope data measured on the acid soluble and insoluble fractions of the shell organic matrix exctracted from modern Mytilus galloprovincialis shells from three sites along an environmental gradient from the beach to the upper estuary.

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