Stable isotopes in the shell organic matrix for (paleo)environmental reconstructions

Stable isotope ratios of mollusc 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 (δ 2 H) and oxygen (δ 18 O) 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 δ 2 H-δ 18 O correlation regressions subparallel to the Global and Local Meteoric Water Lines. These results showed the great potential of the shell organic matrix stable isotopes as possible (paleo)environmental proxies, stimulating further research to better define the fractionation mechanisms involved. Isotope ratios of mollusk shell carbonates are commonly used to reconstruct past environmental conditions, but the shell organic matrix is often overlooked. Here, the authors find the hydrogen and oxygen isotope compositions of the organic matrix of modern Mytilus galloprovincialis shells from sites along a coast-to-upper-estuary environmental gradient reflect environmental variables.