Mussel periostracum as a high-resolution archive of soft tissue δ13C records in coastal ecosystems

Abstract

Stable carbon isotope ratios (δ13C) of mussel soft tissues have been widely used to characterize baseline δ13C isoscapes and identify carbon sources at the base of coastal food webs. Extending soft tissue δ13C records back in time, however, is extremely challenging due to very limited sample availability. Here, we test if the stable carbon isotopic composition of periostracum (the outermost organic layer of the shell) in the Mediterranean mussel (Mytilus galloprovincialis) can be used as an environmental archive, similar to soft tissue records. In general, spatial and seasonal variations of periostracum δ13C values are comparable to those of soft tissues, but apparently the latter are more time-averaged and smoothed. Irrespective of such offset, there is a significant linear correlation between mussel periostracum and soft tissue δ13C values (R2 = 0.608, p < 0.001). Moreover, high-resolution δ13C analysis of mussel periostracum indicates that it integrates much less time than soft tissue due to metabolically inert nature, consequently being able to record predictable events such as tidal changes and also unpredictable ephemeral events in coastal ecosystems. The present study demonstrates the potential of the periostracum as a viable alternative to the most widely used soft tissues in isotopic studies. Most promisingly, given ample collections over time scales up to hundreds of years and usually stored dry, this technique could be used to extend coastal organic carbon δ13C records back in time.