Surface-Water Seasonality from Stable Isotope Profiles of Littorina littorea Shells: Implications for Paleoenvironmental Reconstructions of Coastal Areas

Abstract

Intra-shell stable oxygen and carbon isotope profiles (180, &13C) were established for the common intertidal gastropod Littorina littorea to test if this species, thriving on the border between land and sea, would be suitable for studies on the paleoseasonality of coastal areas. Comparison of the L. littorea data with isotope records of a Mytilus edulis bivalve from the same locality as the L. littorea shells, and with literature data for M. edulis and M. trossulus, indicates that L. littorea would be useful for stable isotope studies on the coastal marine environment, whether concerning variability in physical (temperature and salinity) or biochemical (phytoplankton productivity) processes. The &180 record representing the period prior to sexual maturity for each L. littorea shell is in agreement with the observed seasonality in surface-water temperature. This indicates that L. littorea precipitates shell calcite also during the temperature minimum in the winter. Responses in the shell 8180 to freshwater peaks in the spring suggest that oxygen isotope profiles of L. littorea also may be useful for detecting seasonal freshwater influx. The &13C values for L. littorea are relatively high (0%o to 2.5%o) as compared with Mytilus (below 0.5%o), which may suggest that the calcite of L. littorea is precipitated close to isotopic equilibrium with &13C of the dissolved inorganic carbon (DIC) of the water. This is probably related to a relatively low contribution of metabolic CO2 to the extrapallial fluid of L. littorea. A negative intrashell correlation between 8180 and 813C is found for L. littorea, reflecting the increased 813C of the surface-water DIC in the summer related to the preferential incorporation of 12C during phytoplankton photosynthesis. No correlation between 8180 and 813C is found for the M. edulis specimen, probably because metabolic CO2 overprints the variations in DIC 813C.