USING STABLE NITROGEN AND CARBON ISOTOPE VALUES OF BIVALVE-SHELL-BOUND ORGANICS TO UNDERSTAND FOOD WEB DYNAMICS
Here we present nitrogen and carbon isotope data from shell associated organic matter, serially sampled from modern Spisula solidissima bivalves. The goal of this study is to develop methodology to test the applicability of this approach for higher resolution paleoecological and paleoceanographic studies using organic matter preserved in the shells of ancient bivalves. Bivalves were oxidatively cleaned and sampled using a dental drill. Resulting powders were analyzed directly for δ15N using nano EA-IRMS or acidified in silver cups prior to analysis. Acid digestion increases the variability among replicates and results in anomalous 15N-enrichment, which suggests the loss of 15N-depleted N compounds and is therefore contraindicated. The ontogenetic δ15N range in a single Spisula specimen is ~10‰ (2.5‰ – 12.2‰). This variation could suggest a shift in the nitrogen isotopic signature of the food source for that individual, could be a true biologic effect associated with a change in ecology, or it could reflect a changing mixture of these trophic and basinal signals. However, a range of ~10‰ encompasses at least 2 trophic levels and further complicates the use of stable isotope technique in determining trophic position. Ongoing work attempts to unravel these potential influences on the δ15N values of shell-associated organic matter.