USING STABLE NITROGEN AND CARBON ISOTOPE VALUES OF BIVALVE-SHELL-BOUND ORGANICS TO UNDERSTAND FOOD WEB DYNAMICS

Food webs arrange organisms on the basis of their food or energy source and thus can provide insights about the flow of energy from primary producers to the higher-level consumers. Reconstruction of food webs, however, even in modern environments, is challenging due to the sheer diversity of the living biota and the complex relationships among taxa. In fossil assemblages, trophic positions are assigned largely on the basis of morphology and the ecology of modern analogs. Nitrogen and carbon stable isotopes (δ¹⁵N and δ¹³C) of organic matter can aid in the reconstruction of food webs, and when applied to fossils, can help resolve uncertainty about trophic position and reveal details about basinal conditions and ontogeny.

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.