BASELINE ISSUES IN THE STABLE ISOTOPIC TROPHIC AND DIET STUDY OF FOSSIL ORGANISMS

The heterogeneous distribution of stable isotopic signatures among organic and inorganic materials sets up an isotopic baseline that can be used to estimate trophic position and trace the dietary sources of organisms. The relatively small biological fractionation of carbon (<1‰) makes δ¹³C useful for tracing the relative contribution of these sources, or end-members, whereas the relatively large biological fractionation of nitrogen (~3.4‰) makes δ¹⁵N useful for calculating trophic position (DeNiro and Epstein, 1981; McCutchan et al., 2003; Post, 2002). Both aquatic and terrestrial organisms commonly obtain nutrients from at least two distinct sources (e.g., littoral and pelagic primary producers in aquatic systems; C₃ and C₄ plants in terrestrial systems) which have distinct δ¹³C and δ¹⁵N signatures. These isotopic signatures are known to vary greatly in modern environments and shift with lateral changes along shore, onshore-offshore gradients, water depth, rates of primary productivity, latitude, season, altitude, rainfall, humidity, etc. In contrast, very little is known about the inter-annual variation of isotopic signatures at the base of the food web which is crucial for adequately addressing issues of isotopic baseline in time averaged fossil assemblages. This study documents the variation in isotopic signatures of modern mollusk tissues from Long Island Sound inter-annually and along a steep eutrophication gradient. Observed between-year variation in δ¹³C signature is on the order of 6-7 ‰. This magnitude of variation has severe implications for the use of stable isotopic trophic methods on mollusks in all but the most intensively dated and least time averaged fossil assemblages. Along the eutrophication gradient, a 3-4 ‰ δ¹⁵N shift occurs between New York City and Rhode Island due to the increased nitrogen loading in the west. While reconstructing molluscan food webs would be extremely useful, data on just the isotopic baseline of an area through time may yield important ecological information on changes in biogeochemistry relevant to conservation and management efforts. Archaeological shell midden material is a great potential source for such data because it has the distinct advantage of being relatively recent, is often well-dated, and in some cases deposited within a single year.