Reconstructing stable isotope baselines in paleo-ecosystems: A solution for near shore food webs
Paleoecological research using stable carbon (δ13C) and nitrogen (δ15N) isotopes of bone collagen has become a widely used tool over the past few decades. This research effort involves reconstructing prey sources, trophic levels, past productivity, nutrient sources, and is essential in understanding naturally and anthropogenically driven ecosystem change. One aspect most of these studies have in common, however, is the inability to directly reconstruct the base of the food web due to lack of preservation of plants, especially in the cases of marine ecosystems. This circumstance leads to “floating” food webs since only higher trophic levels are accounted for and it is therefore essential to tie them into data for primary and/or secondary producers. Plant structures (primary producers), do not preserve well, if at all, over long periods of time. However, shellfish (secondary producers) are often preserved and can be used to form the isotopic baseline needed for paleoecological studies. In addition, since shellfish generally live more than one season, the confounding issue of seasonality that one encounters with the use of primary producers in SI analyses can be avoided.
Samples of shell from limpets, chitons, sea urchins, and periwinkle, were crushed with mortar and pestle. Chilled 1 N HCl was added to each sample. Samples were centrifuged, supernatant pipetted off, samples rinsed to neutral, and placed in a freeze drier for 48 hours. Extracted samples were analyzed for δ13C and δ15N composition and percent C and N content with an EA-IRMS. For CSIA-AA, sample powders were prepared by hydrolysis, esterification, and derivitization and then analyzed for nitrogen and isotopic composition and mol% by GC-C-IRMS.
With the exception of limpets, the disparity in bulk SI between modern and archaeological samples, in both δ13C and δ15N is too large to be caused by anything other than diagenesis and/or differences in the composition of the bulk organic matter that was isolated. The δ15N patterns are generally similar in all sample types, indicating that the AA isotopic values are well preserved. Comparing tissue to modern shell, source AA values appear unchanged, while trophic AA appear slightly enriched in δ15N. Together, these results indicate that archaeological shells have great potential as a new archive for reconstructing nitrogen biogeochemical change in littoral systems.