Establishing an Anthropogenic Nitrogen Baseline Using Native American Shell Middens

Paired bulk organic and individual amino acid δ15N analyses of bivalve shell periostracum: A paleoceanographic proxy for water source variability and nitrogen cycling processes

“…Such records have been identified as crucial in understanding ecosystem scale food web dynamics (Casey and Post, 2011). For example, there have been a wealth of studies recently looking at the potential of mollusks to record changes in nitrogen inputs into an ecosystem due to anthropogenic influences, both in the soft tissue of the bivalve (Watanabe et al, 2009;Graniero et al, 2016) as well as the organic matrix of the carbonate shell (Carmichael et al, 2008;Kovacs et al, 2010;Graniero et al, 2016;Oczkowski et al, 2016;Black et al, 2017). One challenge with this marine proxy is that nitrogen isotopes measured in biogenic marine proxy archives such as bivalves are not only a function of  15 N of dissolved nitrogen substrates (nitrate, nitrite, ammonium and urea) but also a function of their diet.…”

“…This technique has also been used in a paleoceanographic context as a proxy for water mass source changes (Sherwood et al, 2011) and to reconstruct the depth of the nitricline through time (Williams et al, 2017). Despite the possible opportunities that  15 N and CSIA-AA  15 N in bivalve shells offers for paleoceanographic studies, there have only been a few reconstructions using these techniques that extend back beyond the instrumental record (Oczkowski et al, 2016;Darrow et al, 2017;Misarti et al, 2017). Darrow et al (2017) demonstrated that oyster (Crassostrea virginica) shells from Native American shell middens on the estuaries at the mouth of the Mississippi reliably preserved changes in the  15 N baseline over time, dating back to 2100 years before present.…”

Compound‐specific amino acid δ15N values in archaeological shell: Assessing diagenetic integrity and potential for isotopic baseline reconstruction

“…These remains have been commonly used for carbonate‐based oxygen and carbon SI analyses, but far fewer studies have reported data from organic material extracted from shell. While organic matter is typically present as a minor constituent within carbonate shell structures in most species, it has been shown that the concentrations are sufficient in many species for bulk SI analyses …”

“…While organic matter is typically present as a minor constituent within carbonate shell structures in most species, it has been shown that the concentrations are sufficient in many species for bulk SI analyses. [15][16][17][18][19][20][21][22][23][24][25][26] Baseline data derived from shell organic matter have several inherent advantages. First, the trophic positions (TPs) of filter feeding or algal grazing shellfish are clearer than for most higher TP consumers, and may also be less susceptible to issues such as ontogenetic isotope ratio variation, providing a more direct connection between measured isotope ratios and the base of local food webs.…”

“…For example, carbonate shell matrices decalcify over long periods of time, 28 meaning that either bacterial degradation (i.e., the selective degradation of native matrix organic matter) or the introduction of exogenous organics might alter organic bulk matter isotope ratios. A number of papers have reported extractions of bulk organic matter or protein from shell for bulk SI analysis using bivalve species, 15,17,18,[20][21][22][23][24][25][26] as well as mussels, urchins, or limpets. 29,30 One way to address such issues is to use molecular level, as opposed to bulk, organic and isotopic analyses.…”