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

Abstract

Developing high resolution, well-dated marine proxies of environmental, climatic, and oceanographic conditions is critical in order to advance our understanding of the ocean's role in the global climate system. While some work has investigated bulk and compound specific stable nitrogen isotopes (δ15N) in bivalve shells as proxies for environmental variability, the small concentrations of nitrogen found in the organic matrix of the shell calcium carbonate (CaCO3) makes developing high resolution records challenging. This study investigates the potential of using the bulk and amino acid δ15N of bivalve periostracum, the protein layer on the outside of the shell, as a proxy archive of nitrogen cycling processes and water source variability. Bulk δ15N values were measured on the periostracum, aragonitic CaCO3, and adductor muscle of Arctica islandicashells collected in the Gulf of Maine. Increased variability of isotopic values across growth lines compared to along growth lines support mechanistic reasoning based on growth processes that periostracum is recording changes in δ15N over the course of the clam's lifetime (up to 500 years). In addition, the statistically significant relationship between periostracum δ15N and contemporaneous carbonate δ15N of the same shell (r= 0.82, p Compound specific δ15N analyses of the periostracum of A. islandica shells were used to determine that the calculated trophic position of the clams in this study (1.4±0.4) did not change significantly between 1783 and 1997. Phenylalanine δ15N values over the last 70 years show similar trends to that of the bulk record, suggesting that changes in bulk δ15N of that time period are related to changes in baseline δ15N. Periostracum δ15N values from shells collected in the western Gulf of Maine have decreased by ∼1‰ since the mid-1920s. This trend (-0.008‰/year) is not statistically different from the trend of previously published δ15N values of deep-sea corals from the entrance to the Gulf of Maine over the same time period. This coral record has been shown to indicate a shift in water mass source to the region and therefore the similarity between the two records suggest that changes in periostracum δ15N values are reflecting broader North Atlantic hydrographic changes. Our study introduces a new, high-resolution and absolutely dated paleoceanographic proxy of baseline δ15N, presenting the opportunity for future reconstructions of aspects of nitrogen cycling and water source changes in the global oceans.

Reconstructing Seasonal and Baseline Nitrogen Isotope Ratios in Riverine Particulate Matter Using Freshwater Mussel Shells

“…In addition, Whitney et al. (2019), showed a strong linear relationship between δ 15 N values of contemporaneously formed CBOM and periostracum in marine bivalves. While it is possible to measure δ 15 N CBOM values at relatively high resolution (see Gillikin et al., 2017; Lueders‐Dumont et al., 2018), the low percentage of organic matter–∼2% organics and 0.2% nitrogen by mass (our calculations; Carmichael, 2004; Carmichael et al., 2008; Goulletquer & Wolowicz, 1989)—often makes it difficult to precisely measure δ 15 N CBOM values without very large sample sizes (∼25 mg shell carbonate per δ 15 N measurement) or newly developed techniques to increase instrument sensitivity (Das et al., 2020).…”

“…Because of this, shell organic matter should record changes in the δ 15 N value of riverine environmental nitrogen and, importantly, allow us to build a detailed history of dietary δ 15 N values derived from SPOM using a time‐series stored in shell material. Because mollusk shells are dense and resistant to weathering, they are able to store these geochemical records across a spectrum of time scales, from modern (Gillikin et al., 2017; Whitney et al., 2019), to historical (Black et al., 2017; Gillikin et al., 2017) to archeological (Andrus, 2011; Darrow et al., 2017; Hallmann et al., 2013; Oczkowski et al., 2016), to Quaternary (Engel et al., 1994; Estrella‐Martinez et al., 2019; Goodfriend, 1988; Goodfriend & Ellis, 2000). This history can be extracted after the fact, without extensive long‐term monitoring, and it can cover decades with seasonal to weekly resolution.…”

Using modern and paleoceanographic isotopic systems to reconstruct Late Holocene temporal oceanographic variability in the rapidly warming Gulf of Maine

“…A. islandica shells are also valuable proxies because these grow in oxygen isotopic equilibrium and record changes in oceanic carbon Reynolds et al, 2017], and nitrogen [Whitney et al, 2019] isotopic systems (see Section 1.4 for a more detailed description of these systems). This fact means that these isotopes measured in A. islandica shells are representative of environmental isotopic signatures at the time of shell formation and therefore can be used to reconstruct past components of the oceanic and climatic systems.…”

“…Each chapter has been written to be a stand-alone journal article, which has been submitted or will be submitted to a peer-reviewed, scientific journal. Chapter 2 is entitled "Using stable isotopes as tracers of water masses and nutrient cycling processes in the Acta [Whitney et al, 2019]. This chapter investigates the periostracum -the outer, protein rich layer of A. islandica shells -as a proxy for nitrogen composition of water.…”

Late Quaternary paleoecology of the North American midcontinent

“…When a download button is clicked, a download progress bar appears in the bottom right corner. (Cerling 1984;Lipp et al 1991;McDermott 2004;West et al 2006;Baker et al 2017), ascertain migratory patterns (Rubenstein et al 2002;Rubenstein and Hobson 2004;Hobson 2016), match organisms to their environments (Bearhop et al 2004;Hogan et al 2014) or diets (Hilderbrand et al 1996;Richards et al 2000;Bearhop et al 2003;Divine et al 2017;Patterson et al 2019), assess food web bioenergetics (Bunn and Boon 1993;Hobson, Piattt, and Pitocchelli 1994;Vander Zanden, Casselman, and Rasmussen 1999;Hershey et al 2017), document ecosystem and environmental changes through time (O'Reilly et al 2003;Reynolds et al 2016Reynolds et al , 2017Whitney et al 2019), measure soil carbon budgets (Hsieh and Yapp 1999;Bridgham et al 2006), assess soil microbial activity (Boschker and Middelburg 2002;Steinbeiss, Gleixner, and Antonietti 2009), and other topics.…”