Presenter: Josie Mottram (University of Connecticut)

Description:
The N isotope composition (δ¹⁵N) of cold-water corals (CWCs) is a promising proxy to reconstruct past marine N cycling. Recent studies document a strong correlation between the δ¹⁵N of organic material in CWC skeletons and the δ¹⁵N of organic matter exported from the surface ocean. However, a sizeable yet unexplained offset of 8-9‰ between the δ¹⁵N of coral skeleton and N export (Wang et al., 2014)–larger than a typical trophic level offset of about +3.4‰ (Minagawa and Wada, 1984)–raises questions regarding the coral proxy’s reliability. The unexplained offset implies gaps in our understanding of factors that influence the δ¹⁵N recorded in corals’ carbonate matrix, such as the food source(s) of CWCs. We conducted culture experiments with the CWC Balanophyllia elegans in order to evaluate the δ¹⁵N recorded in their tissue compared to that of their diet. Under controlled experimental conditions, we determined δ¹⁵N trophic offsets between diet and tissue of no more than 3.1±0.1‰ and offsets between tissue and skeleton of 1.3 ± 0.5‰. In order to infer the composition of B. elegans’ diet in a natural setting, we collected specimens in the San Juan Channel and compared the δ¹⁵N of coral tissues and skeletons to that of ambient nitrate, of suspended particulate organic N collected on filters, and of bulk and size fractionated organic material collected with net tows. Preliminary analyses suggest that coral tissue samples are enriched in δ¹⁵N by 5.7±0.7‰ relative to ambient nitrate and corresponding skeletons by 7.6±0.5‰ to nitrate. We will present a complete isotopic N budget of corals and their potential food sources, further clarifying the relationship between diet and δ¹⁵N recorded in CWC tissue and skeleton, and we will discuss implications of our findings for the CWC as an archive of paleoceanographic proxies. References Minagawa and Wada. 1984. Geochim. Cosmochim. Acta 48:1135-1140 Wang et al. 2014. Earth and Planetary Science Letters 400:243-250