Open Access Senior Thesis
Bachelor of Arts
2022 Sylvia A Bergerud
In the face of anthropogenic climate change, increasing pressure is being mounted on contextualizing current climate altercations compared to those of the past. One key tracer of climate variation is the isotopic ratio of 15N compared to 14N, which tracks biological and chemical reactions throughout the N cycle. However, it is difficult to find accurate palaeoceanographic records of N isotope fractionation, as most sedimentary and organic samples are subject to extensive diagenesis, degradation, and erosive processes. Scleractinian corals present a potential solution to this lack of accurate data. Multiple studies indicate that, due to their protective aragonite skeletons and quality of being rooted to the seafloor, these corals are exempt from many of the contamination and deterioration issues of other sample types. Yet challenges persist when trying to interpret coral-bound nitrogen in corals as proxies of their environment. Unlike other organisms, corals do not display a consistent ~3.5 ‰ trophic offset in N isotope ratio compared to their diet of POM (particulate organic matter). Instead, their offset is approximately ~8.5 ‰, which is 2-3 ‰ higher than the ubiquitous trophic offset of ~3.5‰, but it is unclear what factors are responsible for this difference. Although multiple theories exist, this study examines the potential influence of preferential uptake of suspended PON (particulate organic nitrogen) by corals as a source of higher d15N relative to sinking PON. Combined with the expected ~3.5 ‰ trophic offset, we hypothesized that because suspended PON has high δ15N, this preferential feeding could result in higher δ15N in corals. However, our results indicate that this is not the case. Instead, the unexpected offset could result from high δ15N in starting nitrate in addition to a higher trophic diet than expected. This study provides a starting point from which to pursue nitrate and zooplankton as determining factors in CB- δ15N.
Bergerud, Sylvia, "Nitrogen Isotopes in Deep Sea Corals; A Potential Tracer of Paleoceanographic Conditions" (2023). Scripps Senior Theses. 2151.