Visualising Organics and Trace Elements in Scleractinian Cold-water Corals through Isotope Labelling Culture Experiments

Organic matter (OM) in scleractinian coral skeletons has recently become a target for paleoclimate reconstruction. For example, the 𝛅15N of carbonate bound organic N has become a promising proxy for marine N cycling [1]. In order to capitalise on the proxy potential of OM in coral skeletons, we still need to fully characterise (1) the fine-scale skeletal distribution of organic compounds, (2) the relationship between skeletal OM and coral geochemistry and (3) the incorporation mechanisms of OM in coral aragonite.

We designed experiments to track N incorporation into cold-water coral skeletons. Individuals of the cup coral B. elegans were cultured under controlled temperature and pH for 16 days and fed with 15N-labelled Artemia. During the experiment, corals were also labelled using dyes and trace element-enriched seawater at discrete intervals. This method enabled comparisons of 15N incorporation relative to skeletal growth and geochemistry. In preparation for geochemical analysis, samples were embedded in epoxy, polished and coated with 30 nm gold. Cultured samples were measured using Nanoscale Secondary Ion Mass Spectrometry (NanoSIMS) with both O- and Cs+ beams at the Environmental Molecular Sciences Laboratory (PNNL). We then analysed chemical maps of trace elements (6Li+, 23Na+, 24Mg+, 39K+, 44Ca+, 55Mn+, 88Sr+) and organic-associated isotope ratios (12C15N-/12C14N- , 12C15N-/12C-, 12C14N-/12C-, 24C2-/12C-, 32S-/12C-).

Preliminary results indicate that 15N was successfully incorporated into cultured coral skeletons as shown by enrichments of 15N up to 3 atom %. 15N and other organic-associated isotopes (e.g. 32S [2]) appear to be concentrated in bands perpendicular to the direction of skeletal extension. However, both N and other organic-associated elements do not always appear to be co-located at fine spatial scales, possibly suggesting different incorporation mechanisms and roles in biomineralisation. Comparison of 15N and trace element bands can help us infer average extension rates and possible controls on the incorporation of organics in these corals. Results of this study will ultimately improve our understanding of OM in corals, refining coral paleoclimate proxies that rely on this OM, and aiding in understanding coral biomineralisation.

[1] Wang et al. (2014) GCA.

[2] Cuif and Dauphin (2005) BG.