NanoSIMS mapping of skeletal organic matrix relative to aragonite formation in a scleractinian cold-water coral

Organic matrix (OM) molecules are found in coral skeletons and make up ∼ 1 % of the coral skeleton by weight. These molecules play an important role in the coral biomineralization process and can be used as environmental archives. In this study, we visualize the presence and formation of OM in coral skeletons using NanoSIMS imaging of samples from cold-water corals (CWCs) fed with a¹⁵N-enriched diet and incubated in elementally-enriched seawater. We estimate extension rates of 0.8–2.8 μm/day for our samples, consistent with other estimates of skeletal extension in this species, suggesting that our experimental conditions did not have a noticeable deleterious impact on calcification in this study. While NanoSIMS maps reveal that N, S and organic C are mainly concentrated in thin bands parallel to the growth front of the skeleton, these elements are sometimes enriched in distinct patches and/or diffusely present throughout the skeleton. Quantitative analysis reveals that the areas enriched in N, S, and organic C overlap in space, suggesting that these regions are enriched in organic matter. Our NanoSIMS maps may be useful when considering the potential impact of OM contamination on trace element proxies. Analysis of the level and patterns of¹⁵N enrichment in our coral skeletons indicates that part of the N source that forms the organic matrix may exist as a discrete N pool that is either small or has a short turnover rate. Our¹⁵N data, interpreted in the context of the timing of experimental growth, also suggest that heterotrophic feeding may trigger the formation of organic-rich layers within the skeleton, implying that feeding may support biomineralization by encouraging the synthesis of organic bands.