Nutrient regime shift in the western North Atlantic indicated by compound-specific δ¹⁵N of deep-sea gorgonian corals

Despite the importance of the nitrogen (N) cycle on marine productivity, little is known about variability in N sources and cycling in the ocean in relation to natural and anthropogenic climate change. Beyond the last few decades of scientific observation, knowledge depends largely on proxy records derived from nitrogen stable isotopes (δ¹⁵N) preserved in sediments and other bioarchives. Traditional bulk δ¹⁵N measurements, however, represent the combined influence of N source and subsequent trophic transfers, often confounding environmental interpretation. Recently, compound-specific analysis of individual amino acids (δ¹⁵N-AA) has been shown as a means to deconvolve trophic level versus N source effects on the δ¹⁵N variability of bulk organic matter. Here, we demonstrate the first use of δ¹⁵N-AA in a paleoceanographic study, through analysis of annually secreted growth rings preserved in the organic endoskeletons of deep-sea gorgonian corals. In the Northwest Atlantic off Nova Scotia, coral δ¹⁵N is correlated with increasing presence of subtropical versus subpolar slope waters over the twentieth century. By using the new δ¹⁵N-AA approach to control for variable trophic processing, we are able to interpret coral bulk δ¹⁵N values as a proxy for nitrate source and, hence, slope water source partitioning. We conclude that the persistence of the warm, nutrient-rich regime since the early 1970s is largely unique in the context of the last approximately 1,800 yr. This evidence suggests that nutrient variability in this region is coordinated with recent changes in global climate and underscores the broad potential of δ¹⁵N-AA for paleoceanographic studies of the marine N cycle.