Award: OCE-1949984

Coral skeletons, which provide the framework for vibrant and biodiverse coral reef ecosystems in the surface and deep ocean, also provide archives of ocean history. Similar to tree rings, coral skeletons incorporate chemical elements and isotopes that reflect the environment in which they grow. We call the elements and isotopes that are sensitive to the environment “proxies”. 

One deep-sea coral proxy that has been in development over the last ~10 years is the nitrogen isotope proxy for the marine nitrogen cycle. Having proxies for marine nitrogen is very important. Nitrogen is a limiting nutrient in the oceans, which means that if nitrogen is scarce, it can restrict the growth of organisms and even whole ecosystems. Alongside carbon and other nutrients, nitrogen is incorporated into phytoplankton as they grow in the surface ocean. Some of these phytoplankton sink and decay and others are consumed by organisms higher up in the food chain. As a result, nitrogen and carbon are transported from the surface ocean to the deep ocean as part of the ocean’s ‘biological pump’. Given the close link between nitrogen and carbon, reconstructions of the marine nitrogen cycle in the past can help us understand the cycling of carbon, global climate, and biodiversity through time. Importantly, existing research has identified that the isotopic composition of the nitrogen in the organic material (referred to as ‘particulate organic matter’) transported from surface to deep ocean serves as a tracer for the key nitrogen cycle processes.

While we can learn about marine nitrogen from a range of different proxies, deep-sea coral skeletons are particularly useful archives of proxies because they can be accurately dated using radiometric techniques, are found globally from the tropics to the polar regions, and they are large, which means it is possible to reconstruct multiple environmental variables (seawater pH, sample age, temperature) all from one sample. 

The deep-sea coral proxy for the marine nitrogen cycle relies on measuring nitrogen isotopes (expressed using the notation δ15N) in small amounts of organic matter encapsulated in the coral skeleton. This organic matter is thought to be produced by the coral animal, which itself acquires nitrogen from its food source (assumed to be particulate organic matter sinking from the surface ocean). Most heterotrophic organisms have a slightly elevated δ15N compared to that of their food source, which is referred to as a ‘trophic offset’. The magnitude of a typical trophic offset in animals is ~3 parts per thousand. Our study sought to explain the origins of the unusually large trophic offset observed for deep-sea corals (~ 8.5 parts per thousand). This offset exists between the δ15N of coral skeletons and that of particulate organic nitrogen in regions of coral growth.  

We sought to understand whether this offset was due to coral-specific biochemistry, environmental causes like intermittent starvation, or an unknown dietary source. More broadly, a key aim of this work was to improve the existing mechanistic understanding of how the signatures of marine nitrogen isotopes are transmitted to the coral skeleton. 

Our work involved (1) experiments growing live deep-sea corals under controlled conditions, (2) field work to collect live deep-sea corals and their local seawater, and (3) laboratory measurements of δ15N in water and coral samples. Through lab experiments, we discovered that deep-sea corals exhibit a very typical (~3 parts per thousand) trophic offset which was unchanged by long periods of starvation. Combined with field data, our results suggest that marine nitrogen processes are captured by coral skeletons through their consumption of large zooplankton as a major component of their diet. In other words, the large observed trophic offset for these corals resulted from corals feeding directly on large zooplankton, leading to multiple trophic offsets stacked on top of each other. 

We then conducted an experiment to understand the dynamics of nitrogen incorporation into organic matter within deep-sea coral skeletons by feeding the coral a diet with elevated levels of a rare nitrogen isotope at calibrated time intervals. Key results of this experiment suggest that deep-sea corals rapidly (within hours) incorporate a small portion of dietary nitrogen directly into their skeletons and that organic matter incorporation into the skeleton is likely triggered by feeding. This work improves our mechanistic understanding of how nitrogen is incorporated into the coral skeleton and also improves our understanding of how coral skeletons are built more generally. 

To date, the project has produced one peer-reviewed publication, one manuscript in review, and a third paper in development. One graduate student and one post-baccalaureate student, and ~15 undergraduate students participated in this work and were trained in coral biomineralization, paleoclimatology, isotope geochemistry, and ocean biogeochemistry. In addition research projects with live corals were incorporated into two courses at St. Olaf College, and students produced video documentaries about their work which were shared with local public schools. 

 

 

 

Last Modified: 05/12/2025
Modified by: Anne Gothmann