Nitrogen isotopes in scleractinian corals: Modern ocean studies and paleoceanographic applications

Abstract

Nitrogen is an essential nutrient for every living organism and and the status of biologically available N in the ocean affects the ocean's biological productivity and thus the concentrations of CO2 and O2 in the atmosphere. This dissertation studies the cycling of N in both the modern and past ocean, by developing and applying a novel proxy: the nitrogen isotopes of organic matter bound within the coral carbonate skeleton (coral skeletal 15N, where 15N = [(15N/14N)sample/(15N/14N)air]-1). There are, generally speaking, two types of corals: one living in the sunlit surface ocean, typically in symbiosis with an internal photosynthetic algal population; and another living in the dark, deep ocean. This dissertation studies both types of corals and can be divided accordingly, with the two topics unified by methodology and the ultimate goal of understanding both modern and past environmental conditions.

First, a new method is established for analyzing the coral skeletal 15N, requiring only 5 mg of coral carbonate and yielding a precision of 0.2‰. With this method, the distribution and controlling factors of coral skeletal 15N in the modern ocean are explored for both surface and deep sea corals. A broad range in skeletal 15N (~10‰) is observed in the modern ocean for both types of corals. Further, it is found that the skeletal 15N in surface symbiotic corals are primarily controlled by the 15N of N sources supplied to the reefs. Based on a study of the corals around Bermuda, it is inferred that the efficiency of the internal N recycling between the coral host and its symbionts can lead to a modest (<2‰) variation in skeletal 15N between offshore corals and corals in more food-rich inshore waters on a given reef. The skeletal 15N in deep sea corals is found to be dominantly controlled by the 15N of sinking N exported from the surface ocean, but with the direct N source to the corals being the suspended particulate N that derives from the sinking particles. Based on these results, both types of corals can be used to study the marine N cycle in the past.

This novel proxy was applied to address two questions about the past ocean. In the first application, the skeletal 15N of more than 250 Southern Ocean deep sea fossil corals indicates nearly complete consumption of nitrate in both the Antarctic and Subantarctic Zones of the Southern Ocean during the Last Glacial Maximum, supporting the hypothesis that the entire Southern Ocean contributed to lowering the atmospheric CO2 concentration during the glacial periods. In the second application, skeletal 15N records from offshore Bermuda indicate that anthropogenic N deposition has not had a clear influence on the North Atlantic N cycle since the Industrial Revolution.

This dissertation points the way to a vast range of environmental questions that can be addressed with living and fossil corals.