Modern-ocean ground-truthing of nitrogen isotopes in scleractinian corals: A proxy for surface ocean nutrient conditions

Abstract

Cycling of the essential nutrient nitrogen (N) is governed by a complex suite of transformations that often impart distinctive signals on the isotopic composition (15N/14N; δ15N) of biologically available, or “fixed,” N. Because the marine N cycle is tightly linked to other biogeochemical cycles, the N isotopes can also provide insight into past environmental, ecological, and climatic changes. High-resolution δ15N records have been measured in skeleton-bound organic N in symbiotic stony corals (CS-δ15N). However, although CS-δ15N has been shown to generally reflect the δ15N of N sources to corals, questions remain regarding potential secondary signals from internal N processes that may complicate this proxy. This dissertation uses both laboratory and field approaches to assess how consistently corals record the δ15N of fixed N in both oligotrophic and nutrient-replete environments. In a laboratory experiment on juvenile Porites corals, the most common genus of coral used in geochemical reconstructions, varying feeding frequency and light level has relatively little effect the δ15N difference between the coral and its food, which averages 0.73±0.22‰ and 0.14±0.13‰ for low- and high-fed corals, respectively. A quantitative isotopic model of the coral host/symbiont system presents potential mechanisms for the small (~0.6‰) variations. On the Bermuda platform, across an environmental gradient in food and nutrient availability and δ15N, a multispecies comparison finds that three of the four symbiotic coral species studied track δ15N patterns similarly to benthic autotrophs and heterotrophs, though species-specific δ15N offsets exist. Both studies suggest that nutritional conditions do not alter internal coral-zooxanthellae N cycling in a way that substantially changes coral δ15N. A case study in the nitrate-replete central equatorial Pacific explains the large difference between CS-δ15N and foraminifera shell-bound δ15N observed there as the result of reef-associated water column productivity. The high variability (up to 3‰ in a year) in the CS-δ15N of two decade-long coral cores can reasonably be explained by changes in water column N sources to the corals, driven by the El Niño-Southern Oscillation. The findings of this dissertation bolster confidence in the CS-δ15N proxy in oligotrophic environments, and they support and initiate the application of CS-δ15N to dynamic, nutrient-replete environments.