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The role that western boundary currents (WBCs) play in Earth's climate system by transporting heat and salt from low-to higher latitudes is well-known (

The stable isotopes of nitrogen ( 14 N and 15 N) can offer important insights into present and past changes in the cycling of this key element through organisms, food webs, and environments (

The analyses of the stable isotope ratios of carbon (δ13C), nitrogen (δ15N), and oxygen (δ18O) in animal tissues are powerful tools for reconstructing the feeding behavior of individual animals and characterizing trophic interactions in food webs. Of these biomaterials, tooth enamel is the hardest, most mineralized vertebrate tissue and therefore least likely to be affected by chemical alteration (i.e., its isotopic composition can be preserved over millions of years), making it an important and widely available archive for biologists and paleontologists. Here, we present the first combined measurements of δ13C, δ15N, and δ18O in enamel from the teeth of modern fauna (herbivores, carnivores, and omnivores) from the well-studied ecosystem of Gorongosa National Park (GNP) in central Mozambique. We use two novel methods to produce high-precision stable isotope enamel data: (i) the “oxidation-denitrification method,” which permits the measurement of mineral-bound organic nitrogen in tooth enamel (δ15Nenamel), which until now, has not been possible due to enamel’s low organic content, and (ii) the “cold trap method,” which greatly reduces the sample size required for traditional measurements of inorganic δ13Cenamel and δ18Oenamel (from ≥0.5 to ≤0.1 mg), permitting analysis of small or valuable teeth and high-resolution serial sampling of enamel. The stable isotope results for GNP fauna reveal important ecological information about the trophic level, dietary niche, and resource consumption. δ15Nenamel values clearly differentiate trophic level (i.e., carnivore δ15Nenamel values are 4.0‰ higher, on average, than herbivores), δ13Cenamel values distinguish C3 and/or C4 biomass consumption, and δ18Oenamel values reflect local meteoric water (δ18Owater) in the park. Analysis of combined carbon, nitrogen, and oxygen stable isotope data permits geochemical separation of grazers, browsers, omnivores, and carnivores according to their isotopic niche, while mixed-feeding herbivores cannot be clearly distinguished from other dietary groups. These results confirm that combined C, N, and O isotope analyses of a single aliquot of tooth enamel can be used to reconstruct diet and trophic niches. Given its resistance to chemical alteration, the analysis of these three isotopes in tooth enamel has a high potential to open new avenues of research in (paleo)ecology and paleontology.

Nitrogen isotopes are widely used to study the trophic position of animals in modern food webs, however, their application in the fossil record is severely limited by degradation of organic material during fossilization. In this study, we show that the nitrogen isotopic composition of organic matter preserved in mammalian tooth enamel (δ15Nenamel) records diet and trophic position in modern and fossil ecosystems. The δ15Nenamel of modern African mammals shows a trophic enrichment of 3.7 ‰ between herbivores and carnivores, as well as a strong positive correlation between δ15Nenamel and δ15Nbone-collagen values from the same individuals. δ15Nenamel values of Late Pleistocene fossil teeth record expected dietary patterns, despite complete diagenetic loss of collagen in the same specimens. We demonstrate that δ15Nenamel represents a powerful new paleodietary proxy that could help delineate major dietary transitions in ancient vertebrate lineages, such as the onset and intensification of animal resource use in early hominins.

Nitrogen isotopes are widely used to study the trophic position of animals in modern food webs; however, their application in the fossil record is severely limited by degradation of organic material during fossilization. In this study, we show that the nitrogen isotope composition of organic matter preserved in mammalian tooth enamel (δ15Nenamel) records diet and trophic position. The δ15Nenamel of modern African mammals shows a 3.7‰ increase between herbivores and carnivores as expected from trophic enrichment, and there is a strong positive correlation between δ15Nenamel and δ15Nbone-collagen values from the same individuals. Additionally, δ15Nenamel values of Late Pleistocene fossil teeth preserve diet and trophic level information, despite complete diagenetic loss of collagen in the same specimens. We demonstrate that δ15Nenamel represents a powerful geochemical proxy for diet that is applicable to fossils and can help delineate major dietary transitions in ancient vertebrate lineages.

The stable isotopes of nitrogen ( 14 N and 15 N) can offer important insights into present and past changes in the cycling of this key element through organisms, food webs, and environments (

<p>The Agulhas Current in the southwest Indian Ocean is the strongest western boundary current on Earth. The major role of the Agulhas Current in driving significant heat and salt fluxes is well known, yet its biogeochemical fluxes remain largely uncharacterised. Here, we use nitrate isotopes (δ<sup>15</sup>N, δ<sup>18</sup>O, and Δ(15-18) = δ<sup>15</sup>N-δ<sup>18</sup>O) to evaluate nutrient supply mechanisms that ultimately support new production in the southwest Indian Ocean. Across the greater Agulhas region, thermocline nitrate-δ<sup>15</sup>N is lower (4.9-5.8‰) than the underlying Subantarctic Mode Water source (δ<sup>15</sup>N of 6.9‰) and the upstream source regions (where nitrate-δ<sup>15</sup>N ranges from 6.4-7.0‰), which we attribute to local N<sub>2</sub> fixation. Using a one-box model to simulate the newly-fixed nitrate flux, we estimate a local N<sub>2</sub> fixation rate of 7-25 Tg N.a<sup>-1</sup>, amounting to ~30-95% of the whole Indian Ocean nitrogen gain estimated by models. Thermocline and mixed-layer nitrate Δ(15-18) is also low, due to both N<sub>2</sub> fixation and coupled partial nitrate assimilation and nitrification. This local nitrogen cycling imprints an isotopic signal on Indian Ocean nitrate that persists in Agulhas rings that “leak” into the South Atlantic and are subsequently transported northwards. If this signal is retained in calcifying organisms (e.g., foraminifera) deposited on the seafloor, it could be used to trace past Agulhas leakage, yielding quantitative insights into the strength of the Atlantic Meridional Overturning Circulation over time. In addition to local N<sub>2</sub> fixation, the nitrate isotopes reveal three physical mechanisms of subsurface nitrate supply: i) inshore upwelling driven by the current and winds, ii) entrainment at the edges of a mesoscale eddy, and iii) density-driven overturning at the current edge induced by strong horizontal velocity and density shears. All these nitrate supply mechanisms are evident as incidences of relatively high-Δ(15-18) nitrate in the thermocline and surface yet the intensity and subsurface expression of some of them is not apparent in the physical data, highlighting the utility of the nitrate isotopes for exploring physical ocean processes. The high mesoscale variability that likely drives subsurface nitrate supply to Agulhas Current surface waters is common to all western boundary currents, implying that vertical nitrate entrainment is quantitatively significant in all such systems. We posit that along with N<sub>2</sub> fixation, physical mechanisms of upward nitrate supply enhance ocean fertility and possibly carbon export in the South Indian Ocean. Higher rates of warming, and thus thermal stratification, are expected to decrease Indian Ocean productivity more rapidly in the future than that of other ocean basins. However, a coincident increase in eddy kinetic energy across boundary currents may enhance the upward nutrient supply, partially offsetting the stratification-driven decline in productivity.</p>