Anomalous δ15N values in the Neoarchean associated with an abundant supply of hydrothermal ammonium
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Abstract Unusually high δ 15 N values in the Neoarchean sedimentary record in the time period from 2.8 to 2.6 Ga, termed the Nitrogen Isotope Event (NIE), might be explained by aerobic N cycling prior to the Great Oxidation Event (GOE). Here we report strongly positive δ 15 N values up to +42.5 ‰ in ~2.75 – 2.73 Ga shallow-marine carbonates from Zimbabwe. As the corresponding deeper-marine shales exhibit negative δ 15 N values that are explained by partial biological uptake from a large ammonium reservoir, we interpret our data to have resulted from hydrothermal upwelling of 15 N-rich ammonium into shallow, partially oxic waters, consistent with uranium isotope variations. This work shows that anomalous N isotope signatures at the onset of the NIE temporally correlate with extensive volcanic and hydrothermal activity both locally and globally, which may have stimulated primary production and spurred biological innovation in the lead-up to the GOE.Keywords:
Isotopes of nitrogen
δ15N
Isotopes of nitrogen
δ15N
Norwegian
Isotope Analysis
Isotopic signature
Isotopic ratio
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δ15N
Isotopes of nitrogen
Isotope Analysis
Isotope-ratio mass spectrometry
Omnivore
Isotopic signature
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ABSTRACT The stable carbon (δ 13 C) and nitrogen (δ 15 N) isotope values of bone collagen are frequently used in paleodietary studies to assess the marine contribution to an individual's diet. Surprisingly, the relationship between stable isotope these values characteristics and the percentage of marine foods in diet has never been effectively demonstrated. To clarify this relationship, the stable isotope values and radiocarbon dates of nine humans and one sheep from Herculaneum, all who perished simultaneously during the AD 79 eruption of Vesuvius, were determined. Significant differences were found in the radiocarbon dates which are attributable to the incorporation of “old” carbon from the marine reservoir. The magnitude of the observed differences was linearly correlated with both δ 13 C and δ 15 N values allowing the response of each isotope to increasing marine carbon in collagen to be independently verified. Regression analyses showed that for every 1‰ enrichment in δ 13 C and δ 15 N, 56 years and 34 years were added to the radiocarbon age, respectively. Predictions of the maximum marine reservoir age differed considerably depending on which stable isotope was considered. This discrepancy is attributed to some degree of macronutrient scrambling whereby nitrogen from marine protein is preferentially incorporated in collagen over marine carbon. It is suggested that the macronutrient scrambling explains the observed relationship between δ 13 C and δ 15 N from Roman coastal sites and should be considered when interpreting any diet which is not dominated by protein. Nevertheless, without knowing the degree of macronutrient scrambling in different dietary scenarios, the accuracy of dietary reconstructions is severely compromised. Am J Phys Anthropol 152:345–352, 2013. © 2013 Wiley Periodicals, Inc.
δ15N
Isotopes of nitrogen
Isotope Analysis
Carbon fibers
Scrambling
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δ15N
Isotopes of nitrogen
Isotope Analysis
Human bone
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