ABSTRACT
Nitrogen is the main constituent of the Earth's atmosphere, but its provenance in the Earth's mantle remains uncertain. The relative contribution of primordial nitrogen inherited during the Earth's accretion versus that subducted from the Earth's surface is unclear1-6. Here we show that the mantle may have retained remnants of such primordial nitrogen. We use the rare 15N15N isotopologue of N2 as a new tracer of air contamination in volcanic gas effusions. By constraining air contamination in gases from Iceland, Eifel (Germany) and Yellowstone (USA), we derive estimates of mantle δ15N (the fractional difference in 15N/14N from air), N2/36Ar and N2/3He. Our results show that negative δ15N values observed in gases, previously regarded as indicating a mantle origin for nitrogen7-10, in fact represent dominantly air-derived N2 that experienced 15N/14N fractionation in hydrothermal systems. Using two-component mixing models to correct for this effect, the 15N15N data allow extrapolations that characterize mantle endmember δ15N, N2/36Ar and N2/3He values. We show that the Eifel region has slightly increased δ15N and N2/36Ar values relative to estimates for the convective mantle provided by mid-ocean-ridge basalts11, consistent with subducted nitrogen being added to the mantle source. In contrast, we find that whereas the Yellowstone plume has δ15N values substantially greater than that of the convective mantle, resembling surface components12-15, its N2/36Ar and N2/3He ratios are indistinguishable from those of the convective mantle. This observation raises the possibility that the plume hosts a primordial component. We provide a test of the subduction hypothesis with a two-box model, describing the evolution of mantle and surface nitrogen through geological time. We show that the effect of subduction on the deep nitrogen cycle may be less important than has been suggested by previous investigations. We propose instead that high mid-ocean-ridge basalt and plume δ15N values may both be dominantly primordial features.
ABSTRACT
The corneal endothelium of 179 healthy persons (327 eyes), 4-89 years of age. as photographed through a non-contact specular microscope. From the histograms of the cell areas of each eye, other parameters were derived and subjected to computerized statistical analysis. In addition, photographs of both eyes of one person were taken on two different days and examined to determine the reproducibility of the method. The known age-dependent increase in the mean cell area as well as the decrease of the cell density with age could be confirmed. Other parameters that increase with age are the variability of the mean cell area and the maximum cell area; the latter correlates so well with age and is so easily measured that this parameter may facilitate further morphometric evaluations. For these age-dependent correlations not only regression lines but also third-rank regression curves, which seem to fit better, were calculated. Finally, age-independent parameters were found that may serve as potential additional criteria for a qualitative evaluation of the individual corneal endothelium.
