Astronomy and Astrophysics – Astrophysics
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27..222g&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 222
Astronomy and Astrophysics
Astrophysics
3
Scientific paper
The boundary clays from several marine K/T sites are known to be enriched in nitrogen and to show marked shifts in nitrogen isotopic compositions (1, 2, 3). At Woodside Creek, New Zealand, a 20-fold increase in nitrogen concentration has been observed together with a corresponding shift in delta^15N of +10o/oo, (1, 2). Similarly at Caravaca and Bidart, Spain, ^15N enrichments of between +12 and +18%o have been found (3) although the same study found no ^15N enrichments at Gubbio, Italy, or Stevns Klint, Denmark. The source of this excess nitrogen, which can result in N/C ratios up to 0.1 is still unknown (typical N/C values for sediments are around 0.01). It has been suggested that it may be a signature of nitric acid rain or reflect a rapid onset of anoxic conditions in the world's oceans. In order to try and distinguish between these possibilities we have measured the abundance and isotopic composition of nitrogen and examined the molecular organic geochemistry of samples from a nonmarine site in the Northwest Territory of Canada, Police Island . A total of 10 samples from across the K/T boundary at Police Island were solvent-extracted using MeOH/CH2Cl2; a kerogen/elemental carbon fraction was isolated by HF/HCl digestion of silicates. Analysis of the nitrogen content and isotopic composition of the kerogen reveals an enrichment in nitrogen in the fireball layer and a corresponding shift in delta^15N. Nitrogen concentrations increase by ca. 8-fold to 11,600 ppm in the 0.3-cm-thick fireball layer over the underlying ejecta layer. The lowermost Tertiary samples show a progressive decrease to 6200 ppm then 1800 ppm over the next 2-3 cm. delta^15N values shift by +3% from 3.3% in the ejecta layer to +6.2% in the fireball layer. Both the abundance and isotopic variations are in the same direction as observed at marine sites, though of lesser magnitude. The total MeOH/CH2Cl2 extract was analyzed by gas chromatography/mass spectrometry. The boundary and lowermost Tertiary samples contain a series of n-alkenes, presumably of bacterial origin. However, the gas chromatogram of the fireball layer sample is dominated (50% of the total ion current signal) by a single compound. The mass spectrum of this compound is distinguished by the presence of strong fragment ions at m/z 59 and 72, characteristic of an aliphatic amide. Using m/z 59 as a marker for amides the ejecta layer and lowermost Tertiary samples were analyzed by selected ion monitoring. Trace quantities of the same amide were found in the ejecta layer but no amides were found in the Tertiary samples. Apparently the amide is restricted to the fireball layer. The enrichment in nitrogen observed at the K/T boundary together with the variable enrichment in ^15N is apparently global but not universal. It is not restricted to marine sediments though the enrichments in ^15N would appear to be greater in marine sediments. The onset of anoxic conditions in the oceans could account for the enrichments observed at marine sites; however, for terrestrial sediments this is clearly not possible. The coincidence of the enrichment in nitrogen in the fireball layer and the presence of significant quantities of a single aliphatic amide in this horizon suggest an association between the two. The shock production of HNO2 and HNO3 (4) from N2 and O2 coinciding with the fireball may result in the enrichment in the fireball layer in nitrogen. The reaction of this "acid rain" with dead or dying organic matter may explain the presence of the amide. References 1. Gilmour et al. (1988) Meteoritics 23, 269. 2. Gilmour et al. (1990) In: Global Catastrophes in Earth History. (Sharpton, V. L. and Ward, P. eds). pp. 383-390. 3. Robert et al. (1990) Meteoritics 25, 401. 4. Prinn and Fegley (1987) Earth Planet. Sci. Lett. 83, 387-396.
Gardner A.
Gilmour Iain
Hildebrand Arne
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