Nitrogen 15–enriched Precambrian kerogen and hydrothermal systems

Abstract

[1] Nitrogen isotope data are reported for kerogen from Precambrian black shales at prehnite-pumpellyite to greenschist facies and for micas from postmetamorphic hydrothermal quartz vein systems. Nitrogen in micas was acquired from N-bearing aqueous metamorphic hydrothermal fluids generated, from breakdown of sedimentary kerogen and dehydration of K-silicates, at deeper crustal levels. The 2.7 Ga kerogens and hydrothermal K-micas yield d 15 N values of 15-24%, compared to existing data of 2-6% in Phanerozoic counterparts. Paleoproterozoic equivalents have intermediate d 15 N of 7-12%, implying a secular decrease in d 15 N of shale kerogen. The 15 N-enriched nitrogen in Archean shales and hydrothermal vein systems cannot be caused either by N isotopic shifts accompanying metamorphism or Rayleigh fractionation because premetamorphic and postmetamorphic samples from the same terrane are both enriched and lack covariation of d 15 N with N, C/N ratios, or metamorphic grade. The magnitude of shifts during progressive metamorphism of sedimentary rocks in previous studies is constrained at <1% to greenschist and about 3% to amphibolite facies. Furthermore, 15 N-enriched values cannot stem from long-term preferential diffusional loss of 14 N as samples were selected from terranes where 40 Ar/ 39 Ar ages are within a few million years of concordant U-Pb ages; nitrogen is structurally bound in micas, whereas Ar is not. It is possible that the 15 N-enriched values stem from a different N cycle in the Archean, with large biologically mediated fractionations, yet the magnitude of the fractionations observed exceeds any presently known, and chemoautotrophic communities tend to depleted values. Earlier results on Archean cherts show a range from À6 to 30%. Given the temporal association of chert-banded iron formation with mantle plumes, the range is consistent with mixing between mantle N 2 of À5% and the enriched marine reservoir identified in this study. We attribute the secular trend from Archean kerogen of 15-24% to Phanerozoic kerogen averaging 3-4% to a secondary atmosphere derived from CI-chondrite-like material and comets with d 15 N of 30-42%. Shifts of d 15 N to its present values of 0% can be accounted for by a combination of sequestration of atmospheric N 2 into sedimentary rocks and mantle degassing of À5% mantle N 2 .

The evolution of Earth's biogeochemical nitrogen cycle

“…Data from the 2.2-2.1 Gyr Ashanti Belt of Ghana (Jia and Kerrich, 2004) support these conjectures, as all samples fall between the notably heavy values of +9 to +13‰. Corroborating δ 15 N bulk data come from the 2.09-1.98 Gyr upper Zaonega Formation in the Fennoscandian Shield (Kump et al, 2011) which also reach +7 to +10‰.…”

Biogenic nitrogen and carbon in Fe‐Mn‐oxyhydroxides from an Archean chert, Marble Bar, Western Australia

“…Furthermore, in chert A458 the C is intrinsically linked to N (Figure 4b). An abiotic production of N as ammonia through the [Hayes et al, 1983;Beaumont and Robert, 1999;Ueno et al, 2004;Jia and Kerrich, 2004;van Zuilen et al, 2005;Kerrich et al, 2006]; Neoarchean BIFs [Beaumont and Robert, 1999;; in metacarbonates of Isua, West Greenland [van Zuilen et al, 2005] which could represent abiotic N; and modern marine organic matter [Shen et al, 2006] Haber-Bosch process is a very slow reaction and requires high temperatures (400-500°C) and pressures (from 200 bar to several MPa) [Asperger, 2003]. There is no indication for such conditions being applicable to the studied samples.…”

“…[4] Recent studies on Precambrian sediments [Boyd and Philippot, 1998;Beaumont and Robert, 1999;Jia and Kerrich, 2004;van Zuilen et al, 2005;Papineau et al, 2005;Ueno et al, 2004;Nishizawa et al, 2005;Shen et al, 2006] showed the high potential of N as a biomarker. Yet, they also showed its limitations which are mainly due to our poor understanding of the origin of N in sediments and of the geochemical behavior of nitrogen during early diagenesis [Freudenthal et al, 2001;Lehmann et al, 2002] weathering [Busigny et al, 2005] and metamorphism [Busigny et al, 2003].…”

Can NanoSIMS probe quantitatively the geochemical composition of ancient organic-walled microfossils? A case study from the early Neoproterozoic Liulaobei Formation

“…6. Linear relationship between age and the logarithmic bulk N/C atomic ratios from the literature (Schopf, 1983;Beaumont and Robert, 1999;Jia and Kerrich, 2004;Van Zuilen et al, 2007). The gray shade represents the 95% confidence interval.…”

“…3.8 billion years old were assessed using a simple linear regression (XLSTAT; AddinSoft software). N/C atomic ratios determined in Precambrian (meta)sedimentary rocks (Schopf, 1983;Beaumont and Robert, 1999;Jia and Kerrich, 2004;Van Zuilen et al, 2007) were considered as dependent variables whereas their ages were used as quantitative explanatory variables. About 50% of the variability observed for Precambrian N/C atomic ratios can be explained by their age (p < 0.001).…”