Differential metamorphic effects on nitrogen isotopes in kerogen extracts and bulk rocks

Abstract

The last decade has seen a steady rise in the number of publications on nitrogen isotopes in sedimentary rocks, which have become an established tool for investigating the evolution of life and environmental conditions. Nitrogen is contained in sedimentary rocks in two different phases: bound to kerogen or substituted in potassic minerals (mostly K-bearing phyllosilicates and feldspars). Isotopic measurements and interpretations typically focus either on kerogen extracts alone or on bulk rocks that include both phases. The community is split about which sample type more accurately captures the original composition of the biomass. To address this question, we combined nitrogen isotopes and carbon-to-nitrogen ratios with carbon-to-hydrogen ratios which act as an independent proxy for metamorphic alteration. Our results reveal that metamorphism drives kerogen-bound nitrogen isotopically lighter while silicate-bound nitrogen becomes heavier. For rocks up to greenschist facies, the isotopic effect of this internal partitioning (up to 3-4‰) is larger than the isotopic effect of metamorphic nitrogen loss from the system (up to 1-2‰). The opposite may be true for higher metamorphic grades. We conclude that for low-grade sedimentary rocks with more than 60% of their total nitrogen residing in the silicate phase the primary isotopic composition of the biomass is best approximated by the bulk rock measurement, whereas for high-grade rocks the kerogen extract may be the more accurate proxy. The isotopic difference between nitrogen phases can thus serve as a rough indicator of the degree of metamorphic alteration.

Nitrogen isotope ratios trace high-pH conditions in a terrestrial Mars analog site

“… 15 N does not covary with C/N ratios ( Fig. 3C), indicating that  15 N values have not suffered from metamorphic alteration (27), which is consistent with post-depositional geothermal heating to a maximum of 60°C as inferred from vitrinite reflectance (28). The slight covariance between  15 N and TN ( Fig.…”

Ammonium availability in the Late Archaean nitrogen cycle

“…The rise of atmospheric O2 during the Great Oxidation Event (GOE) at ~2.3 Ga 7 and the subsequent partial oxygenation of the surface ocean is thought to have revolutionised the nitrogen cycle, stimulating widespread nitrification-denitrification. 23 . Detailed sample descriptions and further discussion of data fidelity are provided in the supplementary information (SI).…”

Coupled Nitrate and Phosphate Availability Facilitated the Expansion of Eukaryotic Life at Circa 1.56 Ga

“…In addition, thermal nitrogen volatilization associated with metamorphism may increase the δ 15 N of sediment through preferential removal of 14 N. The associated isotopic fractionation is commonly within 1–2‰ below greenschist facies, 3–4‰ for amphibolite facies, and up to 6–10‰ for the upper amphibolite facies (e.g., Ader et al, 2016; Bebout & Fogel, 1992; Jia, 2006; Mingram & Bräuer, 2001; Stüeken et al, 2017; Thomazo & Papineau, 2013). Previous studies suggest that the metamorphic grade of the Mesoproterozoic succession in North China was below prehnite‐pumpellyite facies (Chu et al, 2007; Li et al, 2003); thus, metamorphism is unlikely to have significantly altered our δ 15 N values.…”