Earth’s sedimentary record has preserved evidence of life in rocks of low metamorphic grade back to about 3.2–3.5 billion years ago (Ga). These lines of evidence include information about specific biological metabolisms, permitting the reconstruction of global biogeochemical cycles in the early Archean. Prior to 3.5 Ga, the geological record is severely compromised by pervasive physical and chemical alteration, such as amphibolite-granulite facies metamorphic overprinting. Despite this alteration, evidence of biogenic organic matter is preserved in rare localities, including meta-turbidites from the 3.8 to 3.7 Ga Isua Supracrustal Belt, Western Greenland. But detailed insights into metabolic strategies and nutrient sources during the time of deposition of these Eoarchean meta-sedimentary rocks are lacking. Here we revisit the Isua meta-turbidites and provide new data for metal abundances as well as organic carbon and nitrogen isotope values. Our results reveal mixing between authigenic and detrital nitrogen phases with the authigenic phase likely fractionated by metamorphic degassing. Rayleigh fractionation models of these 3.7 Ga samples indicate pre-metamorphic δ15N values of between −1 and −10‰. The most plausible initial values are below −5‰, in agreement with a prior study. While the upper endmember of −1‰ could indicate biological N2 fixation at 3.7 Ga, the more plausible lighter values may point toward a distinct biogeochemical nitrogen cycle at that time, relative to the rest of Earth’s history. In light of recent experimental and phylogenetic data aligned with observations from the modern atmosphere, we tentatively conclude that lightning and/or high-energy photochemical reactions in the early atmosphere may have contributed isotopically light nitrogen to surface environment(s) preserved in the Isua turbidites. In this case, recycling of Eoarchean sediments may have led to the isotopically light composition of the Earth’s upper mantle dating back to at least 3.2 Ga.

Thirteen commercially available silicate reference materials (RM) and one in‐house reference material, eleven of which have no previously published values, were analysed for nitrogen mass fraction and isotopic ratios with an Elemental Analyser (EA), and a Sealed Tube Combustion line, coupled to a continuous flow isotope ratio mass spectrometer (IRMS). These materials ranged from < 10 μg g−1 to 1% m/m nitrogen mass fractions and δ15N of −0.5 to +19.8‰. Existing nitrogen RM BHVO‐2, MS#5 and SGR‐1b were used to assess the accuracy of the data from the sealed tube combustion line, which was found to be in good agreement with existing published values. In contrast, the EA‐IRMS failed to fully liberate nitrogen from all silicate rocks and minerals (achieving a mean of 44 ± 10% nitrogen yield) resulting in kinetic fractionation of isotope values by −1.4‰ on average. Therefore, sealed tube combustion is better suited for analyses of silicate‐bound nitrogen. The EA worked reliably for organic samples, but care should be taken when using the EA for silicate nitrogen research. Moving forward, it is recommended that BHVO‐2, Biotite‐Fe, FK‐N and UB‐N be used as quality control materials as they appear to be most reproducible in terms of nitrogen mass fraction (relative error < 10%, 1s), and isotopic composition (< 0.6‰, 1s).

The Archean ocean supported a diverse microbial ecosystem, yet studies suggest that seawater was largely depleted in many essential nutrients, including fixed nitrogen. This depletion was in part a consequence of inefficient nutrient recycling under anoxic conditions. Here, we show how hydrothermal fluids acted as a recycling mechanism for ammonium (NH4+) in the Archean ocean. We present elemental and stable isotope data for carbon, nitrogen, and sulfur from shales and hydrothermally altered volcanic rocks from the 3.24 Ga Panorama district in Western Australia. This suite documents the transfer of NH4+ from organic-rich sedimentary rocks into underlying sericitized dacite, similar to what is seen in hydrothermal systems today. On modern Earth, hydrothermal fluids that circulate through sediment packages are enriched in NH4+ to millimolar concentrations because they efficiently recycle organic-bound N. Our data show that a similar hydrothermal recycling process dates back to at least 3.24 Ga, and it may have resulted in localized centers of enhanced biological productivity around hydrothermal vents. Last, our data provide evidence that altered oceanic crust at 3.24 Ga was enriched in nitrogen, and, when subducted, it satisfies the elemental and isotopic source requirements for a low-N, but 15N-enriched, deep mantle nitrogen reservoir as sampled by mantle plumes.

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