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Condensed aromatic nitrogen (ConAN) is commonly found in soil and aquatic environments. ConAN is broadly viewed by the biogeochemistry community as a combustion-derived product leading to its frequent designation as pyrogenic “black” nitrogen. Here, we provide evidence that ConAN can also form through nonpyrogenic pathways. In an experiment simulating biomass oxidation, pine wood boards assembled with iron nails were exposed to 12 years of natural environmental conditions. Nitrogen concentrations increased from 0.2 to 1.2 wt % with increasing proximity to the nail. Ultrahigh resolution mass spectrometry showed enrichment of nitrogen-containing compounds within the lignin and condensed aromatic domains. X-ray spectroscopic characterization revealed the development of C–N bonds and formation of pyridinic and pyrrolic nitrogen moieties. We propose plausible nonpyrogenic formation mechanisms for ConAN and perform back-of-the-envelope calculations suggesting that biomass oxidation yields a nonpyrogenic ConAN flux to soils of 3–63 Tg-N yr–1. These findings demonstrate that ConAN can form under fire-free environmental conditions, highlighting nonpyrogenic ConAN as an underrecognized but potentially major component of the global nitrogen cycle. More broadly, this study reveals a previously ignored pathway for the long-term incorporation of nitrogen into refractory “stable” organic matter in soils and other environments. 

The three-isotope system of oxygen (16O, 17O, 18O) is a powerful tool to study environmental oxidation chemistry and cycling of oxygen-bearing species (e.g., sulfates, nitrates, carbonates, etc.). Despite its evident utility, little work has focused onextending the triple oxygen isotope (Δ’17O) tool to oxygen contained in organic matter (OM). This is largely due to methodological challenges with isolating OM-bound oxygen and preparing it for isotopic analysis. Herein, we report on a newly developed method for high-precision Δ’17O measurements of OM (Δ’17O precision of 0.020‰) and apply this technique to investigate partial photochemical oxidation of Suwannee River natural OM in air-equilibrated aquatic samples. Through this, we reveal that the oxygen isotope evolution of the Suwannee OM supports a model whereby OM partial photo-oxidation proceeds via one or more reactive oxygen intermediates. Our measurements further highlight the potential of triple oxygen isotope analyses on OM-bound oxygen to fingerprint OM oxidation pathways, redox chemistry, and source and synthesis reactions. 

The export of terrestrial organic matter (TOM) to the ocean has been traditionally viewed to be minimal or only of significance in near-coastal continental margins. The broadly accepted explanation for the widespread loss of terrestrial lignin biomarkers and apparent disappearance of the <−25‰ stable carbon isotopic signature (δ13C) of TOM is that TOM is almost fully degraded before reaching the open ocean. Oceanic carbon (δ13C value ≥ −22‰) is therefore thought to be derived primarily from algae. However, an alternative explanation for the different molecular and δ13C signatures in terrestrial and marine environments may be that oxidative processing transforms TOM to appear marine-like. To test this hypothesis, we subjected eight TOM samples to a strong oxidation gradient. At carbon losses typical of TOM degradation during transport and sedimentation (above 60%), the differentiators of provenance disappeared, leaving a residue that was marine-like both chemically (aliphatic- and nitrogen-rich) and isotopically (δ13C enrichment of 4 to 9‰). This challenges the validity of conventional two-endmember mixing models, suggesting that a much larger fraction than previously estimated of the organic matter found in the ocean may originate from terrestrial sources, impacting global models of carbon cycling and sequestration. 

The geochemical characterization of phytoplankton-derived organic compounds found in marine sediments has been widely used to reconstruct atmospheric pCO2 thoughout the Cenozoic. This is possible owing to a well-established relationship between the carbon isotope ratios of phytoplankton biomass and CO2 concentration in the ambient seawater. An ideal molecular target for such proxy reconstructions would be degradation resistant on geologic timescales and unambiguously associated with known, experimentally tractable, organisms, so that species-specific models can be developed, calibrated, and applied to appropriate material. However, existing organic matter targets do not quite meet these criteria, primarily owing to ambiguity in the source species of recalcitrant compounds in deep time. Here we explore the potential of a novel organic carbon target for isotopic analysis: acidic polysaccharides extracted from the calcite plates (coccoliths) that are produced by all calcifying haptophytes. Carbohydrates are usually rapidly remineralized in sediments, but coccolith-associated polysaccharides (CAPs) are mechanically protected from diagenesis within the coccolith calcite lattice. Coccoliths can be taxonomically separated by size and identified, often to species level, prior to CAP extraction, providing a species-specific record. Coccolith morphology and composition are important additional sources of information, which are then unambiguously associated with the extracted CAPs. We find that carbon isotope ratios of CAPs changed in response to the environmental changes associated with a glacial cycle, which we attribute to temperature-driven changes in average growth rate. Once the underlying biosynthetic processes and the associated isotope effects are better understood, this archive of pristine organic matter has the potential to provide insight into phytoplankton growth rates and atmospheric pCO2 far beyond the Cenozoic, to when the first coccolithophores inhabited the surface ocean over 200 million years ago. 

Abstract Geochemical records of ancient periods of warm climate can be useful to help understand the looming effects of modern anthropogenic warming, including changes to biogeochemical nutrient cycles. Stable nitrogen isotope compositions of marine sediments archive the balance of processes in the global nitrogen cycle. However, the unusual isotopic signals of Mesozoic oceanic anoxic events (OAEs) remain enigmatic, thus hindering our understanding of nitrogen cycle processes and dynamics under conditions of ocean deoxygenation. Here, we present an ammonium “nutrient capacitor” model of the water-column nitrogen cycle to explain the anomalously negative isotopic compositions seen in Mesozoic OAE sediments. Our model applies isotopic inferences derived from high-resolution records of Lake Kivu sediments to show how periodic chemocline overturning of redox-stratified water columns during Mesozoic OAEs may have delivered ammonium to the photic zone in excess of primary producer requirements. Smoothed, stochastic sampling of the changing fluxes within the nitrogen cycle across these events can simulate OAE nitrogen isotope records.