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ABSTRACT Sequential thermal analysis allows for deconvoluting the refractory nature and complexity of carbon mixtures embedded in mineral matrices for subsequent offline stable carbon and radiocarbon (¹⁴C) isotope analyses. Originally developed to separate Holocene from more ancient sedimentary organic matter to improve dating of marine sediments, the Ramped Pyrolysis and Oxidation (RPO) apparatus, or informally, the “dirt burner” is now used to address pressing questions in the broad field of biogeochemistry. The growing interest in the community now necessitates improved handling and procedures for routine analyses of difficult sample types. Here we report on advances in CO₂purification during sample processing, modifications to the instrumentation at the National Ocean Sciences Accelerator Mass Spectrometry (NOSAMS) facility, and introduce sodium bicarbonate procedural standards with differing natural abundance¹⁴C signatures for blank assessment. Measurements from different environmental samples are used to compare the procedure to the different generations of sequential thermal analyses. With this study, we aim to improve the standardization of the procedures and prepare this instrumentation for innovations in online stable carbon isotopes and direct AMS-interface measurements in the future. 

Abstract In Guaymas Basin, organic-rich hydrothermal sediments produce complex hydrocarbon mixtures including saturated, aromatic and alkylated aromatic compounds. We examined sediments from push cores from Guyamas sites with distinct temperature and geochemistry profiles to gain a better understanding on abiotic and biological hydrocarbon alteration. Here we provide evidence for biodegradation of hopanoids, producing saturated hydrocarbons like drimane and homodrimane as intermediate products. These sesquiterpene by-products are present throughout cooler sediments, but their relative abundance is drastically reduced within hotter hydrothermal sediments, likely due to hydrothermal mobilization. Within the sterane pool we detect a trend toward aromatization of steroidal compounds within hotter sediments. The changes in hopane and sterane biomarker composition at different sites reflect temperature-related differences in geochemical and microbial hydrocarbon alterations. In contrast to traditionally observed microbial biodegradation patterns that may extend over hundreds of meters in subsurface oil reservoirs, Guaymas Basin shows highly compressed changes in surficial sediments. 

ABSTRACT This study describes a procedural blank assessment of the ultraviolet photochemical oxidation (UV oxidation) method that is used to measure carbon isotopes of dissolved organic carbon (DOC) at the National Ocean Sciences Accelerator Mass Spectrometry Facility (NOSAMS). A retrospective compilation of Fm and δ 13 C results for secondary standards (OX-II, glycine) between 2009 and 2018 indicated that a revised blank correction was required to bring results in line with accepted values. The application of a best-fit mass-balance correction yielded a procedural blank of 22.0 ± 6.0 µg C with Fm of 0.30 ± 0.20 and δ 13 C of –32.0 ± 3.0‰ for this period, which was notably higher and more variable than previously reported. Changes to the procedure, specifically elimination of higher organic carbon reagents and improved sample and reactor handling, reduced the blank to 11.0 ± 2.75 µg C, with Fm of 0.14 ± 0.10 and δ 13 C of –31.0 ± 5.5‰. A thorough determination of the entire sample processing blank is required to ensure accurate isotopic compositions of seawater DOC using the UV oxidation method. Additional efforts are needed to further reduce the procedural blank so that smaller DOC samples can be analyzed, and to increase sample throughput.