Produced on land by incomplete combustion of organic matter, black carbon (BC) enters the ocean by aerosol and river deposition. It has been postulated that BC resides in the marine dissolved organic carbon (DOC) pool before sedimentary deposition and may attribute to its great 14C age (1500–6500 14C years). Here we report the first radiocarbon measurements of BC in high molecular weight DOC (UDOM). BC exported from rivers is highly aromatic and <500 14C years old, while open ocean samples contain less aromatic BC and have an age of 18,000 ± 3,000 14C years. The low abundance of BC in UDOM (0.5–3.5%) suggests that it is more labile than presently believed and/or the low molecular weight DOC contains a larger proportion of aged BC.

We report measurements of oceanic black carbon (BC) to determine the sources of BC to abyssal marine sediments in the northeast Pacific Ocean. We find that the average 14 C age of BC is older (by 6200 ± 2200 14 C years) than that of the concurrently deposited non-BC sedimentary organic carbon. We investigate sources of aged BC to sediments by measuring a sample of sinking particulate organic carbon (POC) and find that POC may provide the main transport mechanism of BC to sediments. We suggest that aged BC is incorporated into POC from a combination of resuspended sediments and sorption of ancient dissolved organic carbon BC onto POC. Our BC flux estimate represents~8-16% of the global burial flux of organic carbon to abyssal sediments and constitutes a minimum long-term removal estimate of 6-32% of biomass-derived BC using the present day emission flux.

Quantification of black carbon (BC), carbonaceous material of pyrogenic origin, has typically required either chemical or thermal oxidation methods for isolation from heterogeneous matrices, such as sediment or soil. The benzene polycarboxylic acid (BPCA) method involves chemical oxidation of aromatic structures, such as those in BC, into BPCAs. We revised the BPCA method with the intent to quantify BC in marine dissolved organic carbon (DOC). As part of this work, we evaluated the mechanism and yield of the method using nine polycyclic aromatic hydrocarbons (PAHs) and six BC reference materials. After 8 h of oxidation at 180°C, the average carbon yield was 26 ± 7% C and was not correlated to the molecular weight of the PAH oxidized. The majority of observed BPCAs were nitrated, which has serious implications for the quantification of BC. Smaller PAHs favor the formation of less substituted BPCAs, whereas larger PAHs, such as coronene, favor the formation of more fully substituted BPCAs. Time‐course experiments revealed variations of BPCA distributions over time, favoring less substituted BPCAs with longer oxidation times, whereas the carbon yield exhibited little variation. No decarboxylation of fully substituted mellitic acid (B6CA) was observed during time course experiments. Using the model compound anthracene, a potential internal standard, we propose a mechanism for the oxidation reaction based on time‐course experiment data. Quantification of BC in reference materials revealed that this revision of the BPCA method is significantly more efficient than previous versions and is effective for quantifying both char and soot BC.

ABSTRACT. The Keck Carbon Cycle AMS facility at the University of California, Irvine (KCCAMS/UCI) has developed protocols for analyzing radiocarbon in samples as small as ~0.001 mg of carbon (C). Mass-balance background corrections for modern and 14 C-dead carbon contamination (MC and DC, respectively) can be assessed by measuring 14 C-free and modern standards, respectively, using the same sample processing techniques that are applied to unknown samples. This approach can be validated by measuring secondary standards of similar size and 14 C composition to the unknown samples. Ordinary sample processing (such as ABA or leaching pretreatment, combustion/graphitization, and handling) introduces MC contamination of ~0.6 ± 0.3 g C, while DC is ~0.3 ± 0.15 g C. Today, the laboratory routinely analyzes graphite samples as small as 0.015 mg C for external submissions and 0.001 mg C for internal research activities with a precision of ~1% for ~0.010 mg C. However, when analyzing ultra-small samples isolated by a series of complex chemical and chromatographic methods (such as individual compounds), integrated procedural blanks may be far larger and more variable than those associated with combustion/graphitization alone. In some instances, the mass ratio of these blanks to the compounds of interest may be so high that the reported 14 C results are meaningless. Thus, the abundance and variability of both MC and DC contamination encountered during ultra-small sample analysis must be carefully and thoroughly evaluated. Four case studies are presented to illustrate how extraction chemistry blanks are determined.

Radiocarbon (14C) is a radioactive isotope that is useful for determining the age and cycling of carbon-based materials in the Earth system. Compound specific radiocarbon analysis (CSRA) provides powerful insight into the turnover of individual components that make up the carbon cycle. Extraneous or nonspecific background carbon (Cex) is added during sample processing and subsequent isolation of CSRA samples. Here, we evaluate the quantity and radiocarbon signature of Cex added from two sources: preparative capillary gas chromatography (PCGC, CPCGC) and chemical preparation of CSRA of black carbon samples (Cchemistry). We evaluated the blank directly using process blanks and indirectly by quantifying the difference in the isotopic composition between processed and unprocessed samples for a range of sample sizes. The direct and indirect assessment of Cchemistry+PCGC agree, both in magnitude and radiocarbon value (1.1 ± 0.5 μg of C, fraction modern = 0.2). Half of the Cex is introduced before PCGC isolation, likely from coeluting compounds in solvents used in the extraction method. The magnitude of propagated uncertainties of CSRA samples are a function of sample size and collection duration. Small samples collected for a brief amount of time have a smaller propagated 14C uncertainty than larger samples collected for a longer period of time. CSRA users are cautioned to consider the magnitude of uncertainty they require for their system of interest, to frequently evaluate the magnitude of Cex added during sampling processing, and to avoid isolating samples ≤5 μg of carbon.

The hyperarid core of the Atacama Desert is one of the driest and most inhospitable places on Earth, where life is most commonly found in the interior of rocks (i.e., endolithic habitats). Due to the extreme dryness, microbial activity in these habitats is expected to be low; however, the rate of carbon cycling within these microbial communities remains unknown. We address this issue by characterizing the isotopic composition ((13)C and (14)C) of phospholipid fatty acids (PLFA) and glycolipid fatty acids (GLFA) in colonized rocks from four different sites inside the hyperarid core. δ(13)C results suggest that autotrophy and/or quantitative conversion of organic matter to CO2 are the dominant processes occurring with the rock. Most Δ(14)C signatures of PLFA and GLFA were consistent with modern atmospheric CO2, indicating that endoliths are using atmospheric carbon as a primary carbon source and are also cycling carbon quickly. However, at one site the PLFA contained (14)C from atmospheric nuclear weapons testing that occurred during the 1950s and 1960s, indicating a decadal rate of carbon cycling. At the driest site (Yungay), based on the relative abundance and (14)C content of GLFA and PLFA, there was evidence of possible preservation. Hence, in low-moisture conditions, glycolipids may persist while phospholipids are preferentially hydrolyzed.

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