Radiocarbon analysis is a powerful tool for understanding the cycling of individual components within carbon pools, such as black carbon (BC) in dissolved organic carbon (DOC). Radiocarbon (δ14C) measurements of BC in DOC provide insight into one source of aged, recalcitrant DOC. We report a modified solid phase extraction (SPE) method to concentrate 43±6% of DOC (SPE-DOC) from seawater. We used the Benzene Polycarboxylic Acid (BPCA) method to isolate BC from SPE-DOC (SPE-BC) for subsequent 14C analysis. We report SPE-BC δ14C values, SPE-BC concentrations, and the relative BPCA distributions from Milli-Q water process blanks, two riverine reference standards, as well as a coastal and an open ocean surface water sample. The composition of BC is less aromatic in the ocean samples than those in the river standards. We find higher BC δ14C values in the river standards (+148 to -462‰) than BC in the ocean samples (-592 to -712‰), suggesting that BC ages within oceanic DOC. We report that BC is 4.2±1.0% of SPE-DOC in the open ocean surface sample, or 1.4±0.1μM C. This work provides the methodological basis by which global BC concentrations, compositions (e.g. relative abundances of BPCA marker compounds) and 14C values can be assessed.

The standard procedure for storing/preserving seawater dissolved organic carbon (DOC) samples after field collection is by freezing (-20°C) until future analysis can be made. However, shipping and receiving large numbers of these samples without thawing presents a significant logistical problem and large monetary expense. Access to freezers can also be limited in remote field locations. We therefore test an alternative method of preserving and storing samples for the measurement of DOC concentrations ([DOC]), stable carbon (δ13C), and radiocarbon (as δ14C) isotopic values via UV photooxidation (UVox). We report a total analytical reproducibility of frozen DOC samples to be [DOC]±1.3 μM, δ14C±9.4‰, and δ13C±0.1‰, comparable to previously reported results (Druffel et al. 2013). Open Ocean DOC frozen versus acidified duplicates were on average offset by δDOC±1.1 μM, δδ14C± -1.3‰, and δδ13C± -0.1‰. Coastal Ocean frozen vs. acidified sample replicates, collected as part of a long-term (380-day) storage experiment, had larger, albeit consistent offsets of δDOC±2.2 μM, δδ14C±1.5‰, and δδ13C± -0.2‰. A simple isotopic mass balance of changes in [DOC], δ14C, and δ13C values reveals loss of semi-labile DOC (2.2±0.6 μM, δ14C=-94±105‰, δ13C=-27±10‰; n=4) and semi-recalcitrant DOC (2.4±0.7 μM, δ14C=-478±116‰, δ13C=-23.4±3.0‰; n=3) in Coastal and Open Ocean acidified samples, respectively.

We report marine dissolved organic carbon (DOC) ?14C from seawater collected from the North central Pacific Ocean (NCP) in 2015. These measurements show DOC ?14C values averaged -235±5‰ (n=3) in the mixed layer (24-81 m) and -544±5‰ (n=5) in the deep water (1500-5139 m). A comparison of these data with two previously published DOC δ14C profiles from the NCP in 1985 and 1987 reveals that deep DOC δ14C values have decreased. We discuss several possible mechanisms that could cause such a shift in DOC δ14C values, including spatial inhomogeneity and temporal variability due to changes in the dissolution and δ14C value of surface derived particles in the deep sea. We find that forthcoming profiles of DOC δ14C results from the NCP will determine the primary mechanisms controlling deep DOC δ14C distributions, and changes over the past three decades.

Marine organic matter is one of Earth's largest actively cycling reservoirs of organic carbon and nitrogen. The processes controlling organic matter production and removal are important for carbon and nitrogen biogeochemical cycles, which regulate climate. However, the many possible cycling mechanisms have hindered our ability to quantify marine organic matter transformation, degradation and turnover rates. Here we analyse existing and new measurements of the carbon:nitrogen ratio and radiocarbon age of organic matter spanning sizes from large particulate organic matter to small dissolved organic molecules. We find that organic matter size is negatively correlated with radiocarbon age and carbon:nitrogen ratios in coastal, surface and deep waters of the Pacific Ocean. Our measurements suggest that organic matter is increasingly chemically degraded as it decreases in size, and that small particles and molecules persist in the ocean longer than their larger counterparts. Based on these correlations, we estimate the production rates of small, biologically recalcitrant dissolved organic matter molecules at 0.11-0.14 Gt of carbon and about 0.005 Gt of nitrogen per year in the deep ocean. Our results suggest that the preferential remineralization of large over small particles and molecules is a key process governing organic matter cycling and deep ocean carbon storage.

Dissolved organic carbon (DOC) is the largest pool of exchangeable organic carbon in the ocean. However, less than 10% of DOC has been molecularly characterized in the deep ocean to understand DOC's recalcitrance. Here we analyze the radiocarbon (14C) depleted, and presumably refractory, low molecular weight (LMW) DOC from the North Central Pacific using atomic force microscopy to produce the first atomic-resolution images of individual LMW DOC molecules. We evaluate surface and deep LMW DOC chemical structures in the context of their relative persistence and recalcitrance. Atomic force microscopy resolved planar structures with features similar to polycyclic aromatic compounds and carboxylic-rich alicyclic structures with less than five aromatic carbon rings. These compounds comprise 8% and 20% of the measurable molecules investigated in the surface and deep, respectively. Resolving the structures of individual DOC molecules represents a step forward in molecular characterization of DOC and in understanding its long-term stability.