Black carbon (BC), a bi-product of combustion, is a major long-term carbon sink in the Earth system. Known storage pools for BC are marine sediment and soil. Previous studies found significant ¹⁴C age differences between BC and organic carbon in sediments, and projected that BC must reside in an intermediate pool, such as dissolved organic carbon (DOC), before deposition to the sediment. This research applied compound specific radiocarbon analysis (CSRA) of BC using the benzene polycarboxylic acid (BPCA) method, to provide the first estimates of BC cycling in marine DOC.

First, the BPCA method was adapted for CSRA of marine DOC. This method was applied to nine polycyclic aromatic hydrocarbons (PAHs) to examine the oxidation mechanism of the BPCA method. These experiments showed larger BPCAs are preferentially formed for large (>4 ring) PAHs and an average C recovery of 26 ± 7%. Quantification of nitrated BPCAs was found to be essential for accurate assessment of BC. Next, I evaluated the mass and radiocarbon of extraneous carbon (C_ex) added in the processing and isolation of CSRA samples. The C_ex originated equally from column bleed and the processing steps prior to compound isolation. While constant over a few weeks, the mass and radiocarbon signature of C_ex varied over longer time periods and must be frequently re-evaluated.

Finally, the radiocarbon signatures of BC in marine high molecular weight (HMW) DOC samples from a river and five locations in the Atlantic and Pacific Oceans are presented. BC exported from the river was ¹⁴C modern, while ocean samples were uniformly old (average open ocean BC Δ ¹⁴C = -888 ± 25‰, n=6). The concentration of BC in HMW DOC (also known as UDOM) ranged from 0.5 to 3.5% and suggests that a substantial portion of BC should be in the low molecular weight DOC pool. The presence of ¹⁴C-depleted BC in modern HMW DOC demonstrates that there are widely different turnover times for these two pools in the marine carbon cycle.