Radiocarbon Studies of Black Carbon in the Marine Environment
The incomplete combustion of biomass and fossil fuels produces black carbon (BC). BC is recalcitrant and serves as a long term holding pool for carbon, with a mean residence time of one to two orders of magnitude greater than unburnt carbon on land. Yet the known sources of BC are far larger than the known sinks, which led to studies of BC in the ocean’s dissolved organic carbon (DOC) reservoir.
The goal of this dissertation was to measure the abundance and residence times of BC in sediment, sinking particles and DOC in order to understand the cycling of BC in the ocean. I found that BC in northeast Pacific sediments was older by 6,200 ± 2,200 14C years than the concurrently deposited sedimentary organic carbon, suggesting that BC ages within DOC for millennia before deposition to sediments. Sinking particulate organic carbon may provide the main transport mechanism of aged BC into sediments.
A solid phase extraction (SPE) technique was modified to extract DOC from seawater, and provided the methodological basis by which BC composition, concentration and Δ14C values were determined. This SPE method isolated 43 ± 6% of the DOC from seawater. I found that the composition of SPE-BC was less aromatic in the ocean samples than those in a set of river standards. The average concentration of SPE-BC in the surface ocean samples was 1.6 ± 0.1 μM and 1.2 ± 0.1 μM in a deep ocean sample. The average 14C age of surface SPE-BC is 4,500 ± 3,000 14C years, and is much older in the deep water sample from the Sargasso Sea (23,000 ± 3,000 14C years). I calculate that the SPE-BC pool is approximately 14 ± 2 Gt C in the open ocean. This value is a minimum estimate, because it does not include the BC content in 57% of the DOC that was not isolated using SPE methods in this work. BC can explain a small part of the 4,000 – 6,500 14C years age of DOC, but is not the major cause. The range of SPE-BC structures and Δ14C values found in this work suggest that oceanic SPE-BC is not homogeneous, but that it contains several distinct pools of BC with widely ranging residence times.