Distinct Radiocarbon Ages Reveal Two Black Carbon Pools Preserved in Large River Estuarine Sediments.

Black carbon (BC), a group of environmentally concentrated organic pollutants, is widely distributed in marine sediments via riverine run-off and atmospheric deposition. The fate of BC transformation and cycling in marine sediments, however, has not been well studied. Here, we present radiocarbon measurements for sedimentary solid-phase BC (SBC) and porewater-dissolved BC (DBC) in surface sediments collected from the Yangtze and Yellow River estuaries and their adjacent coastal regions. Radiocarbon results revealed that two distinct BC pools in the sediments of the SBC had ancient radiocarbon ages (7110-15,850 years BP) that were 5370-14,935 years older than the 14C ages of porewater DBC. Using a radiocarbon mass balance model, we calculated that modern biomass-derived BC contributed 77-97% of the DBC pool and that fossil material-produced BC accounted for 61-87% of the SBC pools. This discrepancy between modern and dead BC contributions was associated with the BC budget after particulate BC (PBC) deposition; 38 ± 13% of the PBC was transferred to porewater as DBC and 62 ± 13% was sequestrated as SBC in sediments, serving as an important CO2 sink in marine sediments. We also provide evidence suggesting that DBC likely comprises some very fine particulate forms that are not completely dissolved as molecules. The nature and transformation mechanisms of DBC in natural aquatic systems need to be further studied.

Speciation analysis of both inorganic and organic 129I in seawater and its application in the study of the marine iodine cycle.

A complete protocol is presented for the speciation analysis of 129I for both inorganic and organic iodine in seawater using coprecipitation and solid-phase extraction (SPE) combined with accelerator mass spectrometry (AMS). By modifying the iodide separation process and adding a crossover removal step, the improved coprecipitation method significantly reduces the cross-contamination of iodide and iodate to less than 0.05% in the speciation analysis of inorganic 129I, with the separation efficiencies of about 95% and 93% for iodide and iodate, respectively. The SPE-DOI method for the dissolved organic 129I (DO129I) analysis was developed, whereby we report the first direct observation of DO129I/DO127I atom ratios in seawater in this paper. 129I species in seawater from Tokyo Bay were analysed. The 129I results demonstrated that our protocol for speciation analysis of 129I is reliable and provided new insights into understanding the iodine cycle.

Dissolved black carbon is not likely a significant refractory organic carbon pool in rivers and oceans.

Rivers are the major carriers of dissolved black carbon (DBC) from land to ocean; the sources of DBC during its continuous transformation and cycling in the ocean, however, are not well characterized. Here, we present new carbon isotope data for DBC in four large and two small mountainous rivers, the Yangtze and Yellow river estuaries, the East China Sea and the North Pacific Ocean. We found that the carbon isotope signatures of DBC are relatively homogeneous, and the DBC 14C ages in rivers are predominantly young and increase during continuous transport and cycling in the ocean. The results of charcoal leaching experiments indicate that DBC is released from charcoal and degraded by bacteria. Our study suggests that riverine DBC is labile and respired during transport and mixing into the ocean and that residual DBC is cycled and aged on the same time scales as bulk DOC in the ocean.