Sources and cycling of dissolved black carbon (DBC) in rivers and ocean revealed by carbon isotopes

Abstract

Dissolve black carbon (DBC) has been recently recognized as an important fraction of dissolved organic carbon (DOC) in both rivers and ocean. It is estimated that about 10% of the riverine DOC transported by the world rivers could be DBC. The sources and fate of DBC in both rivers and ocean, however, is not well known. In this study, we present radiocarbon (¹⁴C) and stable carbon isotope (¹³C) measurements of DBC in several large rivers in China, and in coastal and open ocean waters. DBC was concentrated using solid phase extraction (SPE) method and quantified by chemothermal oxidation (CTO) method.

Concentrations of DBC varied in rivers depending on the drainage basin of the river and accounted for 3.7-7.6% of the riverine DOC pool. DBC was slightly lower, accounted for 2.9-5.9% of DOC in coastal and open oceans. Carbon isotope results indicate that DBC δ¹³C values were all slightly enriched (by 2-3‰) than the values of DOC in both rivers and ocean. The DBC Δ¹⁴C values varied largely in rivers and the values were significantly higher than DOC Δ¹⁴C values in rivers but similar to DOC Δ¹⁴C values in the ocean. Using a two-end member isotope mass balance model, we calculated that the most DBC (80%) with relatively young ¹⁴C ages in the rivers was derived from biomass burning. Laboratory incubation studies also found that DBC released from recent charcoal was able to be utilized by bacteria, supporting the speculation that river transported DOC could be decomposed during estuaries mixing. Our study suggests that DBC is cycled in the same time scales with the DOC pool in the ocean and no extremely older DBC was identified as reported in other studies previously.