Dissolved black carbon in boreal forest and glacial rivers of central Alaska: assessment of biomass burning versus anthropogenic sources

Abstract

Boreal forests are thought to be an important source and sink for pyrogenic carbon, or black carbon (BC), as fire is very common in this type of ecosystem. However, the reported soil BC content in boreal forests is low, suggesting active removal processes of soil BC possibly through transfer to the dissolved phase. In this study, dissolved black carbon (DBC) concentrations from samples collected from streams in Alaska boreal forest watersheds and in glacial rivers are reported. The DBC levels, as well as its degree of polycondensation, in glacial rivers are much lower than those from boreal forest streams. This suggests source differences (fossil fuel vs. forest fires) as a possible reason for these molecular variations. Our hypothesis is that DBC in boreal forest watersheds is mostly generated from forest fires, while glacial rivers will receive a higher relative contribution of BC from atmospheric deposition, resulting in a more fossil fuel like molecular signature of DBC in the latter. To test the source hypothesis, we compared the DBC molecular characteristics of samples from boreal rives and glacier-fed streams with additional samples from locations isolated from significant fire-derived inputs, where atmospheric deposition of anthropogenic soot may be important. Based on this comparative approach, we suggest that anthropogenic BC, associated with small soot particles, is the most prominent source of DBC in the glacial rivers. A linear positive correlation between dissolved organic carbon (DOC) and DBC concentrations was observed for both boreal forest and glacial samples, once again confirming that DOC and DBC dynamics are closely coupled. The ever increasing export of DOC associated DBC from high latitude boreal forests and glaciers as a result of global warming, may impact DOC quality and ultimately trophic dynamics in the receiving marine environment.