Decoupling of dissolved organic carbon (DOC) and dissolved black carbon (DBC) in a temperate fluvial network

Black carbon (BC) is a significant component of the global carbon cycle both in terrestrial and aquatic systems. Dissolved black carbon (DBC) is a significant portion of the total dissolved organic carbon (DOC) pool and represents a major flux of recalcitrant carbon to the coastal and deep oceans. Dissolved black carbon can originate from multiple sources related to its relative biogeochemical reactivity with the dynamics of highly recalcitrant DBC integral to long-term sequestration. Thus, understanding how the more recalcitrant fractions of DBC varies in diverse catchments is critical and currently underexplored. We used hydrogen pyrolysis to isolate the fraction of DBC with aromatic clusters above 7 rings, representing the more stable components. Here we report the dynamics of DBC_HyPy over a hydrological year in a temperate catchment, with a long history of coal mining extraction. Quarterly measurements of DBC were undertaken from two main channel and four tributary sites. Hydrogen pyrolysis derived DBC comprised a significant percentage of the total DOC flux (3.2% to 28.3%) and included significant spatial variability. Unlike other studies examining more reactive DBC fractions, bulk DOC concentrations and DBC_HyPy were poorly correlated when considered over an annual scale. Rather, DBC_HyPy was correlated with indicators of groundwater such as dissolved inorganic carbon and conductivity. Data suggest a consistent source of DBC_HyPy not subject to the same mobilisation drivers as DOC, which shows substantial seasonality. Rather, our data shows a potentially consistent supply of stable DBC originating from the coal mining-influenced groundwater. Petrogenic sources of DBC have been poorly constrained to date, the data presented here suggests in some catchments it may be significant and yield catchment scale DOC-DBC decoupling. The dynamics of DBC have implications for carbon fluxes, pollution transport and water quality/treatment requirements. These preliminary findings suggest potentially complex drivers in spatially heterogeneous catchments, contrasting with previous work finding tight DOC-DBC mobilisation dynamics.