Transformations of carbon in anoxic marine sediments: Implications from Δ14C and δ13C signatures

Organic-rich nearshore sediment was incubated in sealed vessels under sulfate-reducing conditions to investigate the mechanism of dissolved organic carbon (DOC) production in marine sediments. Concentrations and isotopic signatures (delta C-13 and Delta C-14) of particulate organic carbon (POC), DOC, dissolved inorganic carbon (DIC), and particulate inorganic carbon (PIC) were monitored for 130 d. POC solubilization was largely counterbalanced by respiration, resulting in a net increase in DIC of > 35 mmol L-1. Net accumulation of DOC was relatively muted, yet significant, at similar to 0.04 mmol L-1. All carbon pools exhibited distinct delta C-13 and Delta C-14 signatures prior to the incubation. Once the incubation began, these isotopic values varied with time, reflecting exchanges of isotopically distinct moieties across carbon pools. The C-14-enriched (modern) component of bulk POC was selectively solubilized into DOC, and the majority of this DOC was rapidly respired to DIC. However, net accumulation of DOC was accompanied by a drop in Delta C-14, suggesting that during selective solubilization of the younger component of bulk POC, there was concomitant solubilization of pre-aged organic matter that subsequently accumulated as DOC. PIC was minor in terms of pool size, but likely played a critical role in determining the delta C-13 signature of pore-water DIC through isotope exchange.