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

Tomoko Komada*, Jonathon A. Polly and Leah Johnson

Limnol. Oceanogr., 57(2), 2012, 567-581 | DOI: 10.4319/lo.2012.57.2.0567

ABSTRACT: 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 (δ13C and Δ14C) 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 ∼ 0.04 mmol L-1. All carbon pools exhibited distinct δ13C and Δ14C 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 14C-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 Δ14C, 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 δ13C signature of pore-water DIC through isotope exchange.

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