Determination of radiocarbon in marine sediment porewater dissolved organic carbon by thermal sulfate reduction

Fractional abundances of 14C and 13C in dissolved organic carbon (DOC) in sediment porewaters may hold important clues about organic carbon cycling in sediments. Yet there is a dearth of isotopic signatures for porewater DOC because of the difficulty associated with oxidizing DOC in seawater. At present, marine DOC can be processed for analyses of 14C and 13C with high precision and minimal contamination by ultraviolet (UV) oxidation, but this method is resource intensive and could be difficult to implement. To resolve this, a thermal sulfate reduction (TSR) method, previously developed to determine 13C in seawater DOC (Fry et al. 1996), was modified and tested for 14C and 13C determination in porewater DOC. CO2 yields from six test materials ranged between 90% and 108%. δ13C signatures were reproducible within ± 0.4‰ and agreed with independently known values. The 14C and δ13C values of porewater DOC obtained by TSR and UV oxidation showed excellent agreement. Application of TSR to archived porewater DOC samples from a sediment incubation experiment revealed notable changes in isotopic values that were not readily discerned in the DOC concentration data alone. The total uncertainty in the 14C values after blank correction was ± 0.005 to 0.02 fraction modern (Fm) for sample sizes ranging between 143 and 560 µg C. For systems where precision on the order of ± 0.02 Fm is acceptable, TSR could be a viable alternative to UV oxidation for processing small, concentrated marine DOC samples.