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

Abstract

Fractional abundances of 14 C and 13 C 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 14 C and 13 C 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 13 C in seawater DOC (Fry et al. 1996), was modified and tested for 14 C and 13 C determination in porewater DOC. CO 2 yields from six test materials ranged between 90% and 108%. d 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 14 C 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. *Corresponding author: E-mail: tkomada@sfsu.edu AcknowledgmentsWe thank Ann McNichol for encouraging us to pursue this method and for sharing the protocol that she and Bill Martin developed. Sheila Griffin and Ellen Druffel generated the UV oxidation data, ran our AMS samples, and provided many helpful discussions. Xiaomei Xu provided assistance in radiocarbon data interpretation. Sabrina Crispo ran the TOC-V and assisted in the laboratory and in the field. We thank the captain and crew of R/V Point Sur, J. Polly, R. Paul, D. Burdige, M. Jinuntuya, and A. Pitts for the work conducted in the Santa Monica Basin. Three anonymous reviewers provided comments that helped improve the quality of this manuscript. This work was supported by NSF-OCE0726819 (TK) and James C Kelley Scholarship (LJ).Acknowledgment is made to the Donors of the American Chemical Society Petroleum Research Fund for partial support of this research (ACS-PRF #46120-GB2 to TK). DOI 10.4319/lom.2011.9.485 Limnol. Oceanogr.: Methods 9, 2011, 485-498 © 2011, by the American Society of Limnology and Oceanography, Inc. LIMNOLOGY and OCEANOGRAPHY: METHODSues for marine porewater DOC have been reported in only a handful of studies (Bauer et al. 1995; Alperin et al. 2000).Several different methods have been developed to oxidize DOC to CO 2 for subsequent determination of 14 C and 13 C. As summarized by Bauer (2002), these can be broadly categorized into three types: (1) UV oxidation (Armstrong et al. 1966;Williams 1968; Williams et al. 1969; Williams and Gordon 1970;Druffel et al. 1989; Bauer et al. 1998;Beaupré et al. 2007), (2) sealed-tube dry combustion (Fry et al. 1993; Fry et al. 1996), and (3) flow-through high-temperature oxidation (Bauer ...

Sediment Pore Waters

“…To the best of the authors' knowledge, only a handful of ∆ 14 C and δ 13 C values for marine pore water DOC (∆ 14 C DOC and δ 13 C DOC , respectively) have been published (Bauer et al, 1995;Heuer et al, 2009;Ijiri et al, 2012;Komada et al, 2013;Valentine et al, 2005). The scarcity of data may be due, in part, to the analytical challenges associated with determining natural C isotope ratios in marine DOC, especially ∆ 14 C (Bauer, 2002;Johnson and Komada, 2011; also see Chapter 6).…”

Dissolved organic carbon dynamics in anaerobic sediments of the Santa Monica Basin

“…A subset of frozen DOC samples was further processed for 14 C and 13 C by UV oxidation (UV ox ; Beaupré et al, 2007), and by thermal sulfate reduction (TSR; Johnson and Komada, 2011). Samples from core 6-7 were processed by UV ox at UC Irvine; selected samples from cores 6-7, 8-7, and 10-8 were processed by TSR at SFSU.…”

“…Samples from core 6-7 were processed by UV ox at UC Irvine; selected samples from cores 6-7, 8-7, and 10-8 were processed by TSR at SFSU. For both of these analyses, the entire contents of the vial, consisting of solution and precipitate, were processed to ensure quantitative recovery of DOC (Johnson and Komada, 2011). To account for any 14 C contamination that may have occurred during pore water collection, aqueous solutions of SRM 4990B and IAEA-C7 with concentrations similar to those of pore-water DOC were prepared in the laboratory, and variable volumes of these solutions were processed similarly to pore water samples (aspirated into all-propylene syringes, then filtered), and oxidized by UV ox .…”

“…3). Because this comparison was made only using splits from frozen samples, it is possible that this offset arose from precipitate formation, and the fact that these precipitates tend to be under-sampled during analysis by HTC (Alperin and Martens, 1993;Johnson and Komada, 2011). DOC concentrations in refrigerated samples which did not form precipitates were also relatively low (cores 7-2 and 9-1; Fig.…”

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

“…The evolved CO 2 was dried, quantified, and collected into borosilicate break-seal tubes for determination of 13 C by isotope ratio mass spectrometry (IRMS), and for 14 C by accelerator mass spectrometry (AMS). DOC was oxidized to CO 2 by thermal SO 2{ 4 reduction (Johnson and Komada 2011), then quantified and split for isotopic analyses as described above. The entire contents of each DOC sample vial (solution plus precipitates that formed during frozen storage) were processed to ensure accurate DOC determination.…”