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46120-GB2
The Effect of Substrate 14C-Age on Anaerobic Degradation of Organic Matter in Marine Sediments

Tomoko Komada, San Francisco State University

Findings to Date

This study aims to better understand the mechanism of anaerobic sedimentary organic matter degradation by employing natural ¹⁴C and ¹³C abundances as proxies of organic matter reactivity and origin, respectively.  The key goal is to determine, under sulfate reducing conditions, the Δ¹⁴C and d¹³C signatures of carbon that undergoes solubilization from particulate organic (POC) to dissolved organic (DOC), followed by respiration to dissolved inorganic carbon (DIC).  It is hypothesized that the isotopic signatures of carbon undergoing transformation is distinct from the parent material, such that moieties enriched in ¹⁴C is preferentially utilized during both POC solubilization and DOC respiration. 

We are testing the above hypothesis through sediment incubation experiments carried out under controlled laboratory conditions.  To date, we have completed a 130-day incubation of surface (~5 cm) sediment collected from an estuarine site in San Francisco Bay.  The sediment was sieved, homogenized, and incubated in the dark at in situ temperature in 50 mL glass centrifuge tubes devoid of headspace.  Pore-water SO₄^2- concentration was maintained above 20 mM to avoid methanogenesis.  Over the course of 130 days, DIC and NH₄⁺ concentrations increased from initial values of 7 and 0.1 mM to 50 and 1.8 mM, respectively.  DOC decreased initially from a starting value of 1.2 mM, but steadily increased after t = 9 d, reaching 1.4 mM by t = 130 d.  Particulate inorganic carbon concentration was low throughout the incubation (< 0.05 wt%), and exhibited no clear trend with time. 

To date, out of the total of 12 sampling points (t = 0, 2, 5, 9, 14, 18, 25, 39, 53, 74, 95, 130 d), nine DIC samples have been processed by vacuum distillation, and 5 have been further analyzed for Δ¹⁴C and d¹³C values by accelerator MS (AMS) and isotope ratio MS (IRMS), respectively.  DOC concentrations from all times points have been quantified by high temperature combustion.  We are working on setting up a dry combustion technique that will allow us to convert DOC into CO₂ for further analysis by AMS and IRMS. 

Net accumulation of NH₄⁺, and a 2.0 ± 0.2 stoichiometry between DIC production and SO₄^2- consumption, indicate that sulfate reduction was the primary mode of respiration that took place during the incubation.  A small net increase in DOC over 130 days (0.2 mM) indicate close coupling of POC solubilization and DOC oxidation; net DOC accumulation amounts to only ~ 0.5 % of net DIC accumulation.  Nonetheless, net accumulation of DOC indicates that not all organic carbon released into the porewater is readily available for respiration over the duration of this experiment. 

The Δ¹⁴C signatures of DIC available to date show that the net accumulation of DIC was supported by respiration of organic matter having Δ¹⁴C signatures that range from -30 to +20 ‰.  These values are significantly higher than the bulk Δ¹⁴C value of bulk POC (-160 ‰).  This observation is consistent with our hypothesis, and indicates that ¹⁴C-young organic matter is selectively utilized during POC solubilization and/or DOC respiration.  The d¹³C values of DIC and plant tissue collected from the study site further suggest that modern cordgrass may be an important substrate for respiration at this site.

Impact on Students and the PI's Career

This project provided robust research experience for Mr. Jonathon Polly, a MS candidate who plans to defend in December 2008.  Mr. Polly gained both field (coring) and laboratory (incubation, sample collection, and various chemical analyses) experience.  He is currently working on data synthesis and writing the thesis in peer-reviewed journal format.  Mr. Polly is planning to apply to Ph.D. programs in the ocean sciences.  A new MS candidate, Ms. Leah Johnson, has begun working on a modified version of the DOC dry combustion technique originally described by Fry et al. (Marine Chemistry, 54, 191-201).  Determination of Δ¹⁴C and d¹³C values of pore-water DOC lies at the heart of the current project, but is also the most challenging.  Ms. Johnson will gain invaluable laboratory research experience by setting up this method from scratch.  Ms. Johnson will also gain insight into sedimentary organic matter cycling through data synthesis. 

The current project has allowed the PI to set up essential instrumentation in her laboratory (FIA for DIC and NH₄⁺; spectrophotometer; pH meter).  The determination of Δ¹⁴C value of POC at our study site allowed the PI to complete a project on carbonate removal from sediments, resulting in a publication in Limnology and Oceanography: Methods.  The results obtained to date are promising, and clearly show the usefulness of natural carbon isotopes in understanding organic matter cycling in sediments.  With DOC dry combustion soon to be underway, the outcome of this project is expected to give exciting insight into the nature of pore-water DOC, and provide a solid foundation for further questions for the PI to pursue.

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