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During the reporting time period, our work focused on two areas: (1) kinetic modeling of incubation results; and (2) development of a dry combustion method to oxidize small DOC samples for radiocarbon analysis.    

1. Kinetics of Dissolved Organic Carbon (DOC) Production and Consumption

This portion of the study was completed as a Master of Science thesis (Aug 09), and is currently being put together into a manuscript for peer-review. 

1.1. Major Findings

(a) DOC having limited reactivity accumulated during the incubation experiment.  The amount of net DOC production was ~3% of the total carbon oxidation.  These findings support the working hypothesis that although the majority of the particulate organic carbon (POC) undergoing degradation is labile, a small fraction is resistant to oxidation, and accumulates as DOC in the porewater.

(b) The model results suggest that net accumulation of resistant DOC began as soon as the incubation was initiated.  Production rate of resistant DOC slowed in an exponential manner with time.  These findings imply that resistant DOC is produced directly via dissolution of POC, rather than from internal transformation processes (such as geopolymerization and condensation) in the porewater.  They also suggest that production rate of resistant DOC may be highest near the sediment-water interface.

1.2. Additional Findings

(a) It was possible to simulate DIC, DOC, NH₄⁺, and POC simultaneously by invoking three pools of DOC: (1) DOC₁, a highly reactive portion of POC that is rapidly oxidized to DIC; (2) DOC₂, the resistant fraction that accumulates in porewater; and (3) DOC₀, labile DOC that was likely introduced into the porewater when the sediment was sieved and homogenized prior to the incubation (source of DOC₀ is likely ruptured cells).  The model-derived degradation rate constants of these three fractions decrease in the order k_DOC0 (0.1 d^-1) > k_DOC1 (0.04 d^-1) >> k_DOC2 (4e-4 d^-1). 

(b) Stoichiometric analysis shows that the C/N ratio of oxidized organic matter was significantly higher during early stages of the incubation (t=0-5 d) than in the remainder of the incubation (t=9 d and onwards).  This observation is also supported by the kinetic model, which predicts that DOC₀ is devoid of N, while DOC₁ has a C/N ratio of 24.  These findings suggest that labile carbohydrate-like material may have been the dominant substrate for respiration early in the experiment.

(c) The d¹³C signatures of DIC show that the organic matter oxidized earlier on in the incubation was more enriched in ¹³C relative to that oxidized later in the incubation.  This is consistent with the suggestion based on C/N evidence that carbohydrate-like material was consumed first.  d¹³C values have yet to be incorporated into our kinetic model.

(d) The carbon oxidation rates calculated from the initial rate of DIC increase (65 mmol m^-2 d^-1) is comparable in magnitude to those reported for near-shore sediments.  The production rate of resistant DOC₂ amounted to only ~3% of the total mineralization rate, but was comparable in magnitude to benthic DOC fluxes reported for coastal sediments.   This suggests that an appreciable fraction of DOC escaping organic-rich coastal sediments may be of limited reactivity.

2. Development of a Dry Combustion Method to Oxidize DOC for Radiocarbon Analysis

This portion of the study is currently being carried out by an M.S. candidate (Ms. Johnson).

In order to further probe into the nature of the DOC that accumulated during the incubation experiment, we are working towards setting up a low-cost dry combustion method that will allow oxidation of small, concentrated DOC samples for radiocarbon analysis.  The method is adapted from a 1996 publication by Fry et al. (Marine Chemistry, 54: 191-201) that was developed for the determination of d¹³C signatures of DOC.  Our current method incorporates modifications as per Dr. Ann McNichol, NOSAMS (personal communication) that are intended to reduce entrainment of extraneous carbon during the drying process. 

(a) Method Description:  Briefly, samples are dried in a pre-combusted round-bottom flask necked to a 25 cm long, 9 mm diameter Pyrex tube.  The sample is dried in the flask in the presence of phosphoric acid and an excess of pre-combusted K₂SO₄.  The sample is sealed, combusted, and the resultant gas is scrubbed to remove SO_2.  Using this method, it is possible to process 3-4 samples into CO₂ gas in one week. 

(b) Blank measurements made to date (n=12) indicate a background level of approximately 10 μgC, which is at least a factor of 20 smaller than the unknown sample sizes.  Use of PtCl₂ was avoided, because this appeared to increase the overall background level.  Obtaining lower and more reproducible blanks remains one of our present tasks. 

(c) We have steadily improved the carbon recovery yield.  The most recent carbon yield using reagent-grade glucose is near ~95 %, a significant improvement compared to earlier low values of ~50%.  Recovery tests will continue using two more reagents (tannic acid, humic acid), and a large porewater sample collected from test sediment. 

(d) Next steps:  A major remaining task for Ms. Johnson prior to oxidizing the incubation unknowns is to determine the isotopic composition of the blank carbon.  This will be carried out by oxidation of standards of known d¹³C and Δ¹⁴C signatures.