The relationship between diagenetic cycles of reducible iron and maganese oxides and dissolved organic carbon (DOC) /

• Main Author: Barazzuol, Lisa Nicole.
• Format: Thesis
• Language: English
• Published: McGill University 2005
• Subjects: Geochemistry; Canada; Northwest Territories; Amundsen Gulf; Banks Island
• Online Access: http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=97900

Field evidence suggests that the fate of sedimentary dissolved organic carbon (DOC) is linked to the diagenetic cycles of iron and manganese in marine sediments. Co-variations of their concentrations in sediment porewater, as well as the release of DOC upon the reductive dissolution of authigenic iron and manganese oxides lead us to believe that sorption (i.e., co-precipitation and/or adsorption) onto these oxides may play an important role on the diagenetic behavior of DOC. This DOC sink, if it exists, would considerably alter our views of the mechanisms that regulate DOC fluxes across the sediment-water interface as well as their quantification. Sorption onto authigenic metal oxides may also lead to a molecular and isotopic fractionation of DOC. Oxic surface sediments recovered from the Amundsen Gulf, located south of Banks Island, Northwest Territories, Canada and the Saguenay Fjord, Quebec, Canada were extracted with two different mild reducing agents to determine the amount and composition (molecular and isotopic) of the DOC associated with the authigenic iron and manganese oxides. The reducing agents employed were: (i) 0.25 M hydroxylamine with 0.25 M HCl (HA, pH=1.7) and (ii) 1 N HCl (pH=0). Our results suggest that the sorbed DOC from the Saguenay Fjord sediment is both isotopically (delta13Corg) and molecularly fractionated with respect to the sedimentary particulate organic carbon (POC) pool whereas only molecular fractionation was evident in the Amundsen Gulf samples. The lack of isotopic fractionation in the latter sample set could be attributable to the inefficient extraction of iron oxides (50%) or simply isotopic uniformity, thereby limiting the isotopic fractionation. Molecular analyses by Fourier-Transform Infra-Red Spectroscopy (FTIR) indicate that the porewater and adsorbed DOC pools are most likely dominated by carbohydrates. Our results suggest that the adsorbed DOC pool may buffer the porewater DOC concentration and composition. Based on our preliminary results, we propose that authigenic and detrital iron and manganese oxides that accumulate in oxic sediments scavenge oxygen-rich organic acids diffusing upwards through the sediment column, thereby creating an "aging" reservoir in the oxic sediments. Slow diffusion across the sediment-water interface and desorption upon resuspension of these sediments in the overlying bottom waters may contribute to the "old" DOM pool found in benthic marine waters.