Origin, sedimentation and diagenesis of organic matter in coastal sediments of the southern Beaufort Sea region, Canadian Arctic

The source and fate of organic carbon in coastal sediments of the Beaufort Sea, Canadian Arctic, are examined. Despite the Mackenzie River's discharge to the coastal waters, sediments of the adjacent slope and Amundsen Gulf have a similar organic carbon content and CORG:N ratio to sediments of the continental margin. In contrast, the stable isotope composition of the organic carbon and nitrogen reveal striking differences: low delta 13C and delta15N values are found in Beaufort Shelf sediments indicative of a terrigenous origin, whereas higher values, closer to the marine signature, are observed in the Amundsen Gulf sediments. On the Slope, the isotopic signatures are intermediate and may be interpreted as a mixture of terrigenous and marine organic matter. These results indicate that the terrigenous suspended matter carried by the Mackenzie River's plume and discharged to the Beaufort Sea does not spread to the Amundsen Gulf.;In strongly seasonal environments, organic carbon is typically delivered to the sediment in pulses. To simulate the influence of episodic organic carbon fluxes on the sediment chemistry, closed jar incubation experiments were conducted using both Mn and Fe oxide-poor (Mackenzie Shelf) and -rich sediments (Amundsen Gulf). Experimental Mn reduction rates were fit reasonably well to a rate law that is first order with respect to both organic carbon and Mn oxide concentrations. The Fe reduction rates were also proportional to the organic matter concentration. These results indicate that a sudden incorporation of fresh organic matter to the sediment can deplete the sedimentary pool of Mn and Fe oxide within a few days.;Organic carbon that settles onto the seafloor fuels most early diagenetic reactions. The nature and amount that reaches the sediment determines the vertical zonation of redox reactions within the sediment. In turn, the vertical distribution of available oxidants can be used to infer the flux of organic carbon to the seafloor. The Mn and Fe oxide content of Gulf and Slope sediments are one order of magnitude larger than in Shelf sediments, indicating that the Mn and Fe cycles are maintained well below the sediment-water interface by low accumulation rates of organic carbon. Conversely, in margin sediments, high organic carbon accumulation rates bring the Mn and Fe cycles closer to the interface and allow their escape to the overlying waters.