Sources and turnover of organic carbon and methane in fjord and shelf sediments off northern Norway

Abstract

To better understand the present and past carbon cycling and transformation processes in methane-influenced fjord and shelf areas of northern Norway, we compared two sediment cores from the Hola trough and from Ullsfjorden. We investigated (1) the organic matter composition and sedimentological characteristics to study the sources of organic carbon (C org ) and the factors influencing C org burial, (2) pore water geochemistry to determine the contribution of organoclastic sulfate reduction and methanogenesis to total organic carbon turnover, and (3) the carbon isotopic signature of hydrocarbons to identify the carbon transformation processes and gas sources. High sedimentation and C org accumulation rates in Ullsfjorden support the notion that fjords are important C org sinks. The depth of the sulfate-methanetransition (SMT) in the fjord is controlled by the supply of predominantly marine organic matter to the sediment. Organoclastic sulfate reduction accounts for 60% of the total depth-integrated sulfate reduction in the fjord. In spite of the presence of ethane, propane, and butane, we suggest a purely microbial origin of light hydrocarbons in the sediments based on their low d 13 C values. In the Hola trough, sedimentation and C org accumulation rates changed during the deglacial-to-post-glacial transition from approximately 80 cm ka 21 to erosion at present. Thus, C org burial in this part of the shelf is presently absent. Low organic matter content in the sediment and low rates of organoclastic sulfate reduction (only 3% of total depth-integrated sulfate reduction) entail that the shallow depth of the SMT is controlled mostly by ascending thermogenic methane from deeper sources.

Organic Carbon Origin, Benthic Faunal Consumption, and Burial in Sediments of Northern Atlantic and Arctic Fjords (60–81°N)

“…Highest δ 13 C and highest contributions of marine origin organic matter (~80%) were found in two northern Norway fjords. Similar range of δ 13 C in Ullsfjord were also reported by Sauer et al () and interpreted as indicators of organic matter originating predominantly from marine phytoplankton. Relatively high photosynthetic pigments concentrations in Balsfjord sediments support the findings of Reigstad et al (), who showed that both in Balsfjord and Ullsfjord phytoplankton cells may dominate the vertical fluxes during bloom periods (as opposed to earlier suggestions of very strong zooplankton grazing and low sedimentation of fresh pelagic material to the sea bottom in northern Norwegian fjords).…”

“…The subarctic fjords located in northern Norway (Balsfjord and Ullsfjord) were characterized by much lower C org accumulation (and burial) rates, probably due to lower marine primary production and poorer terrestrial vegetation in this region. The low C org accumulation was also reported for Ullsfjord and adjacent Malagenfjord (Glud et al, 1998;Sauer et al, 2016).…”

Dynamic interactions between iron and sulfur cycles from Arctic methane seeps

“…It is a crossshelf trough with a width of 12km and water depth of around 200m (Sauer et al, 2016). Active methane seepage was documented by geophysical (Chand et al, 2008) and geochemical investigations (Sauer et al, 2015;Sauer et al, 2016). The sedimentation rate is relatively low and the organic matter in the sediments is predominately derived from terrestrial sources 10 (Sauer et al, 2016).…”

“…Active methane seepage was documented by geophysical (Chand et al, 2008) and geochemical investigations (Sauer et al, 2015;Sauer et al, 2016). The sedimentation rate is relatively low and the organic matter in the sediments is predominately derived from terrestrial sources 10 (Sauer et al, 2016). High flux of methane derived from the Late Jurassic and Early Cretaceous source rock (Sauer et al, 2015) results in rapid sulfate turnover through AOM as documented by porewater profiles (Sauer et al, 2016).…”

“…The sedimentation rate is relatively low and the organic matter in the sediments is predominately derived from terrestrial sources 10 (Sauer et al, 2016). High flux of methane derived from the Late Jurassic and Early Cretaceous source rock (Sauer et al, 2015) results in rapid sulfate turnover through AOM as documented by porewater profiles (Sauer et al, 2016). Ullsfjorden is a 70-km long fjord in northern Norway (Fig.…”