Carbon mineralization in Laptev and East Siberian Sea shelf and slope sediment

Abstract

<p><strong>Abstract.</strong> The Siberian Arctic Sea shelf and slope is a key region for the degradation of terrestrial organic material transported from the organic carbon-rich permafrost regions of Siberia. We report on sediment carbon mineralization rates based on O<sub>2</sub> microelectrode profiling, intact sediment core incubations, <sup>35</sup>S-sulfate tracer experiments, porewater dissolved inorganic carbon (DIC), δ<sup>13</sup>C<sub>DIC</sub>, and iron, manganese, and ammonium concentrations from 20 shelf and slope stations. This data set provides a spatial overview of sediment carbon mineralization rates and pathways over large parts of the outer Laptev and East Siberian Arctic shelf and slope, and allowed us to assess degradation rates and efficiency of carbon burial in these sediments. Rates of oxygen uptake and iron and manganese reduction were comparable to temperate shelf and slope environments, but bacterial sulfate reduction rates were comparatively low. In the topmost 20 to 50 cm of sediment, aerobic carbon mineralization dominated degradation and comprised on average 82 % of the depth-integrated carbon mineralization. Oxygen uptake rates and <sup>35</sup>S-sulfate reduction rates were higher in the eastern East Siberian Sea shelf compared to the Laptev Sea shelf. DIC/NH<sub>4</sub><sup>+</sup> ratios in porewaters and the stable carbon isotope composition of remineralized DIC indicated that the degraded organic matter on the Siberian shelf and slope was a mixture of marine and terrestrial organic matter. Based on dual end member calculations, the terrestrial organic carbon contribution varied between 32 % and 36 %, with a higher contribution in the Laptev Sea than in the East Siberian Sea. Extrapolation of the measured degradation rates using isotope end member apportionment over the outer shelf of the Laptev and East Siberian Sea suggests that about 16 Tg C per year are respired in the outer shelf sea floor sediment. Of the organic matter buried below the oxygen penetration depth, between 0.6 and 1.3 Tg C per year are degraded by anaerobic processes, with a terrestrial organic carbon contribution ranging between 0.3 and 0.5 Tg per year.</p>

Around one third of current Arctic Ocean primary production sustained by rivers and coastal erosion

“…Our assumption of instantaneous remineralisation of organic nutrients into their inorganic form when delivered to the ocean by Arctic rivers and coastal erosion (see Methods) clearly leads to an overestimation of the amount of nutrients available to fuel primary production. Based on observed remineralisation rates of organic matter [48][49][50][51][52] and Arctic Ocean water residence times of freshwater (3.5 ± 2.0 years on Siberian shelves 53 , 11 years in the Canada Basin 54 ), we estimate that only 60 (20-80)% of riverine dissolved organic nitrogen and 80 (70-90)% of organic nitrogen derived from coastal erosion (see Methods) would be remineralised rapidly enough to fuel Arctic Ocean NPP. Accounting for this "reaction-limited" effect would decrease the input of nitrogen that is available to fuel marine NPP from 1.0 (0.9-1.1) Tg N yr −1 to 0.7 (0.5-0.9) Tg N yr −1 for riverine fluxes and from 1.6 (1.0-2.5) Tg N yr −1 to 1.4 (0.7-2.3) Tg N yr −1 for coastal erosion fluxes.…”

“…subsea coastal erosion 16 or the riverine particulate matter 22 that does not precipitate in the river delta 33 or due to resuspension of terrestrial organic matter in the nearshore zone 34,56 . Following the observed remineralisation rates for terrigenous organic carbon 51,52 in sediments and the water column, we estimate that 80 (70-90)% of our estimated input of organic nutrients released by coastal erosion will contribute to NPP before leaving the shelf seas.…”

“…The organic matter from coastal erosion is expected to be even more reactive than riverine supplies, in agreement with the findings that thawing permafrost soils release one of the most labile forms of organic matter in nature [80][81][82] . Observationbased estimates of remineralisation rates of terrigenous organic matter in the Laptev Sea and East-Siberian Sea are 0.87 Tg N yr −1 in sediments 52 and 0.38 Tg N yr −1 in the water column 51 . Scaled up to the entire Arctic Ocean, using the primary production sustained by terrigenous nutrients as scaling factor, remineralisation rates of 1.56 Tg N yr −1 in sediments and 0.68 Tg N yr −1 in the water column are obtained.…”

The Importance of Benthic Nutrient Fluxes in Supporting Primary Production in the Laptev and East Siberian Shelf Seas

“…Sediment cores were sampled at 20 stations with a Multicorer (OKTOPUS GmbH, Kiel, Germany) that collected eight cores of about 40 cm length at one time. Only cores from the same cast with well‐preserved sediment surfaces and clear bottom water were used for pore water extraction using rhizons, O 2 microelectrode measurements, and whole‐core flux incubations (Brüchert et al., 2018).…”

“…Western Arctic shelf benthic flux studies have focused more on the productive Chukchi and Beaufort Seas to investigate organic carbon export and transformation, nutrient recycling, and trace element fluxes (Hardison et al., 2017; Kondo et al., 2016; Moran et al., 2005; Vieira et al., 2018), while in the Laptev and East Siberian Sea, sediment biogeochemical studies have focused on assessing the transport, accumulation, origin, and degradation state of terrestrial‐derived organic carbon (Bröder et al., 2019; Karlsson et al., 2015; Vonk et al., 2012). Despite the large size covering about 1.5 million km 2 , there are very few reports of directly measured benthic carbon mineralization rates for the Laptev Sea and East Siberian Seas (Boetius and Damm, 1998; Brüchert et al., 2018) and, to our knowledge, only indirect, model‐derived assessments of benthic nutrient fluxes for this extensive shelf region (e.g., Grebmeier, 2012; Nitishinsky et al., 2007; Terhaar et al, 2020). Given the shelf size and shallow water depth, recycled benthic nutrients are probably important contributors to the Laptev and the East Siberian Sea nutrient inventory.…”