Abstract:We report quantitative carbon isotope evidence showing that porewater dissolved organic carbon (DOC) is produced from different sources in Arctic Ocean sediments, resulting in dramatic changes in both δ13C and Δ14C values of DOC. The very low δ13C (−42.7‰) and Δ14C (−947‰) values measured for porewater DOC indicate that bacteria consume methane hydrate‐derived OC and convert a significant fraction of this δ13C depleted and old OC (44–97%) into DOC in the sediments of the Arctic Ocean. We suggest that the produ… Show more
“…1). The degradation of clam tissues would also lead to the increase of pore water DIC content 30 , which is possibly one main driver of Hg methylation as it in uences both Hg bioavailability and microbial activities 31 .…”
Mercury (Hg) is an important element in seafloor cold seeps that might govern methane emission. However, so far, the knowledge of biogeochemical Hg cycle in it remains poorly understood. In this study, Hg geochemical characteristics and microorganisms involved in Hg biogeochemical cycling were examined in three (active, inactive seep vs reference) different types of sediments sampled from the Haima cold seep in the South China Sea. Sediments in the active seep area were significantly enriched in mercury and methylmercury (MeHg) compared to the reference. Accordingly, abundant genes related to Hg methylation (hgcAB), demethylation (merB) and reduction (merA) were detected in the active seep sediments, phylogenetically associated with various bacterial and archaeal linages (e.g. Desulfobacterota, Gammaproteobacteria and Halobacteriota). Hg odd-mass number isotopes (Δ199Hg and Δ201Hg) pointed to their source from the upper ocean and the occurrence of abiotic dark oxidation. The δ202Hg values indicate Hg mass fractionation, migration and transformation in the active seep sediments. These geochemical and microbial data highlight active Hg biogeochemical cycles in seafloor cold seeps, functioning as important Hg-sinks and MeHg sources in the deep ocean.
“…1). The degradation of clam tissues would also lead to the increase of pore water DIC content 30 , which is possibly one main driver of Hg methylation as it in uences both Hg bioavailability and microbial activities 31 .…”
Mercury (Hg) is an important element in seafloor cold seeps that might govern methane emission. However, so far, the knowledge of biogeochemical Hg cycle in it remains poorly understood. In this study, Hg geochemical characteristics and microorganisms involved in Hg biogeochemical cycling were examined in three (active, inactive seep vs reference) different types of sediments sampled from the Haima cold seep in the South China Sea. Sediments in the active seep area were significantly enriched in mercury and methylmercury (MeHg) compared to the reference. Accordingly, abundant genes related to Hg methylation (hgcAB), demethylation (merB) and reduction (merA) were detected in the active seep sediments, phylogenetically associated with various bacterial and archaeal linages (e.g. Desulfobacterota, Gammaproteobacteria and Halobacteriota). Hg odd-mass number isotopes (Δ199Hg and Δ201Hg) pointed to their source from the upper ocean and the occurrence of abiotic dark oxidation. The δ202Hg values indicate Hg mass fractionation, migration and transformation in the active seep sediments. These geochemical and microbial data highlight active Hg biogeochemical cycles in seafloor cold seeps, functioning as important Hg-sinks and MeHg sources in the deep ocean.
“…In particular, it has been shown that the N and/or P limitation in the subtropical gyres can enhance DOC production by phytoplankton (Moreno & Martiny, 2018;Zakem & Levine, 2019). In the Arctic Ocean, observations (Bussmann & Kattner, 2000;Engel et al, 2019;Fu et al, 2020;Hansell et al, 2004) also suggest that our model underestimates the surface Arctic DOC concentrations by around 30 mmol C/m (Figure 4). This is not surprising as previous studies (Connolly et al, 2020) have shown a strong influence of the relatively refractory terrestrial DOC supplied to the Arctic Ocean by rivers, a component of DOC that is not represented in our model.…”
Section: Fe Doc and Fl Bacteria Biomass And Productionmentioning
Heterotrophic bacteria (hereafter bacteria) account for a significant portion of the ocean carbon biomass (Buitenhuis et al., 2012) and play various roles in oceanic nutrient cycles. Despite the wide variety of bacteria that live under various conditions (Sogin et al., 2006;Zakem et al., 2020), free-living (FL), carbon-oxidizing bacteria have been the focus of previous studies due to their control on the flow of carbon and bio-essential elements through the marine food web (Azam & Malfatti, 2007;del Giorgio & Duarte, 2002;Jiao et al., 2010). Carbon-oxidizing bacteria either transforms a fraction of the organic matter fixed by phytoplankton into a highly refractory form, thus keeping carbon away from the atmosphere for several millennia or respires the fixed organic matter to release inorganic carbon and nutrients back to the seawater, thus sustaining oceanic primary production (Jiao et al., 2010(Jiao et al., , 2014. Other bacterial groups, such as particle-attached, nitrogen-oxidizing, and sulfur-oxidizing bacteria and archaea, carry out various chemical transformations, which modulate the ocean nitrogen and sulfur cycles and produce potent greenhouse gas (nitrous oxide and methane) (
“…2 ) in the Japan Trench are ~6 times higher than those in other oceanic trenches 20 and increase with depth, reaching maximum concentrations (Fig. 2 ) that are comparable to the coastal 21 and continental shelf 22 areas, highlighting a dynamic carbon cycle and huge dissolved carbon storage in the hadal trenches enhanced by earthquakes. Fig.…”
Hadal trenches are unique geological and ecological systems located along subduction zones. Earthquake-triggered turbidites act as efficient transport pathways of organic carbon (OC), yet remineralization and transformation of OC in these systems are not comprehensively understood. Here we measure concentrations and stable- and radiocarbon isotope signatures of dissolved organic and inorganic carbon (DOC, DIC) in the subsurface sediment interstitial water along the Japan Trench axis collected during the IODP Expedition 386. We find accumulation and aging of DOC and DIC in the subsurface sediments, which we interpret as enhanced production of labile dissolved carbon owing to earthquake-triggered turbidites, which supports intensive microbial methanogenesis in the trench sediments. The residual dissolved carbon accumulates in deep subsurface sediments and may continue to fuel the deep biosphere. Tectonic events can therefore enhance carbon accumulation and stimulate carbon transformation in plate convergent trench systems, which may accelerate carbon export into the subduction zones.
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