Dynamic interactions between iron and sulfur cycles from Arctic methane seeps

Abstract

Bioavailable iron is an important micro-nutrient for marine phytoplankton and therefore critical to global biogeochemical cycles. Anoxic marine sediment is a significant source of Fe(II) to the ocean. Here, we investigate how the fluxes of Fe(II), both towards the sedimentary oxic layer and across the sediment-water interface, are impacted by the high concentration and flux of porewater sulfide in cold seep environments. We present new porewater data from four recently documented cold 15 seeps around Svalbard as well as from continental shelves and fjords in northern Norway. We quantitatively investigated porewater data first by calculating the Fe(II) fluxes towards oxidized surface sediments and bottom water and, second, applied a transport-reaction model to estimate the mass balance of several key chemical species. Sedimentary sulfur speciation data from two of the sites were used to constrain Fe(II) consumption in the shallow sediments. We showed that the iron reduction zone is usually confined to the top 10 cm of the sediments from our studied sites due to high sulfate 20 turnover and therefore high sulfide flux. Such a thin iron reduction zone allows proportionally more Fe(II) to reach the bottom water. Rapid precipitation of pyrite occurs at the base of the iron reduction zone, where the downward diffusing Fe(II) meets upward migrating hydrogen sulfide. Dissolved H 2 released during pyrite formation stimulates a small but significant rate of sulfate reduction in the same horizon, which results in faster production of hydrogen sulfide and a positive feedback for iron reduction in the shallow sediment. Deeper in the sediment, where sulfate is actively consumed due to 25 anaerobic methane oxidation, no apparent formation of pyrite is observed from the available measurements and our modeling results. This is mostly due to the relatively low availability of Fe(II) as a result of slower turnover of the less active iron mineral phases. Such an observation may contradict the use of pyrite abundance to deduce the sulfate-methanetransition-zone in past sedimentary records. A series of model sensitivity tests were performed to systematically investigate how the Fe(II) dynamics is impacted by higher deposition rate of iron (oxyhydr)oxides minerals on the seafloor and 30 intensifying methane supply. We showed that the increases in iron reduction rate, pyrite formation rate, and Fe(II) flux are expected with higher seafloor iron (oxyhydr)oxides deposition initially. However, complicated feedbacks between Fe(II) Biogeosciences Discuss., https://doi.2 production and sulfate reduction pose negative feedbacks to pyrite formation in the sediments. With a larger supply of methane, Fe(II) flux towards the oxic surface sediments is initially intensified by the higher production of hydrogen sulfide until such an interplay is too fast that essentially all reactive iron minerals settled on the seafloor dissolve immediately and dissolved iron is fixed through pyrite precipitation. Such an interplay between Fe(II) and sulfide determi...

Microbial communities from Arctic marine sediments respond slowly to methane addition during ex situ enrichments

“…Dissolved iron (II) was determined spectrophotometrically at 595 nm onboard with a ferrospectral complex in 1% ascorbic acid. Details of sulfide and iron (II) analyses are also available in (Latour et al ., ). Porewater sulfate was analysed with a Dionex ICS1100 ion chromatography (IC) at the Geological Survey of Norway.…”

“…TA was calculated from the recorded pH, and the amount of acid was added using the Gran function. Details of the calculation were reported previously in Latour and colleagues (). Increases in porewater alkalinity determined by onboard titrations were used to roughly constrain the SMT depths (within 30 cm) for sampling purposes.…”

Atypical biological features of a new cold seep site on the Lofoten-Vesterålen continental margin (northern Norway)

“…Methods of porewater extraction and analyses are reported in Latour et al . 71 . Briefly, porewater was extracted in a temperature-controlled room (4 °C) with acid-washed rhizon samplers and syringes, and 0.5 to 2 ml of porewater was preserved with Zn(OAc) 2 onboard (<30 minutes after rhizons were disconnected) for further analyses in the lab.…”

Distinct methane-driven biogeochemical regimes in Arctic seafloor gas hydrate mounds

“…TA was calculated from the recorded pH and amount of acid added using the Gran function. Details of the calculation were reported previously 69 . Increases in porewater alkalinity determined by onboard titrations were used to roughly constrain the SMT depths (within 30 cm) for sampling purposes.…”

“…The samples were then kept frozen until shore-based analysis. Details of the analyses were also given in Latour et al 69 . Depending on the factor of dilution, the detection limit is around tens of μM.…”