28 May 2018

28 May 2018

Review status: this preprint was under review for the journal BG but the revision was not accepted.

Dynamic interactions between iron and sulfur cycles from Arctic methane seeps

Pauline Latour1,2, Wei-Li Hong2,3, Simone Sauer2,3, Arunima Sen2, William P. Gilhooly III4, Aivo Lepland3, and Fotios Fouskas4 Pauline Latour et al.
  • 1European Institute for Marine Studies, Université de Bretagne Occidentale, 29280, Plouzané, France
  • 2CAGE – Centre for Arctic Gas Hydrate, Environment and Climate, The Arctic University of Norway in Tromsø (UiT), 9019, Norway
  • 3Geological Survey of Norway, Trondheim, 7040, Norway
  • 4Department of Earth Sciences, Indiana University-Purdue University Indianapolis, Indianapolis, 46202, USA

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 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 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 H2 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 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-methane-transition-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 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) 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 determines the distribution of animals with chemoautotrophic symbionts which rely on sulfide as their energy source.

Pauline Latour et al.

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Status: closed
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Pauline Latour et al.

Pauline Latour et al.


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Short summary
Dissolved iron is one of the most important nutrients for the marine life. The production and consumption of dissolved iron are therefore closely associated with the carbon cycling in the ocean. We present geochemical data and numerical modeling results to discuss how the supply of dissolved iron, from marine sediments to the ocean, is connected to carbon and sulfur cycles and influence the distribution of animals in environments with high methane supply.