Trace element behaviour at cold seeps and the potential export of dissolved iron to the ocean

Type Article
Date 2014-10
Language English
Author(s) Lemaitre Nolwenn1, 2, 3, Bayon Germain1, Ondreas Helene1, Caprais Jean-Claude4, Freslon Nicolas2, Bollinger Claire2, 3, Rouget Marie-Laure2, 3, de Prunele Alexis2, Ruffine LivioORCID1, Olu-Le Roy KarineORCID4, Sarthou Geraldine2, 5
Affiliation(s) 1 : IFREMER, Unite Rech Geosci Marines, F-29280 Plouzane, France.
2 : Univ Europeenne Bretagne, F-35000 Rennes, France.
3 : Univ Bretagne Occidentale, CNRS UMS 3113, Inst Univ Europeen Mer, F-29280 Plouzane, France.
4 : IFREMER, Unite Rech Etud Ecosyst Profonds, F-29280 Plouzane, France.
5 : Univ Brest, IFREMER, CNRS, LEMAR,IRD,IUEM,UMR 6539, F-29280 Plouzane, France.
Source Earth And Planetary Science Letters (0012-821X) (Elsevier Science Bv), 2014-10 , Vol. 404 , P. 376-388
DOI 10.1016/j.epsl.2014.08.014
WOS© Times Cited 32
Keyword(s) iron, rare earth elements, cold seeps, seawater, organic ligands, iron-sulfide nanoparticles
Abstract Seawater samples were collected by submersible above methane seeps in the Gulf of Guinea (Regab and Baboon pockmarks) in order to investigate the behaviour of iron (Fe), manganese (Mn) and rare earth elements (REE) during fluid seepage. Our aim was to determine whether cold seeps may represent potential sources of dissolved chemical species to the ocean. Dissolved (<0.45 μm filtered samples) and total dissolvable (unfiltered samples) concentrations were determined over ∼50 m long vertical transects above the seafloor and at various discrete locations within the pockmarks. We show that substantial amounts of Fe and Mn are released into seawater during seepage of methane-rich fluids. Mn is exported almost quantitatively in the dissolved form (more than 90% of total Mn; mean MnDISS∼12±11 nmol/kg∼12±11 nmol/kg). Although a significant fraction of Fe is bound to particulate phases, the dissolved iron pool still accounts on average for approximately 20 percent of total iron flux at vent sites (mean FeDISS∼22±11 nmol/kg∼22±11 nmol/kg). This dissolved Fe fraction also appears to remain stable in the water column. In contrast, there was no evidence for any significant benthic fluxes of pore water REE associated with fluid seepage at the studied sites. Overall, our results point towards distinct trace element behaviour during fluid seepage, with potential implications for the marine geochemical budget. The absence of any dissolved REE enrichments in bottom waters clearly indicates effective removal in sub-surface sediments. Most likely, precipitation of authigenic mineral phases at cold seeps (i.e. carbonates) represents a net sink for these elements. While Mn appears to behave near-conservatively during fluid seepage, the observed relative stability of dissolved Fe in the water column above seepage sites could be explained by complexation with strong organic ligands and/or the presence of Fe-bearing sulfide nanoparticles, as reported previously for submarine hydrothermal systems. Considering the ubiquitous occurrence of methane vents at ocean margins, cold seeps could represent a previously unsuspected source of dissolved Fe to the deep ocean.
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Lemaitre Nolwenn, Bayon Germain, Ondreas Helene, Caprais Jean-Claude, Freslon Nicolas, Bollinger Claire, Rouget Marie-Laure, de Prunele Alexis, Ruffine Livio, Olu-Le Roy Karine, Sarthou Geraldine (2014). Trace element behaviour at cold seeps and the potential export of dissolved iron to the ocean. Earth And Planetary Science Letters, 404, 376-388. Publisher's official version : , Open Access version :