HR: 14:25h
AN: B23G-04    [Abstracts]
TI: Biogeochemical Cycling of Iron Isotopes at Loihi Seamount
AU: * Rouxel, O J
EM: orouxel@whoi.edu
AF: Woods Hole Oceanographic Institution Marine Chemistry & Geochemistry Dept., MS#25, Woods Hole, MA 02543, United States
AU: Edwards, K J
EM: kje@usc.edu
AF: Department of Biological Sciences University of Southern California, 3616 Trousdale Parkway, Los Angeles, CA 90089-0371, United States
AU: Moyer, C L
EM: cmoyer@hydro.biol.wwu.edu
AF: Western Washington University Dept. of Biology, MS#9160, Bellingham, WA 98225-9160, United States
AU: Wheat, G
EM: wheat@mbari.org
AF: Global Undersea Research Unit, P.O. Box 475, Moss Landing, CA 95039, United States
AB: It is now well recognized that seafloor hydrothermal systems support diverse and unique biological communities capable of using dissolved chemical species, such as Iron (Fe), as well as mineral substrates as sources of metabolic energy. Deep-sea hydrothermal systems such as the Loihi Seamount hydrothermal field are important examples of environments where both chemical and biological oxidation of Fe can occur simultaneously and provide an ideal system in which to test hypotheses on biotic vs. abiotic origin of iron-oxide formation. Here, we applied Fe isotope systematics of hydrothermal fluids and Fe-oxide precipitates to study biogeochemical cycling of iron and the formation of microbial mats at Loihi seamount. Warm hydrothermal fluids (<60°C) and iron oxide precipitates were recovered using the DSV Jason II during FeMO 2006 cruise. Fe-isotope composition of warm hydrothermal fluids yielded δ⁵⁶Fe values near 0.1‰ and are indistinguishable from basalt values defined at 0.09‰. Suspended iron oxide particles in the fluids and seafloor iron oxide sediments (microbial mats) recovered in the vicinity of the vents yielded systematically positive δ⁵⁶Fe values. The enrichment in heavy isotopes between 1.05 to 1.43‰ relative to Fe(II) in vent fluids is slightly higher than those obtained for abiotic Fe oxidation (around 0.9‰) and slightly lower than for bacterial Fe oxidation at circum neutral pH (around 1.5‰). Mass balance considerations also imply that the extent of Fe(II) oxidation is very limited in the vicinity of the vents (<20%) and that most Fe(II) is oxidized later in the water column. These results are consistent with the low oxygen content of seawater (i.e. summit of Loihi is located in the OMZ) and resultant slow kinetics of abiotic Fe oxidation. In contrast, mats supported by very diffuse fluids recovered at the base of the Loihi Seamount (~ 5000m depth) have distinctly negative Fe-isotope values between -0.3 to -1.5‰. These negative values are best explained by near-complete oxidation of isotopically light Fe(II) source. Negative δ⁵⁶Fe values in the source fluid are likely generated by subsurface precipitation of isotopically heavy Fe-oxides during partial Fe(II) oxidation. These results, together with the significant enrichment in Mn-oxides relative to the warm mats, are consistent with the higher oxygen level in deep seawater and suggest extensive microbial Fe(II) oxidation below seafloor. Fe-isotope compositions of microbial mats at Loihi Seamount display a remarkable range between -1.2 to 1.6‰ which enlarges considerably the range of δ⁵⁶Fe values for other hydrothermal Fe-oxide deposits at mid-oceanic ridges (δ⁵⁶Fe values between -0.8 to 0‰). This unique feature at Loihi indicates that Fe isotope compositions of hydrothermal Fe-oxide precipitates are particularly sensitive to oxygen levels in the local environment where they form, and are less sensitive to abiotic vs. biotic origins. The Loihi hydrothermal ecosystem provides an important modern analogue for testing hypotheses about the biogeochemical cycling of Fe-isotopes on early Earth.
DE: 0448 Geomicrobiology
DE: 0450 Hydrothermal systems (1034, 3017, 3616, 4832, 8135, 8424)
DE: 0454 Isotopic composition and chemistry (1041, 4870)
DE: 0461 Metals
DE: 1050 Marine geochemistry (4835, 4845, 4850)
SC: Biogeosciences [B]
MN: 2007 Fall Meeting