2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)
Paper No. 119-3
Presentation Time: 9:00 AM-6:30 PM

IRON ISOTOPE SIGNATURE IN SEDIMENTS OF AN OLIGOTROPHIC LAKE

LIU, Kai1, WU, Lingling2, COUTURE, Raoul-Marie3, LI, Weiqiang4, BEARD, Brian L.5, JOHNSON, Clark, M.5, and CAPPELLEN, Philippe Van6, (1) Earth and Environmental Sciences, University of Waterloo, 200 University Ave W, Waterloo, ON N2L 3G1, Canada, k56liu@uwaterloo.ca, (2) Earth and Environmental Sciences, University of Waterloo, 200 University Ave W, Waterloo, ON N2L3G1, Canada, (3) Norwegian Institute for Water Research, Gaustadalléen 21, Oslo, NO-0349, Norway, (4) Department of Geoscience, University of Wisconsin-Madison, Weeks Hall 1215 W Dayton St, Madison, WI 53706, (5) Department of Geoscience, University of Wisconsin-Madison, NASA Astrobiology Institute, 1215 W. Dayton St, Madison, WI 53706, (6) Earth and Environmental Science, University of Waterloo Waterloo, ON N2L 3G1, Canada, 200 University Avenue West, Waterloo, ON N2L 3G1, Canada

This study investigated iron isotope compositions of pore waters and solid-associated Fe phases in natural lacustrine sediments at Lake Tantaré, Québec, Canada. Lake Tantaré, an oligotrophic lake located in an undisturbed boreal catchment, is a well constrained natural laboratory, where previous modelling work has shown that dissimilatory Fe(III) reduction (DIR) is the main Fe reduction pathway in the sediments with minor contribution from abiotic reduction by sulfides. The Fe concentration profiles for sediments indicate that intense Fe cycling includes upward migration of aqueous Fe(II), generated by DIR, followed by oxidation and precipitation of Fe(III) oxyhydroxides at the sediment-water interface (SWI). The sediment-water interface therefore works as a barrier to prevent upward flux of Fe, which in turn confines sorption of trace elements, limiting transport of many elements across the SWI. Our speciation measurements show that a significant fraction of Fe in pore water likely exists as ferric Fe, complexed with organic matter and/or in colloidal phase. We found isotopically light aqueous Fe(II) in pore waters and sorbed Fe(II) on sediments, in contrast to previous findings of isotopically light Fe only in water column in other lacustrine systems. DIR is likely to be assisted by electron shuttling by humic substances in Lake Tantaré sediments. The highly negative δ56Fe values of aqueous Fe(II) (‒2.12±0.60‰) is consistent with previous experimental work, which showed that δ56Fe values of aqueous Fe(II) produced by DIR can vary between ‒3‰ and ‒1‰ depending on different proportions of Fe species (Fe(II)aq, Fe(II)sorb, Fe(III)am) in the system. The apparent isotope fractionation factor between aqueous Fe(II) and Fe(III) oxyhydroxides is ‒2.7±0.7‰ in Lake Tantaré sediments, which is consistent with previous experimental work on the isotopic fractionations produced by dissimilatory Fe(III) reduction. These findings have provided for the first time a clear Fe isotope signature of DIR from natural lacustrine sediments and have improved our understanding of the stable isotope effects of Fe redox cycling in lacustrine environments.

2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)
General Information for this Meeting
Session No. 119--Booth# 56
Frontiers in Non-Traditional Stable Isotopes: In Honor of Fang-Zhen Teng, Recipient of the 2014 MSA Award (Posters)
Vancouver Convention Centre-West: Exhibition Hall C
9:00 AM-6:30 PM, Monday, 20 October 2014


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