Iron Isotope Constraints on the Early Redox History of the Oceans

Presenter: Olivier Rouxel, Woods Hole Oceanographic Institution
When: February 28, 2005 11AM PST

The response of the ocean redox state to the rise of atmospheric oxygen by
ca. 2.3 Ga ago is poorly constrained although deposition of BIFs until ca.
1.8 Ga suggests at least episodical deep ocean anoxia during the
Paleoproterozoic. Since Fe, along with C and S, are coupled with and
maintain the redox state of the surface environment, Fe seawater
concentration and isotopic composition were likely affected by the change in
the redox state of the atmosphere. Here we will present a study of Fe
isotope composition of sedimentary sulfides over geological time and provide
evidence for a change in the ocean Fe cycle at the same time as atmospheric
redox state changed.

We analyzed Fe isotope compositions of ~ 150 pyrites from 20 black shale
units, specifically focusing on Late Archean to Paleoproterozoic time. δ56Fe
values of handpicked sulfides were obtained using a Neptune MC-ICPMS at WHOI
and are reported relative to IRMM-14 with an external precision of 0.1‰. The
emerged general pattern of Fe isotope record allows dividing the Earth’s
history into three stages which are strikingly similar to the stages defined
by multiple S isotopes as well as other indicators of the redox state of the
atmosphere and ocean. (1) Stage 1 (>2.8 to 2.3 Ga) is characterized by
highly variable and negative δ56Fe values of pyrite (down to –3.5‰) that are
interpreted to reflect the reservoir effects during partial oxidation of
hydrothermally-derived Fe(II) and precipitation of Fe-oxides. (2) Stage 2
(2.3 to ~1.6 Ga) is characterized by δ56Fe values ranging from -0.3 to 1.2‰
that might be related to the increased effect of sulfide precipitation in a
redox-stratified ocean. (3) Stage 3, from 1.6 Ga through the Phanerozoic, is
characterized by sedimentary pyrite having a limited range of δ56Fe
variations (less than 0.5‰ around igneous value at ~0‰) reflecting the
establishment of an Fe-poor oxygenated ocean.

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