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The transition to a sulphidic ocean 1.84 billion years ago


The Proterozoic aeon (2.5 to 0.54 billion years (Gyr) ago) marks the time between the largely anoxic world of the Archean (> 2.5 Gyr ago)1 and the dominantly oxic world of the Phanerozoic (< 0.54 Gyr ago). The course of ocean chemistry through the Proterozoic has traditionally been explained by progressive oxygenation of the deep ocean in response to an increase in atmospheric oxygen around 2.3 Gyr ago. This postulated rise in the oxygen content of the ocean is in turn thought to have led to the oxidation of dissolved iron, Fe(II), thus ending the deposition of banded iron formations (BIF) around 1.8 Gyr ago1,2. An alternative interpretation suggests that the increasing atmospheric oxygen levels enhanced sulphide weathering on land and the flux of sulphate to the oceans. This increased rates of sulphate reduction, resulting in Fe(II) removal in the form of pyrite as the oceans became sulphidic3. Here we investigate sediments from the 1.8-Gyr-old Animikie group, Canada, which were deposited during the final stages of the main global period of BIF deposition. This allows us to evaluate the two competing hypotheses for the termination of BIF deposition. We use iron–sulphur–carbon (Fe–S–C) systematics to demonstrate continued ocean anoxia after the final global deposition of BIF and show that a transition to sulphidic bottom waters was ultimately responsible for the termination of BIF deposition. Sulphidic conditions may have persisted until a second major rise in oxygen between 0.8 to 0.58 Gyr ago4,5, possibly reducing global rates of primary production and arresting the pace of algal evolution6.

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Figure 1: Stratigraphy of the Gunflint and Rove formations and sample horizons.
Figure 2: Iron speciation and S-isotope profiles.
Figure 3: S/C ratios for the Gunflint and Rove formations.

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We thank M. Jirsa from the Geological Survey of Minnesota and the staff at the Department of Natural Resources in Ontario, Canada for help in locating and accessing core material. This research was supported by a Marie Curie Individual Fellowship (S.W.P.) and the Danish National Research Foundation (Danish Grundforskingsfond).

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Correspondence to Simon W. Poulton.

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Supplementary Information

Contains a referenced note; Supplementary Figure S1 (Fe speciation in sediment cores from the Mt. McRae Shale); Supplementary Table S1 (Analytical data for the Rove Formation) and Supplementary Table S2 (Analytical data for the Gunflint Formation). (DOC 211 kb)

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Poulton, S., Fralick, P. & Canfield, D. The transition to a sulphidic ocean 1.84 billion years ago. Nature 431, 173–177 (2004).

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