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2003 Seattle Annual Meeting (November 2–5, 2003)

Paper No. 11
Presentation Time: 11:00 AM

FE2O3/FEO RATIO IN AVERAGE SHALE THROUGH TIME: A REFLECTION OF THE STEPWISE OXIDATION OF THE ATMOSPHERE?


BEKKER, Andrey, Department of Earth and Planetary Sciences, Harvard Univ, 20 Oxford St, Cambridge, MA 02138, HOLLAND, Heinrich D., Earth and Planetary Sciences, Harvard Univ, 20 Oxford Street, Cambridge, MA 02138, YOUNG, Grant M., Univ Western Ontario, Biology & Geology Bldg Rm 128A, London, ON N6A 5B7, Canada and NESBITT, H. Wayne, Department of Earth Sciences, Univ of Western Ontario, London, ON N6A 5B7, Canada, abekker@fas.harvard.edu

The iron content and the Fe2O3/FeO ratio of shales have been compiled from published data for all continents in a search for secular changes in these parameters. The Fe2O3/FeO ratio in all of the >2.3 Ga shales in our data set is £ 1.0. All of the data sets for shales <2.3 Ga contain a significant number with Fe2O3/FeO ratio > 1.0. The average value of the Fe2O3/FeO ratio generally increases with decreasing age. The average (Fe2O3 + FeO) content of shales in all of the data sets is 7.0 ± 1.4 %.

The dramatic increase in the average (Fe2O3/FeO) ratio of shales between 2.3 and 2.1 Ga is consistent with other evidence for the rise of atmospheric oxygen at that time, and accounts for the removal of part of the excess O2 that was generated during the positive d13C excursion between ca. 2.22 and 2.06 Ga. The continued increase in the Fe2O3/FeO ratio in shales since 2.1 Ga probably reflects an increase in the oxidation state of the atmosphere and oceans and perhaps the recycling of progressively more oxidized crust.

Age (Ga)

Average Fe2O3/FeO (gm/gm)

Average Fe2O3 + FeO, wt %

n

>2.3

0.20

7.9

125

2.3-2.1

0.62

6.6

271

2.1-1.0

0.58

8.4

182

1.0-0.54

0.83

6.6

288

<0.54

1.34

5.9

7960

Session No. 19

T119. Revisiting the Biogeochemistry of Black Shales and Oxygen-Deficient Marine Environments
Sunday, 2 November 2003: 8:00 AM-12:00 PM
Geological Society of America Abstracts with Programs. Vol. 35, No. 6, September 2003, p.83
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