T19 Revisiting the Biogeochemistry of Black Shales and Oxygen-Deficient Marine Environments

Sunday, 2 November 2003
8:00 AM - 12:00 PM

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

Paper number: T19-13
Program: Technical Sessions
Time: 11:00 AM – 11:15 AM

Authors

Abstract

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