55 Processes in Biogeochemistry and Biomineralization

Wednesday, 23 March 2016
8:00 AM - 12:00 PM
Meeting Room 4

CARBON, NITROGEN, AND SULFUR ISOTOPES FROM THE 2.1 GA FRANCEVILLE BASIN, GABON

Paper number: 55-4
Program: Theme Sessions
Time: 9:00 AM – 9:20 AM

Authors

Abstract

We analyzed carbon, nitrogen, and sulfur isotopes archived in shallow marine carbonates of the 2.1 Ga Franceville Basin, Gabon to investigate biogeochemical cycling in the interval shortly after the Great Oxidation Event (GOE). The Franceville Basin preserves a sequence of 2.1 Ga sandstones, shales, and carbonates overlying Archaean crystalline basement rock. The region has been stable since the Paleoproterozoic and regional greenschist-facies metamorphism is thought not to have disturbed primary stable isotope signatures of carbonate. This is supported by previous work documenting the lack of correlation between δ13Ccarb and δ18Ocarb, as well as the absence of any stratigraphic trend in δ18Ocarb (Préat et al., 2011). Carbon isotopes of bulk organic matter have a mean value of -33.9‰ (n=35, SD=5.65), with no easily discernable stratigraphic trend. There is little covariation between our δ13Corg data and the δ13Ccarb values of the same samples reported by Préat et al. (2011). The difference between δ13Corg and δ13Ccarb of the same sample ranges widely from 26.33 to 50.28‰ VPDB, with a mean value of 38.30‰ (SD=6.23). Our carbon data are consistent with several explanations, including the variable incorporation of organic matter from multiple sources, incorporation of light carbon from methane metabolisms, as well as organic matter recycling. Pyrite sulfur isotope compositions were very positive and variable δ34Spyr with an average value of +22.03‰ VCDT (n=30; SD=10.55). Such enriched δ34Spyr values are similar to those in other sections in the early Proterozoic and the late Neoproterozoic. Our results are largely in agreement with other recent studies documenting relatively low oxygen levels soon after the GOE.

Geological Society of America Abstracts with Programs. Vol. 48, No. 1

doi: 10.1130/abs/2016NE-272489

© Copyright 2016 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.