Geoscience Reference
In-Depth Information
1000000
100000
GIF
BIF
10000
1000
100
10
1
0.1
500
1000
1500
2000
2500
3000
3500
4000
Age (millions of years)
Figure 9.2. Age distribution (in gigatons, gt, where one gt is 10
9
metric tons) of banded
iron formations (BIFs) and granular iron formations (gIFs). gIFs are more sand-like deposits
and lack the banding of BIFs. Figure redrawn from Bekker et al. (2010), and modified from
raiswell and Canfield (2012).
with an earlier rise of oxygen to present levels. Finally, there was per-
haps another way to explain the loss of the BIFs.
I had been compiling a history of sulfur isotopes through time while
working as a research scientist at the Max Planck Institute for Marine
Microbiology in Bremen, Germany. My real job was to explore the fate
of organic matter in sediments and the various microbial processes
responsible for metabolizing it, but the sulfur isotope history was en-
gaging, and you might call it my night job. hat emerged from this
compilation was a picture of how the sulfur cycle had evolved through
Earth history, and one striking feature was a big increase in the isotope
difference between sulfate and sulfide around the GOE (recall sulfur
isotopes from
chapter 7)
. In fact, Eion Cameron from the Geological
Survey of Canada, was the first to recognize this increase, but the new
compilation emphasized its singularity (
ig. 9.3)
. Eion had suggested
that the jump was due to an increase in the sulfate concentration of the
ocean in response to the GOE and the enhanced oxidation of sulfide
minerals to sulfate during weathering on land. This is very consistent
with what we discussed in the last chapter. The logic here is that sulfate-
reducing bacteria preferentially reduce the light sulfur isotope in sulfate,
