Geoscience Reference
In-Depth Information
Nearly every pre-2.4-billion-year-old sedimentary rock containing sul-
fur was affected by it, so the process(es) producing these weird fraction-
ations was a large-scale global phenomenon. To understand the cause
of these mass-independent fractionations, we need to think about where
sulfur came from and how it got into sediments. Today, rivers supply
most of the sulfur to the oceans in the form of sulfate. The sulfate comes
from the weathering of pyrites in the presence of oxygen and from the
dissolution of sulfate-bearing rocks (most commonly gypsum), which
formed sometime in the past through the evaporation of seawater. Do
you remember our drive in chapter 4 with Dave Des Marais into the
Mexican salt company housing those beautiful cyanobacterial mats?
During that drive, we passed several ponds where gypsum was precipi-
tating, and this occurs before the seawater is concentrated to the point
of salt (NaCl) formation.
Anyway, with a good source from rivers, sulfate can accumulate in
the oceans to pretty high concentrations. Take away the oxygen, how-
ever, and pyrites wouldn't oxidize to sulfate. We saw evidence for this in
the Witwatersrand and related Archean river deposits, where pyrites
were left unoxidized as river sands. Without a riverine sulfate source,
the concentration of sulfate in the oceans becomes very low, 7 and criti-
cally, other sources of sulfate to the oceans likely become significant.
James followed this line of reasoning, and he decided that an atmo-
spheric source of sulfur made sense because the effect is globally dis-
tributed, and he thought that volcanoes might provide a good sulfur
source. Indeed, way back in 1962, Dick Holland had recognized the
possible significance of a volcanic source of sulfur to the early Earth
surface environment. If you measure the gases emanating from volca-
noes today, you find that the major sulfur gas is sulfur dioxide, or SO 2 .
Hydrogen sulfide (H 2 S) is also found in these gases, and whether SO 2
or H 2 S dominates depends, importantly, on the chemistry of Earth's
mantle at the location where the volcanic gases originate. The chemistry
of the mantle, however, has likely changed little through Earth history, 8
so SO 2 was the probably also the most important volcanic sulfur spe-
cies on the early Earth.
Stimulated by his initial discovery, James conducted a series of ex-
periments in which he subjected SO 2 gas to various wavelengths of
ultraviolet light, which has the appropriate energy to transform SO 2 gas
 
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