Geoscience Reference
In-Depth Information
G
O
E
60
40
20
0
-20
-40
-60
500
1000
1500
2000
2500
3000
3500
4000
Age (millions of years)
Figure 9.3. Compilation of sulfur isotopes through time. Diamonds represent individual
analyses of sediment sulfides whereas the parallel lines reflect an estimate for the isotopic
composition of seawater sulfate. the gOe is also marked. Figure modified from raiswell
and Canfield (2012).
the extent to which they do so is greatly reduced at very low sulfate
concentrations.
6
With my compilation in hand, I could therefore only
agree with Eion, but one could take the argument another step. An in-
creased flux of sulfate to the oceans should stimulate more sulfate reduc-
tion, producing more sulfide in the ocean. As a result, more dissolved
iron would be removed in the formation of pyrite (FeS
2
).
Thus, piecing the whole thing together, I reasoned that the GOE
likely increased the flux of sulfate to the oceans through the oxidative
weathering of sulfides on land, thus enhancing rates of sulfate reduc-
tion to hydrogen sulfide in the ocean. The GOE did not, however, at
least in my thinking, produce enough oxygen to oxygenate the oceans.
Therefore, dissolved iron was removed from the oceans not by reaction
with oxygen as Holland and Cloud supposed, but rather by reaction
with sulfide. So in my proposal, the deep oceans remained anoxic and
became rich in sulfide, instead of becoming well oxygenated. Andy
Knoll from Harvard University soon named this sulide-rich oceano-
graphic state the “Canfield Ocean,” and for better or worse the name
has stuck. At the time I made this proposal, however, there was not a
stitch of evidence to back it up, so it became a high priority to look for
evidence to either support or falsify the Canfield Ocean model and to
more generally define the state of Earth surface and ocean chemistry
after the GOE.
