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locked up in the sediments, the level of oxygen began to rise steadily, spawn-
ing the evolution of more advanced species.
Biochemical activity in the ocean was also responsible for stratified sul-
fide deposits (Fig. 38). Sulfur-metabolizing bacteria living near undersea
hydrothermal vents oxidized hydrogen sulfide into elemental sulfur and vari-
oussulfates. Copper, lead, and zinc, which were much more abundant in the
Proterozoic than in the Archean, also reflect a submarine volcanic origin.
PRECAMBRIAN GLACIATION
The Proterozoic was a period of transition, when oxygen generated by pho-
tosynthesis replaced carbon dioxide.Early in the Archean, the Sun’s output
was only about 70 percent of its present value. Large amounts of atmospheric
carbon dioxide 1,000 times greater than current levels kept Earth from freez-
ing solid.When the first microscopic plants evolved, they began replacing car-
bon dioxide in the ocean and atmosphere with oxygen so that today the
relative percentages of these two gases have completely reversed. The loss of
carbon dioxide, an important greenhouse gas, caused the climate to cool even
while the Sun was getting progressively hotter.
The drop in global temperatures soon after the beginning of the Protero-
zoic initiated the first known glacial epoch about 2.4 billion years ago (Table 5)
Figure 38Metal-rich
massive sulfide vein
deposit in ophiolite.
(Photo courtesy USGS)57PROTEROZOIC METAZOANS