Life of the Oceans - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 134

series of layers, each separated by a thin layer of mud or chert, so that
BIFs, when fossilized in stone, resemble an ancient supermarket bar
code. Then, near the beginning of the Proterozoic Eon, a new bio-
logical revolution in photosynthesis produced, for the first time, free
oxygen. This event is associated with the first great snowball Earth
glaciation of the Precambrian, called the Makganyene after the area in
South Africa where its ancient sedimentary deposits are preserved.
Perhaps Makganyene was a direct response to the oxidation of the
greenhouse gas methane in Earth's early atmosphere, plunging the
planet into an icy global climate. As oxygen began to accumulate in
the atmosphere it also changed the weathering of minerals on land,
leading to a host of new mineral forms. Notably, it oxidized iron
pyrite to sulphate, which was delivered to the oceans by rivers (see
Chapter 4). This sulphate leaves the oceans either as pyrite buried in
sediments—the end result of a bacterial transformation deep in the
ocean—or as the evaporitic mineral gypsum, calcium sulphate. The
influx of sulphate to the early Proterozoic oceans is important,
because while the near-surface waters of these oceans may have been
oxygenated, the deep oceans remained oxygen-depleted. This was the
realm of oxygen-loathing bacteria that converted the sulphate back to
its reduced form as hydrogen sulphide: in turn, hydrogen sulphide
lowers the solubility of key trace metals such as molybdenum and
copper that are essential to nitrogen fixation by cyanobacteria. Sul-
phidic ocean waters are thus bad for life in general—and not just for
oxygen-respiring forms.
In such oxygen-depleted oceans, organisms using anoxygenic pho-
tosynthesis in the deep part of the photic zone (the regions to which
light reaches) may have used up much of the nutrient supply, thus
limiting the biological productivity of photosynthesizing cyanobac-
teria in the surface waters. It seems to have been thin pickings all
round. Ocean life evolved only slowly during the almost unimaginably