Geology Reference
In-Depth Information
hundred million years. By the time of the boring billion, the oceans were still anoxic, but
they had lost most of their dissolved iron.
Fast-forward a billion years: photosynthetic algae continued to produce oxygen, which
began to take over the oceans; by six hundred million years ago, most of Earth's oceans
wereoxygenrich,toptobottom.Whathappenedinbetween,thecruxoftheboringbillion,
is known as the intermediate ocean.
In1998geologistDonaldCanfieldoftheUniversityofSouthernDenmarkproposedthat
sulfur, not oxygen, played the major role in Earth's intermediate ocean. (Many scientists
now refer to a sulfur-dominated Mesoproterozoic ocean as the Canfield Ocean.) His pro-
vocative thesis, entitled “A New Model for Proterozoic Ocean Chemistry,” appeared in the
December 3 issue of
Nature
(after almost a year's delay by initially reluctant reviewers)
and soon transformed the way many of us think about the ocean through deep time.
The core idea is simple. The Great Oxidation Event produced enough oxygen to influ-
ence the distribution of many “redox sensitive” elements, including iron, but not nearly
enough to oxygenate the oceans. On the other hand, enhanced weathering and oxidation of
the land introduced lots of sulfate into the oceans. Hence the intermediate ocean became
enriched in sulfur, while it was poor in oxygen and iron, a steady state that persisted for a
billion years.
Hanging On
The fossil record reinforces the view of an ever so slowly changing intermediate ocean.
Some rocks deposited between 2 and 1 billion years ago preserve microscopic fossils of
unprecedented quality. The 1.9-billion-year-old Gunflint chert of North America, the 1.4-
to1.5-billion-year-oldGaoyuzhuangformationofnorthernChina,andthe1.2-billion-year-
old Avzyan formation from the Ural Mountains in Russia contain minute fossil microbes
so sharp and clear, some in the intimate act of dividing, that they look just like their mod-
ern living counterparts. Yet such striking improvement in the quality of some fossils only
reflects their less altered history, not anything intrinsically novel about that time in Earth
history.
Theprotractedanoxic,sulfidicintermediateoceanwasbothgoodnewsandbadnewsfor
life.Ontheplusside,theinfluxofsulfateprovidedanexcellentenergysourceforsomemi-
crobes—they made their living by reducing the sulfate to sulfide. Hints from the fossil re-
cord, including distinctive molecular biomarkers, sulfur isotope data, and even some well-
preserved microbes in chert, all point to a thriving Mesoproterozoic coastal population of
greenandpurplesulfurbacteria.Thesesulfur-eatingmicrobes,whichpersisttodayinsome
anoxic environments, produce organic sulfur compounds that smell just awful—like a tra-
gically failed septic system.
