Some Factors Influencing Biomineralization in Earth History
The history of the earth from first life is reviewed in terms of various factors that could influence biomineralization. The first record of life, prokaryotic organisms, seems to coincide with the first occurrences of sedimentary rocks. Carbon isotopes and the organic content of sediments 3.6 billions of years old are like those of today, attesting to oxygen production because of carbon dioxide reduction and organic Carbon deposition. However, oxygen sinks were so abundant that atmospheric oxygen levels remained low (10−5 Today?) until perhaps 2.0 billions of years ago. Thus there was a very long period (until 2.5–3.8 billion years ago) during which oxidation of iron, sulfur and primordial gases was taking place, increasing the reservoirs of oxygen — sulfates and ferric iron compounds and decreasing oxygen sinks — chiefly ferrous silicates, ferrous carbonates and ferrous sulfides.
Until about 2.0 billion years ago ferrous iron cycled in the hydrologic cycle with calcium, magnesium, and manganese. Sulfides were oxidized irreversibly to sulfates in sedimentary systems; the resultant sulfates may have been reduced by circulation of sea water through basaltic ocean floor, but bacterial reduction of sulfates apparently was not important until 2.7 billion years ago, and perhaps not until about 2.3 billion years ago.
The abundance of siderite in sedimentary deposits until about 2.0 billion years can be interpreted as a result of irreversible sulfur oxidation and perhaps, therefore, generation of a high methane, carbon dioxide atmosphere, which would give an important greenhouse effect, and high global temperatures (40–60° C?).
Many lines of evidence point to 2.0 billion years as the time of the transition from low oxygen levels in the atmosphere (but perhaps coincident with continued high O2 production), and important bacterial reduction of sulfate in the oceans, converting an irreversible oxygen sink to a system in which sulfur oxidation and reduction was reciprocal to carbon oxidation and reduction during sedimentary cycling.
Little is known about the important transitional period from about 2.0 billion years until about 0.7 billion years ago when the first metazoa appeared and the redox system of the world became dominated by sulfur and carbon, with iron taking a subsidiary role.
Since the beginning of the Phanerozoic (here assumed at 0.7 billion years), the earth surface system seems to have lost any truly continuous long term secular trends, but has oscillated over very long time spans between a system with a net increase in sulfate and a decrease in organic carbon and a system reversing that trend. The evolution of vascular plants makes its mark in the sedimentary record with an increase in the ratio of reduced carbon to oxidized carbon in sediments and perhaps an increase in atmospheric O2.
We are on the verge of understanding some emerging Phanerozoic correlations. Reversals in the trend of σ 34S with time (over very long intervals) seem to be related to evaporites and phosphorites. Increases in the ratio of terrestrial to marine sediments seem to correlate with changes in σ 13C of marine organic carbon, and perhaps with increases in atmospheric oxygen levels.
Finally, it must be recognized that diagenetic changes in sediments tend to obscure primary secular changes. Perhaps research will permit separation of the effects of these processes that affect the overall mineralogy, chemistry, and biotic content of ancient sediments.
KeywordsFerrous Iron Sulfur Isotope Banded Iron Formation Evaporite Deposit Ferrous Sulfide
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