It has been suggested that Precambrian cherts have much higher δ 30 Si values than Phanerozoic cherts do and that the former show an increasing trend from 3.5 to 0.85 Ga, reflecting a decrease in ocean temperatures.
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The δ<sup>30</sup>Si peak value discovered in middle Proterozoic chert and its implication for environmental variations in the ancient ocean.
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