Abstract
The silicon isotope composition of chert has recently been used to study the historic evolution of the global ocean. It has been suggested that Precambrian cherts have much higher δ30Si 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. However, cherts have various origins, and their isotopic compositions might be reset by metamorphic fluid circulation; thus, different types of cherts should be distinguished. Here, we present a new set of δ30Si data for cherts from early and middle Proterozoic carbonate rocks from Northern China. We found that cherts of 1.355–1.325 Ga show a peak range of 2.2–3.9‰. Based on these results, we propose that from the Archean to the middle Proterozoic, there was a drastic decrease in silicon content and an increase in the δ30Si value in ocean water due to a temperature decrease and biological activity increase. After that period, the silicon content of the ocean was limited to a low level by a high degree of biological absorption, and their δ30Si values varied in a small range around a significantly lower value.
Highlights
Input from the continent to the ocean became dominant, and the Si input from sea-floor weathering and submarine hydrothermal fluids became less prevalent; peritidal chert may be more significant in tracing the Si isotope composition of the ocean
Where A represents the isotope, R represents the isotope ratio, the subscript Sa refers to the sample and St refers to the standard
The obtained δ18OV-PDB values of dolomites range from −1 2.9‰ to −3 .5‰, with an average of −7 .11 ± 2.15 (1 SD) ‰. These values are similar to those reported in previous studies[22,23], reflecting their normal sedimentary origin in a shallow marine environment
Summary
Input from the continent to the ocean became dominant, and the Si input from sea-floor weathering and submarine hydrothermal fluids became less prevalent; peritidal chert may be more significant in tracing the Si isotope composition of the ocean. We collected a suite of samples of carbonate rocks containing chert bands and nodules from the early Proterozoic Hutuo Group[19] (2.35–2.20 Ga) and the middle Proterozoic Changcheng and Jixian Systems[20,21] (1.63– 1.20 Ga) in Northern China (Fig. 1). These cherts were selected because they showed good preservation of their original structure and composition (Figs 2 and 3); i.e., no obvious effects of metamorphism or weathering were found in the selected samples. The Si and O isotopic compositions of the chert and the C and O isotopic compositions of dolomite were studied systematically
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