The δSi peak value discovered in middle Proterozoic chert and its implication for environmental variations in the ancient ocean.

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 δSi 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 δSi 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 δSi 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 δSi values varied in a small range around a significantly lower value.