Marine carbonatedominated strata from the Jixian section, Tianjin, spanning from the Mesoproterozoic to early Neoproterozoic, are intercalated with a large number of chert bands. Especially, the Wumishan Formation is composed of thickly bedded dolomite intercalated with chert bands and is also characterized by well developed sedimentary rhythm and biological activity. However, the mechanism of formation of the chert bands is debated. We systematically studied the boron isotopic composition of carbonate rocks and the silicon and oxygen isotopes composition of chert bands in the Mesoproterozoic Jixian section. The δ 30 Si NBS28 value of chert bands ranges from 0. 6‰~3. 3‰ (an average of 2. 2‰), which is significantly higher than that of the chemically precipitated chert from hot water environment, but close to the silicon isotope composition of the marine biosedimentary siliceous rocks in shallow water environment. The δ 18 O V- SMOW value of the chert bands is from 21. 7 ‰~27. 8 ‰ (an average value of 25. 5 ‰), obviously higher than that of hydrothermal siliceous rocks, but similar to that of normal marine siliceous rocks. The boron isotopic composition of pristine carbonatic rocks (dolomite and dolomite limestone etc.) associated component ranges from 0. 0‰~11. 0‰ (an average of 4. 8‰), which is obviously lower than that of modern marine carbonate. The δ 11 B values of siliceous banded dolomite samples from Gaoyuzhuang Formation to Wumishan Formation are generally higher ranging from 3. 3‰ to 12. 9‰ with an average of 8. 4‰. Under the acid condition, decrease in local pH would result in more B(OH) 3 with higher δ 11 B incorporated into the lattice of carbonate minerals and an elevated δ 11 B C in carbonate phase. This indicates that siliceous banded dolomite may have formed in a local acidic water environment. Combined with the correlation between carbonate, SiO 2 dissolution/precipitation and pH, this study suggests that siliceous banded dolomite was a synsedimentary biochemical sedimentary origin of chert bands in Jixian section. Mesproterozoic seawater had a high concentration of SiO 2 and was basically in a saturated state, with a high Mg/Ca ratio. Biological activities have become an important factor affecting the marine environment. During the flourishing period of biological activities, a large amount of organic matter was deposited at the bottom of the sea, resulting in rapid decreasing of pH (e. g. , pH<7. 8) at the seawatersediment interface and increasing of the solubility of dolomite and other carbonate; on the contrary, decreased solubility of SiO 2 led to saturation and this gave a rise to the precipitation of silica and form chert bands/lenses. In the weak period of biological activity, the pH value increases to the normal level, with the solubility of SiO 2 increasing and the carbonate solubility decreasing, resulting in precipitation of carbonate such as dolomite and others. The periodic change of biological activity resulted in the formation of striped rhythmic layer of dolomite and chert. The boron isotopic composition of siliceous banded dolomite reflecting δ 11 B and pH of local fluids does not apply to deducing the δ 11 B composition of sea water.