Abstract
In the Early Ordovician Zhaogezhuang (赵各庄) Section of Tangshan (唐山), North China, the Yeli (冶里) Formation is composed of an entire third-order sequence, with facies ranging from the inner ramp restricted platform and open marine to the middle and even outer ramps. The Liangjiashan (亮甲山) Formation is dominated by highstand system tracts (HST) with predominantly inner ramp grain-shoal and lagoon facies. Analyzing the carbon and oxygen isotope during the whole-rock carbonate reveals the δ13C values in the Yeli Formation range from −7.11‰−0.76‰ (PDB), with the mean value at −2.98‰, while the δ18O values range of −9.09‰–−4.65‰ with the mean value at −6.12‰. The δ13C values in the Liangjiashan Formation range of −1.15‰−0.3‰, and the mean value of −0.57‰; the δ18O values are −8.76‰–−7.48‰, and the mean value is −8.06‰. The δ13C values in the Yeli Formation decrease, but at the bottom of the Liangjiashan Formation the values increase steadily. In the middle-upper formation, there is an extended fluctuation between 0–−1.00‰. The δ13C trend in the studied section is similar to that of the contemporary sections, except that it has much lower δ13C values and a more negative excursion. The correlation between the δ13C changes and the eustatic events, as well as the sedimentary facies, indicates that in the Tangshan area, the carbon isotope evolution can be attributabled to the processes of the eutrophic sea/oligotrophic sea, the seafloor organism-mediated oxidation in shallow water and the organic reduction after maximum flooding. The changes in the carbon isotope contents were primarily affected by the regional relative sea level changes. Compared to the other coeval data, the Early Ordovician of the Tang shan area is also severely depleted in 18O, with all of the δ18O sample values being below −5‰, except for one sample with a value of -4.65‰. With the ancient seawater having a δ18O value of −5.5‰ (SMOW), it is reasonable to delineate a temperature of less than 37 °C.
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