Environmental and geochemical significance of carbon and oxygen isotopes of Ordovician carbonate paleokarst in Lunnan, Tarim Basin

Abstract

In the Lunnan area of the Tarim Basin, the fracture cave system of the Ordovician carbonate paleokarst, with strongly heterogeneous well-developed corrosion pores, caves, fissures, and a large underground water system, is buried at depths of 5068.51–5709.74 m. There has been little research on the carbon and oxygen isotopic characteristics of the cave fills of the middle–lower Ordovician karst within this area, and little is known about the regional karst developmental stages and their filling environments. In this work, systematic geochemical studies were conducted on the carbon and oxygen isotopes of 47 karst fill samples, based on which the developmental stages of the paleokarst and their characteristic filling environments were determined. Four different karstification stages were identified together with their filling environments. The results revealed δ13C (PDB) values within the range of 6.06 to −8.69 ‰ with an average of −1.40 ‰ and δ18O values within the range of −5.78 to −17.28 ‰ with an average of −10.74 ‰. The calculated deposition temperatures of the karst fracture caves were between 2.24 and 55.84 °C with an average of 22.06 °C. The Z values of the characteristic medium salinity were between 104.33 and 133.47 with an average of 119.06. Both the δ13C and the δ18O values exhibited broad ranges, resulting in a large span of calculated temperature differences that drift to opposite ends, revealing the complex and multiphase nature of the carbonate karstification process and the respective filling environments within this area.