Carbon and Strontium Isotopes of Late Palaeozoic Marine Carbonates in the Upper Yangtze Platform, Southwest China

Abstract238 marine carbonate samples were collected from seven sedimentary sections of the entire late Palaeozoic (Permian, Carboniferous and Devonian) in the Upper Yangtze Platform, southwest China. Based on the absence of cathodoluminescence and very low Mn (generally < 50 ppm) contents of the samples, it is thought that they contain information on the original sea water geochemistry. The results of isotopic analyses of these samples are presented in terms of δ13C and 87Sr/86Sr ratios versus geological time. The strontium data, consistent with other similar data based on samples from North America, Europe, Africa and other areas in Asia, support the notion of a global consistency in strontium isotope composition of marine carbonates. The strontium data exhibit three intervals of relatively low 87Sr/86Sr ratios in the late Middle Devonian to early Late Devonian, Early Carboniferous and Early Permian, corresponding to global eustatic high sea level stands. The lowest 87Sr/86Sr ratio recorded in the Late Permian was probably caused by substantial basalt eruptions in the Upper Yangtze Platform at the time. Three corresponding periods of relatively high δ13C values at roughly the same time intervals were caused by a relatively high rate of accumulation of organic carbon during sea level rises at these times. The deposition of coal was probably responsible for the increase of sea water δ13C at other times. The δ13C values drop dramatically near the Devonian/Carboniferous, Carboniferous/Permian and Permian/Triassic boundaries, consistent with other similar data, which further support the notion that geological time boundaries are associated with mass extinction and subsequent rejuvenation.