New Paleomagnetic and Stable-Isotope Results from the Nanxiong Basin, China: Implications for the K/T Boundary and the Timing of Paleocene Mammalian Turnover

摘要

The Nanxiong Basin (Guangdong Province, China) preserves the most complete Asian stratigraphic record of the Cretaceous-Paleogene (K/Pg) boundary extinction and the subsequent Paleocene mammalian radiation. Despite extensive study, the precise placement of the K/Pg boundary in the Nanxiong Basin sequence has been controversial, and the timing of subsequent mammalian turnover is poorly constrained. We present new paleomagnetic and geochemical data from the Late Cretaceous Pingling Formation (Nanxiong Group) and the overlying Paleocene Shanghu, Nongshan, and Guchengcun formations (Luofozhai Group). Our samples are directly correlated with previous geochemical and paleontological sampling localities, allowing for easy comparison with other local proxy records. Results indicate that the traditional placement of the K/Pg boundary at the base of a chaotic channel sandstone bed marking the highest stratigraphic appearance of dinosaur eggshell fragments and lowest stratigraphic appearance of Paleocene mammalian fossils lies about two-thirds of the way up Chron C29R, consistent with the placement of the boundary in all other well-documented sections. The average carbon isotope composition of paleosol carbonates decreases by 12‰ in the Early Paleocene, consistent with a major disruption to global carbon cycling after the K/Pg boundary. Constraints on the age of the first major Cenozoic mammalian turnover event in Asia (the Shanghuan-Nongshanian Asian Land Mammal Age boundary) support its placement near the top of Chron C27N, which coincides with a similar turnover in North America and geochemical changes recorded in several deep sea cores.