Middle–Upper Ordovician(Darriwilian–Early Katian) Positive Carbon Isotope Excursions in the Northern Tarim Basin, Northwest China: Implications for Stratigraphic Correlation and Paleoclimate

Three positive carbon isotope excursions are reported from Middle-Upper Ordovician in Tahe oil-gas field, northern Tarim Basin. Based on conodont biostratigraphy, the Middle Darriwilian Isotope Carbon Excursion（MDICE） and the Guttenberg Carbon Isotope Excursion（GICE） are identified from Darriwilian to Early Katian by the aid of whole-rock carbon isotope data from two well cores. Positive excursion within conodont Pygodus anserinus zone is developed in Early Sandbian, and the fluctuation range is no less than MDICE. Because the range of this excursion in the generalized global carbon isotope curve is short, previous studies paid little attention to it, and named Early Sandbian Isotope Carbon Excursion（ESICE） in this paper. Furthermore, these positive excursions are not directly related to sea level fluctuations and the MDICE and GICE identified in northern Tarim can be globally correlated to that in southern China, North America, South America, and Europe. The Saergan Fm. source rocks of Middle-Upper Ordovician in Kalpin Dawangou outcrop are in accord with the geologic time of MDICE and ESICE, and GICE have strong ties to the source rock of Lianglitag Fm. in basin. Abundant organic carbon burial is an important factor in genesis of positive isotope carbon excursions. Positive oxygen isotope excursion, conodont fauna turnover, decreased conodont total diversity, and the change of sedimentary facies indicated that dramatic changes of paleoceanographic environment of Early-Middle Ordovician in Tarim Basin started from the end of Darriwillian, and an obvious icehouse climate of Late Ordovician occurred in ESICE.

Dynamical analysis of an SDOF quasi‐linear system with jump noises and multitime‐delayed feedback forces

In this paper,stochastic dynamics of a single degree‐of‐freedom quasi‐linear system with multitime delays and Poisson white noises are investigated using an approximate procedure based on the stochastic averaging method.The simplified equations,including the averaged stochastic differential equation and the averaged generalized Fokker–Planck–Kolmogorov equation,are obtained to calculate the probability density functions(PDFs)to explore stationary responses.The expression of the Lyapunov exponent is presented to examine the asymptotic stochastic Lyapunov stability.An illustrative example of a quasi‐linear oscillator with two Poisson white noises controlled by two time‐delayed feedback forces is worked out to demonstrate the validity of the proposed method.The approximate stationary PDFs of stochastic responses and asymptotic stochastic stability are demonstrated numerically and theoretically.The results show that the Gaussian white noise has a stronger influence on the dynamics than the Poisson white noise with a small mean arrival rate.Moreover,the influence of the time delay and noise parameters on stochastic dynamics is investigated.It is found that the PDFs under the Poisson white noise approach those under Gaussian white noise as the mean arrival rate increases.The time delay can induce stochastic P‐bifurcation of the system.It is also found that the increase of time delay and the mean arrival rates of the Poisson white noises will broaden the unstable parameter region.The comparison between numerical and theoretical results shows the effectiveness of the proposed method.