Recognition of Milankovitch cycles in XRF core-scanning records of the Late Cretaceous Nenjiang Formation from the Songliao Basin (northeastern China) and their paleoclimate implications

摘要

Cretaceous terrestrial sedimentary records are crucial for our understanding of geological systems' responses to past climate change under greenhouse condition. Numerous publications have documented that Milankovitch cycles were a dominant climate driver over multi-millennial timescales. However, most of these orbital signals were derived from marine records obtained during the Cenozoic geological period, whereas knowledge of Milankovitch cycles preserved in lacustrine sediments prior to the Cenozoic is limited due to the lack of a precise chronological framework, poor preservation rate of terrestrial sediments, limited records, and fewer experts in this research area. This paper reports high-resolution X-ray fluorescence (XRF) elemental records of K, Ti, Rb, Sr, Zr, Zr/Rb, Rb/Sr, and K/Ti from Member 1 (k(2)n(1)) and Member 2 (k(2)n(2)) of the Nenjiang Formation, which were obtained from a near-continuous SK-2 East (SK-2e) borehole drilled in the Songliao Basin (SB) of northeastern (NE) China. Variations of the elemental records reveal a humid-arid-humid-semiarid climatic evolution throughout the deposition of k(2)n(1) and a humid-arid-humid-arid climatic variation throughout the deposition of k(2)n(2). In this context, K(2)n(1) experienced a relatively longer humid period and more pronounced climatic fluctuation than K(2)n(2). A method of average spectral misfit (ASM) was adopted to successfully identify two optimal sedimentation rates of 6.577 and 8.369 cm/ka for K(2)n(1) and K(2)n(2), respectively. Based on these two sedimentation rates, nearly all significant Milankovitch cycles preserved in the Rb/Sr record were recognized. It is suggested that westerly wind was the main climatic driving factor of climate evolution in the SB under the forcing of Milankovitch cycles. The collectively regulation of obliquity and precession increased the seasonal contrasts during humid periods in the Nenjiang Formation and thereby amplified the paleomonsoon effect, thus bringing more moisture towards the SB and lead to enhanced rainfall.