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

Highlights

• Milankovitch cycles were recorded in Rb/Sr series from Member 1 (K₂n¹) and Member 2 (K₂n²) of the Nenjiang Formation.

• K₂n¹ experienced a longer humid period and more pronounced climatic fluctuation than K₂n².

• The mean sedimentation rates of 6.577 and 8.369 cm/ka for K₂n¹ and K₂n² were provided by ASM method.

• The collectively regulation of obliquity and precession amplified the paleomonsoon effect during humid periods in K₂n¹ and K₂n².

Abstract

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 (k₂n¹) and Member 2 (k₂n²) 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 k₂n¹ and a humid-arid-humid-arid climatic variation throughout the deposition of k₂n². In this context, K₂n¹ experienced a relatively longer humid period and more pronounced climatic fluctuation than K₂n². 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 K₂n¹ and K₂n², 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.

Introduction

The Cretaceous Period is considered to be a paradigm of the greenhouse climate period (Skelton et al., 2003, Bice et al., 2006) and detailed studies on Cretaceous climate change provide a potential to improve our understanding of modern global warming. However, compared to widely documented oceanic responses to Cretaceous climate change determined from marine sediments (Grocke et al., 1999, Ostrander et al., 2017, Seibold and Berger, 2017), fewer studies have been conducted using terrestrial settings (Hasegawa, 2003, Heimhofer et al., 2005). In this respect, the Cretaceous Songliao Basin (SB) in NE China is an excellent candidate for researching terrestrial climate change (Chen, 1987). A near-complete Cretaceous lacustrine sedimentary record has been recovered by the International Continental Scientific Drilling Project Of Cretaceous Songliao Basin (Wang et al., 2013). The program was conducted in two stages: SK-1 of Upper Cretaceous, which involves sites SK-1 North (SK-1n) and SK-1 South (SK-1s), and SK-2 of Lower Cretaceous, which involves sites SK-2 East (SK-2e) and SK-2 West (SK-2w) (Fig. 1). The valuable cores achieved from this program are the longest and most continuous Cretaceous continental sedimentary records in the world to date. Thus, studying these sedimentary records provides us a unique chance to explore how the terrestrial system responded to various types of climatic forcing, such as orbital forcing under greenhouse condition.

The effects of orbital changes on solar radiation and the Earth’s climate have been recognized for over a century (Hinnov, 2000). Numerous publications have documented orbital forcing as a dominant climate driver over multi-millennial timescales. The most important effects relate to the Milankovitch cycles of eccentricity with periods of 400 ka and 100 ka, obliquity with a period of ~40 ka, and precession with a period of ~20 ka (Milankovitch, 1941). Numerous studies have focused on Milankovitch theory when addressing long-standing questions regarding underlying mechanisms of climate change in Earth history, but most of them were based on carbon and oxygen isotopes in marine sediments (Holbourn et al., 2005, Holbourn et al., 2007, Kuhnt et al., 2005, Prokoph et al., 2001, Sageman et al., 2006) or concentrated on typical rhythmic deposits (Boulila et al., 2015, Eldrett et al., 2015a). For example, scientists have successfully recognized that orbital signals were preserved in the frequently-occurred Neogene sapropels within the semi-closed basin of the Mediterranean, after which a highly accurate chronostratigraphic framework was established based on these orbital signals (Krijgsman et al., 1999, Lourens et al., 2001, Hilgen et al., 2003). Orbital cycles have also been discovered in lithologic and geochemical records from Eocene Green River Formation within the Green River Basin of Wyoming, USA (Sloan and Morrill, 1998, Morrill et al., 2001). Yet in contrast, except for the floating astronomical time scale constructed for the Late Triassic by Kemp and Coe (2007), the cyclostratigraphic analyses of Upper Cretaceous from Wu et al., 2009, Wu et al., 2013, Wu et al., 2014 based on logging data, the astronomical tuning of the Upper Triassic Xujiahe Formation of South China conducted by Li et al. (2017), and several astrochronologic studies on the Newark basin (Olsen et al., 1996, Olsen et al., 2010), relatively less research has been conducted on orbital cycles using geochemical records when it comes to greenhouse periods.

The Upper Cretaceous Nenjiang Formation provides a geological record spanning 84.7–79.1 Ma (Wan et al., 2013), and the entire sedimentary strata can be divided into five stages (Fig. 2). The two bottom sedimentary stages, named Member 1 and Member 2 of the Nenjiang Formation (K₂n¹ and K₂n²), are representative of the evolution of the SB. For example, the second large-scale lake transgression, the second lake anoxic event, periodic seawater incursion events, and the water salinization event all occurred during the deposition of K₂n¹ and K₂n² (Zhang et al., 1977, Feng et al., 2012, Huang et al., 2013, Cao et al., 2016). It is also known that the hydrocarbon source rocks formed in the sedimentary strata of K₂n² was attributed to anoxic conditions (Huang et al., 1998), and that the sedimentary period of the Nenjiang Formation was one of the three extreme greenhouse climate peaks in the Cretaceous (Gao et al., 1999). Some researchers have found evidence of a rapid climatic cooling event that occurred at the boundary of the Santonian and Campanian stages when global temperatures dropped by about 5–6 °C (Friedrich et al., 2012, Ando et al., 2013). In general, the Nenjiang Formation, acting as an optimum document of climatic evolution during the greenhouse period, is preferred for the Milankovitch research.

X-ray fluorescence (XRF) core scanning has become a widely used geochemical analytical tool for evaluating the elemental composition of sediments (Groudace et al., 2015). Compared with traditional wavelength-dispersive and energy-dispersive XRF analysis, XRF core scanning is more suitable for the continuous drilling of marine and terrestrial sediments. Meanwhile, it has the advantages of enabling a rapid and non-destructive analysis, as well as providing a set of data with unprecedented resolution (Kylander et al., 2011), making XRF core scanning an outstanding method in reconstructing the paleoclimatic and paleoenvironmental evolution from a new perspective (Melles et al., 2012, Wennrich et al., 2014).

This study presents high-resolution XRF element data from K₂n¹ and K₂n² of the SK-2e core with the specific objectives of (1) reconstructing the climate change history during the deposition of K₂n¹ and K₂n²; (2) identifying potential Milankovitch cycles preserved in paleoclimate proxies; (3) exploring possible climatic drivers within the SB that were modulated by Milankovitch cycles.