Obliquity-forced aquifer-eustasy during the late-Cretaceous greenhouse world

The mechanism of short-term and high-magnitude sea-level oscillation has long been debated between glacio- and aquifer-eustasy (Miller et al., 2005; Haq, 2014), largely due to the sparse robust evidence for the aquifer-eustasy, and the little knowledge about hydrological dynamics behind it. Non-marine/ continental greenhouse archives (e.g. lake level) and their temporal correlation to marine successions (e.g. sea level) could give clue to aquifer-eustasy (Wagreich et al., 2014). The Songliao Basin (SLB), in Northeast China, is one of the largest Mesozoic terrestrial inland basins and has deposited the near whole Cretaceous successions (Wang et al., 2013). The greenhouse Late Santonian-Early Campanian Lower Nenjiang Formation (K₂n¹⁺²), recovered from three boreholes in SLB provides a unique opportunity for validating and decoding the aquifer-eustasy. Initially the cyclostratigraphy of logging gamma ray (GR) and Thorium (Th) series from three boreholes was implemented, which in junction with the radioactive ages renewed the chronology framework of SLB. Using the astronomically tuned GR and Th series, the lake level of SLB, which is recovered from sedimentary noise modeling (Li et al., 2019) and presents the water table of groundwater reservoir, shows a clear out-of-phase relationship with the coeval sea level, validating the aquifer-eustasy hypothesis. The lake level shows prominent ~1.2Myr cycles and a well-coupled relationship with sea level and obliquity modulation, indicating that the orbital obliquity drove the lake level and modulated the water exchange between ocean and continent during the Cretaceous greenhouse period. The strong precipitation indicated by the negative excursion of Ostracods δ¹⁸O (Chamberlain et al., 2013) well correlates to the high lake level, high obliquity, and low sea level, suggesting that during obliquity modulation maxima, more moisture was precipitated into the high-latitude continents, consequently recharging the aquifer and raising the lake level while drawing down the sea level and vice versa. The close correspondence between reported marine incursion layers (Hu et al., 2015) and lowstand of sea level casts a doubt on marine incursion hypothesis in the SLB, more work is needed to reconcile this paradox. Overall, this study gives robust geological evidence for aquifer-eustasy and firstly decodes its role on Cretaceous short-term eustasy.