Orbital‐Driven Lake/Sea‐Level Inverse Relationships on Greenhouse Earth: Evidence From the Chang 7 of the Triassic Yanchang Formation (Ordos Basin, China)

ABSTRACT

Astronomical forcing is widely recognized as a key driver of sedimentary cyclicity; however, its control on lacustrine hydrological changes—and the relationship of these changes to global sea‐level fluctuations under greenhouse conditions—remains insufficiently understood. To better investigate these controls and their interrelationships, a high‐resolution floating astronomical timescale spanning 7.676 million years was established for the Chang 7 member of the Triassic Yanchang Formation in the Ordos Basin, which was deposited under greenhouse conditions. This timescale was constructed using the multi‐taper method (MTM), correlation coefficient (COCO), evolutive correlation coefficient (eCOCO), and sedimentary noise modelling methods implemented in Acycle, as well as the average spectral misfit (ASM) method in the Astrochron toolkit. The timescale was obtained by tuning gamma‐ray log data to the 405‐ka eccentricity cycle. The astronomical forcing mechanisms of lake‐level fluctuations and their relationship with global sea‐level changes could thus be established. The lacustrine system showed a stable response to long‐term modulations of Earth's orbital eccentricity and obliquity. The ~2.4‐Myr eccentricity and ~1.2‐Myr obliquity modulation cycles correspond closely with the lake‐level fluctuations, demonstrating that the lacustrine system responded sensitively to Myr‐scale orbital forcing. In the absence of polar ice sheets, the lacustrine hydrological system was highly sensitive to orbital obliquity, showing a stronger response than global sea‐level variations. When orbital obliquity was high, increased precipitation at high latitudes boosted aquifer recharge, raising lake levels, while marine water sequestration into continental aquifers reduced ocean volume. The consequence was a rise in lake level but a drop in the sea level. It implies that orbital modulations, by altering mid‐ to high‐latitude precipitation and groundwater recharge, can induce an inverse relationship between lake‐ and sea‐level fluctuations. This supports the applicability of the aquifer–eustasy mechanism: lacustrine systems, as sensitive recorders of hydroclimatic changes, demonstrate long‐term “hydrological memory”, capable of capturing regionally expressed water redistribution at orbital timescales. These findings provide new geological evidence for reconstructing the Middle to Late Triassic climate and hydrological evolution, and offer theoretical support for understanding the sea/lake system's coupling mechanisms under greenhouse conditions.