Obliquity forcing of sea-level and paleoclimate variability in the mid-neoproterozoic: Insights from the ca. 820-805 Ma Hongzixi Formation, South China block

Long-term obliquity forcing (s₄-s₃) of third-order sea-level variations has been widely reported throughout the Phanerozoic. During icehouse periods, long-term obliquity cycles and third-order sea-level cycles are in phase, whereas under greenhouse, they are antiphase. However, it is uncertain whether these relationships can be extended to the Precambrian due to the scarcity of high-quality cyclostratigraphic data. A global greenhouse climate prevailed during ca. 820–805 Ma, as evidenced by geological and paleomagnetic records, providing a key climatic setting to explore the relationship between obliquity forcing and sea-level changes. Integrated with previously published two precise ages of 815.73 ± 0.81 Ma and 809.52 ± 0.50 Ma from the Hongzixi Formation in the South China Block (SCB), we performed a cyclostratigraphic analysis on a 474-m continuous silty mudstone succession in the lower part of the Hongzixi Formation. Distinct Milankovitch cycles were identified in the high-resolution magnetic susceptibility (MS) series, including 405 kyr and 95 kyr eccentricity cycles, 26.2-31.8 kyr obliquity cycles, and 15.3-18.5 kyr precession cycles. The dynamic noise model of sea-level fluctuations derived from the tuned MS series reveals ∼1.0-Myr cycles. Meanwhile, the obliquity power-to-total power (O/T) and obliquity amplitude modulation (AM) also shows ∼1.0-Myr long-term obliquity cycles. The ∼1.0-Myr long-term obliquity cycles exhibit an antiphase relationship with ∼1.0-Myr sea-level cycles, which is similar to the Phanerozoic records under greenhouse conditions. These results indicate that long-term obliquity forcing of sea-level changes persisted during ca. 820-805 Ma.