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Abstract A cooling trend during the Middle Ordovician may have resulted in significant biotic, atmospheric and hydrologic changes, but the exact timing and forcing of this climate event still remain largely ambiguous. Here, we present astronomically calibrated magnetic susceptibility (MS) and gamma ray (GR) data from the shale and carbonate successions of the Kalpin area in the Tarim Basin, NW China to study the Middle Ordovician climate. The spectral analyses of the 405-kyr calibrated MS and GR time series reveal periodicities close to 405 (calibrated), 100, 31 and 19 kyr, supporting a Milankovitch forcing for the sedimentary cyclicity. The eccentricity and obliquity amplitude modulation cycles show main periodicities of ~2.4 and ~1.2 Myr, respectively. A cooling event along with the middle Darriwilian isotope carbon excursion (MDICE) appears to have been initiated by the alignment of the eccentricity and obliquity amplitude minima at 4.07 Myr prior to the Darriwilian/Sandbian (D/S) boundary. This climate event was in pace with high-amplitude obliquity oscillations during a period when the 1.2 Myr obliquity amplitude modulation cycles controlled the global third-order eustatic sequences. The termination of this cooling event corresponding to a global third-order highstand was initiated by an eccentricity oscillation maximum at 0.63 Myr prior to the D/S boundary. Subsequent climate alleviation was dominated by eccentricity forcing until the earliest Sandbian. We suggest that although the climate changes have been primarily associated with the CO 2 level, a specific sequence of astronomical variations during the Middle Ordovician was responsible for the time when the critical threshold was crossed. Highlights • Onset and termination of a cooling event in the Middle Ordovician were dated. • Obliquity forcing dominated this cooling event. • 1.2 Myr obliquity modulation cycles controlled the third-order eustatic sequences. • Onset and termination of cooling event were controlled by astronomical forcing. [ABSTRACT FROM AUTHOR]

Pliocene diachronism and mismatches between planktic foraminifera and nannofossil bioevents are clearly observed in the Mediterranean Sea. Hence, astronomical tuning combined with calcareous nannofossils and plankton foraminiferal biozones constitutes one of the important tools for regional correlation between different paleolatitudes. Consecutive and high-resolution well logs from the Sapphire-Dd borehole in the Mediterranean contributed a vital role in exploring the astronomically forced sedimentation in Pliocene strata. A detailed cyclostratigraphic analysis is applied to the natural gamma-ray spectrum logging data that is used as a paleoenvironmental and paleoclimatic proxy to construct a floating astronomical time scale (ATS) for the first time in Egypt. The results show that the hierarchy of sedimentary cycles with a ratio of ~20 : 5 : 2 : 1 in the Kafr EI Sheikh Formation can be interpreted as Milankovitch cycles of eccentricity, obliquity, and precession cycles, respectively. The ATS is established by tuning the Gamma Ray log to the target curves of the insolation of the La2004 solution based on the biozones time framework of the Sapphire-Dd core, which indicates the highest occurrence (HO) of calcareous nannofossils Sphenolithus verensis at 4.03 Ma, the HO of Triquetrorhabdulus rugosus at 5.24 Ma, the highest common occurrence at 4.093 Ma; and the lowest common occurrence of planktic foraminifera Globorotalia margaritae at 5.1 Ma, respectively. The new time scale yields more reliable age constraints on the evolution of paleoclimate and paleoenvironment in southeastern Mediterranean area since the Pliocene. • Astronomical tuning combined with calcareous nannofossils and plankton foraminiferal biozones. • Pliocene calcareous nannofossil and planktic foraminiferal biozones. • Diachronous in the stratigraphic positions of marine microfossils between different palaeolatitudes are noted. [ABSTRACT FROM AUTHOR]