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High-resolution (930 samples) magnetic susceptibility (MS) and anhysteretic remanent magnetization (ARM) analyses were conducted on the ~ 49 m thick Middle Permian Maokou Formation at Dukou, South China. Spectral analysis of covariant MS and ARM series reveal Milankovitch cycles with eccentricity, obliquity, and precession. A 5.7 myr floating 405 kyr eccentricity cycle-calibrated astronomical time scale was constructed and used as a geochronometer for estimating the durations of the conodont zones. The results indicate that the durations of the Jinogondolella postserrata , J. altudaensis , J. prexuanhanensis , and J. xuanhanensis conodont zones were 490 kyr, 560 kyr, 820 kyr, and 750 kyr, respectively. The 405 kyr-calibrated MS and ARM series also show prominent ~ 1 myr cyclicity, which may correspond to the secular frequencies of Earth and Mars, s 4 –s 3 , in the Middle Permian. The early Capitanian time interval was synchronous with the Kamura cooling event, and the accentuated obliquity cycling may signal ice-sheet expansion. The modulations of the obliquity have a close relationship with third-order eustatic sequences of the Wordian and Capitanian stages, suggesting a glacioeustatic origin during the Middle Permian. [ABSTRACT FROM AUTHOR]

[Display omitted] • A newly constructed astronomical time scale constrains the duration of the GICE to ∼ 0.82 Myr. • The duration of the Middle Ordovician stratigraphic gap in South China is ∼ 1.38 Myr. • Positive 13C carb excursions during the MDICE and GICE were associated with astronomical forcing. The Middle–Late Ordovician transition was one of the most critical periods throughout the Earth's history, and it involved paleoclimate changes and biological turnover. Here, we conducted cyclostratigraphic analysis on the magnetic susceptibility (MS) from the Kuniutan and Miaopo formations in the Zhenjin section and the Pagoda and Linhsiang formations in the Puxihe section in Yichang, South China. We constructed an astronomical time scale to understand the astronomically forced paleoclimate changes during this key interval. Spectral analyses on the MS series show that the thickness ratios of different sedimentary cycles in the studied sections are consistent with those of Milankovitch cycles during the Middle and Late Ordovician. The floating astronomical time scale of the studied sections was constructed based on astronomical tuning to 405-kyr eccentricity cycles, providing an estimated duration of ∼ 0.82 Myr for the Guttenberg isotopic carbon excursion (GICE). A high-precision and continuous astronomical time scale has previously been established in the Ordovician marine sediments in the Tarim Basin. The cyclostratigraphic correlation of sedimentary successions between the South China Block and Tarim Basin reveals that the duration of the Middle Ordovician gap in South China is ∼ 1.38 Myr. Alignment of the maxima of the ∼ 1.2-Myr obliquity cycle and the 405-kyr eccentricity cycle may have resulted in increased weathering inputs and prolonged periods of ocean anoxia, pacing the enhanced organic carbon burials and the positive isotopic carbon excursion during the middle Darriwilian isotope carbon excursion (MDICE) and GICE. [ABSTRACT FROM AUTHOR]

Abstract: Magnetic susceptibility of Chinese loess–paleosol sequences has been extensively used as a reliable paleoclimatic proxy for reconstruction of the Asian summer monsoon system, and its enhancement is usually attributed to neoformation of fine-grained ferrimagnetic particles via pedogenesis, but there is little understanding of either the formation process of such magnetic particles or their actual form and grain sizes. To investigate these problems, scanning electron microscope observations were conducted on magnetic extracts from pristine loess and strongly-pedogenic paleosol samples from Xifeng on the Chinese Loess Plateau. Both magnetic extracts commonly contained coarse-grained detrital magnetite with grain size ranging from a few μm to several tens of μm. However, the magnetic extract from the mature paleosol sample additionally included coarse silicate particles with ultrafine ferrimagnetic inclusions, while that from the pristine loess sample did not. The size of the ferrimagnetic inclusions mainly ranged from tens to hundreds of nanometers. The temperature-dependent susceptibility and hysteresis loops of the paleosol sample, combined with the stratigraphic variations of rock-magnetic parameters, showed that the pedogenic maghemite inclusions should be a potential contributor to magnetic enhancements in Chinese paleosols. The fine-grained ferrimagnetic inclusions have been pedogenically formed by weathering of coarse-grained Fe-bearing silicate minerals. Our proposed magnetic enhancement mechanism is consistent with the accepted rock magnetic result that magnetic enhancement in Chinese paleosols is caused by pedogenic maghemite grains in superparamagnetic and single-domain size regions. The presence of ultrafine ferrimagnetic grains as inclusions within coarse silicate particles is also consistent with Han and Jiang''s (1999) significant experimental result that magnetic enhancement in Chinese paleosols is mainly contributed by the >0.5μm sediment fractions. [Copyright &y& Elsevier]