[M-IS16] 古気候・古海洋変動

[MIS16-P26] Eccentricity paced increase in the continental weathering in the latest Hauterivian, Early Cretaceous

★Invited Papers

*松本 廣直¹、Coccioni Rodolfo²、Frontalini Fabrizio²、白井 厚太朗¹、黒田 潤一郎¹ (1.東京大学大気海洋研究所、2.ウルビノ大学)

キーワード：オスミウム、白亜紀

The 405-kyr eccentricity is a constant orbital parameter throughout the Phanerozoic and is commonly associated with long-term variations in the global continental weathering mechanism. However, the lack of reliable geological evidence has hampered the understanding of the role of the orbital-parameter on continental weathering during the Cretaceous. Os isotopic ratio (¹⁸⁷Os/¹⁸⁸Os) of sedimentary record reflects the balance between the radiogenic Os derived from continental weathering and unradiogenic Os derived from the unradiogenic sources (e.g., hydrothermal activity, the weathering of mafic rocks, and extraterrestrial sources), and can be therefore regarded as the best proxy for estimation of the global continental weathering. To trace the orbital-paced continental weathering, this study reconstructs the marine Os isotopic records across the Upper Hauterivian to the Lower Barremian (Lower Cretaceous) where previous studies already reported that variation in the clay-mineral composition is paced by the 405-kyr cycle. Our new Os isotopic record shows periodic oscillation of ¹⁸⁷Os/¹⁸⁸Os between 0.7 and 0.9, which corresponds to the 405-kyr Earth’s eccentricity cycle. Since the sedimentary interval with the radiogenic ¹⁸⁷Os/¹⁸⁸Os values (~0.9) corresponds to an interval characterized by intensive humidity in areas surrounding the Tethys, a variation of ¹⁸⁷Os/¹⁸⁸O likely reflects the cyclic changes in the continental weathering caused by the eccentricity-paced intensification of monsoonal activity at low latitude. This variation could have been further amplified by the increase in the radiogenic Os input from higher latitude Paleozoic shale and Pre-Cambrian crust that was caused by the latitudinal shift of the intertropical convergence zone.