Coupling between the Cenozoic west Pacific subduction initiation and decreases of atmospheric carbon dioxides

At the beginning of the Cenozoic,the atmospheric CO_(2)concentration increased rapidly from~2000 ppmv at 60 Ma to~4600 ppmv at 51 Ma,which is 5–10 times higher than the present value,and then continuous declined from~51 to 34 Ma.The cause of this phenomenon is still not well understood.In this study,we demonstrate that the initiation of Cenozoic west Pacific plate subduction,triggered by the hard collision in the Tibetan Plateau,occurred at approximately 51 Ma,coinciding with the tipping point.The water depths of the Pacific subduction zones are mostly below the carbonate compensation depths,while those of the Neo-Tethys were much shallower before the collision and caused far more carbonate subducting.Additionally,more volcanic ashes erupted from the west Pacific subduction zones,which consume CO_(2).The average annual west Pacific volvano eruption is 1.11 km~3,which is higher than previous estimations.The amount of annual CO_(2)absorbed by chemical weathering of additional west Pacific volcanic ashes could be comparable to the silicate weathering by the global river.We propose that the initiation of the western Pacific subduction controlled the long-term reduction of atmospheric CO_(2)concentration.

The formation of explosive volcanos at the circum-Pacific convergent margin during the last century

The circum-Pacific convergent margin is known as"the Ring of Fire",with abundant volcano eruptions.Large eruptions are rare but very disastrous.It remains obscure how are large explosive volcanos formed and where are the danger zones.Three largest eruptions since 1900,the Hunga Tonga-Hunga Ha’apai,the Mt.Pinatubo,and the Novarupta were found to be associated with subductions of volatile-rich sediments and located close to slab windows.Among them,the Hunga Tonga-Hunga Ha’apai is close to subducting seamount chains;the Mt.Pinatubo is right next to subducting fossil ridges.Both seamount chains and fossil ridges have water depths much shallower than the carbonate compensation depths(CCD)in the Pacific Ocean.Seismic image shows that a seamount is subducting towards the Novarupta volcano.Subduction of volatile-rich sediments and a slab window nearby are the two most important favorable conditions for catastrophic eruptions.Slab windows expose the mantle wedge to the hot asthenosphere,which increases the temperature and dramatically promotes the partial melting of the carbonate-fluxed domains,forming volatile-rich magmas that powered explosive eruptions.

冲绳海槽超临界二氧化碳热液的形成

冲绳海槽超临界二氧化碳热液喷口位于俯冲的加瓜海岭之上.菲律宾海盆水深通常超过5000 m,在碳酸盐补偿深度之下,因此洋壳上碳酸盐不稳定.而加瓜海岭的水深在碳酸盐补偿深度之上,碳酸盐稳定.加瓜海岭俯冲携带的碳酸盐被俯冲板片释放的流体所溶解,向上运移、聚焦,高浓度的二氧化碳在冲绳海槽形成超临界二氧化碳喷口.结合高温高压实验结果,我们推断携带富碳酸盐沉积物的俯冲板片可以直接释放大量二氧化碳.被动陆缘是富碳酸盐沉积物形成和存储的最主要的构造背景,其俯冲可能对气候变化起了重要作用.鉴于此,我们认为新生代大气二氧化碳的长期增加是由于新特提斯洋关闭期间,被动大陆边缘富碳酸盐沉积物俯冲造成的.

间冰期最大值期间永久冻土线的退缩

在过去100万年,地球气候总体呈现10万年周期的冰期-间冰期旋回.在此期间未受扰动的永久冻土保存着大量甲烷.我们将间冰期温度最高值期间的永久冻土线定义为PLIM(permafrost line during the interglacial maximum).过去有100万年以来,PLIM以内的永久冻土未受扰动.假设冰下甲烷平均积累速率稳定,则PLIM以内的甲烷浓度超过每10万年至少被扰动一次的区域的10倍.近几十年的全球大气成分观测表明,永久冻土区的大气表现出了高甲烷浓度和最轻碳同位素值,显示永久冻土持续释放甲烷.这种甲烷的大量释放可能是由于人为碳排放推动了PLIM后撤,导致冻土冰下积累的甲烷发生扰动.甲烷释放到大气中会进一步加速冻土的升温和融化,进而释放更多的温室气体.目前,全球气候正处于间冰期临界点,冻土区的这种甲烷释放的正反馈会促进全球升温,很可能打破万年尺度的冰期-间冰期循环,将全球气候推入一个漫长的温室期.

Genesis of High Ba-Sr Yashan Intrusion from the Jiaodong Peninsula,Eastern China:Implications for the Destruction of the North China Craton

The origin of the Mesozoic high Ba-Sr(HBS)granitic magmatism in the Jiaodong Peninsula remains controversial in petrogenesis models and geodynamic settings.Here,we report zircon UPb age,trace element and oxygen isotope compositions,and whole-rock major-trace element and Sr-Nd isotope compositions of the HBS Yashan granodiorite.The zircon U-Pb age of~118 Ma denotes that the Yashan granodiorite belongs to the Weideshan-stage magmatic activity,which is consistent with the age of Mo mineralization in the Yashan intrusion.The low Sr/Y(48.8-115)and high(La/Yb)_(N)(23.8-50.4)ratios of the Yashan granodiorite are analogous to adakitic features derived from the lower-crust.This is also supported by the whole-rock initial^(87)Sr/^(86)Sr ratios(0.7096-0.7103)and zircon δ^(18) O values(6.79‰-8.03‰).Contemporaneous mantle-derived mafic microgranular enclaves indicate the involvement of the metasomatized lithospheric mantle.The high magma oxygen fugacity of the Yashan intrusion as indicated by high zircon Ce^(4+)/Ce^(3+) values suggests the involvement of plate subduction.The obviously lower Dy/Yb,La/Yb and Sr/Y ratios of magmatic rocks in the Weideshan-stage than those in the early-stage imply lithospheric thinning of the eastern North China Craton.We propose that the Yashan HBS granodiorite was formed by crust-mantle interactions during slab rollback.