古生代-中生代之交的水循环演变及驱动机制

Evolution and driving mechanisms of water circulation during the late Paleozoic to early Mesozoic

水循环是连接大气圈、水圈、岩石圈和生物圈的重要物质循环过程.本文根据地质记录和深时气候模拟等数据总结了古生代-中生代之交的水循环演变.结果表明,水循环在晚古生代、早中生代发生了重大变化:(1)地质记录恢复的气候带显示,当时全球的干旱带发生了明显扩张,蒸发岩面积从中-晚二叠世开始扩张,到早三叠世达到顶峰,中三叠世恢复至中-晚二叠世的水平,晚三叠-早侏罗世继续收缩;(2)全球年平均降水量在晚石炭、早侏罗世较高,中-晚二叠世和早三叠世较低,呈现为“V”字形变化,显示出一次重大的波动;(3)全球大陆的聚合可能导致泛大陆巨型陆面风和特提斯区域跨赤道风的形成,进而构成全球性的超级季风;(4)热带辐合带摆动的纬度位置在志留纪-石炭纪以及侏罗纪-现代较低,在二叠纪-三叠纪时期向高纬度大幅度漂移.根据事件发生的时间,这些水循环的重大变化与泛大陆的聚合、华力西造山、晚古生代冰期的结束以及大量碳释放和升温事件吻合,这些因素与当时的大气环流和水循环的变化存在密切联系:泛大陆的聚合改变了当时水储量的空间分布,同时改变了大气环流和“季风”,进而导致中-晚二叠世干旱带开始扩张,降水量减少;晚古生代末期华力西造山运动的减弱增强了热带辐合带的摆动,促进了超级季风的发展;碳释放驱动的升温事件改变了能量传输,影响了大气环流;同时植物面貌的转变影响了碳释放,进而影响水循环.由此可见,古生代-中生代之交水循环的强烈波动可能是该重大地质历史转折期岩石圈(地球深部过程)、大气圈(大气过程)、水圈(水循环)及生物圈(生命活动)相互作用的重要体现.

Water circulation plays a crucial role in the Earth’s systems,linking the atmosphere,hydrosphere,lithosphere,and biosphere.During the Paleozoic-Mesozoic transition,climatic and environmental conditions turned into crises,caused by a sudden and serious global warming,accompanied by the largest known extinction event of all time.Several studies have been conducted to discover the causes of this notable climate change.Nevertheless,the evolution of water circulation processes during the Paleozoic-Mesozoic transition is still unclear.Several lithologic indicators present in paleoclimate records have been described over the last few decades.Based on these data,we defined climate zones and calculated their area values using Thiessen polygons and Geographic Information System technology to visualize climate evolution during the Paleozoic-Mesozoic transition.Results of a few of the climate simulations showed changes in precipitation and the amplitudes of the inter-tropical convergence zone(ITCZ)seasonal migration.The results showed several changes in water circulation during the late Paleozoic to early Mesozoic.(1)The climate zones reconstructed by sedimentary and paleontological records showed a remarkable arid expansion.The distribution area of evaporites began to expand in the Middle to Late Permian and reached a peak in the Early Triassic,returned to Middle-Late Permian levels in the Middle Triassic,and thereafter continued to narrow during the Late Triassic to Early Jurassic.(2)Global precipitation was higher during the Late Carboniferous and Early Jurassic and lower in the Middle to Late Permian and Early Triassic.Such a“V”shape represents considerable swing.(3)The assembly of Pangea might have led to“giant land surface winds”in Pangaea and“trans-equatorial winds”in the Tethys region,resulting in a global“megamonsoon”.(4)Seasonal movements of the ITCZ were located at lower latitudes from the Silurian to Carboniferous and from the Jurassic to the present,while reaching higher latitudes with large seasonal oscillations during the Permian-Triassic.Depending on the timing of events,these major changes in water circulation coincided with the assembly of Pangaea,the Variscan or Hercynian orogeny,the end of the late Paleozoic ice age,major carbon emissions,and global warming.These factors are closely related to the changes in atmospheric and water circulation during the Paleozoic-Mesozoic transition.The assembly of Pangea redistributed water storage and altered atmospheric circulation and the“megamonsoon”,while the elevation reduction of mountains traversing middle Pangea weakened the resistance of north and south winds.These factors promoted the evolution of a“megamonsoon”with strong seasonal swings of the ITCZ,which led to the expansion of arid zones and reduced precipitation during the Middle and Late Permian.Meanwhile,large carbon emissions and dramatic global warming altered the energy transition within global systems,which influenced atmospheric circulation.In addition,transformations of global plant clades affected the carbon budget and altered water circulation.In summary,strong fluctuations in water circulation were present during the Paleozoic-Mesozoic transition,especially as observed through climate zones,precipitation and runoff,the“monsoon”,and ITCZ.These crucial processes reflect the interaction between the lithosphere,atmosphere,hydrosphere,and biosphere.However,we have limited knowledge of the complete process of water circulation because geological records and climate simulations are extremely imprecise for the Paleozoic-Mesozoic transition.Future research could focus on other water circulation processes,such as the evolution of underground water,reserves,and fluxes of lakes,rather than only precipitation and“monsoons”.Moreover,the upgrade and extension of big data platforms and a larger number of climate simulations should be considered in future studies.