藏南碳酸岩脉成因及其气候效应

始新世末期以来,全球大气CO₂浓度持续下降,但长期以来不清楚为何这一时期全球大气CO₂浓度下降,巨量的大气CO₂赋存于何处。深入研究该问题有助于准确理解未来大气CO₂浓度变化的趋势,特别是有助于进一步评估人类自身碳排放的后果。这一时期,小印度陆块持续与大亚洲陆块汇聚,导致了以喜马拉雅为代表的山脉群和青藏高原的形成。很早就有学者从地球表层碳循环的角度提出了"青藏高原的隆升导致了全球变冷"的观点,但这一观点既没有解释清楚"巨量大气CO₂到何处去"的问题,也没有讨论青藏高原本身向大气圈排放CO₂等问题,因此该观点最近受到了强烈的质疑。这些激烈的争论充分反映了传统的地球表层碳循环研究已不能充分满足当前社会的需求。本文从深部碳循环这个视角重新探讨青藏高原在全球碳循环中的作用。在印度与亚洲陆块持续汇聚期间,以喜马拉雅为代表的巨型山脉快速崛起,然后持续遭受化学风化作用,大量消耗大气CO₂。化学风化的产物堆积在喜马拉雅山前的前陆盆地内,形成了巨量含新生碳酸盐矿物和有机碳的西瓦里克沉积杂岩,随后新生的西瓦里克杂岩又随持续平板俯冲的印度陆壳被带入青藏高原内部,与平板俯冲的印度陆壳共同经历高温变质作用。俯冲板片内的(黑)云母等含水矿物发生脱水,形成花岗岩浆。花岗岩浆再与俯冲的西瓦里克杂岩内的碳酸盐岩发生交代反应,释放出含钙、镁离子、以CO₂和水为主的高温流体,本文称其为壳源火成碳酸岩浆。碳酸岩浆沿张性裂隙上侵、冷凝之后形成藏南的碳酸岩脉。虽然青藏高原内部的火山、温泉等均向大气圈排放CO₂,但所排放的碳均为再循环来自大气圈的碳,并且排放量略小于吸收量,否则消耗大气CO₂所新生的碳酸岩脉就不会在青藏高原内部保存下来。藏南大量晚新生代碳酸岩脉的发现充分说明了喜马拉雅山脉和藏南高原是一个巨大的碳储库,在其形成过程中将巨量大气CO₂转化为流体(岩浆)的形式封存于青藏高原内部,从而大幅降低了大气CO₂浓度,最终导致了全球变冷。上述过程充分说明,大气CO₂浓度的变化实质上是受控于地球内部的构造运动。进一步可推论出,"全球变化"只是一个自然现象,虽然它有独特的运行轨迹,但与人类的碳排放量无因果关系。

Petrogenesis of carbonic dykes within southern Tibetan Plateau, and climatic effects

Global climatic change has become one of the hottest issues worldwide.Knowledge of ancient Earth's surface temperature is critical to understanding Earth today and future as well as evaluating effects of mankind's carbon emissions exactly.Earth's surface average temperature has decreased since the end of the Eocene.It is generally accepted that this long-term global cooling is perhaps a consequence of long-term decreasing of global CO₂ concentrations.However,it is still unknown where and how the huge atmospheric CO₂ sinked.The hypothesis of global warming in the near future is,therefore,lack of solid evidence when the questions remain unknown.Since the Cenozoic,Indian continent has continuously flighted northwards and hit Asian continent finially,leading to the close of larger Neo-Tethyan Ocean and subsequent uplift of Himalayan Mountains as well as Tibetan Plateau.A "Raymo" hypothesis that the uplift and subquent erosion of the Himalayan-Tibetan orogen has drawn down atmospheric CO₂ and cooled the globe is,therefore,present.However,this hypothesis has been recently challenged by the studies of degassing of hot springs within Himalayan Mountains.These scientific arguments have revealed that traditional approaches to surface carbon recycling have hardly satisfied the demands of current society.In this study,the role of Himalayan Mountains and south Tibetan Plateau in the global carbon cycling is re-evaluated.During collision between Indian and Asian continents,the Himalayan Mountains have quickly uplifted and hence underwent stronger chemical weathering,leading to the formation of carbon-rich Siwalik formation within the north of Gange foreland basin to the south of Himalayan Mountains.The carbon-rich Siwalik formation,at the expense of huge atmospheric CO₂,has been subsequently transferred into the interior of Tibetan Plateau with flat-subducted Indian crust.Some carbon from the buried Siwalik formation beneath Himalayan Mountains has been released back to atmosphere through hot springs.Most carbon had,however,transferred into deep interior of Tibetan Plateau along with subducted Indian crust.The biotite within the subducted slab underwent dehydration to form granitic magmas beneath south Tibetan Plateau.The metasomatic reactions between the granitic magmas and the subducted carbon-rich Siwalik formation took place to release high-temperature CO₂-rich fluids beneath Tibetan Plateau,regarded as crustal-derived carbonic magmas in this study.The magmas/fluids intruded into south Tibetan upper crust to form carbonic dykes.The huge atmospheric CO₂ has,therefore,been transformed into carbonic magmas within thickened crust of southern Tibetan Plateau during the collision between India and Asia.The carbon emitted by hot springs as well as volcanoes within Tibetan Plateau was originated from atmosphere.It is recycling carbon.The carbon emissions from Tibetan Plateau are slightly less than those sinked by Tibetan Plateau.Otherwise,the carbonic dykes,formed by consuming huge atmospheric CO₂,never occurred within Tibetan Plateau.This clearly suggests that Himalayan Mountains and south Tibetan Plateau are a huge reservoir for atmospheric CO₂,leading to global cooling during Cenozoic times.Moreover,the changing of atmospheric CO₂ was mainly derived by Earth's tectonic activities,and not by mankind.Global changing is only a natural phenomenon,without any relationships carbon emissions of human.