中国东北新生代火山区CO_(2)释放规模与成因

Fluxes and genesis of carbon dioxide emissions from Cenozoic volcanic fields in NE China

板片俯冲过程将地表碳带入地球内部,火山作用将深部碳输送至地球外部圈层,两者构成了地球深部碳循环的主要方式,进而影响了地史时期的气候变化。我国东北新生代火山活动被认为是太平洋板片深俯冲作用的产物,板片俯冲导致岩浆源区强烈的碳酸盐组分交代作用,进而使东北亚上地幔成为一个新生代时期的巨型深部碳库,它的活动和释放将会对全球的气候与环境变化造成重要影响。然而,有关该深部碳库向当今大气圈输送CO_(2)气体的规模及其演化过程尚不清楚,从而影响了进一步定量评估该碳库在全球变化研究中的地位和作用。针对上述科学问题,本文对我国东北长白山、五大连池和阿尔山火山释放CO_(2)气体的规模与成因进行了研究。结果表明,东北新生代火山区的土壤CO2释放通量介于9.6~41.2g·m^(-2)·d^(-1)之间,每年向当今大气圈释放CO2气体约为2.1Mt(其中,长白山火山区为0.94Mt,五大连池火山区为1.2Mt)。气体地球化学研究证实,长白山与五大连池火山气体均起源于太平洋板片深俯冲环境;但是,与长白山相比,五大连池火山气体具有较高比例壳源组分贡献。阿尔山火山气体的成分与长白山和五大连池火山区存在着明显的差异,它们以N_(2)为主(>95%),并且其δ^(15)N_(N2)值高于空气值(1.3‰~1.9‰),^(3)He/^(4)He比值较低(0.14~0.18RA),δ^(13)C_(CO2)较轻(-13.7‰~-6.2‰),表明壳源富氮有机沉积物的贡献占比较大的比例。上述特征进一步表明,阿尔山火山气体在上升经过地下水时可能滞留了较长时间,混染了大量的陆壳组分,其源区并未受到太平洋板片俯冲物质的显著影响。

Carbon was carried into the Earth’s interior by plate subduction,and returned to the exogenic environment mainly by volcanism,which constitute the main process of deep carbon cycles in the Earth and drive the long-term(~Myr)climate change.Cenozoic intraplate volcanism in NE China is deemed to be derived from deep subducted Pacific slab in the mantle transition zone(MTZ),along with large quantities of crustal carbonates,which makes the upper mantle beneath NE China a huge carbon reservoir in Cenozoic era,whose activity and release would have a significant impact on global climate and environmental change.Nevertheless,the total output of volcanic carbon,its origin and evolution process remain controversial.Studies were performed in Cenozoic volcanic fields of Changbai volcanic field(CVF),Wudalianchi volcanic field(WVF)and Aershan volcanic field(AVF)in NE China to better constrain this issue.The fluxes of soil CO_(2) emissions from Cenozoic volcanic fields in NE China are between 9.6 g·m^(-2)·d^(-1) and 41.2 g·m^(-2)·d^(-1),and the total flux of volcanic CO_(2) is reviewed and recalculated to be 2.1 Mt per year,about 0.94 Mt from the CVF and 1.2 Mt from WVF.Chemical compositions and C-He isotopes indicate that CO_(2)-riched gases in CBV and WVF might originate from deeply subducted Pacific slab beneath NE China.However,volcanic gases from WVF show more proportions of crustal components than that in CVF.Bubble gases from hot springs in AVF exhibit enrichment in N_(2)(>95%)and remarkable contribution from crustal nitrogen-rich organic components,as indicated by relatively heavier δ^(15)N_(N2) values(1.3‰~1.9‰),low ^(3)He/^(4)He ratios(0.14~0.18 RA)and light δ^(13)C_(CO2) values(-13.7‰~-6.2‰),which reveals that volcanic gases in AVF may not be prominently influenced by the recycled materials from deeply subducted Pacific slab.We suggest that the primordial gases in AVF are contaminated by large quentities of crustal materials during its long time retention in the groundwater systems.