青藏高原中东部囊谦盆地始新世气候变化及其驱动机制

Eocene Climate Change and Its Driving Mechanism in the Nangqian Basin of the Central-eastern Qinghai-Tibetan Plateau

【目的】古近系以来,伴随着青藏高原隆升、地层大规模逆冲和走滑,在青藏高原北部形成了众多内陆盆地。由于盆地位置的特殊性,盆地内部的古气候和古植被演化对厘清欧亚大陆古近系气候演化特征具有重要意义。【方法】利用青藏高原中东部囊谦盆地古近系沉积地层剖面中的生物标志化合物和总有机碳(TOC)含量重建了51.8~37.5 Ma囊谦盆地古气候和古植被演化历史。【结果与结论】囊谦盆地古近系气候指标演化历史具体可分为三个阶段。阶段Ⅰ(51.8~46.4 Ma),CPI值和TOC值均较低,Paq值偏高,ACL值和nC_(27)/nC_(31)在该时期呈现明显的变化趋势,可能是受到河流所带来的陆源沉积物混入影响,主峰碳大部分为nC22和nC23,植被类型以木本植物为主,气候条件相对较为潮湿;阶段Ⅱ(46.4~42.7 Ma),此阶段Paq值和nC_(27)/nC_(31)值逐渐降低,ACL值逐渐上升,主峰碳碳数逐渐由低碳数向中高碳数转变,植被类型逐渐由木本占优势转变为以草本植物为主,气候条件逐渐变干,同时伴有干湿交替的气候特征;阶段Ⅲ(42.7~37.5 Ma),Paq、nC_(27)/nC_(31)和ACL值变化不大,主峰碳碳数明显增加,高碳数正构烷烃明显增多,干旱化程度进一步增加;同时TOC明显增加,指示植被总生产力增强,可能是对中始新世温暖适宜期(MECO)的响应。此外,通过对比分析研究区以及相邻盆地气候变化的研究结果,认为囊谦盆地51.8~37.5 Ma期间的气候变化主要受全球性气候变化和特提斯海退缩的影响,青藏高原的隆升,海拔的增加对于囊谦盆地水汽影响较大,季风对于水汽影响较小。

[Objective]Since the Paleogene,with the uplift of the Qinghai-Tibetan Plateau and the large-scale thrust and strike-slip of the strata,many inland basins were formed in the northern part of the Qinghai-Tibetan Plateau.Because of their particular location,the evolution of paleoclimate and paleo-vegetation in the basins is very significant for clarifying the climatic evolution in the Paleogene in Eurasia.[Methods]In this study,the evolution history of paleoclimate and paleo-vegetation in the Nangqian Basin in central and eastern Qinghai-Tibetan Plateau(51.8-37.5 Ma)was reconstructed from biomarkers and total organic carbon(TOC).[Results and Conclusions]The evolution history of Paleogene climate index in Nangqian Basin is in three stages,each with different changes of sedimentary facies.Stage I(51.8-46.4 Ma):The sedimentary facies are mainly fluvial and short-term lacustrine,with lower CPI and TOC values and higher Paq values.ACL and nC_(27)/nC_(31)show an obvious change trend in this period,which may have been affected by the mixing of terrestrial sediments brought by rivers.Most of the main peak carbon was nC_(22) and nC_(23);the vegetation type was predominantly woody plants,and the climate was relatively humid.Stage II(46.4-42.7 Ma):The Paq value and nC_(27)/nC_(31)ratio gradually decreased,ACL gradually increased,the carbon number of the main peak gradually changed from low to medium or high,the vegetation gradually changed from woody to herbaceous plants,and the climate gradually became drought alternating with wet conditions.Stage III(42.7-37.5 Ma):Paq,nC_(27)/nC_(31)and ACL changed little,the carbon number of the main peak increased significantly,n-alkanes with high carbon numbers increased significantly,and drought conditions increased.TOC also significantly increased,reflecting increasing total vegetation productivity,which may have been a response to the warm Middle Eocene Climatic Optimum(MECO).In addition,from comparisons with previous reports of climate change in the study area and adjacent basins,it is considered that the climate change in the Nangqian Basin during 51.8-37.5 Ma was mainly affected by global climate change and the retreat of the Tethys Sea.The uplift of the Qinghai-Tibetan Plateau and its higher elevation greatly influenced water vapor in the Nangqian Basin,whereas monsoons have little influence on water vapor.