青藏高原印度季风传输途径中表土叶蜡氢同位素的高度效应：古高度重建意义

The paleoaltimeter of soil wax-derived n-alkane δD values from south Tibetan Plateau is Key for lifting history of Tibetan Platean and climate change. The δDwax values of the continuous sequence of Cenozoic sediments from Lunpola basin (53.2-18.3Ma, our the last NSFC grant) challenge isotopic evidences that points to Late Eocene paleoelevations comparable to the present 4500 m elevation, and demonstrate later (ca.34- 25Ma) Lunpola basin uplift. However, continental and monsoon-derived moisture, along as aridity are the causes for enriched wax-derived n-alkane δD values (δDwax) (relative to equivalent elevations to the south), as well as the reduced isotopic lapse rate and increased isotopic fractionation εwax-p on the northwestern Plateau, and highlight the large uncertainties associated with paleoelevation reconstructions based solely on δDwax paleorecords. Based on the water vapor entering the plateau mainly from the southeast plateau and the middle Himalaya Mountains, this project collect and analyze superficial soil samples for δDwax, along three moisture channels from the southeastern Tibetan Plateau and Nepalese Himalayas (mainly influenced by the Indian Monsoon). Paired river water and plant samples are also collected during this period and analyzed for modern river water δD (δDRW) and δDwax. Studying the modern relationships between the δDRW and δDwax values with regional climate change, transport distance from the source and topography, we emphasize to assess the main control other than elevation change on the δD (δDRW) and δDwax values, such as the monsoon and vegetation type changes in the region, the amount of precipitation, temperature and evaporative enrichment. We believe this would much improve our ability to use δDwax values for quantitative reconstruction of the Lunpola basin.

稳定同位素方法是古高程定量重建的最重要方法之一。伦坡拉盆地沉积物叶蜡正构烷烃δDwax值(53.2-18.3Ma，我们前一个基金)显示其在约34Ma才开始出现较大幅度的隆升，质疑了以往国外伦坡拉同位素古高度恢复的晚始新世就达到目前高度的结果。然而。现有研究表明，叶蜡正构烷烃δDwax值及其高程递减率受到水汽来源、纬向分馏、局地再循环和山地垂直地带性等各种因素的影响，说明只根据δD wax古记录的变化进行古高度恢复具有较大的不确定性。因此，本项目拟通过对比印度季风进入青藏高原的三条主要传输途径中，δDwax值响应相同高度变化的差异，确定相关区域气候参数、植被类型、不同传输距离和纬度等主要特征因素对δDwax值高度效应的不同影响程度,进一步估算伦坡拉盆地定量高度历史, 补充和完善δDwax值恢复青藏高原古高度、古气候技术手段。

青藏高原南部古高度重建对理解理解印度和欧洲大陆板块的构造碰撞的过程和机理极为重要。稳定同位素方法是古高程定量重建的最重要方法之一。然而, 沉积物的稳定同位素一般比化石恢复的古高度要高出较多。因此，需要建立当地的同位素区域高度关系，以精确恢复青藏高原古高度。本研究选取喜马拉雅山南坡(1660~5050 m, 代表喜马拉雅南坡气团)，以及组合代表雅鲁藏布江雅鲁藏布江气团的察隅~波密(ZB; 1468~4800 m) 和八一~拉萨高度断面(BL; 3050 ~ 4970 m)，分别建立了各断面表土和植物叶蜡烷烃δDwax、河水δ18O 和δD值的高程递减率，以探讨各断面气候参数、传输距离和纬度等主要因素对δDwax及其高程递减率和表观分馏值εwax-p的影响程度。结果表明：虽然在各断面δDwax值及其高度关系有变化，但是，整体上藏东南地区高度仍然是δDwax值的主控因素，其高程递减率为−2.2‰/100m (R2= 0.78; n=76)。而且，喜马拉雅南坡以及高原南部δDwax 值和河水氢同位素(δDrw)之间的同位素分馏值(εwax/rw) 稳定在−99 和 –110‰之间（均值为–104.5‰)，证明了河水(代表降水)同位素值的高度效应是控制喜马拉雅南坡以及高原南部δDwax值高度变化的主要因素，证明δDwax值在这一地区可以作为极具潜力的古高度定量指标。然而，由于各断面气候参数、传输距离和湿气来源、纬度以及山形等因素的影响，不同断面同位素值分布及其高程递减率有明显差异，说明即使在湿润的高原东南部，青藏高原古高度恢复研究需要考虑这些因素的影响。同时，我们额外建立了藏南表土MBT′/CBT–MAT和MBT′5ME–MAT高度关系，并推导出该区温度校准公式(在研基金41571014)，为藏东南地区沉积物古高度和古温度计精确估算奠定了坚实的基础。