叶蜡正构烷烃氢同位素和甘油四醚脂揭示的晚中新世以来喜马拉雅山隆升历史

The paleoelevation reconstructions have been made in recent years suggest that the Himalayan mountains (HM) obtained elevations close to those of the present by the early Miocene. However, there are intense debates as to whether there was a critical elevation of the HM was attained then a loss of elevation or a continue uplift in elevation since the Miocene? Thus, the continuous paleoelevation history of the Himalayan mountains using new paleoaltimeters and evidence elsewhere will highlight the significance of to work out above differences. Based on empirically derived lapse rate equations of leaf wax-derived n-alkanes hydrogen isotope values (δDwax), and branched glycerol dialkyl glycerol tetraethers (brGDGTs) MBT′5ME-derived temperatures (MBT′5ME-MAT) of surficial soil from the south Tibetan Plateau, we select continuous late Miocene sediments from Zhada and Gyirong basins with systematic magnetostratigraphic dating on the Himalayan mountains, to measure δDwax values and the distribution of brGDGTs, and obtaining high resolution paleo-hydrology and paleo-temperature continuous records since late Miocene, respectively. Comparative analysis of δDwax and MBT′5ME-MAT of Karnali River section from Himalayan mountain front basin in western Nepal at the same period, we probe into Indian monsoon evolution history. After remove the influences of the global cooling and Monsoon, we constrain and (or) validate accurately the evolution of the late middle Miocene paleoelevations of the Gyirong Basin and Zande Basin. This study will supply new clues and evidence to understand the elevation history of the Himalayan mountains since Late Miocene, which promises insight into the mechanisms and dynamics that develop and sustain high topography. Based on these data, we will carry out detailed correlation analyses of major events in Indian monsoon evolution and paleoelevation history of the Himalayan mountains since the Miocene, particularly revealing their possible coupling of climatic change- tectonomorphologic evolution.

最新结果表明喜马拉雅造山带在早中新世可能就已达到现今高度。然而，晚中新世以来喜马拉雅山发生了伸展垮塌、持续隆升或更复杂的高度变化仍众说纷纭。这就需要新指标从不同角度给出新的证据，最好是连续的高度演化历史。本研究拟在已建立的青藏高原南部表土δDwax和GDGTs高度计及其应用方法基础上，选取已进行过系统磁性地层年代研究的札达盆地和吉隆盆地进行系统采样，测定连续地层沉积物中δDwax值和GDGTs温度指标的变化趋势，建立喜马拉雅山晚中新世以来的古水文和古温度连续变化记录；同时，对比分析喜马拉雅山前尼泊尔Karnali剖面同时期同类指标记录的印度季风演化信息；在排除全球降温和季风影响后，估算出札达和吉隆盆地的高程连续变化历史。这将为揭示喜马拉雅山晚中新世以来古高度变化历史和模式提供新的证据和见解；并为进一步探讨晚中新世印度季风强化事件与喜马拉雅山古高度变化的气候-构造耦合关系提供新的背景资料。

晚中新世以来喜马拉雅山究竟发生了伸展垮塌、持续隆升或更复杂的高度变化众说纷纭，急需新指标从不同角度给出新的证据。本研究通过已建立的青藏高原南部表土叶蜡正构烷烃氢同位素（δDwax）和GDGTs高度计及其应用方法基础上，应用于已进行过系统磁性地层年代研究的吉隆盆地和札达盆地，首先重建其古温度和古水文信息。其次，对比分析喜马拉雅山前尼泊尔地区同时期同类指标记录，以探讨如何排除背景环境变化对古高度重建的影响。最后，探讨背景环境和物源等对各古高度计的影响，通过多指标对比重建晚中新世以来喜马拉雅山古高度变化，为揭示喜马拉雅隆升历史和模式提供新的证据和见解。.基于GDGTs指标的古温度、古pH值重建和干湿古气候指标Ri/b均指出吉隆盆地和札达盆地晚中新世至上新世相比现今更为暖湿，暗示喜马拉雅山当时海拔低于现今，与δDwax和碳酸盐氧同位素（δ18Oc）重建古高度高出现今约1-2km的情况明显不同。GDGTs古温度重建指示尼泊尔地区晚中新世以来未表现明显的升温或降温趋势，指示这种暖湿的环境不能完全由背景气候变化解释。通过指标对比分析，GDGTs古温度在晚中新世至上新世中期的暖湿环境下不易出现暖季偏差，而由于当时的降水模式可能与现今不同，因而δDwax和δ18Oc古高度计存在系统高估风险。因此，本研究采信GDGTs古高度计。通过对分别代表陆源有机质输入区的土壤型和代表湖表面的水生型brGDGTs的区分，重建吉隆盆地在7.0-3.2Ma间盆地平均底部高度为2.5±0.8km，而周围山体海拔可能超过3.6±0.6km，指示上新世中期后喜马拉雅山中段仍需隆升约1.5km，支持了生态证据的晚期隆升观点。本研究不仅为认识喜马拉雅山晚中新世以来的隆升历史提供了新证据，同时也为在青藏高原中南部新生代地层中是普遍存在的氢氧同位素古高度计体系与生态证据的不匹配提供了新的解决思路。