基于洞穴钙板碳、氧同位素和Δ47温标的青藏高原东南缘MIS7.2时段气候变化过程与机制探讨

CLIMATE CHANGE DURING MARINE ISOTOPE STAGE 7.2(MIS7.2) RECORDED BY CARBON,OXYGEN AND CLUMPED ISOTOPE IN CAVE FLOWSTONE FROM SOUTHEASTERN TIBETAN PLATEAU

本研究基于云南香格里拉石卡雪山高原面洞穴钙板SK下部约8.2 cm厚的7个^(230)Th年龄、81组碳氧同位素和9个Δ47温度数据,恢复了MIS 7.2时段的气候变化历史。SK-δ^(18)O序列重建了213.75~199.74 ka B.P.时段的印度夏季风演变过程,平均分辨率约180 a,该序列显示δ^(18)O值在209.17 ka B.P.和205.00 ka B.P.前后显著正偏,清晰记录了两个千年尺度季风减弱事件。通过与亚洲季风区石笋记录的对比发现,在MIS 7.2开始阶段印度夏季风与东亚夏季风同步变化,但在结束阶段,SK记录早于三宝洞记录约4000 a,考虑到样品年代误差及分辨率等因素,认为该时段轨道尺度上亚洲季风具有一致变化,但是准确时间还有待进一步厘清。Δ47古温度曲线与δ^(18)O、δ^(13)C记录的对比显示出MIS 7.2结束时段水热异步变化特征,与黄土-古土壤序列结论类似,不同的是SK记录的温度直接响应北半球太阳辐照度变化,而不是响应南极温度变化。SK记录显示夏季温度从207.36 ka B.P.左右逐渐回升,同期印度夏季风缓慢增强,但被大幅度的千年尺度季风减弱事件中断,印度夏季风显著增强滞后温度变化约6000 a,该过程与末次消冰期中海因里希事件(Heinrich events)或新仙女木事件(YD)类似,其机制可能与高纬气候变化有关,即冰盖融解导致大量淡水注入北大西洋,影响了北大西洋深水(NADW)和南极底层流(AABW)的生成,从而导致温盐环流(THC)减弱或中断,并通过大气-海洋环流传递到低纬地区,触发千年尺度印度夏季风减弱事件。

High-resolution terrestrial climate index over Marine Isotope Stage 7.2(MIS 7.2)could improve our understanding of glacial-interglacial climate change.Speleothem-based record from the East Asian Summer Monsoon(EASM)region has been well reconstructed.In contrast,records with high resolution and well dating constrained from the Indian Summer Monsoon(ISM)domain are still scarce.In this paper,we use carbon(δ^(13)C),oxygen isotope(δ^(18)O)and clumped isotope(Δ47)records of the flowstone to reconstruct the history of climate change during MIS 7.2.The cave(27°48'N,99°36'E;4449 m a.s.l)is located on the plateau of Shika Snow Mountains,Shangri-La City,Yunnan Province,southeastern of Tibetan Plateau.Precipitation of this regional is strongly affected by ISM.Most of precipitation(more than 70%)occurs during summer monsoon season(June to September).Mean annual temperature and precipitation recorded by the Shangri-La meteorological station are 6.3℃and 650 mm(1981~2010).Flowstone SK,105 mm in height,was collected at 5 meters from the entrance of the sinkhole.It's composed of densely pure calcite,with milky white in colour,excepting for top 24 mm.All powder samples were collected on the white and densely part,total about 82 mm.Total of 7^(230)Th dating were performed on MC-ICPMS,and 81 stable isotope were analyzed using MAT 253 coupling with Gasbench-Ⅱ.In addition,9 clumped isotope(Δ47)experiments were determined on a dual injection mode of isotope ratio mass spectrometer(IRMS;Thermo ScientificT M 253 Plus)for paleotemperature reconstruction.The SK-δ^(18)O values vary from-14.93‰to-10.50‰,with an average value being-11.98‰.Its variability is interpreted as the change in the intensity of the Indian summer monsoon.Therefore,the SK-δ^(18)O time series recorded the evolution of the Indian Summer Monsoon with an average resolution of 180 years from 213.75 ka B.P.to 199.74 ka B.P.Two millennial-scale weak monsoon events,centering in 209.17 ka B.P.and 205.00 ka B.P.respectively,can be clearly identified.Compared to the stalagmiteδ^(18)O records of IMS domain,Bittoo Cave from India and Xiaobailong Cave from central Yunnan,SK-δ^(18)O sequence shows similar trends on glcial-intergalcial scale,although they are difficult to be compared on detail due to rough resolution and poor time constraining of these referenced records on overlapping time period.Indeed,the second weak monsoon event recorded by SK-δ^(18)O is consistent with the record from Xiaobailong Cave within the error range.Compared spelethem-based ISM records with EASM record from Sanbao Cave,the SK-δ^(18)O record shows similar variation pattern with Sanbao Cave record,with two distinct millennial scale weak monsoonal events.The first event is synchronous for both records,but there is clear time offset between two records for the second one,with 4000 a earlier in SK record than in the Sanbao Cave record.This indicates that EASM is synchronous with the Indian summer monsoon at the beginning phase of the MIS 7.2,but unsynchronous at the ending phase.However,considering factors such as age error,local noise or sample resolution,we incline to suggest that change of the Asian monsoon during MIS 7.2 is consistent on orbital scale,but it still needs more longer record with better dating constrained to clarify this issue.SK-δ^(13)C ranges from-1.94‰to 2.04‰,with an average value of 0.25‰.The SK-δ^(13)C series is similar,but not totally consistent,withΔ47 temperature curve,which mainly reflects the local summer temperature change and modulated by precipitation to some extent.Combined with theδ^(18)O,δ^(13)C record and theΔ47 paleotemperature curve,SK records give us a chance to access the moisture and heat variation during the termination of MIS 7.2 without dating bias.It shows that the summer temperature in phase in time with the North hemispheric insolation,with continually increasing since 207.36 ka B.P.However,the slowly intensification of ISM at the same time was then interrupted by a millennium scale abruptly ISM broken event,which make it six thousand years lag the temperature change.The decouple process of heat and moisture variation during termination of MIS 7.2 is similar,but not all,with those in the last deglacial recorded by Chinese loess sequence,indicating that heat and moisture may be forced by different mechanism.Different to loess records,which show the summer temperature variation responds to Antarctic temperature change,our SK record illustrates summer temperature over the southwestern of Tibetan Plateau follows the summer insolation change directly.The possible reasons for this discrepancy at least include age uncertain from all records,different ice volume in MIS 7.2 and Last Glacial Maximum etc.,and need to be further explored with more high quality records.The abrupt millennial event identified in our SK-δ^(18)O record during the termination of MIS 7.2 is similar to those been found in the Heinrich events or the Younger Dryas(YD),supporting the previous hypothesis proposed by Cheng et al.(2009)who suggested that the lag may have been caused by cold anomalies generated by disintegrating ice sheets and sea-ice formation in the North Atlantic region,resulting in slowdown of the Thermohaline Circulation(THC)and triggering millennial scale cold eveuts through ocean-atmosphere process,finally weakening the Asian Summer Monsoon.