青藏高原新仙女木事件的气候与环境

近年来的研究结果表明，发生于１１－１０ｋａＢＰ的新仙女木降温事件呈全球性变化，青藏高原在这一事件中气候与环境也发生了急剧变化，由于青藏高原巨大的高度和脆弱的冰冻圈结构，使新仙女木事件的敏感性和作用被放大了，这对同纬度地区和全球产生极大的影响。     

东北亚的新仙女木和全新世气候演变的初步证据：一个湖泊岩心？…

内蒙古合同察汁淖碱湖的磁化率和稳定同位素资料的初步证据表明,在东北亚也经历了新仙女木寒冷事件,此外,９０００－６０００ｂＢＰ的潮湿气候环境可能是由于季风活动加剧所致,最后,一个逐渐干化的气候条件一直延续到现在。     

青藏高原腹地湖泊沉积纪录的“仙女木期”古气候颤动事件

通过对青藏高原腹地钻孔岩心剖面的最新研究,用层序地层学与年代地层学和气候地层学相结合的方法,分辨出可可西里地区苟弄措湖泊沉积记录(孔深7.25 m)约30 ka BP以来的古气候变化,并在孔深3.32～1.98 m处识别出“仙女本期”的3个气候颤动事件,即:①老仙女术事件(孔深3.32～2.62 m);②中仙女本事件(孔深2.62～2.43 m);③新仙女本事件(孔深2.43～1.98 m)。这3个气候颤动事件都是以冷干时段开始、以冷湿时段结束的,并有越向后期其冷干的时段越长的变化趋势,而其后的冷湿时段却具有等时的变化趋势。岩心沉积物的有机碳、有机氮和磁化率以及化学元素含量的变化,都与仙女本期气候变化具有十分吻合的变化态势。仙女术期气候在湖泊沉积物中的响应,在青藏高原环境演化中还是首次发现,为研究第四纪末次冰期冰消期以来的气候变化提供了新的信息。     

大兴安岭地区新仙女木事件的表现特征

新仙女木事件是末次冰消期期间发生的一次气候转冷事件,该事件对整个北半球的中高纬度地区的气候变化有明显的影响。然而,在该事件造成东亚中高纬地区温度下降的统一认识下,对于其季节性特征仍较为缺乏认识。本文用大兴安岭中部月亮湖末次冰消期多指标记录,揭示了大兴安岭地区在新仙女木事件期间植被中的北方针叶林成分增加、森林发育、冬季温度下降、东亚冬季风明显增强。

     

全球新仙女木事件的恢复及其触发机制研究进展

大约发生在12.9~11.6 ka BP的新仙女木事件(YD)在气候突变的研究中占有重要地位,然而对其触发时间、触发地点、触发机制以及YD事件是否是一个全球性的事件一直存在很大争议,并且该事件触发机制的研究对目前全球温度快速升高背景下未来气候变化的预测也有很大的意义.综述了近年来世界范围内YD事件恢复的最新研究,分析了该事件在全球不同区域发生时间和强度的异同,目前北半球对该事件的记录比较一致,南半球还有很大分歧.介绍了YD事件的可能触发机制,YD事件的触发除了北大西洋温盐环流终止之外,也可能受到诸如太阳辐射减少和赤道大洋的影响.最后讨论了有待加强的研究区域和未来的研究方向.     

东北亚的新仙女木和全新世气候演变的初步证据── 一个湖泊岩心的古环境变化的同位素和磁性记录

内蒙古合同察汗淖碱湖的磁化率和稳定同位素资料的初步证据表明,在东北亚也经历了新仙女木（ＹｏｕｎｇｅｒＤｒｙａｓ）寒冷事件。此外,９０００～６０００ａＢＰ的潮湿气候环境可能是由于季风活动加剧所致,随后,一个逐渐干化的气候条件一直延续到现在。     

晚冰期新仙女木事件的研究历史及现状

发生于１０－１１ｋａ左右的新仙女木事件是一次全球笥的气候态然变冷事件。目前的有关其成因的各种假说都不能完全解释它的形成机制。本通过对比世界各地有关此事件的地质记录，综合评判各种假说，目前关于此事件的成因尚未搞清。现有的知识表明，洋流的改变及与之相伴的随的海－气系统的重新调整。对理解此事件及与此类似的气候突发事件具指导意义。     

新仙女木末期南海北部天然气水合物分解事件

深海天然气水合物分解与全球变暖密切相关.南海北部是重要的天然气水合物蕴藏区,ZHS-176、ZHS-174、17940和MD2905孔CaCO₃含量记录均表明,11.3~8.0 ka B.P.神狐海域存在一次典型的“低钙事件”(CM),该事件具有“快速降低、缓慢升高”不对称的变化结构,CaCO₃含量降幅高达9%.“低钙事件”期间,底栖有孔虫Cibicidoides wuellerstorfi和Cibicidoides kullenbergi壳体δ13C分别负偏了1.4 ‰和0.7 ‰,海底有机碳的堆积速率(MAR)也突然升高了1倍.综合分析表明,新仙女木末期南海北部天然气水合物很可能发生了一次较大规模的快速分解,大量甲烷气体从天然气水合物中逸散,氧化后使底层海水快速酸化,从而导致了神狐海域碳酸盐的溶解.底层水团温度上升很可能是神狐海域天然气水合物分解的主要触发因素.      

新仙女木事件在罗布泊湖相沉积物中的记录

通过对罗布泊CK-2钻孔湖相沉积物磁化率、粒度、碳酸盐和有机质含量的测定以及地球化学元素的多元统计分析, 探讨了我国西北极端干旱区域的环境对全球气候变化的响应.结果显示各代用指标均一致揭示了发生在12.8~11.6kaBP间非常明显的低温期, 且降温幅度大, 变化迅速.这次冷事件在时间坐标以及发生特点上都与格陵兰冰心记录的末次冰消期中的新仙女木事件相对应, 为新仙女木事件的全球性特征提供了新的证据.这说明在我国新疆的极端干旱区, 其气候与环境的变化也与北半球高纬度地区有着密切的联系; 由主成分分析结果, 揭示了我国西北地区的西风气候区有别于东南季风气候区的水热配置特点, 即冷期湿润, 暖期干旱, 并提出了该事件信号从北大西洋传送到罗布泊地区的潜在机制.      

赣北石笋记录的新仙女木事件

新仙女木事件（Younger Dryas,简称YD）是末次冰消期升温过程中发生的一次北半球急剧降温事件,目前全球范围多个地区的地质记录中均有发现该事件的印记。然而,受限于定年误差或采样分辨率等因素,关于YD事件的精确起止时间、精细内部结构和触发机制仍存在争议。本研究利用江西北部神农宫溶洞内一支文石石笋SN29（总长度426 mm,其中距离顶部220~388 mm用于本研究）,通过高精度²³⁰Th定年和氧、碳稳定同位素测试分析,建立了13614~10811 a B.P.期间平均分辨率12年的石笋氧同位素（δ¹⁸O）变化序列。该记录最为显著的特征是δ¹⁸O值在12851~11575 a B.P.时段偏正1.7 ‰,指示了一次显著的弱季风突变事件,响应于北大西洋YD事件。SN29为高铀含量文石石笋且生长速率较快,精确界定了响应于YD事件的东亚弱季风事件的起止时间和精细内部结构。结果表明：YD期间,SN29记录的东亚弱夏季风事件的开始时间为12851±16 a B.P.,经历了约249年;结束过程开始于11575±19 a B.P.,经历了约146年;整个YD弱季风事件共持续了1276±35年。东亚季风区各石笋δ¹⁸O记录的YD弱季风事件在误差范围内基本一致,且与格陵兰冰芯δ¹⁸O记录和低纬度其他记录的发生时间一致,支持YD时期东亚夏季风减弱事件可能是对大量淡水进入北大西洋导致径向翻转流减弱、北半球变冷、ITCZ南移的快速响应的假说。

     

The Younger Dryas Event and Holocene Climate Fluctuations Recorded in a Stalagmite from the Panlong Cave of Guilin

Comprehensive studies on a stalagmite from the Panlong cave, Guilin, have shown that the isotopic records and sedimentary characteristics can reflect the changes of both palaeotemperatures and palaeorainfall, that is to say, it is possible to get some information about the changes in climate of the area from the speleothem. The results suggest that: (1) the Younger Dryas event might have persisted in the area from 11 300 to 10 800 a B.P.; (2) from 9000 to 7000 a B.P., the climate got warmer and wetter, and the summer monsoon was gradually enhanced; (3) from 7000 to 4500 a B.P., the climate was warm and wet, and the summer monsoon prevailed; and (4) from 4500 a B.P. on, the summer monsoon was weakened and the modern climate pattern appeared, but there were several cold and dry periods, namely, from 4000 to 2500 a B.P., ca. 2400 a B.P. and < 1000 a B.P.     

Younger Dryas事件与北黄海泥炭层的形成

形成于海平面变化处于停滞阶段和湿润气候条件下的泥炭层是古环境变化信息的重要载体。对北黄海4个含泥炭层的沉积剖面进行研究后发现,其均集中在渤海海峡入口处,水深变化在50～54m。泥炭层的AMS14C年龄在10650～1010014 CaBP,与发生在11000～1000014 CaBP间的末次冰消气候回冷事件——Younger Dryas(YD)事件在年代上非常吻合,表明北黄海泥炭层的形成可能与YD事件的全球效应密切相关,可作为YD事件在北黄海陆架响应的一个重要证据。泥炭层在北黄海的集中出现说明,冰后期的海平面上升过程中在YD事件期间存在停滞阶段,这一时期海面已经达到渤海海峡外侧,并可能在此徘徊了近千年。此外,泥炭层的大量出现和孢粉记录表明YD事件发生期间约为10600～1020014 CaBP,此时北黄海可能处在寒冷而湿润的环境。这一发现与全球范围内大部分YD事件的海陆记录存在明显差异,说明不同地区对YD事件的响应存在差异,不能简单地利用单一的干冷模式来分析YD事件在区域上的响应特征和过程。     

新仙女木(YD)事件区域特征及动力机制研究新进展

新仙女木事件(Younger Dryasevent,YD事件)为末次冰消期发生的快速降温事件,作为典型突变气候事件,它一直是国际古气候关注热点,其研究的开展促进和深化了科学界对千年尺度突变气候事件的理解和认识。近年来,随着高分辨率地质记录涌现,YD事件时空差异性日益突出。对比和认识这些区域之间事件细节结构及转型模式异同有助于甄别其相位关系,探讨不同环境快速重组对高、低纬突变气候事件的响应方式,进而验证早期驱动—响应假说,为其动力学机制建立及未来相似边界条件下气候预测提供基本地质证据和理论模型。本文从YD事件区域响应方式入手,通过总结和对比模拟研究及地质记录,对YD事件已有研究存在的争议进行回顾,提出有待加强的研究区域和未来研究方向。     

Rapid Ice Margin Fluctuations during the Younger Dryas in the Tropical Andes

Radiocarbon dated lacustrine sequences in Perú show that the chronology of glaciation during the late glacial in the tropical Andes was significantly out-of-phase with the record of climate change in the North Atlantic region. Fluvial incision of glacial-lake deposits in the Cordillera Blanca, central Perú, has exposed a glacial outwash gravel; radiocarbon dates from peat stratigraphically bounding the gravel imply that a glacier advance culminated between 11,280 and 10,990 ¹⁴C yr B.P.; rapid ice recession followed. Similarly, in southern Perú, ice readvanced between 11,500 and 10,900 ¹⁴C yr B.P. as shown by a basal radiocarbon date of 10,870 ¹⁴C yr B.P. from a lake within 1 km of the Quelccaya Ice Cap. By 10,900 ¹⁴C yr B.P. the ice front had retreated to nearly within its modern limits. Thus, glaciers in central and southern Perú advanced and retreated in near lockstep with one another. The Younger Dryas in the Peruvian Andes was apparently marked by retreating ice fronts in spite of the cool conditions that are inferred from the ∂¹⁸O record of Sajama ice. This retreat was apparently driven by reduced precipitation, which is consistent with interpretations of other paleoclimatic indicators from the region and which may have been a nonlinear response to steadily decreasing summer insolation.     

The Younger Dryas and the Sea of Ancient Ice

We propose that prior to the Younger Dryas period, the Arctic Ocean supported extremely thick multi-year fast ice overlain by superimposed ice and firn. We re-introduce the historical term paleocrystic ice to describe this. The ice was independent of continental (glacier) ice and formed a massive floating body trapped within the almost closed Arctic Basin, when sea-level was lower during the last glacial maximum. As sea-level rose and the Barents Sea Shelf became deglaciated, the volume of warm Atlantic water entering the Arctic Ocean increased, as did the corresponding egress, driving the paleocrystic ice towards Fram Strait. New evidence shows that Bering Strait was resubmerged around the same time, providing further dynamical forcing of the ice as the Transpolar Drift became established. Additional freshwater entered the Arctic Basin from Siberia and North America, from proglacial lakes and meltwater derived from the Laurentide Ice Sheet. Collectively, these forces drove large volumes of thick paleocrystic ice and relatively fresh water from the Arctic Ocean into the Greenland Sea, shutting down deepwater formation and creating conditions conducive for extensive sea-ice to form and persist as far south as 60°N. We propose that the forcing responsible for the Younger Dryas cold episode was thus the result of extremely thick sea-ice being driven from the Arctic Ocean, dampening or shutting off the thermohaline circulation, as sea-level rose and Atlantic and Pacific waters entered the Arctic Basin. This hypothesis focuses attention on the potential role of Arctic sea-ice in causing the Younger Dryas episode, but does not preclude other factors that may also have played a role.     

Iceberg discharge to the Chukchi shelf during the Younger Dryas

The extent of glaciation in northwestern Alaska, the source of sediment supply to the Chukchi shelf and slope, and the movement of sea ice and icebergs across the shelf during the last glacial maximum (LGM) remain poorly constrained. Here we present geophysical and geological data from the outer Chukchi margin that reveal a regionally extensive, heavily ice-scoured surface ∼ 5-8 m below the modern seafloor. Radiocarbon dating of this discrete event yields age estimates between 10,600 and 11,900 ¹⁴C yr BP, indicating the discharge event occurred during the Younger Dryas. Based on mineralogy of the ice-rafted debris, the icebergs appear to be sourced from the northwestern Alaskan margin, which places important constraints on the ice extent in northern Alaska during the LGM as well as existing circulation models for the region.