Exposure dating and glacial reconstruction at Mt. Field,Tasmania, Australia,identifies MIS 3 and MIS 2 glacial advances and climatic variability

Tasmania is important for understanding Quaternary climatic change because it is one of only three areas that experienced extensive mid‐latitude Southern Hemisphere glaciation and it lies in a dominantly oceanic environment at a great distance from Northern Hemisphere ice sheet feedbacks. We applied exposure dating using ³⁶Cl to an extensive sequence of moraines from the last glacial at Mt. Field, Tasmania. Glaciers advanced at 41–44 ka during Marine oxygen Isotope Stage (MIS) 3 and at 18 ka during MIS 2. Both advances occurred in response to an ELA lowering greater than 1100 m below the present‐day mean summer freezing level, and a possible temperature reduction of 7–8°C. Deglaciation was rapid and complete by ca. 16 ka. The overall story emerging from studies of former Tasmanian glaciers is that the MIS 2 glaciation was of limited extent and that some glaciers were more extensive during earlier parts of the last glacial cycle. Copyright © 2006 John Wiley & Sons, Ltd.     

Direct comparison of UK temperatures and Greenland snow accumulation rates, 15000—12000 yr ago

A palaeotemperature record based on fossil coleopteran evidence and dated by accelerator mass spectroscopy (AMS) ¹⁴C determinations on plant macrofossils has been obtained for the last glacial-interglacial transition from the site of Gransmoor in eastern England. Calibration of the radiocarbon measurements enabled a direct comparison to be made between this palaeotemperature curve and the snow accumulation record from the GISP-2 Greenland ice-core. The similarity between the two data sets suggests a degree of correspondence in the pattern and timing of climatic change in Greenland and the British Isles at the end of the last cold stage.     

Radiocarbon calibration beyond 20,000 C yr B.P. by means of planktonic foraminifera of the Iberian Margin

We present a new set of ¹⁴C ages obtained by accelerator mass spectrometry (AMS) on planktonic foraminifera from a deep-sea core collected off the Iberian Margin (MD952042). This site, at 37°N, is distant from the high-latitude zones where ¹⁴C reservoir age is large and variable. Many independent proxies — alkenones, magnetic susceptibility, ice-rafted debris, foraminifera stable isotopes, abundances of foraminifera, pollen, and dinoflagellates — show abrupt changes correlative with Dansgaard-Oeschger and Heinrich events of the last glacial period. The good stratigraphic agreement of all proxies — from the fine to the coarse-size fractions — indicates that the foraminifera ¹⁴C ages are representative of the different sediment fractions. To obtain reliable ¹⁴C ages of foraminifera beyond 20,000 ¹⁴C yr B.P., we leached the shells prior to carbonate hydrolysis and subsequent analysis. For a calendar age scale, we matched the Iberian Margin U₃₇^K′ profile with that of Greenland Summit δ¹⁸O. Both are proxies for temperature, which in models varies synchronously in the two areas. The match creates no spurious jumps in sedimentation rate and requires only a limited number of tie points. Except for ages older than 40,000 ¹⁴C yr B.P., Greenland's GISP2 and GRIP records yield similar calendars. The ¹⁴C and imported calendar ages of the Iberian Margin record are then compared to data — from lacustrine annual varves and from corals and speleothems dated by U-Th — previously used to extend the calibration beyond 20,000 ¹⁴C yr B.P. The new record follows a smooth pattern between 23,000 and 50,000 cal yr B.P. We find good agreement with the previous data sets between 23,000 and 31,000 cal yr B.P. In the interval between 33,000 and 41,000 cal yr B.P., for which previous records disagree by up to 5000 cal yr, the Iberian Margin record closely follows the polynomial curve that was previously defined by an interpolation of the coral ages and runs between the Lake Suigetsu and the Bahamian speleothem data sets.     

Heinrich事件和末次冰期气候的不稳定性

对北大西洋深海沉积物的研究表明：末次冰期北大西洋沉积物记录中有多次洋面温度降低、有孔虫含量减少、盐度降低和粗颗粒的碳酸盐碎屑快速堆积的现象，这些时间跨度上千年或几百年的气候快速波动被称为Heinrich事件^{［１－４］}，很难用古气候的米兰科维奇理论来解释。最近对末次冰期中国马兰黄土堆积的调查发现，在末次冰期之中东亚冬季风也有多次加强，它与北大西洋沉积物中的Heinrich事件对应很好，暗示着东亚季风的变迁更直接受控于北半球冰量的变化，而不是过去人们认为的东亚古季风气候变化与地球轨道变化引起太阳辐射变化直接相关。      

Quantitative estimates of temperature and precipitation changes over the last millennium from pollen and lake-level data at Lake Joux, Swiss Jura Mountains

This paper presents quantitative climate estimates for the last millennium, using a multi-proxy approach with pollen and lake-level data from Lake Joux (Swiss Jura Mountains). The climate reconstruction, based on the Modern Analogue Technique, indicates warmer and drier conditions during the Medieval Warm Period (MWP). MWP was preceded by a short-lived cold humid event around AD 1060, and followed by a rapid return around AD 1400 to cooler and wetter conditions which generally characterize the Little Ice Age (LIA). Around AD 1450 (solar Spörer minimum), the LIA attained a temperature minimum and a summer precipitation maximum. The solar Maunder minimum around AD 1690 corresponded at Joux to rather mild temperatures but maximal annual precipitation. These results generally agree with other records from neighbouring Alpine regions. However, there are differences in the timing of the LIA temperature minimum depending on the proxy and/or the method used for the reconstruction. As a working hypothesis, the hydrological signal associated with the MWP and LIA oscillations at Lake Joux may have been mainly driven by a shift around AD 1400 from positive to negative NAO modes in response to variations in solar irradiance possibly coupled with changes in the Atlantic meridional overturning circulation.     

大理苍山世界地质公园第四纪冰碛物特征及年代测定

云南大理苍山世界地质公园具有较高的美学价值与地学科研价值,在地学研究领域中苍山又被称为点苍山。本次研究的主要区域是点苍山玉局峰北坡和点苍山西北部的罗坪山大小海子地区,本文对点苍山玉局峰北坡冰碛物和罗坪山第四纪沉积物进行了光释光年代测定,讨论了光释光测年方法在点苍山地区第四纪冰碛物测年中的适用性,测得玉局峰北坡搬运距离较远的三条冰碛垄的年龄为(31.68±3.22)~(28.01±1.95)ka BP,属于末次冰期。对罗坪山地区第四纪沉积物进行石英砂扫描电镜分析,观察到大小海子附近第四纪沉积物石英砂表面具有一定的冰川成因特征,推断罗坪山大小海子地区曾发育过第四纪冰川作用,通过光释光测年得到大小海子附近冰川沉积物年代为(136.36±10.16)ka BP,相当于倒数第二次冰期。结合前人研究,推断点苍山地区也具有发育倒数第二次冰期的可能性。     

江西沟1号风成剖面地球化学元素特征及古环境意义

通过对江西沟风成沉积剖面的地球化学分析,并结合OSL年代、粒度和磁化率数据,探讨了末次冰消期以来青海湖南岸气候的演变。剖面常量元素氧化物含量呈SiO_2Al_2O_3CaOFe_2O_3MgOK_2ONa_2O变化特征,常量元素氧化物及化学元素综合参数与地层有较好的对应性。不同的环境指标对气候变化的敏感性不同,总体上看,地球化学元素的敏感性高,全新世以来粒度和磁化率的变幅较小,即使在全新世大暖期,其值变化都很小。地球化学元素记录所反映的气候变化过程如下。Ⅰ.末次冰消期~12 ka BP,13.2 ka BP前总体上呈相对温暖状态,可能受B/A暖期的影响,13.2~12 ka BP气候不断向干冷方向发展,寒冷程度不断加剧并在12 ka BP达到最强,可能反映出12 ka BP左右的新仙女木事件;Ⅱ.12~9 ka BP,气候呈波动回升状态,暖湿程度呈增加趋势,表现出温凉的气候变化特征;Ⅲ.9~4.5 ka BP,气候温暖湿润,与全新世大暖期相对应,该阶段水热组合达到最佳,但存在阶段性的变化;Ⅳ.4.5~2 ka BP,气候总体向冷干方向转变,3 ka BP前变化幅度较小,气候较温暖湿润,3~2 ka BP气候凉偏干。     

末次盛冰期长江南京段河槽特征及古流量

The stratigraphical cross-sections of the Yangtze River incised-valley near the No.1,No.3 and No.4 Nanjing Yangtze River bridges were established with respective bore date and documents.By 14C age analysis of the samples of four drilling cores near the No.4 Bridge(to be built),we can find that the time range of paleo-valley is dated in the LGM at a depth of-60 m to-90 m near Nanjing.It is also indicated that the deep incised-valley channel was narrow and the river flowed swiftly.The ancient Yangtze River deep channel presented partially and deeply incised features near the No.1 Bridge.According to previous publications,much research has been done on the main paleo-channel of the Yangtze River,but few results have been achieved on discharge estimation.In this paper,the incipient velocity and average velocity of the LGM was calculated with Vc = 4.60 d 1/3 h1/6,95 Vc 1.28lg 13.15.h.gd,d = 6 * 90 V 6.5u h≈ d,etc.,in terms of the river shape,sedimentary grain size and sequences near the No.3 and No.1 bridges.Moreover,the discharge in Nanjing reach of the Yangtze River during the LGM has been estimated to be around 12,000-16,000 m3/s according to the relationship of discharge,velocity of flow and cross-section.     

青海湖鸟岛北原生砂楔的发现及其古气候意义

青海湖鸟岛北中更新世冲洪积砂砾石层中发育7个原生砂楔。原生砂楔中的细粒砂ESR测年结果表明,该地区至少经历两次冰期:砂楔剖面南侧3个楔体发育时代分别为228±2ka、228±22ka和143±14ka,形成于倒数第二次冰期;剖面北侧3个楔体发育时代分别为17±1ka、59±10ka和60±6ka,形成于末次冰期。估算青海湖盆地在倒数第二次冰期时的年平均气温不高于-9.5~-11.0℃,降温幅度较大,比现今年均气温低11.2~12.7℃;末次冰期的年平均气温不高于-7.5~-9.0℃,较现在低9.2~10.7℃。     

NAO在末次冰盛期和全新世对AMOC影响的对比研究

大西洋经向翻转环流(AMOC)的年代际变率对气候变化起着重要的调制作用。在现代气候背景下, 北大西洋涛动(NAO)加强能使AMOC增强, 但这种关系在更长时间尺度上是否成立尚不清楚。本研究利用TraCE-21ka模拟资料, 对比分析末次冰盛期(LGM)和全新世时期NAO对AMOC影响的异同。结果表明, LGM时期较全新世时期经向温度梯度偏强, NAO位置偏南, 这导致NAO与AMOC关系的不同: NAO的增强在LGM时期可以使AMOC增强, 而在全新世使AMOC减弱。具体地, 在LGM时期NAO的加强使北大西洋副极地气旋性环流增强, 其南支导致向北的高盐海水输送增加, 从而使北大西洋副极地区域密度升高, AMOC增强。与此同时, NAO正位相还能在中纬度激发异常的Ekman下沉流使AMOC加强。相反, 在全新世时期, NAO正位相导致北大西洋副极地地区气旋性环流减弱, 这导致中纬度向高纬度输送的高盐度海水减少, AMOC减弱。本研究表明NAO与AMOC的关系在很大程度上取决于不同气候背景下NAO的位置。