The Age of Rift-Related Basalts in East Antarctica

The Lambert Rift, which is a large intracontinental rift zone in East Antarctica, developed over a long period of geological time, beginning from the Late Paleozoic, and its evolution was accompanied by magmatic activity. The latest manifestation of magmatism is eruption of alkaline olivine–leucite basalts on the western side of the Lambert Rift; Rb–Sr dating referred its time to the Middle Eocene, although its genesis remained vague. In order to solve this problem, we found geochronometer minerals in basaltic samples and 68 apatite grains appeared to be suitable for analysis. Their ages and ages of host basalts, determined by the U–Pb local method on the SIMS SHRIMP-II, were significantly different (323 ± 31 Ma) from those assumed earlier. This age corresponds to the earliest stage of crustal extension in East Antarctica and to most of Gondwana. The new data crucially change the ideas about the evolution of Lambert Rift and demonstrate the ambiguity of К–Ar dates of the alkali effusive formed under long-term rifting.     

雷达高度计探测东南极地区冰面变化

利用美国SeaSat和GeoSat星载高度计,采用再跟踪算法完成雷达高度计数据处理,对大气影响、固体地球潮汛、坡度和水汽进行误差纠正,提取了南极东南部研究区两个年代的从海岸到72°S的数条截面多条冰面剖面.结果表明:Lambert冰川/Amery冰架在1978-1986年间,Lambert冰川以西冰面平均高度上升0.92m,Lambert冰川以东冰面平均高度上升0.47m.     

东南极内陆格罗夫山地区冰厚及冰下地形特征

中国第30次南极科学考察队格罗夫山分队(CHINARE30, 2013-2014年)利用雪地车载深层探冰雷达在东南极格罗夫山地区开展了测线总长度超过200 km大范围、高分辨率的冰厚及冰下地形调查, 获得了哈丁山北部和萨哈罗夫岭与阵风悬崖之间详细的冰厚及冰下地形特征. 通过对雷达数据分析表明, 哈丁山北部区域平均冰厚为580 m, 最大冰厚超过1 000 m, 出现在该区域的东北方向, 而东南方向冰厚相对较小; 萨哈罗夫岭与阵风悬崖之间区域的平均冰厚为610 m, 最大冰厚超过1 100 m, 该区域槽谷发育十分成熟, 槽谷形态近似呈U型. 通过对雷达剖面影像的筛选和分析, 推测在格罗夫山地区可能存在2个液态冰下湖泊.     

Study of inter-annual variations in surface melting over Amery Ice Shelf,East Antarctica,using space-borne scatterometer data

The widespread retreat of glaciers can be considered as a response to the climate change. Being the largest retreating glacier–ice shelf system in East Antarctica, the Amery Ice Shelf–Lambert Glacier system plays an important role in contributing to sea level rise as well as the surrounding environment and climate. The present study is focused on the investigation of surface melting over the ice shelf using QuikSCAT Ku-band scatterometer data for more than 100 months covering the period from January 2000 to July 2009. The corresponding weather data of Davis Station was obtained from the website of Australian Antarctic Division. Very prominent dips in the backscatter observed in the month of January form a distinct signature caused by physical process of significant melting of the ice/snow surface. The steep increase again in February is attributed to the initiation of the freezing phenomenon. The derived melting index compared well with the passive microwave-based melting index derived by other researchers. A strong relationship was found between the scatterometer-derived melting index and the cumulative monthly mean air temperature. The highest melting was observed in the summer (January) of 2004, and thereafter gradual cooling appeared to take place in the subsequent years. The snow pack thickness, inferred from the backscatter variations, was found to be higher during winters (June) of 2004 and 2005, compared with other years.     

Abrupt climate warming in East Antarctica during the early Holocene

We report a centennial-scale warming event between 8600 and 8400 cal BP from Amery Oasis, East Antarctica, that is documented by the geochemical record in a lacustrine sediment sequence. The organic carbon content, the C/S ratio, and the sedimentation rate in this core have distinctly elevated values around 8500 y ago reflecting relatively warm and ice-free conditions that led to well-ventilated conditions in the lake and considerable sedimentation of both autochthonous and allochthonous organic matter on the lake bottom. This abrupt warming event occurred concurrently with reported warm climatic conditions in the Southern Ocean while the climate in central East Antarctic remained cold. The comparison of the spatial and temporal variability of warm climatic periods documented in various terrestrial, marine, and glacial archives from East Antarctica elucidates the uniqueness of the centennial-scale warming event in the Amery Oasis. We also discuss a possible correlation of the Amery warming event with the abrupt climatic deterioration around 8200 cal BP on the Northern Hemisphere.     

东南极Lambert冰川-Amery冰架系统平衡通量分布的计算

通过对Lambert冰川盆地(LGB)考察路线上约1 700 km长的LGB剖面和距冰架末端约50 km、长150 km的A剖面, 分别利用GPS冰流速值及雷达测厚值进行冰通量的计算得出:每年流过LGB剖面的冰通量为43.95 Gta-1, 而通过A剖面的冰通量仅为26. 42 Gt*a-1, Amery冰架底部净融化量为7.8 Gt*a-1. 整个Lambert冰川-Amery冰架系统(LAS)地区的表面净物质平衡总量约为90 Gt*a-1; LGB地区的表面净物质平衡总量为46 Gt*a-1. 通过分析得出, 整个LAS地区及LGB地区均处于物质正平衡状态, 而LAS流域的上游区域S'则处于物质负平衡状态.     

The last interglacial as represented in the glaciochemical record from Mount Moulton Blue Ice Area,West Antarctica

Understanding climate during the last interglacial is critical for understanding how modern climate change differs from purely naturally forced climate change. Here we present the first high-resolution ice core record of the last interglacial and transition to the subsequent glacial period from Antarctica and the first glaciochemical record for this period from West Antarctica. Samples were collected from a horizontal ice trench in the Mt. Moulton Blue Ice Area (BIA) in West Antarctica and analyzed for their soluble major anions (Cl^?, NO₃^?, SO₄^2-), major and trace elements (Na, Mg, Ca, Sr, Cd, Cs, Ba, La, Ce, Pr, Pb, Bi, U, As, Al, S, Ti, V, Cr, Mn, Fe, Co, Cu, Zn) and water hydrogen isotopes (δD). The last interglacial is characterized by warmer temperatures (δD), weakened atmospheric circulation (dust elements, seasalts aerosols), decreased sea ice extent (Na, nssSO₄^2-) and decreased oceanic productivity (nssSO₄^2-). A combined examination of Mt. Moulton seasalts, dust, nssSO₄^2- and δD records indicates that the last interglacial was extremely stable compared to glacial age climate events and it ended through a long period of gradual cooling unlike that projected for future Holocene climate.     

Animals and humans in the European Russian Arctic towards the end of the last Ice Age and during the mid‐Holocene time

Here, we present and discuss results from geo‐archaeological and palaeo‐zoological investigations at the Palaeolithic site Pymva Shor, in the Russian Arctic. As many as 3324 vertebrate fauna remains were recovered during two excavations. This includes bones of mammals, birds and fish. Radiocarbon dates were obtained from 26 specimens. The results show ages in the range 30–3 cal. ka BP. Hare and reindeer are the best represented amongst the identified mammalian species, whilst ptarmigan and various wader species dominate the avian bones. The Pleistocene assemblage includes herbivorous herd animals such as horse, bison and musk ox. These species are typical of the treeless tundra‐steppe landscape that existed during the Lateglacial. Of particular interest is a cave lion specimen that has been radiocarbon dated to approximately 15.5 cal. ka BP. According to our knowledge, this is one of the latest dated examples of this species in Eurasia. The faunal composition in the Holocene assemblage is strikingly different and includes distinct forest taxa such as beaver and pine marten. The avifauna also supports a forested environment with the presence of black grouse. A few stone artefacts were found within the strata, and have been radiocarbon dated to 16–15 cal. ka BP, suggesting that there were humans in the Pymva Shore area at that time. We identified impact notches and cut marks on some radiocarbon‐dated reindeer and bison bones, showing that humans were present twice during the Younger Dryas period. A fourth occupation phase is identified during the mid‐Holocene (6–5 cal. ka BP). We also investigated river terraces and obtained a series of luminescence dates. These have been used to reconstruct the geological history and the relationship to the find‐bearing strata.     

Evidence for delayed poleward expansion of North Atlantic surface waters during the last interglacial (MIS 5e)

The Last Interglacial (Marine Isotopic Stage or MIS 5e) surface ocean heat flux from the Rockall Basin (NE Atlantic) towards the Arctic Ocean was reconstructed by analysing dinoflagellate cyst (dinocyst) assemblages in four sediment cores. Together with records of stable isotopes and ice-rafted detritus, the assemblage data reflect the northward retreat of ice(berg)-laden waters and the gradual development towards interglacial conditions at the transition from the Saalian deglaciation (Termination II) into MIS 5e. At the Rockall Basin, this onset of the Last Interglacial is soon followed by the appearance of the thermophilic dinocyst species Spiniferites mirabilis, with relative abundances higher than those observed at present in the area. North of the Iceland-Scotland Ridge, however, S. mirabilis only appears in significant numbers during late MIS 5e, between ～118 and 116.5 ka. Hence, fully marine Last Interglacial conditions with most intense Atlantic surface water influence occurred during late MIS 5e in the Nordic seas, and consequently also farther north in the Arctic Ocean, and at times when northern hemisphere summer insolation was already significantly decreased. The stratigraphic position of this Late Interglacial optimum is supported by planktic foraminifers and contrasts with the timing of the early Holocene climatic optimum in this area. We interpret the delayed northward expansion of Atlantic waters towards the polar latitudes as a result of the Saalian ice sheet deglaciation and its specific impact on the subsequent water mass evolution in this region.     

Geochemical evidence for a large methane release during the last deglaciation from Marmara Sea sediments

Recent methane inventories have revealed the potential impact of gas hydrates on the global carbon cycle, and hence in climate change (Milkov, 2004). However, only a few studies have traced methane release in the geologic record. Here, we show geochemical evidence for a large scale methane release at mid-latitudes during the last deglaciation. The Sea of Marmara, an enclosed sea between the Mediterranean and Black Seas, is located in a tectonically active basin with gas hydrate expulsion and the formation of shallow gas hydrates. Since depths in the basin are shallower than 1100 m, future global temperatures are expected to have a great influence in destabilizing methane clathrates. Among the suite of biomarkers, we have focused on diplopterol and diploptene profiles in core MD012430, retrieved from the central basin in the Marmara Sea. Our results indicate that during the last 15,000 years, hopanoids showed important concentration variations with a pronounced peak during the deglaciation.The lack of a relationship between diplopterol/diploptene and phytoplanktonic biomarker concentrations, as well as a depleted isotopic composition, have linked the hopanoid maxima to methanotrophic activity, suggesting that an intense methane release occurred at the onset of deglaciation in the Marmara Sea. The vulnerability of the hydrate stability zone to changes in temperature and pressure under this range of shallow water depths, as well as the relative timing of the hopanoid maxima and sea surface temperature rise, points to thermal destabilization of hydrates as a trigger for methane release in the water column.     

Climate change on the Yucatan Peninsula during the Little Ice Age

We studied a 5.1-m sediment core from Aguada X'caamal (20° 36.6′N, 89° 42.9′W), a small sinkhole lake in northwest Yucatan, Mexico. Between 1400 and 1500 A.D., oxygen isotope ratios of ostracod and gastropod carbonate increased by an average of 2.2‰ and the benthic foraminifer Ammonia beccarii parkinsoniana appeared in the sediment profile, indicating a hydrologic change that included increased lake water salinity. Pollen from a core in nearby Cenote San José Chulchacá showed a decrease in mesic forest taxa during the same period. Oxygen isotopes of shell carbonate in sediment cores from Lakes Chichancanab (19° 53.0′N, 88° 46.0′W) and Salpeten (16° 58.6′N, 89° 40.5′W) to the south also increased in the mid-15th century, but less so than in Aguada X'caamal. Climate change in the 15th century is also supported by historical accounts of cold and famine described in Maya and Aztec chronicles. We conclude that climate became drier on the Yucatan Peninsula in the 15th century A.D. near the onset of the Little Ice Age (LIA). Comparison of results from the Yucatan Peninsula with other circum-Caribbean paleoclimate records indicates a coherent climate response for this region at the beginning of the LIA. At that time, sea surface temperatures cooled and aridity in the circum-Caribbean region increased.     

近千年东亚夏季风演变*

在分析东亚地区夏季海平面气压场与中国东部6区域干湿指数关系的基础上,重建了公元960—2000年近千年东亚地区夏季海平面气压场的格点资料,并对重建效果进行了检验。同时,依据重建的海平面气压场资料定义了近千年东亚夏季风强度指数,探讨了近千年东亚夏季风的变化特征。结果表明：（1）重建的近千年东亚地区的海平面气压场具有一定的可信度,它为研究更长时间尺度的东亚夏季风变化特征提供了基础;（2）东亚夏季风指数存在60～70 a、30～40 a、10～20 a的显著周期变化;（3）近千年东亚夏季风的强度指数主要经历过9次明显的趋势突变。其中,13世纪30年代东亚夏季风的强度指数经历了最显著的振动。     

Palaeobotanical evidence for warm summers in the East Siberian Arctic during the last cold stage

Plant macrofossils from the “Mamontovy Khayata” permafrost sequence (71°60′N, 129°25′E) on the Bykovsky Peninsula reflect climate and plant biodiversity in west Beringia during the last cold stage. 70 AMS and 20 conventional ¹⁴C dates suggest sediment accumulation between about 60,000 and 7500 ¹⁴C yr B.P. The plant remains prove that during the last cold-stage arctic species (Minuartia arctica, Draba spp., Kobresia myosuroides) coexisted with aquatic (Potamogeton vaginatus, Callitriche hermaphroditica), littoral (Ranunculus reptans, Rumex maritimus), meadow (Hordeum brevisubulatum, Puccinellia tenuiflora) and steppe taxa (Alyssum obovatum, Silene repens, Koeleria cristata, Linum perenne). The reconstructed vegetation composition is similar to modern vegetation mosaics in central and northeast Yakutian relict steppe areas. Thus, productive meadow and steppe communities played an important role in the Siberian Arctic vegetation during the late Pleistocene and could have served as food resource for large populations of herbivores. The floristic composition reflects an extremely continental, arid climate with winters colder and summers distinctly warmer than at present. Holocene macrofossil assemblages indicate a successive paludification possibly connected with marine transgression, increased oceanic influence and atmospheric humidity. Although some steppe taxa were still present in the early Holocene, they disappeared completely before 2900 ¹⁴C yr B.P.     

小冰期气候的湖北石笋碳同位素记录

研究季风区小冰期的结构特征和区域响应有助于深入了解季风系统与地球内外驱动力的耦合关系。利用湖北神农架永兴洞YX275石笋的7个230Th年龄和120个碳同位素数据,建立了1 360~1 955 AD期间5年分辨率的石笋碳同位素序列。石笋δ13C与δ18O记录在长期趋势上有很好的对应关系,对小冰期气候响应明显,δ13C记录在大尺度季风环流影响下主要反映了局域湿度变化特征。δ13C序列在1 453~1 890 AD显著正偏,表明小冰期时湿度明显降低。此外,石笋δ13C与亚洲夏季温度、南方涛动指数和热带辐合带记录有较好的一致性,表明亚洲大陆夏季温度和太平洋水汽可能通过影响夏季风的强弱来调控湖北地区的湿度变化。在小冰期内部,δ13C记录在1 450~1 550 AD和1 790~1 830 AD出现进一步正偏,这些振荡分别对应于太阳活动的Sp?rer和Dalton极小期,暗示太阳活动减弱期对中国中部小冰期水文振荡的进一步调控作用。     

小冰期时中国南方地区降水模式的差异研究

小冰期是过去一千年中全球气候变化的重要事件之一。关于小冰期时中国季风区和西风影响区气候变化的对比研究众多,但是缺乏中国南方地区降水模式时空差异的研究,难以了解中国南方地区降水变化规律。为了系统地了解小冰期时中国南方地区降水的复杂性,本文将中国南方地区划分为东南—华南沿海地区、中部地区以及西南地区三个区域,总共选取了19条高分辨率的古气候记录进行对比研究,主要有以下几点认识:(1)相对于中世纪暖期而言,小冰期期间中国南方东南—华南沿海地区的气候偏湿,这可能与雨带在中国南方的滞留时间延长和沿海地区受台风的影响增强有关。(2)中国中部地区秦岭南麓和神农架高山林区在小冰期时期主要呈"冷湿"的模式,差异在于秦岭南麓区域主要在小冰期中后期偏湿,这与中部其他区域偏"冷干"的模式不同。这种区域差异可能是由于地形地势和大气环流的复杂性导致。(3)中国西南地区受印度夏季风和东亚夏季风的共同影响,且该区域地形复杂,其气候变化在小冰期时期存在更加明显的空间差异,没有呈现出比较一致的降水模式。与小冰期期间的降水变化不同的是,近30年东南—华南沿海地区除了台湾和雷州半岛,其他区域降水明显减少,可能受气温和人类活动等因素的影响。通过结合高分辨率的古气候记录,我们系统分析了中国南方小冰期的干湿模式在时空上的差异及其可能的影响因子,这对于认识小冰期时中国南方不同区域降水的复杂性及未来旱涝灾害的防控具有一定意义。     

Quaternary changes in level of the upper Taylor Glacier, Antarctica: implications for paleoclimate and East Antarctic Ice Sheet dynamics

Glacial drifts perched alongside outlet glaciers that drain through the Transantarctic Mountains constrain inland polar plateau ice elevations. The Taylor Glacier, which heads in the Taylor Dome (a peripheral dome of the East Antarctic Ice Sheet), drains East Antarctic ice into the Dry Valleys sector of Transantarctic Mountains and terminates in central Taylor Valley, about 24 km west of the Ross Sea. Five gravel-rich drifts (including 39 distinct moraine ridges) fringe a lateral lobe of the Taylor Glacier in the lower Arena Valley, Quartermain Mountains, southern Victoria Land. ³He and ¹⁰Be exposure age dating (from Brook et al . 1992), together with Arena Valley stratigraphy and soil morphologic data, provide chronologic control for these drifts and constrain maximum Quaternary thickening of the inland Taylor ice dome to less than 160 m. These minor Quaternary expansions of Taylor Glacier were out-of-phase with outlet glaciers that pass through the Transantarctic Mountains and terminate in the Ross Sea north and south of the Dry Valleys region. Textural analyses suggest that drift deposition occurred from cold-based ice, even though Taylor Glacier advances most likely occurred during global interglaciations. The thermal regime of former Taylor Glacier ice lobes, the character of geomorphic features superimposed on individual drifts, the chemical composition of soils developed on Taylor drifts, and the stability of in situ moraine ridges on steep valley walls suggest that the present cold-desert climate in Arena Valley has persisted for at least the last 2.2 Ma.