Environmental magnetic studies of lacustrine sediments

In recent decades, environmental magnetism has become an important method for studying past global climatic and environmental changes. For an environmental magnetic study, it is important to understand the magnetic properties, of various magnetic minerals in lacustrine sedimcuts and the processes involved in the formation, transport,and preservation of these magnetic minerals. The magnetic response of lacustrine sediments to climatic change may vary widely from lake to lake. To reconstruct the processes contributing to paleoclimatic and paleoenvironmental change, it is, therefore, necessary to derive a complete understanding of the potential origin of the magnetic signal observed.     

长江三角洲与东海岛屿黄土研究综述

黄土是记录古气候和古环境变化信息的良好载体,长江三角洲平原及东海岛屿晚更新世黄土地层剖面的发现, 证明了末次冰期以来长江三角洲地区及海域均广泛存在风尘黄土堆积的事实,研究表明长江三角洲平原埋藏黄土和东海岛屿黄土是下蜀黄土上部地层在中国东部沿海和海域的延伸和继续.中国东部风尘黄土记录了我国晚更新世以来东部沿海地区的气候变化及东亚季风演变,与中国西北地区黄土地层相比,东部黄土具有沉积厚度小、成因复杂、次生作用强烈的特点;粒度和磁化率等参数垂向变化规律没有西部明显;粘土矿物和常量化学成分显示了东部黄土经历了较强地淋溶作用.在东部风尘黄土研究过程中,更需注意区域环境特征、地貌地形特点以及沉积期后环境变化对风尘黄土沉积物的影响效应,通过多指标的综合验证才能够正确解读黄土地层赋含的环境变化信息以及认识我国东部季风区晚更新世以来的古环境演变过程及规律.     

Mid-Late Quaternary loess-paleosol sequence in Lantian’s Yushan,China: An environmental magnetism approach and its paleoclimatic significance

The application of rock magnetism methods to investigating the variations of magnetic minerals in the sediments is an important approach to the reestablishment of paleoclimate evolution. Thus we performed fine magnetic measurements on the loess-paleosol sequence (from L15 upwards to S5, in which L is short for Loess, S is short for Paleosol, the same hereinafter) of Yushan stratigraphic section, which is on the southeastern margin of Chinese Loess Plateau, in Lantian County of China’s Shaanxi Province, and the thickness of which is ca. 40 m. Our study shows that the primary magnetic carriers of loess and paleosol in this section are magnetite, maghemite, hematite and goethite. Thermomagnetic analyses on the samples of representative horizons show that the higher pedogenesis degree of the sediments, the smaller variations of magnetization there will be before and after heating, probably related to the pedogenic alteration of loess sediments. Analyses of several magnetic parameters show a significant discrepancy between the paleoclimatic conditions recorded in the strata from the loess unit L15 upward to the paleosol unit S5 in the study area and those recorded in the relative strata of other sections on the Chinese Loess Plateau, and those recorded in marine sediments, indicating the great impact of regional geological background. Similarly, the rapid and intensive change recorded in the segment from L15 to S9-1, and the significant difference between the paleoclimate evolutions of the two periods before and after the change (from L15 to S9-1, and from L9 to the base of S6) indicate the strong alteration of magnetic carriers in the study area as a result of the alternations of summer and winter monsoons in East Asia.     

辽南滨海地区黄土磁化率特征及其古气候分析

通过对辽南滨海地区第四纪黄土沉积物的磁化率测量和分析,发现该区马兰黄土的磁化率值不高,反映了该区晚更新世受末次冰期影响,气候条件干冷,黄土粉尘沉积速率快,成壤作用微弱;时间更早的离石黄土磁化率值偏高,反映其成土环境较为温暖潮湿,黄土粉尘堆积速率慢,成壤作用好。磁化率曲线的波动变化反映了古气候发生多次旋回变化;黄土-古土壤序列很好的指示了干冷、暖湿气候的交替,也对应了冬季风、夏季风的交替变化。根据磁化率值对古气温和古降水量的回归计算得到古气温应介于5.85℃～13.0℃（＋1.5℃）,古降水量介于403.22mm～735mm,其标准都低于当今水平。反演辽南滨海地区中、晚更新世时期的黄土沉积环境是比较干冷的,并且经历了多次的小规模的干冷、暖湿交替变化。     

Mechanism of variations in environmental magnetic proxies of lake sediments from Nam Co, Tibet during the Holocene

High-resolution environmental records from the Tibetan Plateau are essential to understand past global climatic and environmental changes. Magnetic minerals in lake sediments are important proxies to reconstruct environmental and climatic changes. Nam Co (lake) is a typical great lake in the transitional region of southwest monsoon in the Tibetan Plateau. Previous studies have extensively focused on geochemistry, microfossils, sedimentology and biochemistry analysis of Nam Co, which provides sound interpretation of paleoclimatic and paleoenvironmental changes. However, up to now, no systematic environmental magnetic studies have been carried out. Therefore, high-resolution and systematic magnetic studies combined with geochemical parameters were carried on lake sediments of core NC 08/01 from Nam Co for the Holocene period (11.3 cal ka BP) in order to explore how magnetic properties of the sediments respond to climatic changes. Based on variations of magnetic proxies, the sequence can be separated into 3 units. Unit 1 (236-199 cm, 11.3-7.8 cal ka BP) contains dominantly coarse-grained magnetite with homogeneous grain size. A positive correlation between magnetite and Ti strongly suggests that these coarse-grained detrital magnetites reflect detrital input signals due to insignificant effects of postdepositional dissolution processes on these coarse-grained magnetite particles. For Unit 2 (198-102 cm, 7.8-2.1 cal ka BP), magnetic grain size is finer and the corresponding concentration of magnetite is also reduced. This is mainly due to significant dissolution of these fine-grained detrital magnetite particles, which were transported under reduced water flow conditions during this period. For Unit 3 (101-0 cm, 2.1-0 cal ka BP), the bulk magnetic properties are dominated by a mixture of single domain biogenic magnetite and detrital magnetite. The concentration of magnetic minerals is not correlated with the Ti content. In conclusion, the preservation of magnetic minerals in the lake sediment and thus the corresponding magnetic properties are related to the initial grain size. Combination of magnetic properties (including variation of grain size and concentration) and other proxies of detrital inputs (e.g. Ti) can be used to infer the variation of redox conditions in Nam Co. These results provide a viable framework for reconstructing the paleoenvironmental changes of this lake.     

The rapid drift of the Indian tectonic plate

The breakup of the supercontinent Gondwanaland into Africa, Antarctica, Australia and India about 140 million years ago, and consequently the opening of the Indian Ocean, is thought to have been caused by heating of the lithosphere from below by a large plume whose relicts are now the Marion, Kerguelen and Réunion plumes. Plate reconstructions based on palaeomagnetic data suggest that the Indian plate attained a very high speed (18-20 cm yr(-1) during the late Cretaceous period) subsequent to its breakup from Gondwanaland, and then slowed to approximately 5 cm yr(-1) after the continental collision with Asia approximately 50 Myr ago. The Australian and African plates moved comparatively less distance and at much lower speeds of 2-4 cm yr(-1) (refs 3-5). Antarctica remained almost stationary. This mobility makes India unique among the fragments of Gondwanaland. Here we propose that when the fragments of Gondwanaland were separated by the plume, the penetration of their lithospheric roots into the asthenosphere were important in determining their speed. We estimated the thickness of the lithospheric plates of the different fragments of Gondwanaland around the Indian Ocean by using the shear-wave receiver function technique. We found that the fragment of Gondwanaland with clearly the thinnest lithosphere is India. The lithospheric roots in South Africa, Australia and Antarctica are between 180 and 300 km deep, whereas the Indian lithosphere extends only about 100 km deep. We infer that the plume that partitioned Gondwanaland may have also melted the lower half of the Indian lithosphere, thus permitting faster motion due to ridge push or slab pull.     

贵州一石笋TIMS--铀系定年及其古气候研究

通过对贵州七星洞ZFQX1石笋进行TIMS--铀系定年及其生长率分析,获得了距今43.02-1 2.65 ka的古气候信息.该石笋反映了此阶段贵州地区经历了暖湿--冷干--暖湿的气候演变趋势,这与全球古气候变化波动基本一致,同时也具有地区性的气候特点.     

新疆巴里坤湖 9.0cal.ka BP来沉积物地球化学元素分布特征与古气候环境演化

根据中国西北干旱区巴里坤湖BLK-1剖面沉积物的地化元素分析,采用SPSS因子分析法提取了对沉积环境变化敏感的地化元素和氧化物组分. 在 R、Q 型因子分析的基础上,结合沉积物的硅铁铝率（SiO2/（Al2O3+Fe2O3））、淋失系数（SiO2/（MgO+K2O））、CaO/MgO以及腐殖化度等指标,提取出古气候环境信息. 初步研究结果表明:近9.0cal ka BP以来,巴里坤湖地区气候环境仍以干旱化为主,全新世期间出现过多次不同程度的干湿变化,经历了5个气候阶段:9.0～7.5cal kaBP期间,气候干旱;7.5～5.8cal kaBP期间,气候温暖湿润,为研究区全新世最佳适宜期;5.8～3.0cal kaBP期间,气候干旱;3.0～1.0cal kaBP期间,气候湿润;1.0～0cal kaBP期间,气候干旱.     

Relations between sedimentary sequence and paleoclimatic changes during last 200 ka in the southern South China Sea

Core NS-93-5 was taken from the mild slope terrace of the southern South China Sea (SCS), which has preserved the steadiness depositional record of the normal marine environment since late Quaternary. Sedimentary sequence and oxygen isotopic stratigraphy of high resolution in the near 200 ka of the southern SCS has been established. By the comparative analysis with GISP2's ice core, the depositional record of D/O's events 1-21 and Heinrich's events H1-H6 in the southern SCS that reflected the quick climate change in short time scale since the last interglacial stage is revealed, which indicates that in the last 200 ka in the southern SCS and the Arctic area there was tele-connection of paleoclimate and the unstability of the Western Pacific Warm Pool. This note shows that the sedimentary layers order of this sea area classified by color identification and by oxygen isotope stratum has the very good corresponding relation. The color feature of sediment changes along with the climate and the former arranges a stagnant time of 1-1.5 ka generally. It was suggested that the paleoclimatic changes resulted in the rise and fall of sea level, the open and close of strait thoroughfare and the upwelling activity, which causes the changes of the oxidation-reduction condition of deep water along with the dynamic environment in this sea area, thus producing the different sediment features of sea bed. In addition, a volcanic ash layer, about 17 cm thick, has been found in the transition of oxygen isotope 4/5 stage, which is related with the Toba's volcanic eruption.     

浙江中全新世古气候古环境变化与河姆渡古人类

初步讨论了浙江东部中全新世古气候古环境变化及其对河姆渡古人类发展的影响。研究结果表明,中全新世大暖期是促使河姆渡古人类定居、发展和稻作农业在本区出现发展的重要条件,古气候古环境不同变化阶段与河姆渡古人类的发展阶段可以相互对比,而当时近于热带的古气候古环境变化,包括其导致的自然灾害的加强,对河姆渡古人类的生产,生活面貌的变化和发展有深刻影响,特别是缓慢降温后紧接着急剧升温的变化过程是引起人类发展面貌发生重大改变的一个重要因素。     

南极大气环流特征探讨

本文应用日本南极昭和基地１９８２年的探空资料＾［１］，讨论了从地面至２０ｈｐａ高度之间的风、温和４００ｈｐａ以下的湿度场的月平均年变化结构及大气环流特征。对位温、相当位温和饱和相当位温与大气环流的关系也进行了分析探讨。为研究全球大气环流的变化与异常及南北半球大气环流的相互作用的时空尺度遥相关提供了参考。     

公元1450年以来巢湖湖泊沉积物与区域和全球气候变化对比研究

湖泊沉积物物理、化学和生物特征是反演区域古环境的有效载体。本文选择巢湖西湖区CH-1钻孔连续湖泊沉积物的粒度和磁化率数据,结合公元1450年以来该区域旱涝灾害文献资料和全球气候变化代用指标中的Intcal 98 ~（14）C同位素数据,对500 cala.BP以来巢湖流域古气候演变,特别是降水演替进行分析。研究表明1450-1949年间,巢湖流域降水变化经历了少-多-少-多的变迁,湖芯钻孔粒度、磁化率变化很好的响应了区域和全球气候变化,环境变化与人类活动密切相关。     

新疆巴里坤湖9.0 cal ka BP以来沉积物地球化学元素分布特征与古气候环境演化

根据中国西北干旱区巴里坤湖BLK-1剖面沉积物的地化元素分析,采用SPSS因子分析法提取了对沉积环境变化敏感的地化元素和氧化物组分.在R、Q型因子分析的基础上,结合沉积物的硅铁铝率(SiO2/(Al2O3+Fe2O3))、淋失系数(SiO2/(MgO+K2O))、CaO/MgO以及腐殖化度等指标,提取出古气候环境信息.初步研究结果表明:近9.0 cal ka BP以来,巴里坤湖地区气候环境仍以干旱化为主,全新世期间出现过多次不同程度的干湿变化,经历了5个气候阶段:9.0—7.5 cal ka BP期间,气候干旱;7.5—5.8 cal ka BP期间,气候温暖湿润,为研究区全新世最佳适宜期;5.8—3.0 cal ka BP期间,气候干旱;3.0—1.0 cal ka BP期间,气候湿润;1.0—0 cal ka BP期间,气候干旱.     

南极地区Landsat8卫星影像的阴影检测方法

阴影在光学遥感影像中普遍存在,会对后续一系列图像信息提取等过程产生影响.在南极地区,由于其特殊的地理位置和环境,光学遥感影像中存在的阴影不可忽视.本研究在已有的阴影检测方法的基础上,根据Landsat8卫星数据的特点构建了一个新的阴影检测方法:首先对所有数据进行反射率转换处理,利用南极地区成像时的阴影在各波段的反射率特征,选取第1波段Coastal/Aerosol(0.43~0.45μm)和第5波段NIR(0.85~0.88μm)建立了能够有效识别阴影的阴影检测指数R51,并选取不同地物的训练样本进行统计分析,根据阈值提取法提取阴影为二值化图像,并从影像中剔除少量易与阴影混淆的冰面融池.通过选取南极不同地区的影像进行精度评价,发现该方法针对Landsat8数据在南极地区的阴影检测具有较高的提取精度和较好的普适性.本文利用阴影检测方法对收集的覆盖南极地区的1127景Landsat8影像进行阴影提取与统计、分布特征分析,最终提取阴影面积约15000km2,主要分布于裸岩山体集中区和大 型冰川所在地.      

夏季南极长城湾沿岸海域浮游硅藻群落定量分析

对南极硅藻进行定量研究较有必要,以客观准确地了解它的组成和分布特征。在南极长城湾沿岸海域共设置6个采样点,每个采样点定量采集3个浮游硅藻样本。结果显示:长城湾海域共发现浮游硅藻31种。浮游硅藻群落的优势种为Cocconesi fasciolata(18.5%)、Amphora holsatica(13.6%)。次优势种为Cocconeis costata(7.1%)、Cocconeis imperatrix(6.8%)、Thalassiosira antarctica(6.4%)、Rhoicosphenia genuflexa(6.1%)、Licmophora antarctica(5.1%)。各样点浮游硅藻细胞密度总平均值为1.92×10~5ind·L~(-1),变幅为1.44×10~5~2.56×10~5ind·L~(-1)。浮游硅藻的分布从湾心到湾口呈现出一定趋势:浮游硅藻的物种丰度,湾心处共15种,湾口处共25种;浮游硅藻的细胞密度,湾心处平均为2.51×10~5ind·L-1,湾口处平均为1.48×10~5ind·L~(-1)。通过相关性分析结果可知,浮游硅藻由湾心到湾口,物种丰度和硅藻细胞密度呈负相关。     

Low sea level rise projections from mountain glaciers and icecaps under global warming

The mean sea level has been projected to rise in the 21st century as a result of global warming. Such projections of sea level change depend on estimated future greenhouse emissions and on differing models, but model-average results from a mid-range scenario (A1B) suggests a 0.387-m rise by 2100 (refs 1, 2). The largest contributions to sea level rise are estimated to come from thermal expansion (0.288 m) and the melting of mountain glaciers and icecaps (0.106 m), with smaller inputs from Greenland (0.024 m) and Antarctica (- 0.074 m). Here we apply a melt model and a geometric volume model to our lower estimate of ice volume and assess the contribution of glaciers to sea level rise, excluding those in Greenland and Antarctica. We provide the first separate assessment of melt contributions from mountain glaciers and icecaps, as well as an improved treatment of volume shrinkage. We find that icecaps melt more slowly than mountain glaciers, whose area declines rapidly in the 21st century, making glaciers a limiting source for ice melt. Using two climate models, we project sea level rise due to melting of mountain glaciers and icecaps to be 0.046 and 0.051 m by 2100, about half that of previous projections.     

成都平原4 ka以来地层磁化率特征及气候变化意义

对成都金沙遗址IT6814探坑约4 m厚地层的5 cm间距磁化率分析,建立了成都平原4 ka以来的质量磁化率曲线和频率磁化率曲线.根据质量磁化率和频率磁化率特征结合粒度中值曲线对比,探讨了金沙遗址4 000 a来的气候环境变化,划分出成都平原的三个气候阶段:4 000～3 380 aBP,气候偏湿;3 380～1 700 aBP,气候条件相对湿热;1 700 aBP～20世纪90年代初,气候逐渐偏干冷.     

中国北方新近纪红黏土记录的古气候意义——与动物群化石序列对比

目的研究中国北方新近纪红黏土序列的古气候演化特征。方法对中国北方4个新近纪红黏土剖面沉积物的粒度、磁化率和化石动物群时间序列进行区域对比。结果黄土高原新近纪红黏土沉积以来发生了5次大的气候事件,气候事件的分界线在粒度、磁化率年龄曲线上特征明显,反映的气候特点与化石动物群演替性质完全一致。结论中国北方新近纪红黏土序列古气候演化过程的气候事件均与青藏高原隆升和东亚季风的发展相联系。时间上,南北气候演化事件基本同步;空间上,北部较南部地区变化剧烈。5次气候事件从下到上表现为半湿润、半干旱、湿润、干旱与湿润交替、干旱的气候演化特征;相应动物群生态类型表现为草原-疏林草原、草原为主的疏林草原、森林、草原荒漠与森林交替到草原的演化特点。     

Partial anhysteretic remanent magnetization （pARM） synthetic single- and multi-domain magnetites and its paleoenvironmental significance

In order to isolate magnetic signals carried by single-domain （SD） ferrimagnetic （FM） minerals from multi-domain （MD） FM minerals, we developed a few parameters using partial anhysteretic remanent magnetization （pARM）. Because MD fraction contains only soft （easy to be demagnetized） coercivity spectrum, pARM（〉20 mT） was sensitive in eliminating MD contributions. Ratio of pARM（5 mT, 10 mT）/pARM（0, 5 mT） is useful in quantifying a relative abundance of mass ratio between SD and MD fractions. These new proxies can quickly characterize the details of grain size distribution of magnetic minerals in paleoclimatic and paleomagnetic studies.     

Cenozoic motion between East and West Antarctica

The West Antarctic rift system is the result of late Mesozoic and Cenozoic extension between East and West Antarctica, and represents one of the largest active continental rift systems on Earth. But the timing and magnitude of the plate motions leading to the development of this rift system remain poorly known, because of a lack of magnetic anomaly and fracture zone constraints on seafloor spreading. Here we report on magnetic data, gravity data and swath bathymetry collected in several areas of the south Tasman Sea and northern Ross Sea. These results enable us to calculate mid-Cenozoic rotation parameters for East and West Antarctica. These rotations show that there was roughly 180 km of separation in the western Ross Sea embayment in Eocene and Oligocene time. This episode of extension provides a tectonic setting for several significant Cenozoic tectonic events in the Ross Sea embayment including the uplift of the Transantarctic Mountains and the deposition of large thicknesses of Oligocene sediments. Inclusion of this East-West Antarctic motion in the plate circuit linking the Australia, Antarctic and Pacific plates removes a puzzling gap between the Lord Howe rise and Campbell plateau found in previous early Tertiary reconstructions of the New Zealand region. Determination of this East-West Antarctic motion also resolves a long standing controversy regarding the contribution of deformation in this region to the global plate circuit linking the Pacific to the rest of the world.