中国黄土研究新进展 (二)古气候与全球变化

本文扼要介绍最近几年来在中国黄土高分辨率气候地层、第四纪古气候演化型式以及冷期和暖期中黄土高原古气候空间格局研究方面的一些新进展。根据土壤地层学与地球化学、地球物理学手段的结合,2.5 Ma来的中国黄土系列可划分出37个大的气候旋回,其中含74个气候阶段,这74个气候阶段又可细分为110个次级阶段。第四纪气候振荡的幅度受气候周期长短所控制。从冷期到暖期的气候转换具有突变性,从暖期到冷期的气候转换具有渐变性。冷期堆积的黄土与暖期形成的土壤在性状上都有地带性分布的特征。     

最后两个冰期旋回季风-沙漠系统不稳定性的高分辨率黄土记录

黄土高原西北部黄土沉积的粒度变化同时受控于沙漠进退和冬季风强度波动,因此,此区粒度可视为季风-沙漠系统变迁的替代性指标。本研究以2cm为间隔,对李家塬和新庄源S₂以上部分做了粒度分析。结果表明,倒数第二次冰期中的冰阶时段与末次冰期类似,存在着频繁的千年尺度的气候波动;而末次间冰期这类变化并不明显。     

Late Pleistocene paleoclimates of North America as inferred from stable isotope studies of speleothems

Some speleothems (CaCO₃ cave deposits) can be demonstrated to have been formed in oxygen isotopic equilibrium with their parent seepage waters and thus a record of relative fluctuations in depositional temperature can be obtained from the δ¹⁸O variations in successive growth layers of such deposits. These temperature fluctuations reflect variations in the average annual air temperature at the surface above the cave, and therefore permit inference of past continental climate changes. Equilibrium deposits have been obtained from caves in San Luis Potosi, Bermuda, Kentucky, West Virginia, Iowa, and Alberta, ranging in age from 200,000 years BP to the present, as determined by ${}^{230}\text{Th}{}^{234}\text{>U}$ dating of the speleothems. The δ¹⁸O time curves for the six sites show the following synchronous climatic fluctuations: warm periods from 190 to 165 and from 120 to 100 Ka, at 60 and 10 Ka, and cold periods from 95 to 65 and from 55 to 20 Ka. The periods of thermal maxima correspond in time to the interglacial periods of the marine foraminiferal isotopic and faunal temperature records and to periods of high sea stand as observed from radiometric dating of raised coral reefs. Maxima and minima in insolation appear to be synchronous with this record as well.     

A seasonally ''dry'' interglacial climate in eastern England during the early Middle Pleistocene: palaeopedological and stable isotopic evidence from Pakefield, UK

The pedology and the isotope geochemistry of the early Middle Pleistocene interglacial 'rootlet bed' at Pakefield, eastern England, record the climate when this unit accumulated. Horizons of nodular pedogenic calcrete indicate the existence of a strongly seasonal precipitation regime that produced a pronounced net soil moisture deficit during which secondary carbonate was precipitated. The isotopic composition of these nodules confirms the seasonality of annual precipitation, as this indicates that intense soil moisture evaporation was a major mechanism in secondary carbonate formation. The climatic interpretation is enhanced by the palaeoecology of the unit that indicates average summer temperatures significantly warmer than those of the present. The climate of this period was, therefore, different from that of the present day in terms of both temperature and the seasonality of precipitation. Although it has previously been shown that certain interglacial periods in northwest Europe were warmer than the Holocene, this is the first study to show that the precipitation regime of one such episode may also have been significantly different.     

五十万年来毛乌素沙漠的变迁

对沙漠-黄土边界带的古风成砂-黄土-古土壤序列的研究表明，毛乌素沙漠至少在0.5MaB.P.就已出现，但也并非从其出现之初就持续至今。沙漠-黄土边界带的风成沉积所记录的沙丘活化和固定的多次转变是第四纪时期毛乌素沙漠对气候振荡响应的结果。五十万年来，沙漠-黄土边界带的石峁剖面记录了13层古风成砂，代表了13次沙漠大规模的南侵。沙漠的南侵不仅可以发生在冰期鼎盛之时，同样也可以发生在间冰期中的寒冷气候幕出现之时。此外，鉴于沙漠-黄土边界带具有气候敏感性的特点，石峁剖面的地层记录还表明了第四纪时期不仅有冷期与暖期的多次旋回，而且冷期和暖期发生时还有次一级的气候波动。     

Modern,Sangamon and Yarmouth soil development in loess of unglaciated southwestern Illinois

The Thebes Section in unglaciated southwestern Illinois contains a well preserved ∼500 kyr loess–paleosol sequence with four loesses and three interglacial soils. Various magnetic, mineralogical, and elemental properties were analyzed and compared over the thickness of soil sola. These proxies for soil development intensity have the following trend: Yarmouth Geosol>Sangamon Geosol>modern soil. Quartz/plagioclase, Zr/Sr, and TiO₂/Na₂O ratios were most sensitive to weathering. Frequency dependent magnetic susceptibility and anhysteretic remanent magnetization, greatest in A horizons, also correspond well with soil development intensity. Neoformed mixed-layered kaolinite/expandables, suggestive of a warm/humid climate, were detected in the Sangamon and Yarmouth soil sola. Clay illuviation in soils was among the least sensitive indicators of soil development. Differences in properties among interglacial soils are interpreted to primarily reflect soil development duration, with climatic effects being secondary. Assuming logarithmic decreases in weathering rates, the observed weathering in the Sangamon Geosol is consistent with 50 kyr of interglacial weathering (Oxygen Isotope Stage 5) compared to 10 kyr for the modern soil (Oxygen Isotope Stage 1). We propose that the Yarmouth Geosol in the central Midwest formed over 180 kyr of interglacial weathering (including oxygen isotope stages 7, 9, and 11).     

Comparison of the Climate during Marine Isotope Stage 9 and 11 Inferred from a Speleothem Isotope Record from Northern Norway

A stalagmite from northern Norway is dated with 12 thermal ionization mass spectrometry U-Th dates, and at least four separate growth periods are identified that correspond with marine isotope stages 9, 11, 13, and probably 15. The calcite is tested for isotopic equilibrium with the Hendy test. Oxygen isotope measurements on 231 subsamples on a vertical transect are used as a paleotemperature proxy. The detailed isotopic record from MIS 9 show apparent similarities to a Holocene record from the same cave, both in the climatic evolution and the overall temperatures: both show temperature oscillations changing from high-frequency, low-amplitude cycles in the beginning of the interglacial period to lower frequency, higher amplitude cycles in the later part of the interglacial period. The isotope record from MIS 11 shows a distinct isotopic event toward heavier values. The isotopic record together with the porous, humus-rich calcite are interpreted as indicating a warmer than present interglacial period with several episodes of heavy rainfall.     

Effects of Climate Change on Soils in Glacial Deposits, Wind River Basin, Wyoming

Soil chronofunctions are often quasilinear, but those relationships may be only gross, fortuitous generalizations that ignore changes in soils resulting from climatic and other cyclic external influences that govern soil formation or soil degradation. In the Rocky Mountains, soil development has been extensively used to estimate the age of sediments, but the number of soils is usually few, and large ranges in values for soil data may be dismissed as “normal variation.” In a detailed study of soils in the Wind River Range and Wind River Basin, characteristics of near-surface horizons do not follow age trends. However, the underlying carbonate-bearing horizons do have age-related characteristics. This “soil paradox” may be related to glacial–interglacial cycles in which (1) wind erodes near-surface horizons and then provides new parent material and (2) cryoturbation disrupts carbonate horizons, remobilizing carbonate and changing carbonate morphology but with only minimal loss of carbonate from the soil.     

Paleoecology of two marine oxygen isotope stage 7 sites correlated by the Sheep Creek tephra, northwestern North America

Pollen analysis at two sites, correlated by the presence of the 190,000 yr-old Sheep Creek tephra, documents fluctuations in vegetation and climate consistent with this date and indicates that the records span marine oxygen isotope stage 7 and the stage 6/7 transition. Dawson Cut, near Fairbanks, Alaska, provides a 5.2-m-long pollen record of interglacial boreal forest succeeded by shrub tundra and then forest/tundra. Ash Bend, Stewart River, central Yukon, provides a 9.5-m-long record of interglacial boreal forest succeeded by forest/tundra, shrub tundra, and herbaceous tundra. The replacement of forest at both sites by more open or tundra vegetation indicates warm interglacial conditions giving way to cold and arid climate. It is not clear whether stage 7 was warmer than the present. The warm-cool-warm climate oscillation evident at both sites may correlate to Lake Baikal substages 7a, 7b, and 7c. Sheep Creek tephra fell on forest/tundra vegetation.     

Mineral magnetic properties of Holocene lake sediments and soils from the Kårsa valley,Lappland, Sweden,and their relevance to palaeoenvironmental reconstruction

A mineral magnetic study of lacustrine sediments and soils from the Kårsa valley in Lappland, Sweden was undertaken to estimate the potential of mineral magnetic techniques in elucidating the variability of climate, recorded by glacier fluctuations and soil development. Sediment cores from two lake basins receiving sediment input from a glacial outflow stream, and their respective catchments, show a mineral magnetic record that can be interpreted in terms of glacial activity and soil development. Dissolution of magnetite is indicated during periods of climatic amelioration and causes a high S-ratio. During periods of glacial advance (and colder, wetter climate) the minerogenic sediment supply is increased and magnetite is preserved in the sediment, resulting in a low S-ratio. Variations in the S-ratio can then be used as a proxy-climate indicator.     

Rock magnetic response to climatic changes in west Philippine Sea for the last 780 ka: Discussion based on relative paleointensity assisted chronology

We conducted rock magnetic and paleomagnetic research on two deep-sea sediment cores from the west Philippine Sea, located to the east of Benham Rise with the length of 4 m and water depth of over 5000 m. At the bottom of core 146 occurs a reversal of inclination and deflection of relative declination, which is recognized as Brunhes-Matuyama Polarity Boundary (MBPB). No reversal occurs in core 89, which implies a younger bottom age than that of core 146. Rock magnetic results reveal magnetic uniformities in mineralogy, concentration and grain size along the two cores, thus relative paleointensity variations are acquired. The three normalizers-anhysteresis remanent magnetization (ARM), magnetic susceptibility (k) and saturation isothermal remanent magnetization (SIRM) are used for normalization to obtain relative paleointensities. The three normalization results are averaged to indicate the paleoitensity of the cores and are further stacked together to get a synthetic curve for west Philippine Sea (named asWPS800 in this paper). Based on the magnetic correlation between cores and paleointensity to Sint800, we transfer the changes of rock magnetic parameters from depth to time. Furthermore, the astronomically tuned oxygen isotope from ODP site 1143 in the south China Sea is used for the glacial and interglacial indicator. Three concentration proxies (ARM, k and SIRM) and grain size indicators (k _ARM/SIRM, k _ARM/k) are examined according to the paleointensity-assisted chronology. The grain size changes in the two cores display a consistent pattern with the climatic changes embodied by oxygen isotope. The magnetic sizes are usually coarser in glacial periods and finer in interglacial times, which may reflect the influence of chemical erosion rather than fining from sea level rising on the source sediment. Furthermore, the sub-peaks and sub-troughs in interglaciations almost correspond with that of oxygen isotope records, which means sedimentation can reflect the subtle changes in interglaciations. This kind of revelation of climatic fluctuation by magnetic size is also found in the South China Sea, which shows a common pattern of magnetic signals to climate at least within East Asia. The concentration of ARM (representing more about fine grain) also shows similar response to glacial and interglacial cycles, that is, high in interglacial cycle and low in glacial cycle; but k and SIRM (reflecting more about coarse grain) lack the response to the climatic cycles. At the same time, S-ratio lacks the correlation with aeolian dust record and rhythmic changes, indicating the dominant source of main magnetic carrier (low coercivity magnetite) is the suspended matter instead of dust. The decreasing trend of sedimentation rate from west to east also reveals that the sediments are mainly from west Luzon and adjacent land. Grain sizes first became coarse and then stable around 400 ka B.P., and at the same time all the magnetic contents lowered and amplitude of magnetic mineral changes increased. The magnetic transition around 400 ka B.P. is simultaneous with the decreases of carbonate content, reflecting a global carbonate dissolution event, i.e. mid Brunhes event. The synchronization of magnetic content and grain size with climatic cycles of glacials and interglacials imply the validity of paleointesnityassisted chronology. Also, the response of rock magnetic signals to stable oxygen isotope changes and carbonate variation reveals that rock magnetismmethod can be an effective tool for paleoclimatic and paleoceanographic research. __________ Translated from Quaternary Sciences, 2007, 27(6): 1040–1052 [译自 : 第四纪研究]     

Investigation of high-level glaciofluvial terraces and re-evaluation of the established soil stratigraphy for Early and Middle Pleistocene surfaces, central Yukon, Canada

Palaeosols developed on the highest Yukon River glaciofluvial terraces were investigated in order to reconstruct the Plio-Pleistocene evolution of the river valley beyond Late Pleistocene glacial limits. A record of at least two pre-Reid (> 311 kyr) glaciations is chronicled by the presence of two populations of glaciofluvial terraces within the study area. The populations of pre-Reid terraces were identified based on their degree of soil development and elevation. Pre-Reid terraces 200–250 m above river level have preserved morphological and mineralogical features of the Wounded Moose palaeosol, a palaeosol previously associated with pre-Reid surfaces in central Yukon. Clay mineralogy and colour indicate that the Wounded Moose palaeosol developed in part during warm and sub-humid as well as temperate and humid interglacials. A second set of pre-Reid terraces between 110 and 30 m above river level are characterized by the presence of the less-developed Diversion Creek palaeosol, a palaeosol previously associated with only Reid-aged (< 311 kyr) surfaces in central Yukon. In contrast to the Wounded Moose palaeosol, the Diversion Creek palaeosol developed during comparatively cool and humid interglacial conditions. The presence of Diversion Creek palaeosols on pre-Reid outwash terraces suggests that a transition from dominantly warmer to cooler interglacial conditions occurred prior to 311 kyr in Yukon Territory. In addition, the presence of a Diversion Creek palaeosol cannot be used to differentiate stable Reid and stable pre-Reid surfaces across central Yukon.     

黄土剖面中古气候变化的地球化学记录

作者通过对陕西洛川黄土剖面中碳酸盐总量、氧化物比值和元素演化周期性的分析，讨论了黄土高原地区２．４Ｍａ以来古气候的演化模式。对渭南黄土剖面高密度连续采集的样品，着重分析了ＣａＣＯ_３、Ｆｅ₂Ｏ_３／ＦｅＯ值和有机碳的高分辨记录。指出它们的变化彼此同步(但趋势不尽相同)，与黄土一古土壤系列变化一致，井与深海沉积物氧同位素可进行良好的对比。它们综合反映了该区１３４５００年来有五次万年级的暖湿(或凉湿)一冷干的气候波动，其中各期又有若干次级的小颤动。它们的变化与季风影响强弱程度明显有关。     

Vegetation context and climatic limits of the Early Pleistocene hominin dispersal in Europe

The vegetation and the climatic context in which the first hominins entered and dispersed in Europe during the Early Pleistocene are reconstructed, using literature review and a new climatic simulation. Both in situ fauna and in situ pollen at the twelve early hominin sites under consideration indicate the occurrence of open landscapes: grasslands or forested steppes. The presence of ancient hominins (Homo of the erectus group) in Europe is only possible at the transition from glacial to interglacial periods, the full glacial being too cold for them and the transition interglacial to glacial too forested. Glacial–interglacial cycles forced by obliquity showed paralleled vegetation successions, which repeated c. 42 times during the course of the Early Pleistocene (2.58–0.78 Ma), providing 42 narrow windows of opportunity for hominins to disperse into Europe.The climatic conditions of this Early Pleistocene vegetation at glacial-interglacial transitions are compared with a climatic simulation for 9 ka ago without ice sheet, as this time period is so far the best analogue available. The climate at the beginning of the present interglacial displayed a stronger seasonality than now. Forest cover would not have been hampered though, clearly indicating that other factors linked to refugial location and soils leave this period relatively free of forests. Similar situations with an offset between climate and vegetation at the beginning of interglacials repeated themselves throughout the Quaternary and benefitted the early hominins when colonising Europe.The duration of this open phase of vegetation at the glacial–interglacial transition was long enough to allow colonisation from the Levant to the Atlantic.The twelve sites fall within rather narrow ranges of summer precipitation and temperature of the coldest month, suggesting the hominins had only a very low tolerance to climate variability.     

Late Pleistocene Environments of the Central Ukraine

The Vyazivok loess sequence from the Dnieper Plain, Ukraine, documents regional environmental changes during the late Pleistocene and Holocene. Pedological and palynological analyses and low-field magnetic susceptibility document changes from dense temperate forest during the last interglacial maximum to open, harsh, loess–steppe during the latest Pleistocene. The Vyazivok section overlies hillwash derived from a lower Pleistocene terrace and consists of two stratified soil complexes (Kaydaky and Pryluky; marine isotope stage [MIS] 5 equivalent) separated by a layer of eolian dust (Tyasmyn silt). The lower soils in both complexes formed within forest. These soils are overlain by the Uday (MIS4) and Bug (MIS2) loess units, which are separated by boreal soils of the Vytachiv (MIS3) complex. The coldest conditions within the record occurred in the youngest loess. Holocene soils cap the Bug loess. The Vyazivok section shows remarkable similarities with other classical loess sequences in western Europe, the Czech Republic, and Austria. The Kaydaky, Pryluky, and Vytachiv deposits, correlate with the PKIII, PKII, and PKI soil complexes, respectively, of the Czech Republic. The Tyasmyn and Prylyky silt layers correspond to marker horizons from central Europe.     

Magnetic properties of lake sediments from Lake Chalco,central Mexico,and their palaeoenvironmental implications

Lake Chalco (99.0°W, 19.5°N) in the Basin of Mexico, was formed during the Pleistocene after the emplacement of the Chichinautzin volcanic field that closed the former drainage system. The lake sediment record has been influenced by a number of factors, including glacial–interglacial cycles, local volcanism, erosion of soils and anthropogenic disturbances. The magnetic properties of the lake sediments and the associated tephra layers of the last 16500 yr have been studied. It is found that the magnetic properties of the Lake Chalco sediments are very distinctive. Magnetic concentration varies by a factor of 1000 and magnetic stability also varies over an extremely wide range. The predominant magnetic mineral is titanomagnetite in addition to an imperfect antiferromagnetic phase, possibly goethite. An unusually large range of coercivities is found in certain of the tephras. Down‐core variations in magnetic properties closely follow climatic/environmental changes previously established by other proxy methods. The late Pleistocene and late Holocene lake sediments display a higher concentration of magnetic minerals than the early–middle Holocene sediments. In the non‐volcanic sediments, fluctuations in the magnetic concentration reflect changes in both the intensity of erosion, as represented by ferrimagnetic and paramagnetic minerals, and by the maturing of soils, as represented by geothite. Copyright © 2000 John Wiley & Sons, Ltd.     

Stratigraphy and palaeoecology of a possible interglacial site,northernmost Ellesmere Island,Canada

An extensive stratigraphic section at Cape Alfred Ernest on the Wootton Peninsula, northwest Ellesmere Island contains six lithofacies which appear to record two glacial phases separated by an organic layer. (1) A lower massive gravel records a pre-ice advance outwash phase; (2) massive fine-grained sediments record a period of non-glacial marine deposition when sea-level was higher than present; (3) a massive diamicton records the advance of ice across the site; (4) intermediate stratified beds record supraglacial and proglacial outwash, and include an organic layer; (5) massive diamicton grading down-valley to stratified diamicton and then massive, sheared diamicton, overlain by laminated fine-grained sediments with dropstones, recording the last (late Wisconsinan) glaciation; (6) upward-coarsening sands and gravels record proglacial outwash and grade to raised marine deltas. Radiocarbon dates of 39270 ± 640 and > 51000 yr BP were obtained on samples from the organic layer by accelerator mass spectrometry and conventional radiocarbon dating, respectively. Palaeoecological data suggest that the organics accumulated in a wet sedge meadow environment when the climate was warmer than present. Stratigraphic considerations suggest that the organic layer represents an interglacial interval which, if valid, indicates that the site constitutes the northernmost interglacial stratigraphy in the Canadian Arctic. Alternatively, the organic layer may date to Plio-Pleistocene times.     

Palaeoclimatic record in the loess–palaeosol sequence of the Strelitsa type section (Don glaciation area,Russia) deduced from rock magnetic and palynological data

Until now, palaeoclimatic reconstructions for the major stages in the development of the Quaternary loess–palaeosol sequence on the Russian Plain have been based on pedological, palynological and faunal (vertebrates and molluscs) analyses. In order to demonstrate the palaeoclimatic influence on the magnetic properties of this sequence, the magnetic susceptibility signature of the Strelitsa type section in the Upper Don basin is compared with a detailed landscape – climate reconstruction of loess and soil from palynological data. Large amplitude fluctuations of palaeoclimate and palaeoenvironment are reflected clearly in the lithology and in the rock magnetic properties, which usually are enhanced in wet and warm interglacial periods, but stay at low levels during cold glacial epochs. Palynological climate zonation, however, is sometimes in conflict with the pedologic–magnetic record. Strong climate fluctuations, as indicated by changing pollen assemblages, are not always paralleled by corresponding changes in lithology and/or rock magnetic properties. Alternatively, light coloured illuvial horizons with low magnetic signal sometimes appear to have formed during early stages of interglacials, and the top parts of some palaeosols apparently formed during glacial stages. Copyright © 2000 John Wiley & Sons, Ltd.     

最后4个冰期旋回中国黄土记录的东亚冬季风变化

开展高分辨率的黄土记录研究, 是揭示过去气候变迁和变率的主要途径.选择黄土高原中部的灵台和赵家川剖面, 对L5以上的黄土-古土壤序列进行了石英颗粒的提取和粒度分析.结果表明, 石英粗颗粒(> 43μm) 体积分数变化揭示了最后4个冰期旋回东亚冬季风存有的快速变化特征, 并同深海氧同位素记录具有良好的对应关系; 然而, 在每一个冰期时段, 石英粗颗粒体积分数的变化幅度和频率存有较大差异, 可能意味着对应的冰期下垫面状况和气候系统内部的非线性响应机制并不相同.值得注意的是由石英粗颗粒体积分数反映的东亚冬季风变化, 在某些时段不仅同磁化率记录的夏季风变化不协调, 而且同深海氧同位素记录存有明显差异.深入研究这种由不同代用指标反映的古气候变化间的差异, 对理解黄土记录的全球性意义显得尤为重要.      

Geology of the dry creek site; a stratified early man site in Interior Alaska

The Dry Creek archeologic site contains a stratified record of late Pleistocene human occupation in central Alaska. Four archeologic components occur within a sequence of multiple loess and sand layers which together form a 2-m cap above weathered glacial outwash. The two oldest components appear to be of late Pleistocene age and occur with the bones of extinct game animals. Geologic mapping, stratigraphic correlations, radiocarbon dating, and sediment analyses indicate that the basal loess units formed part of a widespread blanket that was associated with an arctic steppe environment and with stream aggradation during waning phases of the last major glaciation of the Alaska Range. These basal loess beds contain artifacts for which radiocarbon dates and typologic correlations suggest a time range of perhaps 12,000–9000 yr ago. A long subsequent episode of cultural sterility was associated with waning loess deposition and development of a cryoturbated tundra soil above shallow permafrost. Sand deposition from local source areas predominated during the middle and late Holocene, and buried Subarctic Brown Soils indicate that a forest fringe developed on bluff-edge sand sheets along Dry Creek. The youngest archeologic component, which is associated with the deepest forest soil, indicates intermittent human occupation of the site between about 4700 and 3400 ¹⁴C yr BP.