n-alkanol ratios as proxies of paleovegetation and paleoclimate in a peat-lacustrine core in southern China since the last deglaciation

High resolution records of long chain n-alkanol biomarkers were obtained from a peat-lacustrine core from the Dingnan profile in southern China. The n-alkanol distributions are characterized by the predominance of even-over-odd carbon number and maximize at C₂₄ or C₂₆. On the basis of the reported n-alkanol records in the literature and the n-alkane record in our samples, we concluded that the n-alkanol ratio of C₂₆/C₃₀ varying from 1.25 to 6.48, together with the n-alkanol ratio C₂₂/C₂₄ less than unity, is indicative of the presence of a dominant forest paleovegetation. A 2000-year cycling in the variation of the n-alkanol ratio C₂₆/C₃₀ is identifiable in our profile, and probably results from the change in the abundance of the grass relative to trees induced by a cyclic paleoclimate. The n-alkanol ratio C₂₄/C₂₆ appears to be more sensitive to change in precipitation than in temperature, and may be a potential indicator of precipitation/humidity, with increased values being associated with relatively dry conditions. The paleovegetation and the paleoclimate reconstructed on the basis of the n-alkanol records for the recent 18000 cal a BP in general accord with the pollen data and other lipid evidence recorded in the Dingnan region in southern China. In particular, both the n-alkanol records and the pollen data infer the different paleoclimate conditions for the two peat sequences, with a cool and wet climate dominating in the lower peat deposition formed during the latest Pleistocene and a change to a drier and cooler climate occurring in the upper peat sequence in mid-Holocene.     

Hydrogen isotopic ratios of plant wax n-alkanes in a peat bog deposited in northeast China during the last 16 kyr

Based on paleoclimatic reconstructions using various proxies, the Holocene Climate Optimum (10.5–6 ka) has been characterized as a warmer and wetter period in most of East Asia. The summer monsoons associated with the East Asian Monsoon evidently intensified and extended further inland from the Pacific Ocean, a source region of moisture. A notable exception to this general pattern exists in northeast China, where less wet conditions are recorded. We determined molecular compositions of individual plant wax hydrocarbons and their hydrogen isotope compositions (δD values) in a radiocarbon-dated peat core recovered from the Hani marsh in Jilin Province (China) and confirmed that the temperature-dependent effective precipitation in northeast China decreased during the Holocene Climate Optimum. A combination of P_aq, an indicator of the relative contribution of aquatic to terrestrial plants, and the difference in δD between low (C₂₃, C₂₅ and C₂₇) and high molecular weight (C₃₁) n-alkanes in the Hani peat bog indicates a dramatic change in vegetation from the deglaciation to the Holocene. No significant differences were observed between the δD values of low and high molecular weight n-alkanes with relatively high δD values and low P_aq during the early Holocene, indicating that all n-alkanes were produced by evapotranspiration-sensitive terrestrial plants during that time. However, lower δD values of mid-chain n-alkanes (C₂₃, C₂₅ and C₂₇) relative to the long chain n-alkane (C₃₁), together with higher P_aq values during the deglaciation (14–11 ka), suggest an increase in the contribution of aquatic plants and a higher water level during the period. The study demonstrates that northeast China was under a markedly wetter climate condition during the late deglaciation. For the 16 kyr record in the Hani peat sequence, we infer that moisture delivery by the East Asian Monsoon was relatively invariable in northeast China, but increased evaporation during the warmer Holocene Climate Optimum reduced the effective precipitation, defined by the balance between precipitation and evaporation.     

n-Alkan-2-one distributions in a northeastern China peat core spanning the last 16 kyr

Most research on long chain methyl ketones has focused on their origins and distributions. Their application in paleoclimate studies is less common than that of other n-alkyl lipids. The goal of this research was to explore this potential by studying n-alkan-2-ones from the Hani peat sequence in northeastern China. They were identified using gas chromatography-mass spectrometry (GC-MS) and showed a distribution ranging from C₁₉ to C₃₁ with a strong odd/even predominance. This type of distribution is considered to derive from Sphagum and microbial oxidation of n-alkanes. Comparison with climate sensitive indicators and macrofossil analysis shows that microbial oxidation of n-alkanes derived from higher plants was enhanced during the warm early Holocene period. This led us to develop three n-alkan-2-one proxies - C₂₇/ΣC_23-31 (C₂₇/HMW-KET), carbon preference index (CPI_H-KET) and average chain length (ACL_(27-31)-KET) - as possible indicators of paleoclimate in the peat-forming environment. These proxies, in combination with C₂₇n-alkane δD values and peat cellulose δ¹⁸O records, might allow examination of paleo-ecosystem behavior during climatic evolution in northeastern China over the past 16,000 yr.     

若尔盖高原全新世气候序列的类脂分子化石记录*

基于可靠的年代标尺,结合青藏高原泥炭沉积剖面的有机碳、孢粉等相关资料,首次在该区利用类脂分子化石指标建立了全新世以来的气候与环境演化序列。结果表明,正构烷烃、脂肪酸和脂肪醇等类脂分子化石指标存有意义的规律性变化。其中,反映低等菌藻生物和高等植物相对变化的轻组分与重组分之比L/H以及平均碳链长度ACL具有很好的古气候意义,即L/H高,ACL低,指示气候偏暖湿;L/H低,ACL高,指示气候偏干冷。各个类脂分子化石指标记录的气候信息在细节上具有一定的差异,可能是不同类脂物分子的地球化学行为和生物组成结构的不同以及对气候因素温湿组合变化响应各异的体现。同时,也较好地揭示了类脂分子化石具有自己的敏感性,在细节上较传统的有机碳指标在记录气候信息上更为敏感。该研究进一步突出了这些泥炭类脂分子化石在第四纪研究中的重要地位和应用潜力。     

Environmental evolution recorded by lipid biomarkers from the Tawan loess–paleosol sequences on the west Chinese Loess Plateau during the late Pleistocene

This study provides a reconstruction of the environmental evolution since 128 ka recorded by the lipid biomarkers of the C₁₅–C₃₅ n-alkanes, the C₁₃–C₃₃ n-alkan-2-ones and the C₁₂–C₃₀ n-alkanols isolated from the Tawan loess section, Northwest China. Variations in paleoenvironment are reconstructed from the values of the carbon preference index (CPI), the average chain length (ACL), the L/H (ratio of lower-molecular-weight to higher-molecular-weight homologues), the n-alkane C₂₇/C₃₁ ratios, and the n-alkan-2-one C₂₇/C₃₁ ratio. These parameters indicate the dominance of grasses over the west Chinese Loess Plateau (CLP) during the late Pleistocene. Lower values of the CPI and the ACL values, respectively, indicate stronger microbial reworking of organic matter and changes in plant species, which are both indicative of a warmer-wetter environment. Furthermore, the fluctuations of environment recorded in the Tawan section exhibit ten phases that show obvious cycles between warm periods and cold intervals. This study reveals that changes in the biomarker proxies agree well with changes in the magnetic susceptibility and grain size, and it indicates a huge potential for paleoenvironmental reconstructions by using the n-alkan-2-one and n-alkanol proxies.     

Quantitative climate and vegetation trends since the late glacial on the northeastern Tibetan Plateau deduced from Koucha Lake pollen spectra

Quantitative information on vegetation and climate history from the late glacial-Holocene on the Tibetan Plateau is extremely rare. Here, we present palynological results of a 4.30-m-long sediment record collected from Koucha Lake in the Bayan Har Mountains, northeastern Tibetan Plateau. Vegetation change has been traced by biomisation, ordination of pollen data, and calculation of pollen ratios. The application of a pollen-climate calibration set from the eastern Tibetan Plateau to Koucha Lake pollen spectra yielded quantitative climate information. The area was covered by alpine desert/steppe, characteristic of a cold and dry climate (with 50% less precipitation than today) between 16,700 and 14,600 cal yr BP. Steppe vegetation, warm (∼ 1°C higher than today) and wet conditions prevailed between 14,600 and 6600 cal yr BP. These findings contradict evidence from other monsoon-influenced areas of Asia, where the early Holocene is thought to have been moist. Low effective moisture on the northeastern Tibetan Plateau was likely due to high temperature and evaporation, even though precipitation levels may have been similar to present-day values. The vegetation changed to tundra around 6600 cal yr BP, indicating that wet and cool climate conditions occurred on the northeastern Tibetan Plateau during the second half of the Holocene.     

Role of late glacial to mid-Holocene climate in catchment weathering in the central Tibetan Plateau

The lightness (L*) and concentrations of Rb, Sr and organic carbon (C_org) have been measured in the age-constrained lake sediment cores recovered from Co Ngoin in the central Tibetan Plateau. Dissolved Sr flux is a dominant control on the variation of Rb/Sr ratios in the sediments. Variations in color and geochemical proxies of Co Ngoin sediments display a continuous history of late glacial to mid-Holocene chemical versus physical weathering intensity in response to past climatic changes between approximately 13,500 and 4500 cal yr B.P. A lower chemical weathering under a late glacial climate was followed by a higher weathering during the Holocene Optimum. Weathering intensity in the central Tibetan Plateau catchment also responds to well-known climatic events, such as the Younger Dryas (YD), and possibly the Holocene Event 5 (HE-5). Although there are differences in time or duration of the climatic events, many of the well-known late glacial to mid-Holocene events occurred in high-elevation Co Ngoin where atmospheric circulation might play a hemispherical role in climatic forcing. The sediment hiatus since c. 4200 ¹⁴C yr B.P. in the Co Ngoin indicates a period of desiccation that was probably associated with a sharp decrease in summer monsoon strength. Our lascustrine results not only imply catchment weathering variations in response to late glacial to mid-Holocene climatic conditions in the central plateau, but also provide further evidence for global connections between regional climates.     

牛首山-罗山断裂带北段柳木高断裂第四纪活动特征

牛首山-罗山断裂带是青藏高原东北缘弧形断裂系最外缘断裂带,自南向北由固原断裂、罗山东麓断裂、牛首山断裂及三关口断裂组成。通过遥感解译、野外调查及探槽揭露等方法对牛首山断裂北段柳木高断裂第四纪几何学、运动学特征进行了研究,并通过断层截切地层关系及年代学测试,限定了该断裂第四纪演化过程及全新世的地震事件。研究结果表明,柳木高断裂上新世至晚更新世自南西向北东逆冲,晚更新世至全新世左行走滑逆冲,表现为正花状构造,而全新世则发生了正倾滑运动。全新世期间,1690±30 yr BP（公元320-415）之后发生了一次古地震事件,推测柳木高断裂可能是公元876年青铜峡南6.5级地震的发震断裂。柳木高断裂第四纪早期活动特征与固原断裂、罗山东麓断裂及牛首山断裂一致,是青藏高原北东向持续扩展引起的,而全新世的正倾滑运动可能与银川地堑的伸展作用有关。      

n-alkane distribution coupled with organic carbon isotope composition in the shell bar section, Qarhan paleolake, Qaidam basin, NE Tibetan Plateau

Lipids extracted from lacustrine deposits in the paleolake Qarhan of the Qaidam basin in the northeastern Tibetan Plateau were determined by conventional gas chromatography-mass spectrometry. Several series of biomarkers were identified, mainly including n-alkanes, n-alkan-2-ones, n-alkanoic acids, branched alkanes, triterpenoids and steroids, indicative of various biogenic contributions. On the basis of cluster analysis, the n-C₁₅, n-C₁₇, n-C₁₉ alkanes were proposed to be derived from algae and/or photosynthetic bacteria, the n-C₂₁, n-C₂₃, n-C₂₅ homologues from aquatic plants, and the n-C₂₉, n-C₃₁ homologues from vascular plants. In contrast, the n-C₂₇ alkane is not categorized in the n-C₂₉ and n-C₃₁ group of alkanes, probably due to more complex origins including both aquatic and vascular plants, and/or differential biodegradation. Stratigraphically, layers-2, 4 and 5 were found to show a close relationship in n-alkane distribution, associated with a positive shift in carbon isotope composition of bulk organic matter (δ¹³C_org), inferring a cold/dry period. Layers-1 and 6 were clustered together in association with a negative δ¹³C_org, excursion, probably indicating a relatively warm/humid climate. The potential coupling between the n-alkane distributions and δ¹³C_org, suggests a consequence of vegetation change in response to climate change, with the late MIS3 being shown to be unstable, thought to be the climatic optimum in the Tibetan Plateau. Our results suggest that the cluster analysis used in this study probably provides an effective and authentic method to investigate the n-alkane distribution in paleolake sediments.     

Reconstructing climate variability on the northeastern Tibetan Plateau since the last Lateglacial – a multi-proxy,dual-site approach comparing terrestrial and aquatic signals

A sediment core from a closed basin lake (Lake Kuhai) from the semi-arid northeastern Tibetan Plateau was analysed for its pollen record to infer Lateglacial and post glacial vegetation and climatic change. At Lake Kuhai five major vegetation and climate shifts could be identified: (1) a change from cold and dry to relatively warmer and more moist conditions at 14.8 cal ka BP; (2) a shift to conditions of higher effective moisture and a stepwise warmer climate at 13.6 cal ka BP; (3) a further shift with increased moisture but colder conditions at 7.0 cal ka BP; (4) a return to a significantly colder and drier phase at 6.3 cal ka BP; (5) and a change back to relatively moist conditions at 2.2 cal ka BP. To investigate the response of lake ecosystems to climatic changes, statistical comparisons were made between the Lake Kuhai pollen record and a formerly published ostracod and sedimentary record from the same sediment core. Furthermore, the pollen and lacustrine proxies from Lake Kuhai were compared to a previously published pollen and lacustrine record from the nearby Lake Koucha. Statistical comparisons were done using non-metric multidimensional scaling and Procrustes rotation. Differences between lacustrine and pollen responses within one site could be identified, suggesting that lacustrine proxies are partly influenced by in-lake or local catchment processes, whereas the terrestrial (pollen) proxy shows a regional climate signal. Furthermore, we found regional differences in proxy response between Lake Kuhai and Lake Koucha. Both pollen records reacted in similar ways to major environmental changes, with minor differences in the timing and magnitude of these changes. The lacustrine records were very similar in their timing and magnitude of response to environmental changes; however, the nature of change was at times very distinct. To place the current study in the context of Holocene moisture evolution across the Tibetan Plateau, we applied a five-scale moisture index and average link clustering to all available continuous palaeo-climate records from the Tibetan Plateau to possibly find general patterns of moisture evolution on the Plateau. However, no common regional pattern of moisture evolution during the Holocene could be detected. We assign this to complex responses of different proxies to environmental and atmospheric changes in an already very heterogeneous mountain landscape where minor differences in elevation can cause strong variation in microenvironments.     

Late Quaternary environmental change of Yellow River Basin: An organic geochemical record in Bohai Sea (North China)

Bulk geochemical characterization (total organic carbon, grain size distribution, carbon isotope composition) and molecular biomarkers (lignin phenols, straight chain aliphatic hydrocarbons, glycerol dialkyl glycerol tetraethers) were analyzed for a 21 m core from the Bohai Sea (North China), spanning ca 21 ka BP. These paleo-proxies presented remarkable differences between the late glacial period and the Holocene, reflecting continental and coastal environments, respectively. Two peat layers were deposited during the period of ca 9000-8460 yr BP. Thereafter the core site has been consistently covered by seawater until recent reclamation of land from the sea. The occurrence of a total organic carbon maximum from ca 6000-3800 yr BP was attributed to delivery of organic carbon enriched sediments via the Yellow River, consistent with increased vegetation density and higher development of soil under warm and humid mid-Holocene climate conditions. The distributions of lignin phenol compositions and C₃₁/C₂₉n-alkane ratio suggested the largest expansion of woody plants between ca 5300 and 4000 yr BP, corresponding to the extremely favorable climatic conditions. Since ca 3800 yr BP, an abrupt increase in the C₃₁/C₂₉n-alkane ratio suggested higher abundance of grasses, consistent with a drying climate trend after the mid-Holocene. Since our coastal sediments close to the Yellow River outflow contain catchment-integrated environmental signals of the river basin, molecular proxies demonstrate that the variability of vegetation distributions in the Holocene is a widespread phenomenon in those areas adjacent to Yellow River Basin.     

Spatial–temporal differences in climate change at different altitudes,northeastern Qinghai-Tibetan Plateau during the Holocene period

The history of climate change and related driving mechanisms of the Gonghe Basin, northeastern Qinghai-Tibetan Plateau, China, was reconstructed in the Holocene epoch, based on the comprehensive analysis of multi-proxies consisting of magnetic susceptibility, grain size, and geochemical elements from eolian and peat deposits at different altitudes. The results indicate that Holocene climate change at different altitudes is both consistent and different: a synchronous record of an increased warm–humid phase (10.0–8.5 ka) and a cold phase around 8.2 ka in the Early Holocene; an optimal warm–humid condition in marginal mountains of the Gonghe Basin in the Mid-Holocene; and a gradual decline in temperature and humidity in the Late Holocene. The Gonghe Basin interior in the Mid-Holocene was relatively arid, with increased moisture in the Late Holocene. On this basis, we compared our results to the paleoclimatic record in the low-latitude Asian monsoonal region, which indicates that, in addition to the Asian summer monsoonal strength having influenced regional climate change, the upward and subsidence motion of airflow over the Qinghai-Tibetan Plateau induced by topographic conditions, evaporation, and the feedback effect by the substrate was also influential. The latter was especially important for spatial–temporal differences in Middle and Late Holocene climatic changes at different altitudes.     

The influence of climate,hydrology and permafrost on Holocene peat accumulation at 3500 m on the eastern Qinghai–Tibetan Plateau

Peatland of the eastern Qinghai–Tibetan Plateau lies at the convergence of the East Asian and Indian monsoon systems in eastern Asia. To understand the evolution of this peatland and its potential to provide new insights into the Holocene evolution of the East Asian monsoon a 6 m peat core was collected from the undisturbed central part of a peat deposit near Hongyuan. The age-depth profile was determined using 16 ¹⁴C-AMS age dates, the peat analysed for a range of environmental variables including carbon, nitrogen and hydrogen concentration, bulk density, δ¹³C and the associated spring water analysed for hydrogen and oxygen isotopes. The age-depth profile of the recovered peat sequence covers the period from 9.6 to 0.3 kyr BP and is linear indicating that the conditions governing productivity and decay varied little over the Holocene. Using changes in carbon density, organic carbon content and its δ¹³C, cold dry periods of permafrost characterised by low density and impeded surface drainage were identified. The low δ¹⁸O and δD values of the spring water emanating around the peat deposit, down to ?13.8 and ?102‰ (VSMOW), respectively, with an inverse relationship between electrical conductivity and isotopic composition indicate precipitation under colder and drier conditions relative to the present day. In view of the current annual mean air temperature of 1 °C this suggests conditions in the past have been conducive to permafrost. Inferred periods of permafrost correspond to independently recognised cold periods in other Holocene records from across China at 8.6, 8.2–7.8, 5.6–4.2, 3.1 and 1.8–1.5 kyr BP. The transition to a cold dry climate appears to be more rapid than the subsequent recovery and cold dry periods at Hongyuan are of longer duration than equivalent cold dry periods over central and eastern China. Light–dark banding peat on a scale of 15–30 years from 9.6 to 5.5 kyr BP may indicate a strong influence of decadal oscillations possibly the Pacific Decadal Oscillation and a potential link between near simultaneous climatic changes in the northwest Pacific, ENSO, movement of the Intertropical Convergence Zone and the East Asian Monsoon.     

Mid‐Holocene frozen ground in China from PMIP3 simulations

Extensive degradation of frozen ground in the mid‐Holocene is widely assumed on the basis of sparse proxy data. Here, the simulated soil temperature from the Paleoclimate Modelling Intercomparison Project Phase 3 is used to address this issue over China. By comparing with the results of a preindustrial (0 ka, baseline) simulation, we show that frozen ground in the mid‐Holocene (6 ka) simulation is degraded mainly in northeast China and on the northern Tibetan Plateau. The change follows closely orbitally induced variations in insolation. Quantitatively, permafrost area reduces by 0.02×10⁶ km² in northeast China in response to an orbitally induced increase in boreal summer insolation but increases by 0.08×10⁶ km² on the southern Tibetan Plateau due to local summer cooling. Changing values of active layer thickness vary greatly amongst different locations. On average, they are 3 and 4 cm thicker than the preindustrial values in northeast China and on the Tibetan Plateau, respectively. No degradation in seasonally frozen ground is detected over China as a whole. Regionally, its coverage increases by 0.21×10⁶ km² near the middle and lower reaches of the Yangtze River valley. In addition, the maximum depth of seasonal frost penetration is on average 8.5 cm deeper than preindustrial values due to widespread winter cooling. The changes in frozen ground are consistent amongst models. However, the models disagree with proxy data in terms of not only the changes in frozen ground but also climate. Further modelling improvements and adequate proxy data are both needed to fill in the gaps between models and the data in our knowledge of the mid‐Holocene frozen ground.     

青藏高原东北缘隆德观音店剖面碎屑锆石年龄序列及地质意义

青藏高原东北缘古近纪—新近纪地层的精确时代和高原隆升扩展何时影响到东北缘地区一直是地质学家争论的焦点。此次以青藏高原东北缘隆德观音店剖面为研究对象,采用碎屑锆石U-Pb测年方法,结合区域古生物化石鉴定、古地磁测年结果,针对上述两个问题提出了新的认识。测试分析发现,青藏高原东北缘古近系清水营组上部碎屑锆石年龄序列中最年轻的峰值为17.8 Ma,研究得出如下两个认识:清水营组的沉积时代应归属于晚渐新世—早中新世,区域上具有穿时性;青藏高原隆升向东北方向的扩展推挤作用在早中新世时期就开始影响到了海原-六盘山断裂带以西地区,区域上构造-沉积-气候之间具有协同演化的耦合关系。这一研究成果为青藏高原隆升向东推挤扩展影响到东北缘地区的起始时限提供了新的时间标尺。      

Holocene vegetation and climate histories in the eastern Tibetan Plateau: controls by insolation-driven temperature or monsoon-derived precipitation changes?

The climates on the eastern Tibetan Plateau are strongly influenced by direct insolation heating as well as monsoon-derived precipitation change. However, the moisture and temperature influences on regional vegetation and climate have not been well documented in paleoclimate studies. Here we present a well-dated and high-resolution loss-on-ignition, peat property and fossil pollen record over the last 10,000 years from a sedge-dominated fen peatland in the central Zoige Basin on the eastern Tibetan Plateau and discuss its ecological and climatic interpretations. Lithology results indicate that organic matter content is high at 60–80% between 10 and 3 ka (1 ka = 1000 cal yr BP) and shows large-magnitude fluctuations in the last 3000 years. Ash-free bulk density, as a proxy of peat decomposition and peatland surface moisture conditions, oscillates around a mean value of 0.1 g/cm³, with low values at 6.5–4.7 ka, reflecting a wet interval, and an increasing trend from 4.7 to 2 ka, suggesting a drying trend. The time-averaged mean carbon accumulation rates are 30.6 gC/m²/yr for the last 10,000 years, higher than that from many northern peatlands. Tree pollen (mainly from Picea), mostly reflecting temperature change in this alpine meadow-forest ecotonal region, has variable values (from 3 to 34%) during the early Holocene, reaches the peak value during the mid-Holocene at 6.5 ka, and then decreases until 2 ka. The combined peat property and pollen data indicate that a warm and wet climate prevailed in the mid-Holocene (6.5–4.7 ka), representing a monsoon maximum or “optimum climate” for the region. The timing is consistent with recent paleo-monsoon records from southern China and with the idea that the interplays of summer insolation and other extratropical large-scale boundary conditions, including sea-surface temperature and sea-level change, control regional climate. The cooling and drying trend since the mid-Holocene likely reflects the decrease in insolation heating and weakening of summer monsoons. Regional synthesis of five pollen records along a south–north transect indicates that this climate pattern can be recognized all across the eastern Tibetan Plateau. The peatland and vegetation changes in the late Holocene suggest complex and dramatic responses of these lowland and upland ecosystems to changes in temperature and moisture conditions and human activities.     

青藏高原东北缘武威盆地内部全新世伸展构造特征及其成因机制

武威盆地位于青藏高原东北缘北祁连山与龙首山之间的河西走廊东端,全新世期间处于北东向挤压环境中。野外地质调查发现,武威盆地内部发育有两组走向近于垂直的正断层,即北西西走向和北东走向的正断层。光释光测年结果表明,两组正断层在0.70 ka、0.49~0.18 ka发生了两期构造活动。分析结果认为,北西西走向正断层是由武威盆地内部坟门山隆起持续隆升所产生的垂直于地层层面的差异应力作用所形成;北东走向正断层可能是盆地两侧近东西走向左旋走滑断裂控制下形成的张性破裂（T破裂）,也不排除是由在青藏高原北东向挤压作用下,与挤压应力相垂直方向上的伸展作用形成。因此,晚全新世期间武威盆地的构造变形受到青藏高原东北缘构造的控制。      

A multiproxy record of late Holocene natural and anthropogenic environmental change from the Sphagnum peat bog Dürres Maar,Germany: implications for quantitative climate reconstructions based on pollen

Pollen data are well established for quantitative climate reconstructions over long timescales, including the Holocene and older interglacials. However, anthropogenically induced environmental change in central Europe was strong during the last 4 ka, challenging quantitative reconstructions of this time period. Here we present quantitative climate reconstructions based on pollen analyses and evaluate them with the peat humification record and the stable carbon isotopes of Sphagnum plant material (δ¹³C_cellulose). All analyses were carried out on the same 7.5 m long, largely ombrotrophic peat bog section from Dürres Maar. Three different methods were used for the quantitative climate reconstructions on the basis of the pollen data: (1) a probabilistic indicator taxa approach (the ‘pdf method’); (2) a modern analogue technique based on pollen taxa from modern surface samples (cMAT); and (3) a modern analogue technique expanded by plant functional types (pMAT). At Dürres Maar the peat humification is only affected by peat cutting during the Roman period and the Middle Ages. The stable carbon isotopes are seemingly unaffected by human impact. Thus both proxies provide independent data to evaluate the reconstructions on the basis of pollen data. The quantitative climate reconstructions on the basis of the individual methods are in general relatively similar. Nevertheless, distinct differences between the individual approaches are also apparent, which could be attributed to taxa that reflect human impact on a local to regional scale. While the pdf method appears to be relatively robust to all observed anthropogenically induced vegetation changes, it potentially underestimates climate variability. This method is therefore expected to be independent of local site characteristics and to provide robust quantitative estimates of climatic trends rather than of climatic variability of small amplitude. This is of value for palaeoclimate reconstructions of older interglacials, for which neither multiple sites nor independent climate proxies are available for comparison. Copyright © 2009 John Wiley & Sons, Ltd.     

Synchronous pattern of moisture availability on the southern Tibetan Plateau since 17.5 cal. ka BP – the Tangra Yumco lake sediment record

A possible asynchronicity of the spatial and temporal moisture availability on the Tibetan Plateau has been a controversial subject of discussion in recent years. Here we present the first attempt to systematically investigate possible spatial and temporal variations in moisture availability by examining two lakes, Tangra Yumco and Nam Co, on an east–west transect on the southern Tibetan Plateau using identical proxies for palaeoenvironmental reconstruction. In this study, an independent record from Tangra Yumco was analysed applying a multi‐proxy approach to reconstruct variations in moisture availability since the Lateglacial. Results were subsequently compared with previously published records from Nam Co and additional records from Tso Moriri (northwestern Himalaya) and Naleng Co (southeastern Tibetan Plateau). Our results show that Tangra Yumco was at least partially ice covered prior to 17.1 cal. ka BP. A temperature rise after 17.1 cal. ka BP probably resulted in thawing of the permafrost. At 16.0 cal. ka BP moisture availability increased, representing an initial monsoonal intensification. Warmer conditions between 13.0 and 12.4 cal. ka BP and cooler conditions between 12.4 cal. ka BP and the onset of the Holocene reflect the Bølling‐Allerød and Younger Dryas. At the onset of the Holocene moisture availability rapidly increased, with moisture highest prior to 8.5 cal. ka BP when temperatures were also highest. After 8.5 cal. ka BP the moisture availability gradually decreased and showed only minor amplitude variations. These findings are consistent with the records from large lakes like Nam Co, Tso Moriri, and Naleng Co, revealing a synchronous pattern of moisture availability on the southern Tibetan Plateau.