Vegetation evolution and its response to climatic change during 3.15–0.67 Ma in deep-sea pollen record from northern South China Sea

In order to study vegetation evolution and environmental change during 3.15-0.67 Ma,a total of 608 pollen samples with an average time resolution of 4 ka has been analyzed from ODP Site 1145(water depth 3175 m),northern South China Sea.The deep-sea sediments(213.62-91.9 m) cover from 3.15 to 0.67 Ma based on micropaleonotological stratigraphy.The characteristic features of the pollen diagram include that,pollen influx rises twice obviously around 2.58 Ma and during 2.0-1.8 Ma,in response to global climatic cooling and winter monsoon enhancement.Before 2.58 Ma,tropical and subtropical vegetation,mainly evergreen Quercus(E) and Altingia,predominated around northern SCS with much more tropical montane conifers and ferns relatively.From 2.58 Ma,temperate vegetation increased obviously,implying climatic cooling and winter monsoon enhancement.Spectral analysis shows that percentage variation in tropical and subtropical taxa has a strong 20 ka procession cycle probably due to the summer monsoon;while pollen influx has 100,41 and 19 ka cycles,indicating the sea level and winter monsoon change response to the ice-sheet variations.     

Deep-water Oligocene pollen record from South China Sea

Leg 184 of ODP recovered a record of deepwater sediments spanning the past 32.8 Ma from the South China Sea (SCS). The sediments of the Ofigocene (32.8-23.8 Ma) at Site 1148 contain relatively abundant fossil pollen. The pollen analysis at Site 1148 has established the first pollen assemblage sequence of deep-water Oligocene in the China Sea. The pollen assemblages of the Ofigocene are dominated by montane conifer tree pollen. The abundances of broad-leaved tree pollen are lower in the assemblages.Both of the montane conifer and broad-leaved tree pollen groups include mainly tropical-subtropical components. The pollen of cold and drought-enduring plants is present in lower content. The distinct change in pollen assemblage sequence of deep-water Oligocene of the SCS occurred at 32.0 Ma, indicative of an important shift of the Oligocene climate in the SCS region. The characteristics of the pollen flora of the deep-water Oligocene indicate the tropical montane ralnforest and lowland ralnforest developed on the areas around the SCS before 32.0 Ma, reflecting the warm and wet climatic condition. In the pollen flora of the Oligocene after 32.0 Ma, the temperate montane conifer and cool and drought-en- during deciduous tree taxa remarkably increased, indicating that the climate in the SCS region became comparatively cool and dry.     

Pollen records and time scale for the RM core of the Zoige Basin, northeastern QinghaiTibetan Plateau

A continuous pollen record from the Zoige Basin in the northeastern Qinghai-Tibetan Plateau not only provides information on the vegetation and climate changes during the last two glacial/interglacial cycles, hut also gives proof to establish the time scale of the upper 60 m of the RM core. Subalpine spruce-fir forests colonized the Zoige Basin during the interglacials and interstadials, implying warm and wet climate conditions. Alpine periglacial desert or dry desert may have existed during the penultimate glacial and the last glacial maxima, respectively. Alpine sedge meadow dominated the landscape during MIS 4. The MIS 3 is punctuated by a number of stadials similar to those documented in the Guliya and GISP2 ice cores, as indicated by repeated rise and fall of subalpine spruce-fir forests. Our pollen record reveals a regional climate history similar to those from the neighboring sites, including the Arabian Sea and the Guliya ice core, and thus supports the notion that the Qinghai-Tibetan Plateau acts as an important link between climatic events in the North Atlantic realm and the Asian monsoon domain.     

Pollen-inferred Holocene vegetation and climate histories in Taro Co,southwestern Tibetan Plateau

A 310-cm-long sediment core, covering the last 10,200 years, was collected from Taro Co on the south- western Tibetan Plateau and analyzed for pollen, grain size and total inorganic carbon content. The pollen data showed that vegetation changed from alpine steppe to alpine meadow during 10,200-8,900 cal a BP, to alpine steppe dominated by Artemisia during 8,900-7,400 cal a BP, to alpine meadow during 7,400-3,300 cal a BP and to alpine steppe after 3,300 cal a BP. Correspondingly, the pollen, grain size and total inorganic carbon content results revealed climatic change in this area over four stages. The initial stage was from 10,200 to 8,900 cal a BP, during which the climate changed from cold-dry to warm-humid. The second stage （8,900-7,400 cal a BP） was characterized by a warm and dry climate. However, at approximately 7,400 cal a BP, the climate began to become cold andhumid, which continued until 3,300 cal a BP. The last stage, from 3,300 cal a BP to present, was characterized as cold and increasingly arid. Climatic events of the early and mid-late Holocene showed that the area was significantly affected by the westerlies. However, the mid-Holocene climate in Taro Co was controlled by the Indian monsoon. The mid-late Holocene depositional environment record of Taro Co was very important to further elaborate the degree of influence by the westerlies or Indian monsoon.     

Evolution of the South China Sea and monsoon history revealed in deep-sea records

As the third summary report of ODP Leg 184 to the South China Sea (SCS), this paper discusses the evolution of the East Asian monsoon and the SCS basin. A multi-proxy approach, involving geochemistry, micropale-ontology, pollen and other analyses, was adopted for reconstructing the evolutionary history of the East Asian monsoon, which was characterized by a series of paleo-climate events especially at 8, 3.2, 2.2 and 0.4 Ma. The new record indicates similar stages in the development of the East and South Asian monsoons, with an enhanced winter monsoon over East Asia being the major difference. The rich spectrums of monsoon variability from the southern SCS also reveal other characteristic features of the low latitude ocean. Evidence for the evolution of the SCS includes the hemipelagic Oligocene sediments, implying the existence of deep water environments during the early seafloor spreading stage of the SCS basin. The four major unconformities and some remarkabl ediagenetic features in upper Oligocene deposits indicate the strongest tectonic events in the region. From a careful comparison of lithologies and sedimentation rates, we conclude that the prominent differences in sedimentary environments between the southern and northern SCS were established only by ～3 Ma.     

Vegetation and climate change during Marine Isotope Stage 3 in China

Fossil pollen records from 45 sites across China were evaluated and synthesized to document Vegetation and climate change during Marine Isotope Stage 3 （MIS 3） at 60-27 ka and to understand the large-scale controls. During MIS3, vegetation type was dominantly forest in eastern China, forest steppe/meadow in the north and Tibetan Plateau, and steppe desert in northwestern arid China. We developed a semi-quantitative vegetation index to reflect change in plant abundance （by inferring the general climate conditions）, with a vegetation score from 1 to 3 based on the different vegetation types inferred from pollen data at individual sites at intervals of 2,000 years.The reconstructed vegetation index shows higher values during MIS 3, especially during the period 53-40 ka, than at the Last Glacial Maximum. Our results also suggest that climate on the basis of vegetation change was cooler and drier during MIS 3 than during the Holocene optimum; however, MIS 3 vegetation was probably similar to modern vegetation. The close relationship between vegetation change, insolation and Asian summer monsoon strength suggests that climate variations, probably in both temper- ature and precipitation, are the primary drivers of regional vegetation change. Additional well-dated, high-resolution palaeoclimate records from many locations across China will be needed to understand the vegetation change and climate forcings on millennial and centennial scales within MIS 3.     

Deep-sea pollen research in China

This paper briefly presents the progress of deep-sea pollen research in China since the beginning of ninetieths of the last Century. All the deep-sea pollen contri-butions mainly come from the South China Sea (SCS) and the East China Sea (ECS). The German-Chinese joint cruise (Sonne 95) and ODP 184 cruise initiated by Chinese scientists in the SCS provided excellent material for the deep-sea pollen research. So far a number of pollen results of 20-30 ka and million years from the SCS have been published. A couple of deep-sea pollen records from Okinawa Through of the ECS also came out. The high resolution pollen records obtained from the continuous deposits with high sedimentation rates and reliable age control of the deep-sea sediments provided a high time resolution history (hundred to millennial scales) of vegetation, environment and monsoon evolution of the pollen source areas (southern China and Japan). Spectral analysis of deep-sea pollen records from the SCS discovered orbital (100, 41, 23, 10 ka) and suborbital cyclicities(Heinrich and Dansgaard/Oscheger-O/D events) in the vegetation changes. Moreover, cross spectral analysis showed that the trend of vegetation changes in northern SCS was regulated mainly by changes of the ice volume in the Northern Hemisphere. The pollen record of the last 20 ka from the Okinawa Through of the ECS indicates that the marine environmental change lagged that on the terrestrail by about 1000 year. The asynchronous environmental changes between land and sea were probably caused by the time difference in thermohaline circulation. This study underscored the role of the deep-sea plant fossils as a bridge across the land and sea.     

Evidences of rapid erosion driven by climate in the Yarlung Zangbo （Tsangpo） Great Canyon, the eastern Himalayan syntaxis

Climate and tectonism are both particularly intense in the Yarlung Zangbo (Tsangpo) Great Canyon in the eastern Himalayan syntaxis,which is characterized by the most rapid landscape evolution of anywhere in the world.Thus,the eastern Himalayan syntaxis is one of the best locations to study the interactions between climate and tectonics.This paper investigates the cooling ages of the Doxong La-Baibung profile using apatite fission track (AFT) dating on 11 bedrock samples at elevations ranging from 4210 to 710 m.There are topographic,climatic,metamorphic,and thermochronological gradients in the profile,providing good conditions to study interactions between climate and tectonics.AFT ages ranged from 4.6±0.6 Ma to 1.7±0.3 Ma,and the mean fission track lengths ranged from 11.0 to 12.4 μm.It was found that the cooling rates revealed by AFT ages increased with decreasing elevation.However,the tendency of the cooling rates revealed by the 40 Ar-39 Ar ages was different from that indicated by the AFT ages.Moreover,for most districts of the eastern Himalayan syntaxis,the compiled AFT age distribution correlates well with the annual average precipitation,indicating the coupling of the cooling and erosion rates of the near-surface rock and precipitation.The geothermal history modeling results indicate an obvious increase in the cooling and erosion rate between 1.0 and 0.5 Ma.This age is consistent with other research findings for this time,when the vapor channel of the Yarlung Zangbo Great Canyon began to take effect.These evidences suggest that climate,especially precipitation,has acting as a key factor influencing the rapid cooling and erosion in the Yarlung Zangbo Great Canyon since 1-0.5 Ma.     

The plant diversity and its relationship with paleoenvironment since 2.78 Ma revealed by pollen records in the Heqing deep drilling core

Multifold palynological diversity indices calculated by pollen data of the Heqing deep drilling core are synthetically analyzed, the results show that the plant diversity rapidly increased and its fluctuant am- plitude and frequency evidently largened during the stages of 2.729--2.608 Ma, 1.176--0.342 Ma and since 0.118 Ma respectively, while during the other periods the plant diversity was relatively low with a gentle fluctuation. To discuss further the relation between ancient plant diversity and paleoenvironment, it is considered that the plant diversity in the Heqing region lies on increase or decrease of vertical vegetational belts in mountains around the studied area and richness of components of these vegeta- tional belts, and is controlled by amplitude of tectonic uplift of mountains and climatic changes. The high plant diversity during 2.729--2.608 Ma responded to the strong uplift of mountains at the stage, while the plant diversity greatly increased and its fluctuant amplitude and frequency markedly largened at about 1.176 Ma was obviously response to the Middle Pleistocene Transition. The plant diversity and its fluctuant amplitude and frequency increased further at about 0.991 Ma may be superposably affected by both the Middle Pleistocene Transition and the tectonic uplift. The plant diversity increased since 0.118 Ma may indicate that the southwest monsoon strengthened and humid climate more fitted many kinds of plants to grow.     

Environmental evolution of Xingkai （Khanka） Lake since 200 ka by OSL dating of sand hills

Crossing the Sino-Russian boundary, Xingkai Lake is the largest freshwater lake in Northeast Asia. In addition to the lakeshore, there are four sand hills on the north side of the lake that accumulated during a period of sustainable and stable lacustrine trans gression and were preserved after depression. Analysis of well-dated stratigraphic sequences based on 18 OSL datings combined with multiple index analysis of six sites in the sand hills revealed that the north shoreline of Xingkai Lake retreated in a stepwise fashion since the middle Pleistocene, and that at least four transgressions (during 193-183 ka, 136-130 ka, 24-15 ka and since 3 ka) and three depressions occurred during this process. The results of this study confirmed that transgressive stages were concur- rent with epochs of climate cooling, whereas the period of regression corresponded to the climatic optima. Transgressions and regressions were primarily caused by variations in the intensity of alluvial accumulation in the Ussuri River Valley and fluctuations in regional temperature and humidity that were controlled by climatic change. Moreover, one obvious transgressive process that occurred in MIS3 may have been related to enhanced precipitation that was reportedly widespread in the west of China, while shortterm fluctuations in the lake level might well be a direct response to regional precipitation variations on the millennial scale.     

Variations in air temperature during the last 100 years revealed by δ~(18)O in the Malanice core from the Tibetan Plateau

By comprehensive analyses,it was found that the variations in δ^18O recorded in Malan ice core from the Kekexili Region on the Tibetan Plateau could represent the changes in air temperature during the summer half year (from May to October) over the Kekexili Region and the northern margin of the Tibetan Plateau.The general increase trend in δ^18O in the ice core during the past century indicated climate warming,and it was estimated that air temperature during the summer half-year rose about 1.2℃ over there then.However,this ice core record documented that the study area has been cooling while most of the world has been dramatically warming since the late 1970s. A tele-connection was found between the variations in δ^18O in the Malan ice core and the North Atlantic Oscillation.Moreover,the variations in δ^18O in this ice core were similar to that in the summer half-year air temperature over the southern Tibetan Plateau on the centurial time scale,but opposite on the multidecadal time scale.     

Environmental changes in the upper-reaches of Thurumqi River in the past 10,000 years, Tianshan Mountains, China

The sediments, at bore-hole T-3 in the upper reaches of the Urumqi River, are 10.7 m thick, and are composed mainly of till, slope-washed materials, solifluction and glacial-water deposits. The borehole can be divided into six sections based on the pollen distribution, magnetic susceptibility , δ~(13)C determinations and the diatoms distribution. There were three cold and dry periods during the past 10.0 Ka (10~3 years): before 9.3 KaBP, from 8.1 to 5.6 KaBP and 3.0 to 1.0 KaBP. The three warmer periods were from 9.3 to 8.1 KaBP, 5.6 to 3.0 KaBP and since 1.0 KaBP. The Holocene maximum was between 5.6 and 3 KaBP. However, the vegetation compositions, mainly C_3 plants, the magnetic susceptibility and the δ~(13)C were little changed in the study region in the past 10 Ka. Indications are that the climate was relatively cold and stable during the past 10 Ka.     

Palaeoenvironmental changes from pollen record in deep sea core PC-1 from northern Okinawa Trough, East China Sea during the past 24 ka

A pollen record of core PC-1 from the northern Okinawa Trough, East China Sea （ECS）, provides information on vegetation and climate changes since 24 cal. kaBP. A total of 103 samples were palynologically analyzed at 8 cm intervals with a time resolution of 230 a. Four pollen zones are recognized： zone Ⅰ （812-715 cm, 24.2-21.1 cah kaBP）, zone Ⅱ （715-451 cm, 21.1-15.2 cah kaBP）, zone Ⅲ （451-251 cm, 15.2-0.8 cah kaBP）, zone Ⅳ （251-0 cm, 10.8-0.3 cal. kaBP）, corresponding to Late MIS 3, Last Glacial Maximum （LGM）, deglaciation and Holocene, respectively. The LGM is characterized by the dominance of herbs, mainly Artemisia, and high pollen influx, implying an open vegetation on the ex- posed continental shelf and a cool and dry climate. The deglaciation is a climate warming stage with Pinus percentage increased and Artemisia percentage decreased and a rapid sea-level rise. The Holocene is characterized by predominance of tree pollen with rapid increase in Castanea-Castanopsis indicating the development of mixed evergreen and deciduous broad-leaved forest and a warm, humid climate. Low pollen influx during the Holocene probably implies submergence of the continental shelf and retreat of the pollen source area. The vegetation indicated by pollen assemblage found in this upper zone is consistent with the present vegetation found in Kyushu, Japan. Originating from the humid mountain area of North Luzon of the Philippines, Tasmania and New Zealand, Phyllocladus with sporadic occurrence throughout PC-1 core probably suggests the influence of Palaeo-Kuroshio Current or intense summer monsoon. The observed changes in Pinus and Herbs percentage indicate fluctuations of the sea level, and high Pinus percentage corresponds to high sea level. Spectrum analysis of the pollen percentage record reveals many millennial-scale periodicities, such as periodicities of 6.8, 3.8, 2.2, 1.6 ka.     

Climate change over southern Qinghai Plateau in the past 500 years recorded in Sabina tibetica tree rings

Two tree ring-width chronologies of Sabina tibetica were developed based on cores sampled in Qumalal and Zhiduo, southern Qinghai Plateau. The response function analysis showed that the chronologies were sensitive to temperature and precipitation from April to June in the plateau. Moisture index (MI) was defined, reconstructed and extended back to AD 1550. The cross-validation method was used to check the stability of the calibration equation, and the result indicated that the equation was stable. Six severe dry periods were found in this region in the past 453-year reconstruction, which were 1592 to 1610, 1649 to 1665, 1687 to 1697, 1740 to 1750, 1818 to 1829 and 1918 to 1933. Five severe wetting periods were 1669 to 1682, 1700 to 1709, 1800 to 1814, 1898 to 1909 and 1935 to 1950. Spectrum analysis indicated that there existed long-term cycles of 60.4 and 53.4 a, solar cycle of 11 a and short-term cycles of 8, 6 and 4 a in the reconstructed series.     

The early Holocene optimum inferred from a hish-resolution pollen record of Husuansyan Maar Lake in southern China

A high-resolution pollen record of the past 13000 a from Huguangyan Maar Lake reveals the vegetation and environment changes in southern China during the Holocene. It shows that （i） pollen percentage of trees and shrubs reached 56% during the early Holocene （11600-7800 cal a BP）, of which the pollen percentage of tropical trees reached a maximum at 9500-8000 cal a BP, reflecting a hot and wet environment; （ii） during the mid-Holocene （7800-4200 cal a BP）, the pollen percentage of montane coniferous trees and herbs increased, while the percentage of tropical-subtropical trees decreased, indicating lower temperature and humidity; （iii） in the late Holocene spanning from 4200 to 350 cal a BP, the pollen percentage of herbs and montane conifer increased significantly, indicating a marked decrease of temperature and humidity. Our pollen data reveal that the time period 9500-8000 cal a BP in southern China represents a climatic optimum for the Holocene characterized by hot and wet conditions. This is consistent with the Holocene optimum found in lower latitude regions globally. We speculate that strong insolation might cause the northward migration of the ITCZ and subtropical summer monsoon front, which resulted in an early Holocene optimum in the Huguangyan area. The dry tendency and climate fluctuations of the middle and late Holocene could be associated with a decrease in solar insolation and frequent ENSO event.     

Weldability and liquation cracking behavior of ZhS6U superalloy during electron-beam welding

The weldability of the ZhS6U nickel-based superalloy, which is prone to solidification cracking during electron-beam welding(EBW) repair processes, was investigated. The effects of two different pre-weld heat-treatment cycles on the final microstructure before and after welding were examined. Welds were made on flat coupons using an EBW machine, and the two heat-treatment cycles were designed to reduce γ′ liquation before welding. Microstructural features were also examined by optical and scanning electron microscopy. The results showed that the change in the morphology and size of the γ′ precipitates in the pre-weld heat-treatment cycles changed the ability of the superalloy to release the tensile stresses caused by the matrix phase cooling after EBW. The high hardness in the welded coupons subjected to the first heat-treatment cycle resulted in greater resistance to stress release by the base alloy, and the concentration of stress in the base metal caused liquation cracks in the heat-affected zone and solidification cracks in the weld area.     

Mechanism of stratospheric decadal abrupt cooling in the Early 1990s as influenced by the Pinatubo eruption

Studies have suggested that one volcanic eruption can influence seasonal to inter-annual climate variations.This study indicates that the Pinatubo eruption in 1991 may have actually induced the stratospheric decadal cooling recorded in the early 1990s.Using the NCEP/NCAR reanalysis and TOMS/SBUV satellite data,a decadal abrupt cooling of stratospheric tropical air temperature was found to have occurred in the early 1990s during a long-term descending trend.We generated the spatio-temporal structures of the decadal abrupt changes(DACs) for the stratosphere,and explored the relationship between the Pinatubo volcano eruption in 1991 and stratospheric DACs in the early 1990s.Our results suggest that the eruption of Pinatubo prompted a decadal decrease of ozone by the activation of nitrate and sulfate volcanic aerosols on ClO free radicals.The stratospheric heat absorbed by ozone decreased over a decadal time scale.As a result,decadal abrupt cooling of stratospheric tropical air temperatures occurred in the early 1990s,and may be attributed to the Pinatubo eruption.The results therefore indicate that one strong volcanic eruption can induce stratospheric decadal climate variation.     

Pulleniatina Minimum Event during the last deglaciation in the southern South China Sea

The planktonic foraminiferal faunal census of core MD 05-2894 （7°2.25′N, 111°33.11′E, water depth 1982 m）, retrieved from the southern South China Sea （SCS） during the ＂Marco Polo＂ cruise in 2005, was performed to investigate the abundance changes of a subsurface dweller, Pulleniatina obliquiloculata. The results display that the abundance of P. obliquiloculata nearly declines to zero during 16.0--14.9 ka, corresponding to the Heinrich 1 （H1） cold interval. The unexpected decrease of P. obliquiloculata occurs in the adjacent cores, roughly between 17 and 14.8 ka based on the previous studies. Accordingly, the Pulleniatina Minimum Event in the last deglaciation can serve as a good stratigraphical indicator, at least in the southern SCS. To further explore the changes of sea surface temperature （SST） and subsurface seawater temperature （SSST）, we made parallel Mg/Ca measurements on surface dweller Globigerinoides tuber and subsurface dweller P. obliquiloculata tests. Since the last deglaciation, the SSTs show a continuous increasing trend towards the late Holocene, while the warming of the subsurface water is punctuated by a 2℃-cooling interval across the deglacial Pulleniatina Minimum Event. Both increased 5180 differences between G. ruber and P. obliquiloculata, and increased temperature differences between surface and subsurface water suggest a shoaling of the mixed layer during the deglacial Pulleniatina Minimum Event. Therefore, we consider that the significant changes in the upper ocean structure are responsible for the Pulleniatina Minimum Event during the last deglaciation in the southern SCS.     

High—resolution records of Holocene from the Shuangchi Maar Lake in Hainan Island

The results of sedimentary characteristics,grain-size,clay mineral,C/N and pollen analysis reveal the evolution history of the Shuangchi Maar Lake of Hainan Island as well as the short-term climate variations in tropical areas.It is testified that the Maar sediment system was sensitive in recording the rapid climate changes in the Holocene.These multidisciplinary results demonstrate that the thermal maximum in the Holocene climate lasted from 7200 to 2700 aBP,with serveral abrupt ephemeral vibrations occurring between 7250-6120aBP and at around 4460,3850 and 2700 aBP.In the late Holocent,particularly at -2700 aBP,a distinct environmental change indicating an important climate transition in the tropical eastern Asia was recorded,probably responding to the enhancement of ENSO phenomena in the Pacific Ocean.In addition,the high-resolution algae record clearly indicates periodical climate fluctuations at 500 a cyclicity during the last 2500 years.