PP21A-1389
Establishment of 14C chronology of Lake Qinghai and the application of Mean Value Concept to removal of Old Carbon Effect
The largest inland lake in China, Lake Qinghai, has experienced apparently continuous sedimentation since the Pliocene. Its unique geologic setting and potential for high-resolution records makes Lake Qinghai an ideal site to study past environmental changes in the inland continental setting. Since 2004 "Scientific Drilling at Lake Qinghai in the Northeastern Tibetan Plateau: High-resolution Paleoenvironmental Records of Eastern Asia Linked to Global Change" (QHDP) was on-schedule. One of the multi-disciplinary studies for the QHDP project is to establish a reliable and high resolution geochronology stratigraphy, which is crucial to quantify the time resolution of the proxies, and will be the foundation for all subsequent studies of climate, environment, and tectonic. However, there is no varve or dated archeological remains available in the core sequence from lake Qinghai, and the living fish and algae seeds there all have been polluted by the nuclear explosion, which have made it difficult to calibrate the chronology for Qinghai core. Furthermore, the traditional regression analysis method to establish the lake chronology did not consider the different accumulation rate in the core sequence, which would introduce the significant error into the established chronology. In this paper, we use 14C-AMS technique to measure organic carbon, non-organic carbon and plant seeds from drilling core sequences, 1F(36°48'40.7"N,100° 08'13.5"E)with 18m deep in Qinghai lake. By introducing the "mean value concept" in the linear regression analysis of a multi-variable geological system into the traditional regression method and dividing the 18m deep core sequence into two sections according to their different accumulation rates, we have established the 14C chronology of the Qinghai lake and determined the average dead carbon age of about 260 yr and 650 yr for the core sequence above and below about 12m depth respectively. The results above the 12m depth are compared with ones obtained from the wiggle matching method and in good coincidence with each other.
PP21A-1390
Magnetic properties of Lake Qinghai Sediment
Lake Qinghai, the largest lake in China, locates at an elevation of 3196 m, covers an area of 4400 km2, with an average depth of 21 m and a drainage area of about 29,660 km2. It sits near the northeastern margin of the Tibetan Plateau and is positioned on the conjunction region where East Asian winter and summer monsoon, Indian Monsoon, and Westerlies interact. Lake Qinghai Drilling Project (LQDP) initiated by Institute of Earth Environment, Chinese Academy of Sciences had extracted a series of shallow cores from Lake Qinghai basin in the summer of 2005. Upper 6m of Core 1F (36°48¡ä40.7¡åN, 110°08¡ä13.5¡åE) contains a relatively uniformed greenish gray silty clay sediment with several darker layers and with a grayish brown section at 4.2-5m, preliminary chronology work (Zhou et al.,2008) indicate the upper 6m sediment spans to about 14ka. Some typical samples are selected for magnetic properties tests (K-T, J-T and Hysteresis loops), environmental magnetism parameters (magnetic susceptibility, ARM, SIRM, S-rations) are measured at 1cm intervals of core 1F. Rock magnetism tests identified that hematite and magnetite are dominant magnetic minerals below 5m, which may imply a detrital origin of magnetic properties related to terrestrial, especially eolian sources. In contrast paramagnetic iron sulfides e.g pyrites, normally formed in reduced condition are common in upper 5m. Magnetic susceptibility value varies between 4-8 10-8m3/kg below 5m, while that of the upper 5m are even lower (about 1-4 10-8m3/kg). Such magnetic susceptibility value change is attributed to changes of magnetic minerals and sedimentary conditions, such as oxidation-reduction alternation and carbonate dilution in the lake. Comparison with grain size and other climate proxies show that Lake Qinghai was generally in a drier environment before Holocene and became a deeper lake since Holocene. Key words: Lake Qinghai, magnetic minerals and magnetic susceptibility
PP21A-1391
Records of Long-chain Alkenones From the Drilling Core in Lake Qinghai Over the Last 13,000 Years
We tended to concentrate on alkenone distribution patterns in Lake Qinghai region around 2005 and found the long-chain alkenones were widely distributed in surface water and modern sediments. Alkenone proxy could be used in palaeoenvironment change research in this region. In here, we present the C37 content, %C37:4 value and Uk'37 of lacustrine sediments from a drilling core LQDP05-1F of Lake Qinghai. (The 14C chronology is from Zhou et al, unpublished data). C37 contents range from 0.6 to 60¦Ìg/g around the average value of 10¦Ìg/g and represent several significant higher values stages in the Holocene, which reflects greater biomass level of LCAs producer algae's. In the other words, it is records the producer algae's suitable growth period and the information of algae growth environment. The long period existence of LCAs in drilling core also indicates that it can be used as a continuous biological proxy in Lake Qinghai over late Pleistocene. Based on the recognition of %C37:4 distribution of modern Lake Qinghai, there is negative correlation between %C37:4 value and salinity within a certain salinity range. The salinity indicated by %C37:4 values represent three stages of different change trends over the last 13 kyr. From late Pleistocene to early Holocene, %C37:4 range from 5.9 to 42.7% and salinity shows several significant fluctuations. During middle Holocene, %C37:4 values firstly stabilize on the lower level around the average value 17% and increase to 60%, a lower salinity exhibit in this period. During the late Holocene, the salinity change strongly with significant fluctuations of %C37:4, the variation range over 40%. We suggest it can be used to explain the salinity changes in three stages by the interaction of the climate change and the evolution of Lake Qinghai. Uk'37 values change slightly around the average value of 0.17 during the late Pleistocene generally. However, it shows several significant peaks of higher values respectively, which is difficult to explain by the usual recognition of temperature change during the Holocene period. We suggest that Uk'37 is still strongly effected by other environment factors in Lake Qinghai or other closed interior brackish-salt lakes, except for the temperature of lake water. It needs more mechanisms research of modern LCAs distribution in Lake Qinghai. Keywords: long-chain alkenones, Lake Qinghai, drilling core
PP21A-1392
Elemental Chemistry of Sediments From Two Drill Cores in Lake Qinghai, China, From Scanning XRF Analyses
The Lake Qinghai Drilling Project (LQDP) obtained cores from a number of sites in Lake Qinghai, China, under the auspices of the International Continental Drilling Program and the Chinese Academy of Sciences. Although thick sands were encountered at depth at all sites, fine-grained sediments as much as 18-20 m thick were obtained, 2-3 times more than in any previous coring efforts. A well behaved sequence of radiocarbon ages, extending to more than 22 cal kyr BP, have been obtained for the upper 9 m of the 18 m of core from site 1, suggesting that the fine-grained sediments extend well past the Last Glacial Maximum. Elemental analyses of the sediments by scanning XRF methods at 2-5 mm intervals have been obtained for cores from drill sites 1 and 2. The overall trends in most elements from the two sites are remarkably similar, suggesting that the sedimentary records from the two sites correlate well. Differences in detail are relatively minor. One significant difference is the fact that the gypsum layer associated with the Younger Dryas in previous coring studies near drill site 1 appears in the XRF data up as a major peak in sulfur abundance at site 1, but no such peak exists at site 2. Ca abundance in the late glacial-to-Holocene section appears to correlate well with carbonate analyses from previous studies of nearby cores. Ca abundance varies slightly but remains low in the glacial-age sediments, and Ca/Ti ratios are nearly constant, suggesting an absence of authigenic carbonate. Ca variations in this interval appear to be related to variations is silica, probably from detrital (eolian?) quartz. Authigenic carbonate, inferred from Ca abundance and Ca/Ti ratios, begins to increase about ca. 14.5 cal kyr BP, and reaches its highest values ca. 10.5-7 cal kyr BP ka. This result is consistent with this interval being the warmest, wettest period of the Holocene. Inferred authigenic carbonate progressively decreases after 7 ca kyr BP, but remains well above glacial values.
PP21A-1393
Initial Results From a Multi-Proxy Investigation of a Core From the Southeast Basin of Lake Qinghai, China
The Lake Qinghai Drilling Project (LQDP), under the auspices of the International Continental Drilling Program (ICDP) and the Chinese Academy of Science (CAS), aims to uncover past variations in Asia monsoon and climate, on both orbital and millennial time scales. To supplement that project with records of modern to Holocene sediments, in May/June 2007, we recovered a 3.55-m-long Uwitec sediment core (LQDP07-1A) and a 70-cm-long Mackereth core (LQDP07-7B) from the southeastern basin of Lake Qinghai, at the site of LQDP site 2. Radiocarbon dates indicate that these cores provide a record extending back to ca. 20 ka. Data from multi-proxy analysis including high resolution magnetic susceptibility, optical images, bulk density, XRF and X-ray radiographs (ITRAX X-ray Fluoresence Core Scanner), water content (freeze dried), grain size (Beckman Coulter), carbonate content (coulometry), and isotope analyses of carbonate and organic matter (EA-IRMS) provide clues from which to decipher past changes in regional climate and in the Asian monsoon system. Lithological and chemical proxies are yielding highly consistent records of distinct glacial and Holocene climatic features. The glacial period is characterized by fine-grained uniform medium grey mud with low calcium carbonate contents, whereas Holocene sediments are highly variable in both sedimentary facies and grain size, and are relatively high in carbonate. We are currently undertaking 3-cm pollen analyses for the Uwitec core, and will work to synthesize these results to unravel the climatic signals stored in Lake Qinghai sediments.
PP21A-1394
Modern Pollen Study in Lake Qinghai Valley
This study shows results of modern pollen in Lake Qinghai valley in the northeastern Tibetan Plateau. The records suggest that most pollen assemblages are dominated by shrub and herb pollen (mostly>84%). Seventeen taxa of indicator pollen are significantly correlated with the vegetation types of this area. In the pollen spectra, Artemisia is over-represented, while Poaceae, Cyperaceae and Polygonaceae are low- represented. Artermisia/Chenopodiaceae (A/C) radios with the regional vegetation characteristic can be used as a proper index to reconstruct the history of vegetation and climate in Lake Qinghai valley. Modern pollen in the lake mainly comes from the nearby vegetation. The pollen spectra is controlled by the directions and velocity of the wind and influenced by rivers. The distribution of modern pollen in Lake Qinghai tends to be similar in most part of the lake. The difference of pollen sedimentation process in the lake can be potentially influenced by the focusing function of the lake, river streams, and lake current.
PP21A-1395
Quaternary Fluvial History of the Lake Qinghai Basin, Northeastern Tibet
Detailed stratigraphic and sedimentologic analyses (vertical and lateral facies analysis, section measuring, sampling, etc.) of approximately 25 localities on the major tributaries around Lake Qinghai, northeastern Tibet allow us to reconstruct the record of Holocene fluvial sedimentation in the Lake Qinghai Basin. This record provides evidence of both climate and lake level change and changes in regional runoff during the Holocene Our preliminary analysis of the Lake Qinghai Basin fluvial strata suggests that a high-resolution record of changes in water volumes is indeed present in the basin. For example, the broad flat surface west/northwest of Lake Qinghai (in the upstream direction from near the mouth of the Buha River) represents a higher-than-modern paleo lake-level. The sediments in this region are organic-rich, fluviodeltaic in origin, and about 10 m above the modern lake level. Radiocarbon dates in these and similar strata show an interesting clustering, broadly at times of alpine glacier advance ca 3400-2200 BP (middle Neoglacial) and ca 1500-400 BP (early Medieval and Little Ice age). In addition to providing the first good record of Holocene fluvial sedimentation in the basin, these high-stand deposits likely coincide with the most negative values of δ18O in lacustrine carbonates and other high-stand indicators measured in the Lake Qinghai drill cores – as well as with other regional climate records. OSL dating of these strata and of other (and older) strata in the fluvial valleys around Lake Qinghai is in progress. Together, the radiocarbon dates, OSL dates, and detailed sedimentary facies analysis will allow a detailed reconstruction of correlations between lake-level change, changes in runoff and precipitation, and changes in regional (or global) climate.
PP21A-1396
Lake Level Reconstruction Using the Water Depth Related Distribution of Ostracoda in two Tibetan Lakes, Nam Co and Donggi Cona
Changes in the Asian Monsoon are reflected in paleo-lake level changes that can be reconstructed from ostracode associations of lake sediment cores. Ostracodes are one of the main micropaleontological proxies in paleolimnological research. They show high abundance and continuous presence in the often slightly saline high-altitude lakes on the Tibetan Plateau (TP). A comprehensive knowledge about the ecology and present day depth distribution of species, however, is a prerequisite for the use of this proxy. Therefore, we studied Recent ostracode associations from two lakes on the Tibetan Plateau for actualistic reference. Nam Co is situated on the south-eastern TP (30.5° N; 90.7° E) at 4719 m altitude. The surficially closed lake covers an area of 1961 km2 and has a maximum depth of 105 m. The water is slightly brackish (0.8 psu). The other lake, Donggi Cona, is located on the north-eastern TP (35.3° N; 98.9° E) at 4144 m altitude. Covering 250 km2; Donggi Cona is much smaller and shallower (38 m maximum depth) than Nam Co. Donggi Cona is a through-flow system and thus characterized by prevailing freshwater conditions. Ostracode diversity and species composition of both lakes are different; we identified six species in Nam Co and thirteen species in Donggi Cona sediments; four of the species we found in Nam Co were found in Donggi Cona as well. The most abundant species (70-95%) in Nam Co is Leucocytherella sinensis Huang, 1982, which is absent from Donggi Cona. There, Eucypris gyirongensis Huang, 1982 (shallow water indicator) and Leucocythere mirabilis (Kaufmann, 1892) (deep water indicator) are dominant. Despite these faunistic differences, our results suggest that Limnocythere inopinata (Baird, 1843) prefers shallow water and Leucocythere dorsotuberosa Huang, 1982, deep water below the thermocline in both lakes. The depth dependent distribution of ostracode species provides, together with autecological information on preferences and tolerances of species a database to establish a transfer function for the reconstruction of water depths and thus past lake levels. These transfer functions cover water depths between 3.7 m and 64 m in Nam Co, and between 0.2 m and 35 m in Donggi Cona. The standard error is approximately 7 m. Ostracodes of a 2.5 m long core and two short cores from Nam Co, for example, indicate lake levels 10 m to 20 m lower than today before 4 ka BP followed by a high stand of initially more than 10 m above the modern lake level. A distinct drop of approximately 25 m is reconstructed for the Little Ice Age. Since then until today, the lake level has been rising. Faunistic and also some ecological differences of species associations stress the need of regional modern data sets necessary for the establishment of transfer functions and their application to long sedimentary sequences from Tibetan Plateau lake systems.
PP21A-1397
A late glacial and Holocene lake record from the eastern Tibetan Plateau and inferences of lake, glacier and climate evolution
A 12.81 m sediment core from Lake Ximencuo provides insight into the late glacial and Holocene evolution of the lake and the Nianbaoyeze glacier in its catchment, and the regional climate history since ~19 ka. Lake Ximencuo was a permanent deep lake throughout its history. In contrast to numerous lakes on the Tibetan Plateau which experienced shallow lake levels or even desiccation during the late glacial, Lake Ximencuo was fed by large meltwater quantities in the late glacial. The existence of glaciated upper catchment areas seems to have been a prerequisite for lakes on the Tibetan Plateau which maintained relatively high water levels during the generally cold and dry period following the global LGM (~21 ka). A minor re-advance of the Nianbaoyeze glacier was recorded during the latter half of the Oldest Dryas (~16.4 and 14.5 ka), followed by rapid Bolling/Allerod warming. Most favourable conditions were recorded at Lake Ximencuo during the early Holocene which was punctured by a remarkable pulse of climate deterioration around 8.3 ka. This spell represents the 8.2 ka event of the North Atlantic region and proves that it had a significant impact on the Tibetan Plateau. Less favourable conditions of longer duration occurred from 4.7-3.7 ka, apparently in phase with numerous records signalling colder and drier conditions on the Tibetan Plateau. Two minor spells of less favourable conditions and probably catchment erosion were recorded in the late Holocene between 2.0 and 1.4 ka and between 0.5 and 0.1 ka with the latter representing the Little Ice Age. It remains open, whether human activities may have accelerated or even solely triggered the late Holocene erosion events.
PP21A-1398
Regionalization of Tibetan Plateau precipitation and its relation to the Asian Monsoon
Many paleoclimate records from the Tibetan Plateau link past changes in local precipitation to Southwest (SW, or Indian) and East (E) Asian Monsoon variability. However, few of these records are correlated with instrumental records of local or regional monsoon variability. And, although the majority of Tibetan precipitation occurs in the summer months, a dearth of station data limits the connection of instrumental precipitation variability across Tibet to the Asian Monsoon regimes. To properly interpret proxy climate records, a quantitative understanding of Asian Monsoon influences on the Tibetan Plateau is required. With this goal in mind, we investigated precipitation variability across the Tibetan Plateau using monthly gridded merged precipitation (CMAP) and outgoing longwave radiation (OLR) datasets to quantify the relationship between summer precipitation on the Tibetan Plateau and the SW and E Asian Monsoons. Average summer OLR and precipitation are significantly correlated at the 95% confidence level between southwest, northwest, southeast Tibet and the central Himalayas, but average summer OLR and precipitation in northeast Tibet is only significantly correlated with OLR and precipitation in southeastern Tibet. OLR over the central Himalayas, southwest Tibet, and northwest Tibet correlates at the 95% confidence level with the Indian Monsoon Index, and precipitation in the central Himalayas correlates at the 95% confidence level with the Webster-Yang Index of SW Monsoon variability. OLR and precipitation over the central Himalayas, southwest Tibet, and northwest Tibet also significantly correlate with OLR and precipitation over India, as well as surface wind speed, 850 mb zonal, and 850 mb meridional wind speeds over the southwest Arabian Sea. These significant correlations indicate precipitation variability over western Tibet and the central Himalayas is related to SW Asian Monsoon variability. Correlations between southeastern and northeastern Tibet OLR/precipitation and climate south of the Himalayas are weaker and fewer in number. However, OLR over northeastern Tibet is significantly correlated with East Asian Monsoon Indices, with less precipitation in northeast Tibet during stronger East Asian Monsoon seasons that bring anomalously high precipitation to other parts of China.
PP21A-1399
Westerly jet-Asian Monsoon Coupling During the Last 30 kyr
Increasing evidence has revealed that the East Asian Summer Monsoon (EASM) intensity varies in millennial timescale during the last deglaciation and the Holocene as well as the last glacial period (e.g., Hu et al., 2008). However, the cause and nature of these EASM variations are not well understood. At present, the Asian summer monsoon front, the major convection/rainfall band, moves northward with the northward migration of the westerly jet from April to September (Liang and Wang, 1998), transporting moisture to inner Asia. Thus, it is possible that behavior of the westerly jet strongly influences on variability of the EASM. Then, here we reconstructed the variation in the westerly jet during the last 30 kyr, and examine its relation to the EASM changes. The Japan Sea is located on the eastern margin of the Eurasian continent under the influence of Asian winter monsoon wind and the westerly jet that passes over dry areas in the Eurasian continent. Therefore, it receives significant amount of eolian dust emitted from these dry areas. In order to examine the variability of the westerly jet, here we reconstructed latitudinal changes of the eolian dust provenance using the two sediment cores D-GC-6 and KR07-12-PC8 derived from south and central parts of the Japan Sea, respectively. We use two parameters of quartz, Electron Spin Resonance signal intensity and Crystallinity Index, for the estimation of the eolian dust provenance. The Gobi desert in Mongolia (Mongolian Gobi) and the Taklimakan desert are estimated as the dominant source areas of eolian dust at both sites of the Japan Sea. Latitudinal change of the eolian dust provenance varied in harmony with the EASM variations. Namely, larger contributions of eolian dust from the Taklimakan desert at the south site than the central site are found during the Heinrich 1 and 2, Younger Dryas, and some intervals of the Holocene, which correspond to the periods of weak EASM. Since the latitudinal change of the eolian dust provenance reflects the position of the westerly jet, our result supports the idea that there is a close linkage between the westerly jet position and intensity of the EASM in millennial timescale.
PP21A-1400
Climate changes in northeastern margin of Tibetan Plateau since 8ka BP --- The speleothem records
We analyzed two stalagmites (XN-2 and XN-5) collected from Xiannv (Fairy) Cave, southeast of Qinghai Province. Twenty 230Th dating results shown that stalagmite XN-2 grew through 7600-130 yr. BP except a hiatus between 4600-920 yr. BP and XN-5 deposited through 4590-2470 yr. BP. Tests for equilibrium conditions show that their δ18O variations are primarily caused by climate change. Thus, we propose that calcite δ18O were dominated by the isotope composition of drip water, i.e., amount weighted mean of precipitation δ18O, because the 3.5‰ shift of δ18O value within the Holocene could not be ascribed to temperature change in cave alone, as it needs 11 °C change which is practically too large. Comparison of our record with adjacent δ18O record of Hongyuan peat cellulos and speleothem records of Asian summer monsoon (ASM), indicates that all these records correlated remarkably from 4000 yr BP to 7600 yr BP, implying that ASM, was the primary climate system influencing the precipitation in the study region, and monsoon front could reach this region and even northwestward during this period. However, since 4000 yr BP, our speleothem and peat records sharing similar pattern disagree with that of speleothem records of ASM. As our study cave and Hongyuan peat site located in the north border of summer monsoon front, it is plausible to suggest that this difference may be caused by the southward retreat of monsoon front as ASM weakening with the NH insolation decreasing, and the summer precipitation might not be dominated by ASM during this period. Further study need to clarify which climate systems, including the westerlies and air masses activity over the Tibetan Plateau, was the primary system influencing summer precipitation in this region during the late Holocene. Moreover, climate oscillations on centennial time-scales in all these records well correlate, implying that driving mechanisms for these variations might be the same during the Holocene.
PP21A-1401
Evolutionary Process of the Climate of Lop Nur Region, Xinjiang, China, Since Mid- Pleistocene
Climate variation since Mid-Pleistocene is studied based on a multi-proxy in core LS2 drilled from the Lop Nur region (88°23'18.2"E, 39°46'39.3"N), in the eastern Tarim Basin, Xinjiang, China. Geochemical properties, including carbonate content, total organic carbon content, total nitrogen content and carbon / nitrogen, have been determined to reconstruct the paleoenvironmental evolution of the area since Mid- Pleistocene. Our results indicate that the paleoclimate fluctuation can be divided into seven stages: I, depth of 115-103m, the climate changed from warm-arid to cold- arid; II, depth of 102-82 m, dominated by cold-humid; III, depth of 81-67 m, characterized by warm-arid; IV, depth of 66-57m, extreme cold-humid period; V, depth of 56-46 m, warm-arid period; VI, depth of 45-18 m, cold-humid episode with three warm-arid periods sub-fluctuations; VII, depth of 17-0 m, the climate gradually turned to extreme arid. The paleoclimate evolutionary process indicates the instability of the climate system and the hydro-thermal characteristic of climate variation between cold-humid and warm-arid in the region since Mid-Pleistocene. Also, the paleoclimate variation in Lop Nur area correlates well with climate change records indicated by marine oxygen isotope. This indicates the climate in Lop Nur area responds to global climate change and maybe influenced by the global ice volume. This study gives information about the paleoclimate evolutionary process in Lop Nur area and also has contribution to the global paleoclimate change investigation. Keywords: Lop Nur; carbonate content; total organic carbon content; total nitrogen content; Carbon / Nitrogen; climate.
PP21A-1402
Contrasting Moisture Shifts at Low- and High-Elevations Inferred from Annual-to- Centennial Resolution Fossil Pollen Data from the Qaidam Basin, Northeastern Tibetan Plateau
We review fossil pollen records from our three study lakes in the Qaidam Basin on the NE Tibetan Plateau and investigate the possible different responses of high- and low-elevation moisture conditions to large-scale climate forcing by comparing with the reconstructed precipitation data from the surrounding mountains. The near-annual resolution pollen data for the last 50 years at Gahai Lake show that large changes in A/C (Artemisia/Chenopodiaceae) pollen ratios correlate with relative humidity from instrumental data at nearby Delingha, likely reflecting changes in pollen production in response to interannual shifts in effective moisture. The pollen data for the past 1000 years at decadal resolution from Hurleg and Toson lakes mainly reflect changes in relative plant abundances, especially of dominant Artemisia and Chenopodiaceae, and regional moisture. Fossil pollen A/C ratios from these two lakes show similar centennial-scale moisture oscillations as in lithology, stable isotopes and ostracode shell abundance during the last millennium. Similarly, Holocene pollen data at centennial sampling resolution from a long core at Hurleg Lake show major shifts in vegetation types between steppe desert and desert, caused by multi-millennial scale climate changes. A/C ratios suggest that climate was dry at 10.2-8.7 ka, relatively wet at 8.7-6.5 ka, dry and variable at 6.5-3 ka, and relatively wet after 3 ka. Other proxies (carbonate, oxygen isotopes of authigenic carbonates, and oxygen isotopes and Mg/Ca ratios of ostracode shells) from the same core support the climate interpretations from pollen data. The wet/dry climate patterns recorded in our lakes in the Qaidam Basin on all time scales are antiphase with tree-ring-based precipitation reconstruction from the surrounding mountains and to pollen and snow accumulation data from Dunde ice core, especially in the 1970s, around 1600 AD and in the early Holocene when the moisture experienced extreme changes. This contrasting pattern implies that topography might have played an important role in mediating regional moisture response to large-scale forcing factors, including summer insolation, monsoon intensity and the westerlies. The insights from these paleorecords will help us understand and project possible future droughts in this arid region.
PP21A-1403
Objective spatiotemporal proxy-model comparisons of the Asian monsoon for the last millennium
The Asian monsoon system can be studied using a complementary proxy/simulation approach which evaluates climate models using estimates of past precipitation and temperature, and which subsequently applies the best understanding of the physics of the climate system as captured in general circulation models to evaluate the broad-scale dynamics behind regional paleoclimate reconstructions. Here, we use a millennial-length climate field reconstruction of monsoon season summer (JJA) drought, developed from tree- ring proxies, with coupled climate simulations from NCAR CSM1.4 and CCSM3 to evaluate the cause of large- scale persistent droughts over the last one thousand years. Direct comparisons are made between the external forced response within the climate model and the spatiotemporal field reconstruction. In order to identify patterns of drought associated with internal variability in the climate system, we use a model/proxy analog technique which objectively selects epochs in the model that most closely reproduce those observed in the reconstructions. The concomitant ocean-atmosphere dynamics are then interpreted in order to identify and understand the internal climate system forcing of low frequency monsoon variability. We examine specific periods of extensive or intensive regional drought in the 15th, 17th, and 18th centuries, many of which are coincident with major cultural changes in the region.
PP21A-1404
Holocene megafloods? Stories of the lacustrine strata along the Nyang River, Tibet
The occurrence of lacustrine sediments along the Yarlung-Tsangpo, southern Tibet, has evoked the arguments regarding the megafloods scenario and paleo-lakes dammed by monsoon driven valley glaciers in the early Holocene. According to our field survey of the terraces constituted by lacustrine deposits on the top along the middle Yarlung-Tsangpo and its tributary, Nyang River, we are able to establish a successive litho-column that reveals the evolution of the paleo-lake. The 150 m sedimentary sequence was reconstructed by compiling the outcrops exposed among the terraces on the elevation of ca. 2900-3100 m. The strata mainly consist of alternating sand and silt units and distinctly record two remarkable varve sections. Our resulted stratigraphy shows at least two sets of coarsening- upwards sequence beginning from the varve layers and then gradually transiting to interbeded sand/silt with abundant ripple rich cross-beds, parallel lamination, syn-depositional deformation and even showing channel gravels in certain horizons. The facies identification presents the evolution of depositional environments from pro-delta to the beach ridges of the frontal floodplain and indicates at least two aggradational stages of the glacier-related outwash delta. Radiocarbon and OSL dates reveal an aging-downwards trend as the stratigraphic order, confirming the assumption that the recorded sequence was deposited under the successive deposition of the investigated terrace. They further suggest that the younger paleo-lake took place before 32 ka and the older one occurred no earlier than 50 ka, both are far older than Holocene. As a summary, from Marine Isotope Stage 4 to 2, two sedimentary sequences of delta evolution from pro-delta, delta front to floodplain are recorded along the Nyang River; this is believed as a result of glacier-damming events occurring in the middle reach of the Yarlung-Tsangpo.
PP21A-1405
Lake-System Response To Late Quaternary Environmental Dynamics On The Northeastern Tibetan Plateau
In the scope of the joint Chinese-German research programme TiP (Tibetan Plateau: Formation, Climate, Ecosystems), our project deals with the reconstruction of landscape development in response to changes in the monsoon climate system. In a first step, geomorphological and limnogeological investigations were undertaken at Lake Donggi Cona on the NE Tibetan Plateau at 4200 m elevation. The 30 km long, 8 km wide, and 90 m deep open-basin lake is of tectonic origin and situated along the Kunlun Fault. Sub-bottom profiling of the lake basin revealed the presence of graben structures, conjugate faults, subaquatic terraces, and ancient fan systems, draped by 4 to 5 m thick postglacial lacustrine muds. The catchment area with elevations of more than 5000 m is also dissected by tectonic faults along the surrounding mountain ridges. A prominent moraine arc in the vicinity of the lake gives evidence of a glacial advance during the last glacial maximum, as also suggested by a palaeo Gilbert Delta at 20-30 m water depth in the modern lake. Several terrace levels around the lake shore document former stages of higher lake level. The timing of regional postglacial environmental changes is well documented by sedimentological and geochemical proxy data in radiocarbon-dated sediment cores from the lake bottom. The basal age of lacustrine sediments above the last glacial delta sequences indicates local deglaciation and initial lake-level rise to have started at about 18.7 ka BP. The lacustrine environment was characterized by a well-ventilated fresh-water lake with high detrital sediment influx and deposition of siliciclastic laminites. Between 15.4 and 14.4 ka BP, the lake environment switched to a poorly ventilated closed-basin system, dominated by laminated aragonite precipitates. This change went along with a strong lake-level rise, enhanced biological productivity, increased preservation of organic matter, and pyrite formation in the oxygen-poor bottom sediments. The timing of change is consistent with the onset of enhanced summer monsoon and increased regional summer insolation at the beginning of the Bolling-Allerod interstadial. The sedimentary conditions remained stable until the mid-Holocene. However, small-scale variations in composition and layer thickness point to pervasive short-term variability of climate conditions at centennial to decadal time scales. An abrupt change in sedimentation patterns took place at around 4.5 ka BP towards the modern well-ventilated open- basin lake system, dominated by massive silty clays with calcite micrites. This sudden change can be attributed to a threshold response in the course of continuous climate deterioration with the decline in summer insolation. A neotectonic pulse or erosional effect at the outflow of the lake, however, cannot be ruled out so far.
PP21A-1406
History and Variability of aridity in the Tarim Basin since Late Miocene and its links to Tibetan Growth and global ice-volume change
Modern climate environmental system in China is composed of monsoon region in the south-east where the annual precipitation is more than 400mm,, arid region in the north-west where the annual precipitation is less than 200mm, and the transitional zone between them. Results of numerical simulation show that the monsoon-arid environmental system of China has been influenced greatly by the growth of the Tibetan Plateau since Miocene (An et al., 2006). Numerous climate information, including the precipitation and temperature changes, has been obtained in the eastern monsoon-effected region, but that are very scarce in the western arid region. As the classical representative of the arid region, the Tarim Basin is very important to understand the coupling mechanisms among Tibet uplift, monsoon-arid environmental evolution, and global climate change. Lop Nor is located in the east part of the Tarim Basin, an ideal location to monitor drying of Asian inland. In year 2004, the LS2 core (39°46.65'N, 88°23.3'E, 1050.60m long) in the Lop Village was retrieved under the support of the project of Continental Environmental Scientific Drilling of China. Magnetostratigraphic result indicates that the age of the bottom of LS2 core is around 7.2Ma. Multiple proxies such as magnetic susceptibility, grain size, total organic carbon, total nitrogen, δ13Corg, and carbonate content as well as δ13C and δ18O of carbonate were generated to reconstruct the history and variability of aridity in the Tarim basin since 7.2Ma. The results reveal a remarkable drying in the Tarim basin around 5.5-5.3 Ma, possibly related to the growth of the Tibetan Plateau. Significant humid-dry variability under gradual drying trend is closely linked to global ice- volume change represented by marine δ18O records during the Plio-Pleistocene.
PP21A-1407
Extending the Record of Millennial-scale Variability of East Asian Summer Monsoon Back to 650 ka and Beyond
Millennial-scale abrupt climatic changes known as Dansgaard-Oeschger [D-O] events are northern hemispheric phenomena of global significance. Asian summer monsoon [ASM] shows distinct millennial-scale variability associated with D-O events, and recent ultra-high resolution analyses of NGRIP ice core demonstrated that the decreases in dust emission from Asian deserts slightly lead abrupt warming events in Greenland during deglaciation. If true, it is essential to know the origin of millennial-scale variability of ASM and its tele-connection mechanism with Greenland climate. In this respect, obtaining a long, continuous, high- resolution record of ASM is critical. Hemipelagic sediments of the Japan Sea are characterized by cm to m- scale alternations of dark and light layers. Previous studies demonstrated that deposition of the dark and light layers is synchronous and basin-wide. Correlation of the gray scale profile with δ18O profile of Greenland ice cores revealed surprising similarity in millennial-scale variation pattern with darker intervals corresponding to D-O interstadials. The similarity continues back to the last interglacial maximum although the amplitude of gray scale variation seems slightly reduced. Nutrient budget calculation of the Japan Sea suggests that nutrient influx from the East China Sea [ECS] through the Tsushima Strait (sill depth of ca. 130m) is the major control of the surface productivity and deposition of the dark layers. Nutrient influx from the ECS is controlled by relative contribution of the nutrient-rich ECS coastal water and nutrient-poor Kuroshio water. ECS coastal water expands to the entrance of the Tsushima Strait during summer when intense precipitation over south China increases Yangtze River discharge. Thus the nutrient influx from the ECS into the Japan Sea is considered as reflecting summer monsoon intensity. Nutrient supplied to the Japan Sea was effectively trapped in the sea when the sill depth was between ca. 40 and 100m, yielding a favorable condition to deposit the dark and light layers. Leakage of nutrient-rich subsurface water have occurred when sill depth became deeper than ca. 100m, and resulted in the reduction in sensitivity of organic burial to the nutrient influx. When the sill depth was shallower than ca. 40m, the Japan Sea became isolated and deepwater became euxinic, which resulted in further reduction in sensitivity of organic burial to the nutrient influx. The observation that gray scale profile of the Japan Sea sediments faithfully records millennial-scale variability of ASM is further supported by excellent correlation between the gray scale profile and ƒÂ18O of stalagmites in Chinese caves during the last 220 ka. We further correlated the gray scale profile with CH4 record of Antarctic ice cores during the last 650 ka, since CH4 record is claimed as the best proxy to extend the D-O signals back to 800 ka. The result confirms overall correlation although absence of CH4 peaks is observed in several intervals. Thus, gray scale profile of the Japan Sea sediments is the most reliable and long record of the millennial-scale variability of ASM. Previous result of ODP Leg 127 suggests appearance of the dark and light layers at ca. 1.6 Ma, suggesting high potential of the Japan Sea sediments to extend the record further back. 1.6 Ma could represent onset timing of millennial-scale variability of ASM. Alternatively, it may represent the opening of Tsushima Strait or shift in the course of the Yangtze River. IODP drilling of the Japan Sea and the northern ECS will give us an answer.
PP21A-1408
Reconstructing Late Holocene Climate Variability in North East China From Varved Maar Lake Sediments
Reconstructing climatic variability over the past c. 2 ka years is recognised as a key PAGES timeframe (focus 2). However few high-resolution records exist from the climate sensitive region of N) China which receives the majority of its precipitation from the east Asian summer monsoon (EASM). Interactions between the EASM and the global climate system have great resonance. Such examples include how the EASM responded to changes in climate over the documented e.g. "Medieval Warm Period" (c. AD 900 - 1300), "Little Ice Age" (c. AD 1350-1850) and recent warming. At present, literature remains contradictory to such environmental changes in NE China over this time-frame due to poor chronological control, low resolution of existing studies and even due to the inexact terminology of these climatic periods. Xiaolongwan Lake (XLW) is a small, closed, maar lake located in the Long Gang Volcanic Field, NE China (42°18'N; 126°19'E). It is at an elevation of 655 m a.s.l. with a maximum depth of 15 m. A varve chronology has been created for a 143 cm composite core (2 cores collected in 2006), and here we present diatoms and organic geochemistry (δ13C, TOC, C/N) evidence for environmental change over the past c. 2 ka years. Results show a gradual change in diatom species, moving from a composition where opportunistic species (e.g. Achnanthidium minutissimum) dominate (between c. 100 BC to 500 years AD) at the beginning of the record to one comprised of benthic/epiphytic species (e.g. Staurosira construens var venter, Punctastriata discoidea, Gomphonema parvulum). The introduction after c. 1850 years AD of the planktonic diatom species, Discotella woltereckii, not previously seen in the record, coincides with recent warming. This may be a response to changing limnological conditions, such as decreasing duration of lake ice-cover. Bulk organic δ13C results conducted on a short core collected from XLW in summer 2007, show that over the past c. 350 years there is a distinct decreasing trend in lake macrophytes. At the same time, trends in decreasing mean annual precipitation and an increasing trend in mean annual temperatures for the region are documented from meteorological records. It is possible that reduced EASM intensity may be affecting the limnology and therefore biology of XLW as seen in more recent years. Quantitative techniques (siZer) will later provide a method of assessing periodicities of EASM from this high-resolution record in order to compare with other independent palaeorecords.
PP21A-1409
Holocene Asian and African Monsoon Strength Recorded in O-18 of Atmospheric Oxygen
Prior work using trapped gases in ice cores has shown that the oxygen-18/16 ratio of atmospheric molecular oxygen (δ18Oatm) is sensitive to orbital-scale monsoon and ice volume variations (Bender et al., 1994) and millennial-scale monsoon variations during Marine Isotope Stage 4 (Landais et al., 2007). Here we extend these findings with a high-resolution record from the Siple Dome ice core, West Antarctica, covering the last 60 kyr. The early Holocene sample resolution is ~30 yr and precision is ±0.009‰, revealing previously unrecognized centennial-scale variations in δ18Oatm. Removal of the seawater δ18O record (Waelbroeck et al., 2002) yields a record of past changes in the Dole Effect, the difference between the δ18O of air and seawater (+23.8‰ today). A further calculation using the derivative of the δ18Oatm data and an assumed 1000-yr O2 turnover time allows a deconvolution of the implied effective value of δ18O of O2 introduced to the atmosphere by the terrestrial biosphere (δ18Oland). Photosynthesis transfers the δ18O of chloroplast water (H218O/H216O) directly to the O2 that is produced (Guy et al., 1997). Strong monsoons are expected to produce low values of δ18Oland and thus a weak Dole Effect by 1) the low δ18O of heavy rainfall, 2) weak evaporative enrichment of 18O in leaf water due to the high humidity in which monsoon photosynthesis generally occurs, and 3) weak respiratory fractionation in wet tropical soils (Angert et al., 2003). In fact, our calculated δ18Oland correlates surprisingly well with Asian monsoon strength indicators such as the Dongge Cave record (Wang et al., 2005) at periods >0.2 kyr, except at 6 ka, where a prominent increase in δ18Oland coincides with the drying of the Sahara. These observations suggest that δ18Oland from ice cores may be a useful proxy of past Asian and African monsoon variations that integrates over large spatial scales (because the atmosphere is well-mixed, and photosynthesis is ubiquitous). Interestingly, δ18Oland suggests strengthening monsoons over the past ~2 kyr, probably caused by the upturn in May-June 30°N insolation, which may obviate the need to call for early anthropogenic emissions to explain rising methane in the last 2 millennia (Ruddiman, 2003).
PP21A-1410
C4 Plants Decline in the Himalayan Basin Since the LGM: Implications for the Evolution of the Monsoon
The Bengal Fan turbiditic system, supplied by the Ganga-Brahmaputra (G-B) river system, provides an integrated record of Himalayan erosional system. Recently, we showed that riverine organic carbon delivered by G-B is very efficiently buried in Bengal Fan sediments (Galy et al. 2007, Nature v450 p407). Thus, thanks to minor inputs of marine organic matter, organic matter buried in Bengal Fan sediments is a proxy of Himalayan basin paleo-vegetation. The active channel levee system of the middle fan documents the last 19 ka and allows the reconstruction of vegetation change in the Himalayan basin since the Last Glacial Maximum (LGM). We measured δ13C of organic carbon (OC) in order to track changes in the proportions of C3 and C4 plants in the Himalayan basin (Galy et al. 2008, QSR v27 p1396). From LGM to mid-Holocene, δ13C of bulk OC shifts of 3 to 4‰ towards more negative values. Relative abundance of individual n-alkanes reveals that terrestrial higher plant inputs represent a dominant fraction of OC buried in Bengal Fan sediments. δ13C of higher plants biomarkers mimic that of bulk OC showing that the later can be used as continental paleo-vegetation proxy. δ13C negative shift from LGM to mid- Holocene, mostly indicates the transition from a dominant input of C4 plants to a dominant input of C3 plants and therefore reveals that C4 plants were more abundant in the basin under glacial conditions. Vegetation repartition in the basin simulated using the CARAIB dynamic vegetation model is consistent with OC data. The model indicates a dominance of C4 plants in the Gangetic plain during the LGM while eastern part of the basin remains dominated by C3 plants. The comparison between our basin scale record and proxies of regional paleo-climate suggests that the large decline of C4 plants after the LGM was due to combined increase of atmospheric CO2 and humidity levels. Integrated record of Himalayan basin paleo-vegetation suggests more arid conditions during the LGM than during the mid-Holocene and agrees with reconstructions of the monsoon indicating stronger SW monsoon during interglacial and stronger NE monsoon during glacial periods.