PP24A-01
A Pleistocene Indian Monsoon record from Heqing Basin, SW China
Heqing Basin (100°06'-100°16'E, 26°28'-26°46'N) is situated in southeastern margin of Tibetan Plateau, a geological conjunction zone of three tectonic units separated by Jinshajiang, Honghe and Xiaojinhe-Lijiang fault belts. Modern climate in this region is mainly influenced by Indian monsoon circulation. In Year 2002, a 665.83 m long core was retrieved from the Heqing basin under the support of Chinese Environmental Scientific Drilling program, which permits a high-resolution reconstruction of the Indian monsoon evolution from a continental perspective. The core mainly consists of gray clay, silty clay and silt. Magnetostratographic result generated by both thermal and alternating-field demagnetization methods indicates that the bottom age of the Heqing core is about 2.78 Myr. Multiple proxies (magnetic susceptibility, grain size, CaCO3 content, loss of ignite, pollen concentrations, and major/trace elements) were generated to reconstruct regional climate change and its dynamical links to Indian summer monsoon and solar insolation forcing. The results suggest that during glacial periods, this region is characterized by reduced vegetation cover (e.g., low total pollen concentration) and enhanced physical weathering (e.g., high Rb/Sr ratio), whereas during interglacial times, vegetation cover was extensive and chemical weathering is relatively strong around Heqing basin. Good correlation between variations in proxy indicators from Heqing core and stacked Indian summer monsoon record from Arabian Sea (Clemens and Prell, 2003) indicates that over the last 0.35 Myr, Heqing basin is predominantly influenced by Indian summer monsoon. Unlike Indian monsoon records from the Arabian Sea and Indian Ocean which resolution is relatively low, our high-resolution proxy variations permit a robust understanding of the Indian summer monsoon variations over the last 2.6 Myr. Comparisons of monsoon proxies from land and ocean indicate that solar insolation is the dominant factor controlling the Indian summer monsoon variation, particularly low-latitude radiation difference between northern and southern Hemisphere. However, an evident 100-kyr cycle occurred around 1.2 Myr and became remarkable after 0.5 Myr, implying that global ice volume might have a significant influence on the monsoon records in Heqing basin since at least mid- Pleistocene.
PP24A-02
Variations of East Asian Summer Monsoon Deduced from the Results of Mg/Ca and Oxygen Isotope of Planktic Foraminifera in the East China Sea Since the Last Deglaciation
Variability and extremity of the East Asian summer monsoon (EASM) is of prime importance because one third of the world population lives in the area strongly influenced by EASM. Previous studies in China demonstrated that the timing of the precipitation maximum is different from place to place through the Holocene. However, most of the studies about EASM intensity focused on the terrestrial record, and EASM intensity has never been reconstructed continuously with high-resolution from marine record. The modern sea-surface salinity (SSS) in the northern part of the East China Sea (ECS) reflects the volume of the Changjiang (Yangtze River) discharge. Because the drainage area of the Changjiang occupies a major part of South China where EASM has a storong influence, the discharge of the Changjiang during summer is considered as a good indicator of EASM intensity. Piston core KY0704 PC-1 (128°56E, 31°38N), which we used in this study, was retrieved from the northern part of the ECS at a water depth of 758m. In order to reconstruct paleo-SST and paleo-SSS, the planktic foraminifera Globigerinoides ruber was used for analyses of oxygen isotope (δ18Oruber) and Mg/Ca. We estimated the oxygen isotope ratio of the ambient past seawater (δ18Osw) from δ18Oruber and Mg/Ca- derived SST. The result suggested that SST in the studied location increased by 3-4°C while δ18Osw decreased by approximately 1.2 permil from the last glacial to the Holocene. Millennial-scale variations between relatively warm and saline water and relatively cold and less saline water were recognized, suggesting the changes in the mixing ratio between the Changjiang Diluted water and the Kuroshio water during the Holocene. In order to remove the influence of the Kuroshio water, we subtracted δ18Osw of the central Okinawa Trough studied by Sun et al.(2005) from δ18Osw of our site. The difference of δ18Osw (δ18Odif) can be regarded as reflecting changes due to the influence of dilution of Changjiang discharge. Comparison of δ18Odif with the Holocene lake level record by An et al.(2000) revealed that δ18Odif changes showed a trend similar to the lake level changes in the middle and lower reaches of the Changjiang. This suggested that the δ18Osw record in the northern ECS reflects precipitation changes in the drainage basin of the middle and lower reaches of the Changjiang river. Therefore changes in the EASM intensity are recorded with high-resolution in marine cores from the northern ECS.
PP24A-03 INVITED
Spatial Reconstructions of Asian Monsoon Climate Variability Over the Past Millennium from Long Tree-Ring Records
We present the first spatial reconstructions of Asian monsoon climate variability over the past millennium from long tree-ring records. The reconstructions, for both the monsoon (JJA) and pre-monsoon (MAM) seasons, are based on a 534-point grid of instrumental Palmer Drought Severity Indices (PDSI) covering all of monsoon Asia and an irregular network of 312 annual tree-ring chronologies over most of the same domain. The seasonal reconstructions were initially estimated at each grid point using a local "point-by-point regression" (PPR) method that has been used successfully in reconstructing drought over North America. Different levels of predictor variable screening applied in PPR produced a 5-member ensemble of reconstructions for each season. The estimated noise level in these reconstructions was relatively high (average cross-validation R2 over the 534 grid point domain typically <0.30). In addition, the lengths of the grid point reconstructions varied over space due to the variable length tree-ring series available for use in PPR. For these reasons, each ensemble member was iteratively refined using a local variant of PPR to improve its reconstructions, with missing values imputed as necessary, to produce complete fields extending back to AD 1000 over all 534 grid point locations. An ensemble average for each season, with estimated uncertainties, was then calculated and used for analysis. The reconstructions reveal the occurrence of some persistent "megadroughts" in the past that appear to be unprecedented in the instrumental records. These megadroughts are not restricted to any particular part of "Monsoon Asia", but the ones in Southeast Asia stand out particularly strong. Comparisons made between these drought reconstructions and a companion field of SST reconstructions for the tropical Pacific back to AD 1400, based on independent tree-ring data from the American Southwest and Mexico, suggest that unusual ENSO variability is a contributor to the development of past severe droughts in monsoon Asia. An association between monsoon drought variability and explosive volcanism is also indicated. Implications for future work, especially related to expanding and improving the tree-ring network for reconstruction in certain regions, are also discussed.
PP24A-04 INVITED
Records from Lake Qinghai: Holocene climate history of Northeastern Tibetan Plateau linking to global change
Lake Qinghai (99°36'-100°16'E, 36°32'-37°15'N ) of the north eastern margin of Tibet Plateau is the largest inland lake of China. It sits on the transitional zone of Asian monsoon- arid areas, receives influences of Asian monsoons and Westerlies, thus sensitive to global climate changes. Although previous studies had investigated Holocene climate change of Lake Qinghai area, it is rare to see precise Holocene climatic sequences of Lake Qinghai, nor in-depth discussions on controlling factors of Lake Qinghai climate changes. In Year 2005, with support from ICDP, Chinese Academy of Sciences (CAS), Chinese Ministry of Science and Technology (MOST) and National Science Foundation of China (NSFC), Drilling, Observation and Sampling of the Earths Continental Crust Corporation (DOSECC) and Institute of Earth Environment, Chinese Academy of Sciences (IEECAS) took a series of shallows cores from the southern basin of Lake Qinghai. West sub-basin sediments display Holocene lacustrine feature for the upper 5m, while the 5-18m are interbeded sediments of shallow lake, eolian-lacustrine and eolian loess. Chinese and US scientists with support from NSFC, MOST, CAS and NSF analysed 1F core from west sub-basin depocenter of the south basin with multiple physical, chemical, biological approaches. By comparing with modern process observation records, we obtained proxies that respectfully reflect precipitation, temperature and lake salinity changes, etc., reconstructed high resolution time sequences of magnetic susceptibility, colour scale, grain size, Corg, C/N, δ13Corg, carbonate, δ13C and δ18O of carbonate and ostracodes, elements, char-soot,Uk'37 and %C37:4 as well as pollen of the last 13Ka. They indicate the climatic change history of Lake Qinghai since past 13Ka, and agreeable evidences are found from adjacent tree ring and stalagmite records. Comparison of Lake Qinghai Holocene climate change sequence with those from high altitude ice core, stalagmites and ocean records for East Asian monsoon and Indian monsoon show that, in accordance with Asian monsoon climate changes, at 11-5ka cal. 14C BP Lake Qinghai revealed the warm and humid Optimal climate, while since 5ka cal.14C BP the Lake showed relatively cold and dry climate of New Glaciation, this orbital climate trend resembled northern hemisphere summer solar insolation changes. Lake Qinghai millennial-centennial climate events in Holocene are linked with Westerlies changes, and with East Asian summer monsoon front shift as well as winter monsoon, on centennial-decadal scale Lake Qinghai climate changes are controlled more by solar activities.
PP24A-05
Peat Plant-Wax n-Alkane D/H Ratios Document Early Holocene Arrival of Indian Ocean Monsoon in Southern China
Changes in climate are recorded in organic compounds present in the peatlands that developed in temperate northern latitudes after post-glacial continental wetness increased. Peat sequences in China yield evidence of changes in moisture delivery by the Asian monsoon since 15 ka from molecular and isotopic differences in plant-derived peat layers. Proportions of submergent and emergent plants vary in peat bogs as water levels rise and fall in response to changes in precipitation and its temperature-sensitive evaporation. Changes in the water level are recorded in the relative abundances of C23 and C25 submergent-plant n-alkanes and C27, C29, and C31 emergent-plant n-alkanes. Compound-specific D/H analyses of these plant-wax n- alkanes improve our understanding of past changes in delivery of Asian monsoonal precipitation and in temperature. n-Alkane molecular analyses reveal that the warmer climate of the Holocene Climatic Optimum (10.5 to 6 ka) was accompanied by increases in monsoonal precipitation that raised the water level of the Hongyuan bog on the northeast edge of the Tibetan Plateau. At the same time, the climate at Hani in northeast China became less-wet. Comparison of n-alkane delD values from the Hongyuan and Hani bogs reveals that both regions received moisture predominantly from the East Asian Monsoon until ca 10.5 ka. However, after this time, delivery of isotopically light Indian Ocean Monsoon moisture was found to dominate over the Tibetan Plateau, signaling an important change in the Asian monsoon system. Indian Ocean moisture remained dominant at Hongyuan through the late Holocene, although increases in n-alkane delD values indicate that its climate shifted to less wet, implying diminished monsoonal precipitation on the Tibetan Plateau.
PP24A-06 INVITED
Reconciling Cave, Marine, and Loess Proxies for Summer Monsoon Strength at the Precession Band
Cave speleothem δ 18O records from southeastern China have been interpreted as the ratio between summer and winter monsoon precipitation (Cheng et al. 2006, Geology v34; Wang et al. 2001, Science v294), the removal of water vapor from air masses between the tropical Indo-Pacific and southeastern China (Yuan et al. 2004, Science v304), and changes in East Asian summer monsoon intensity as a direct response to Northern Hemisphere summer insolation (Wang et al. 2008, Nature v451). The latter two interpretations are inconsistent with a multiproxy phase analysis of 15 other published summer monsoon proxies from the Arabian Sea, the South China Sea, and the Chinese Loess Plateau. The interpretation involving the ratio of summer- and winter-monsoon precipitation (Cheng et al. 2006 and Wang et al. 2001), although simplified, is more consistent with the modern seasonal δ 18O of precipitation in southeastern China and can be reconciled with precession-band phase results from other marine and terrestrial proxy data as follows. Multiproxy analysis of 16 published proxies for late Pleistocene Indian and East Asian summer monsoon strength indicate that 15 of the proxies cluster about a phase of - 124±18° relative to Precession minimum (ω = 90°; maximum NH Summer insolation). One proxy, Hulu-Sanbao cave δ 18O, is an outlier, plotting with phase of -42±5°. This apparent phase discrepancy is reconciled by interpreting the cave δ 18O as a mix of isotopically distinct seasonal precipitation sources. GNIP and NCAR/NCEP reanalysis data indicate three seasonal wind and precipitation regimes, each with a distinct isotopic composition. Summer monsoon months (JJA) are characterized by southerly flow from the South China Sea, account for 51% of the total annual precipitation, and have an average δ 18Oprecip of -9.5 ‰. Spring intermonsoon months (AM) are characterized by easterly flow off the Pacific, account for 14% of the total annual precipitation, and have an average δ 18Oprecip of -3.7 ‰. The remaining fall and winter months (SONDJFM) are characterized by north-northwesterly flow from the continental interior, account for 35% of the total annual precipitation, and have an average δ 18Oprecip of -7.2 ‰. Isotopically-weighted precipitation models indicate that all three seasonal precipitation sources are required to account for the full range exhibited in the Hulu-Sanbao composite δ 18O record. On this basis, interpreting the Hulu-Sanbao δ 18O record as a direct summer monsoon proxy is unjustified (Wang et al., 2008), a conclusion supported by the phase discrepancy between Hulu-Sanbao δ 18O and the 15 other summer monsoon proxies. A simplified vector addition model incorporating the amplitude and phase of summer (-124°) and winter (0°) forcing vectors predicts a precession-band phase of - 63° for the Hulu-Sanbao record when based on modern summer and winter GNIP precipitation weightings (50% each). The measured phase (-42°) indicates a 60% contribution of winter precipitation and 40% summer precipitation, averaged over the past 224,000 years, consistent with the Wang et al. (2001) findings. Thus, the precession-band phase of Hulu-Sanbao δ 18O differs from the 15 other marine and terrestrial summer monsoon proxies because Hulu-Sanbao δ 18O incorporates, at a minimum, both summer and winter monsoon signals, each with different amplitudes and phases.
PP24A-07
Holocene Increase in the Bay Branch of the Monsoon Inferred From δ18O of Speleothems From Central India
Monsoon rainfall reconstructed using stable oxygen isotope ratio (δ18O) of two 230Th-dated stalagmites from the Dandak cave, central India, spanning three epochs during the Holocene (the present to ~1.6 kyr BP, ~2.8 to 5.6 kyr BP and ~9.1 to 10.4 kyr BP) indicate an increasing trend, in contrast to the reduction in (i) the boreal summer insolation and (ii) inferred monsoon rainfall using speleothems from Oman and China. The observed increase in geomagnetic shielding of cosmic rays during the Holocene probably decreased global cloudiness, thus increasing Sea Surface temperatures; the Bay branch of the monsoon, fed mainly be large depressions, could have enhanced from such an effect. Shorter- term monsoon fluctuations during the early Holocene are relatable to solar variability. Although human habitations were significantly influenced by the vagaries of the monsoon, they appear to have survived all through, as evidenced by radiocarbon-dated charcoal collected from the caves.
PP24A-08 INVITED
New 230Th dating methods applied to Chinese Caves: the Asian Monsoon on glacial to cultural timescales
We present a 350,000-year record of the oxygen isotopic composition of cave calcite from from Hulu, Dongge, Sanbao, and Wanxian Caves. The record is improved in resolution (oxygen isotope resolution: 3 y - decades), range, and dating precision over published results. It is, in essence, a history of the oxygen isotopic composition of meteoric precipitation, which is related to monsoonal precipitation. Portions of the chronology were established with new generation 230Th dating methods employing multi-collector, inductively-coupled plasma mass spectrometry, capable of yielding precisions in age (on ideal cave samples) of ±1 y at 1000 y, ±10 y at 10 ky, ±100 y at 100 ky, and ±10 ky at 640 ky, with a range well in excess of 700 ky. Key points include high ionization/transmission efficiency for U and Th (1-2%) yielding high precision on calcite with low U concentrations and new half-life values for 234U and 230Th. The monsoon is dominated by orbital-scale variability throughout, and millennial-scale variability during glacial periods. The monsoon follows northern summer insolation with no discernable phase shift, supporting a direct link between seasonal heating and the monsoon. At millennial scales, the last glacial record correlates strikingly with that observed in Greenland, with Chinese correlatives to all 25 Greenland interstadial events, and broadly similar sequences observed for penultimate and antepenultimate glacial periods. The Holocene monsoon correlates significantly with proxies for solar irradiance, linking some monsoon variability to changes in the sun. The highest resolution portion of the record (last 1800 y) establishes links between the cultural history of China and climate. The monsoon exhibits remarkable relationships with atmospheric methane (tied to low-latitude wetland methane production), the isotopic composition of atmospheric O2 (linking low latitude hydrology to the Dole Effect), and Heinrich Events in the North Atlantic. These links allow correlations among ice core, marine, and monsoon records, thereby establishing, for key periods, the timing and sequence of events recorded around the globe in different surface environments. Using these strategies, we have determined the timing and sequence of events during the terminations of the last 4 glacial cycles. Our correlations place new constraints on the causes of glacial terminations, a problem that still remains unsolved despite decades of study. The relationships that we observe suggest that terminations are triggered by small orbitally-induced changes in the seasonal distribution of insolation, coupled with multiple positive feedbacks within the climate system.