Mid-Holocene Climate Variations Recorded by Palaeolake in Marginal Area of East Asian Monsoon： A Multi-proxy Study

Traditionally, the mid-Holocene in most parts of China was thought to be warmer with higher precipitation, resulting from a strong Asian summer monsoon. However, some recent researches have proposed a mid-Holocene drought interval of millennial-scale in East Asian monsoon margin areas. Thus whether mid-Holocene was dry or humid remains an open issue. Here, Zhuyeze palaeolake, the terminal lake of the Shiyang River Drainage lying in Asian monsoon marginal areas, was selected for reconstructing the details of climate variations during the Holocene, especially mid-Holocene, on the basis of a sedimentological analysis. Qingtu Lake (QTL) section of 6.92m depth was taken from Zhuyeze palaeolake. Multi-proxy analysis of QTL section, including grain size, carbonate, TOC, C/N and δ13C of organic matter, was used to document regional climatic changes during 9-3 cal ka B.P. The record shows a major environmental change at 9.0-7.8 cal ka B.P., attributed to a climate trend towards warmth and humidity. This event was followed by a typical regional drought event which occurred during 7.8-7.5 cal ka B.P. And a warm and humid climate prevailed from 7.5 to 5.0 cal ka B.P., attributed to the warm/humid Holocene Optimum in this region. After that, the climate gradually became drier. Moreover, comparison of the climate record from this paper with the summer insolation at 30°N indicates that the climate pattern reflecting the Asian monsoon changes was caused by insolation change.     

Millennial-centennial scales climate changes of Holocene indicated by magnetic susceptibility of high-resolution section in Salawusu River valley,China

The upmost segment(Holocene series)of the Milanggouwan stratigraphic section(MGS1)in the Salawusu River valley shows 11 sedimentary cycles of dune sands and fluvio-lacustrine facies,or dune sands and paleosols.The analysis of the magnetic susceptibility of this segment suggests that there are 11 magnetic susceptibility cycles with the value alternating from low to high,in which the layers of the dune sands correspond to the lower value of the magnetic susceptibility and the layers of fluvio-lacustrine facies and paleosols correspond to the higher peaks.The study reveals that the low and high magnetic susceptibility values indicate the climate dominated by cold-arid winter monsoon and warm-humid summer monsoon of East Asia,respectively,and the study area has experienced at least 22 times of mil-lennial-centennial scales climate alternation from the cold-arid to the warm-humid during the Holocene.In terms of the time and the climate nature,the variations basically correspond to those of the North Atlantic and some records of cold-warm changes in China as well.They might be caused by the alternation of winter and summer monsoons in the Mu Us Desert induced by global climate fluctuations in the Holocene.     

用大气热源表征的东亚夏季风指数的研究

利用1965—2007年NCEP／NCAR再分析资料和同期的中国160站降水资料,在讨论大气热源的气候特征基础上,用整层积分的大气热源定义了一个东亚夏季风指数,并用该指数研究了东亚夏季风和中国气候的关系。研究表明：定义的大气热源季风指数能反映夏季风的异常变化,高（低）指数年对应的东亚夏季风偏强（弱）;该指数与长江中下游降水存在高度的同期负相关,对长江中下游夏季降水有较强的分辨能力。     

用大气热源表征的东亚夏季风指数的研究

利用1965—2007年NCEP／NCAR再分析资料和同期的中国160站降水资料,在讨论大气热源的气候特征基础上,用整层积分的大气热源定义了一个东亚夏季风指数,并用该指数研究了东亚夏季风和中国气候的关系。研究表明：定义的大气热源季风指数能反映夏季风的异常变化,高（低）指数年对应的东亚夏季风偏强（弱）;该指数与长江中下游降水存在高度的同期负相关,对长江中下游夏季降水有较强的分辨能力。     

EVIDENCE FOR ABRUPT CLIMATIC CHANGES ONNORTHWESTERN MARGIN OF EAST ASIAN MONSOON REGION DURING LAST DEGLACIATION

Based on investigations of the Zhongwei Nanshan aeolian section situated in the southeastern margin of Tengger Dcsert, carbon-14 and TL (thermoluminescence) dating results and paleoclimatic proxies such as magnetic susceptibility and grain size, we inferred that the northwestern margin of East Asian monsoon region experienced abrupt climatic changes during the last deglaciation. Six oscillation events were identified: Oldest Dryas, Belling, Older Dryas, AllerФd, lntra-AllerФd Cold Period (1ACP) and Younger Dryas (YD). The summer monsoon was weaker during Oldest Dryas and Younger Dryas when the winter monsoon was stronger. However, during the B/A (BФlling/AllerФd) period, the summer monsoon strengthened, reflected by magnetic susceptibility, when the winter monsoon also became strong, which is different from the paleoclimatic pattern established in the East Asian monsoon region. Furthermore,the summer monsoon was nearly in phase with the climate changes inferred from the oxygen isotopic records of Greenland ice cores. It could be speculated that the variations of the sea ice cover in the high latitudes of the North Hemisphere affected the high pressure of Asian continent and the changes of the winter monsoon inland. On the other hand,the sea ice cover variations might have indirectly caused the occurrence of ENSO events that has tightly been related to the summer monsoon in northwest margin of East Asian monsoon region.     

Anti-Phase Relationship Between the East Asian Winter Monsoon and Summer Monsoon During the Holocene?

The relationship between the East Asian winter monsoon (EAWM) and East Asian summer monsoon (EASM) during the Holocene is complicated and remains controversial. In this study, analysis of grain size and benthic foraminiferal oxygen isotope, as well as accelerator mass spectrometry ¹⁴C dating was performed on a sediment core retrieved from the newly revealed muddy deposit on the northern South China Sea continental shelf. The history of the EAWM and EASM were reconstructed for the last 8200 a BP. Further analysis in conjunction with previously published paleo-climate proxies revealed that the relationship between the EAWM and EASM during the Holocene is more complex than a simple and strict anti-phase one-both negative and positive correlations were identified. The EAWM and EASM are negatively correlated around 7500, 4800, 4200, 3200, and 300 a BP (cooling periods), while positively correlated around 7100, 3700, and 2100 a BP (warm periods). In particular, both the EAWM and EASM intensified during the three positive correlation periods. However, we also found that the relationship between these two sub-monsoons is anti-phase during the final phase of particularly hot periods like Holocene Optimum and Medieval warm period. The possible impact from variations of solar irradiance on the relationship between the EAWM and EASM was also discussed.     

南海北部近海柱状沉积物多环芳烃组成及来源分析

【目的】研究南海北部近海区域柱状沉积物多环芳烃组成及分布特征,讨论全新世早期火历史及气候变化。【方法】利用AMS ¹⁴C定年技术结合有机地球化学分析手段对全新世早期南海北部近海沉积物柱状样品中多环芳烃(PAHs)分布特征进行研究。【结果】南海北部近海沉积物中16种PAHs总浓度范围为8.58~17.48 ng/g,在约10000 a B.P.的全新世早期呈现先增大后减小波动变化,与TOC变化基本同步。【结论】沉积物中多环芳烃主要来源于南海北部近海陆源区域自然火灾产生的焦炭残渣。PAHs的沉积浓度变化间接指示了全新世早期东亚季风的强度变化。     

Sedimentary Characteristics of the Second Marine Layer During the Late Marine Isotope Stage 3 in Southern Yellow Sea and Their Response to the East Asian Monsoon

We use the particle size of sediments in core YS01 A to study the sedimentary environment of the mud deposit in the central South Yellow Sea of China during late Marine Isotope Stages 3(MIS3;40.5 kyr–31.3 kyr).In addition,the East Asian Monsoon and its relationship with the North Atlantic Ocean climate change are discussed based on the sensitive grain-size calculation and the spectrum analysis.The results show that during late MIS3,the muddy area in the central South Yellow Sea experienced the evolution of coastal facies,shallow marine facies,coastal facies,and continental facies,with weak hydrodynamic conditions.Compared with other climate indicators,we found that there were many century to millennium-scale climate signals documented in the muddy area sediments in the central South Yellow Sea.According to our particle size results,three strong winter monsoon events occurred at 37.6 kyr,35.6 kyr and 32.2 kyr.The East Asian Winter Monsoon records in core YS01 A are consistent with the Greenland ice core and the Hulu cave stalagmite δ~(18)O.The millennial and centennial scale cycles,which are 55 yr,72 yr,115 yr,262 yr respectively,correspond to solar activity cycles,while the 1049 yr and 2941 yr cycles correspond to the Dansgaard-Oeschger cycles.These cycles indicate that the paleoclimate evolution of the area was controlled by the solar activities,with the high-latitude driving thermohaline circulation as the main energy conveyor belt,followed by the sea-air-land amplification of the winter monsoon variation in the central Yellow Sea in the late MIS3.     

A singular spectrum analysis on Holocene climatic oscillation from lake sedimentary record in Minqin Basin, China

The total organic carbon (TOC) content series from the lake sediment of Minqin Basin (100°57′–104°57′E, 37°48′–39°17′N) in northwestern China, which has a 10 000-year-long paleo-climatic proxy record, was used to analyze the Holocene climate changes in the local region. The proxy record was established in the Sanjiaocheng (SJC), Triangle Town in Chinese, Section (103°20′25″E, 39°00′38″N), which is located at the northwestern boundary of the present Asian summer monsoon in China, and is sensitive to global environmental and climate changes. Applying singular spectrum analysis (SSA) to the TOC series, principal climatic oscillations and periodical changes were studied. The results reveal 3 major patterns of climate change regulated by reconstructed components (RCs). The first pattern is natural long-term trend of climatic change in the local area (Minqin Basin), indicating a relatively wetter stage in early Holocene (starting at 9.5 kaBP), and a relatively dryer stage with a strong lake desiccation and a declined vegetation cover in mid-Holocene (during 7–6 kaBP). From 4.0 kaBP to the present, there has been a gradually decreasing trend in the third reconstructed component (RC3) showing that the local climate changed again into a dryer stage. The second pattern shows millennial-centennial scale oscillations containing cycles of 1 600 and 800 years that have been present throughout almost the entire Holocene period of the last 10 000 years. The third pattern is a millennial-centennial scale variation with a relatively smaller amplitude and unclear cycles showing a nonlinear interaction within the earth’s climate systems.     

Characteristics of Thermal and Geopotential Height Differences Between Continent and Ocean and Its Role in the Strength of the Asian Summer Monsoon

The interdecadal factors affecting the summer monsoon winds over Somalia and the South China Sea were studied. Global geopotential heights and wind velocity fields of the 850-hPa and 200-hPa pressure levels, as well as sea surface temperature anomaly data and correlation coefficients were analyzed. The monsoons over Somalia and the South China Sea were found to be two different monsoon systems, operating on different mechanisms and being affected by different ocean-atmosphere interactions. The intensity of the Asian subtropical summer monsoon is influenced by the intensity of the summer monsoon over Somalia in the month of June and by the intensity of the summer monsoon over the South China Sea in the months of June and July. The summer monsoon wind strength over Somalia is affected by regional factors, such as the heating of the Tibetan plateau, and by global mechanisms, such as the subtropical heat exchange with Antarctica. The summer monsoon over the South China Sea is affected by different ocean-atmosphere interactions. The Somalia and subtropical summer monsoons have wind blowing down the pressure gradient from area over ocean to that over land, like typical summer monsoons. The South China Sea summer monsoon has winds that blow down the pressure gradient from area over land to that over ocean. The South China Sea summer monsoon is affected by the Kuroshio Current off the east coast of Japan.     

陕西榆林风沙滩区全新世气候和环境变迁

榆林长城沿线处于环境变化敏感带,其风沙滩区沉积剖面的磁化率和粒度分析结果显示,该区全新世早期11 500~10 000 a BP随着全球气候的逐渐转暖,磁化率、黏粒和粉沙含量逐渐增高;全新世中期的10 000~4 500 a BP气候相对比较湿润,植被覆盖度增大,风沙活动减弱,地表沉积物当中粉沙和黏土成分含量显著增大,风化成壤作用增强,地表发育沙质弱古土壤;全新世中期后段的4 500~3 000 a BP期间,气候以比较温和湿润为主,但在4 200~4 000 a BP出现一个气候干旱事件,风沙活动强烈;近3 000 a以来,气候显著干旱化,波动频繁,地表风沙沉积物显著变粗,表现出有5次沙漠扩张。该区生态环境演化、人文历史变迁基本上是自然因素与人为活动综合作用的结果,以自然演化为主,近代人为因素对环境变化起着加强和放大作用。     

东亚季风区石笋δ18O解译:基于夏季风与夏季风降雨的思考

近些年,对于东亚季风区石笋δ18O的气候环境指示意义的争论较多,主要在东亚季风区石笋δ18O代表夏季和风强度、夏季风降水还是水汽源变化。基于中国东部华北地区降水与长江中下游地区降水反相变化和长江中下游地区降水与菲律宾海降水反相变化(遥相关),从年际-年代际到千年-轨道尺度对石笋δ18O与夏季风降水、厄尔尼诺-南方涛动(ENSO)的相互关系进行了探讨分析。通过对比石笋δ18O记录与华北和梅雨区降水,发现石笋δ18O偏负对应华北降水增加,梅雨区降水减少;石笋δ18O偏正对应华北降水减少,梅雨区降水增加。这种对应关系不仅存在年际-年代际尺度,而且在千年-轨道尺度同样存在,石笋δ18O不仅反映夏季风强弱变化,同时与中国东部区域降水关系是明确对应的。通过降水的空间相互关系,发现ENSO活动主要通过影响中国东部降水的空间分布格局而作用于石笋δ18O。La Ni?a态导致南海及菲律宾海对流加强,西太副高位置偏北,长江中下游地区梅雨期缩短,华北夏季降水增加,东亚季风区石笋δ18O偏负。El Ni?o态,南海和菲律宾海对流受到抑制,西太副高位置南移,长江中下游地区梅雨期延长,华北夏季降水减少,东亚季风区石笋δ18O偏正。另外,水汽源分析发现,菲律宾海水汽输送对东亚季风区降水及降水δ18O贡献相对较小。因此,综合分析认为,东亚季风区石笋δ18O主要反映了亚洲夏季风的强弱变化。      

Difference in the influence of Indo-Pacific Ocean heat content on South Asian Summer Monsoon intensity before and after 1976/1977

Monthly ocean temperature from ORAS4 datasets and atmospheric data from NCEP/NCAR Reanalysis I/II were used to analyze the relationship between the intensity of the South Asian summer monsoon(SASM) and upper ocean heat content(HC) in the tropical Indo-Pacific Ocean.The monsoon was differentiated into a Southwest Asian Summer Monsoon(SWASM)(2.5°–20°N,35°–70°E) and Southeast Asian Summer Monsoon(SEASM)(2.5°–20°N,70°–110°E).Results show that before the 1976/77 climate shift,the SWASM was strongly related to HC in the southern Indian Ocean and tropical Pacific Ocean.The southern Indian Ocean affected SWASM by altering the pressure gradient between southern Africa and the northern Indian Ocean and by enhancing the Somali cross-equatorial flow.The tropical Pacific impacted the SWASM through the remote forcing of ENSO.After the 1976/77 shift,there was a close relationship between equatorial central Pacific HC and the SEASM.However,before that shift,their relationship was weak.     

Holocene abrupt climate shifts and mid-Holocene drought intervals recorded in Barkol Lake of Northern Xinjiang of China

Study results in this paper have indicated that the Holocene climate in Xinjiang, Northwestem China has been alternating between wet and dry conditions, and was punctuated with a series of abrupt climate shifts. A sediment core taken from Barkol Lake in the northern Xinjiang of Northwest China was analyzed at 1 cm interval for grain-size distribution. Abrupt climate shifts revealed by the grain-size proxy occurred at ca 1.4, 3.0, 4.3, 5.6, 8.0 cal kyr B.E, which were well correlated to both the abrupt shifts recorded in the North Atlantic Ocean （NAO） and the Holocene sea surface temperature （SST） cooling events in the Arabian Ocean. The correlation indicated that the climatic changes in the extreme arid Northwest China were associated with the NAO, probably via the North Atlantic Oscillation-affected westerly winds. The strength and position of westerly winds probably modulated the Siberian-Mongolian high- pressure system （winter monsoon）, and played an important role in climate change of Northwest China. Moreover, an evident drought interval during the middle Holocene was also revealed by grain-size proxy.     

天水谢家湾全新世环境变迁及人地关系演化

在野外详细观测的基础上，对天水谢家湾全新世黄土剖面以2cm为间距进行了连续采样，并对所有样品作了磁化率和粒度分析，结果综合反映出研究区自全新世以来，经历了早期升温、中期基本暖湿、晚期趋于干冷的气候环境演化阶段，其中全新世中期在总体温湿的大背景下，存在频繁的气候波动。以此为基础，结合对渭河文化的历史研究资料，探讨了区内约1万年以来的人地关系演化历史，指出人地关系的和谐与统一，对人类社会发展具有重要意义。     

RELATIVE BRIGHTNESS INDEX AND IT‘S CLIMATIC SIGNIFICANCE FROM LACUSTRINE SEDIMENT OF NAPAHAI LAKE，NORTHWESTERN YUNNAN PLATEAU，CHINA

Information on the palaeoenvironment from Late Pleistocene to Holocene in northwestern Yannan Plateau has been deduced from a study of a 28.81m-long core taken from Napahai Lake.The results from Relative Brightness In-dex(RBI) as well as those from the lithological analyses of bulk sediments,total organic carbon and granulometric analy-ses have been used to reconstruct the environmental and climatic evolution of the area.The ages were provided by three ^14C datings.The record suggested a climate fluctuation between warm-dry and cool-wet from ca.57 to 32ka B.P.which led a shallowing and swamping of the lake.The water level again increased quickly at ca.32ka B.P.,reached itˊs peak during LGM(Last Glacial Maximum,ca.18-20kaB.P.)and remained relative high until ca.15kaB.P.The high wa-ter level at LGM is attributed to cold-wet conditions.The area experienced an abrupt and unstable climatic changes dur-ing the transition period form 15 to 10ka B.P.with a dominated littoral environment.Awarm-dry climate led to the contrac-tion of the lake during the Holocene and reed-swamps became dominant..After a minor wet-cool pulse during the Late Holocene,the modern climate became to be established.     

Middle Holocene Organic Carbon and Biomarker Records from the South Yellow Sea: Relationship to the East Asian Monsoon

The East Asian monsoon system influences the sedimentation and transport of organic matter in East Asian marginal seas that is derived from both terrestrial and marine sources. In this study, we determined organic carbon (OC) isotope values, concentrations of marine biomarkers, and levels of OC and total nitrogen (TN) in core YSC-1 from the central South Yellow Sea (SYS). Our objectives were to trace the sources of OC and variations in palaeoproductivity since the middle Holocene, and their relationships with the East Asian monsoon system. The relative contributions of terrestrial versus marine organic matter in core sediments were estimated using a two-end-member mixing model of OC isotopes. Results show that marine organic matter has been the main sediment constituent since the middle Holocene. The variation of terrestrial organic carbon concentration (OCter) is similar to the EASM history. However, the variation of marine organic carbon concentration (OCmar) is opposite to that of the EASM curve, suggesting OCmar is distinctly influenced by terrestrial material input. Inputs of terrestrial nutrients into the SYS occur in the form of fluvial and aeolian dust, while concentrations of nutrients in surface water are derived mainly from bottom water via the Yellow Sea circulation system, which is controlled by the East Asian winter monsoon (EAWM). Variations in palaeoproductivity represented by marine organic matter and biomarker records are, in general, consistent with the recent EAWM intensity studies, thus, compared with EASM, EAWM may play the main role to control the marine productivity variations in the SYS.     

The study on climatic changes and lake level fluctuations of Qinghai lake in holocene

According to loess and palaeosol climatic record, field observation, analysis data and¹⁴C dating, we discuss the climatic changes and the water level fluctuations of Qinghai Lake. It is pointed out that there were four relatively warm and moist stages in Qinghai Lake basin during the Holocene. They formed in the periods from 10,300 yr.B.P. to 8,500 yr.B.P., 7,000 yr.B.P. to 3,500 yr.B.P., 2,800 yr.B.P. to 2,000 yr.B.P. and from 1,300 yr.B.P. up to now. the climate in the Holocene optimum period, from 7,000 yr.B.P. to 3,500 yr.B.P., was much warmer and moister than that today. Polypodium plant grew luxuriantly around Qinghai Lake. The annual temperature was 2.5°C higher than that today, but there was no forest at Qinghai Lake shore. It is found that there was a good relationship between precipitation and water level fluctuation. In warm and moist period water level was high and in the cold and dry period it was low in the Holocene. There were four high water level periods for Qinghai Lake in the Holocene, but the highest level was less than 30m above present water level. This research is supported by Youth Foundation of Natural Science Foundation of China (No. 49101015).     

Major mid-late Holocene cooling in the East China Sea revealed by an alkenone sea surface temperature record

Although the mid-late Holocene cold and dry event about 4000 years ago(the 4 ka event) has been observed almost globally, it was most prominent in terrestrial climate proxies from the lower latitudes. Here we evaluate the oceanic response to this event in terms of a Holocene sea surface temperature(SST) record reconstructed using the K'37U index for Core B3 on the continental shelf of the East China Sea. The record reveals a large temperature drop of about 5℃ from the mid-Holocene(24.7℃ at 5.6 ka) to the 4 ka event(19.2℃ at 3.8 ka). This mid-late Holocene cooling period in Core B3 correlated with(i) decreases in the East Asia summer monsoon intensity and(ii) the transition period with increased El Nino/Southern Oscillation activities in the Equatorial Pacific. Our SST record provides oceanic evidence for a more global nature of the mid-late Holocene climate change, which was most likely caused by a southward migration of the Intertropical Converge Zone in response to the decreasing summer solar insolation in the Northern Hemisphere. However, the large SST drop around Core B3 indicates that the mid-late Holocene cooling was regionally amplified by the initiation/strengthening of eddy circulation/cold front which caused upwelling and resulted in additional SST decrease. Upwelling during the mid-late Holocene also enhanced with surface productivity in the East China Sea as reflected by higher alkenone content around Core B3.     

Does the Asian monsoon modulate tropical cyclone activity over the South China Sea?

To investigate whether the Asian monsoon influences tropical cyclone (TC) activity over the South China Sea (SCS), TCs (including tropical storms and typhoons) over the SCS are analyzed using the Joint Typhoon Warning Center dataset from 1945 to 2009. Results show an increasing trend in the frequencies of TC-all (all TCs over the SCS) and TY-all (all typhoons over the SCS), due mainly to an increase in the number of TCs moving into the SCS after development elsewhere. Little change is seen in the number of TCs that form in the SCS. The results of wavelet analysis indicate that the frequency of typhoons (TY) shows a similar oscillation as that of TCs, i.e., a dominant periodicity of 8-16 years around the 1970s for all TC activity, except for TC-mov (TCs that moved into the SCS from the western North Pacific). To examine the relationship between typhoon activity and the summer monsoon, a correlation analysis was performed that considered typhoons, TCs, and five monsoon indexes. The analysis reveals statistically significant negative correlation between the strength of the Southwest Asian summer monsoon and typhoon activity over the SCS, which likely reflects the effect of the monsoon on TC formation in the western North Pacific (WNP) and subsequent movement into the SCS. There is a statistically significant negative correlation between TY-loc (typhoons that developed from TCs formed over the SCS) and the South China Sea summer monsoon and Southeast Asian summer monsoon.