黄土-红粘土化学风化与晚新生代气候变化——甘肃平凉白水剖面为例

Late Cenozoic has witnessed a series of climate-environmental change which ends with a transitional shift from greenhouse to icehouse conditions. In last two decades, scientists began to employ the tectonic uplift and its weathering effect to interpret the climatic changes during the late Cenozoic. However, this endeavor has partly been restricted by the lacking of regional and global chemical weathering data. The loess-red clay deposit in the Loess Plateau may record the weathering features of the detritus material from the wide range upwind of the Loess Plateau. Therefore geochemistry of the loess-plaeosol and red clay sequences may provide insights into the regional chemical weathering regime and the connection between the chemical weathering and the late Cenozoic climate-environmental change Here we selected 319 samples from the Baishui section near the Pingliang City, Gansu Province, and analyzed them with X-ray fluorescence. Based on the result, we reconstruct the chemical weathering history of the Baishui section since 6Ma. We chose CIA as the proxy for chemical weathering intensity. The CIA ratio in soil units is higher than in adjacent loess horizons, but lower than in the red clay, in good agreement with the field observation. The CIA ratios of the Baishui section correlates well with the global ice volume fluctuations, indicating that the global cooling may contribute a lot to the chemical weathering variations in Chinese Loess Plateau. There are at least 3 million-year time scale variations that can be identified in the chemical weathering intensity curve, i.e., between 3.3 to 2.1 Ma, 1.7 to 0.9 Ma and from 0.9Ma. We think these may reflect the combined effect of the tectonic uplift and ice sheets on monsoon intensity. Other time scale variations can be also observed. In the period between 2.4 and 0.8 Ma, the CIA record display the 400,000 years cycle, which may be resulted from the Tibetan uplift during the Pliocene-early Pleistocene which have significantly amplified the monsoon response sensitivity to the orbital-scale variations in insolation. From 1.2 Ma, the 100,000 years period became intensifying, and particularly after 0.8 Ma, the earlier monsoon response at 400,000 year periodicity was overwhelmed by the ice sheet forcing at 100,000 year periodicity. These may indicate that the expansions of the Northern Hemisphere ice sheets may have crossed a threshold, which enforce the monsoon responding at the 100,000 year periodicity.

中国西部大气和雪冰黑碳及其传输过程研究

Glaciers in west China are the sources of the major great rivers in Asia, and the solid water resources are crucial to China and South Asia. Black carbon (BC) results in very complex climate effects not only in the atmosphere, but accelerates the melting after its deposit on the surface of snow/ice. As the main distributed area of glaciers in China, the Tibetan Plateau (TP) and Xinjiang region are abutted by South Asia, Central Asia, and Russia, and east China, and the atmospheric environment would be influenced by the BC emitted from these regions. Whereas, the BC’s temporal and spatial distributions for concentration in the mid and top troposphere in west China, its transport, and its radiative forcing after deposited on the snow/ice surface are not well understood at the present. In the field, we collected samples from surface snow, snow pits, ice core, and aerosol in the glaciers, analyzed BC content mainly by the thermo-oxidized method in the laboratory, and discussed temporal and spatial distributions for BC concentrations in glaciers, the transport, and its impacts on the environment. Several conclusions were derived as follows: 1_Spatial distribution and the impact on albedos for BC concentrations in snow/ice: the BC concentrations in the surface snow for the investigated glaciers could be placed in areas, the Tianshan Mountains ＞ the central TP ＞ the Pamirs ＞ the Qilian Mountians ＞ the Himalayas. This distribution could be attributed to the elevation of the glaciers, the topography of the TP, and more regional emissions. Probably significant impacts on the albedos of the glacier surface could be caused by BC deposits, and the estimated reduced albedos on the glaciers are 9.8% (the Zhadang glacier), 8.7% (the Miao’ergou Riverhead No.3 glacier), and 6.8% (the Kuitun River Haxilegen No.48 glacier), and 6.2% (the Dongkemadi glacier), and 5.3% (the La’nong glacier), and 4.2% (the Muztagata glacier), etc. 2_The temporal variance of BC concentrations in ice of the East Rongbuk Glacier (ERG) and its climatic implications: major cations and anions (e.g., SO42- and Ca2+) concentrations in aerosols during summer monsoon seasons showed their close relationships with the sources of air masses, in which the variance of SO42- concentrations suggested the atmospheric environment over the ERG was significantly influenced by the aerosols from South Asia. BC record based on an ice core suggested its deposit was dominantly transported by monsoons in summers and by westerlies in other seasons, and the BC from South Asia in summers dominated the varying trend of its concentrations in the ice core and caused higher concentrations in summers than those in other seasons. In the past 50 yrs, BC concentrations showed fluctuations, whereas showed an increasing tread in the most recent decade, and exceeded 50 μg kg-1 in the summer of 2001; correspondingly, the radiative forcing caused by BC showed an increasing trend since 1990s, and exceeded 4.5 W m-2 in the summer of 2001. 3_Cabonaceous aerosols in the Nam Co region: organic carbon (OC) concentration accounted for ～95% and BC for ～5% in the total carbonaceous aerosol concentration, which was significantly influenced by summer precipitations. OC was dominantly derived from fossil fuel burning and BC from both fossil fuel and biomass burning. Trajectory analysis and aerosol optical depth suggested the atmospheric environment in the Nam Co region was most probably influenced by the emissions from South Asia. The potential source regions of air pollutants in the Nam Co regions in summers might be Bangladesh and east India, and in winters might be the Indo-gangetic basin. The scavenging ratio of atmospheric BC by rainfalls was less than those at other sites. West China is a less-developed region for industry, where BC concentrations in the atmosphere and snow/ice could be significantly influenced by the emissions from the abutted regions with rising industries (South Asia, Central Asia, and Russia). For example, snow/ice BC concentrations in the glaciers of the Parmirs, the Tianshan Mountains, and the Qilian Mountains in the northeast margin of the TP might be more influenced by the emissions from Centrial Asia (transported by the westerlies), those in the glaciers of the Himalayas might be more influenced by the emissions from South Asia (transported by the monsoons and the westerlies), and atmospheric carbonaceous aerosols might also be more influenced by the emissions from South Asia (transported by the monsoons and the westerlies). The BC concentrations in some glaciers might cause significant impacts on the albedos for the glaciers, and therefore enhanced the radiative forcings, for example, the ERG. The research on the relationships among atmospheric and snow/ice BC and its radiative forcing, variance of snow cover, mass balance of glaciers, and climate forcing would be needed in future.

东北四海龙湾玛珥湖啊1500年来长链烯酮不饱和度温标与古水温重建

The alkenone unsaturation paleothermometer is an important proxy to reconstruct water temperature, and is widely applied to reconstructing sea surface temperature in most oceanographic settings. Recent research indicates that long chain alkenone is preserved in lacustrine sediments, and the alkenone unsaturation has good relationship with mean annual temperature in studied lakes. Thus, the alkenone unsaturation could be used as a temperature proxy to reconstruct temperature in limnic systems. In this study, we analyzed long chain alkenone from the varved sediments in Lake Sihailongwan, northeastern China. Based on the counting varves, we established time scale during the past 1500 years. The distribution pattern in the sediment is similar with the previous study in lacustrine environment. The ratio of C37:4 methyl ketone to the sum of C37 alkenones is high. Based on the published temperature- alkenone unsaturation equation, we reconstructed the mean air temperature and July water temperature during the past 1500 years. Three major cold periods are in AD560-950, AD 1540-1600 and AD1800-1920. Three major warm periods are AD450-550, AD 950-1400 and AD 1600-1800. The Medieval Warm Period was a significant warm periods. However, the traditional “Little Ice Age” was not a persistent cold period, and interrupted by relative longer warm period. The temperature variations in this study show a general similar pattern with the summer temperature reconstruction from Shihua Cave and the winter temperature from historical documents. The temperature variations from long chain alknone record show a good agreement with solar activity (10Be data from ice core and sunspot number from tree rings). It may suggest that solar activity is most important forcing in the studied area.

2200万年来风尘沉积的粒度变化及古气候意义

Three eolian deposit formations, including Quaternary loess (QL, Liu et al.3 1985), Hipparion red earth (HRE, also called red clay, Liu et al., 1985) and Miocene loess (ML, Guo et al., 2002) constitute a set of unique paleoclimatic archives in northern China dated back to at least 22Ma ago. The Miocene loess is a recently discovered loess-soil sequence. Detailed investigation has been made on its origin, chronology and paleoclimatic significance (Guo et al., 2002), but further work is still needed to obtain detailed paleoclimate information, and mechanical links behind paleoclimatic changes. In this study, grain size analysis of QL, HRE and ML has been conducted on two sections: Qinan and Xifeng. The objective is focused on comparison of the grain size distribution characteristics (GSDC) among different eolian deposit formations, and reconstruction of the Asian monsoon circulation in the past 22 Ma. Results show that GDSC of ML resembles that of QL and HRE, and GDSC of ML is especially similar to HRE. Both ML and HRE contain a significant proportion of fine fraction, however, QL has a large amount of coarse sediments. This is mainly due to that the wind system transported aeolian dust was weaker in the late Tertiary than that in the Quaternary. Grain size difference between loess and paleosol in ML is much smaller than that in QL, indicating that the climatic fluctuations during the late Tertiary were much smaller than that happened in the Quaternary The grain size records of the past 22 Ma reveal several evolution phases of the Asia winter monsoon. -2.7 Ma BP is the most important boundary in the process of the winter monsoon evolution: the wind strength have significantly enhanced since 2.7 Ma ago. During a period between -22.0 and -3.6 Ma, three periods with relatively stronger winter monsoon are recorded in the QA-I section, between 21.2 and 19.9, and 16.0 and 13.3, and 8.7 to 6.9 Ma, respectively. From 3.6 to 2.7, the winter monsoon was enhanced gradually. In the Miocene time, the intensified winter monsoon phases (between 21.2 and 19.9, and 16.0 and 13.3, and 8.7 and 6.9 Ma) seemed to have a close relationship with the uplift of the Tibetan Plateau and/or the ongoing global cooling, but the forcing mechanism behind the Asia winter monsoon evolution need to be further investigated. During the Pliocene-Pleistocene time, the Asia winter monsoon strengthened at 3.6 and 2.7Ma ago are in good agreement with the ongoing global cooling and the Arctic ice sheet development. In the mean time, much evidence suggests that an intense uplift of the Tibetan Plateau occurred at ~3.6 Ma, which is synchronous with a major increase in Asia winter monsoon. Therefore, two major factors may be invoked to explain the winter monsoon enhancement: Arctic ice sheet development and Tibetan uplift. We propose that changes in location and intensity of the Siberian-Mongolian high that were caused by the Tibetan uplift and Arctic ice sheet development might be an important factor for Asian winter monsoon evolution in the Pliocene-Pleistocene.

甘肃西峰晚第三纪红粘土地层中的蜗牛化石记录及其古环境意义

In this study, 260 mollusk fossil samples from a Red Clay sequence at Xifeng, Gansu province, in the northern China were analyzed quantitatively. 12 fossil species and four fossil zones have been identified. Three main ecological groups were determined based on ecological requirement of each mollusk taxon. According to fossil composition and succession of three ecological groups, the author discussed the origin and sedimentary environment of the red clay deposits, and the process of ecological environmental changes as well as the variations of the East Asia monsoons during 6.2-2.4 Ma in the Loess Plateau. A preliminary study on periodicity of paleoclimatic changes was also conducted by using spectral analysis method. The main results and conclusions are presented as follows:A continuous land mollusk fossil sequence of 6.2-2.4 Ma from Xifeng Red Clay Formation has been established, which provided a basic data for studying the environmental changes during late Miocene to Pliocene.The study of composition and preservation condition of mollusk fossils reveals a terrestrial in situ ecological population in the Red Clay Formation. All of identifiable mollusk species are composed of terrestrial taxa, which support the view that the Red Clay is an eolian origin, similar to the overlying Quaternary loess deposits.The mollusk record reveals the processes of ecological and environmental changes during 6.2-2.4 Ma in the Loess Plateau. Climatic changes experienced cold and dry from 6.2-5.4 Ma, warm and wet during 5.4-4.5 Ma, mild and moderate from 4.5-3-4 Ma, to rapid cooling and drying after 3.4 Ma. From '5.4- 2.4 Ma, climate was stepwise cooling. The cooling trend is in good agreement with a general1 0global cooling trend during this period, as documented by marine 5 0 records.4. Three remarked ecological shifts took place in mollusk assemblages from 6.2-2.4 Ma, focused on about 5.4, 4.5 and 3.4 Ma. The warming shift around 5.4 Ma was probably related to the rising of the global temperature. The cooling shifts around 4,5 and 3.4 Ma however might be closely linked to the uplift of Tibet Plateau and the development of Northern Hemisphere ice sheet.The succession in mollusk ecological groups also recorded the variability of the East Asian winter and summer monsoon. The winter monsoon dominated two periods from 6.2-5.4 Ma and from 3.4-2.4 Ma, while the summer monsoon was strong during 5.4-4.5 Ma. The variations in winter and summer monsoons were in phase during 4.5-3.4 Ma. Monsoon regimes changed with the duration about 1 Ma, which roughly corresponds to the cycle driven by tectonic activity on the time scales of ICP-IO7 years. In addition, mollusk fossils recorded the large amplitude and high frequency fluctuations overlapped on 105-107 years climate cycle.The maximum entropy spectral analysis and filter-band analysis of total mollusk individuals and three typical ecological groups suggest that the climate changes controlled mainly by solar insolation had periods about 70 ka and 40 ka on the time scales of 105 during late Miocene-Pliocene. Climatic periodicity intensified from 4.0 Ma, which reflected strengthened forcing by high latitude ice volume.