Paleoclimate of the Earth and solar activity

The paper focuses on climate variations caused by the orbital effect and solar activity over the last one million years and oscillations (warming or cooling) of the climate since the last ice age retreat. Attention is paid to a significant discrepancy in the trend of global temperature change during the modern interglacial epoch (Holocene) obtained by various methods. A long-term cooling trend was observed in the summer temperature of the Northern Hemisphere during the last 2000 years.     

Preliminary reconstruction of the desert and sandy land distributions in China since the last interglacial period

The desert and sandy land are the products of arid climate. The spatial distribution of modern deserts and sandy land in China and their relation to climate show following characteristics: arid and hyper-arid desert zones, at isohyet of less than 200 mm, are dominated by mobile dunes; semi-arid steppe and arid desert steppe with the precipitation between 200-400mm, are dominated by semi-fixed and fixed sand dunes; the precipitation of sub-humid forest grassland and humid forest zones with scattered fixed sand land is higher than 400 mm. With this as reference, in combination with considerable amount of paleoclimatic data in desert regions and adjacent regions, the distributions of desert and sandy land in China during the last interglacial period, the last glacial maximum (LGM), and the Holocene megathermal, were preliminarily reconstructed. The results compared with that of today show that the distribution of desert and sandy land in China was greatly dwindled during last interglacial period, and the mobile dune area was about two-thirds of that of today's, but greatly expanded during LGM. However, the dwindling area of desert and sandy land in the Holocene megathermal was smaller than that in the last interglacial period. The forcing mechanism was mainly related to the changes of East Asian winter and summer monsoon, south-northward swing of the westerlies and the variations of the Qinghai-Tibet Plateau monsoon intensity caused by global climate changes during the cold and warm intervals since the last interglacial period.     

Preliminary reconstruction of the desert and sandy land distributions in China since the last interglacial period

The desert and sandy land are the products of arid climate. The spatial distribution of modern deserts and sandy land in China and their relation to climate show following characteristics: arid and hyper-arid desert zones, at isohyet of less than 200 mm, are dominated by mobile dunes; semi-arid steppe and arid desert steppe with the precipitation between 200–400 mm, are dominated by semi-fixed and fixed sand dunes; the precipitation of sub-humid forest grassland and humid forest zones with scattered fixed sand land is higher than 400 mm. With this as reference, in combination with considerable amount of paleoclimatic data in desert regions and adjacent regions, the distributions of desert and sandy land in China during the last interglacial period, the last glacial maximum (LGM), and the Holocene megathermal, were preliminarily reconstructed. The results compared with that of today show that the distribution of desert and sandy land in China was greatly dwindled during last interglacial period, and the mobile dune area was about two-thirds of that of today’s, but greatly expanded during LGM. However, the dwindling area of desert and sandy land in the Holocene megathermal was smaller than that in the last interglacial period. The forcing mechanism was mainly related to the changes of East Asian winter and summer monsoon, south-northward swing of the westerlies and the variations of the Qinghai-Tibet Plateau monsoon intensity caused by global climate changes during the cold and warm intervals since the last interglacial period.     

Problem of the length of the current interglacial

The climate during the past hundreds of thousands of years has been characterized by a rather distinct periodicity of about 100000 yr with brief warming periods (interglacials) lasting approximately 10000–12000 yr. Today, mankind is living in an interglacial period that began about 11 ka ago. In light of the discussion about global warming observed in recent decades, which advocates of an anthropogenic impact associate with emission of greenhouse gases due to combustion of fossil fuel, the question arises concerning the duration of the current interglacial. The data available on climate change and solar radiation on a time scale of the last millions of years are critically analyzed in this article and the problem of the length of the current interglacial is discussed.     

The Earth's Climate in the Next Hundred Thousand years (100 kyr)

One of the most striking features of the Quaternary paleoclimate records remains the so-called 100-kyr cycle which is undoubtedly linked to the future of our climate. Such a 100-kyr cycle is indeed characterised by long glacial periods followed by a short-interglacial (10–15 kyr long). As we are now in an interglacial, the Holocene, the previous one (the Eemian, which corresponds quite well to Marine Isotope Stage 5e, peaking at 125 kyr before present, BP) was assumed to be a good analogue for our present-day climate. In addition, as the Holocene is 10 kyr long, paleoclimatologists were naturally inclined to predict that we are quite close to the next ice age. Simulations using the 2-D climate model of Louvain-la-Neuve show, however, that the current interglacial will most probably last much longer than any previous ones. It is suggested here that this is related to the shape of the Earth's orbit around the Sun, which will be almost circular over the next tens of thousands of years. As this is primarily related to the 400-kyr cycle of eccentricity, the best and closest analogue for such a forcing is definitely Marine Isotopic Stage 11 (MIS-11), some 400 kyr ago, not MIS-5e. Because the CO₂ concentration in the atmosphere also plays an important role in shaping long-term climatic variations – especially its phase with respect to insolation – a detailed reconstruction of this previous interglacial from deep sea and ice records is urgently needed. Such a study is particularly important in the context of the already exceptional present-day CO₂ concentrations (unprecedented over the past million years) and, even more so, because of even larger values predicted to occur during the 21st century due to human activities.     

Late quaternary monsoon patterns on the Loess Plateau of China

China's Loess Plateau was formed under special conditions. The tectonic movement, topographical characteristics, and monsoon patterns combined to create a favourable environment for the accumulation of thick loessic deposits. The Loess Plateau itself is part of the ‘Monsoon Triangle’ of China, a region very susceptible to climatic changes. Throughout the Upper Pleistocene the palaeoenvironment on the Loess Plateau alternated from steppe, to deciduous forest and coniferous forest, in response to shifts in the atmospheric circulation. Three monsoon patterns appear to be indicated: (1) a full glacial monsoon pattern (18000–15000 yr BP) which induced a cold and dry climate favouring loess accumulation in steppe conditions; (2) an interglacial monsoon pattern (last interglacial and Holocene) in which a warm humid climate prevailed with deciduous forests, leaving palaeosols interbedded within the loess sequence; and (3) a transitional or interstadial monsoon pattern (50 000–23 000 yr BP) in which the climate was cold and humid in the Loess Plateau, encouraging the development of coniferous forest.     

Rapid age assessment in the Namib Sand Sea using a portable luminescence reader

A rapid assessment of burial age for sedimentary materials is useful to aid in-situ interpretation of sites and sequences during fieldwork. This can assist with targeted field sampling strategies for full dating back in the laboratory, for example when the study is concerned with reconstructing landscape dynamics during a specific time period. Field-based luminescence measurements are possible using a portable luminescence reader; the challenge is translating relative portable luminescence reader signal intensities of samples into an estimate of age. This study uses a portable luminescence reader for the first time in the analysis of African dunefield sediments. Samples from the Namib Sand Sea (NSS) with established luminescence ages are used to assess what in-situ information about relative sample age can be gleamed at and between sites using the portable luminescence reader, and to establish whether first-order estimates of sample age can be obtained. Two sites in the NSS, which are of modern, very late Holocene and last interglacial age were selected for this assessment and a simple calibration between portable luminescence reader signals and sample age is made. Results show that portable luminescence reader signals differ by over two orders of magnitude between late Holocene and last interglacial age samples and that useful relative-age information can be established using bulk material in the field. Predicted ages from portable luminescence reader signals using a linear regression appear to be indicative and useful. Further development of this calibration using a wider range of sample ages would confirm its applicability in the NSS, and a similar approach is applicable to other sand sea environments.     

Expansion and contraction of Chinese deserts during the Quaternary

Episodic dune formations during the Quaternary are found in many deserts of China. The causes of desert expansions on different time scales are not the same. Desert extension at about 1.1 and 0.9 Ma ago were the response to the active tectonic movements, whereas the desert evolutions on the ten-thousand years time scale were the response to the orbital scale climatic changes. Spatial scale studies on desert evolution indicate that desert margins shifted greatly during the last glacial maximum (LGM) and the Holocene optimum, its changing from 125°E of the LGM to 105°E of the climatic optimum. Historical desertification in the semiarid China is not a response to climate drought but largely associated with the human impacts (mainly over-cultivation) since about 2300 years ago, which leads to the reworking of the underlying LGM sands.     

African monsoon variability during the previous interglacial maximum

Little is known about centennial- to millennial-scale climate variability during interglacial times, other than the Holocene. We here present high-resolution evidence from anoxic (unbioturbated) sediments in the eastern Mediterranean Sea that demonstrates a sustained ∼800-yr climate disturbance in the monsoonal latitudes during the Eemian interglacial maximum (∼125 ka BP). Results imply that before and after this event, the Intertropical Convergence Zone (ITCZ) penetrated sufficiently beyond the central Saharan watershed (∼21°N) during the summer monsoon to fuel flooding into the Mediterranean along the wider North African margin, through fossil river/wadi systems that to date have been considered only within a Holocene context. Relaxation in the ITCZ penetration during the intra-Eemian event curtailed this flux, but flow from the Nile - with its vast catchment area - was not affected. Previous work suggests a concomitant Eurasian cooling event, with intensified impact of the higher-latitude climate on the Mediterranean basin. The combined signals are very similar to those described for the Holocene cooling event around 8 ka BP. The apparent type of concurrent changes in the monsoon and higher-latitude climate may reflect a fundamental mechanism for variability in the transfer of energy (latent heat) between the tropics and higher latitudes.     

研究季风区历史时期湖泊环境演化的意义

本文首先讨论了历史时期季风区湖泊环境演化研究在古全球变化研究中的地位及其意义,着重强调人与自然相互作用研究以及进行定量研究是该领域的关键所在。综合中国近年来在湖泊沉积与古全球变化研究中取得的成果,提出在该领域研究的一些新的认识与亟待解决的问题。     

用地貌学方法研究贺兰山山前断层全新世活动状况

本文根据穿过断层的45条冲沟中的裂点和阶地测量资料,论述贺兰山山前断层全新世以来的活动状况。我们认为全新世以来该断层至少有8次活动。最后一次断层活动错断了距今400年的长城,并在冲沟中形成一个裂点,该裂点现今已距断层陡坎约5米远。假定这次断层活动和1739年平罗大地震有关,求出裂点的溯源侵蚀平均速度为2.0—2.5厘米/年.根据两裂点间的距离和裂点的溯源侵蚀平均速度,并考虑到我国近5000年的气候有逐渐变干的趋势,求出断层活动的周期(表3)。从断层陡坎附近的各级阶地面之间的高差,求出断层各次活动的幅度(表4)。     

Palaeomonsoon vicissitudes in eastern desert region of China since last interglacial period

Based on the geological records, the palaeomonsoon in eastern desert region of China is divided into three major evolution stages, i.e. summer monsoon prevailing stage of last interglacial period (130-70kaB.P.), winter monsoon prevailing stage of last glacial period (70-10 kaB. P.), and unstable summer monsoon prevailing stage of postglacial period (10 kaB. P. to present) and further divided into several substages. The conversion between summer monsoon and winter monsoon in the region is dominated by the sudden change process. The north limit of summer monsoon in the region retreated to the north limit of sandy loess zone of the Loess Plateau in the last glacial period from the Mazong Mts. -Ulan Bator of last interglacial period, then it entered Shandan-Yabrai region in the optimum period of the Holocene, and finally it retreated to the present extended line from north piedmont of the Yinshan Mts to Hulun Buir. This shows that the summer monsoon caused by East Asian monsoon circulation tends to be weakened fluctuationally. However, the factors affecting the monsoon vicissitudes are complex, so special attention should be paid to the study of the short-period climatic fluctuations of the Holocene.     

Holocene valley aggradation driven by river mouth progradation: examples from Australia

Since the end of the post‐glacial sea level rise 6800 years ago, progradation of river mouths into estuaries has been a global phenomenon. The responses of upstream alluvial river reaches to this progradation have received little attention. Here, the links between river mouth progradation and Holocene valley aggradation are examined for the Macdonald and Tuross Rivers in south‐eastern Australia. Optical and radiocarbon dating of floodplain sediments indicates that since the mid‐Holocene sea level highstand 6800 years ago vertical floodplain aggradation along the two valleys has generally been consistent with the rate at which each river prograded into its estuary. This link between river mouth progradation and alluvial aggradation drove floodplain aggradation for many tens of kilometres upstream of the estuarine limits. Both rivers have abandoned their main Holocene floodplains over the last 2000 years and their channels have contracted. A regional shift to smaller floods is inferred to be responsible for this change, though a greater relative sea level fall experienced by the Macdonald River since the mid‐Holocene sea level highstand appears to have been an additional influence upon floodplain evolution in this valley. Copyright © 2006 John Wiley & Sons, Ltd.     

Variations in the cosmic ray fluxes,modulated by the solar and terrestrial magnetic fields,and climate changes. Part 3: A time interval of 1.5 Myr,including the pleistocene

The most reliable data on a change in the intensity of cosmic rays and geomagnetic field on large time scales have been analyzed, and the relations between changes in these processes and climate during the last 1.5 Myr have been studied. An analysis indicated that the climate of the Earth is affected by changes in the Earth’s orbit parameters and geomagnetic dipole values; however, the climate responds to these changes with a delay of 10 kyr and immediately, respectively. In this case about two thirds of the effect of eccentricity on ¹⁸O is implemented via an intermediate chain: virtual axial dipole moment, changes in which can be related to changes in eccentricity. Thus, an analysis of the accumulated data on the processes, proceeding in the Earth’s atmosphere during the interaction with cosmic rays on the scales of several years to several hundreds of thousand years, indicates that the cosmophysical factor of influence on climate cannot be rejected. To make the conclusion more convincing, it is necessary to collect data for the studied time interval in a much wider region, to more accurately date samples, and to study the response of the climatic system to the external influence.     

Instability characteristics of the East Asian Monsoon recorded by high-resolution loess sections from the last interglacial (MIS5)

The selection of high-resolution loess sections is needed in order to determine the climatic variability of the East Asian Monsoon during the last interglacial. Two sequences of S1 on the eastern and west-ern sides of the Liupan Mountain were both composed of five paleosol layers and four loess layers,indicating that there were five strong summer monsoon events and four strong winter monsoon events in MIS5. This corresponds with other records of the East Asian Monsoon,along with NGRIP and the North Atlantic records,implying that the climate of the Northern Hemisphere was very instable during the last interglacial. Two layers of paleosols and one layer of loess had developed during MIS5a and MIS5c. Compared with MIS5e,the climate in MIS5a and MIS5c fluctuated more intensively on a millen-nial scale,whereas the climate was relatively stable in MIS5e.     

The Y. D. and climate abrupt events in the early and middle Holocene: Stalagmite oxygen isotope record from Maolan,Guizhou, China

Changes of climate, especially the abrupt changesof precipitation and temperature lead frequently tograve effect in human productive and living activity.Since the 1950s, results of monitoring indicate thatatmosphere CO2 concentration rises by 1.4×10?6g/gevery year. The atmospheric greenhouse effect leadsprobably to global warming since the 1940s, whichbrings furthermore special attention of scientists andrelevant government organizations in various coun-tries. In the near 20 years, the …     

The record of paleoclimatic change from stalagmites and the determination of termination II in the south of Guizhou Province,China

A high-resolution climate record from 163.00 kaBP to 113.80 kaBP has been obtained through TIMS-U series dating and carbon and oxygen isotope analysis of the three large stalagmites from two caves in the south of Guizhou Province, China. The record of the oxygen isotopes from the stalagmites reveals that the undulation characteristics between the cooling event of the glacial period and the warming event of the interglacial period in the research area can compare well to those of ice cores, lake sediments, loess and deep sea sediments on the scale of ten-thousand years or millennium time scale. The climate undulation provided by the record of the stalagmites has a coherence with the global changes and a tele-connection to the paleoclimate changes in the north polar region. Our results suggest that the direct dynamics of paleo-monsoon circulation changes reflected in the record of the stalagmites might be caused by changes of the global ice volume, and in turn related to various factors, including the solar radiation strength at the mid-latitudes in the Northern Hemisphere, the southern extension of the ice-rafted event in the North Atlantic, and changes of the equatorial Pacific sea surface temperature at the low-latitudes. Using °¹⁸O values, we have calculated the temperatures and the results show that the temperature difference between the penultimate glacial period (with an average temperature of 8.1°C, and a minimum temperature range from 0.65°C to-1.43°C at stage 6) and the last interglacial period (with an average temperature of 18.24°C at sub-stage 5e) was about 10°C. This temperature difference from the record of the stalagmites corresponds in general to the record temperature variation (about 10°C) of measured ice cores. The climate records from the three stalagmites in the two caves have shown that the circulation strength of the Asian summer monsoon and the winter monsoon in the penultimate glacial period and the last inter-glacial period had a clear change. With the TIMS-U series method, termination II of the penultimate glacial period has been precisely dated at an age of (129.28± 1.10) kaBP for the three stalagmites in the south of Guizhou Province, China. This borderline age represents the beginning of the last interglacial period or the boundary between the Middle Pleistocene and the Late Pleistocene, and corresponds to the beginning age of the last interglacial period shown by the ice cores and in the SPECMAP curve of the marine oxygen isotopes. The chronology determination of termination II is not only of stratigraphic and chronological significance, but also lays an important foundation for discussing the short time scales of climate oscillation and rapidly changing events of paleoclimate in the circulation region of the East Asian monsoon.     

Holocene Dune Activity in the Thar Desert,India

The Thar Desert dune system in north-west India and eastern Pakistan provides a rich archive of past environmental, geomorphological and climatic change. Much of the knowledge about the timing of dune accumulation in the Thar stems from scattered and sporadic records, based on older luminescence dating protocols. If the Thar dune record is to be incorporated within a growing multiproxy framework of past climate and environmental dynamics, it is necessary to generate a systematic record of the timing of dunefield accumulation. From this, relationships to climate and other drivers of dune activity may then be better established. To this end, an intensive programme of field sampling and optically stimulated luminescence (OSL) dating was carried out from a dunefield in the east-central Thar Desert. This study presents the first detailed Holocene dune accumulation history from the region, and sheds light on the development of the multi-generational parabolic dune systems. In contrast to previously published work, we identify the importance of the Holocene and the last millennium as periods with a number of preserved accumulation phases. OSL ages suggest that accumulation was persistent during the early and mid-Holocene (within 11.7-5.5 ka), late Holocene (2-1 ka), as well as two major phases in the last millennium (600 – 200 a and within the last 70 a). Potential drivers of dune mobility in the last century include a strong anthropogenic dimension. Rapid net accumulation is recorded in the last 70 years, with rates varying between 2 and 5 m/year, in an environment where agricultural pressures have increased dramatically with the advent of irrigation schemes expanding into dunefield areas. © 2019 John Wiley & Sons, Ltd.     

Organic carbon isotope and molecular fossil records of vegetation evolution in central Loess Plateau since 450 kyr

Significant uncertainties remain regarding the temporal evolution of natural vegetation during the Quaternary, and drivers of past vegetation change, on the Chinese Loess Plateau (CLP). This study presents analyses of total organic carbon isotopic composition (TOC) and n-alkane ratios (C31/C27) from the Lingtai loess-palaeosol sequence on the central CLP over the last 450 kyr. The results demonstrate that the vegetation in this region comprised a mix of C₃ and C₄ plants of herb and woody growth-form. C₃ plants dominated for most of the last 450 kyr, but this did not lead to extensive forest. C₃ woody plants were more abundant in MIS9 (S3 period) and MIS5 (S1 period) during warm and humid climate conditions. Herbs increased in the region since 130 kyr, possibly as a result of increased aridity. On the orbital timescales, there was a reduction of C₃ herbal plants in MIS11 (S4) than in MIS12 (L5), and in Holocene than in the last glacial period. Our isotope and n-alkane proxy records are in agreement with Artemisia pollen changes in the region, which is/was the dominant species in this area and varying due to different heat and water conditions between glacial and interglacial periods. Though the climate in MIS1 (S0) was similar to that in MIS11 (S4), a significant increase in woody plants during the Holocene suggests the impact of human activities and ecological effects of changes in fire activity.     

Clay aquitards as archives of holocene paleoclimate: delta18O and thermal profiling

Paleoclimatic conditions in the Holocene were reconstructed from a detailed vertical profile of pore water delta(18)O and a series of downhole thermal profiles at a thick, hydrogeologically simple, aquitard research site in the Northern Great Plains of Saskatchewan. Reconstructions were obtained using the theory of one-dimensional diffusive transport and an empirical Bayesian inversion technique. Inversion of the delta(18)O profile shows that input signal consists of a sudden increase of +6 per thousand (corresponding to a temperature increase of about 18 degrees C) at about 12,000 years before present (BP), after which no measurable change in delta(18)O is apparent to present day. This research shows, at this location, that there is no evidence of large amplitude temperature shifts in the Holocene and supports the commonly used assumption in ground water studies of constant atmospheric delta(18)O values throughout the Holocene. Inversion of the temperature profiles suggests the ground surface temperature increased primarily in the last half of the 20th century, with a peak temperature (about 3 degrees C) circa 1990. For both profiles, the ability to resolve historical variability decays rapidly with time. For the temperature profiles, the decay in resolution precluded the development of reliable estimates of climatic conditions prior to about 1950 and, in the case of the delta(18)O profile, it prevented the precise definition of climate changes (e.g., Hypsithermal and Little Ice Age) in the Holocene.