Yougui Song

Interplay between the Westerlies and Asian monsoon recorded in Lake Qinghai sediments since 32 ka

Two atmospheric circulation systems, the mid-latitude Westerlies and the Asian summer monsoon (ASM), play key roles in northern-hemisphere climatic changes. However, the variability of the Westerlies in Asia and their relationship to the ASM remain unclear. Here, we present the longest and highest-resolution drill core from Lake Qinghai on the northeastern Tibetan Plateau (TP), which uniquely records the variability of both the Westerlies and the ASM since 32 ka, reflecting the interplay of these two systems. These records document the anti-phase relationship of the Westerlies and the ASM for both glacial-interglacial and glacial millennial timescales. During the last glaciation, the influence of the Westerlies dominated; prominent dust-rich intervals, correlated with Heinrich events, reflect intensified Westerlies linked to northern high-latitude climate. During the Holocene, the dominant ASM circulation, punctuated by weak events, indicates linkages of the ASM to orbital forcing, North Atlantic abrupt events, and perhaps solar activity changes.

Pacific freshening drives Pliocene cooling and Asian monsoon intensification

The monsoon is a fundamental component of Earths climate. The Pliocene warm period is characterized by long-term global cooling yet concurrent monsoon dynamics are poorly known. Here we present the first fully quantified and calibrated reconstructions of separate Pliocene air temperature and East Asian summer monsoon precipitation histories on the Chinese Loess Plateau through joint analysis of loess/red clay magnetic parameters with different sensitivities to air temperature and precipitation. East Asian summer monsoon precipitation shows an intensified trend, paradoxically at the same time that climate cooled. We propose a hitherto unrecognized feedback where persistently intensified East Asian summer monsoon during the late Pliocene, triggered by the gradual closure of the Panama Seaway, reinforced late Pliocene Pacific freshening, sea-ice development and ice volume increase, culminating in initiation of the extensive Northern Hemisphere glaciations of the Quaternary Ice Age. This feedback mechanism represents a fundamental reinterpretation of the origin of the Quaternary glaciations and the impact of the monsoon.

Magnetostratigraphic determination of the age of ancient Lake Qinghai, and record of the East Asian monsoon since 4.63 Ma

Lake Qinghai, in North China, is the largest interior plateau lake in Central Asia, and is sensitive to climate change and the environmental effects of Tibetan Plateau uplift. We have obtained an almost continuous 626 m long sediment core from an in-filled part of the southern lake basin, which documents both the age of the origin of the lake and the evolution of the East Asian monsoon during the Late Cenozoic. High-resolution magnetostratigraphy provides a chronology back to ca. 5.1 Ma. Analysis of lithofacies and depositional environments reveal that the change from eolian to lacustrine facies occurred at ca. 4.63 Ma, corresponding to a shift from an arid or semi-arid to a more humid climate, which resulted in the origin of Lake Qinghai. Changes in sediment lithology and mean grain size indicate that the lake level fluctuated considerably, superimposed on a long-term trend from higher to lower levels in response to variations in the East Asian Monsoon. This archive is a significant additional source of information on regional and global environmental change, complementing the existing records from north China, which are mainly based on analysis of loess deposits.

Dominant 100,000-year precipitation cyclicity in a late Miocene lake from northeast Tibet

Paleoclimatic records reveal >6–million year earlier onset of dominant 100–thousand year eccentricity cycles. East Asian summer monsoon (EASM) precipitation received by northern China over the past 800 thousand years (ky) is characterized by dominant 100-ky periodicity, mainly attributed to CO2 and Northern Hemisphere insolation–driven ice sheet forcing. We established an EASM record in the Late Miocene from lacustrine sediments in the Qaidam Basin, northern China, which appears to exhibit a dominant 100-ky periodicity similar to the EASM records during the Late Quaternary. Because evidence suggests that partial or ephemeral ice existed in the Northern Hemisphere during the Late Miocene, we attribute the 100-ky cycles to CO2 and Southern Hemisphere insolation–driven Antarctic ice sheet forcing. This indicates a >6–million year earlier onset of the dominant 100-ky Asian monsoon and, likely, glacial and CO2 cycles and may indicate dominant forcing of Northern Hemisphere climate by CO2 and Southern Hemisphere ice sheets in a warm world.

Rapid and cyclic dust accumulation during MIS 2 in Central Asia inferred from loess OSL dating and grain-size analysis

Due to lack of reliable proxies from the Westerlies-dominant region, the strength change of Northern Hemisphere Westerlies remains poorly understood. The aim of this study is to provide a reliable paleoclimatic proxy about the Northern Hemisphere Westerlies change. Here we report a 30.7 m thick loess section from the Ili basin directly controlled by the Westerlies. Based on optically stimulated luminescence (OSL) and high resolution grain-size records, we reconstruct the change history of the Westerlies strength during the last glacial period (mainly Marine Isotope Stages 2, MIS2), being similar with the Westerlies index recorded in the Qinghai Lake sediments. Within error limits, all ages are in stratigraphic order. We further compare the climatic records among the Ili loess, Qinghai Lake and the NGRIP, their similarity shows a good climatic coupling relationship among the Central Asia, East Asia and the North Atlantic, and the Westerlies plays a critical influence in transporting the North Atlantic signal to Central and East Asia.

Magnetostratigraphy of Late Tertiary sediments from the Chinese Loess Plateau and its paleoclimatic significance

The Late Tertiary red clay/bed sediments underlying the Quaternary loess-paleosol in the Chinese Loess Plateau possesses high-resolution paleoclimatic changes related to the uplift of the Tibetan Plateau. Magnetostratigraphy and susceptibility measurements are discussed in this paper. The paleomagnetic results show that the red clay/bed began to accumulate at about 8.1 Ma, which represents the oldest Late Tertiary deposit continuously in the central Loess Plateau. The magnetic susceptibility curves show stepwise increases since the initiation of red clay, superimposed on several peaks. Moreover, the magnetic susceptibility increased abruptly since 3.8 Ma, which probably indicates the inception of the modern East Asia monsoon system. Between 3.8 and 2.6 Ma, the stepwise increases of susceptibility may imply progressively intensified East Asia summer monsoon activity related with the stepwise uplift process of the Tibetan Plateau.

Timing and Spatial Distribution of Loess in Xinjiang, NW China.

Central Asia is one of the most significant loess regions on Earth, with an important role in understanding Quaternary climate and environmental change. However, in contrast to the widely investigated loess deposits in the Chinese Loess Plateau, the Central Asian loess–paleosol sequences are still insufficiently known and poorly understood. Through field investigation and review of the previous literature, the authors have investigated the distribution, thickness and age of the Xinjiang loess, and analyzed factors that control these parameters in the Xinjiang in northwest China, Central Asia. The loess sediments cover river terraces, low uplands, the margins of deserts and the slopes of the Tianshan Mountains and Kunlun Mountains and are also present in the Ili Basin. The thickness of the Xinjiang loess deposits varies from several meters to 670 m. The variation trend of the sand fraction (>63 μm) grain-size contour can indicate the local major wind directions, so we conclude that the NW and NE winds are the main wind directions in the North and South Xinjiang, and the westerly wind mainly transport dust into the Ili basin. We consider persistent drying, adequate regional wind energy and well-developed river terraces to be the main factors controlling the distribution, thickness and formation age of the Xinjiang loess. The well-outcropped loess sections have mainly developed since the middle Pleistocene in Xinjiang, reflecting the appearance of the persistent drying and the present air circulation system. However, the oldest loess deposits are as old as the beginning of the Pliocene in the Tarim Basin, which suggests that earlier aridification occurred in the Tarim Basin rather than in the Ili Basin and the Junggar Basin.

Clay mineral records of the Erlangjian drill core sediments from the Lake Qinghai Basin, China

Located at the northeastern margin of the Qinghai-Tibet Plateau (QTP) in the Asian interior, the Lake Qinghai is sensitive to environmental change and thus an outstanding site for studying paleoenvironmental changes. Thick deposits in the Lake Qinghai provide important geological archives for obtaining high-resolution records of continental environmental history. The longest drilling core obtained from the Lake Qinghai, named Erlangjian (ELJ), reached about 1109 m and was investigated to determine its clay mineral assemblage and grain size distributions. Clay mineralogical proxies, including type, composition, and their ratios, as well as the illite crystallinity (KI) and chemical index (CI), in combination with grain size data, were used for reconstructing the history of paleoenvironmental evolution since the late Miocene in the Lake Qinghai Basin. The clay mineral records indicate that the clay mainly comprise detritus originating from peripheral material and has experienced little or no diagenesis. The proportion of authigenic origin was minor. Illite was the most abundant clay mineral, followed by chlorite, kaolinite, and smectite. Variations of clay mineral indexes reflect the cooling and drying trends in the Lake Qinghai region, and the grain size distribution is coincided with the clay minerals indexes. The paleoclimatic evolution of the Lake Qinghai Basin since the late Miocene can be divided into five intervals. The climate was relatively warm and wet in the early of late Miocene, then long-term trends in climate change character display cooling and drying; later in the late Miocene until early Pliocene the climate was in a short relatively warm and humid period; since then the climate was relatively colder and drier. These results also suggest multiple tectonic uplift events in the northeastern QTP.

Loess Magnetic Susceptibility in Central Asia and its Paleoclimatic Significance

For standard loess sections in the Chinese Loess Plateau (CLP), paleosols always have higher magnetic susceptibility (MS) than adjacent loesses. But this characteristic is not always suitable to MS records in loess-paleosol sequences from Central Asia. Here we report MS results of two loess sections in Yili basin, Central Asia. The MS variation of one loess section with an elevation of 1432m is identical with that of loess-paleosol sequences in the CLP, but the other one at 875m have opposite trend. Based on field investigation, pedogenesis characteristics observations, carbonate, lithological and grain-size correlations, the authors suggest that the differences of regional paeloprecipitation related to altitudes during last interglacial period between the two sections are responsible for their differences of MS. Carbonate deposits also have influence on decrease MS. The susceptibility enhancement of pedogenesis model for loess-paleosol sequences in the CLP can not interpret completely the MS variations of loess sections from Central Asia area. It should be careful to reconstruct paleoclimatic change using susceptibility as a proxy in Central Asia area.

A Review of Recent Advances in Red-Clay Environmental Magnetism and Paleoclimate History on the Chinese Loess Plateau

The red-clay sequence on the Chinese Loess Plateau (CLP) was deposited during the late Miocene-Pliocene and is encoded with important information of past climate changes. However, it has received much less study in comparison to the overlying Pleistocene loess-paleosol sequence. In this paper, we review recent progress in characterizing the environmental magnetic parameter-based paleoclimate history recorded by the red-clay sequence. Several key conclusions are as follows. 1) the red clay and the loess-paleosol sequences have similar magnetic enhancement mechanisms but magnetic minerals in the red clay sequence have experienced a higher degree of oxidation than in the loess-paleosol sequence. 2) The CLP experienced a cooling and wetting trend from 4.5 to 2.7 Ma, caused by ice sheet expansion and East Asian summer monsoon intensification, respectively. 3) The above conclusions benefit from backfield remanence curve unmixing and comparison of magnetic grain size/concentration records, which are particularly useful in separating the temperature from the precipitation signal. A clear need in future studies is to explore the concentration and the grain size variations of hematite and goethite in the red-clay sequence and their formation mechanisms. The payback would be a clear understanding of climate history during the late Miocene-Pliocene, a possible analog for future warmer climate.