Chengqi Zhang

Reworking effects in the Holocene Zhuye Lake sediments: A case study by pollen concentrates AMS 14 C dating

Transported by wind and water, the relatively old sediments can deposit in the terminal lake of an inland drainage basin. The reworking effect can affect the lake sediments 14C dating and explanations for proxies. The Zhuye Lake is the terminal lake of the Shiyang River Basin. Previous studies indicated that sediments in different locations of the lake basin showed different climatic change patterns. And then, some radiocarbon dates were inverted for some Late Pleistocene sections. Whether this phenomenon is related with the reworking effect? The pollen concentrates 14C dating can avoid the reservoir effect, which is an ideal method for studying the reworking effect. In this study, we used the pollen concentrates as dating materials and dated five Holocene sections in the Zhuye Lake Basin. Based on the 14C dates comparison between the pollen concentrates, organic matter, and shells, the pollen concentrates dates are relatively older than other dating materials. Based on the result, the reworking effect worked in the Zhuye Lake Basin during the Holocene; however, in different locations of the lake basin the reworking effects were in different levels. Furthermore, the Holocene lacustrine deposits were formed mostly during the early and middle Holocene. This study provided clues for reworking effect studies of other lakes in arid China.

Climatic and environmental change in Yanchi Lake, Northwest China since the Late Glacial: A comprehensive analysis of lake sediments

Modern climate research has shown that the Asian summer monsoon water vapor transport is limited to the eastern part of the Qilian Mountains. On the Holocene millennial-scale, whether the northwest boundary of the summer monsoon varies according to climate change is a key scientific issue. Yanchi Lake is located in the northern Qilian Mountains and the middle of the Hexi Corridor, where the modern climate is less affected by the Asian summer monsoon. It is a key research area for examining the long-term variations of the Asian summer monsoon. Paleoclimatic data, including AMS 14C dates of pollen concentrates and bulk organic carbon, lithology, grain-size, mineral composition and geochemical proxies were acquired from sediments of Yanchi Lake. The chronological results show that the lower part of the lacustrine section is formed mainly in the Late Glacial and early Holocene period, while the proxies’ data indicate the lake expansion is associated with high content of mineral salts. The middle part of this section is formed during the transitional period of the early and middle Holocene. Affected by the reworking effect, the pollen concentrates AMS 14C dates from the middle part of the section are generally older than those from the lower part. Since the mid-Holocene, Yanchi Lake retreated significantly and the deposition rate dropped obviously. The Yanchi Lake record is consistent with the Late Glacial and Holocene lake records in the Qinghai-Tibet Plateau and the climatic records in typical monsoon domain, which indicate the lake expansion and the strong Asian summer monsoon during the Late Glacial and early Holocene. The long-term monsoonal pattern is different from the lake evolution in Central Asia on the Holocene millennial-scale. This study proves the monsoon impacts on the northwestern margin of the summer monsoon, and also proves the fact that the northern boundary of the summer monsoon moves according to millennial-scale climate change.

Early Holocene environment at a key location of the northwest boundary of the Asian summer monsoon: a synthesis on chronologies of Zhuye Lake, Northwest China

The intensified monsoon increases summer rainfall and creates wet conditions in the Asian summer monsoon region during the early Holocene. Along with millennial-scale changes of the monsoon intensity, it is still unclear whether the boundary of the monsoon region changes according to monsoon variability. Investigations into the early Holocene environment in monsoon marginal zones are crucial for understanding the monsoon boundary changes. Zhuye Lake is located at the northwest edge of the Asian summer monsoon, the northern Qilian Mountains, which are less affected by modern summer monsoon water vapor. Previous studies have reached different conclusions regarding the early Holocene climatic and environmental changes based on different dating methods (14C and OSL (optically stimulated luminescence)) and materials (shells, carbonate, pollen concentrates and bulk organic carbon). In this study, we synthesized 102 14C dates and 35 OSL dates from ten Holocene sedimentary sections and ten paleo-shorelines in the lake basin. A comparison between ages from different dating methods and materials generally shows that carbon reservoir effects are relatively slight in Zhuye Lake while the disordered chronologies are mainly related to the erosion processes and reworking effects. In addition, proxy data, including lithology, pollen, total organic carbon and carbonate, were collected from different sites of Zhuye Lake. According to the new synthesis, the early Holocene environment was relatively humid, associated with high runoff and lake water levels. The result indicates that the monsoon boundary moves to the north during the period of the intensified monsoon. A typical arid-area lake was formed during the mid-Holocene when carbonate accumulation and high organic matter contents were the main features of this period. The lake retreated strongly during the late Holocene, showing a drought trend. Overall, the lake evolution is generally consistent with the Holocene Asian summer monsoon change, showing the monsoon influence to monsoon marginal zones.

An Abrupt Centennial-Scale Drought Event and Mid-Holocene Climate Change Patterns in Monsoon Marginal Zones of East Asia

The mid-latitudes of East Asia are characterized by the interaction between the Asian summer monsoon and the westerly winds. Understanding long-term climate change in the marginal regions of the Asian monsoon is critical for understanding the millennial-scale interactions between the Asian monsoon and the westerly winds. Abrupt climate events are always associated with changes in large-scale circulation patterns; therefore, investigations into abrupt climate changes provide clues for responses of circulation patterns to extreme climate events. In this paper, we examined the time scale and mid-Holocene climatic background of an abrupt dry mid-Holocene event in the Shiyang River drainage basin in the northwest margin of the Asian monsoon. Mid-Holocene lacustrine records were collected from the middle reaches and the terminal lake of the basin. Using radiocarbon and OSL ages, a centennial-scale drought event, which is characterized by a sand layer in lacustrine sediments both from the middle and lower reaches of the basin, was absolutely dated between 8.0–7.0 cal kyr BP. Grain size data suggest an abrupt decline in lake level and a dry environment in the middle reaches of the basin during the dry interval. Previous studies have shown mid-Holocene drought events in other places of monsoon marginal zones; however, their chronologies are not strong enough to study the mechanism. According to the absolutely dated records, we proposed a new hypothesis that the mid-Holocene dry interval can be related to the weakening Asian summer monsoon and the relatively arid environment in arid Central Asia. Furthermore, abrupt dry climatic events are directly linked to the basin-wide effective moisture change in semi-arid and arid regions. Effective moisture is affected by basin-wide precipitation, evapotranspiration, lake surface evaporation and other geographical settings. As a result, the time scales of the dry interval could vary according to locations due to different geographical features.

Lake evaporation: A possible factor affecting lake level changes tested by modern observational data in arid and semi-arid China

Qinghai Lake and Zhuye Lake, ∼400 km apart, are located in the northwest margin of the Asian summer monsoon. Water of these two lakes mostly comes from the middle and eastern parts of the Qilian Mountains. Previous studies show that the Holocene climate changes of the two lakes implied from lake records are different. Whether lake evaporation plays a role in asynchronous Holocene climate changes is important to understand the lake records. In this paper, we used modern observations beside Qinghai Lake and Zhuye Lake to test the impact factors for lake evaporation. Pan evaporation near the two lakes is mainly related to relative humidity, temperature, vapor pressure and sunshine duration. But temperature has different impacts to lake evaporation of the two lakes, which can affect Holocene millennial-scale lake level changes. In addition, differences in relative humidity on the millennial-scale would be more significant, which also can contribute to asynchronous lake records.

Holocene millennial-scale erosion and deposition processes in the middle reaches of inland drainage basins, arid China

In inland drainage basins, many studies have been conducted regarding the geomorphologic evolution processes in the upper and lower reaches, few of them have been done on the middle reaches, which are the pivotal areas connecting the erosion effects of the upper reaches and the deposition processes in terminal areas. In this study, we presented a millennial-scale study from middle reaches of the Shiyang River and the Shule River drainage basins during the Holocene. Several sedimentary sections are selected to calculate millennial-scale erosion and deposition rates. Results show that the middle reaches of the Shiyang River and the Shule River are characterized by deposition processes in the early and middle Holocene, but erosion processes prevailed during the late Holocene. At the same time, the sedimentary facies, grain size, geochemical proxies and pollen proxies of the sediments from middle reaches of the two drainages were used to complement the erosion and deposition rates, which are good for understanding the environment background during the geomorphologic evolution processes. Besides, the main factor affecting the basin-wide geomorphologic evolution is millennial-scale climate change, and the responses of the middle reaches to climate change are different from those of the upper and lower reaches.

Ecological responses to holocene millennial-scale climate change at high altitudes of east and Central Asia: A case study of Picea/Abies pollen changes in lacustrine sediments

Ecosystem response to climate change in high-altitude regions is a focus on global change research. Picea/Abies forests are widely distributed at high altitudes of East and Central Asia, and their distribution changes are sensitive to climate change. Humidity is an important climatic factor that affects high-altitude ecosystems; however, the relationship between distribution changes of Picea/Abies forests and millennial-scale variability of humidity is still not clear. Palynological records can provide insights into millennial-scale paleovegetation changes, which have been successfully used to reconstruct past climate change in East and Central Asia. In this study, we synthesized 24 Picea/Abies pollen and humidity/moisture changes based upon Holocene lake records in East and Central Asia in order to explore the response of high-latitude ecosystem to millennial-scale climate change. The changing pattern of Holocene lacustrine Picea/Abies pollen in arid Central Asia differs from that of monsoonal East Asia, which can be due to different millennial-scale climate change patterns between monsoonal and arid Central Asia. Then, the relationship between changes in Picea/Abies pollen and humidity/moisture conditions was examined based on a comparison of pollen and humidity/moisture records. The results indicate that millennial-scale Picea/Abies distribution changes are mainly controlled by moisture variability at high altitudes, while the temperature effect plays a minor role in Picea/Abies distribution changes. Moreover, this research proves that lacustrine Picea/Abies pollen can be used as an indicator of millennial-scale humidity/moisture evolution at high altitudes in East and Central Asia.

Holocene Climate Cycles in Northwest Margin of Asian Monsoon

In the mid-latitude regions of the Asian continent, Zhuye Lake is located in the northwest margin of the Asian monsoon, where the modern climate is affected by the Asian monsoon and Westerlies. In this study, we investigated the absolutely dated Holocene records in Zhuye Lake for detecting the Holocene climate cycles. Totally, 14 14C dates and 6 optically simulated luminescence (OSL) dates are obtained from the QTH01 and QTH02 sections. The proxies of grain-size, total organic carbon content (TOC), C/N and δ13C are used for wavelet analysis, and the results show obvious ∼256, ∼512 and ∼1024-year climate cycles, which are consistent with the Holocene millennial and centennial scale climate cycles in the typical Asian summer monsoon domain. In different parts of the Zhuye Lake, the Holocene sediments show variable climate cycles that are affected by the lake basin topography. In the Zhuye Lake, the Holocene climate cycles are mainly correlated with the solar-related Asian summer monsoon variability and the North Atlantic ice-rafting events.

Temporal and spatial evolution of Holocene vegetation and lake hydrological status, China

Studying past climate change has important scientific significance in exploring long-term climatic variability, assessing the nature and human’s contribution to climate warming, and predicting future climate change. Here, we presented a data synthesis of pollen records, organic geochemical proxies, as well as lake-level records, along with numerical climate classification and lake-level simulations to show temporal and spatial evolution of past vegetation and hydrologic change during the Holocene in China. Both geological data and numerical modeling indicated that the evolution of vegetation shows out-of-phase relationships with lake hydrological status in various regions. China as a whole experienced optimal vegetation cover in the mid-Holocene. However, areas dominated by the Asian monsoon and the Westerlies did not uniformly show an early Holocene hydrological optimum. For the Westerlies-controlled regions, optimal conditions prevailed during the mid-Holocene. These apparent contradictions were closely related to various driving factors in different geographical regions. Precipitation has been recognized as a major influence on lake hydrological status of arid and semi-arid in northeast China, whereas in other regions of China, the combined effect of evaporation and precipitation was more influential. Likewise, the limiting factor of vegetation conditions was asynchronous in China, for example, moisture in northwestern China, temperature in the Qinghai-Tibet Plateau, as well as moisture and temperature in the monsoon marginal zones and monsoonal regions. The results provide insights into the underlying climate-forcing mechanisms, and demonstrate a new perspective on Quaternary environment change research.

Long-Term Fine-Grained Sediment Records in a Drainage System in Arid China: A New Perspective from Paleo-Climatological Records and Simulations

Atmospheric particulate matter (PM 2.5 and PM 10) can adversely affect human health and also has impacts on climate and precipitation. Much research has been done on the transport of human-made fine-grained matter in modern times. It is still unclear, though, what controls the transport process of fine-grained sediment from natural sources on the millennial scale. In this study, we present Holocene basin-wide fine-grained sediment records from the Shiyang River drainage basin system in arid China. Six Holocene sedimentary sequences were collected from various geomorphological units of the drainage system. A total of 1,043 sediment samples were obtained for analysis of fine-grained sediment; fifty-eight radiocarbon dates were acquired for establishing the geochronological frames. In addition, we synthesized the results from transient paleo-climate simulations to understand environmental backgrounds of the Holocene. Our records and simulations indicated that fine-grained sediment 2.5 μm content was relatively stable and less affected by monsoon intensities and circulations. Millennial-scale fine-grained sediment 10 μm content varied according to sedimentary facies, and it was negatively correlated with the winter monsoon intensity at eolian sediments. The fine-grained sediment 10 μm content increases dramatically at lacustrine layers in lake and alluvial sediments from the middle and lower drainage basin, showing its relationship with long-term moisture conditions that are closely related to monsoon precipitation based on climate simulations. This finding contributes to our understanding of the fine-grained matter trends against the backdrop of global warming.