Xiayun Xiao

Glacial-Interglacial Indian Summer Monsoon Dynamics

Indian summer monsoon changes during the Pleistocene were influenced by dynamic effects originating in both hemispheres. The modern Indian summer monsoon (ISM) is characterized by exceptionally strong interhemispheric transport, indicating the importance of both Northern and Southern Hemisphere processes driving monsoon variability. Here, we present a high-resolution continental record from southwestern China that demonstrates the importance of interhemispheric forcing in driving ISM variability at the glacial-interglacial time scale as well. Interglacial ISM maxima are dominated by an enhanced Indian low associated with global ice volume minima. In contrast, the glacial ISM reaches a minimum, and actually begins to increase, before global ice volume reaches a maximum. We attribute this early strengthening to an increased cross-equatorial pressure gradient derived from Southern Hemisphere high-latitude cooling. This mechanism explains much of the nonorbital scale variance in the Pleistocene ISM record.

Diatom response to Asian monsoon variability during the Late Glacial to Holocene in a small treeline lake, SW China

Analyses of diatoms, grain size, magnetic susceptibility, total organic carbon, and total nitrogen were applied to a 9.26 m long sediment core, spanning the last 12.2 kyr, from a small treeline lake (Tiancai Lake, ~3898 m a.s.l.) in southwest China. Diatom assemblages are dominated by Cyclotella distinguenda, Aulacoseira species, and small fragilarioid taxa, all of which are sensitive to changes in water pH and light conditions that are probably related to vegetation development and runoff processes triggered by variations in the Asian monsoon. High abundances of C. distinguenda and Pseudostaurosira brevistriata reflected cold and dry climates during the Late Glacial (12.2–11.4 kyr BP). In the early Holocene (11.4–9.4 kyr BP), a steep decline in C. distinguenda and a visible increase in Aulacoseira alpigena responded to a strengthening monsoon intensity. The persistent increases in A. alpigena mirrored strong monsoon intensity in the middle Holocene (9.4–4.6 kyr BP). After 4.6 kyr BP, the reduction of A. alpigena was related to weak monsoon intensity in the late Holocene. The main trends of diatom evolution show a general correspondence to variations in solar insolation. Three visible excursions, with an increase in P. brevistriata and a drop in A. alpigena, centered at around 8.4, 2.5, and 0.3 kyr BP, correlate with low sunspot numbers and known cold events in the North Atlantic. Some similarities and correlations between the Holocene diatom data, the North Atlantic record, and solar insolation indicate that variations in the Asian monsoon response to changes in solar forcing and the North Atlantic climate.

Vegetation history and dynamics in the middle reach of the Yangtze River during the last 1500 years revealed by sedimentary records from Taibai Lake, China

Pollen and charcoal records of a 150 cm long lake sediment core from Taibai Lake in the middle reach of the Yangtze River reveal seven major changes in regional vegetation over the last 1500 years. During the period c. ad 480–710, evergreen broadleaved forest dominated by Castanopsis/Lithocarpus and evergreen oaks occurred in the Taibai Lake catchment. From c. ad 710 to 1050, the vegetation shifted to a mixed conifer and broadleaved forest, with Pinus expanding at the expense of Castanopsis/Lithocarpus. Between c. ad 1050 and 1320, evergreen broadleaved forest reoccupied the studied area. From c. ad 1320 to 1650, the area of primary forest decreased markedly with synchronous reduction in broadleaved trees and Pinus. Between c. ad 1650 and 1740, the biomass declined rapidly, while secondary Pinus forest began to expand. During c. ad 1740–1950, forest extent increased slightly compared with the previous stage, but the landscape was still secondary forest with the minimum proportion of broadleaved trees. After c. ad 1950, the biomass in the surrounding area was very low, with vegetation types similar to that at present (secondary Pinus forest and mixed conifer and broadleaved forest). A detrended correspondence analysis (DCA) with 125 samples and 34 main pollen types is used to separate human and climatic impacts on vegetation. Then, the pollen assemblage, DCA, charcoal record, and magnetic susceptibility are combined to discuss the key factors inducing these vegetation changes. The vegetation changes were mainly controlled by the climatic changes, with the weak impacts of human activities before c. ad 1320. Since then, the intensity of human influences on vegetation increased gradually, entering a transitional period of main controlling factors of vegetation changes from nature to human activities. After c. ad 1740, the vegetation changes were chiefly controlled by human activities, and the climatic signal was weak.

Evidence of Holocene climatic change and human impact in northwestern Yunnan Province: High-resolution pollen and charcoal records from Chenghai Lake, southwestern China

High-resolution pollen and charcoal records from Chenghai Lake, northwestern Yunnan Province of China, are presented in this study. These records reveal changes in climate and human activity since 8220 cal. yr BP, and the drivers of climatic change are further discussed based on spectral analysis of Tsuga and evergreen oak pollen percentages. The pollen record indicated that the climate in the low altitude area was relatively warm and moderately dry, whereas the climate in the high altitude area was relatively cool and humid during the period 8220–4670 cal. yr BP, corresponding to the Holocene climate optimum, although the seasonality of precipitation increased between 5180 and 4670 cal. yr BP. From 4670 to 3470 cal. yr BP, the temperature declined, and the precipitation and its seasonality decreased. Between 3470 and 1860 cal. yr BP, the temperature was still relatively low, and precipitation appeared to decrease further. At the same time, human activity represented by agricultural planting began to influence this study area, but the intensity of human activity was relatively weak. After 1860 cal. yr BP, the climate was gradually becoming drier and the intensity of human activity such as deforestation and slash-and-burn agriculture increased. Spectral analysis of Tsuga and evergreen oak pollen percentages shows statistically significant periodicities of ~1500 and ~200 years at the ≥95% confidence level, which suggests that insolation and feedback interactions on millennial to centennial scales are the primary forcing mechanisms of the southwest monsoon and past climatic change during the Holocene in northwestern Yunnan Province, southwestern China.

Sediment environment of surface pollen in sea area to the west of Guangdong and east of Hainan

The study area is an open gulf in the west of Pearl River Estuary, and to the Qiongzhou Strait and east coast of Hainan Island. Here, we palynologically analyzed 400 surface sediment samples collected from this sea area. All these samples contain abundant pollen and spores. According to concentration(weight concentrations) distributions of the total pollen, arboreal and terrestrial herbaceous pollen and fern spores, the study area can be divided into 10 pollen regions. In addition, three high-concentration and three low-concentration subregions can be distinguished within these pollen regions, with the consideration of extremely high values and/or low values of pollen concentrations. Based on the submarine topography and marine dynamic conditions, the 10 pollen regions are summarized as four types of depositional environment, namely, along-shore zone, nearshore zone, offshore zone and steep-slope zone of island. Our results show that depositional environment of along-shore zone(I, VI, IX and X pollen regions) is relatively complex with high-concentration and low-concentration sugregions. Herb pollen percentages in the zone are the highest. The pollen concentration of nearshore zone(II, III, V, VII pollen regions) is moderately high, and pollen percentages of trees(especially Pinus) and ferns there increase slightly. The pollen concentration of offshore zone(IV pollen region) is low, and its pollen assemblage is highlighted by high percentages of herbs. In steep-slope zone of the island(VIII pollen region), pollen assemblages appear as a mixed characteristics of the other three zones with moderately high pollen percentages. High-concentration subregion is related to activities of water masses. For example, high-concentration subregion A is located in oceanic front belt where the Pearl River freshwater diffuses westward and mixes with the shelf seawater, and high-concentration subregions B and C in the counterclockwise circulation(the low-velocity zone) of the eastern water of Leizhou Peninsula. Low-concentration subregion is usually related to activities of wave and tidal. For instance, low-concentration subregion D is situated in a strong wave-erosion zone along the coast of Leizhou Peninsula, low-concentration subregion E in a strong tide zone in the east of Qiongzhou Strait, and low-concentration subregion F in a zone of frequent internal tides and waves near continental slope.