Junyan Sun

Pedostratigraphy and paleomagnetism of a ∼7.0 Ma eolian loess–red clay sequence at Lingtai, Loess Plateau, north-central China and the implications for paleomonsoon evolution

Abstract A 305-m-thick loess–red clay sequence was discovered recently at Lingtai, which is located in the middle part of the Chinese Loess Plateau. It consists of a complete Pleistocene loess–soil sequence with a thickness of about 175 m and 130 m of Tertiary red clay deposits. The red clay sequence at Lingtai is the thickest one presently known in the Loess Plateau. Field observations show that the Lingtai red clay contains over 110 couplets of pedogenic B horizons and horizontal carbonate nodule horizons, and that it can be regarded as an extremely thick soil complex. Paleomagnetic studies of 680 samples suggest that the basal age of the Lingtai loess–red clay sequence is about 7.05 Ma. Grain size analysis of samples taken at 3.3 cm intervals indicates that the red clay has the same sedimentological characteristics as those observed in the loess, thus suggesting a wind-blown origin of the Tertiary red clay and continuous atmospheric dust deposition in the Loess Plateau during the last 7.05 Ma. The pedogenic characteristics of the paleosols within the Pleistocene loess and the B horizons in the red clay suggest that the East-Asia summer monsoon in the latest Miocene may have already been slightly stronger than that during the Holocene. This implies that at about 7.5 Ma BP, the Tibetan Plateau could have been uplifted to a critical height in maintaining the East-Asia summer monsoon system. Observations of the loess–red clay sequence also suggest that the long-term changes in the East-Asia summer monsoon strength have been nonlinear since the latest Miocene.

Re-arrangement of atmospheric circulation at about 2.6 Ma over northern China: evidence from grain size records of loess-palaeosol and red clay sequences

Abstract Recent studies have shown that the red clay sequence underlying the Quaternary loess of the Chinese Loess Plateau is wind-blown in origin. Continuous atmospheric dust deposition in the past 7.0xa0Ma has been documented. To address the wind system that transported the Tertiary red clay, two north–south transects were studied in the Chinese Loess Plateau. One of the transects was designed to study spatial changes in grain size of the last glacial–interglacial loess records, and the other to observe particle changes of the Tertiary red clay underlying the Quaternary loess. The loess transect consists of nine sections, and the red clay transect of four sections. Analyses of closely spaced samples show that there is a strong southward decrease in grain size of both loess and palaeosol horizons of the Late Pleistocene, which is consistent with the idea that the aeolian materials of the Quaternary in the Loess Plateau are transported by the northerly winter monsoonal winds. Grain size distribution of the red clay sequences, however, does not show such a change. From north to south along the red clay transect, the particle size distribution is almost identical in the four sections, suggesting that the winter monsoonal winds might have played a less important role in transporting the red clay material. It is suggested that the red clay may have been transported by the westerlies from the dust-source regions of northwestern China onto the Loess Plateau. A remarkable re-arrangement of atmospheric patterns at about 2.6xa0Ma, therefore, has been recorded by the red clay-loess shift. It is speculated that this re-arrangement of atmospheric patterns may have been caused by the onset of glaciation in the Northern Hemisphere.

Tree-ring hydrologic reconstructions for the Heihe River watershed, western China since AD 1430

Based on the tree-ring-width analysis, the total precipitation from previous July to current June of the Qilian Mountains from 1634 to 2000 AD and the average runoff from previous September to current June in the middle section of the Heihe River from 1430 to 2007 have been reconstructed. This allowed detailed examination of the hydrologic history of the watershed of the Heihe River in western China. Precipitation, runoff and groundwater level were found to be significantly correlated with each other on the decadal scale. The three curves display quite synchronous trends of natural variation before AD 1940 to present before the onset of man-made disturbances. A remarkable period is AD 1925-1940 when the precipitation is low in the upper section, the runoff decreases in the middle section, and the groundwater level declines in the downstream section. After 1940, the groundwater level shows a lag effect, which may be a result of high water consumption in the middle and downstream sections. All three tree-ring based hydrologic indices commonly display the most significant periodicities around 80 (78-82), 50 (49-58) and 2 year. These cycles correspond to large-scale oscillation found in the climate system and appear mainly related to ocean-atmosphere interaction.

Reconstruction of May–July precipitation in the north Helan Mountain, Inner Mongolia since A.D. 1726 from tree-ring late-wood widths

By analyzing statistical characteristics of five tree-ring standard chronologies, early-wood ring width (EWW), late-wood ring width (LWW), total ring width (TRW), minimum early-wood density (MinD), maximum late-wood density (MaxD) and, their climatic response respectively, we reconstructed the May to July precipitation using late-wood ring width (LWW) over the north Helan Mountain since A.D. 1726. The explained variance is 42% (R2adj = 41%,F= 31.46,p < 0.000001). After 11-a moving average, the explained variance reaches 82% (F= 156.9,p < 0.05). On the decadal scale, the rainfall reconstruction of the northern Helan Mountain displays a quite similar variation pattern with that of the April to early July precipitation in Baiyinaobao, east of Inner Mongolia for the last 150 years. It may reflect the intensity variation of the East Asia Summer Monsoon front to a certain extent. Spectrum analysis shows 11-a and 22-a periodicities in the May to July precipitation reconstruction at the north Helan Mountain.

Tree-Ring-Derived Precipitation Records from Inner Mongolia, China, Since A.D. 1627

Two Chinese pine (Pinus tabulaeformis) tree-ring width chronologies up to 375 years long were used to reconstruct rainfall from February to early July for the Wu Dangzhao region and from February to mid-July for the La Madong region, Inner Mongolia, China. The predictor variables account for 44.3% and 42.7% of the variance in precipitation, respectively. Both historical records and two other tree-ring based precipitation reconstructions from the environmentally sensitive zone (the northern Helan Mountain range and Baiyinaobao) confirm our results. After applying a 10-year moving average, the trends of four tree-ring based precipitation reconstructions vary synchronously. Periods with below-normal precipitation occurred during the 1720s–1730s, 1740s–1750s, 1790s, early 1810s, late 1830s–1860s, 1880s–1910s, late 1920s–1930s and after the late 1960s–early 1970s. Periods with above-normal precipitation occurred in the 1760s to early 1770s, 1820s to early 1830s, 1870s–1880s, early 1920s, 1940s to early 1960s, and 1990s. The late 1920s period was the most severe drought over a broad area in north China in the last 375 years. In contrast, the wettest period was in the late 1990s.

Recent enhancement of central Pacific El Nino variability relative to last eight centuries

The far-reaching impacts of central Pacific El Niño events on global climate differ appreciably from those associated with eastern Pacific El Niño events. Central Pacific El Niño events may become more frequent in coming decades as atmospheric greenhouse gas concentrations rise, but the instrumental record of central Pacific sea-surface temperatures is too short to detect potential trends. Here we present an annually resolved reconstruction of NIÑO4 sea-surface temperature, located in the central equatorial Pacific, based on oxygen isotopic time series from Taiwan tree cellulose that span from 1190 AD to 2007 AD. Our reconstruction indicates that relatively warm Niño4 sea-surface temperature values over the late twentieth century are accompanied by higher levels of interannual variability than observed in other intervals of the 818-year-long reconstruction. Our results imply that anthropogenic greenhouse forcing may be driving an increase in central Pacific El Niño-Southern Oscillation variability and/or its hydrological impacts, consistent with recent modelling studies.

Tree-ring based PDSI reconstruction since 1853 AD in the source of the Fenhe river basin, Shanxi province, China

A robust tree-ring-width chronology was developed from two Pinus tabulaeformis sampling sites in the source of the Fenhe River, Shanxi Province, China. Based on the tree-ring-width indices, a 157-year long Palmer Drought Severity Index (PDSI) was reconstructed, which explains 53.7% of the variance of the modeled PDSI over the common period 1957-2008. The drought periods in the study area include 1914–1931 and 1970 to the present, whereas the wet periods were 1866-1892 and 1932–1969. The drought of 1914–1931 was a severe long-lasting drought with a low inter-annual variability, and the drought of 1970–2009 was an overall long-term drought with a high inter-annual variability. The period of 1866-1892 is a continuously wet period with a low inter-annual variability and the period of 1932–1969 is an overall long-term wet period with a high inter-annual variability. The reconstructed PDSI series in the source of the Fenhe River shows synchronous variations with the regional drought/wetness indices. Spatial correlation analyses indicate that the higher correlations lie exclusively in the Fenhe River Basin. This indicates that the reconstructed PDSI has regional representativeness and can represent the drought history of the entire Fenhe River Basin to some extent. Furthermore, the reconstructed PDSI matches with the variability of the per unit yield of summer grain crops in Shanxi Province very well and they have significant correlation. From a long-term perspective the reconstructed PDSI series could supply scientific and valuable information to the water resources management and then help the sustainable development in agricultural production, economic development, and ecosystem balance.

Annual precipitation in Liancheng, China, since 1777 AD derived from tree rings of Chinese pine (Pinus tabulaeformis Carr.)

Precipitation from the previous August to the current June over the last 232xa0years in Liancheng, China, was reconstructed by a transfer function based on the correlation between tree-ring widths and local meteorological data. The explained variance was 45.3xa0%, and fluctuations on both annual and decadal scales were captured. Wet periods with precipitation above the 232-year mean occurred from 1777 to 1785, 1802 to 1818, 1844 to 1861, 1889 to 1922 and 1939 to 1960. Dry periods (precipitation below the mean) occurred from 1786 to 1801, 1819 to 1843, 1862 to 1888 and 1923 to 1938. The reconstruction compares well with a tree-ring-based precipitation reconstruction at Mt. Xinglong; both of them showed the well-known severe drought in the late 1920s. The rainfall series also shows highly synchronous decreasing trends since the 1940s, suggesting that precipitation related to the East Asian summer monsoon at these two sites has decreased by large spatial and temporal (decadal) scales. Power spectrum analysis of the reconstruction showed remarkable 21.82-, 3.48-, 3.12-, 3.08- and 2.31-year cycles for the past 232xa0years; the 22-year cycle corresponds to the solar cycle and is expressed widely in tree ring/precipitation reconstructions on the China Loess Plateau. This may suggest a solar influence on the precipitation variations on the Loess Plateau, although the mechanisms are not well understood.

A Standardized Precipitation Evapotranspiration Index Reconstruction in the Taihe Mountains Using Tree-Ring Widths for the Last 283 Years.

Tree-ring samples from Chinese Pine (Pinus tabulaeformis Carr.) that were collected in the Taihe Mountains on the western Loess Plateau, China, were used to analyze the effects of climate and drought on radial growth and to reconstruct the mean April-June Standardized Precipitation Evapotranspiration Index (SPEI) during the period 1730–2012 AD. Precipitation positively affected tree growth primarily during wet seasons, while temperature negatively affected tree growth during dry seasons. Tree growth responded positively to SPEI at long time scales most likely because the trees were able to withstand water deficits but lacked a rapid response to drought. The 10-month scale SPEI was chosen for further drought reconstruction. A calibration model for the period 1951–2011 explained 51% of the variance in the modeled SPEI data. Our SPEI reconstruction revealed long-term patterns of drought variability and captured some significant drought events, including the severe drought of 1928–1930 and the clear drying trend since the 1950s which were widespread across northern China. The reconstruction was also consistent with two other reconstructions on the western Loess Plateau at both interannual and decadal scales. The reconstructed SPEI series showed synchronous variations with the drought/wetness indices and spatial correlation analyses indicated that this reconstruction could be representative of large-scale SPEI variability in northern China. Period analysis discovered 128-year, 25-year, 2.62-year, 2.36-year, and 2.04-year cycles in this reconstruction. The time-dependency of the growth response to drought should be considered in further studies of the community dynamics. The SPEI reconstruction improves the sparse network of long-term climate records for an enhanced understanding of climatic variability on the western Loess Plateau, China.

Responses of tree-ring growth and crop yield to drought indices in the Shanxi province, North China

In this paper, we analyze the relationships among the tree-ring chronology, meteorological drought (precipitation), agricultural drought (Palmer Drought Severity Index PDSI), hydrological drought (runoff), and agricultural data in the Shanxi province of North China. Correlation analyses indicate that the tree-ring chronology is significantly correlated with all of the drought indices during the main growing season from March to July. Sign test analyses further indicate that the tree-ring chronology shows variation similar to that of the drought indices in both high and low frequencies. Comparisons of the years with narrow tree rings to the severe droughts reflected in all three indices from 1957 to 2008 reveal that the radial growth of the trees in the study region can accurately record the severe drought for which all three indices were in agreement (1972, 1999, 2000, and 2001). Comparisons with the dryness/wetness index indicate that tree-ring growth can properly record the severe droughts in the history. Correlation analyses among agricultural data, tree-ring chronology, and drought indices indicate that the per-unit yield of summer crops is relatively well correlated with the agricultural drought, as indicated by the PDSI. The PDSI is the climatic factor that significantly influences both tree growth and per-unit yield of summer crops in the study region. These results indicate that the PDSI and tree-ring chronology have the potential to be used to monitor and predict the yield of summer crops. Tree-ring chronology is an important tool for drought research and for wider applications in agricultural and hydrological research.