Guoqiang Chu

Periodicity of Holocene climatic variations in the Huguangyan Maar Lake

There exist five primary periods of 2 930, 1 140, 490, 250 and 220 a in the Holocene climatic variations in the Huguangyan Maar Lake, according to the energy-spectrum and filter analyses of high-resolution time sequences (10–15 a) of the sediment dry density. The peak values of the three temperature-decreasing periods with the 2 930 a cycle occur at about 7 300, 4 250 and 1 200 Cal. aBP. There are 7–8 temperature-decreasing periods with the 1 140 a cycle, and the climate fluctuation range is largest in the early Holocene, and reduces gradually in the middle and late Holocene. The millennial-scale climatic change in the Holocene may adjust the global water cycle and the thermohaline circulation intensity through the harmonic tones of the earth’s precession cycle, which in turn influences the global climate change.

Seasonal temperature variability during the past 1600 years recorded in historical documents and varved lake sediment profiles from northeastern China

Seasonal temperature variability over longer timescales could offer new insights into understanding different forcing factors and response processes in the climate system. Here we report an alkenone-based temperature reconstruction for growing season over the past 1600 years from the varved sediment in Lake Sihailongwan, northeastern China. The most notable cold spells occurred during the periods ad 480–860, ad 1260–1300, ad 1510–1570 and ad 1800–1900 with a temperature decrease of about 1°C compared with the 20th century. Based on the historical evidence such as ‘snow or frost in the summertime’ and ‘no ice during the wintertime’, we compile extreme cold summer events and warm winter events over the past 1600 years. The ‘Little Ice Age’ time period experienced more extreme cold summer/warm winter events, while the ‘Medieval Warm Period’ had milder winters. Comparatively, the natural proxy data show a general similar pattern with historical documents at decadal time scales, except for between ad 1620 and 1720. Our results show multidecadal to centennial variations in seasonal temperature, possibly caused by interactions of external natural forcing and atmosphere–ocean circulations.

Alkanes, compound-specific carbon isotope measures and climate variation during the last millennium from varved sediments of Lake Xiaolongwan, northeast China

Annually laminated sediments in Lake Xiaolongwan, northeast China, contain a suite of n-alkanes (C17–C33) with a strong odd over even carbon number predominance. Biomarker n-alkane proxies (average chain length, Paq ratio, grass/tree ratio, carbon preference index and compound-specific δ13C values) were used to reconstruct climate changes that occurred over the last millennium. Compound-specific δ13C values show large differences between the distinctive chain length groups of n-alkanes that originate from algae, aquatic macrophytes and terrestrial plants. Long-chain n-alkanes (C27–C33) are predominantly derived from leaf wax lipids in the forest. Variations in long-chain n-alkane δ13C values may mainly record water-use efficiency, inasmuch as the contribution from C4 plants is negligible in the Lake Xiaolongwan catchment. Short- and middle-chain n-alkanes are mainly from algae and aquatic plants. They are strongly depleted in 13C. This feature may be linked to a methane-derived, negative δ13C pool and lake overturn, which regulates dinoflagellate blooms. Parallel fluctuations are observed in δ13C27–31 values, Paq, and the grass/tree ratio throughout the record. Variations in δ13C27–31 values and Paq are in agreement with historical documents on summer and winter climate conditions. They support earlier suggestions that δ13C values in the long-chain n-alkanes and Paq may be useful indicators of effective precipitation or drought stress in this forested area. The δ13C27–31 index and Paq show distinct decadal variations. Periods with high δ13C27–31 values and a low Paq index correspond with the warm phases of the Pacific Decadal Oscillation (PDO). Values are reversed during PDO cool phases. At the decadal timescale, summer monsoon rainfall in northeast China over the last millennium may have been regulated mainly by the PDO.

The two-step monsoon changes of the last deglaciation recorded in tropical Maar Lake Huguangyan,southern China

The concentrations of biogenic silica, total organic carbon, total nitrogen and total hydrogen inferred from the sediments of tropical Maar Lake Huguangyan, southern China, provide a climate record of the last déglaciation with century resolution. The records fully demonstrate the existence of the two-step shape of the last déglaciation in tropic East Asia, and they point out noticeable differences between the low and high latitudes in the Northern Hemisphere. Thus, the Boiling first warming at the last deglaciation in the low latitude may have preceded that of the high latitude, whereas the cooling of the Younger Dryas occurred synchronously in the two regions. These results likely suggest that the links between the low and high latitude climates in the Northern Hemisphere during this period are complexity.

Vegetation and Climate Change during the Last Deglaciation in the Great Khingan Mountain, Northeastern China

The Great Khingan Mountain range, Northeast China, is located on the northern limit of modern East Asian Summer Monsoon (EASM) and thus highly sensitive to the extension of the EASM from glacial to interglacial modes. Here, we present a high-resolution pollen record covering the last glacial maximum and the early Holocene from a closed crater Lake Moon to reconstruct vegetation history during the glacial-interglacial transition and thus register the evolution of the EASM during the last deglaciation. The vegetation history has gone through distinct changes from subalpine meadow in the last glacial maximum to dry steppe dominated by Artemisia from 20.3 to 17.4 ka BP, subalpine meadow dominated by Cyperaceae and Artemisia between 17.4 and 14.4 ka BP, and forest steppe dominated by Betula and Artemisia after 14.4 ka BP. The pollen-based temperature index demonstrates a gradual warming trend started at around 20.3 ka BP with interruptions of several brief events. Two cold conditions occurred around at 17.2–16.6 ka BP and 12.8–11.8 ka BP, temporally correlating to the Henrich 1 and the Younger Dryas events respectively, 1and abrupt warming events occurred around at 14.4 ka BP and 11.8 ka BP, probably relevant to the beginning of the Bølling-Allerød stages and the Holocene. The pollen-based moisture proxy shows distinct drought condition during the last glacial maximum (20.3–18.0 ka BP) and the Younger Dryas. The climate history based on pollen record of Lake Moon suggests that the regional temperature variability was coherent with the classical climate in the North Atlantic, implying the dominance of the high latitude processes on the EASM evolution from the Last Glacial Maximum (LGM) to early Holocene. The local humidity variability was influenced by the EASM limitedly before the Bølling-Allerød warming, which is mainly controlled by the summer rainfall due to the EASM front covering the Northeast China after that.

A 150-year Record of Heavy Metals in the Varved Sediments of Lake Bolterskardet, Svalbard

Abstract Laminated sediments from Bolterskardet Lake on Svalbard provide a new 150-year record of heavy metals in the Arctic. Independent data of 137Cs and 210Pb indicate that these laminations are annually deposited varves. The high sedimentation rate and varved sediments make Lake Bolterskardet a good site for studying history of heavy metal pollution in the region. A suite of heavy metals (Pb, As, Cd , Cu, Cr, Co, Ni, and Sn) were studied. The variations of Cu, Cr, Co, and Ni concentrations show an inverse pattern with the median grain size. It suggests that the particle size has a significant role in the accumulation and enrichment of heavy metals in the sediments. In the concentration profiles of studied heavy metals, only Pb concentrations show a significant increase from the lower parts to the upper parts of the core. Profiles of “total,” “lithogenic,” and “anthropogenic” Pb flux also show an increasing pattern. Anthropogenic Pb flux varies between 0.1 μg cm−2 yr−1 and 12.3 μg cm−2 yr−1, with a mean value of 2.4 μg cm−2 yr−1. The anthropogenic Pb fluxes were relatively low at around 0.7 μg cm−2 yr−1 prior to 1945, slowly increased after 1945, and reached a sidestep (between 1940s and 1970s) with mean value of 1.8 μg cm−2 yr−1. Second high value period was between 1980s and 1990s with mean value of 5.9 μg cm−2 yr−1. The variation of anthropogenic Pb fluxes before 1970 in Bolterskardet Lake coincides with the sulfate record and Pb concentration data from Greenland snow and ice, and the acidity concentration in the Svalbard ice cores. However, the anthropogenic Pb fluxes during the last two decades show a rapidly increasing trend. This regional pattern may suggest that local source is an important factor for heavy metal contamination in Svalbard, and the complex of long-range transport contaminations for regional and global process.

Minor element variations during the past 1300 years in the varved sediments of Lake Xiaolongwan, north-eastern China

Synchrotron radiation X-ray fluorescence measurement provides a nondestructive in situ analysis of elements. We present a high-resolution minor element dataset covering the past 1300 years from the varved sediments in Lake Xiaolongwan. The variations of lithogenic elements (Rb, Sr, Ti, Zn, As and Fe) in the lake sediment show distinct decadal to centennial features during the past 1300 years. The Medieval Warm Period has a low value of lithogenic elements. In contrast, the Little Ice Age may be a period with stronger physical weathering and a higher value of lithogenic elements. Principal components analysis of our geochemical dataset suggests a link between high Rb/Sr ratio and Zr abundance in the sediment that could be related with both chemical weathering process and dust input. The variations of biogenic bromine (factor-3) show a pattern similar to the decadal drought index in Korea. Wet conditions favor bromine accumulation in lake sediment because high precipitation may transport more bromine from forest soil and increase plankton production in the lake. Spectral analysis of factor-3 yields notable periodicities of 2.6, 3.5, 53–55, 87–89 and 105–110 years, implying that precipitation variability for the past 1400 years could be associated with El Niño–Southern Oscillation and solar activity.

High-resolution magnetic and palynological records of the last deglaciation and Holocene from Lake Xiarinur in the Hunshandake Sandy Land, Inner Mongolia

High-resolution mineral magnetic and pollen records from overlapping piston cores from Lake Xiarinur (42°37′N, 115°28′E) document detailed changes in environment and vegetation since the last deglaciation in the Hunshandake Sandy Land, Inner Mongolia. The formation of Lake Xiarinur commenced during the Bølling–Allerød warming, as reflected by an abrupt shift in sedimentary facies from eolian sands to lacustrine sediments at a core depth of 3.5 m (~14.1 cal. kyr BP). The pollen records demonstrate that desert vegetation with sparse herbs occurred before 14.1 cal. kyr BP, which was succeeded by meadow grassland vegetation from ~14.1 –13.3 cal. kyr BP (the Bølling -Allerød). A dramatic decrease in pollen concentration occurred between ~13.3 and 11.7 cal. kyr BP corresponding to the Younger Dryas. The remarkable increases in pollen and Pediastrum concentrations at ~11.7 cal. kyr BP suggest that the environment began to ameliorate, and the most humid conditions prevailed until ~8.5 cal. kyr BP. The maxima of magnetic concentration–dependent and magnetic-grain-size-dependent parameters, together with high pollen concentrations, between ~10 and 9 cal. kyr BP are interpreted as a brief interval of high annual precipitation. Our results indicate a stepwise decrease in effective moisture in the mid-Holocene and an accelerated drying trend in the late Holocene, characteristics typical of an East Asian summer monsoon–influenced region. The broad similarities between our data and other lake records from central-eastern Inner Mongolia, well-dated speleothem δ18O records from southern and central China, and summer insolation at 45°N, support the idea that vegetation and climate changes in the Hunshandake Sandy Land were controlled mainly by fluctuations in the response of the East Asian monsoon to Northern Hemisphere summer insolation.

Discrimination of two kinds of sedimentary laminae in maar lakes of China

Varved lacustrine sediments have been found from high latitude to low latitude, even in tropical area. Paleoenvironmental and paleoclimatic research has increasingly relied on the use of varved sediments to provide high-resolution geochronologies and climatic reconstructions. However, a special depositional environment is necessary for varve formation and preservation. Generally, varves can be found in the sediments of maar lakes because of their special geological and hydrological environment. The preliminary research showed that a few carbonate laminations are developed in the Huguangyan maar lake, south China. They are poorly developed and not annual laminations, and may be related to periodically drought. Diatomaceous laminations are found in the maar lakes in northeast China. Diatomaceous laminations with cyclic nature corresponding with seasonal diatom blooming are the credible natural clock.

Surface soil phytoliths as vegetation and altitude indicators: a study from the southern Himalaya

Phytoliths represent one of the few available altitudinal vegetation proxies for mountain ecosystems. This study analyzed 41 topsoil phytolith samples collected from five altitudinal zones in the southern Himalaya as far as, and beyond, the timberline, from tropical forest (up to 1,000 m a.s.l.) to subtropical forest (1,000–2,000 m a.s.l.), to temperate forest (2,000–3,000 m a.s.l.), to subalpine forest (3,000–4,100 m a.s.l.) and finally to alpine scrub (4,100–5,200 m a.s.l.). The statistical results show a good correlation between phytolith assemblages and these five altitudinal vegetation zones: the five phytolith assemblages identified effectively differentiated these five altitudinal vegetation zones. In particular, coniferous phytoliths accurately indicated the timberline. Additionally, we tested the phytolith index Ic (a proxy for estimating the percentage of Pooideae vis-à-vis the total grass content) as a quantifier of phytolith variety versus altitude. Ic increased along altitude, as expected. An investigation of phytoliths provided an initial basis for the analysis of the composition of gramineous vegetation. Furthermore, redundancy analysis and discriminant analysis also suggested a significant correlation between phytolith assemblages and altitude. Our research therefore provides an up-to-date analogue for the reconstruction of changes to palaeovegetation and palaeoaltitude in mountainous areas.