中国内陆区湖泊沉积所反映的全新世干湿变化

湖泊沉积记录的环境演变是全球变化的重要研究领域之一,通过对中国内陆区中30 个湖泊研究成果的总结和梳理,探讨了全新世以来该地区干湿变化的规律和区域分异。通过降水量结合传统分区方法将中国内陆区分为西北干旱区、东亚季风边缘区和青藏高原区。对每个湖泊样点以500 年为时间间隔,以孢粉为主要干湿指标,综合氧同位素、有机质及碳酸盐等,将湖泊干湿状况划分3 个干湿等级（干旱,半湿润,湿润）,建立区域干湿指数。结果表明,中国内陆不同区域全新世可能经历了不同的干湿变化过程：①西北干旱区基本上是早中全新世干旱,中晚全新世相对湿润,但区域差异明显;②东亚季风边缘区早全新世早期干旱,早全新世晚期和中全新世相对湿润,晚全新世干旱;③青藏高原区的湿润时期主要发生在早中全新世,但是不同地区有所不同。对比分析显示：西北干旱区的干湿变化可能受西风环流控制,但在时间和强度上区域内部差异较明显;东亚季风边缘区可能主要受东亚季风控制;青藏高原地区早中全新世的湿润可能与印度季风的增强相关。     

Holocene climate and environmental history of East Greenland inferred from lake sediments

Prediction of future Arctic climate and environmental changes, as well as associated ice-sheet behavior, requires placing present-day warming and reduced ice extent into a long-term context. Here we present a record of Holocene climate and glacier fluctuations inferred from the paleolimnology of small lakes near Istorvet ice cap in East Greenland. Calibrated radiocarbon dates of organic remains indicate deglaciation of the region before ~10,500 years BP, after which time the ice cap receded rapidly to a position similar to or less extensive than present, and lake sediments shifted from glacio-lacustrine clay to relatively organic-rich gyttja. The lack of glacio-lacustrine sediments throughout most of the record suggests that the ice cap was similar to or smaller than present throughout most of the Holocene. This restricted ice extent suggests that climate was similar to or warmer than present, in keeping with other records from Greenland that indicate a warm early and middle Holocene. Middle Holocene magnetic susceptibility oscillations, with a ~200-year frequency in one of the lakes, may relate to solar influence on local catchment processes. Following thousands of years of restricted extent, Istorvet ice cap advanced to within 365 m of its late Holocene limit at ~AD 1150. Variability in the timing of glacial and climate fluctuations, as well as of sediment organic content changes among East Greenland lacustrine records, may be a consequence of local factors, such as elevation, continentality, water depth, turbidity, and seabirds, and highlights the need for a detailed spatial array of datasets to address questions about Holocene climate change.     

Holocene palaeoecology of southwest Greenland inferred from macrofossils in sediments of an oligosaline lake

We retrieved a lake sediment record from an oligosaline, meromictic lake in southwest Greenland. The record spans the last 8,200 cal. years and was radiocarbon dated and analysed for macroscopic remains of plants and animals. The record extends the known history of several invertebrate species back in time, and provides minimum ages for their immigration to Greenland after the last deglaciation. Shells of the ostracod Ilyocypris bradyi were found in sediments dated to the time interval c. 7,000–6,500 cal. years BP. Shells of this species were found previously in a nearby oligosaline lake, where its occurrence was dated to about the same short time interval. The species is a thermophilous, non-arctic taxon that is absent from the present day Greenland fauna, and we suggest that its former occurrence in west Greenland marks the peak of the Holocene thermal maximum. This is in agreement with other records from Greenland.     

Late Holocene paleoenvironmental changes in subtropical Taiwan inferred from pollen and diatoms in lake sediments

We analyzed pollen, spores, diatoms, organic carbon, nitrogen, and δ¹³C of organic matter in lake sediments to infer climate changes and reconstruct the paleo-environment of subtropical Taiwan over the past ∼1300 years. A 31.5-cm sediment core that represents deposition from 650 AD to present was taken from a mountain lake, Duck Pond, located 760 m a.s.l. We differentiated five zones using cluster analysis on pollen and spore assemblages. Fluctuations in the relative abundances of arboreal taxa, herbaceous plants, and ferns reflect changes in the relative amounts of woody versus grassland vegetation. Such shifts are associated with changes in temperature and humidity and are consistent with climatic periods reported for the temperate region of central China. Climate changes inferred from the pollen assemblages are also correlated well with changes in the ratios of fern spores to pollen, with organic carbon to nitrogen, and with the δ¹³C values in the sediments. Fluctuations in these data throughout the entire core were in good agreement with the changes in pH inferred from diatom assemblages. This study provides evidence of climate change in northern Taiwan over the past 1.3 millennia, assuming that climate can be inferred from the ratio of arboreal to non-arboreal pollen and from the pH of the aquatic environment.     

Holocene paleostorms identified by particle size signatures in lake sediments from the northeastern United States

The frequency and timing of Holocene paleofloods in the hilly terrain of New Hampshire and Maine are identified using ¹⁴C and high-resolution (cm-by-cm) particle size analysis of sediment cores taken from six post-glacial lakes (~0.1–1.4 km²). A total of nine sediment cores (4.5–6 m long) were taken near the base of stream delta foreslopes. End-member modeling of the particle-size frequency distributions from each core produces 3–5 representative end member distributions, or end members (EMs). Concurrent increases in mean and median particle size, and in the relative abundance of the coarsest EM(s), indicate increased transport capacity of inflowing tributaries, resulting from rainstorms. In all 9 cores, particle size data show clear signs of episodic, high-energy sediment transport events where proxy measurements such as loss-on-ignition and magnetic susceptibility do not, demonstrating the sensitivity of particle size analysis in paleostorm investigations made using lake sediment cores. Floods caused by storms in this region peaked around 1.4, 2.1, 3.0, 3.9, 6.8, 8.2, and 11.5 ka cal BP, and presently appear to be increasing in frequency. Periods of storminess in New Hampshire and Maine correlate well with other records of precipitation and climate in the northeastern United States during the Holocene, further supporting modern records which show tropical air masses as a primary driver of extreme precipitation events in New England (Ludlum 1996; Konrad 2001; Sisson and Gyakum 2004).     

A record of late Holocene precipitation on the Central Tibetan Plateau inferred from varved lake sediments

Journal of Paleolimnology - Lake sediments on the Tibetan Plateau are important natural archives for studying past climate and environment changes. A precise sediment core chronology is a...     

Tardigrade remains from lake sediments

Remains of tardigrades have rarely been reported to preserve in sediments, resulting in the absence of important ecological and biogeographic information that they could provide. However, a study of faunal microfossils in Antarctic lake sediment cores has shown that tardigrade eggs and occasionally exuvia can be abundant. Eggs from at least five tardigrade species were identified in sediment cores from six lakes from across the continent, with abundances up to 6,000 (g⁻¹ dry wt.). It is likely that the cold temperatures and absence of benthic grazers in Antarctic lakes results in particularly good preservation conditions, though it may also be a function of population density. The conservation of tardigrade eggs and exuvia in lake sediments enables a better understanding of paleodistributions and effects of environmental changes for this phylum that cannot otherwise be obtained.     

Deciphering climatic history from lake sediments

Reconstructing climate from lake sediments can be challenging, because the response of lakes and various components of lake systems are mediated by non-climatic factors, such as geomorphic and hydrologic stetting. As a result, the magnitude of lake response to climatic forcing may be non-linear. In addition, changes in the lake system associated with the aging process or non-climatic influences may alter the response to a given climate perturbation. These non-linear and non-stationary characteristics can produce spatial heterogeneity in the pattern and timing of inferred change. One approach for generating regionally robust climatic interpretations from lakes is to increase coordinated efforts to generate and synthesize large data sets, so that localized influences can be more clearly distinguished from broad-scale regional patterns. This approach will be most successful for evaluating climate variation at multi-decadal or longer temporal scales; the climatic interpretation of higher frequency limnological variation can be more complicated, because of dating uncertainties and differential response times of individual proxies and systems. This paper is based on a plenary talk at the 10th International Paleolimnology Conference in Duluth, Minnesota in July 2006.     

A first estimate of organic carbon storage in Holocene lake sediments in Alberta, Canada

This paper reports a first estimate of the Holocene lake sediment carbon pool in Alberta, Canada. The organic matter content of lake sediment does not appear to depend strongly on lake size or other limnological parameters, allowing a simple first estimate in which we assume all Alberta lake sediment to have the same organic matter content. Alberta lake sediments sequester about 15 g C m^-2 yr^-1, for a provincial total of 0.23 Tg C yr^-1, or 2.3 Pg C over the Holocene. Alberta lakes may represent as much as 1/1700 of total global, annual permanent carbon sequestration.     

The Little Ice Age recorded in sediments of a small dystrophic mountain lake in southern Poland

We inferred the temperature and environmental conditions of Smreczynski Staw Lake in the Tatra Mountains, southern Poland, from a sediment record covering the last 1,500 years. Paleobiological methods (cladocera, chironomid, and diatom analyses) were used together with sedimentological analysis and dating. These studies provide new information about the timing and character of climate fluctuations during the Little Ice Age (LIA). The Medieval Warm Period ended in the Tatra region at the beginning of the thirteenth century, followed by the first episode of the LIA. The LIA was a relatively long but unstable period. The first part of the LIA was cold in the Tatra Mountains, without evidence of increasing precipitation, while the second part, after AD 1540, was cold and humid. The LIA terminated in the Tatra Mountains at the beginning of the twentieth century, although some aspects of its climatic and sedimentological regime continued until the 1920s. We also found some evidence of warming and acidification during the twentieth century.     

Statistical comparison of palaeomagnetic directional records from lake sediments

Summary. This paper presents three statistical methods for testing the reproducibility or compatibility of palaeomagnetic directional records in undated cores of sediment. These methods are based on: (a) the intrinsic equations (curvature and torsion as functions of arc-length) of the curve defining changes in direction; (b) an F -test derived from multiple linear regression theory and (c) the construction and comparison of confidence bands. All methods utilize a projection of palaeomagnetic unit vectors on to the tangent plane to their mean direction. Since preliminary alignment to a common depth scale is necessary both for method (b) and for subsequent pooling of data, some theoretical results are presented for this problem, allowing for the possibility of missing segments in either core. Methods (b) and (c) are applied to two cores from Loch Lomond, Scotland. Both methods show that, after suitable relative rotation and alignment, these palaeomagnetic directional records are indeed compatible, demonstrating that the Loch Lomond sediments can provide reliable palaeosecular variation data of igh quality.     

Palaeomagnetism of Palaeozoic ultrabasic rocks from the Sudetes Mts (SW Poland): tectonic implications

A complex palaeomagnetic, rock-magnetic and mineralogical study of ultrabasic rocks from the Sowie Góry Block (GSB) and Jordanów–Gogołów Serpentinite Massif (JGSM) revealed the presence of several components of natural remanent magnetization (NRM). The authors found three groups of Palaeozoic as well as Triassic and Recent components of the geomagnetic field. The Palaeozoic components of NRM are carried mainly by magnetite of several generations formed during several serpentinization episodes. Permo-Carboniferous component (A1) present overall the Sudetes was isolated in one JGSM and two GSB exposures, whereas the Late Devonian–Early Carboniferous component (A2) was found in two exposures from the GSB. The corresponding remanent components were already revealed in palaeontologically dated sediments from other West Sudetic units. In the GSB, it was probably acquired during its unroofing dated isotopically for ca. 370–360 Ma. The newly determined group of Palaeozoic directions (A3) was found in three localities from JGSM and in two from GSB is interpreted as the oldest overprint. In JGSM, it was acquired probably shortly after the first oceanic serpentinization phase dated isotopically for ca. 400 Ma. Its acquisition in GSB corresponds to the time of emplacement of ultrabasic xenoliths dated isotopically at ca. 390 Ma. So we suppose that the mean A3 calculated for five exposures corresponds to the 380–400 Ma time span and that at that period both massifs formed one microplate. Mean inclination of A3 places this microplate at 380–400 Ma at the palaeolatitude of 23°S, whereas the West Sudetes were situated during the Early Devonian at 16°S. We suggest that during the Early Devonian the microplate comprising GSB and JGSM massifs was situated to the south from the West Sudetes and accreted them during Middle–Late Devonian.     

Holocene lake sediment records of Arctic hydrology

Although paleoclimatic research in the Arctic has most often focused on variations in temperature, the Arctic has also experienced changes in hydrologic balance. Changes in Arctic precipitation and evaporation rates affects soils, permafrost, lakes, wetlands, rivers, ice and vegetation. Changes in Arctic soils, permafrost, runoff, and vegetation can influence global climate by changing atmospheric methane and carbon dioxide concentrations, thermohaline circulation, and high latitude albedo. Documenting past variations in Arctic hydrological conditions is important for understanding Arctic climate and the potential response and role of the Arctic in regards to future climate change. Methods for reconstructing past changes in Arctic hydrology from the stratigraphic, isotopic, geochemical and fossil records of lake sediments are being developed, refined and applied in a number of regions. These records suggest that hydrological variations in the Arctic have been regionally asynchronous, reflecting the impacts of different forcing factors including orbitally controlled insolation changes, changes in geography related to coastal emergence, ocean currents, sea ice extent, and atmospheric circulation. Despite considerable progress, much work remains to be done on the development of paleohydrological proxies and their application to the Arctic.     

Laminated lake sediments in northeast Poland: distribution, preconditions for formation and potential for paleoenvironmental investigation

Glacial landscapes of the Land of Great Masurian Lakes and Suwa?ki Lakelands in northeast Poland are characterized by very high abundance of lakes. These two areas were surveyed for lakes containing laminated sediments. Using bathymetry as a criterion, 60 small, deep lakes, representing preferred conditions for formation and preservation of lacustrine non-glacial varves, were selected for gravity coring. We found laminated sediments in 24 of the lakes, 15 in the Land of Great Masurian Lakes and 9 in the Suwa?ki Lakeland. Seven of these 24 sediment records were laminated in the topmost part only. Analysis of lake morphometric variables showed that the relation between surface area and maximum water depth can be used to identify lakes with laminated sediments. Most of the newly discovered lakes with laminated deposits have surface areas ≤0.3 km² and maximum depths of 15–35 m. Multivariate statistical analysis (Linear Discriminant Analysis) of the lake dataset identified the morphological features of lake basins and their catchments that largely control preservation of laminated sediments. Microscopic and geochemical analyses revealed a biogenic (carbonaceous) type of lamination typical for lakes in northeast Poland. Such lakes are characterized by a spring-summer lamina that is rich in calcium carbonate and an autumn-winter lamina composed of organic and minerogenic detritus. This pattern may be modified by multiple periods of calcite deposition during a single year or substantial contribution of clastic material. Laminations and high sedimentation rates offer the possibility of high-resolution investigation of past climate and environmental changes through application of myriad biological, isotopic and geochemical proxies.     

Holocene sediments of Sägistalsee, a small lake at the present-day tree-line in the Swiss Alps

Sägistalsee is a small lake located at the modern tree-line in the Swiss Alps. A 13.5 m long core taken in the central part of the 9.5 m deep basin consists of clayey silts and sands and dates back to about 9000 cal. BP. These sediments have a low organic content that steadily increases from 4–8% loss-on-ignition at 550 °C towards the top of the core, whereas the carbonate content decreases from 20 to about 10% loss-on-ignition at 950 %C. We outline the aims of an interdisciplinary research project centred on the Holocene sediments of Sägistalsee. We also present information about the lake, its sediments, and its catchment that forms the basis for different biotic and abiotic multi-proxy studies carried out on the sediments of Sägistalsee.     

Palynological study of Holocene sediments from Lake Doirani in northern Greece

A palynological investigation was conducted on two cores with Holocene sediments collected from the northeastern littoral part of the border Lake Doirani in northern Greece. The radiocarbon dates indicated that the analyzed sediments accumulated during the last 5000 yrs. The pollen-stratigraphic record revealed the environmental changes in the catchment area, starting from a natural undisturbed landscape to one modified by increasing anthropogenic influences. The tree vegetation dominated by Quercus woods in the lowlands and byPinus, Abies, and Fagus at higher altitudes, lasted for the period 2900 - 830 cal. B.C. Subsequently it was replaced by xerothermic herb and tree vegetation as a result of intensive human activity - and farming and stock-breeding. The accumulation of sediments with more sand and gravel in historical time was the result of increased erosion.