Holocene climate changes in southern Greenland: evidence from lake sediments

A Holocene lake sediment record is presented from Lake N14 situated on Angissoq Island 15 km off the main coast of southern Greenland. The palaeoclimatic development has been interpreted on the basis of flux and percentage content of biogenic silica, clastic material, organic material and sulphur as well as sedimentation rate, moss content and magnetic susceptibility. A total of 43 radiocarbon dates has ensured a reliable chronology. It is argued that varying sediment composition mainly reflects changing precipitation. By analogy with the present meteorological conditions in southern Greenland, Holocene climate development is inferred. Between 11 550 and 9300 cal. yr BP temperature and precipitation increase markedly, but this period is climatically unstable. From 9300 yr BP conditions become more stable and a Holocene climatic optimum, characterised by warm and humid conditions, is observed from 8000 to 5000 cal. yr BP. From 4700 cal. yr BP the first signs of a climatic deterioration are observed, and from 3700 cal. yr BP the climate has become more dry and cold. Superimposed on the climatic long‐term trend is climate variability on a centennial time‐scale that increases in amplitude after 3700 cal. yr BP. A climatic scenario related to the strength and position of the Greenland high‐pressure cell and the Iceland low‐pressure cell is proposed to explain the Holocene centennial climate variability. A comparison of the Lake N14 record with a terrestrial as well as a marine record from the eastern North Atlantic Ocean suggests that the centennial climate variability was uniform over large areas at certain times. Copyright © 2004 John Wiley & Sons, Ltd.     

Selective chemical extraction of carbonate-associated metals from recent lacustrine sediments

Acidic ion-exchanger was applied to the selective extraction of heavy metals in carbonate phases of recent lake sediments. Cr was not detected and Fe and Cu contents were reduced by factors of three to ten, relative to non-carbonate fractions of the sediment samples. Values obtained for Mn and Zn exemplify the effects of dilution was well as enrichment, both originating from increased carbonate percentages. Determinations on grain-sized fractions from Bodensee samples suggest that elevated inputs of dissolved Zn, which partly originate from human activities, may be immobilized by authigenic carbonate precipitation.     

A Lateglacial and Holocene organic carbon isotope record of lacustrine palaeoproductivity and climatic change derived from varved lake sediments of Lake Holzmaar,Germany

A composite varve-dated 11.4 m long sediment sequence from Lake Holzmaar, situated in the Eifel region of western Germany, was investigated for total organic matter content, total nitrogen content and stable organic carbon isotopes. Mean time resolution is 75 years for TN and 14 years for δ¹³C_org. On millennial time scales primary production of lacustrine algae strongly depends on the delivery of nutrients from the catchment. The respective carbon isotope record is characterized by marked variations of δ¹³C_org ranging from −36.0‰ to −27.0‰ and includes a number of pronounced shifts. Reactions of the lacustrine system and the catchment to changes of environmental parameters, e.g. runoff, solar radiation and temperature, induce changes of algal associations and of lacustrine primary production which are reflected in the sediments as carbon isotope variations. Clear evidence of ecosystem reorganizations is detected by the carbon isotope record around 14,200, 10,400, 9600, 5500, 2700, 1700 and 900 varve years BP. In particular, the Holocene events of 9600, around 5500 and 2700 are interpreted as the expression of massive changes of the climate system. The steady rise of δ¹³C_org values during the mid-Holocene is interpreted as a continuous climatic amelioration reaching an optimum around 6500 varve years BP. Rapid and large changes of δ¹³C_org values from 2700 varve years BP to the present indicate major disturbances in the catchment area. These are most probably related to deforestation or reforestation and runoff changes, presumably in conjunction with human impact. Carbon isotopes, thus, characterize the Holzmaar ecosystem in time revealing lacustrine palaeoproductivity as well as providing palaeoenvironmental and palaeoclimatic information.     

Late-glacial climatic oscillations as recorded in Swiss lake sediments

Regional pollen assemblage zones for the late-glacial period of the Swiss Plateau are introduced and defined. They include four major zones (Artemisia, Juniperus—Hippophaë, Betula, Pinus PAZ) with several subzones. Pollen and oxygen-isotope analyses on lacustrine sediments from several lakes in the area reveal four distinct phases of climatic oscillation in the time period of 13 000-9500 yr BP. The first oscillation, termed the Aegelsee fluctuation, occurs shortly before 12 000 yr BP and varve counts suggest its duration was ca. 100 yr. It is characterised by a short decrease in the oxygen isotopes as well as a short increase in NAP associated with a depression in birch pollen values. The second oscillation, which occurs in the δ¹⁸O record shortly before the deposition of the Laacher See Tephra (ca. 11 000 yr BP), is termed the Gerzensee fluctuation. It occurs during a pine-dominated phase and its vegetational effects cannot be determined palynologically. The most prominent regressive phase is the Younger Dryas biozone (ca. 10 700-10 000 yr BP) characterised by an increase in heliophilous NAP and low δ¹⁸O values. The Younger Dryas biozone can often be subdivided palynologically into two parts: a first part rich in grasses and juniper and a second part with higher Filipendula and birch values. During the Preboreal biozone another distinct oscillation is evidenced only in the oxygen isotope ratios. Comparison of the Swiss oxygen isotope profiles with the Greenland Dye 3 record suggests that not only the three major shifts in the δ¹⁸O curves but also the minor ones are closely comparable, suggesting some common climatic control.     

Mineralogy and geochemistry of the Holocene lacustrine sediments in Nam Co, Tibet

The carbonates, clays and major chemical compositions of lacustrine sediments in Nam Co (Lake) were examined by X-ray diffraction, scanning electron microscopy and chemical analysis. Carbonates include monohydrocalcite (MHC, first report from China and in a high-altitude lake), low-Mg calcite and traces of dolomite. MHC in Nam Co is developed in water (1.8 g/L) with high Mg/Ca molar ratios (10.03–15.03), high pH (8.04–9.72) and the presence of bacteria, algae, diatom and ostracoda. Illite and Mg-chlorite provide a strong evidence for physical weathering in the Holocene. Most Ca and Sr in sediments originate from carbonates as the molar ratios of Ca and CO₃²⁻ are all less than 1 and the curve of Sr is very similar to that of Ca. However, most of the Mg, Fe and Rb are from clays. The lake water shifted from a fresh water environment to an evaporative, alkaline environment by 2.06 cal. ka BP. There was a depositional event that the depositional rate changed from 0.134 to 1.639 mm/a at about 2 cal. ka BP.     

Holocene alpine glaciation inferred from lacustrine sediments on northeastern Baffin Island,Arctic Canada

With accelerated melting of alpine glaciers, understanding the future state of the cryosphere is critical. Because the observational record of glacier response to climate change is short, palaeo‐records of glacier change are needed. Using proglacial lake sediments, which contain continuous and datable records of past glacier activity, we investigate Holocene glacier fluctuations on northeastern Baffin Island. Basal radiocarbon ages from three lakes constrain Laurentide Ice Sheet retreat by ca. 10.5 ka. High sedimentation rates (0.03 cm a^?1) and continuous minerogenic sedimentation throughout the Holocene in proglacial lakes, in contrast to organic‐rich sediments and low sedimentation rates (0.005 cm a^?1) in neighbouring non‐glacial lakes, suggest that glaciers may have persisted in proglacial lake catchments since regional deglaciation. The presence of varves and relatively high magnetic susceptibility from 10 to 6 ka and since 2 ka in one proglacial lake suggest minimum Holocene glacier extent ca. 6–2 ka. Moraine evidence and proglacial and threshold lake sediments indicate that the maximum Holocene glacier extent occurred during the Little Ice Age. The finding that glaciers likely persisted through the Holocene is surprising, given that regional proxy records reveal summer temperatures several degrees warmer than today, and may be due to shorter ablation seasons and greater accumulation‐season precipitation. Copyright © 2009 John Wiley & Sons, Ltd.     

Long-chain alkenone distributions and temperature dependence in lacustrine surface sediments from China

Long-chain alkenones (LCK) of lacustrine surface sediments were analyzed in 37 lakes from China. The results obtained were complemented by published data from 13 other Chinese lakes. These lakes are located across large temperature and precipitation gradients, therefore allowing for an assessment of the distribution pattern of LCK and their temperature dependency. Different distribution patterns of LCK (C₃₇ predominant pattern and C₃₈ predominant pattern) were detected in the surface sediment samples. The ratio of C_37:4 methyl ketone to the sum of C₃₇ alkenones observed in the different lakes is highly variable (5%-96%, with mean value of 55%), and more than that seen in marine systems. The finding that some of the ocean LCK precursor algae (Gephyrocapsa oceanica, Coccolithus pelagicus) were also present in the limnic systems suggested that both systems might have similar biosynthetic sources. Empirical relationships between the alkenone unsaturation index U₃₇^k′ and different temperature sets (mean annual air temperature, mean annual air temperature in different seasons, and lake surface water temperature of July) were tested. The best correlation between U₃₇^k′ and temperature was obtained using mean annual air temperature. A general linear regression of U₃₇^k′ and MAAT can be expressed as U₃₇^k′ = 0.0328 × T + 0.126 (n = 38, r² = 0.83). Although questions such as species-uncertainty and other unknown factors for U₃₇^k′ temperature dependence still remain, the equation might be representative of the average contribution of LCK to sediments for these data over a wide range of surface temperatures, water chemistry and different alkenones-producer algal populations. The general relationship of U₃₇^k′ and mean annual air temperature is consistent with that in marine systems. It supports the suggestion that the biosynthetic pathway of alkenones and the mechanism of their temperature signal may be similar in both marine and limnic systems. LCK might be used as an important paleotemperature proxy in limnic environment.     

内蒙古中部达尔罕茂明安联合旗腾格尔诺尔湖泊 沉积物记录的全新世晚期环境演变

通过对内蒙古中部腾格尔诺尔湖泊沉积、粒度组成、孢粉组合等特征的综合分析,重建了该地区全新世晚期1700aBP以来的环境演变过程。结果表明,该地区1700aBP以来的气候环境变化可以分为4个阶段:1700～1170aBP,气候温暖较湿润;1170～840aBP,为温干气候期;840～200aBP,气候转为温暖湿润,间有短期的冷干波动;200aBP以来气候以温干为主,晚期气候有向凉干转变的趋势。     

内蒙古中部达尔罕茂明安联合旗腾格尔诺尔湖泊沉积物记录的全新世晚期环境演变

通过对内蒙古中部腾格尔诺尔湖泊沉积、粒度组成、孢粉组合等特征的综合分析,重建了该地区全新世晚期1700aBP以来的环境演变过程。结果表明,该地区1700aBP以来的气候环境变化可以分为4个阶段:1700～1170aBP,气候温暖较湿润;1170～840aBP,为温干气候期;840～200aBP,气候转为温暖湿润,间有短期的冷干波动;200aBP以来气候以温干为主,晚期气候有向凉干转变的趋势。     

鲁西孟家屯岩组中发现红柱石和锌尖晶石

侯增谦、高永丰、黄卫于２００１年发表的《西藏高原雅鲁藏布江北岸蛇绿岩带的发现及其地质意义》一文中，把所发现的蛇绿岩带描述为“谢通门蛇绿岩带东起大竹卡区，西至谢通门以西，长数百千米，宽３～５km，由一系列自北而南逆冲推覆的构造岩片构成，其中，基性玄武熔岩岩片、巨厚辉长岩岩片、超镁铁质辉石岩－纯橄岩岩片，依次由雅江近岸至冈底斯弧内部呈叠瓦状排布。变质橄榄岩出露零星，少许孤立的岩体出露于雅江近岸，发育强烈碳酸盐化和蛇纹石化；大量散布的岩群出露于堆积岩岩片的北侧，多在冈底斯弧花岗岩基和弧火山－沉积岩系中呈…     

Rice domestication and climatic change: phytolith evidence from East China

Fossil rice phytoliths have been identified from a lateglacial to Holocene sequence of epicontinental sediments in the East China Sea that were probably transported by the Yangtze River from its middle and/or lower reaches. The rice phytoliths occurred first in the sequence at about 13900 cal. yr BP and disappeared during the period of 13000-10000 cal. yr BP, implying the earliest domesticated cereal crops of the world ever reported. Based on the records of phytoliths, pollen, diatoms and foraminifera from the sequence, the climate between 13000 and 10000 cal. yr BP was notably colder (Younger Dryas). The coincidence of disappearance of domesticated rice phytoliths with cold climate conditions may suggest a great climatic influence on human activities during that time. Warmer and wetter conditions during the period 13900 to 13000 cal. yr BP and after 10000 cal. yr BP have probably favoured rice domestication in the area.     

青藏高原昆仑山垭口盆地晚上新世以来沉积物磁化率与气候变化

通过对青藏高原昆仑山垭口盆地磁化率、频率磁化率环境重要参考指标的分析,并与黄土—古土壤和深海δ18O记录的气候变化进行对比,结合剖面的岩性特征和年代序列以及前人所获得的粒度和孢粉资料,揭示了3.6Ma～0.5Ma期间昆仑山垭口盆地气候变化经历了6个不同的阶段:3.6Ma～2.48Ma气候相对冷干,2.48Ma～2.14Ma气候偏凉湿,2.14Ma～2.07Ma气候相对冷干,2.07Ma～1.16Ma气候总体相对较湿润(其间有数次冷干波动),1.16Ma～0.78Ma气候偏冷干以及0.78Ma～0.5Ma气候向冷湿发展。     

Rapid Holocene climate changes in the North Atlantic: evidence from lake sediments from the Faroe Islands

Holocene records from two lakes on the Faroe Islands were investigated to determine regional climatic variability: the fairly wind-exposed Lake Starvatn on Streymoy and the more sheltered Lake Lykkjuvötn on Sandoy. Sediment cores were analysed for content of biogenic silica, organic carbon and clastic material, and magnetic susceptibility. In addition, a new qualitative proxy for past lake ice cover and wind activity was developed using the flux of clastic grains that are larger than 255 μm. Both long-term and short-term climatic developments were similar between the two lakes, suggesting a response to a regional climate signal. The long-term climate development is characterized by early Holocene rapid warming followed by Holocene climatic optimum conditions ending around 8300 cal. yr BP. A more open landscape as evidenced from increased sand grain influx in the period 8300–7200 cal. yr BP could reflect the aftermath of the 8200 cal. yr BP event, although the event itself is not recognized in either of the two lake records. From around 7200 cal. yr BP the mid-Holocene climate deterioration is observed and from 4200 cal. yr BP the climate deteriorated further with increased amplitude of centennial cooling episodes.     

Late Holocene variations in Arctic shelf hydrology and sea-ice regime: evidence from north of the Lena Delta

Diatom assemblage studies are used to interpret past changes in river runoff (salinity) and sea-ice regime in the vicinity of the vast Lena River delta, southern Laptev Sea shelf. On the basis of their distribution in surface sediments, the shelf region outside the strong influence of riverine waters is characterized by a dominance in sea-ice diatoms and other marine species. Their numbers increase steeply (>20%) within the area of drifting pack ice. In contrast, the marginal zone of the delta, where exceedingly low salinities prevail, is marked by freshwater diatoms showing values higher than 70%. Using the environmental information from the surface sediments, the downcore distribution patterns of the main ecological groups of diatoms were investigated on a sediment core that covers the past 2800 cal. years BP. Although the freshwater group indicates some temporal variations in salinities, the study site north of the Lena River delta remained under a dominantly riverine influence for most of the three recognized phases. In contrast, the relative abundance of sea-ice species gives evidence that pack-ice conditions were more severe during the oldest phase (older than ~2700 cal. years BP). The most significant changes are observed in the uppermost core section (younger than ~300 cal. years BP) when the relative abundance of freshwater diatoms decreases from 80% down to below 20%. This dramatic decrease is interpreted as a major shift from a more northward-directed to the modern, dominantly eastern outflow pattern. Because the dispersal and fate of riverine waters and its role on the ice regime as well as on water mass properties is a central issue in understanding short- and longer-term climatic changes in the Arctic and beyond, it needs to be tested using more cores if this most recent change in outflow pattern from the delta is connected to climate change or simply a result of channel migration within the delta.     

Holocene environmental and climatic changes inferred from Wulungu Lake in northern Xinjiang, China

Sedimentological, geochemical and palynological data from Wulungu Lake in northern Xinjiang, China, are used to reconstruct environmental and climate changes since 9550 cal yr BP. High abundance of Sparganium and Poaceae, low Md (median diameter) and δ¹³C_organic values indicate aridity between 9550 and 6730 cal yr BP. High Md and δ¹³C_organic values, and the prevalence of desert-steppe and steppe vegetation between 4200 and 560 cal yr BP, indicate that effective moisture increased after 6730 cal yr BP, peaking at 4200 and 560 cal yr BP. Low Md values, a negative excursion of δ¹³C_org, and the transition from steppe to desert vegetation since 560 cal yr BP reflect a decrease in effective moisture during the latest Holocene. Late Holocene human activities were indicated by sharp increase in the abundance of Pediastrum then. Variations in carbonate contents indicate that temperature was generally high between 9550 and 7740 cal yr BP, low between 7740 and 6730 cal yr BP, intermediate between 6730 and 560 cal yr BP, and low during the last 560 yr. Regional comparison indicates that the Asian monsoon did not extend to Wulungu Lake and westerlies were the main factor in determining the moisture availability during the Holocene.     

Lateglacial and early Holocene climatic oscillations on the western Svalbard margin,European Arctic

A multi proxy sediment core record on the continental margin off western Svalbard, European Arctic, reflects large climatic and oceanographic oscillations at the Lateglacial–early Holocene transition. Based on studies of planktonic foraminifera, their stable oxygen and carbon isotopic composition and ice rafted debris, we have reconstructed the last 14 cal. ka BP. The period 14–13.5 cal. ka BP was characterized by highly unstable climatic conditions. Short-lived episodes of warming alternated with meltwater pulses and enhanced iceberg rafting. This period correlates to a regional warming of the northern North Atlantic. An overall decrease in meltwater took place during the deglaciation (14–10.8 cal. ka BP). The late Younger Dryas and subsequent transition into the early Holocene is characterized by a reduced flux of planktonic foraminifera and increased iceberg rafting. A major warming took place from 10.8 to 9.7 cal. ka BP, the influence of meltwater ceased and the flux of warm Atlantic Water increased. From 9.7 to 8.8 cal. ka BP, the western Svalbard margin surface waters were significantly warmer than today. This warm period, the thermal maximum, was followed by an abrupt cooling at 8.8. cal. ka BP, caused by an increased influence of Arctic Water from the Arctic Ocean. The results document that the European Arctic was very sensitive to climatic and oceanographic changes at the end of the last glacial and during the Holocene.     

Black Sea chemocline oscillations during the Holocene: molecular and isotopic studies of marginal sediments

We measured δ¹³C values of free and sulfur-bound lipids and framboidal pyrite-size distributions in three sediment cores from the southern margins of the Black Sea. The margin cores show a marked difference in the occurrence of biomarkers from green sulfur bacteria compared with the deep-basin cores, as a result of deepening of the chemocline resulting from enhanced mixing and/or decreased light-penetration as a consequence of high turbidity and productivity in shelf waters. Quantitation of biomarkers suggests that photic-zone anoxia along the shelf margin was generally absent during the deposition of unit I, although occurred during the deposition of Unit IIb at two sites.     

全新世岱海流域化学风化及其对气候事件的响应

通过对岱海 12.08 m沉积岩芯 210Pb和 AMS- 14C精确定年及地球化学、物理指标的综合分析,重建了包括 8.2 ka冷期、中世纪暖期 (MWP)和小冰期 (LIA)等典型气候事件在内的全新世以来所经历的化学风化及其环境演化过程.全新世早-中期 (9.0～ 3.5 ka),岱海流域处于温暖湿润气候环境,主要环境特征表现为流域化学风化显著增强、生物生产力逐步提高、湖泊水位大幅度抬升.在 7.90～ 8.25 ka期间存在一次冷气候事件,其发生时间与来自湖沼、海洋、生物组合和极地冰芯等在内的全球环境记录基本一致,具体表现为流域化学风化减弱、湖泊生产力降低以及湖泊水位下降. 2.5 ka以来,岱海沉积物中环境指标的显著变化,表明进入气候波动更频繁的新冰期.岱海沉积物中的指标变化也展示了本世纪以来的气候增暖过程,但其化学风化强度还没有达到 MWP的程度.