Jan Mayen

Beliggenhet. Omgitt av 2000—3000 meter dypt hav på alle kanter ligger øen Jan Mayen ute i Norskehavet på 71° nordlig bredde og 8° 30′ lengde vest for Greenwich, d. v. s. omtrent på samme bredde som Nordkapp, og samme lengde som Færøyene. Avstanden fra Tromsø er ca. 555 nautiske mil og omtrent samme avstand er det også fra Isfjordmunningen på Spitsbergen. Avstannen til Langanes på Island er 290 og til Liverpoolkysten på Grønland 250 nautiske mil. Til sammenligning kan anføres at fra norskekysten til Bjørnøya er det 230 naut. mil og til Isfjorden 465 naut. mil.     

Expanding spread profile at the northern Jan Mayen Ridge

An expanding spread seismic profile at the central northern Jan Mayen Ridge, ESP-5, has yielded a crustal seismic velocity distribution which is similar to observations from the thinned continental crust at the Norwegian continental margin. The profile reveals a post-early Eocene sedimentary sequence, about 1. 5 km thick, overlying 1 km of volcanic extrusives and interbedded sediments. Below, there are about 3 km of pre-opening sediments above the seismic basement. The results indicate that the main ridge block is underlain by a thinned crust, possibly only 13.5 km thick. The results are compatible with a continental nature for the main ridge complex.     

Holocene climate and glacier variability at Hallet and Greyling Lakes,Chugach Mountains,south-central Alaska

Evidence from lake sediments and glacier forefields from two hydrologically isolated lake basins is used to reconstruct Holocene glacier and climate history at Hallet and Greyling Lakes in the central Chugach Mountains of south-central Alaska. Glacial landform mapping, lichenometry, and equilibrium-line altitude reconstructions, along with changes in sedimentary biogenic-silica content, bulk density, and grain-size distribution indicate a dynamic history of Holocene climate variability. The evidence suggests a warm early Holocene from 10 to 6 ka, followed by the onset of Neoglaciation in the two drainage basins, beginning between 4.5 and 4.0 ka. During the past 2 ka, the glacial landforms and lacustrine sediments from the two valleys record a remarkably similar history of glaciation, with two primary advances, one during the first millennium AD, from ~500 to 800 AD, and the second during the Little Ice Age (LIA) from ~1400 to 1900 AD. During the LIA, the reconstructed equilibrium-line altitude in the region was no more than 83 ± 44 m (n = 21) lower than the modern, which is based on the extent of glaciers during 1978. Differences between the summer temperature inferred from the biogenic-silica content and the evidence for glacial advances and retreats suggest a period of increased winter precipitation from 1300 to 1500 AD, and reduced winter precipitation from 1800 to 1900 AD, likely associated with variability in the strength of the Aleutian Low.

The origin and age of driftwood on Jan Mayen

Analysis of the wood anatomy of 481 driftwood specimens from Jan Mayen shows that Larix spp. constitute approximately 70% of the trees, while sawn logs are dominated by Pinus spp. by approximately 69%. A total of 356 driftwood samples from Jan Mayen and a small number of samples from Bjørnøya in the Barents Sea and the Troynoy Island in the Kara Sea were analysed by dendrochronological methods. A driftwood Pinus chronology was dated absolutely using chronologies from living trees of Pinus sylvestris in the lower proximity of the Angara River, a tributary of the Yenisey in Siberia. About 27% of the pine logs measured on Jan Mayen were found to originate in the same region, with end years concentrated in the 1940s and 1950s. A similar source was also found for Pinus driftwood logs on Bjørnøya and Troynoy. The results confirm and further delimit the source areas of the Yenisey driftwood established earlier from driftwood logs on Svalbard and Iceland. A subordinate source of both Pinus and Picea logs on Jan Mayen is northwest Russia, from the Kola Peninsula to the Pechora River. The Transpolar Drift Stream is believed to be the main distributor of driftwood from Siberian and northwest Russian sources to Jan Mayen, via the East Greenland Current. Dendrochronological dating reveals a strong, continuous input of ice-rafted driftwood from the Kara Sea. Radiocarbon datings from Jan Mayen show surface deposits of driftwood to be less than 500 years old, due mainly to extensive degradation of older wood and little or no land uplift.     

北极扬马延岛大气边界层高度的气候特征分析

基于1973—2015年间的全球综合无线电探空资料(IGRA),采用总体理查逊数(Bulk Richardson Number)方法,分析了北极扬马延岛43年间的大气边界层高度变化特征,并对其多年月均大气边界层高度变化以及年均大气边界层高度变化进行深入分析探讨。结果显示,扬马延岛白天对流边界层高度高于夜晚稳定边界层高度,夏季多年月均大气边界层高度远低于冬春季节的高度,夏季平均高度仅为262 m,而冬春季节高度在600 m附近。大气边界层高度的变化与地面相对湿度的变化呈现较好的反相关关系。由于受到墨西哥湾暖流的影响,岛屿全年温差较小,夏季的相对湿度较大,导致潜热通量较多,抑制了边界层内的对流过程,造成夏季大气边界层高度较低。此外,其年均高度在1973—1988年间出现波动下降,而后在1988—1995年快速上升,最后于1995—2015年间变化平稳。     

Polychaetes from Jan Mayen (Annelida, Polychaeta)

A thorough literature review has been undertaken to establish the first complete account of polychaetes recorded from the area around the volcanic island of Jan Mayen. The annotated checklist lists 121 species-level taxa, representing an increase from the 75 species previously recorded. The checklist is based on existing records, supplemented with material sampled in 1999, from which 42 species new to the area were reported. Some previously reported species from the area have been excluded because of inadequate documentation. The polychaete fauna of Jan Mayen is comparable with that of the mainland Norwegian coast and the Svalbard area. No taxa unique to the island were found. However, knowledge of the marine invertebrate fauna in general at Jan Mayen is sparse because few surveys have been undertaken there. It is expected that future expeditions will reveal further new taxon records for the area.     

Late Holocene linkages between decade–century scale climate variability and productivity at Lake Tanganyika, Africa

Microlaminated sediment cores from the Kalya slope region of Lake Tanganyika provide a near-annually resolved paleoclimate record between ∼∼2,840 and 1,420 cal. yr B.P. demonstrating strong linkages between climate variability and lacustrine productivity. Laminae couplets comprise dark, terrigenous-dominated half couplets, interpreted as low density underflows deposited from riverine sources during the rainy season, alternating with light, planktonic diatomaceous ooze, with little terrigenous component, interpreted as windy/dry season deposits. Laminated portions of the studied cores consist of conspicuous dark and light colored bundles of laminae couplets. Light and dark bundles alternate at decadal time scales. Within dark bundles, both light and dark half couplets are significantly thinner than within light bundles, implying slower sediment accumulation rates during both seasons over those intervals.Time series analyses of laminae thickness patterns demonstrate significant periodicities at interannual–centennial time scales. Longer time scale periodicities (multidecadal to centennial scale) of light and dark half couplet thicknesses are coherent and in some cases are similar to solar cycle periods on these time scales. Although laminae thickness cycles do not strongly covary with the actual Δ¹⁴C record for this same time period, two large Δ¹⁴C anomalies are associated with substantial decreases in both light and dark laminae thickness. In contrast to the multidecadal– centennial time scale, significant annual to decadal periodicities, which are broadly consistent with ENSO/PDO forcing and their impact on East African climate, are not coherent between light and dark half couplets. The coherency of light–dark couplets at decadal–centennial time scales, but not at shorter time scales, is consistent with a model of a long-term relationship between precipitation (recorded in wet season dark laminae thickness) and productivity (light laminae thickness), which is not manifest at shorter time scales. We hypothesize that this coupling results from long-term recharging of internal nutrient loading during wet periods (higher erosion of soil P) and reduced loading during drought intervals. The relationship is not expressed on short time scales during which the dominant control on productivity is wind-driven, dry season upwelling, which is uncorrelated with wet-season precipitation. Our record greatly extends the temporal record of this quasi-periodic behavior throughout the late Holocene and provides the first evidence linking decade- to century-scale episodes of enhanced productivity to enhanced precipitation levels and nutrient recharge in a productive tropical lake.     

Younger Dryas glaciolacustrine rhythmites and cirque glacier variations at Kråkenes, western Norway: depositional processes and climate

Kråkenes faces the open sea on the west coast of Norway. During the Younger Dryas a cirque glacier deposited a large outer and a smaller inner moraine in a cirque at the site and melt-water entered a small lake depositing glaciolacustrine sediments. The glaciolacustrine succession can be divided into three sub-units corresponding to the advance, the still-stand and the retreat phases of the glacier. The sediment succession contains both varves and other types of rhythmites, the latter being mainly deposited as turbidity underflows caused by localized slumping events. Lee-side accumulation of snow by wind and avalanching into the cirque was crucial to form and maintain the cirque glacier once summer temperatures were low enough. At maximum, the glacier likely was in equilibrium with climate. The initial retreat from the maximum position might have been triggered by fall-out of volcanic ash from Iceland, but the continued retreat was due to increased ablation season temperatures. The most rapid change in climate at the Younger Dryas/Preboreal transition occurred after the cirque glacier had melted away completely.     

Late Holocene climate change and human disturbance on Andros Island, Bahamas

A 2 m sediment core from Church's Blue Hole on Andros Island, Bahamas provides the first paleoecological record from the Bahama Archipelago. The timing of events in the lower portion of the core is uncertain due to inconsistencies in the radiocarbon chronology, but there is evidence that a late Holocene dry period altered the limnology of Church's Blue Hole and supported only dry shrubland around the site. The dry period on Andros may correlate with a widespread dry period in the Caribbean from 3200 to 1500 yr BP. After the dry period ended, a more mesic climate supported tropical hardwood thicket around Church's Blue Hole. At c. 740 radiocarbon yr BP there is a sudden rise in charcoal concentration and a rapid transition to pinewoods vegetation, while at c. 430 radiocarbon yr BP charcoal concentration drops, but is higher again near the top of the core. Although climatic shifts could have caused these changes in vegetation and charcoal concentration, the changes post-date human colonization of the Bahamas and may reflect human arrival, followed by the removal of humans c. 1530 AD and the recolonization of Andros Island c. 200 years later.This is the 12th in a series of papers published in this special AMQUA issue. These papers were presented at the 1994 meeting of the American Quaternary Association held 19–22 June, 1994, at the University of Minnesota, Minneapolis, Minnesota, USA. Dr Linda C. K. Shane served as guest editor for these papers.     

Late Holocene change in climate and atmospheric circulation inferred from geochemical records at Kepler Lake, south-central Alaska

Climate records during the last millennium are essential in placing recent anthropogenic-induced climate change into the context of natural climatic variability. However, detailed records are still sparse in Alaska, and these records would help elucidate climate patterns and possible forcing mechanisms. Here we present a multiple-proxy sedimentary record from Kepler Lake in south-central Alaska to reconstruct climatic and environmental changes over the last 800?years. Two short cores (85 and 101?cm long) from this groundwater-fed marl lake provide a detailed stable isotope and sediment lithological record with chronology based on four AMS ¹⁴C dates on terrestrial macrofossils and ²¹⁰Pb analysis. The ??¹⁸O values of inorganic calcite (CaCO₃) range from ?17.0 to ?15.7???, with the highest values during the period of 1450?C1850 AD, coeval with the well-documented Little Ice Age (LIA) cold interval in Alaska. The high ??¹⁸O values during the cold LIA are interpreted as reflecting shifts in atmospheric circulation. A weakening of the wintertime Aleutian low pressure system residing over the Gulf of Alaska during the LIA would have resulted in ¹⁸O-enriched winter precipitation as well as a colder and possibly drier winter climate in south-central Alaska. Also, elevated calcite contents of >80?% during the LIA reflect a lowering of lake level and/or enhanced seasonality (warmer summer and colder winter), as calcite precipitation in freshwater lakes is primarily a function of peak summer temperature and water depth. This interpretation is also supported by high ??¹³C values, likely reflecting high aquatic productivity or increased residence times of the lake water during lower lake levels. The lower lake levels and warmer summers would have increased evaporative enrichment in ¹⁸O, also contributing to the high ??¹⁸O values during the LIA. Our results indicate that changes in atmospheric circulation were an important component of climate change during the last millennium, exerting strong influence on regional climate in Alaska and the Arctic.     

Study of Holocene glacier degradation in central Asia by isotopic methods for long-term forecast of climate changes

This article presents a summary of our studies of Holocene moraines and glaciers of the Tien-Shan, Pamir, and Himalaya mountains with the purpose of providing pattern regularity of the Holocene glaciation decomposition. We developed a method for obtaining reliable radiocarbon dating of moraines with the use of autochthonous organic matter dispersed in fine-grained morainic material, as well there were shown new possibilities of isotope-oxygen and isotope-uranium analysis for the Holocene glaciations dynamics. We found that Holocene glaciations disintegrate stadially according to the decaying principle, and seven main stages may be distinguished. We achieved the absolute dating of the first three stages, identifying these periods as 8,000, 5,000, and 3,400 years ago. The application of the above-mentioned isotope methods of the Holocene glaciations and moraines study will allow researchers to improve the offered model of the Holocene glaciations disintegration; it will be great contribution to salvation of the problem of long-term climatic and glaciations forecast.     

近40年来天山东段冰川变化及其对气候的响应

利用经1959/1961年航片校正的地形图、1972年航片和1992年TM、2001年ETM+遥感影像,通过遥感图像处理和人工目视解译,分析了天山东段哈尔里克山区1959/1961-2001年的冰川变化.结果表明,1959/1961-2001年冰川面积和储量减少量分别占1959/1961年的11.4%和12.3%.其中,20世纪50年代末到70年代初,冰川退缩幅度大,冰川面积和储量年减少率分别约为0.51%和0.508%,20世纪70年代初到90年代初,退缩大幅减缓,冰川面积和储量年减少率为0.1%和0.13%,90年代以后退缩速度又有加剧趋势,冰川面积和储量年减少率增加到0.31%和0.34%.对流域气象站气候资料分析发现,1959/1961-1972年的冰川面积减少率大,主要与1959-1966年时段气温偏高、而降水偏少有关.升温幅度的增大是影响20世纪90年代初以来研究区冰川退缩加剧的根本原因.     

Late glacial and Holocene vegetation history at Poets hill lake,western Tasmania

Pollen analysis of 3.25 m of late glacial and Holocene sediments gives a mid‐altitude (600 m) record of vegetation development after the last or Margaret Glaciation. Alpine herbfield, coniferous heath and Nothofagus gunnli scrub developed on the moraines until 11,400 BP. Wet montane forest and heath then developed with Phyllocladus aspleniifolius, Nothofagus cunninghamii and Eucalyptus until c. 10,000 BP. After 10,000 BP a mosaic of N. cunninghamii rainforest, Myrtaceae and Proteaceae scrub and Sprengelia incarnata heath occurred. The development of the vegetation from alpine communities to temperate rainforest, which is near its limit at 600 m, occurred under the influence of improving climatic conditions with rapid upslope migration or local expansion of taxa during the late glacial. Temperatures were warm enough for the development of rainforest at 600 m by 10,000 BP, if not earlier. The development of a mosaic of rainforest, scrub and heath vegetation rather than extensive rainforest after 10,000 BP reflects the influence of poor soils, bad drainage and fires. Comparison with similar pollen diagrams from western Tasmania suggests that the development of pollen/vegetation associations was time transgressive with altitude during the late glacial when climatic influences and migration rates were important, and that the mosaic of vegetation communities became more complex during the Holocene because of adjustment to or control by local ecological factors.     

Palaeomagnetic record in Late Pleistocene and Holocene dry lake deposits at Tlapacoya, Mexico

Summary. A palaeomagnetic investigation of Carbon-14-dated marsh and near-shore lacustrine sediments deposited between about 25000–5000 yr bp at Tlapacoya, Mexico, reveals normal polarity of the geomagnetic field in all samples measured. At one site, anomalous palaeomagnetic directions in a mud unit dated about 14500 yr bp raised the possibility of a geomagnetic excursion, but subsequent work at six additional sites in the unit revealed no abnormal directions. Thus the anomalous directions are most likely not a true reflection of geomagnetic field behaviour, although no specific alternative explanation is entirely convincing. The preliminary Tlapacoya data of anomalous directions have been cited by others as positive evidence for an excursion. We strongly recommend it no longer be considered as such.     

Late Pleistocene climate inferred from the reconstruction of the Taylor River glacier complex, southern Sawatch Range, Colorado

Ice surface topography of a late Pleistocene glacier complex, herein named the Taylor River Glacier Complex (TRGC), was reconstructed on the basis of detailed mapping of glacial landforms combined with analyses of aerial photos and topographic maps. During the last glacial maximum (LGM), the TRGC covered an area of 215 km² and consisted of five valley or outlet glaciers that were nourished by accumulation in cirques basins and/or upland ice fields.Equilibrium-line altitudes (ELAs) for the glaciers of the TRGC were estimated using the accumulation-area ratio method, assuming that ratio to be 0.65 ± 0.05. ELAs thus derived ranged from about 3275 to 3400 m, with a mean of 3340 ± 60 m. A degree-day model (DDM) was used to infer the climatic significance of the LGM ELA. With no appreciable differences in precipitation with respect to modern climate, the ELA implies that mean summer temperatures during the LGM were 7.6 °C cooler than today. The DDM was also used to determine the temperatures required to maintain steady-state mass balances for each of the reconstructed glaciers. The required reductions in summer temperature vary little about a mean of 7.1 °C. The sensitivity of these results to slight (± 25%) changes assumed for LGM precipitation are less than ± 0.5 °C. Even under an LGM climate in which precipitation is assumed to be substantially different (± 50%) than the present, mean summer temperatures must be on the order of 7.0 to 8.5 °C lower to depress equilibrium lines to LGM altitudes. The greater sensitivity of the ELA to changes in temperature suggests that glaciation in the region was driven more by decreases in summer temperature rather than increases in precipitation.     

Five short pollen diagrams of soils from Jan Mayen, Norway: a testimony of a dynamic landscape

Five soil cores varying in length from 30 to 42 cm and seven surface samples were analysed for pollen and spores. The soil layers of four cores were probably formed through redeposition of other eroded soils. Only in the Batvika core is the organic fraction of probable local origin, and here a chronology could be established. A total long-distance pollen influx of 14-22.5 grains/cm²/year was calculated. Nearly 2, 000 long-distance pollen grains were counted; the ratios of the dominant pollen types were calculated. Around Batvika the past environment was relatively stable; only one major shift in sedimentation environment is apparent from the diagram. In another diagram, expansion of Taraxacum species could be correlated with anthropogenic soil disturbance. The former presence of Lycopodium alpinum and Selaginella selaginoides on Jan Mayen is indicated by frequent spore finds; the latter species has not been found on the island before. Two unknown spore types are discussed.     

Holocene millennial-scale climate variations documented by multiple lake-level proxies in sediment cores from Hurleg Lake,Northwest China

Millennial-scale climate variability has not been well documented in arid northwest China due to the scarcity of high-resolution, well-dated paleoclimate records. Here we present multi-proxy records from sediment cores taken in freshwater Hurleg Lake on the northeastern Tibetan Plateau, which reveal millennial-scale lake-level and climate variations over the past 8,000 years. This high-elevation region is very sensitive to large-scale climate change, thus allowing us to better understand Holocene climate variations in East Asia. The lake-level record, derived from lithology, magnetic mineralogy, carbonate isotopes, ostracode shell isotopes and trace elements, X-ray fluorescence (XRF), and gray scale data, indicates a highly variable and generally dry climate from 7.8 to 1 ka (1 ka = 1,000 cal year BP), and a relatively stable and wet climate after 1 ka. Superimposed on this general trend, six dry intervals at 7.6–7.2 ka, 6.2–5.9 ka, 5.3–4.9 ka, 4.4–3.8 ka, 2.7–2.4 ka, and 1.7–1.1 ka were detected from the high-resolution carbonate content and XRF data. The generally dry climate between 7.8 and 1 ka was almost synchronous with the decrease of East Asian and Indian monsoon intensities shortly after 8 ka. The six dry intervals can be correlated with weak monsoon events recorded in the East Asia and Indian monsoon regions, as well as the North Atlantic cold events. Our data suggest that millennial-scale monsoon variations could cause highly variable climate conditions in arid northwest China during the Holocene. These millennial-scale climate variations may reflect changes in solar variation and/or changes in oceanic and atmospheric circulation.     

Late Glacial and Holocene temperature changes at Egelsee,Switzerland, reconstructed using subfossil chironomids

A temperature reconstruction using chironomids was attempted at Egelsee, Switzerland, a site where pollen and macrofossil records showed a correspondence between vegetation and climatic changes inferred by other proxies in Europe. The general pattern of temperature changes inferred from chironomids during the Late Glacial [i.e. cold temperatures between ca. 16,500 and 14,800 cal BP, close to present-day temperature between 15,000 and 13,000 cal BP and colder temperatures during the Younger Dryas (YD)], and the major temperature changes of the Holocene (i.e. the Younger Dryas–Holocene transition and the Late Holocene cooling trend) at Egelsee, were mirrored in other European climate reconstructions using various proxies. However, the amplitude of temperature changes during the YD was smaller than reconstructed by other proxies at various sites, and the 8,200 years BP event was not apparent. These differences between records were probably due to the dominance of Corynocera ambigua, with percentages reaching 60% in parts of the Egelsee sequence. This taxon was not present in any of the 103 lakes used for the transfer function and its absence may have yielded less accurate inferences. Its presence in samples only associated with cold inferences at Egelsee suggests that this taxon is a cold indicator. However, it was also found in warm Danish lakes and the factors that determine the presence of C. ambigua remain unexplained. Most samples had a poor fit to temperature and instead, dissolved organic carbon seemed to be a factor influencing the chironomid assemblages during the Holocene. These results illustrate the need to better understand the ecology of chironomids and to disentangle the various factors that affect chironomid communities through time. Ultimately, such information will lead to more accurate temperature reconstructions.     

中国冰川系统对气候变化响应的敏感性分析

Data of 44 glacier systems in China used in this paper were obtained from Chinese Glacier Inventories and the meteorological data were got from Meteorological Atlas of Plateau of west China. Based on the statistical analysis and functional model simulation results of the 44 glacier systems in China, the glacier systems were divided into extremely-sensitive glacier system, semi-sensitive glacier system, extremely-steady glacier system and semi-steady glacier system in terms of glacier system＇s level of water-energy exchange, rising gradient of the equilibrium line altitudes and retreating rate of area to climate warming, their median size and vertical span distribution, and their runoff characteristics to climate warming. Furthermore the functional model of glacier system to climate warming was applied in this paper to predict the average variation trends of the 4 types of glacier systems, which indicate that different sensitivity types of glacier systems respond to the climate warming differently.