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21.
April S. Dalton Minna Väliranta Peter J. Barnett Sarah A. Finkelstein 《Boreas: An International Journal of Quaternary Research》2017,46(3):388-401
We examine pollen, macrofossils and sedimentological proxies from the Ridge Site, an 18‐m sequence of glacial and non‐glacial sediments exposed along the bank of the Ridge River in the southern Hudson Bay Lowlands (HBL), Canada. As the HBL is located in the previously glaciated region of North America, palaeorecords from this region have important implications for understanding ice‐sheet palaeogeography and climate for the late Pleistocene. Two diamicton units were interpreted as subglacial till deposited by a glacier flowing toward the south‐southwest (lower diamicton) and west‐southwest (upper diamicton), respectively. Confined between these tills is a 6‐m non‐glacial unit, constrained to Marine Isotope Stage 3 (MIS 3; c. 57 000 to c. 29 000 a BP) by three radiocarbon dates. Quantitative analyses of the pollen record (dominated by Sphagnum, Cyperaceae, Pinus, Picea, Salix, Alnus and Betula) suggest that average summer temperature (June, July, August) was 14.6±1.51 °C, which is similar to that of the present day at the site. Total annual precipitation was 527±170 mm as compared to 705 mm present‐day. The macrofossil record confirmed the local presence of Betula, Salix and conifers. Our results, in combination with other records from the periphery of the Laurentide Ice Sheet, suggest that vast boreal forest‐type vegetation, along with a drier interstadial climate, existed in the region during MIS 3. We also compare pollen‐derived palaeoclimate reconstructions from the Ridge Site with reconstructions from a previously published site along the Nottaway River, HBL, which was dated to MIS 5a–d (c. 109 000 to c. 82 000 a BP). This comparison suggests that, with additional data, it may be possible to differentiate MIS 3 and MIS 5 deposits in the HBL on the basis of relative continentality, with MIS 3 characterized by lower total annual precipitation, and MIS 5 by values similar or greater than present‐day. 相似文献
22.
J. Sakari Salonen Heikki Sepp Minna Vliranta Vivienne J. Jones Angela Self Maija Heikkil Seija Kultti Handong Yang 《Quaternary Research》2011,75(3):501-511
To investigate the Holocene climate and treeline dynamics in the European Russian Arctic, we analysed sediment pollen, conifer stomata, and plant macrofossils from Lake Kharinei, a tundra lake near the treeline in the Pechora area. We present quantitative summer temperature reconstructions from Lake Kharinei and Lake Tumbulovaty, a previously studied lake in the same region, using a pollen–climate transfer function based on a new calibration set from northern European Russia. Our records suggest that the early-Holocene summer temperatures from 11,500 cal yr BP onwards were already slightly higher than at present, followed by a stable Holocene Thermal Maximum (HTM) at 8000–3500 cal yr BP when summer temperatures in the tundra were ca. 3°C above present-day values. A Picea forest surrounded Lake Kharinei during the HTM, reaching 150 km north of the present taiga limit. The HTM ended with a temperature drop at 3500–2500 cal yr BP associated with permafrost initiation in the region. Mixed spruce forest began to disappear around Lake Kharinei at ca. 3500 cal yr BP, with the last tree macrofossils recorded at ca. 2500 cal yr BP, suggesting that the present wide tundra zone in the Pechora region formed during the last ca. 3500 yr. 相似文献
23.
海南交通可达性的测度与空间分异格局分析 总被引：6，自引：4，他引：2
设定交通可达性指数及交通区位优势度、内部交通连通度和对外通达性3个分指数,全面测度海南各县级行政区的交通可达性,在此基础上,采用Global Moran’s I和Getis-Ord Gi*指数分析与全局趋势分析方法,从多个维度系统研究海南交通可达性空间分异现状格局。结果显示,目前海南各县级行政区交通可达性差异很明显;海南岛交通可达性总体呈沿海内陆分异与北南方向类似U型分异格局,交通可达性的3个分指标均存在鲜明的空间分异特征,即交通区位优势度与交通可达性总体分异格局基本一致,内部交通连通度呈北部较高、中部内陆到西南沿海较低而东南沿海居中的分异格局,对外通达性呈沿海内陆分异及北南和东西方向双重类似U型分异格局;三沙市是海南一个特殊的沿海地级市,其交通可达性很差。地形地貌、经济发展水平、人口和政治因素对海南交通可达性空间分异产生了明显的影响。推进海南经济发展需要充分利用交通条件进行经济空间布局引导,并逐步增强交通布局的空间均衡性。 相似文献
24.
Minna Ma Jie Dong Fawei Zhang Wenwen Cai Hongqin Li 《International Journal of Digital Earth》2018,11(11):1166-1183
Vegetation gross primary production (GPP) is an important variable for the carbon cycle on the Qinghai-Tibetan Plateau (QTP). Based on the measurements from 12 eddy covariance flux sites, we validated a light use efficiency model (i.e. EC-LUE) to evaluate the spatial-temporal patterns of GPP and the effect of environmental variables on QTP. In general, EC-LUE model performed well in predicting GPP at different time scale over QTP. Annual GPP over the entire QTP ranged from 575 to 703 Tg C, and showed a significantly increasing trend from 1982 to 2013. However, there were large spatial heterogeneities in long-term trends of GPP. Throughout the entire QTP, air temperature increase had a greater influence than solar radiation and precipitation (PREC) changes on productivity. Moreover, our results highlight the large uncertainties of previous GPP estimates due to insufficient parameterization and validations. When compared with GPP estimates of the EC-LUE model, most Coupled Model Intercomparison Project (CMIP5) GPP products overestimate the magnitude and increasing trends of regional GPP, which potentially impact the feedback of ecosystems to regional climate changes. 相似文献
25.
Heather A. Binney Katherine J. Willis Mary E. Edwards Shonil A. Bhagwat Patricia M. Anderson Andrei A. Andreev Maarten Blaauw Freddy Damblon Paul Haesaerts Frank Kienast Konstantin V. Kremenetski Sergey K. Krivonogov Anatoly V. Lozhkin Glen M. MacDonald Elena Y. Novenko Pirita Oksanen Tatiana V. Sapelko Minna Väliranta Ludmila Vazhenina 《Quaternary Science Reviews》2009,28(23-24):2445-2464
We present a database of late-Quaternary plant macrofossil records for northern Eurasia (from 23° to 180°E and 46° to 76°N) comprising 281 localities, over 2300 samples and over 13,000 individual records. Samples are individually radiocarbon dated or are assigned ages via age models fitted to sequences of calibrated radiocarbon dates within a section. Tree species characteristic of modern northern forests (e.g. Picea, Larix, tree-Betula) are recorded at least intermittently from prior to the last glacial maximum (LGM), through the LGM and Lateglacial, to the Holocene, and some records locate trees close to the limits of the Scandinavian ice sheet, supporting the hypothesis that some taxa persisted in northern refugia during the last glacial cycle. Northern trees show differing spatio-temporal patterns across Siberia: deciduous trees were widespread in the Lateglacial, with individuals occurring across much of their contemporary ranges, while evergreen conifers expanded northwards to their range limits in the Holocene. 相似文献
26.
Abstract. Aquarium experiments were performed to test for critical oxygen levels in relation to the predation efficiency of C. crangon . Short-term (60 h) experiments where done in normoxia (> 95%). 50, 40.30, and 20% oxygen saturation with the amphipod Bathyporeiapilosa as prey. A significantly reduced predation rate was detected at 30% oxygen. Sublethal effects of C. crangon on adult Macoma balthica (mainly by siphon cropping) were studied by measuring the condition of the clams (morphometric, somatic, and biochemical). To test for these effects experiments were conducted under normoxia and moderate hypoxia (40% oxygen); Condition and siphon indices revealed no change in condition after a 3-week exposure to moderate hypoxia, while a significant reduction in the condition of adult clams was found in both normoxia and hypoxia when subjected to siphon cropping by C. crangon . No enhanced siphon cropping could be detected due to hypoxia.
The results indicate that C. crangon is able to maintain its predatory role until a level of 30% oxygen in short-term exposure. After a 20% oxygen level, however, the abiotic stress alters the functional role of C. crangon as a predator, indicating a threshold of 20–302 oxygen for the normal functioning of the predator-prey food webs. 相似文献
The results indicate that C. crangon is able to maintain its predatory role until a level of 30% oxygen in short-term exposure. After a 20% oxygen level, however, the abiotic stress alters the functional role of C. crangon as a predator, indicating a threshold of 20–302 oxygen for the normal functioning of the predator-prey food webs. 相似文献
27.
This study presents a multiproxy record of Holocene environmental change in the region East of the Pechora Delta. A peat plateau profile (Ortino II) is analyzed for plant macrofossils, sediment type, loss on ignition, and radiocarbon dating. A paleosol profile (Ortino III) is described and radiocarbon dated. A previously published peat plateau profile (Ortino I) was analyzed for pollen and conifer stomata, loss on ignition, and radiocarbon dating. The interpretation of the latter site is reassessed in view of new evidence. Spruce immigrated to the study area at about 8900 14C yr B.P. Peatland development started at approximately the same time. During the Early Holocene Hypsithermal taiga forests occupied most of the present East-European tundra and peatlands were permafrost free. Cooling started after 5000 14C yr B.P., resulting in a retreat of forests and permafrost aggradation. Remaining forests disappeared from the study area around 3000 14C yr B.P., coinciding with more permafrost aggradation. The retreat of forests resulted in landscape instability and the redistribution of sand by eolian activity. The displacement of the Arctic forest line and permafrost zones indicates a warming of at least 2–3°C in mean July and annual temperatures during the Early Holocene. At least two cooling periods can be recognized for the second half of the Holocene, starting at about 4800 and 3000 14C yr B.P. 相似文献