Evidence for a mid‐Pleistocene shift of ice‐drift pattern in the Nordic seas

Sediment proxy records from a continuous, 1.5 million year long deep‐sea sediment core from a site in the western Norwegian Sea were used to obtain new insights into the nature of palaeoceanographic change in the northern North Atlantic (Nordic seas) during the climatic shift of the Mid‐Pleistocene Revolution (MPR). Red‐green sediment colour and magnetic susceptibility records both reveal significant differences in their mean values when comparing the intervals older than 700 000 yr (700 ka) with those from the past 500 kyr. The timing and duration of these changes indicates that the MPR in the Nordic seas is characterised by a gradual transition lasting about 200 kyr. Together with further sedimentological evidence this suggests that the mid‐Pleistocene climate shift was accompanied by a general change in ice‐drift pattern. It is further proposed that prior to the onset of the major late Pleistocene glaciations in the Northern Hemisphere a significant proportion of the ice in the eastern Nordic seas originated from a southern provenance, whereas later it dominantly came from the surrounding landmasses. Copyright © 2003 John Wiley & Sons, Ltd.     

量子化学方法研究氯代苯酚类化合物对水生物毒性的构效关系

对20个氯代苯酚类化合物进行了CNDO/2量子化学计算,讨论了化合物电子结构与其对水生物翻车鱼(Bluegill)、花鳉鱼(Guppy)、和虾(Shrimp)半致死量之间的关系,分别获得了表示其构效关系的三个线性方程,它们的显著性均远高于α=0.01水平。结果表明:氯代苯酚化合物苯环上碳原子的兀电荷之和(∑Qπ_R)越大苯环上碳原子的净电荷之和(∑Q_R)越大,或LUMO轨道能(E_(LUMO))越低,均使化合物对Bluegill和Guppy水生物的毒性增大;而化合物苯环上羟基邻位的碳原子的兀电荷布居(P_3)越大,同时HOMO轨道能E_(HOMO)越高,则化合物对Shrimp毒性也越大。据此,可预测氯代苯酚类化合物对上述三种水生物的毒性。     

Late Holocene great earthquakes and relative sea‐level change at Kenai,southern Alaska

Kenai, located on the west coast of the Kenai Peninsula, Alaska, subsided during the great earthquake of AD 1964. Regional land subsidence is recorded within the estuarine stratigraphy as peat overlain by tidal silt and clay. Reconstructions using quantitative diatom transfer functions estimate co‐seismic subsidence (relative sea‐level rise) between 0.28±0.28 m and 0.70±0.28 m followed by rapid post‐seismic recovery. Stratigraphy records an earlier co‐seismic event as a second peat‐silt couplet, dated to ～1500–1400 cal. yr BP with 1.14±0.28 m subsidence. Two decimetre‐scale relative sea‐level rises are more likely the result of glacio‐isostatic responses to late Holocene and Little Ice Age glacier expansions rather than to co‐seismic subsidence during great earthquakes. Comparison with other sites around Cook Inlet, at Girdwood and Ocean View, helps in constructing regional patterns of land‐level change associated with three great earthquakes, AD 1964, ～950–850 cal. yr BP and ～1500–1400 cal. yr BP. Each earthquake has a different spatial pattern of co‐seismic subsidence which indicates that assessment of seismic hazard in southern Alaska requires an understanding of multiple great earthquakes, not only the most recent. All three earthquakes show a pre‐seismic phase of gradual land subsidence that marked the end of relative land uplift caused by inter‐seismic strain accumulation. Copyright © 2005 John Wiley & Sons, Ltd.     

40Ar/39Ar and oxygen isotope studies of polymetamorphism from Tinos Island, Cycladic blueschist belt, Greece

Abstract Petrological, oxygen isotope and ⁴⁰Ar/³⁹Ar studies were used to constrain the Tertiary metamorphic evolution of the lower tectonic unit of the Cyclades on Tinos. Polyphase high-pressure metamorphism reached pressures in excess of 15 kbar, based on measurements of the Si content in potassic white mica. Temperatures of 450–500° C at the thermal peak of high-pressure metamorphism were estimated from critical metamorphic assemblages, the validity of which is confirmed by a quartz–magnetite oxygen isotope temperature of 470° C. Some ⁴⁰Ar/³⁹Ar spectra of white mica give plateau ages of 44–40 Ma that are considered to represent dynamic recrystallization under peak or slightly post-peak high-pressure metamorphic conditions. Early stages in the prograde high-pressure evolution may be documented by older apparent ages in the high-temperature steps of some spectra. Eclogite to epidote blueschist facies mineralogies were partially or totally replaced by retrograde greenschist facies assemblages during exhumation. Oxygen isotope thermometry of four quartz–magnetite pairs from greenschist samples gives temperatures of 440–470° C which cannot be distinguished from those deduced for the high-pressure event. The exhumation and overprint is documented by decreasing ages of 32–28 Ma in some greenschists and late-stage blueschist rocks, and ages of 30–20 Ma in the lower temperature steps of the Ar release patterns of blueschist micas. Almost flat parts of Ar–Ar release spectra of some greenschist micas gave ages of 23–21 Ma which are assumed to represent incomplete resetting caused by a renewed prograde phase of greenschist metamorphism. Oxygen isotope compositions of blueschist and greenschist facies minerals show no evidence for the infiltration of a δ¹⁸O-enriched fluid. Rather, the compositions indicate that fluid to rock ratios were very low, the isotopic compositions being primarily controlled by those of the protolith rocks. We assume that the fundamental control catalysing the transformation of blueschists into greenschists and the associated resetting of their isotopic systems was the selective infiltration of metamorphic fluid. A quartz–magnetite sample from a contact metamorphic skarn, taken near the Miocene monzogranite of Tinos, gave an oxygen isotope temperature of 555° C and calculated water composition of 9.1%. The value of δ¹⁸O obtained from this water is consistent with a primary magmatic fluid, but is lower than that of fluids associated with the greenschist overprint, which indicates that the latter event cannot be directly related to the monozogranite intrusion.     

Rapid Lateglacial tree population dynamics and ecosystem changes in the eastern Baltic region

A growing body of evidence implies that the concept of 'treeless tundra' in eastern and northern Europe fails to explain the rapidity of Lateglacial and postglacial tree population dynamics of the region, yet the knowledge of the geographic locations and shifting of tree populations is fragmentary. Pollen, stomata and plant macrofossil stratigraphies from Lake Kurjanovas in the poorly studied eastern Baltic region provide improved knowledge of ranges of north‐eastern European trees during the Lateglacial and subsequent plant population responses to the abrupt climatic changes of the Lateglacial/Holocene transition. The results prove the Lateglacial presence of tree populations (Betula, Pinus and Picea) in the eastern Baltic region. Particularly relevant is the stomatal and plant macrofossil evidence showing the local presence of reproductive Picea populations during the Younger Dryas stadial at 12 900–11 700 cal. a BP, occurring along with Dryas octopetala and arctic herbs, indicating semi‐open vegetation. The spread of Pinus–Betula forest at ca. 14 400 cal. a BP, the rise of Picea at ca. 12 800 cal. a BP and the re‐establishment of Pinus–Betula forest at ca. 11 700 cal. a BP within a span of centuries further suggest strikingly rapid, climate‐driven ecosystem changes rather than gradual plant succession on a newly deglaciated land. Copyright © 2009 John Wiley & Sons, Ltd.     

River sediments,great floods and centennial‐scale Holocene climate change

A new analysis of all 346 published ¹⁴C dated Holocene alluvial units in Britain offers a unique insight into the regional impacts of global change and shows how surprisingly sensitive British rivers have been to relatively modest but repeated changes in climate. Fourteen major but probably brief periods of flooding are identified bracketed within the periods 400–1070, 1940–3940, 7520–8100 and at ca. 10 420 cal. yr BP. There is a strong correspondence between climatic deteriorations inferred from mire wet shifts and major periods of flooding, especially at ca. 8000 cal. yr BP and since ca. 4000 cal. yr BP. The unusually long and complete British record also demonstrates that alterations in land cover have resulted in a step change in river basin sensitivity to variations in climate. This has very important implications for assessing and mitigating the impact of increasing severe flooding. In small and medium‐sized river basins land use is likely to play a key role in either moderating or amplifying the climatic signal. Copyright © 2003 John Wiley & Sons, Ltd.     

Short-term Climate Characteristics at Ny-(A)lesund over the Arctic Tundra Area

Based on the Germany Koldwey Station's 1994-2003 conventional observation hourly data, this paper conducts a statistical analysis on the short-term climate characteristics for an arctic tundra region (Ny-(A)lesund island) where our first arctic expedition station (Huanghe Station) was located. Affected by the North Atlantic warming current, this area has a humid temperate climate, and the air temperature at Ny-(A)lesund rose above 0 ℃ even during deep winter season during our research period. The wind speed in this area was low and appeared most at southeast direction. We find that the temperature at Ny-(A)lesund rose in the faster rate (0.68 ℃/10 a) than those at the whole Arctic area. Compared with the floating ices where our expedition conducted in the Arctic, Ny-(A)lesund was warmer and more humid and had lower wind speed. Comparison of the near surface air temperature derived by NCEP/NCAR reanalysis to the conventional measurements conducted at the Koldwey site in Ny-(A)lesund area shows a good agreement for winter season and a significant difference for summer season.     

Isobaric cooling and anti-clockwise P–T paths in the Variscan Odenwald Crystalline Complex, Germany

Mineral assemblages in Al₂O₃‐rich, SiO₂‐ and K₂O‐poor metapelitic rocks from the western Odenwald Crystalline Complex (Variscan Mid‐German Crystalline Rise, southern Germany) include corundum, spinel, cordierite, sillimanite, garnet and staurolite. Quartz is absent from almost all samples. Therefore, the applicability of conventional geothermobarometry is very limited or even impossible. Detailed petrographic investigation on selected samples permits inference of the sequence of appearance and disappearance of several mineral assemblages. The recognition of such partial re‐equilibration stages and their associated mineral assemblages, together with mineral stabilities predicted from KFMASH pseudosections, enables the determination of the pressure‐temperature (P–T) trajectories experienced by these rocks during the Variscan metamorphism. The rocks were metamorphosed under low‐P/high‐T conditions and underwent an anti‐clockwise P–T evolution. A pressure increase from about 2 kbar to 4 ± 0.5 kbar was accompanied by heating. Peak metamorphic conditions were reached at pressures of 4 ± 0.5 kbar and temperatures of at least 640 °C, probably even higher. The retrograde evolution is characterised by near‐isobaric cooling from ≥ 640 °C to approximately 550 °C. The rocks underwent the anti‐clockwise evolution in a subduction‐related magmatic arc setting. The close spatial association of the low‐P/high‐T rocks with recently discovered metabasic eclogites in the eastern part of the Odenwald Crystalline Complex may indicate a fossil paired metamorphic belt in the Central European Variscides.     

同震阶变异常的特征与机制分析

本文对徐州地震台JB—72水平摆倾斜仪1988年11月至1989年5月间所记录到的24次同震阶变进行了深入分析,认为:a.造成同震阶变的原因是多方面的,而反应了地球应变场准静态调整的同震阶变才是有价值的价变异常;b.多观测手段的地震台站,同震阶变应该在不止一台仪器上得到响应。     

Rapid vegetation change during the early Holocene in the Faroe Islands detected in terrestrial and aquatic ecosystems

High‐resolution pollen, plant macrofossil and sedimentary analyses from early Holocene lacustrine sediments on the Faroe Islands have detected a significant vegetation perturbation suggesting a rapid change in climate between ca. 10 380 cal. yr BP and the Saksunarvatn ash (10 240±60 cal. yr BP). This episode may be synchronous with the decline in δ¹⁸O values in the Greenland ice‐cores. It also correlates with a short, cold event detected in marine cores from the North Atlantic that has been ascribed to a weakening of thermohaline circulation associated with the sudden drainage of Lake Agassiz into the northwest Atlantic, or, alternatively, a period with distinctly decreased solar forcing. The vegetation sequence begins at ca. 10 500 cal. yr BP with a succession from tundra to shrub‐tundra and increasing lake productivity. Rapid population increases of aquatic plants suggest high summer temperatures between 10 450 and 10 380 cal. yr BP. High pollen percentages, concentrations and influx of Betula, Juniperus and Salix together with macrofossil leaves indicate shrub growth around the site during the initial phases of vegetation colonisation. Unstable conditions followed ca. 10 380 cal. yr BP that changed both the upland vegetation and the aquatic plant communities. A decrease in percentage values of shrub pollen is recorded, with replacement of both aquatics and herbaceous plants by pioneer plant communities. An increase in total pollen accumulation rates not seen in the concentration data suggests increased sediment delivery. The catchment changes are consistent with less seasonal, moister conditions. Subsequent climatic amelioration reinitiated a warmth‐driven succession and catchment stabilisation, but retained high precipitation levels influencing the composition of the post‐event communities. Copyright © 2003 John Wiley & Sons, Ltd.