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Annually laminated sediments (glacial varves) from Lake Silvaplauna, a High Alpine proglacial lake in the Central Swiss Alps, were compared with glacier monitoring data and instrumental climate data from 1864 to 1990. Long-term and short-term responses to climatic change as well as anthropogenic influence can be traced separately in the varve succession. Economic development in the lake catchment has resulted in higher autochthonous production in recent years. Autochthonous components contribute around 10% to the total amount of sediment accumulated annually since 1960 but their contribution is negligible before this date. Decadal-scale varve thickness trends correlate with glacier size-variations. A stepwise, running multiple regression analysis demonstrates that interannual changes in varve thickness are strongly correlated with changes in mean summer temperatures, but cannot be sufficiently explained without considering summer precipitation and the number of days with snow per year. The wide range of observed correlation coefficients reveals the sensitivity of the archive to temporal variability of the climatic forcing factors and makes the development of transfer functions ambiguous. 相似文献
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Mario A. VESCONI Shawn P. WRIGHT Mauro SPAGNUOLO Robert JACOB Carlos CERRUTTI Luciana GARCIA Evangelina FERNANDEZ William A. CASSIDY 《Meteoritics & planetary science》2011,46(7):935-949
Abstract– More craters may be discovered in the future, but as it is currently known, the Campo del Cielo crater field is 18 km long by 4 km at its widest point. Such a distribution of craters suggests that the parent meteoroid entered and traversed the atmosphere at a very low angle relative to horizontal. The crater field contains at least 20 small craters produced by the larger fragments of the parent meteoroid. Four of these are explosion analog craters and the rest are penetration funnels. During four field seasons, we have constructed topographic and magnetic maps of four of the penetration funnels as found, and then dug trenches across them to learn their original structures and recover meteorites preserved within them. Structures of these penetration funnels indicate very low angles of impact, i.e., 9–16° relative to horizontal. This supports the idea that the parent meteoroid traversed the atmosphere at a low angle. Data given here for the four penetration funnels include projectile masses, lengths, widths, depths, and estimates of impact angles and azimuths. One of the penetration funnels described here (No. 6) can almost be classified as an explosion analog crater. 相似文献
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JOHANNES SIMSTICH HELMUT ERLENKEUSER INGO HARMS ROBERT F. SPIELHAGEN VLADIMIR STANOVOY 《Boreas: An International Journal of Quaternary Research》2005,34(3):252-263
River discharge of Ob and Yenisei to the Kara Sea is highly variable on seasonal and interannual time scales. River water dominates the shallow bottom water near the river mouths, making it warmer and less saline but seasonally and interannually more changeable than bottom water on the deeper shelf. This hydrographic pattern shows up in measurements and modelling, and in stable isotope records (δ18O, δ13C) along the growth axis of bivalve shells and in multiple analyses of single benthic foraminiferal shells. Average isotope ratios increase, but sample-internal variability decreases with water depth and distance from river mouths. However, isotope records of bivalves and foraminifera of a sediment core from a former submarine channel of Yenisei River reveal a different pattern. The retreat of the river mouth from this site due to early Holocene sea level rise led to increasing average isotope values up core, but not to the expected decrease of the in-sample isotope variability. Southward advection of cold saline water along the palaeo-river channel probably obscured the hydrographic variability during the early Holocene. Later, when sediment filled the channel, the hydrographic variability at the core location remained low, because the shallowing proceeded synchronously with the retreat of the river mouth. 相似文献
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NAJA MIKKELSEN HELMUT ERLENKEUSER JOHN S. KILLINGLEY WOLFGANG H. BERGER 《Boreas: An International Journal of Quaternary Research》1982,11(2):163-171
The ahermatypic coral Lophelia pertusa which produces aragonitic skeletons is widely distributed along the Norwegian coast. Specimens from a number of localities have been analyzed for oxygen and stable carbon isotope composition and 14 C age. Stable isotope ratios of recent corals provide information on growth rate and seasonality of oceanographic conditions. Lophelia can be useful in paleoenvironmental reconstructions. 14 C dates of fossil Lophelia from Drøak in the Oslofjord, collected from 20 m ahove and 40 m below present day sea-level, indicate regional extinction between 8700 and 7800 years ago. We suggest that the extinction resulted from the cut-off of deep waters by a rising sill in connection with the postglacial shoreline displacement. Radiocarbon dating of coral bushes suggests a fairly rapid growth rate as older and younger parts of recent corals do not reveal any difference in activity despite the short time scale of the history of bomb-produced 14 C in the oceans. 相似文献
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Julien STODOLNA Damien JACOB Hugues LEROUX Mark J. BURCHELL 《Meteoritics & planetary science》2012,47(4):696-707
Abstract– Particles from comet 81P/Wild 2 were captured with silica aerogel during the flyby Stardust mission. A significant part of the collection was damaged during the impact at hypervelocity in the aerogel. In this study, we conducted impact experiments into aerogel of olivine and pyroxene powder using a light‐gas gun in similar conditions as that of the comet Wild 2 particles collection. The shot samples were investigated using transmission electron microscopy to characterize their microstructure. Both olivine and pyroxene samples show evidence of thermal alteration due to friction with the aerogel. All the grains have rounded edges after collection, whereas their shape was angular in the initial shot powder set. This is probably associated with mass loss of particles. The rims of the grains are clearly melted and mixed with aerogel. The core of olivine grains is fairly well preserved, but some grains contain dislocations in glide configuration. We interpret these dislocations as generated by the thermal stresses that have emerged due to the high temperature gradients between the core and the rim of the grains. Most of the pyroxene grains have been fully melted. Their high silica concentration reflects a strong impregnation with melted aerogel. The preferential melting of pyroxene compared with olivine is due to a difference in melting temperatures of 300°. This melting point difference probably induces a bias in the measurements of the ratio olivine/pyroxene in the Wild 2 comet. The proportion of pyroxene was probably higher on Wild 2 than expected from the samples collected into aerogel. 相似文献
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HENNING A. BAUCH HEIDEMARIE KASSENS HELMUT ERLENKEUSER PIETER M. GROOTES JÖRN THIEDE 《Boreas: An International Journal of Quaternary Research》1999,28(1):194-204
The Holocene depositional setting of the Laptev Sea was studied using three marine sediment cores from water depths between 77 and 46 m. Based on sedimentary parameters (TOC content, δ13 Corg , sedimentation rates) controlled by radiocarbon age models the palaeoenvironment of a strongly coupled river-shelf system was reconstructed since ˜11 ka BP. Caused by a transgressing sea after the last glaciation, all cores reveal progressive decreases in sedimentation rates. Using the sedimentary records of a core from the Khatanga-Anabar river channel in the western Laptev Sea, several phases of change are recognized: (1) an early period lasted until ˜10 ka BP characterized by an increased deposition of plant debris due to shelf erosion and fluvial runoff; (2) a transitional phase with consistently increasing marine conditions until 6 ka BP, which was marked at its beginning near 10 ka BP by the first occurrence of marine bivalves, high TOC content and an increase in δ13 Corg ; (3) a time of extremely slow deposition of sediments, commencing at ˜6 ka BP and interpreted as Holocene sea-level highstand, which caused a southward retreat of the depositional centres within the now submerged river channels on the shelf; (4) a final phase with the establishment of modern conditions after ˜2 ka BP. 相似文献
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A carbonate buildup of Middle Triassic age, the Esino Limestone, outcrops in the Southern Calcareous Alps of Lombardy (N Italy). Along its margin and within the open subtidal facies, the Esino Limestone contains calcite cement-filled cavities of cm to m size. These features, known as evinosponges, may form pervasive networks within the host rock. The filling consists of concentric, isopachous layers of fibrous low-Mg calcite crystals characterized by strong undulose extinction and bent cleavages. The cement crusts are non-luminescent under cathodoluminescence, but both cements and host rock are cross-cut by micro-fractures filled with bright-luminescent calcite, related to late void-filling sparite. Mixing of different carbonates is reflected in stable isotope data. On the hand specimen scale, the oxygen and carbon isotope compositions of cements and host rock show little variation. When compared on a regional scale, the values cover a broad range from δ18O(PDB)=?5‰ to ?12‰ and from δ13O =0‰ to +3‰. The linear covariant trends defined by the oxygen and carbon isotope data for different sampling regions reflect the admixture of late, isotopically depleted calcite with an isotopically enriched non-luminescent calcite of early diagenetic origin. The Esino Limestone fibrous cements, which were probably precipitated in the marine or marine-meteoric phreatic environment, were affected by late diagenetic processes that caused mineral deformation and isotopic depletion through recrystallization and the admixture of a later calcite. These later calcites precipitated from penetrative fluids possibly related to Late Triassic volcanic activity and/or to the Late Cretaceous/Early Palaeogene alpine orogeny. 相似文献