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991.
William O. Hobbs Stefan V. Lalonde Rolf D. Vinebrooke Kurt O. Konhauser R. Paul Weidman Mark D. Graham Alexander P. Wolfe 《Journal of Paleolimnology》2010,44(2):613-628
The quality and interpretability of the paleobiological record depends on the preservation of morphological and geochemical
fossils. Siliceous microfossils and sedimentary pigments are often cornerstones in paleoecology, although the microbial and
geochemical processes conducive to their preservation remain poorly constrained. We examined sediments from an alpine lake
in Banff National Park (Alberta, Canada) where diatom frustules are completely dissolved within 50 years of deposition. Diatom
dissolution, silica recycling, and diagenetic alteration of algal pigments were investigated, in conjunction with porewater
geochemistry and microelectrode profiling of the sediment–water interface. Analysis of sediment trap material showed ~90%
of biogenic silica (BSi) production is lost prior to burial. Silica flux calculations, based on dissolved silica (as H4SiO4) in pore-waters, show a further ~6% of total BSi is returned to the water column from the upper 4 cm of sediments, implying
that only ~4% of total BSi is permanently archived in sediments. In situ sediment pH and O2 profiles reveal that aerobic respiration by bacteria fully consumes oxygen by a depth of 4 mm into the sediment, with associated
strong pH and redox gradients. During sedimentation and early diagenesis, diatoms undergo loss of extracellular polymeric
substances that coat their frustules, promoting silica dissolution and leading to the loss of the microfossil record by a
depth of 3.25 cm. Sedimentary pigments similarly undergo rapid degradation, but diatom-related carotenoids persist below the
depth of silica dissolution. This work provides new insights on diagenetic processes in lakes, with broad implications for
the interpretation of sedimentary proxies for algal production. 相似文献
992.
Jonas Bergman Stefan Wastegrd Dan Hammarlund Barbara Wohlfarth Stephen J. Roberts 《第四纪科学杂志》2004,19(3):241-249
This paper presents one of the most extensive Holocene tephra records found to date in Scandinavia. Microtephra horizons originating from Icelandic eruptions were recorded in two ca. 2 m thick peat profiles at Klocka Bog in west‐central Sweden. Five of the microtephra horizons were geochemically correlated to the Askja‐1875, Hekla‐3, Kebister, Hekla‐4 and Lairg A tephras respectively. Radiocarbon‐based dating of these tephras broadly agree with previously published ages from Iceland, Sweden, Germany and the British Isles. The identification of the Lairg A tephra demonstrates a more widespread distribution than previously thought, extending the usefulness of Icelandic Holocene tephrochronology further north into west‐central Scandinavia. Long‐lasting snow cover and seasonal wind distribution in the lower stratosphere are suggested as factors that may be responsible for fragmentary tephra deposition patterns in peat deposits of subarctic Scandinavia. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
993.
A low-order model for the response of the Atlantic thermohaline circulation to climate change 总被引:1,自引:0,他引:1
Concern has been expressed that anthropogenic climate change may lead to a slowdown or even collapse of the Atlantic thermohaline circulation (THC). Because of the possibly severe consequences that such an event could have on the northern North Atlantic and northwestern Europe, integrated assessment models (IAMs) are needed to explore the associated political and socioeconomic implications. State-of-the-art climate models representing the THC are, however, often too complex to be incorporated into an integrated assessment framework. In this paper we present a low-order model of the Atlantic THC which meets the main requirements of IAMs: it (1) is physically based, (2) is computationally highly efficient, (3) allows for comprehensive uncertainty analysis and (4) can be linked to globally aggregated climate models that are mostly used in IAMs. The model is an interhemispheric extension of the seminal Stommel model. Its parameters are determined by a least-squares fit to the output of a coupled climate model of intermediate complexity. Results of a number of transient global warming simulations indicate that the model is able to reproduce many features of the behaviour of coupled ocean–atmosphere circulation models such as the sensitivity of the THC to the amount, regional distribution and rate of climate change.Responsible Editor: Richard Greatbatch 相似文献
994.
Nicolas Deichmann Manfred Baer Jochen Braunmiller Cécile Cornou Donat Fäh Domenico Giardini Monika Gisler Sarah Huber Stephan Husen Philipp Kästli Urs Kradolfer Martin Mai Silvio Maraini Ivo Oprsal Thomas Schler Danijel Schorlemmer Stefan Wiemer Jochen Wössner Adrian Wyss 《Eclogae Geologicae Helvetiae》2004,97(3):447-458
995.
The marine benthic fauna and the δ18Oc of foraminifers and ostracods from six sites situated on a west–east transect through central Sweden have been analysed in order to estimate the palaeosalinity and palaeocirculation in this shallow‐marine environment. The measurements have been undertaken on material from the early Preboreal, when the Baltic Basin was in contact with the North Sea through straits in central Sweden. The δ18Oc values have a more negative value towards the east, indicating decreasing salinity. This was the result of limited possibilities for marine water to penetrate into the Baltic Basin and the mixing with freshwater from the melting Fennoscandian ice‐sheet. Four water masses existed in the area: a surface layer of freshwater, marine water from the North Sea, brackish–marine intermediate water on the Swedish west coast and brackish Yoldia Sea water in the Baltic Basin. The chronology is based on radiocarbon dates of marine fossils and, at one site, on the occurrence of the Vedde Ash (10 400–10 300 14C yr BP). This is the first record from marine settings in Sweden. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
996.
997.
998.
Stefan Wastegrd 《Quaternary International》2005,130(1):49
Studies of Late Quaternary sediments in south and central Sweden have yielded a detailed tephrochronology for the Last Glacial–Interglacial transition (LGIT; ca. 15,000–10,000 cal. yr BP) and the Holocene. More than ten tephra layers have been detected and geochemically characterised. The most widespread tephra from the LGIT is the rhyolitic phase of the Vedde Ash (ca. 12,000 cal. yr BP) which has been found in lacustrine sediments and marine clays south of the Younger Dryas moraines in south Sweden. Other horizons from the LGIT identified to date include the Borrobol tephra (ca. 14,400 cal. yr BP), the Hässeldalen tephra (ca. 11,500 cal. yr BP), the 10-ka Askja tephra (ca. 11,300 cal. yr BP) and the Högstorpsmossen tephra (ca. 10,200 cal. yr BP). The most significant Holocene isochrones are Hekla-4 (ca. 4260 cal. yr BP), Hekla-Selsund/Kebister (ca. 3750 cal. yr BP), Hekla-3 (ca. 3000 cal. yr BP) and Askja-1875. Two new Late Holocene tephra horizons (the Stömyren tephra, ca. 2100 cal. yr BP and the Gullbergby tephra; ca. 2700 cal. yr BP) were identified in single sites and are so far less valuable as marker horizons, but are potentially important for the future. 相似文献
999.
Earthquakes in Switzerland and surrounding regions during 2004 总被引:1,自引:0,他引:1
Manfred Baer Nicolas Deichmann Jochen Braunmiller Stephan Husen Donat F?h Domenico Giardini Philipp K?stli Urs KRadolfer Stefan Wiemer 《Eclogae Geologicae Helvetiae》2005,98(3):407-418
This report of the Swiss Seismological Service summarizes the seismic activity in Switzerland and surrounding regions during
2004. During this period, 677 earthquakes and 96 quarry blasts were detected and located in the region under consideration.
With 22 events with ML≥2.5, the seismic activity in the year 2004 was close to the average over the last 30 years. As in previous years, most of
the activity was concentrated in the Valais and in Graubünden. In addition, several moderate earthquakes occurred in the lower
crust below the northern Alpine foreland. Unusual was that five earthquakes were sufficiently strong to cause ground shaking
of intensity IV over large portions of the territory. Two were located in Switzerland (Liestal, ML 3.8, and Brugg, ML 4.0). The epicenters of the other three strong events were located outside Switzerland (Besan?on in the French Jura, ML 4.8, Waldkirch in southern Germany, ML 5.1, and Lago di Garda in northern Italy, ML 5.3). 相似文献
1000.
It is investigated how abrupt changes in the North Atlantic (NA) thermohaline circulation (THC) affect the terrestrial carbon
cycle. The Lund–Potsdam–Jena Dynamic Global Vegetation Model is forced with climate perturbations from glacial freshwater
experiments with the ECBILT-CLIO ocean–atmosphere–sea ice model. A reorganisation of the marine carbon cycle is not addressed.
Modelled NA THC collapses and recovers after about a millennium in response to prescribed freshwater forcing. The initial
cooling of several Kelvin over Eurasia causes a reduction of extant boreal and temperate forests and a decrease in carbon
storage in high northern latitudes, whereas improved growing conditions and slower soil decomposition rates lead to enhanced
storage in mid-latitudes. The magnitude and evolution of global terrestrial carbon storage in response to abrupt THC changes
depends sensitively on the initial climate conditions. These were varied using results from time slice simulations with the
Hadley Centre model HadSM3 for different periods over the past 21 kyr. Changes in terrestrial storage vary between −67 and
+50 PgC for the range of experiments with different initial conditions. Simulated peak-to-peak differences in atmospheric
CO2 are 6 and 13 ppmv for glacial and late Holocene conditions. Simulated changes in δ13C are between 0.15 and 0.25‰. These simulated carbon storage anomalies during a NA THC collapse depend on their magnitude
on the CO2 fertilisation feedback mechanism. The CO2 changes simulated for glacial conditions are compatible with available evidence from marine studies and the ice core CO2 record. The latter shows multi-millennial CO2 variations of up to 20 ppmv broadly in parallel with the Antarctic warm events A1 to A4 in the South and cooling in the North. 相似文献