The Arctic as a trigger for glacial terminations   总被引：1，自引：0，他引：1  

Douglas G. Martinson  Walter C. Pitman III 《Climatic change》2007,80(3-4):253-263

We propose a hypothesis to explain the very abrupt terminations that end most of the glacial episodes. During the last glaciation, the buildup and southerly expansion of large continental ice-sheets in the Northern Hemisphere and extensive cover of sea ice in the N. Pacific and the N. Atlantic imposed a much more zonal climatic circulation system than exists today. We hypothesize that this, in combination with the frigid (dry) polar air led to a significant decrease in freshwater runoff into the Arctic Ocean. In addition the freshwater contribution of the fresher Pacific water was completely eliminated by the emergence of the Bering Strait (sill depth 50 m). As the Arctic freshwater input was depleted, regions of the Arctic Ocean lost surface stability and eventually overturned, bringing warmer deep water to the surface where it melted the overlying sea ice. This upwelled water was quickly cooled and sank as newly formed deep water. For sustained overturn events, such as might have occurred during the peak of very large glacial periods (i.e. the last glacial maximum), the voluminous deep water formed would eventually overflow into the Nordic Seas and North Atlantic necessitating an equally voluminous rate of return flow of warmer surface waters from the North Atlantic thus breaking down the Arctic's zonal isolation, melting the expansive NA sea ice cover and initiating oceanic heating of the atmosphere over the ice-sheets bordering the NA. We suggest that the combined effect of these overturn-induced events in concert with a Milankovitch warming cycle, was sufficient to drive the system to a termination. We elaborate on this proposed sequence of events, using the model for the formation of the Weddell Sea polynya as proposed by Martinson et al. (1981) and various, albeit sparse, data sets from the circum-Arctic region to apply and evaluate this hypothesis to the problem of glacial terminations.  相似文献   

Land subsidence in Mashhad Valley, northeast Iran: results from InSAR, levelling and GPS     

Mahdi Motagh  Yahya Djamour  Thomas R. Walter  Hans-Ulrich Wetzel  Jochen Zschau  Siavash Arabi 《Geophysical Journal International》2007,168(2):518-526

  相似文献   

Big lake records preserved in a little lake’s sediment: an example from Silver Lake,Michigan, USA     

Timothy G. Fisher  Walter L. Loope  William Pierce  Harry M. Jol 《Journal of Paleolimnology》2007,37(3):365-382

We reconstruct postglacial lake-level history within the Lake Michigan basin using soil stratigraphy, ground-penetrating radar (GPR), sedimentology and ¹⁴C data from the Silver Lake basin, which lies adjacent to Lake Michigan. Stratigraphy in nine vibracores recovered from the floor of Silver Lake appears to reflect fluctuation of water levels in the Lake Michigan basin. Aeolian activity within the study area from 3,000 years (cal yr. B.P.) to the present was inferred from analysis of buried soils, an aerial photograph sequence, and GPR. Sediments in and around Silver Lake appear to contain a paleoenvironmental record that spans the entire post-glacial history of the Lake Michigan basin. We suggest that (1) a pre-Nipissing rather than a Nipissing barrier separated Silver Lake basin from the Lake Michigan basin, (2) that the Nipissing transgression elevated the water table in the Silver Lake basin about 6,500 cal yr. B.P., resulting in reestablishment of a lake within the basin, and (3) that recent dune migration into Silver Lake is associated with levels of Lake Michigan. This is the fourth in a series of ten papers published in this special issue of Journal of Paleolimnology. These papers were presented at the 47th Annual Meeting of the International Association for Great Lakes Research (2004), held at the University of Waterloo, Waterloo, Ontario, Canada. P.F. Karrow and C.F.M. Lewis were guest editors of this special issue.  相似文献   

Defining probability of saturation with indicator kriging on hard and soft data     

Steve W. Lyon  Arthur J. Lembo Jr.  M. Todd Walter  Tammo S. Steenhuis 《Advances in water resources》2006

In humid, well-vegetated areas, such as in the northeastern US, runoff is most commonly generated from relatively small portions of the landscape becoming completely saturated, however, little is known about the spatial and temporal behavior of these saturated regions. Indicator kriging provides a way to use traditional water table data to quantify probability of saturation to evaluate predicted spatial distributions of runoff generation risk, especially for the new generation of water quality models incorporating saturation excess runoff theory. When spatial measurements of a variable are transformed to binary indicators (i.e., 1 if above a given threshold value and 0 if below) and the resulting indicator semivariogram is modeled, indicator kriging produces the probability of the measured variable to exceed the threshold value. Indicator kriging gives quantified probability of saturation or, consistent with saturation excess runoff theory, runoff generation risk with depth to water table as the variable and the threshold set near the soil surface. The probability of saturation for a 120 m × 180 m hillslope based upon 43 measurements of depth to water table is investigated with indicator semivariograms for six storm events. The indicator semivariograms show high spatial structure in saturated regions with large antecedent rainfall conditions. The temporal structure of the data is used to generate interpolated (soft) data to supplement measured (hard) data. This improved the spatial structure of the indicator semivariograms for lower antecedent rainfall conditions. Probability of saturation was evaluated through indicator kriging incorporating soft data showing, based on this preliminary study, highly connected regions of saturation as expected for the wet season (April through May) in the Catskill Mountain region of New York State. Supplementation of hard data with soft data incorporates physical hydrology of the hillslope to capture significant patterns not available when using hard data alone for indicator kriging. With the need for water quality models incorporating appropriate runoff generation risk estimates on the rise, this manner of data will lay the groundwork for future model evaluation and development.  相似文献   

Physical model for the downhole orbital vibrator (DOV) – I. Acoustic and borehole seismic radiation     

P. C. Leary  L. A. Walter 《Geophysical Journal International》2005,163(2):647-662

  相似文献   

Decapod crustaceans from the Middle Jurassic Opalinus Clay of northern Switzerland, with comments on crustacean taphonomy     

Walter Etter 《Eclogae Geologicae Helvetiae》2004,97(3):381-392

  相似文献   

Thermal and chemical variations in subcrustal cratonic lithosphere: evidence from crustal isostasy   总被引：3，自引：0，他引：3  

Walter D. Mooney  John E. Vidale 《Lithos》2003,71(2-4):185-193

The Earth's topography at short wavelengths results from active tectonic processes, whereas at long wavelengths it is largely determined by isostatic adjustment for the density and thickness of the crust. Using a global crustal model, we estimate the long-wavelength topography that is not due to crustal isostasy. Our most important finding is that cratons are generally depressed by 300 to 1500 m in comparison with predictions from pure crustal isostasy. We conclude that either: (1) cratonic roots may be 50 to 300 °C colder than previously suggested by thermal models, or (2) cratonic roots may be, on average, less depleted than suggested by studies of shallow mantle xenoliths. Alternatively, (3) some combination of these conditions may exist. The thermal explanation is consistent with recent geothermal studies that indicate low cratonic temperatures, as well as seismic studies that show very low seismic attenuation at long periods (150 s) beneath cratons. The petrologic explanation is consistent with recent studies of deep (>140 km) mantle xenoliths from the Kaapvaal and Slave cratons that show 1–2% higher densities compared with shallow (<140 km), highly depleted xenoliths.  相似文献   

Forest‐Cover Change from Labor‐ and Capital‐Intensive Commercial Logging in the Southern Chocó Rainforests*     

Rodrigo Sierra  Milton Tirado  Walter Palacios 《The Professional geographer》2003,55(4):477-490

This article examines two key aspects of land‐cover change in the south of the Chocó region. First, it assesses and compares the local impact on forest condition of labor‐intensive and capital‐intensive commercial logging. Second, it assesses the regional significance and permanency of these changes. Studies of land‐cover change associated with commercial logging have focused almost exclusively on capital‐intensive extraction and have assumed that after logging, degraded forests are transformed into agricultural cover. This study shows that both capital‐ and labor‐intensive logging result in similar land‐cover changes (i.e., forest degradation) if the timber sought is the same. However, labor‐intensive loggers also seek timber species not sought by capital‐intensive loggers, and this impact is statistically different from the impact of the extraction of the first group of species. Results also show that only a small fraction (20–30 percent) of the area logged is later converted to agricultural cover types. The persistence of logged forests means that up to 20 percent of the remaining forest cover could correspond to forests with significant and lasting levels of degradation. Furthermore, the different production requirements for each group of species also mean that there is a spatial differentiation in the impact of logging in the region. Logged forests are arranged into two consecutive corridors on each side of access routes (e.g., rivers). The first corridor corresponds to a narrow (approximately 1‐km) band of high‐intensity degradation. The second, broader (approximately 2‐km) forest band, with lower levels of degradation, extends inland along first‐tier corridors. A key factor determining the permanency of this land‐cover pattern is the strong control local communities have over the land in the region. This limits the spread of patterns observed in other frontier areas, especially the conversion of logged forests into agricultural cover.  相似文献   

New age constraints on the timing of volcanism and tectonism in the northern Main Ethiopian Rift–southern Afar transition zone (Ethiopia)     

Tadiwos Chernet  William K. Hart  James L. Aronson  Robert C. Walter 《Journal of Volcanology and Geothermal Research》1998,80(3-4)

Forty new K-Ar and ⁴⁰Ar/³⁹Ar isotopic ages from the northern Main Ethiopian Rift (MER)–southern Afar transition zone provide insights into the volcano-tectonic evolution of this portion of the East African Rift system. The earliest evidence of volcanic activity in this region is manifest as 24–23 Ma pre-rift flood basalts. Transition zone flood basalt activity renewed at approximately 10 Ma, and preceded the initiation of modern rift margin development. Bimodal basalt–rhyolite volcanism in the southern Afar rift floor began at approximately 7 Ma and continued into Recent times. In contrast, post-subsidence volcanic activity in the northern MER is dominated by Mio-Pliocene silicic products from centers now covered by Quaternary volcanic and sedimentary lithologies. Unlike other parts of the MER, Mio-Pliocene silicic volcanism in the MER–Afar transition zone is closely associated with fissural basaltic products. The presence of Pliocene age ignimbrites on the plateaus bounding the northern MER, whose sources are found in the present rift, indicates that subsidence of this region was gradual, and that it attained its present physiography with steep escarpments only in the Plio-Pleistocene. Large 7–5 Ma silicic centers along the southern Afar and northeastern MER margins apparently formed along an E–W-oriented regional structural feature parallel to the already established southern escarpment of the Afar. The Addis Ababa rift embayment and the growth of 4.5–3 Ma silicic centers in the Addis Ababa area are attributed to the formation of a major cross-rift structure and its intersection with the same regional E–W structural trend. This study illustrates the episodic nature of rift development and volcanic activity in the MER–Afar transition zone, and the link between this activity and regional structural and tectonic features.  相似文献   

Nuclear stellar discs in early-type galaxies — I. HST and WHT observations     

Frank C. van den Bosch  Walter Jaffe  & Roeland P. van der Marel 《Monthly notices of the Royal Astronomical Society》1998,293(4):343-363

  相似文献