首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
Reconstruction of Mediterranean sea level fields for the period 1945–2000   总被引：1，自引：1，他引：0  
The distribution of sea level in the Mediterranean Sea is recovered for the period 1945–2000 by using a reduced space optimal interpolation analysis. The method involves estimating empirical orthogonal functions from satellite altimeter data spanning the period 1993–2005 that are then combined with tide gauge data to recover sea level fields over the period 1945–2000. The reconstruction technique is discussed and its robustness is checked through different tests. For the altimetric period (1993–2000) the prediction skill is quantified over the whole domain by comparing the reconstructed fields with satellite altimeter observations. For past times the skill can only be tested locally, by validating the reconstruction against independent tide gauge records. The reconstructed distribution of sea level trends for the period 1945–2000 shows a positive peak in the Ionian Sea (up to 1.5 mm yr− 1) and a negative peak of − 0.5 mm yr− 1 in a small area to the south-east of Crete. Positive trends are found nearly everywhere, being larger in the western Mediterranean (between 0.5 and 1 mm yr− 1) than in the eastern Mediterranean (between 0 and 0.5 mm yr− 1). The estimated rate of mean sea level rise for the period 1945–2000 is 0.7 ± 0.2 mm yr− 1, i.e. about a half of the rate estimated for global mean sea level. These overall results do not appear to be very sensitive to the distribution of tide gauges. The poorest results are obtained in open-sea regions with intense mesoscale variability not correlated with any tide gauge station, such as the Algerian Basin.  相似文献   

2.
A set of 13 new unspiked K–Ar dates has been obtained for the Quaternary basaltic volcanism in the Kula area of western Turkey, providing improved age control for the fluvial deposits of the Gediz River that underlie these basalt flows. This dating is able, for the first time, to resolve different ages for the oldest basalts, assigned to category β2, that cap the earliest Gediz deposits recognised in this area, at altitudes of 140 to 210 m above present river level. In particular, the β2 basalt capping the Sarnıç Plateau is dated to 1215 ± 16 ka (± 2σ), suggesting that the youngest underlying fluvial deposits, 185 m above present river level, are no younger than marine oxygen isotope stage (MIS) 38. In contrast, the β2 basalt capping the adjacent Burgaz Plateau is dated to 1014 ± 23 ka, suggesting that the youngest underlying fluvial deposits, 140 m above present river level, date from MIS 28. The staircase of 11 high Gediz terraces capping the latter plateau is thus dated to MIS 48-28, assuming they represent consecutive 40 ka Milankovitch cycles, although it is possible that as many as two cycles are missing from this sequence such that the highest terrace is correspondingly older. Basalt flows assigned to the β3 category, capping Gediz terraces 35 and 25 m above the present river level, have been dated to 236 ± 6 ka and 180 ± 5 ka, indicating incision rates of 0.15 mm a− 1, similar to the time-averaged rates since the eruptions of the β2 basalts. The youngest basalts, assigned to category β4, are Late Holocene; our K–Ar results for them range from zero age to a maximum of 7 ± 2 ka.This fluvial incision is interpreted using numerical modelling as a consequence of uplift caused by a regional-scale increase in spatial average erosion rates to 0.1 mm a− 1, starting at 3100 ka, caused by climate deterioration, since when a total of 410 m of uplift has occurred. Parameters deduced on this basis from the observed disposition of the Early Pleistocene Gediz terraces include the local effective viscosity of the lower crust, which is 2 × 1018 Pa s, the Moho temperature of 660 °C, and the depth of the base of the brittle upper crust, which is 13 km. The thin lithosphere in this area results in high heat flow, causing this relatively shallow base of the brittle upper crust and the associated relatively thick lower-crustal layer, situated between depths of 13 and 30 km. It estimated that around 900 ka, at the start of the 100 ka Milankovitch forcing, the spatial average erosion rate increased slightly, to 0.12 mm a− 1; the associated relatively sluggish variations in uplift rates are as expected given the relatively thick lower-crustal layer.This modelling indicates that the growth of topography since the Pliocene in this study region has not involved a steady state. The landscape was significantly perturbed by the Middle Pliocene increase in erosion rates, and has subsequently adjusted towards—but not reached—a new steady state consistent with these increased erosion rates. It would not be possible to constrain what has been occurring from the Middle to Late Pleistocene or even the Early Pleistocene uplift response alone; information regarding the starting conditions is also essential, this being available in this region from the older geological record of stacked fluvial and lacustrine deposition. This result has major implications for the rigorous modelling of uplift histories in regions of rapid erosion, where preservation of information to constrain the starting conditions is unlikely.  相似文献   

3.
Unspiked K–Ar dating makes the age of the Çakmaközü basalt in eastern Turkey 1818 ± 39 ka (± 2σ). This basalt overlies a staircase of four terraces of the River Murat, a Euphrates tributary, each separated vertically by  20 m. We deduce from the relationship with the basalt that these fluvial deposits aggraded during successive  40 ka climate cycles around the Pliocene–Pleistocene boundary (probably MIS 72-66). The incision and rock uplift at  0.5 mm a− 1, thus indicated, are roughly consistent with the  500 m of entrenchment of this  1.8 Ma Murat palaeo-valley into a former lake basin since the Mid-Pliocene climatic optimum. We infer that the  130 m of incision in this locality since  1.8 Ma dramatically underestimates the associated rock uplift, estimated as  600 m. The  1100 m of rock uplift and  800 m of surface uplift thus estimated since the Mid-Pliocene indicate (assuming Airy isostatic equilibrium)  5 km of thickening of the continental crust, from  37 km to the present 42 km. Eastern Anatolia was thus at a much lower altitude in the Mid-Pliocene than at present, consistent with the low-relief lacustrine palaeo-environment. We infer that the subsequent development of topography and excess crustal thickness are being caused by coupling between surface processes and induced flow in the lower crust: climate change following the Mid-Pliocene climatic optimum resulted in faster erosion that has drawn mobile lower crust beneath the study region.  相似文献   

4.
Altimetry measurements over the Ionian region and tide gauge records along the southern Italian coasts have been combined to analyse the negative sea level trend over the Ionian basin in the last decades. The apparent decreasing trend should be better understood as an abrupt sea level drop in 1998 probably linked to changes in the surface circulation in the Ionian basin induced by the Eastern Mediterranean Transient, which changed from anticyclonic to cyclonic about March 1998. From then onwards, a rising rate of 7.9 ± 0.9 mm/year is observed over the basin.  相似文献   

5.
In this study, more than 13 yr of merged altimetry sea level anomalies (SLA) data were used to analyze the trends of sea level variations in the South China Sea (SCS). The result shows that the mean sea level over the SCS has a rise rate of 11.3 mm/yr during 1993–2000 and a fall rate of 11.8 mm/yr during 2001–2005. The geographical distribution of the sea level variations over the SCS is asymmetric with a pronounced variation existing in the deep water. The trends of thermosteric sea level variations were also examined using Ishii data and MITgcm assimilation data. The result indicates that the thermal change of the upper layer of the SCS has a significant contribution to the sea level variations. Heat budget analysis suggests that heat advection may be a key factor influencing the thermal change. Apart from thermal contribution, the effect of water exchange on the sea level variations was also studied.  相似文献   

6.
Sea level trends and inter-annual variability in the Mediterranean Sea for the period 1960–2000 is explored by comparing observations from tide gauges with sea level hindcasts from a barotropic 2D circulation model, and two full primitive equation 3D ocean circulation models, a regional one and the Mediterranean component of a global one,. In the 2D model, 50% of the sea level variance was found to result from the wind and atmospheric pressure forcing. In the 3D models, 20% of the sea level variance was explained by the steric effects. The sea level residuals at the tide gauges locations, calculated by subtraction of the 2D model output from the sea level observations are significantly correlated (r = 0.4) with the steric signals from the 3D models. After the removal of the atmospheric and the steric contributions the tide-gauge sea level records indicate a period where sea level was stable (1960–1975) and a period where sea level was rising (1975–2000) with rates in the range 1.1–1.8 mm/yr. A part of the residual trend can be explained by the contribution of local land movements (0.3 mm/yr) while its major part indicates a global signal, probably mass addition, appearing after 1975.  相似文献   

7.
The available evidence regarding the disposition and chronology of Pliocene–Pleistocene fluvial terraces, coastal rock flats, raised beaches and lacustrine sediments adjoining the Anti-Atlas coastline of Morocco has been reviewed and supplemented by additional information from our own field reconnaissance. It is thus suggested that the study region has experienced uplift by  130 m since the Mid-Pliocene climatic optimum ( 3.1 Ma), by  90 m since the latest Pliocene ( 2 Ma), and by  45 m since the Mid-Pleistocene Revolution ( 0.9 Ma). Each of these phases of uplift correlates with a phase of global climate change known independently, and it is thus inferred that the observed uplift is being driven by climate through mechanisms such as erosional isostasy and the associated induced lower-crustal flow. Numerical modelling of the observed uplift history indicates that the mobile lower-crustal layer in the study region is  9 km thick, with a temperature at its base of  500 °C. The base of this mobile layer is inferred to be at  24 km depth, the deepest crust consisting of a layer of mafic underplating that does not flow under ambient conditions. The principal landform in the study region, the coastal rock platform at  60 m a.s.l., thus formed during a succession of interglacial marine highstands in the late Early Pleistocene when uplift rates were low. Although control on the ages of young sediments and landforms is currently extremely limited, being dependent on regional correlation schemes rather than on absolute dating, the study region fits the pattern, emerging worldwide, that climate change is driving the systematic growth of topographic relief evident during the Late Cenozoic.  相似文献   

8.
Two Mediterranean sea level distributions spanning the last decades are examined. The first one is a reconstruction of sea level obtained by a reduced-space optimal interpolation applied to tide gauge and altimetry data. The second distribution is obtained from a 3D (baroclinic) regional circulation model. None of the two representations includes the mechanical atmospheric forcing. Results are presented for two different periods: 1993–2000 (for which altimetry data are available) and 1961–2000 (the longest period common to both distributions).The first period is examined as a test period for the model, since the reconstruction is very similar to altimetry observations. The modelled sea level is in fair agreement with the reconstruction in the Western Mediterranean and in the Aegean Sea (except in the early nineties), but in the Ionian Sea the model departs from observations. For the whole period 1961–2000 the main feature is a marked positive trend in the Ionian Sea (up to 1.8 mm yr− 1), observed both in the reconstruction and in the model. Also the distribution of positive trends in the Western Mediterranean (mean value of 1.1 mm yr− 1) and the smaller trends in the Aegean Sea (0.5 mm yr− 1) are similar in the reconstruction and in the model, despite the first implicitly accounts for sea level variations due to remote sources such as ice melting and the second does not. The interannual sea level variability associated with key regional events such as the Eastern Mediterranean Transient is apparently captured by the reconstruction but not by the model (at least in its present configuration). Hence, the reconstruction can be envisaged as a useful tool to validate further long-term numerical simulations in the region.  相似文献   

9.
In the present paper, we report on micropaleontological (dinocysts) and isotopic (18O and 13C in foraminifers) analyses performed in Holocene sediments from fifteen cores raised from the central and northwest North Atlantic. Sea-surface temperature (SST), sea-surface salinity (SSS), thus potential density, and sea-ice cover are reconstructed based on dinocyst assemblages. After proper calibration, oxygen isotope data on the mesopelagic foraminifer Neogloboquadrina pachyderma left coiled (Npl) are converted into potential density values deeper in the water column, thus allowing documentation of vertical density gradients and identification of intervals favourable for winter convection to occur with formation of intermediate Labrador Sea Water (LSW). The most important findings from this study include: (1) the existence of an early-mid Holocene thermal optimum with positive anomalies up to 6 °C above present along the main SW–NE axis of the North Atlantic Current, but no significant SST maximum at most sites along eastern Canadian margins; (2) the evidence for larger than present amplitude of annual SSTs during the early Holocene, thus for a stronger seasonality; (3) minimum sea-ice cover from 11 500 to 6000 cal years BP, and a slight increase of sea-ice variability, and average seasonal duration of 0.5 to 1 month per year afterwards; (4) variable SSS during the entire Holocene, suggesting changes in the routing and rates of freshwater–meltwater discharges from the Arctic and eastern Canada; (5) the setting of conditions compatible with LSW production after 8 ka only, and likely a more steady production during the late Holocene; (6) an overall trend for a potential density increase of the Labrador Sea, throughout the Holocene, matching a decreasing trend eastward, thus suggesting a progressive enhancement of the western branch of the Atlantic Meridional Overturning with respect to its northeastern route; and (7) indication of maximum production and fast dispersal of LSW in the entire North Atlantic during recent times only, as suggested by linearly-converging δ18O-values of Npl from all sites, towards its modern relatively homogeneous composition ( 2.5/2.6‰). The overall picture of the Holocene North Atlantic arising from this study is that of a basin marked by a strong regionalism with large discrepancies in hydrographical trends and high frequency oscillations, at least partly controlled by freshwater–meltwater routes and rates of export from the Arctic.  相似文献   

10.
Influence of upper air conditions on the Patagonia icefields   总被引：1，自引：1，他引：0  
Upper-air conditions archived in the NCEP-NCAR Reanalysis have been used to investigate changes in precipitation and snowfall over the Patagonia icefields during 1960–99. Apparently, whereas total precipitation has not changed, warming has caused a decrease in the amount falling as snow. Precipitation at a site is taken to be proportional to the product of the relative humidity and the component of the wind in a particular critical direction, both at 850 hPa ( 1400 m) at a point over the ocean to the west of the icefields; whether it falls as rain or snow is assumed to depend on whether the temperature at the elevation of the site is above or below + 2 °C. The critical direction is assumed to be 270°, which is perpendicular to the north–south trending Andes and is also the prevailing wind direction in this zone of strong westerlies. Because of the scarcity of precipitation records on or near the icefields, the constant of proportionality cannot be determined, so the investigation is limited to examining relative changes in those upper air variables. Warming at 850 hPa has been 0.5 °C over the 40 years, both winter and summer, with the effects that it has: (1) shifted from snow to rain 5% of the precipitation, the total of which has changed little, and (2) increased annual melt in the ablation areas by 0.5 m w.e. The icefields have been losing mass since at least 1870, so this 40-year trend represents only an acceleration of the longer-term trend of adjusting to climate change since the Little Ice Age.  相似文献   

11.
Here we present a crustal folding or buckling mechanism to explain the rootless 3–5 km high Alborz Mountains in northern Iran as well as  10 km of Late Miocene to recent subsidence in the south Caspian basin and  3–6 km of subsidence in the central Iranian basin in the context of the middle Miocene to recent Arabia–Eurasia collision. A key element of the mechanism is the presence of lateral and vertical lithospheric strength contrasts between the north Iranian continental and south Caspian oceanic crusts: when compression from the collision is applied across the region, the strong south Caspian oceanic crust, buried under > 10 km of premiddle Miocene sediment, interacts with the bottom of the mechanically strong continental upper crust of northern Iran, resulting in upward buckling of the continental crust and downward buckling of the oceanic crust. We test this mechanism using a finite-element numerical model with a Maxwell rheology and obtain results that are consistent with the geological and geophysical observations. The observations compiled here and the model results demonstrate the potential for using this region as a natural laboratory for studying the early stages of continent–oceanic collision, including processes like basin inversion, fault localization and, potentially, subduction initiation.  相似文献   

12.
High thinning rates (up to − 4.0 ± 0.97 m a− 1) have been measured at Campo de Hielo Patagónico Norte (CHN) or Northern Patagonia Icefield, Chile between 1975 and 2001. Results have been obtained by comparing a Digital Elevation Model (DEM) derived from regular cartography compiled by Instituto Geográfico Militar of Chile (IGM) based upon 1974/1975 aerial photographs and a DEM generated from Advanced Space-borne Thermal Emission and Reflection Radiometer (ASTER) satellite images acquired in September 2001. A complete cloud-free Landsat ETM+ satellite image mosaic acquired in March 2001 was used to update the available glacier inventory of the CHN, including all glaciers larger than 0.5 km2 (48 new glaciers). A new delineation of ice divides was also performed over the accumulation areas of glaciers sharing the high plateau where the existing regular cartography exhibits poor coverage of topographic information. This updated glacier inventory produced a total ice area for 2001 of 3953 km2, which represents a decrease of 3.4 ± 1.5% (140 ± 61 km2 of ice) with respect to the total ice area of the CHN in 1979 calculated from a Landsat MSS satellite image. Almost 62% of the total area change between 1979 and 2001 took place in glaciers located at the western margin of the CHN, where the maximum area loss was experienced by Glaciar San Quintín with 33 km2. At the southern margin, Glaciar Steffen underwent the largest ice-area loss (12 km2 or 2.6% of the 1979 area), whilst at the eastern margin the greatest area loss took place in Glaciares Nef (7.9 km2, 5.7% of the 1979 area) and Colonia (9.1 km2, 2.7% of the 1979 area). At the northern margin of the CHN the lower debris-covered ablation area of Glaciar Grosse collapsed into a new freshwater lake formed during the late 1990s. The areal changes measured at the CHN are much larger than previously estimated due to the inclusion of changes experienced in the accumulation areas. The CHN as a whole is contributing melt water to global sea level rise at rates  25% higher than previous estimates.  相似文献   

13.
In this paper we compare sea level trends observed at a few selected tide gauges of good quality records with thermosteric (i.e., due to ocean temperature change) sea level trends over 1950–1998 using different gridded ocean temperature data sets from Levitus et al. (2000) [Levitus, S., Stephens, C., Antonov, J.I., Boyer, T.P., 2000. Yearly and Year-Season Upper Ocean Temperature Anomaly Fields, 1948–1998. U.S. Gov. Printing Office, Washington, D.C. pp. 23.], Ishii et al. (2003) [Ishii, M., Kimoto, M., Kachi, M., 2003. Historical ocean subsurface temperature analysis with error estimates, Mon. Weather Rev., 131, 51–73.] and Levitus et al. (2005) [Levitus S., Antonov, J.I., Boyer, T.P., 2005. Warming of the world ocean, 1955–2003. Geophys. Res. Lett. 32, L02604. doi:10.1029/2004GL021592.]. When using the Levitus data, we observe very high thermosteric rates at sites located along the northeast coast of the US, north of 37°N. Such high rates are not observed with the Ishii data. Elsewhere, thermosteric rates agree reasonably well whatever the data set. Excluding the northeast US coastline sites north of 37°N, we compare tide gauge-based sea level trends with thermosteric trends and note that, in spite of a significant correlation, the latter are too small to explain the observed trends. After correcting for thermosteric sea level trends, residual (observed minus thermosteric) trends have an average value of 1.4 ± 0.5 mm/year, which should have an eustatic (i.e., due to ocean mass change) origin. This result supports the recent investigation by Miller and Douglas (2004) [Miller, L., Douglas, B.C., 2004. Mass and volume contributions to 20th century global sea level rise. Nature 428, 406–408.] which suggests that a dominant eustatic contribution is needed to explain the rate of sea level rise of the last decades observed by tide gauges, and shows that Cabanes et al. (2001) [Cabanes, C., Cazenave, A., Le Provost, C., 2001. Sea level rise during past 40 years determined from satellite and in situ observations. Science 294, 840–842.] arrived at an incorrect conclusion due to peculiarities in the gridded Levitus et al. (2000) [Levitus, S., Stephens, C., Antonov, J.I., and Boyer, T.P., 2000. Yearly and Year-Season Upper Ocean Temperature Anomaly Fields, 1948–1998. U.S. Gov. Printing Office, Washington, D.C. pp. 23.] data set.  相似文献   

14.
Seasonal and inter-annual change in land water storage (expressed in terms of water volume change) over 27 large river basins worldwide are estimated from monthly GRACE geoids solutions computed at GFZ from February 2003 to February 2006. The largest annual water volume change is found in the Amazon basin, followed by the Parana, Ob, Orinoco, Tocantins, Niger, Congo, Ganges, Mekong, and Brahmaputra. In terms of trend over the 3-year period, positive and negative values are observed but in a number of cases computed trends are at the noise level. However significant negative trends are found in the Amazon, Ganges, Mississippi, Nile, Parana, and Zambezi basins, indicating water mass loss over that period. Positive trends (water mass gain) are marginally significant. We have computed the land water contribution to sea level change. On average over the 3-year time span, we find that the net effect is positive (net loss of water in terrestrial reservoirs), on the order of 0.19 +/− 0.06 mm/yr. If sustained over a longer time span than considered here, such a value may become comparable to the ice sheets contribution to sea level rise.  相似文献   

15.
The problem of correcting the tide gauge records for the vertical land motion upon which the gauges are settled has only been partially solved. At best, the analyses so far have included model corrections for one of the many processes that can affect the land stability, namely the Glacial-Isostatic Adjustment (GIA). An alternative approach is to measure (rather than to model) the rates of vertical land motion at the tide gauges by means of space geodesy. A dedicated GPS processing strategy is implemented to correct the tide gauges records, and thus to obtain a GPS-corrected set of ‘absolute’ or geocentric sea-level trends. The results show a reduced dispersion of the estimated sea-level trends after application of the GPS corrections. They reveal that the reference frame implementation is now achieved within the millimetre accuracy on a weekly basis. Regardless of the application, whether local or global, we have shown that GPS data analysis has reached the maturity to provide useful information to separate land motion from oceanic processes recorded by the tide gauges or to correct these latter. For comparison purposes, we computed the global average of sea-level change according to Douglas [Douglas, B.C., 2001. Sea level change in the era of the recording tide gauge. Int. Geophys. Ser., 75, pp. 37–64.] rules, whose estimate is 1.84 ± 0.35 mm/yr after correction for the GIA effect [Peltier, W.R., 2001. Global glacial isostatic adjustment and modern instrumental records of relative sea level history. Int. Geophys. Ser., 75, pp. 65–95.]. We obtain a value of 1.31 ± 0.30 mm/yr, a value which appears to resolve the ‘sea level enigma’ [Munk, W., 2002. Twentieth century sea level: an enigma. Proc. Natl. Acad. Sci. U.S.A., 99(10), pp. 6550–6555].  相似文献   

16.
The Brazilian Northeast affords good opportunities for obtaining reliable timings and rates of landscape evolution based on stratigraphic correlations across a vast region. The landscape formed in the context of an episodically fluctuating but continuously falling base level since the Cenomanian. After formation of the transform passive margin in Aptian times, landscape development was further driven by a swell-like uplift with its crest situated  300 km from the coastline. The seaward flank of this swell or broad monocline between the interior Araripe and coastal Potiguar basins was eroded, and currently forms a deeply embayed plain bordered by a semi-circular, north-facing erosional escarpment. The post-Cenomanian uplift caused an inversion of the Cretaceous basins and generated a landscape in which the most elevated landforms correspond either to resistant Mesozoic sedimentary caprock, or to eroded stumps of syn-rift Cretaceous footwall uplands. Denudation in the last 90 My never exceeded mean rates of 10 m·My− 1 and exhumed a number of Cretaceous stratigraphic unconformities. As a result, some topographic surfaces at low elevations are effectively Mesozoic land surfaces that became re-exposed in Cenozoic times. The Neogene Barreiras Formation forms a continuous and mostly clastic apron near the coast. It testifies to the last peak of erosion in the hinterland and coincided with the onset of more arid climates at  13 Ma or earlier. The semi-circular escarpment is not directly related to the initial breakup rift flanks, which had been mostly eroded before the end of the Mesozoic, but the cause and exact timing of post-Cenomanian crustal upwarping are poorly constrained. It could perhaps have been a flexural response of the low-rigidity lithosphere to sediment loads on the margin, and thus a slowly ongoing process since the late Cretaceous. Uplift could instead be the consequence of a more discrete dynamic event related either to Oligocene magmatism in the region, or to continental-scale far-field stresses determined by Andean convergence.  相似文献   

17.
Airborne laser altimetry survey of Glaciar Tyndall, Patagonia   总被引：1，自引：1，他引：0  
The first airborne laser altimetry measurements of a glacier in South America are presented. Data were collected in November of 2001 over Glaciar Tyndall, Torres del Paine National Park, Chilean Patagonia, onboard a Twin Otter airplane of the Chilean Air Force. A laser scanner with a rotating polygon-mirror system together with an Inertial Navigation System (INS) were fixed to the floor of the aircraft, and used in combination with two dual-frequency GPS receivers. Together, the laser–INS–GPS system had a nominal accuracy of 30 cm after data processing. On November 23rd, a total of 235 km were flown over the ablation area of Glaciar Tyndall, with 5 longitudinal tracks with a mean swath width of 300 m, which results in a point spacing of approximately 2 m both along and across track. A digital elevation model (DEM) generated using the laser altimetry data was compared with a DEM produced from a 1975 map (1:50,000 scale — Instituto Geográfico Militar (IGM), Chile). A mean thinning of − 3.1 ± 1.0 m a− 1 was calculated for the ablation area of Glaciar Tyndall, with a maximum value of − 7.7 ± 1.0 m a− 1 at the calving front at 50 m a.s.l. and minimum values of between − 1.0 and − 2.0 ± 1.0 m a− 1 at altitudes close to the equilibrium line altitude (900 m a.s.l.). The thinning rates derived from the airborne survey were similar to the results obtained by means of ground survey carried out at  600 m of altitude on Glaciar Tyndall between 1975 and 2002, yielding a mean thinning of − 3.2 m a− 1 [Raymond, C., Neumann, T.A., Rignot, E., Echelmeyer, K.A., Rivera, A., Casassa, G., 2005. Retreat of Tyndall Glacier, Patagonia, over the last half century. Journal of Glaciology 173 (51), 239–247.]. A good agreement was also found between ice elevation changes measured with laser data and previous results obtained with Shuttle Radar Topography Mission (SRTM) data. We conclude that airborne laser altimetry is an effective means for accurately detecting glacier elevation changes in Patagonia, where an ice thinning acceleration trend has been observed during recent years, presumably in response to warming and possibly also drier conditions.  相似文献   

18.
Owing to the very gently sloping nature of the flood plain in the lower White Nile valley, which is underlain by a former lake-bed, the depositional record in that area is unusually well preserved. In Egypt and along the Blue Nile phases of erosion have destroyed segments of the sedimentary record, but the White Nile sequence is a good proxy for both the main Nile and the Blue Nile. During the last 15 ka, at least, times of high flow in the Blue Nile and main Nile were synchronous with those in the White Nile.Not all the White Nile flood deposits have been preserved but calibrated radiocarbon dates obtained on fossil freshwater and amphibious Pila shells and fish bones indicate that White Nile levels were high around 14.7–13.1 ka, 9.7–9.0 ka, 7.9–7.6 ka, 6.3 ka and 3.2–2.8 ka. The Blue Nile record is more fragmentary and that of the main Nile even more so except for the Holocene Nile delta. Calibrated radiocarbon ages for high Blue Nile flows indicate very high flood levels towards 13.9–13.2 ka, 8.6 ka, 7.7 ka and 6.3 ka.Incision by the Blue Nile and main Nile has caused progressive incision in the White Nile amounting to at least 4 m since the terminal Pleistocene  15 ka ago and at least 2 m over the past 9 ka. The Blue Nile seems to have cut down at least 10 m since  15 ka and at least 4 m since 9 ka. The time-transgressive and relatively late inception of plant domestication in the Nile valley may partly reflect this history of incision. Nile incision would propagate upstream into the White Nile valley, draining previously swampy areas along the valley floor, which would then become accessible to cultivation.  相似文献   

19.
Sea level observed by altimeter during the 1993–2004 period, thermosteric sea level from 1945 through 2004, and tide gauge records are analyzed to investigate the interannual variability of sea level in the South China Sea (SCS) and its relationship with ENSO (El Niño and Southern Oscillation). Both the interannual variations of the observed sea level and the thermosteric sea level are closely related to ENSO. An ‘enigma’ that the SST and sea level in the SCS have inverse response to ENSO is revealed. It is found that the thermosteric sea level has an excellent correspondence to seawater temperature at 100 m depth, and their variations are unsynchronized to SST. Detailed analysis denotes that the warming of seawater occurs only in the upper 75 m during and after the mature phase of El Niño, while the cooling appears in the layers deeper than 75 m during El Niño years. The volume transports between the SCS and the adjacent oceans and the anomalous Ekman pumping contribute a lot for the sea level fall in the developing stage of El Niño, while the mass exchange, which is dominated by precipitation, plays a more significant role in the following continuous negative sea level anomalies.  相似文献   

20.
The Transkei Basin deposits document the sediment transport around South Africa and, hence, reveal details of palaeocurrent activity of this region for the past 36 my. Thermohaline driven water masses like the North Atlantic Deep Water (NADW) and the Antarctic Bottom Water (AABW), which are part of the global conveyor belt and the main motor for the heat transfer worldwide, have to pass the southern tip of Africa. A large amount of their sedimentary freight is deposited in the submarine Transkei Basin. By the investigation of high resolution seismic reflection data from central Transkei Basin sediments, we reconstructed depocentres and interface outlines for five different time slices since Cretaceous times. Since at least Late Eocene times, we observe an increasing activity of proto-AABW and later proto-NADW in the Transkei Basin. The current's settings were recurrently modified by various large scale global events, like the opening of the Drake Passage Gateway and the Tasman Gateway ( 34 Ma), respectively, or the closure of the Panama Isthmus ( 3 Ma). The investigations reveal a strong influence of global tectonic and climatic events on NADW and (proto-) AABW and their influence on Transkei Basin deposition.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号