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1.
Late Holocene palaeoceanography in Van Mijenfjorden, Svalbard   总被引：2，自引：0，他引：2  
Based on detailed stratigraphical analysis of sediment cores spanning the last ca. 4000 calendar years, we reconstruct the palaeoceanograhic changes in the fiord Van Mijenfjorden, western Svalbard. Benthic foraminiferal δ18O indicate a gradual reduction in bottom water salinities between 2200 BC and 500 BC. This reduction was probably mainly a function of reduced inflow of oceanic water to the fiord, due to isostatic shallowing of the outer fiord sill. Stable salinity conditions prevailed between 500 BC and. 1300 AD. After the onset of a major glacial surge of the tidewater Paulabreen (Paula Glacier) system (PGS) around 1300 AD, there was a foraminiferal faunal change towards glacier proximal conditions, associated with a slight bottom water salinity depletion. During a series of glacial surges occuring from 1300 AD up the present salinity in the fiord has further decreased, corresponding to a δ18O depletion of 0.5 %o. This salinity decrease corresponds to the period when the PGS lost an equivalent of 30 – 40 % of its present ice volume, mainly through calving in the fiord.  相似文献   

2.
Magnetic susceptibility ( χ ) variations and the behaviour of the ratio of susceptibility to saturation magnetization ( χ/J s ) along the loess/palaeosol section at Koriten (NE Bulgaria) are used to deduce climatic changes during the Pleistocene in southeastern Europe. A good correlation of susceptibility variations with the astronomically tuned oxygen isotope record from ODP site 677 enables us to propose a more precise dating of the upper part of the Bulgarian loess complex. Close correspondence between susceptibility and δ 1 8 O records demonstrates the global significance of the palaeoclimatic signal recorded, although differences in relative amplitudes of χ and χ/J s and δ 1 8 O create difficulties in making quantitative estimates of the climatic humidity in the past. The role of local factors affecting the palaeoclimatic mineral magnetic record deduced from the profile studied in Bulgaria is discussed.  相似文献   

3.
Summary. This note presents an exact analytical formula for determining the magnitude of coseismic surface volume change (δ V ) of earthquake faults in a half-space. For a Poisson solid, the formula is remarkably simple; δ V = M zz |8μ, where M zz is one of the moment tensor elements of the source. Maximum δ V values derive from dip slip on faults plunging 45°. For these events, surface volume changes of 0.0001 and 4.3 km3 can be expected for magnitude 5 and 8 earthquakes respectively. All of the coseismic surface volume change is recovered in the interseismic period through relaxation of the Earth and rebound of the surface. A useful rule of thumb for estimating the magnitude of vertical rebound in 45° dip slip events is δ h p=Δ s /24, where Δ s is the coseismic slip on the fault.  相似文献   

4.
This paper outlines the results of stable isotope (δD-δ18O) analysis of snow and glacier ice undertaken as part of a larger study concerning structural glaciology, debris entrainment and debris transport patterns at Midtre Lovénbreen, Svalbard. Samples of fresh snow were collected from the glacier surface in spring 1999 and samples of surface glacier ice and basal ice samples were collected in summer 1999. When plotted on bivariate co-isotopic diagrams (δD-δ18O), the slopes obtained for snow and unmodified glacier ice (6.4 and 6.9, respectively) are less steep than those for the basal ice layer and transverse ice layers on the ice surface (7.6 and 7.7, respectively). The difference in the slope of these lines is not statistically significant at the sample size (50) used in this study. The results indicate that although stable isotope analysis clearly has potential for studies of debris entrainment, transport and structural glaciology, difficulties remain with applying this technique. It is therefore not possible to apply these isotopic techniques to ice facies of unknown origins. In particular, large sample numbers are required to establish statistical differences and high-resolution sampling of specific ice facies may be necessary to establish isotopic differences.  相似文献   

5.
The traveltime perturbation equations for the quasi-compressional and the two quasi-shear waves propagating in a factorized anisotropic inhomogeneous (FAI) media are derived. The concept of FAI media simplifies considerably these equations. In the FAI medium, the density normalized elastic parameters a ijkl ( X i ) can be described by the relation a ijkl ( X i) = f 2( x i ) A ijkl, where A ijkl are constants, independent of coordinates x i and f 2( x i) is a continuous smooth function of x i . The types of anisotropy ( A ijkl ) and inhomogeneity [ f ( x i)] are not restricted. The traveltime perturbations of individual seismic body waves ( q P , qS 1 and qS 2) propagating in the FAI medium depend, of course, both on the structural pertubations [δ f 2( x i)] and on the anisotropy perturbations (δ A ijkl ), but both these effects are fully separated. The perturbation equations for the time delay between the two qS -waves propagating in the FAI medium are simplified even more. If the unperturbed (background) medium is isotropic, the perturbation of the time delay does not depend on the structural perturbations (δ f 2( x i) at all. This striking result, valid of course only in the framework of first-order perturbation theory, will simplify considerably the interpretation of the time delay between the two split qS -waves in inhomogeneous anisotropic media. Numerical examples are presented.  相似文献   

6.
PALAEOCLIMATOLOGICAL SURVEY OF STALAGMITES FROM COASTAL AREAS IN TANZANIA   总被引：1，自引：0，他引：1  
ABSTRACT. The aim of this study was to assess the potential of Tanzanian stalagmites as palaeoclimatic archives. Twelve stalagmites were collected in cave systems in the coastal areas. U-series dating, combined with published 14C-data, shows that one of the stalagmites was precipitated during the latter half of the latest glacial, between approximately 39 and 27 ka. Stable isotope analysis results indicate millennial-scale fluctuations in atmospheric CO2 level during that time; and these fluctuations show a pattern very similar to that of the δ18O variations in the GRIP and Byrd ice-cores. The other stalagmites analysed turned out to be too porous and/or contained too much detritus to allow for reliable U-series dating. Due to low uranium content, the precision of the dating results is generally also poor. Samples from the five most compact and clean specimens yielded ages from c. 105 ka to c. 17 ka, though sometimes with an inverse age stratigraphy, indicating that the stalagmite precipitates were partly not compact enough to prevent post-depositional uranium leakage.  相似文献   

7.
Summary. Attenuation of earthquake intensities with epicentral distance was studied by analysing the intensity data for 39 earthquakes in the United States. Attenuation of MM intensity ( I ) with distance (Δ km) follows a simple relation of the type log I = log I 0 - m Δ, where I 0 is the intensity at the epicentre and m is a constant. Slope m is found to be inversely proportional to the square of the focal depth. Intensity attenuation pattern in the United States in general can be represented by a unified relation I/I 0= exp [-(0.8999/ h 2+ 0.0014)Δ] where 16km ≤ h ≤ 60km. Intensities were calculated with the help of this equation and a good agreement with the observed intensities were found. A comparative study has also been made between the attenuation relations applicable to India and the United States.  相似文献   

8.
207Pb/206Pb single-grain zircon, 40Ar/39Ar single-grain hornblende and biotite, and 40Ar/39Ar bulk-sample muscovite and biotite ages from the Nelshoogte trondhjemite pluton located in eastern Transvaal, South Africa, show that this granitoid had a protracted thermal history spanning 3213±4  Ma to about 3000  Ma. Whole-rock 40Ar/39Ar ages from cross-cutting dolerite dykes indicate that these were intruded at about 1900  Ma. There is no evidence of this or other, later events significantly affecting the argon systematics of the minerals from the pluton dated by the 40Ar/39Ar method.
  The pluton has a well-defined palaeomagnetic pole which is dated at 3179±18 (2 σ ) Ma by 40Ar/39Ar dating of hornblende. This pole (18°N, 310°E, A 95=9°) yields a palaeolatitude of 0°, significantly different from other Archaean poles from the Kaapvaal Craton. The palaeolatitude difference implies that there was significant apparent polar wander during the Archaean. A second, overprinting magnetization seen in the pluton is also seen in the lower-Proterozoic dolerite dykes, and is consistent with other lower-Proterozoic (2150–1950  Ma) poles for southern Africa.  相似文献   

9.
ABSTRACT. Hydrovolcanic deposits, interbedded tills and recycled microfossils, together with erosion anomalies in the Marie Byrd Land (MBL) landscape, each provide a portion of the record of glaciation and deglaciation events from late Oligocene to the present. We have attempted to synthesize these data sources to provide a more complete record, and to reconcile them with climatic events recorded elsewhere in Antarctica and the deep sea. The MBL data suggest that the late Oligocene was marked by the development of an ice cap at Mount Petras, where the MBL dome was beginning to rise from a near-sea-level position. Furthermore, unusually advanced cirque development in the dome crest area is difficult to explain unless there was a period of effective cirque erosion in that area between c. 25 and 15 Ma BP. These inferences are consistent with evidence from the Ross Sea for an expansion of the West Antarctic Ice Sheet (WAIS) in mid-Miocene (15–17 Ma BP) time.
The deep sea oxygen isotope proxy record has been interpreted to show the inception of West Antarctic glaciation around 6 Ma BP. This can perhaps be reconciled with the terrestrial record if one considers (1) the observation that large volume changes in the WAIS cannot produce a ω18O signal that is significantly outside limits of error, and (2) that the landscape in West Antarctica has evolved from very low regional relief in the Oligocene, to more than 2 km of local relief in the present day, as a result of dome uplift since c. 27 Ma BP, and the growth of large volcanoes since c. 19 Ma BP.  相似文献   

10.
Approaches to Modelling the Surface Albedo of a High Arctic Glacier   总被引：1，自引：0，他引：1  
Broadband surface albedo measurements, made during the summer melt season at three weather stations on John Evans Glacier (79°40 ' N, 74°00 ' W), varied strongly with the solar zenith angle ( θ z ). Tests were carried out to assess the impact of diurnal variations in surface albedo on seasonal net shortwave radiation ( K * ) totals. Removing the diurnal signal from albedo measurements by daily averaging of hourly measurements, or by applying midday measurements to all hours of the day, changed K * by up to 16%. Ignoring measurements made at θ z & 75°, to account for measurement (cosine) error at high θ z , decreased K * by between 5 and 18%. Given the sensitivity of K * to diurnal patterns in surface albedo, experiments were carried out with two albedo models. One model accounted for albedo variations with θ z and one did not. The model driven by θ z , when implemented within a surface energy balance model for John Evans Glacier, produced better melt estimates. This suggests that diurnal variations in surface albedo should be accounted for in energy balance models of glacier melt.  相似文献   

11.
以南沙群岛永暑礁的现代滨珊瑚YSL24（生长时间为1985—1999年）与YSL2A（生长时间为1971—1999年）为材料,通过对其骨骼开展高分辨率的Sr/Ca、δ18O和δ13C分析,探讨其对极端高温事件的响应特征。有记录表明,1971—1999年,南海南部在1973、1983和1998年发生了极端高温事件,滨珊瑚骨骼Sr/Ca序列清楚地记录了这3次高温事件,其相应条带Sr/Ca周期数值显著低于序列其他年份。而δ18O对高温事件的响应不太明显,其原因可能与δ18O序列同时受温度和盐度变化的影响有关。δ13C在这3个高温年份均在夏季发生了异常负偏,应该是由高温胁迫下共生藻光合作用强度下降所致。因此,珊瑚骨骼的Sr/Ca和δ13C对南沙群岛高温胁迫具有明显的响应,可作为记录过去白化事件的可靠指标。  相似文献   

12.
Summary. Titanomagnetites of composition Fe2.4-δAlδTi0.6O4 and Fe2.6-δAlδTi0–4O4(δ=0, 0.1 and 0.2 in both cases) were prepared in the monodomain state by pulverization of sintered synthetic material. In low fields, the thermoremanence (TRM) was found to be linear with inducing field and of high enough intensity to account for typical natural remanent magnetizations of fresh submarine basalts. The higher field TRM acquisition curves follow the Néel model curve for an assemblage of non-interacting identical particles in a general way only, the differences being due to interactions, or the range of particle blocking temperatures and volumes or other features of the samples not included in the model. The unblocking temperatures of low field TRM lie in a narrow range below the Curie point. The low field TRM is very resistant to alternating field demagnetization and provides a very striking illustration of the strength of the TRM mechanism in preserving a stable record of a weak magnetic field. The result of a Lowrie-Fuller test on the material is consistent with the monodomain state.  相似文献   

13.
Seismic sources with observable glut moments of spatial degree two   总被引：1，自引：0，他引：1  
Let ζΛ and r Λ. be the hypocentral position and time of an extended indigenous seismic source. Backus showed that the force moment tensors of the source, Γ( m +1, n )(ζΛ, r Λ), determine and are determined by the motion which the source produces. For small m + n , only the long-period motion is relevant. The glut moment tensor Λ( m,n ) (ζΛ, r Λ.) can be calculated uniquely from γ( m +1, n )(ζΛ r Λ) only if m = 0 or m = 1. The tensor G =Λ(2,0)(ζΛ) gives the spatial variance tensor WΛ of the source, and WΛ. roughly describes the size, shape and orientation of the source region. Therefore the failure of the observed F =Γ(3,0)(ζΛ) to determine G uniquely is of seismological interest. In the present paper we show that F determines G uniquely if we assume the source to be a simple straight line source (SSLS) or an ideal fault in an isotropic medium with isotropic prestress (IFIMIP). We give tests on F which determine whether it can come from a SSLS, from an IFIMIP or from a simple plane surface source (SPSS). If we assume the source to be a SPSS then knowing F and the fault plane determines G to within an unknown scalar multiple of a certain tensor tangent to the fault plane. Moreover F determines the fault plane uniquely unless F can come from a SSLS. If it can, then F determines this virtual source line uniquely, and F permits the fault plane to be any plane containing the virtual source line.  相似文献   

14.
Summary. A new asymptotic formula is obtained for the spectrum of an isolated normal mode multiplet nSl or nTl , with n ≪ l , on a laterally heterogeneous Earth. The principal feature of this formula is that it is uniformly valid on the Earth's surface, including near the epicentre and its antipode. The formal conditions for its validity are that | δm / m 0|≪ 1 and s max≪ l ≪ s min| δm / m 0|–1, where | δm / m 0| is the relative magnitude of the lateral heterogeneity, and s min and s max are the minimum and maximum significant degrees in its spherical harmonic expansion. As well as providing a basis for the geographical interpretation of near-epicentral or near-antipodal long-period recordings, the new formula also unifies the asymptotic theory and adds insight into the phenomena which govern the details of multiplet spectra in general.  相似文献   

15.
Summary. The potential function ø for a magnetic body of susceptibility μ in a medium of susceptibility μ* satisfies the integral equation
Here Φ* is the potential function for the region without the heterogeneity and R is the distance from the point of observation to the point on the surface, s , of the body. δΦ /δn is the normal derivative, in the direction of the outward normal. The equation allows for the effects of demagnetization. For numerical purposes the surfaces can be divided into N facets over which δΦ/δ n is a constant. The unknown quantities δΦ/δnj can be found from the system of equations defined by:
The prime on the summation sign denotes that the summation does not include the i th element. The magnetic field in the direction of the unit vector P( P 1, P 2, P3 ) is given by:   相似文献   

16.
The deglaciation history of Balsfjord, northern Norway, and post-glacial mass movement events were investigated. Radiocarbon dates indicate that the Balsfjord glacier retreated from the Tromsø–Lyngen moraines about 10.4 14C Ky BP. Between ca. 10.3 14C Ky BP and 9.9 14C Ky BP, deposition of a distinct end moraine–the Skjevelnes moraine–in the central part of Balsfjord occurred. The transition from glacimarine to open marine sedimentary environment took place before 9.6 14C Ky BP. Between ca. 9.5 14C Ky BP and 8.4 14C Ky BP, at least one local and three regional mass movement events occurred. After this period, no gravity flow activity is preserved in the cores. The high frequency of mass movements in the early post-glacial period is presumed to be due to fast sea level changes and/or tectonic activity induced by rapid isostatic uplift.  相似文献   

17.
Summary. Differences between estimated average heat flow values for the Mesozoic and Cenozoic formations ( Q 1) and estimated average heat flow values for the Palaeozoic formations below the erosional unconformity ( Q 2) are calculated for the Alberta part of the western Canadian sedimentary basin. Significant heat flow differences exist for these two intervals and the map of Δ Q = Q 1– Q 2 shows that Q 2 is generally greater than Q 1 in the western and south-western part of Alberta, while in the northern part of the province Q 2 is generally less than Q 1. The regional variations of Δ Q are large, with standard deviation of 26 mW m−2 and average value –13.5 mW m−2. A regional trend of Δ Q correlates with topographic relief and the hydraulic head variations in the basin. It is shown that there is a heat flow increase with depth in water recharge areas and a decrease in heat flow with depth in the low topographic elevation water discharge areas when comparing the average heat flow in Mesozoic + Cenozoic and Palaeozoic formations.  相似文献   

18.
Summary. The paper gives the results of a study of the anisotropy of seismic wave velocities within the Ashkhabad test field in Central Asia. The anisotropy was studied by analysing variations in the values of apparent velocities of first arrivals for epicentral distances ranging from 30 to 130 km and by analysing the delays (Δ ts1-s2 ) between the arrival times of shear waves with different polarizations.
The velocities of P -waves vary with azimuth from 5.3 to 6.27 km s-1 and the velocities of S -waves vary from 3.15 to 3.5 km s-1.
The delay times Δ tS1 - S2 depend on the direction of the propagation. The character of the variation of the propagation velocity of the longitudinal wave, the presence of two differently polarized shear waves S 1 and S 2 propagating at different velocities, and the character of the distribution of Δ tS1 - S2 on the stereogram suggest that the symmetry of the anisotropic medium is close to hexagonal with a nearly horizontal symmetry axis coinciding with the direction of maximal velocity. The azimuth of the symmetry axis of the medium is 140° and coincides with the direction of geological faults.  相似文献   

19.
Sediment textural properties and total organic carbon (TOC) contents of three sediment cores from Maxwell Bay, King George Island, West Antarctica, record changes in Holocene glaciomarine sedimentary environments. The lower sedimentary unit is mostly composed of TOC-poor diamictons, indicating advanced coastal glacier margins and rapid iceberg discharge in proximal glaciomarine settings with limited productivity and meltwater supply. Fine-grained, TOC-rich sediments in the upper lithologic unit suggest more open water and warm conditions, leading to enhanced biological productivity due to increased nutrient-rich meltwater supply into the bay. The relationship between TOC and total sulfur (TS) indicates that the additional sulfur within the sediment has not originated from in situ pyrite formation under the reducing condition, but rather may be attributed to the detrital supply of sand-sized pyrite from the hydrothermal-origin, quartz-pyrite rocks widely distributed in King George Island. The evolution of bottom-water hydrography after deglaciation was recorded in the benthic foraminiferal stable-isotopic composition, corroborated by the TOC and lithologic changes. The Ø18O values indicate that bottom-water in Maxwell Bay was probably mixed gradually with intruding 18O-rich seawater from Bransfield Strait. In addition, the Ø13C values reflect a spatial variability in the carbon isotope distribution in Maxwell Bay, depending on marine productivity as well as terrestrial carbon fluxes by meltwater discharge. The distinct lithologic transition, dated to approximately 8000 yr BP (uncorrected) and characterized by textural and geochemical contrasts, highlights the postglacial environmental change by a major coastal glacier retreat in Maxwell Bay.  相似文献   

20.
We use annual GPS observations on the Reykjanes Peninsula (RP) from 2000 to 2006 to generate maps of surface velocities and strain rates across the active plate boundary. We find that the surface deformation on the RP is consistent with oblique plate boundary motion on a regional scale, although considerable temporal and spatial strain rate variations are observed within the plate boundary zone. A small, but consistent increase in eastward velocity is observed at several stations on the southern part of the peninsula, compared to the 1993–1998 time period. The 2000–2006 velocities can be modelled by approximating the plate boundary as a series of vertical dislocations with left-lateral motion and opening. For the RP plate boundary we estimate left-lateral motion  18+4−3 mm yr−1  and opening of  7+3−2 mm yr−1  below a locking depth of  7+1−2 km  . The resulting deep motion of  20+4−3 mm yr−1  in the direction of  N(100+8−6)°E  agrees well with the predicted relative North America–Eurasia rate. We calculate the areal and shear strain rates using velocities from two periods: 1993–1998 and 2000–2006. The deep motion along the plate boundary results in left-lateral shear strain rates, which are perturbed by shallow deformation due to the 1994–1998 inflation and elevated seismicity in the Hengill–Hrómundartindur volcanic system, geothermal fluid extraction at the Svartsengi power plant, and possibly earthquake activity on the central part of the peninsula.  相似文献   

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