排序方式： 共有17条查询结果，搜索用时 140 毫秒
1.
Meir Abelson Amotz Agnon Ahuva Almogi-Labin 《Earth and Planetary Science Letters》2008,265(1-2):33-48
The Eocene/Oligocene boundary, at about 33.5 Myr ago, marks the transition from ‘greenhouse-’ to ‘icehouse-world’, accompanied by a sudden cooling of ocean bottom-water. We show that this global event is simultaneous with a deep rooted mantle process: an abrupt suppression of the Iceland plume triggered rapid deepening of the Greenland–Scotland Ridge (GSR) — the sill moderating deep circulation between the Nordic seas and North Atlantic. Striking coincidence of several sets of events reflects the abrupt suppression of the Iceland plume and a rapid removal of its influence on the nearby Reykjanes Ridge (RR): 1) A sudden segmentation of the paleo-RR seen on seafloor magnetic anomalies, 2) a drop in spreading rate of the North Atlantic, 3) a transition from thick to normal oceanic crust, and 4) a rapid deepening and accelerated subsidence of the GSR, inferred from the sedimentary record of DSDP site 336. The plume suppression and the concomitant GSR deepening coincide with the initiation of North Atlantic Deep Water (NADW) at the Eocene/Oligocene (E/O) transition, attested by onset of drift sedimentation in the Faroe–Shetland Channel (FSC), the deepest spill-point on the GSR, and in the North Atlantic, the Feni Drift. These processes have influenced global deepwater composition and temperature as indicated by the striking correlation with the jump in global δ18O (> 1‰) measured on benthic foraminifers that reflects the E/O global cooling, and with enrichment of unradiogenic Nd isotopes in the southeastern Atlantic and Southern Ocean. The initiation of Atlantic thermohaline circulation at that time is inferred from the abrupt split between planktonic and benthic δ18O, indicating the building of ocean-water stratification. This scenario is further corroborated by a reversal in benthic δ18O at the late Oligocene, coincident with the renewal of vigorous Iceland plume some 25 Myr ago, causing a considerable retardation in NADW fluxes. The plume renewal is inferred from the emergence of the Iceland plateau, the transition to oblique-unsegmented RR axis, the cessation in deepening of the GSR, and rapid increase in spreading rate of the North Atlantic. These events coincide with decreasing difference in planktonic–benthic in global δ18O by the late Oligocene. All these inferences suggest the role of the NADW sourced at the Nordic seas to form background cooler conditions in the long time scale since the early Oligocene, or to form permanent conditions of invigorated thermohaline circulation that forces CO2 trap in the oceans. 相似文献
2.
D. Kesten M. Weber Ch. Haberland Ch. Janssen A. Agnon Y. Bartov I. Rabba The DESERT Group 《International Journal of Earth Sciences》2008,97(1):153-169
The left-lateral Dead Sea Transform (DST) in the Middle East is one of the largest continental strike-slip faults of the world.
The southern segment of the DST in the Arava/Araba Valley between the Dead Sea and the Red Sea, called Arava/Araba Fault (AF),
has been studied in detail in the multidisciplinary DESERT (DEad SEa Rift Transect) project. Based on these results, here,
the interpretations of multi-spectral (ASTER) satellite images and seismic reflection studies have been combined to analyse
geologic structures. Whereas satellite images reveal neotectonic activity in shallow young sediments, reflection seismic image
deep faults that are possibly inactive at present. The combination of the two methods allows putting some age constraint on
the activity of individual fault strands. Although the AF is clearly the main active fault segment of the southern DST, we
propose that it has accommodated only a limited (up to 60 km) part of the overall 105 km of sinistral plate motion since Miocene
times. There is evidence for sinistral displacement along other faults, based on geological studies, including satellite image
interpretation. Furthermore, a subsurface fault is revealed ≈4 km west of the AF on two ≈E–W running seismic reflection profiles.
Whereas these seismic data show a flower structure typical for strike-slip faults, on the satellite image this fault is not
expressed in the post-Miocene sediments, implying that it has been inactive for the last few million years. About 1 km to
the east of the AF another, now buried fault, was detected in seismic, magnetotelluric and gravity studies of DESERT. Taking
together various evidences, we suggest that at the beginning of transform motion deformation occurred in a rather wide belt,
possibly with the reactivation of older ≈N–S striking structures. Later, deformation became concentrated in the region of
today’s Arava Valley. Till ≈5 Ma ago there might have been other, now inactive fault traces in the vicinity of the present
day AF that took up lateral motion. Together with a rearrangement of plates ≈5 Ma ago, the main fault trace shifted then to
the position of today’s AF. 相似文献
3.
A. Matmon Y. Shaked N. Porat Y. Enzel R. Finkel N. Lifton E. Boaretto A. Agnon 《Earth and Planetary Science Letters》2005,240(3-4):803-817
In this study, we explored the spatial and temporal relations between boulders and their original in-situ locations on sandstone bedrock cliffs. This was accomplished by combining field observations with dating methods using cosmogenic isotopes (10Be and 14C) and optically stimulated luminescence (OSL). Our conclusions bear both on the landscape evolution and cliff retreat process in the hyperarid region of Timna and on the methodology of estimating exposure ages using cosmogenic isotopes.
We recognize three discrete rock fall events, at 31 ka, 15 ka, and 4 ka. In this hyperarid region, the most plausible triggering mechanism for rock fall events is strong ground acceleration caused by earthquakes generated by the nearby Dead Sea fault (DSF). Our record, however, under represents the regional earthquake record implying that ongoing development of detachment cracks prior to the triggering event might be slower than the earthquake cycle.
Cliff retreat rates calculated using the timing of rock fall events and estimated thickness of rock removed in each event range between 0.14 m ky− 1 and 2 m ky− 1. When only full cycles are considered, we derive a more realistic range of 0.4 m ky− 1 to 0.7 m ky− 1. These rates are an order of magnitude faster than the calculated rate of surface lowering in the area. We conclude that sandstone cliffs at Timna retreat through episodic rock fall events that preserve the sharp, imposing, landscape characteristic to this region and that ongoing weathering of the cliff faces is minor.
A 10%–20% difference in the 10Be concentrations in samples from matching boulder and cliff faces that have identical exposure histories and are located only a few meters apart indicates that cosmogenic nuclide production rates are sensitive to shielding and vary spatially over short distances. However, uncertainties associated with age calculations yielded boulder and matching cliff face ages that are similar within 1 σ . The use of external constraints in the form of field relations and OSL dating helped to establish each pair's age. The agreement between calculated 14C and 10Be ages indicates that the accumulation of 10Be at depth by the capture of slow deep-penetrating muons was properly accounted for in the study. 相似文献
4.
Vladimir Lyakhovsky Yehuda Ben-Zion Amotz Agnon 《Geophysical Journal International》2005,161(1):179-190
5.
Down‐faulting at the north‐west margins of the Gulf of Aqaba is inferred to have triggered a catastrophic sedimentary event at 2.3 ka that killed the Elat fringing coral reef. Whereas segments of the Holocene reef were perfectly fossilized and preserved beneath a veneer of siliciclastic sediments, other segments were abraded, settled by nomads, and later re‐submerged under 4 m of water. Repeated damage triggered by down‐throwing earthquakes degenerate the fringing reefs of the north‐west end of the gulf. Conversely, on the north‐eastern and southern parts of the gulf, where earthquakes uplift the margins, modern reefs are thriving, attached to uplifted fossil reef terraces. Therefore, coastal subsidence moderates the development of fringing coral reefs during the late Holocene sea‐level stand still. 相似文献
6.
The complementary mild-slope equation (CMSE) is a depth-integrated equation, which models refraction and diffraction of linear time-harmonic water waves. For 2D problems, it was shown to give better agreements with exact linear theory compared to other mild-slope (MS) type equations. However, no reference was given to 3D problems. In contrast to other MS-type models, the CMSE is derived in terms of a stream function vector rather than in terms of a velocity potential. For the 3D case, this complicates the governing equation and creates difficulties in formulating an adequate number of boundary conditions. In this paper, the CMSE is re-derived using Hamilton's principle from the Irrotational Green–Naghdi equations with a correction for the 3D case. A parabolic version of it is presented as well. The additional boundary conditions needed for 3D problems are constructed using the irrotationality condition. The CMSE is compared with an analytical solution and wave tank experiments for 3D problems. The results show very good agreement. 相似文献
7.
The freshwater Lake Kinneret (Sea of Galilee) and the hypersaline Dead Sea are remnant lakes, evolved from ancient water bodies that filled the tectonic depressions along the Dead Sea Transform (DST) during the Neogene-Quartenary periods. We reconstructed the limnological history (level and composition) of Lake Kinneret during the past ∼40,000 years and compared it with the history of the contemporaneous Lake Lisan from the aspect of the regional and global climate history. The lake level reconstruction was achieved through a chronological and sedimentological investigation of exposed sedimentary sections in the Kinnarot basin trenches and cores drilled at the Ohalo II archeological site. Shoreline chronology was established by radiocarbon dating of organic remains and of Melanopsis shells.The major changes in Lake Kinneret level were synchronous with those of the southern Lake Lisan. Both lakes dropped significantly ∼42,000, ∼30,000, 23,800, and 13,000 yr ago and rose ∼39,000, 26,000, 5000, and 1600 yr ago. Between 26,000 and 24,000 yr ago, the lakes merged into a unified water body and lake level achieved its maximum stand of ∼170 m below mean sea level (m bsl). Nevertheless, the fresh and saline water properties of Lake Kinneret and Lake Lisan, respectively, have been preserved throughout the 40,000 years studied. Calcium carbonate was always deposited as calcite in Lake Kinneret and as aragonite in Lake Lisan-Dead Sea, indicating that the Dead Sea brine (which supports aragonite production) never reached or affected Lake Kinneret, even during the period of lake high stand and convergence. The synchronous level fluctuation of lakes Kinneret, Lisan, and the Holocene Dead Sea is consistent with the dominance of the Atlantic-Mediterranean rain system on the catchment of the basin and the regional hydrology. The major drops in Lake Kinneret-Lisan levels coincide with the timing of cold spells in the North Atlantic that caused a shut down of rains in the East Mediterranean and the lakes drainage area. 相似文献
8.
Ron Shaar Erez Ben-Yosef Hagai Ron Lisa Tauxe Amotz Agnon Ronit Kessel 《Earth and Planetary Science Letters》2011,301(1-2):297-306
The intensity of the geomagnetic field varies over different time scales. Yet, constraints on the maximum intensity of the field as well as for its maximum rate of change are inadequate due to poor temporal resolution and large uncertainties in the geomagnetic record. The purpose of this study is to place firm limits on these fundamental properties by constructing a high-resolution archaeointensity record of the Levant from the 11th century to the early 9th century BCE, a period over which the geomagnetic field reached its maximum intensity in Eurasia over the past 50,000 years. We investigate a 14C-dated sequence of ten layers of slag material, which accumulated within an ancient industrial waste mound of an Iron Age copper-smelting site in southern Israel. Depositional stratigraphy constrains relative ages of samples analyzed for paleointensity, and 14C dates from different horizons of the mound constrain the age of the whole sequence. The analysis yielded 35 paleointenisty data points with accuracy better than 94% and precision better than 6%, covering a period of less than 350 years, most probably 200 years. We construct a new high-resolution quasi-continuous archaeointensity curve of the Levant that displays two dramatic spikes in geomagnetic intensity, each corresponding to virtual axial dipole moment (VADM) in excess of 200 ZAm2. The geomagnetic spikes rise and fall over a period of less than 30 years and are associated with VADM fluctuations of at least 70 ZAm2. Thus, the Levantine archaeomagnetic record places new constraints on maximum geomagnetic intensity as well as for its rate of change. Yet, it is not clear whether the geomagnetic spikes are local non-dipolar features or a geomagnetic dipolar phenomenon. 相似文献
9.
10.