Chad Basin: Paleoenvironments of the Sahara since the Late Miocene

Since the mid 1990s, the Mission paléoanthropologique francotchadienne (MPFT) conducts yearly paleontological field investigations of the Miocene-Pliocene of the Chad Basin. This article synthesizes some of the results of the MPFT, with focus on the Chad Basin development during the Neogene. We propose an overview of the depositional paleoenvironments of this part of Africa at different scales of time and space, based on a multidisciplinary approach (sedimentary geology, geomorphology, geophysic, numerical simulations and geochronology). The Miocene-Pliocene paleoenvironments are examined through the sedimentary archives of the early hominids levels and the Holocene Lake Mega-Chad episode illustrates the last major paleoenvironmental change in this area. The sedimentary record of the Chad Basin since the Late Miocene can be schematized as the result of recurrent interactions from lake to desert environments.     

东地中海黎凡特盆地油气勘探潜力分析

地中海海域油气储量规模大,但分布不均,可采储量主要分布在地中海中部和东部地区。其中,位于东地中海沿岸的黎凡特盆地近年来陆续发现了多个大油气田,资源量巨大,吸引了国内外学者的广泛关注。但黎凡特盆地存在未来勘探方向不明确等问题,因此本文基于该区域油气资源概况、盆地构造、沉积、成藏组合等的研究,对未来勘探有利区进行了预测。本文认为以色列深水区中生界砂岩、碳酸盐岩及古近系碎屑岩体系等均具有一定的勘探潜力,可作为未来勘探开发的重点区域。     

Budget for dissolved silica in the Mediterranean Sea

Measurement of the silica flux through the Straits of Gibraltar shows a loss of 3.6 × 10³ moles/second. Estimated influx from rivers and the Black Sea is 3.9 × 10³ moles/sec. Apparently no substantial fraction of dissolved silica is lost as the rivers flow into the sea, nor from the water in the sea.Latest flow measurements at Gibraltar suggest a residence time of 170 yr for water in the sea. Diversion of Nile River water—8 % of the total silica input—should have no measurable effect on the silica concentration of the Mediterranean for a long time.     

Tsunamigenic zones in the Mediterranean Sea

A list of 300 tsunamis and similar phenomena known in the Mediterranean is given. Data reliability and wave intensity are estimated; mechanisms of tsunami generation are indicated and data from literature sources on the coordinates and magnitudes of tsunamigenic earthquakes are cited. Eighteen zones of excitation and manifestation of tsunamis are identified which can be integrated into four groups with respect to the recurrence period and maximum intensity of the tsunamis. The strongest tsunamis are excited in the Aegean Sea, and the Hellenic and Calabrian island arcs. The focal depth of the earthquake-generating tsunamis in the Mediterranean is, on average, less than that in the Pacific. Correspondingly, the magnitude of tsunamigenic earthquakes is lower. According to preliminary estimates, the Mediterranean tsunamis attenuate with distance more rapidly than do those in the Pacific Ocean.     

Sea level in the Mediterranean Sea: seasonal adjustment and trend extraction within the framework of SSA

The sea level change is a crucial indicator of our climate. The spatial sampling offered by satellite altimetry and its continuity during the past years are the major assets to provide an improved vision of the Mediterranean sea level changes. In this paper, an automatic signal extraction approach, based on Singular Spectrum Analysis (SSA), is utilized for analysis and seasonal adjustment of the Mediterranean Sea level series. This automatic approach enables us to overcome the difficulties of visual identification of trend constituents that sometimes we encounter when using the conventional SSA method. The results indicate that the Mediterranean mean sea level is dominated by several harmonic components. The annual signal is particularly strong and almost covers 73.62 % of the original sea level series variation whiles its amplitude is about 15 cm. The extracted trend also indicates that the Mediterranean main sea level has significantly been raised during the period 1993–2012 by 2.44?±?0.4 mm yr^?1. As an important consequence, considering the current situation, if this trend continues, the Mediterranean Sea level will be raised about 22 cm by the end of this century, which makes a dramatic effect on several issues such as land, flora, fauna, and people activities established along the Mediterranean coastlines.     

Geoid model in the Western Mediterranean Sea

In this work a geoid model is presented over the Western Mediterranean area. It has been computed using marine and terrestrial gravimetric data. Differences between results including several kinds of data are also studied. Altimetric data from a year of ERS-1 mission are used to test the precision of the results, overall close to the coastal line. A first approximation to the sea surface topography in the area is made with both results: altimetric mean sea surface and gravimetric geoid.     

Deposition of fine-grained sediments in the abyssal environment of the Algéro-Balearic Basin, Western Mediterranean Sea

Two depositional processes control the mud accumulation on the southern Balearic Abyssal Plain: pelagic settling at a rate of 10 cm/1000 years, and turbidity currents at an average frequency of > 3 per 2000 years. Thermo-haline bottom flow has little effect on the abyssal sediment distribution. Just over half of the Late Quaternary section is made up of turbidite mud. Distinctive properties of turbidite mud are: structural, textural, and compositional continuity from the underlying turbidite sand-silt layer into the overlying mud, grading within the mud layer, a ratio of carbonate percent with the underlying turbidite sand-silt layer of about 0.5, and a proportion of sand of > 1%. Those of (hemi)pelagic mud are: bioturbation, an average of 8% of sand consisting largely of remains of foraminifera and pteropods, a grain size distribution which is virtually normal with a median around 9 φ, and very poor sorting; in general, the properties of (hemi)pelagic muds are the same in widely separated localities and depths in cores. In some instances the clay mineral ratios of the turbidite mud layer are markedly different from those of the overlying (hemi)pelagic mud layer.     

Deglaciation of the Irish Sea Basin: a critique of the glaciomarine hypothesis

The glaciomarine model for deglaciation of the Irish Sea basin suggests that the weight of ice at the last glacial maximum was sufficient to raise relative sea‐levels far above their present height, destabilising the ice margin and causing rapid deglaciation. Glacigenic deposits throughout the basin have been interpreted as glaciomarine. The six main lines of evidence on which the hypothesis rests (sedimentology, deformation structures, delta deposits, marine fauna, amino‐acid ratios and radiocarbon dates) are reviewed critically. The sedimentological interpretation of many sections has been challenged and it is argued that subglacial sediments are common rather than rare and that there is widespread evidence of glaciotectonism. Density‐driven deformation associated with waterlain sediments is rare and occurs where water was ponded locally. Sand and gravel deposits interpreted as Gilbert‐type deltas are similarly the result of local ponding or occur where glaciers from different source areas uncoupled. They do not record past sea‐levels and the ad hoc theory of ‘piano‐key tectonics’ is not required to explain the irregular pattern of altitudes. The cold‐water foraminifers interpreted as in situ are regarded as reworked from Irish Sea sediments that accumulated during much of the late Quaternary, when the basin was cold and shallow with reduced salinities. Amino‐acid age estimates used in support of the glaciomarine model are regarded as unreliable. Radiocarbon dates from distinctive foraminiferal assemblages in northeast Ireland show that glaciomarine sediments do occur above present sea‐level, but they are restricted to low altitudes in the north of the basin and record a rise rather than a fall in sea‐level. It is suggested here that the oldest dates, around 17 000 yr BP, record the first Late Devensian (Weichselian) marine inundation above present sea‐level. This accords with the pattern but not the detail of recent models of sea‐level change. Copyright © 2001 John Wiley & Sons, Ltd.     

On the origin of the western Mediterranean Sea basins

This is a report of a symposium organized by the Netherlands Commission for the Upper Mantle Project. The data relative to the generation of the western Mediterranean Sea basins, presented during this symposium, are summarized in the Appendix.

Several modes of origin have been discussed:

1. (1) the basins are remnants of a former larger ocean;

2. (2) they are formed in the wake of drifting continental blocks;

3. (3) by an erosion and denudation of a continental crust;

4. (4) by an upheaval and later subsidence of an ocean floor; or

5. (5) by sub crustal erosion of a continental crust.

It is concluded that, although many data are in agreement with the drift model, this process cannot have been the sole agent in the generation of the basins.     

Application of pyrolysis–GC/MS for the characterisation of suspended particulate organic matter in the Mediterranean Sea: a comparison with the Black Sea

Suspended particulate organic matter (SPOM) both from the oligotrophic Mediterranean and from the more productive Black Sea has been analysed by pyrolysis–gas chromatography/mass spectrometry (Py–GC/MS) and by such conventional techniques as elemental analysis and fluorometry. The data achieved by the two approaches generally confirmed and complemented each other. The relative concentrations of pyrolysis products (termed markers) characteristic of chlorophyll (CHL), lipid, carbohydrate (CBH), and protein (PROT) components of SPOM have been determined. The vertical distribution of lipid markers was more uniform in the Mediterranean surface waters. Their relative abundance increased rapidly in the oxycline of the Black Sea, reaching their highest levels in the suboxic zone, where the protein composition of the SPOM changed significantly. In both seas, the relative concentrations of CHL markers increased consistently within the CHL maximum zone where the CBH markers were less abundant. No lignin markers from terrestrial vegetation were detected at significant levels in the pyrograms.     

南海构造变形分区及成盆过程

为了系统认识新生代南海沉积盆地形成演化过程和成盆机制, 在对南海及其周缘区域构造和沉积研究进展调研的基础上, 利用覆盖南海主要盆地新近采集和重处理的地震剖面开展详细的构造-地层分析.基于盆地结构构造、演化特征和成盆动力学的差异性, 以红河-越东-Lupar线大型走滑构造带为界, 将南海及其周缘沉积盆地划分为古南海俯冲拖拽构造区沉积盆地群和挤出-逃逸构造区沉积盆地群, 前者主要是古南海俯冲及其所引起的区域构造变形形成的盆地, 包括北部湾、琼东南、珠江口、曾母、北康、文莱-沙巴和礼乐等盆地, 后者是印度-欧亚大陆碰撞导致印支地块挤出和旋转形成的盆地, 如莺歌海、湄公、中建南、万安等盆地.最后, 结合周缘板块动力学事件和本次对盆地构造研究的成果, 特别是盆地中重要界面属性的重新厘定, 建立了南海及其周缘沉积盆地演化过程.      

Late Quaternary sapropel sediments in the eastern Mediterranean Sea: Faunal variations and chronology

Distinctive planktonic foraminiferal assemblages which characterize particular late Quaternary sapropel layers in deep basin sediments from the eastern Mediterranean Sea have been identified using cluster analysis. Three distinct clusters allow for identification and intercore correlation of the nine sapropels deposited during the last 250,000 yr. Cluster 1, representing sapropel layers S1 and S9, exhibits low abundances of Neogloboquadrina dutertrei and high abundances of Globigerinoides ruber; Cluster 2, which groups S3, S5, and S7, contains high abundances of G. ruber, N. dutertrei, and Globigerina bulloides, and Cluster 3, which includes samples from S4, S6, and S8, is marked by extremely abundant N. dutertrei and G. bulloides, and rare G. ruber. Analysis of sedimentation rates in 14 cores reveals the following approximate ages for the sapropel layers: S2 = 52,000 yr B.P.; S3 = 81,000–78,000 yr B.P.; S4 = 100,000–98,000 yr B.P.; and S5 = 125,000–116,000 yr B.P. As previously suggested, sedimentation rates on the Mediterranean Ridge were determined to be relatively constant during the last 127,000 yr. In contrast, basin sedimentation rates have fluctuated markedly from lower rates during interglacial stage 5 to higher rates during the last glacial episode. These glacial/interglacial differences are most pronounced in the northern Ionian Basin, because of increased terrigenous sediment deposition during glacial episodes. Unusually high biogenic sedimentation rates occurred in an arc south of Crete during the deposition of sapropel S5, probably due to higher productivity in this region.     

Crustal structure of the Levant Basin, eastern Mediterranean

A seismic refraction/wide-angle reflection experiment was undertaken in the Levant Basin, eastern Mediterranean. Two roughly east–west profiles extend from the continental shelf of Israel toward the Levant Basin. The northern profile crosses the Eratosthenes Seamount and the southern profile crosses several distinct magnetic anomalies. The marine operation used 16 ocean bottom seismometers deployed along the profiles with an air gun array and explosive charges as energy sources. The results of this study strongly suggest the existence of oceanic crust under portions of the Levant Basin and continental crust under the Eratosthenes Seamount. The seismic refraction data also indicate a large sedimentary sequence, 10–14 km thick, in the Levant Basin and below the Levant continental margin. Assuming the crust is of Cretaceous age, this gives a fairly high sedimentation rate. The sequence can be divided into several units. A prominent unit is the 4.2 km/s layer, which is probably composed of the Messinian evaporites. Overlying the evaporitic layer are layers composed of Plio–Pleistocene sediments, whose velocity is 2.0 km/s. The refraction profiles and gravity and magnetic models indicate that a transition from a two layer continental to a single-layer oceanic crust takes place along the Levant margin. The transition in the structure along the southern profile is located beyond the continental margin and it is quite gradual. The northern profile, north of the Carmel structure, presents a different structure. The continental crust is much thinner there and the transition in the crustal structure is more rapid. The crustal thinning begins under western Galilee and terminates at the continental slope. The results of the present study indicate that the Levant Basin is composed of distinct crustal units and that the Levant continental margin is divided into at least two provinces of different crustal structure.     

Empirical relationships between aftershock area dimensions and magnitude for earthquakes in the Mediterranean Sea region

Fault dimension estimates derived from the aftershock area extent of 36 shallow depth (≤ 31 km) earthquakes that occurred in the Mediterranean Sea region have been used in order to establish empirical relationships between length, width, area and surface-wave/moment magnitude. This dataset consists of events whose aftershock sequence was recorded by a dense local or regional network and the reported location errors did not exceed on average 3–5 km. Surface-wave magnitudes for these events were obtained from the NEIC database and/or published reports, while moment magnitudes as well as focal mechanisms were available from the Harvard/USGS catalogues. Contrary to the results of some previously published studies we found no evidence in our dataset that faulting type may have an effect on the fault dimension estimates and therefore we derived relationships for the whole of the dataset. Comparisons, by means of statistical F-tests, of our relationships with other previously published regional and global relationships were performed in order to check possible similarities or differences. Most such comparisons showed relatively low significance levels (< 95%), since the differences in source dimension estimates were large mainly for magnitudes lower than 6.5, becoming smaller with increasing magnitude. Some degree of similarity, however, could be observed between our fault length relationship and the one derived from aftershock area lengths of events in Greece, while a difference was found between our regional and global fault length relationships. A calculation of the ratio defined as the fault length, derived from our relationships, to the length estimated from regional empirical relationships involving surface ruptures showed that it can take a maximum value of about 7 for small magnitudes while it approaches unity at Ms 7.2. When calculating the same ratio using instead global empirical relationships we see the maximum value not exceeding 1.8, while unity is reached at Mw 7.8, indicating the existence of a strong regional variation in the fault lengths of earthquakes occurring in the Mediterranean Sea region. Also, a relationship between the logarithms of the rupture area and seismic moment is established and it is inferred that there is some variation of stress drop as a function of seismic moment. In particular, it is observed that for magnitudes lower than 6.6 the stress drop fluctuates around 10 bar, while for larger magnitudes the stress drop reaches a value as high as 60 bar.     

Tsunami hazard in the Eastern Mediterranean: strong earthquakes and tsunamis in Cyprus and the Levantine Sea

A modern tsunami catalogue has been compiled for the region of Cyprus-Levantine Sea in which 24 certain or possible local tsunamis are listed from antiquity up to the present time, while six regional tsunamis, generated in the Hellenic arc, are documented which affected the region. Another set of 13 doubtful events not included in the catalogue are discussed. Tsunami intensities k and K were re-evaluated using the classic 6-grade and the new 12-grade intensity scales, respectively. The strongest tsunamis reported in the region of interest are those of 551 AD, 749, 1068, 1201, 1222, 1546 and 1759, all occurring along the Levantine coast from Gaza northward, with the exception of the 1222 wave which occurred in the Cyprean arc. The causative earthquakes, however, occur on land and are associated with the left-lateral strike-slip Levantine rift and, as such, remain unexplained. In this paper we speculate on the mechanism of these events. A second tsunami zone follows the Cyprean arc, where the situation of subaqueous seismogenic sources favours the generation of tsunamis by co-seismic fault displacements. Submarine or coastal earth slumping, however, may be an additional tsunamigenic component. Based on historical data, the average tsunami recurrence in the Cyprus-Levantine Sea region is roughly estimated to be around 30 years, 120 years and 375 years for moderate (k/K ≥ 2/3), strong (k/K ≥ 3/5) and very strong (k/K ≥ 5/8) events, respectively. The rate of tsunami occurrence equals 0.033, 8.3 × 10⁻³ and 2.7 × 10⁻³ events/year for intensity k/K ≥ 2/3, 3/5 and 5/8, respectively. For a Poissonian (random) process the probabilities of observing at least one moderate, strong or very strong tsunami are 0.28, 0.01 and 3 × 10⁻³ within 1 year, 0.81, 0.34 and 0.13 within 50 years and 0.96, 0.56 and 0.24 within 100 years, respectively. The tsunami potential in the Cyprus-Levantine Sea area is low relative to other Mediterranean tsunamigenic regions. However, the destructiveness of some historical events indicates the need to evaluate tsunami hazard by all available means. In addition, remote tsunamigenic sources, such as those of 1303 and 1481 in the eastern Hellenic arc, are able to threaten the coasts of the Cyprus-Levantine region and, therefore, such regional tsunamis should be taken into account in the evaluation of the tsunami risk of the region.     

Micro- and macroseismicity of the Dead Sea rift and off-coast eastern Mediterranean

The seismicity of Israel has been evaluated from documented earthquake records of the present century and two years of routine monitoring of microearthquake activity by means of eleven stations spreading from the Gulf of Elat to northern Galilee.

The Dead Sea rift asserts itself as the tectonic feature that accounts for the seismicity of our region. The activity peaks at zones where the fault branches sideways or at a junction with other fault systems. In particular, the crescent fault of Wadi Faria seems to be a zone of high strain accumulation. This is probably the site of many historical earthquakes which caused inland and coastal damage. It is thus found that the most active fault today which constitutes the greatest seismic risk to Israeli metropolitan areas extends along the Dead Sea rift from 31.2°N to 33.4°N.

The seismicity around the Dead Sea conforms with the proposed movement along en-echelon faults. While the southwest segment is presently inactive, most of the seismic activity there is limited to the neighbourhood of its eastern shore with extreme seismicity at its southern tip near the prehistorical site of Bab-a-Dara'a. The seismicity of the Arava is much lower than the Jordan-Dead Sea section. The seismicity of the Israeli coast was found to be somewhat higher than that of the Arava.     

黄海盆地构造特征及形成机制

黄海盆地研究程度极低,其形成机制一直存在争议。本文通过收集、整理前人的构造资料,对黄海盆地的断裂系统进行重新划分,认为北黄海盆地和南黄海盆地主体断裂均为北北东向右行走滑断裂,控制盆地发育,而这些走滑断层的分支断裂控制着盆地内部的隆坳格局,走滑断层派生出的次一级正断层则以雁列状出现,主体为北东东-近东西向,控制盆地内的凹陷和凸起。黄海盆地及其周边断裂的平面组合及盆地内部凹陷和凸起的展布特征指示该盆地在新生代是在右行右阶走滑断裂体系下形成的拉分盆地。渤海湾盆地和东海陆架盆地等在新生代也具有相同的成因机制,因此,中国东部可能在新生代整体统一的右行张扭性体制下形成了这一系列拉分盆地。     

The last glacial-interglacial transition and dinoflagellate cysts in the western Mediterranean Sea

Using the analysis of dinoflagellate cysts in three deep-sea sediments cores situated in the Sicilian-Tunisian Strait, in the Gulf of Lions and in the Alboran Sea, we reconstruct the paleoenvironmental changes that took place during the last glacial-interglacial transition in the western Mediterranean Sea. The development of the warm microflora Impagidinium aculeatum and especially Spiniferites mirabilis appears to be an important proxy for recognizing warm periods as the Bölling/Alleröd and the Early Holocene. Bitectatodinium tepikiense, Spiniferites elongatus and Nematosphaeropsis labyrinthus mark the end of the Heinrich event 1 and the Younger Dryas. This cold microfloral association confirms the drastic climate changes in the western Mediterranean Sea synchronous to the dry and cold climate which occurred in the South European margin. The dinocyst N. labyrinthus shows high percentages in all studied regions during the Younger Dryas. Its distribution reveals a significant increase from the South to the North of this basin during this cold brief event. Thus, we note that this species can be considered as a new eco-stratigraphical tracer of the Younger Dryas in the western Mediterranean Sea.     

Delivery of transuranic elements by rain to the Mediterranean Sea

The concentrations of ²³⁸Pu, ^{239 + 240}Pu, ²⁴¹Am and ¹³⁷Cs were determined in rain samples collected at Monaco in the course of 1978–1979. Based on these data, the annual deliveries of these radionuclides to the Mediterranean by rain are computed to be 0.18 ± 0.01 pCim^?2 for ²³⁸Pu, 8.1 ± 0.1 pCim^?2 for ^{239 + 240}Pu, 0.58 ± 0.02 pCim^?2 for ²⁴¹Am and 351 ± 4 pCim^?2 for ¹³⁷Cs.Comparing the delivery data with the mixed layer inventories of ^{239 + 240}Pu and ²⁴¹Am in the Mediterranean, the upper limits of the mean residence time of these radionuclides in the mixed layer were estimated to be 12.3 yr for ^{239 + 240}pu and 2.9 yr for ²⁴¹Am. These values are consistent with the conclusion deduced from the vertical distribution pattern of these transuranic elements in the Mediterranean.Based on delivery values, the annual activity ratios for ${}^{238}\text{pu}{}^{\mathrm{239\; +\; 240}}\text{Pu}$, ${}^{241}\text{Am}{}^{\mathrm{239\; +\; 240}}\text{Pu}$ and ${}^{\mathrm{239\; +\; 240}}\text{pu}{}^{137}\text{Cs}$ are found to be 0,022, 0.072 and 0.023 respectively. The ${}^{238}\text{pu}{}^{\mathrm{239\; +\; 240}}\text{pu}$ and ${}^{\mathrm{239\; +\; 240}}\text{Pu}{}^{137}\text{Cs}$ activity ratios vary within relatively narrow ranges with time, while a much wider variation was observed for the ${}^{241}\text{Am}{}^{\mathrm{239\; +\; 240}}\text{Pu}$ activity ratio. The cause of the wider variation of the ${}^{241}\text{Am}{}^{\mathrm{239\; +\; 240}}\text{Pu}$ ratio may be related to the difference in the mean age of fallout brought down in different seasons.     

The Alboran Sea (Western Mediterranean) revisited with a view from the Moroccan Margin

This paper presents a new depth to acoustic basement map based on a grid of seismic profiles. In the Western Alboran Basin the basement is more than 10 km deep. An examination of a seismic profile of this region suggests that Late Oligocene–Early Miocene 5 km thick synrift formation has been accumulated in this basin. This West Alboran basin has been formed in an extensional setting, but in a fore‐arc position, south of the Balearic Islands then westwards transported behind a subduction zone.