Present-day Position Research Articles

Large dunes on the outer shelf off the Zambezi Delta, Mozambique: evidence for the existence of a Mozambique Current

The existence of a continuously flowing Mozambique Current, i.e. a western geostrophic boundary current flowing southwards along the shelf break of Mozambique, was until recently accepted by oceanographers studying ocean circulation in the south-western Indian Ocean. This concept was then cast into doubt based on long-term current measurements obtained from current-meter moorings deployed across the northern Mozambique Channel, which suggested that southward flow through the Mozambique Channel took place in the form of successive, southward migrating and counter-clockwise rotating eddies. Indeed, numerical modelling found that, if at all, strong currents on the outer shelf occurred for not more than 9 days per year. In the present study, the negation of the existence of a Mozambique Current is challenged by the discovery of a large (50 km long, 12 km wide) subaqueous dune field (with up to 10 m high dunes) on the outer shelf east of the modern Zambezi River delta at water depths between 50 and 100 m. Being interpreted as representing the current-modified, early Holocene Zambezi palaeo-delta, the dune field would have migrated southwards by at least 50 km from its former location since sea level recovered to its present-day position some 7 ka ago and after the former delta had been remoulded into a migrating dune field. Because a large dune field composed of actively migrating bedforms cannot be generated and maintained by currents restricted to a period of only 9 days per year, the validity of those earlier modelling results is questioned for the western margin of the flow field. Indeed, satellite images extracted from the Perpetual Ocean display of NASA, which show monthly time-integrated surface currents in the Mozambique Channel for the 5 month period from June–October 2006, support the proposition that strong flow on the outer Mozambican shelf occurs much more frequently than postulated by those modelling results. This is consistent with more recent modelling studies comparing the application of slippage and non-slippage approaches—they suggest that, when applying partial slippage, a western boundary current can exist simultaneously with the southward migrating eddies. Considering the evidence presented in this paper, it is concluded that a quasi-persistent, though seasonally variable Mozambique Current does exist.

The Oligocene gap in the formation of Co-rich ferromanganese crusts and sedimentation in the Pacific Ocean and the effects of bottom currents

The Marcus Wake and Magellan guyots formed about 129–74 Ma ago at 10°–30° S and drifted 1700–4400 km to their present-day latitudinal position across the equatorial zone of maximum deposition. Cooling of the Pacific plate brought these guyots to the northern arid zone during the Turonian–Maastrichtian, to depths at which sediment accumulation rates were low and the conditions promoted precipitation of Co-rich Fe–Mn crusts from the Campanian to the present. Nonprecipitation of Co-rich Fe–Mn crusts during the Oligocene was caused by the action of bottom currents. The presence of a hiatus identified in cores from drill holes was used as the basis for reconstruction of the directions of bottom currents in the Oligocene.

Paleocene-earliest Eocene Larger Benthic Foraminifera and Ranikothalia-bearing carbonate paleo-environments of Costa Rica (South Central America)

Here we present a biostratigraphic and systematic revision of American Ranikothalia, based on outcrop collections form Costa Rica end westernmost Panama and on type collections of the Naturhistorisches Museum Basel. Paleocene-lowermost Eocene Ranikothalia-bearing shallow-water limestones occur in three distinct palaeogeographic settings along the Southern Central American convergent margin: 1. The Barra Honda Limestone (Tempisque Basin, Costa Rica) is, according to 87Sr/86Sr-ratios and biostratigraphy of mid Selandian to late Thanetian (? earliest Ypresian) age. Tectonic uplift and temporary demise of a Middle American Arc segment during the Selandian allowed the formation of the pure carbonates of the Barra Honda Limestone. The upper well-bedded subunit exposes a higher energy facies, in which Neodiscocyclina spp. and Ranikothalia catenula group (redefined here) are locally abundant. Open marine facies with Morozovella velascoensis interfinger in a small scaled facies pattern in the topmost part. Tectonic subsidence and resumed arc activity resulted in rapid eutrophication/drowning. 2. Herradura – Quepos carbonate shoals on oceanic seamounts. The Herradura and Quepos terranes may represent the earliest seamounts formed by the Galapagos hotspot plume tail activity, >1000km to the SW of their present-day position. Pillowed lava flows and subaerial volcanic breccias occur with Upper Palaeocene-lowermost Eocene shallow-water limestones rich in Ranikothalia gr. catenula, Neodiscocyclina spp. and coralline algae. 3. Burica Peninsula – carbonate shoal on an accreted oceanic plateau. An Upper Paleocene carbonate shoal established on part of the Inner Osa Igneous Complex after its accretion to the western edge of the CLIP (Caribbean Large Igneous Province) and its possible emersion. In situ rhodoid limestones of the Río Palo Blanco (W-Panama) contrast with redeposited material in turbidites and debris flow breccias, interbedded with siliceous pelagic limestones and arc-derived mudstones. A morphometric study on 150 axial sections of Ranikothalia spp. from all studied settings reveals a morphologic continuum between megalospheric (A) forms and microspheric (B) forms previously described as several species, here grouped as the R. gr. catenula. We conceive that these co-occurring morphotypes belong to one biological species. The separation of this group into 2 or 3 genera seems highly artificial, since many samples contain transitional forms between the “sub-evolute” microsphreric forms “Caudrina†soldadensis (R. catenula soldadensis herein) and the involute “Chordoperculinoides†bermudezi (R. c. bermudezi herein). Neither from previous studies nor from our material we can conclude on a biostratigraphic relevance of the various morphotypes. In particular, we cannot confirm a stratigraphic succession from flatter (R. c. soldadensis) to more robust (R. c. bermudezi) forms. Shape and shell thickness distribution of morphotypes of the Ranikothalia gr. catenula can be interpreted in terms of hydrodynamic energy and light availability in their habitat, in comparison with observations made on living LBF. This interpretation matches the hydrodynamic energy deduced from the microfacies and the abundance of photosynthetic red algae in our thin sections. Flat lenticular delicate A-forms (R. c. catenula) occur together with delicate B-forms (R. c. soldadensis) both in shallow protected and deeper off-shore calm water areas. The most robust B-form R.c. bermudezi may be a good indicator of shallow, high energy palaeo-environments. Possible Paleoceme dispersal routes for Amerucan Ranikothalia are discussed.

Paleomagnetism from Deception Island (South Shetlands archipelago, Antarctica), new insights into the interpretation of the volcanic evolution using a geomagnetic model

Deception Island shows the most recent exposed active volcanism in the northern boundary of the Bransfield Trough. The succession of the volcanic sequence in the island is broadly divided into pre- and post-caldera collapse units although a well-constrained chronological identification of the well-defined successive volcanic episodes is still needed. A new paleomagnetic investigation was carried out on 157 samples grouped in 20 sites from the volcanic deposits of Deception Island (South Shetlands archipelago, Antarctic Peninsula region) distributed in: (1) volcanic breccia (3 sites) and lavas (2 sites) prior to the caldera collapse; (2) lavas emplaced after the caldera collapse (10 sites); and (3) dikes cutting pre- and the lowermost post-caldera collapse units (5 sites). The information revealed by paleomagnetism provides new data about the evolution of the multi-episodic volcanic edifice of this Quaternary volcano, suggesting that the present-day position of the volcanic materials is close to their original emplacement position. The new data have been combined with previous paleomagnetic results in order to tentatively propose an age when comparing the paleomagnetic data with a global geomagnetic model. Despite the uncertainties in the use of averaged paleomagnetic data per volcanic units, the new data in combination with tephra occurrences noted elsewhere in the region suggest that the pre-caldera units (F1 and F2) erupted before 12,000 year BC, the caldera collapse took place at about 8300 year BC, and post-caldera units S1 and S2 are younger than 2000 year BC.

TIDAL STREAM MORPHOLOGY AS AN INDICATOR OF DARK MATTER HALO GEOMETRY: THE CASE OF PALOMAR 5

This paper presents an example where the morphology of a single stellar stream can be used to rule out a specific galactic potential form without the need for velocity information. We investigate the globular cluster Palomar5 (Pal 5), which is tidally disrupting into a cold, thin stream mapped over 22 degrees on the sky with a typical width of 0.7 degrees. We generate models of this stream by fixing Pal 5's present-day position, distance and radial velocity via observations, while allowing its proper motion to vary. In a spherical dark matter halo we easily find models that fit the observed morphology. However, no plausible Pal 5 model could be found in the triaxial potential of Law & Majewski (2010), which has been proposed to explain the properties of the Sagittarius stream. In this case, the long, thin and curved morphology of the Pal5 stream alone can be used to rule out such a potential configuration. Pal 5 like streams in this potential are either too straight, missing the curvature of the observations, or show an unusual morphology which we dub stream-fanning: a signature sensitive to the triaxiality of a potential. We conclude that the mere existence of other thin tidal streams must provide broad constraints on the orientation and shape of the dark matter halo they inhabit.

The possible role of Brazilian promontory in Little Ice Age

The Gulf Stream, one of the strongest currents in the world, transports approximately 31Sv of water (Kelly and Gille, 1990; Baringer and Larsen, 2001; Leaman et al., 1995) and 1.3×1015W (Larsen, 1992) of heat into the Atlantic Ocean, and warms the vast European continent. Thus any change of the Gulf Stream could lead to the climate change in the European continent, and even worldwide (Bryden et al., 2005). Past studies have revealed a diminished Gulf Stream and oceanic heat transport that was possibly associated with a southward migration of intertropical convergence zone (ITCZ) and may have contributed to Little Ice Age (AD ∼1200 to 1850) in the North Atlantic (Lund et al., 2006). However, the causations of the Gulf Stream weakening due to the southward migration of the ITCZ remain uncertain. Here we use satellite observation data and employ a model (oceanic general circulation model – OGCM) to demonstrate that the Brazilian promontory in the east coast of South America may have played a crucial role in allocating the equatorial currents, while the mean position of the equatorial currents migrates between northern and southern hemisphere in the Atlantic Ocean. Northward migrations of the equatorial currents in the Atlantic Ocean have little influence on the Gulf Stream. Nevertheless, southward migrations, especially abrupt large southward migrations of the equatorial currents, can lead to the increase of the Brazil Current and the significant decrease of the North Brazil Current, in turn the weakening of the Gulf Stream. The results from the model simulations suggest the mean position of the equatorial currents in the Atlantic Ocean shifted at least 180–260km southwards of its present-day position during the Little Ice Age based on the calculations of simple linear equations and the OGCM simulations.

Rates of Organic Carbon Burial in a Floodplain Lake of the Lower Yellow River Area During the Late Holocene

The rapid outward and upward growth of the world's large fluvial sedimentary systems during the second half of the Holocene is a remarkable geologic process that may have buried considerable areas of pre-existing riparian wetlands, which in turn would sequester massive carbon. However, the role of floodplain lakes in the global carbon budget has long been neglected. This article demonstrates the potential of organic carbon burial due to floodplain aggradation during the late Holocene by analyzing a sediment core from a buried floodplain lake in the lower Yellow River area. Based on detailed radiocarbon dating, this study inferred that landscape development in the study area has experienced three disparate stages closely related to the displacement of the lower Yellow River channel. The first stage (∼2250–1700 cal yr BP) represents a widespread pedogenic process while the Yellow River discharged to the northern Bohai Sea through a course much farther north from the present-day position. The subsequent stage (∼1700–1000 cal yr BP) broadly corresponds to the calm period of the Yellow River while it discharged to the southern Bohai Sea through a course slightly north from the present-day position. A lacustrine environment prevailed during this period, sequestering organic carbon at a rate of ∼0.58 kg m2yr1. The final stage (∼1000 cal yr BP to present) is marked by the rapid growth of the floodplain due to the frequent rerouting of the lower Yellow River. This analysis suggests that fluvial sedimentary systems should be integrated into the terrestrial carbon budget when accounting for the aberrant rise of the atmospheric CO2in the face of global cooling during the second half of the Holocene.

Eastern segment of the Kiselevka-Manoma terrane (Northern Sikhote Alin): Paleomagnetism and geodynamic implications

The comprehensive geological-geophysical study of the lower Cretaceous volcanosedimentary rocks of the Kiselevka block of the Kiselevka-Manoma lithotectonic terrane made it possible to reach the following conclusions: (1) The composition of the volcanogenic rocks and the lithology of the sediments of the Kiselevka block indicate their formation in a within-plate oceanic setting; the petro- and geochemical characteristics of the studied volcanic rocks are similar to those of the Hawaiian hot spot. (2) The distinguished characteristic component of the natural remanent magnetization of the volcanosedimentary rock complex of the Kiselevka block yields a positive fold test and age similar to that of the rocks. According to the orientation of this characteristic component, the paleolatitudes of the rock formation (18° ± 5° N) and the coordinates of the paleomagnetic pole (Plat = 18.6°, Plong = 222.4°, dp = 5.2, dm = 9.1°) of the Kiselevka Block were determined. (3) The kinematic reconstructions based on the obtained and published data indicate that, (1) in the Valanginian-Albian, the Kiselevka Block migrated northwestward with the Izanaga Plate at a velocity of 15–20 cm/yr, passing over 5 thou. km up to the Eurasian margin (the Korean Peninsula); (2) in the Albian-Campanian, the block, as a fragment of the Kiselevka-Manoma accretionary wedge, moved along the Eurasian transform margin with a velocity of 4–5 cm/yr to its present-day position, where it was integrated into the continental plate.

Hydrological Changes in West Amazonia over the Past 6 Ka Inferred from Geochemical Proxies in the Sediment Record of a Floodplain Lake

The Amazon is responsible for the higher discharge of water and sediment from continent to ocean in the world, but the Amazon basin embodies a large variety of regions with different seasonal climate, hydrological regimes and vegetation. Some of these compartments may be considered as the most critical zones because they encompass severe changes in the recent past and one of the geomorphological features mostly impacted by these changes are the Amazon floodplains. In this study we report data from a floodplain lake, Quistococha, in Peru. The sediment was dated, radiographed, described and bulk density, granulometry, mineral composition, and total organic carbon were determined. Two distinctive sedimentary depositional phases were identified. During the first one, between 6100 and 5600cal years BP, Quistococha Lake was strongly influenced by the Amazon River constraining high sediment accumulation rates. The influence of Amazon River decreased and ceased around 5600cal years BP, due to an avulsion of the River toward its present-day position. Between 5600 and 2600cal years BP a gap in sedimentation is observed, probably related to the dry mid-Holocene climate conditions. The sedimentation re-started about 2600cal years BP, now the lake is an isolated lake.

Kinematics of the South Atlantic rift

Abstract. The South Atlantic rift basin evolved as a branch of a large Jurassic–Cretaceous intraplate rift zone between the African and South American plates during the final break-up of western Gondwana. While the relative motions between South America and Africa for post-break-up times are well resolved, many issues pertaining to the fit reconstruction and particularly the relation between kinematics and lithosphere dynamics during pre-break-up remain unclear in currently published plate models. We have compiled and assimilated data from these intraplated rifts and constructed a revised plate kinematic model for the pre-break-up evolution of the South Atlantic. Based on structural restoration of the conjugate South Atlantic margins and intracontinental rift basins in Africa and South America, we achieve a tight-fit reconstruction which eliminates the need for previously inferred large intracontinental shear zones, in particular in Patagonian South America. By quantitatively accounting for crustal deformation in the Central and West African Rift Zones, we have been able to indirectly construct the kinematic history of the pre-break-up evolution of the conjugate west African–Brazilian margins. Our model suggests a causal link between changes in extension direction and velocity during continental extension and the generation of marginal structures such as the enigmatic pre-salt sag basin and the São Paulo High. We model an initial E–W-directed extension between South America and Africa (fixed in present-day position) at very low extensional velocities from 140 Ma until late Hauterivian times (≈126 Ma) when rift activity along in the equatorial Atlantic domain started to increase significantly. During this initial ≈14 Myr-long stretching episode the pre-salt basin width on the conjugate Brazilian and west African margins is generated. An intermediate stage between ≈126 Ma and base Aptian is characterised by strain localisation, rapid lithospheric weakening in the equatorial Atlantic domain, resulting in both progressively increasing extensional velocities as well as a significant rotation of the extension direction to NE–SW. From base Aptian onwards diachronous lithospheric break-up occurred along the central South Atlantic rift, first in the Sergipe–Alagoas/Rio Muni margin segment in the northernmost South Atlantic. Final break-up between South America and Africa occurred in the conjugate Santos–Benguela margin segment at around 113 Ma and in the equatorial Atlantic domain between the Ghanaian Ridge and the Piauí-Ceará margin at 103 Ma. We conclude that such a multi-velocity, multi-directional rift history exerts primary control on the evolution of these conjugate passive-margin systems and can explain the first-order tectonic structures along the South Atlantic and possibly other passive margins.

Double-saloon-door tectonics in the North Fiji Basin

Fiji Platform rotated counter clockwise from at least 10.2Ma until 1.56Ma, while Vanuatu Arc rotated clockwise from 12/10Ma until the present. A revised model incorporating these opposite rotations explains the distribution of magnetic anomalies in the North Fiji Basin (NFB). The conjugate margin of southwest Fiji Platform is southeast Vanuatu Arc. Previous models which associate NW-oriented anomalies off southwest Fiji with similarly oriented anomalies northeast of Vanuatu Arc are therefore wrong. Secondly, these models propose a NW-oriented spreading ridge extending to 19°S, 177.5°E, almost 500km southeast of the mapped extension of rifted island-arc crust on Vanuatu Arc. This creates an unacceptable overlap with Fiji Platform in pre-rift reconstructions. Thirdly, anomalies off SW Fiji which are NW-oriented in their present-day position were oriented NNE during initial breakup. Rather than aligning with NW-oriented anomalies in the western NFB, they are matched by NNE-oriented anomalies off SE Vanuatu Arc. With further rotation, these conjugate anomaly sets form a fan shape in the southern NFB. Fourthly, previous models which recognise a triple junction only from 3Ma do not explain early WNW–ESE separation of Vanuatu Arc and Fiji Platform required by well-documented opposite rotations. NFB characteristics which match the double-saloon-door tectonic model include opposite rotations of island arc terranes, backarc seafloor spreading which is both arc-parallel and arc-perpendicular, and rifts propagating north, south, northeast and northwest. Features which do not match the double-saloon-door model include the North Fiji Fracture Zone and the West Fiji spreading centre. Both initiated post–1.5Ma, when Fiji Platform stopped rotating, and only one terrane, Vanuatu Arc, continued to rotate. The NFB developed via double saloon door tectonics from 12/10Ma until 1.5Ma, whereas post-1.5Ma opening constitutes single saloon door rotation.

The interplay between adjacent Adige and Po alluvial systems and deltas in the late Holocene (Northern Italy)

The alluvial plain behind the southern Venice Lagoon, in northern Italy, is characterised by the presence of a complex network of alluvial ridges formed by the aggradation of channel deposits and natural levees. They are the geomorphological products of the interaction between the Adige and Po during the late Holocene. New geomorphological and stratigraphic data provided a detailed reconstruction of the evolution of the Po-Adige alluvial plain and allowed for defining the relation with the migration of delta lobes in the southern Venice Lagoon and near the Adriatic coast. Three cross sections (total of 28 manual boreholes) were obtained using a hand auger through two alluvial ridges built by the Adige River: the first two on the Conselve ridge (in the locations of Conselve and Santa Margherita, respectively) and the third across the modern Adige alluvial ridge. Radiocarbon dating was carried out on 9 peat samples. These data, coupled with a DTM, allowed the identification of a confluence between the Adige and Po during the Bronze Age. They also enabled the identification of the timing of major Adige and Po avulsions and the correlation with the 5.0-1.5 ka cal. BP development of a delta-mouth sedimentary system recognised in the southern Venice Lagoon. The results show that major avulsive events in the upstream tracts of the Po and Adige Rivers forced the migration of delta lobes. The delta system in the southern Venice Lagoon was fed by the Saline-Cona Po branch from about 4-3 ka cal. BP. This implies that the Po delta extended as far as 30 km north of the present-day river position. In this time frame, the Adige did not directly reach the sea, as it was a tributary of the Po at Agna. The northernmost lobe of the Po delta was thus fed by the sedimentary input of both the Po and Adige. The subsequent deactivation of the Saline-Cona Po branch through avulsion just upstream of Rovigo, around 3 ka cal. BP, led to a southwards shift of the Po delta system. The Adige still flowed through the Conselve ridge and kept its mouth in the same area. From 3 ka cal. BP to Roman times, the river constructed its own delta in the southern Venice Lagoon, prograding on the previous northern Po delta lobe. The Adige avulsion at Bonavigo during the early Middle Ages led to the abandonment of the Montagnana-Este-Conselve course long the southern foot of the Euganean Hills. As a consequence, the delta in the southern Venice Lagoon was also definitely abandoned and the Adige River started to construct another delta in its present-day position, about 15 km further to the south.

Seismic evidence for the preservation of several stacked Pleistocene coastal barrier/lagoon systems on the Gulf of Valencia continental shelf (western Mediterranean)

The focus of this study is the analysis of coastal sand barriers and associated coastal lagoons on the inner continental shelf of the Gulf of Valencia (western Mediterranean), based on two W–E seismic profiles recorded seaward of the Albufera de Valencia coastal lagoon. Seismic facies identified include a number of coastal sand barriers with landward lagoons draped by contemporary continental shelf deposits. The barrier systems have been grouped into two sedimentary systems tracts, the older one corresponding to a prograding/aggrading highstand systems tract involving at least four paleo-coastal sand barrier/lagoon systems, followed landward by a transgressive systems tract comprising three such systems. All the systems have been allocated a Tyrrhenian age, the formation of individual barrier systems having been associated with successive sea-level stillstands, and their present-day position being explained by the very high regional subsidence rate. In summary, this study demonstrates that the Quaternary stratigraphic record of the Gulf of Valencia inner continental shelf is composed of littoral sand facies, in particular coastal sand barrier and lagoon deposits. These findings are in agreement with corresponding observations on other continental shelves of the western Mediterranean, showing that the formation of coastal sand barriers was a characteristic feature of this region during the Quaternary.

Anatomy of Piton de la Fournaise volcano (La Réunion, Indian Ocean)

The aim of this work is to propose a general model of Piton de la Fournaise volcano using information from geological and geophysical studies. Firstly, we make a graphical compilation of all available geophysical information along a W–E profile. Secondly, we construct a geological section that integrates both the geophysical information and the geological information. The lithosphere beneath Piton de la Fournaise is not significantly flexed, and the crust is underlain by an underplating body, which might represent the deep magma reservoir for La Reunion volcanism. Piton de la Fournaise is a relatively thin volcano lying on a huge volcanic construction attributed mostly to Les Alizes volcano. Indeed, if the differentiated rocks observed at the bottom of the Riviere des Remparts are the top of Les Alizes volcano, the interface with Piton de La Fournaise may be located at about sea level beneath the summit area. The endogenous constructions (intrusive complexes) related to Les Alizes and Piton de la Fournaise volcanoes represent a large volume. The huge intrusive complex of Les Alizes volcano probably rests on the top of the oceanic crust and appears to have a buttressing effect for the present eastern volcano-tectonic activity of Piton de la Fournaise. The early Piton de la Fournaise edifice was built around a focus located beneath the Plaine des Sables area. The center subsequently moved 5–6 km eastward to its current location. The dense, high-velocity body beneath the Plaines des Sables and the western part of the Enclos probably corresponds to the hypovolcanic intrusive complex that developed before the volcanic center shifted to its present-day position. Magma reservoirs may have existed, and may still exist, as illustrated by the March 1998 crisis, at the mechanical and density interface between the oceanic crust and the Les Alizes edifice. Strong evidence also exists for the presence of a shallower magma reservoir located near sea level beneath the summit. The March 1998 pre-eruptive seismic pattern (location and upward migration) seems to be evidence for a transfer of magma between the two reservoirs. The dominant structural feature of the central zone is a collapse structure beneath the summit craters, above the inferred magma reservoir near sea level. The collapsed column constitutes a major mechanical heterogeneity and concentrates most of the seismic, intrusive, and hydrothermal activity because of its higher permeability and weaker mechanical strength.

Geological structure and petroleum resource potential of the North Sea basin

The North Sea basin occupies a spacious depression almost isometric in shape. In the west and northwest, the basin is bordered by the continental crust consolidated during the Precambrian, Caledonian, and Hercynian orogenic epochs, which now forms epiplatformal orogenic structures. They are represented by the London-Brabant uplift and the Arden massif in the southwest and south and the Baltic Shield in the east and northeast. The North Sea basin may be considered as an ancient aulacogen that was transformed in the Early Mesozoic into a complex system of continental rifts and grabens. The sedimentary cover of the basin is represented by a thick (8.5–12.5 km) Ordovician-Quaternary sequence. Oil and gas generation in the sedimentary cover of the basin is likely connected with four main productive sequences: the coaliferous Upper Carboniferous (Westphalian), the subsalt Zechstein, the Jurassic-Lower Cretaceous (Lotharingian, Toarcian, Kimmeridgian, and Weldian bituminose shales), and the shaly Cenozoic. The large oil and gas reserves in the North Sea’s sedimentary cover (over 280 fields) implies that the above-mentioned sequences have realized their oil-generating potential. The present-day position of the main oil and gas generation zones in the sedimentary section of the North Sea explains the distribution of the oil and gas fields through the basin from the genetic standpoint. The petroleum resource potential of the basin is still significant. In this regard, most promising are the spacious shelf areas, turbidite sediments, deep Paleozoic sequences, and continental slopes in the northern part of the basin, which remains insufficiently investigated.

The origin and orbit of the old, metal-rich, open cluster NGC 6791

NGC 6791 is a unique stellar system among Galactic open clusters being at the same time one of the oldest open clusters and the most metal rich. Combination of its properties is puzzling and poses question of its origin. One possible scenario is that the cluster formed close to the Galactic Center and later migrated outwards to its current location. In this work we study the cluster's orbit and investigate the possible migration processes which might have displaced NGC 6791 to its present-day position, under the assumption that it actually formed in the inner disk. To this aim we performed integrations of NGC 6791's orbit in a potential consistent with the main Milky Way parameters. In addition to analytical expressions for halo, bulge and disk, we also consider the effect of bar and spiral arm perturbations, which are expected to be very important for the disk dynamical evolution, especially inside the solar circle. Starting from state-of-the art initial conditions for NGC 6791, we calculate 1000 orbits back in time for about 1 Gyr turning on and off different non-axisymmetric components of the global potential. We then compare statistical estimates of the cluster's recent orbital parameters with the orbital parameters of 10^4 test-particles originating close to the Galactic Center (having initial galocentric radii in the range of 3-5 kpc) and undergoing radial migration during 8 Gyr of forward integration. We find that a model which incorporates a strong bar and spiral arm perturbations can indeed be responsible for the migration of NGC 6791 from the inner disk (galocentric radii of 3-5 kpc) to its present-day location. Such a model can provide orbital parameters which are close enough to the observed ones. However, the probability of this scenario as it results from our investigations is very low.

Rapid response of Helheim Glacier, southeast Greenland, to early Holocene climate warming

Recent changes in speed, thinning, and retreat rates of marine-terminating outlet glaciers have raised concerns about the future stability of the Greenland Ice Sheet. Establishing a longer term record of outlet glacier retreat rates is essential to provide a context for present-day observations and to improve and constrain numerical models of outlet glacier behavior. New exposure dating (10Be) of streamlined bedrock surfaces and glacial erratic boulders of Sermilik Fjord, southeast Greenland, the present-day drainage route of Helheim Glacier, documents rapid retreat (∼80 m a−1) of this major marine-terminating outlet glacier at the close of the last glaciation. The glacier front retreated ∼80 km to within 20 km of the present-day (2010) position of Helheim Glacier in <1 ka, ca. 10.8 ± 0.3 ka ago. Retreat followed rapidly rising air temperatures at the start of the Holocene, and at this temporal resolution there is no evidence that fjord geometry influenced glacier behavior. The significant response to climatic amelioration at the end of the last glacial suggests a high sensitivity to abrupt temperature increases, which has major implications for the future stability of present-day Greenlandic outlet glaciers in a warming climate.

Paleomagnetic data support Early Permian age for the Abor Volcanics in the lower Siang Valley, NE India: Significance for Gondwana-related break-up models

Confusion exists as to the age of the Abor Volcanics of NE India. Some consider the unit to have been emplaced in the Early Permian, others the Early Eocene, a difference of ∼230 million years. The divergence in opinion is significant because fundamentally different models explaining the geotectonic evolution of India depend on the age designation of the unit. Paleomagnetic data reported here from several exposures in the type locality of the formation in the lower Siang Valley indicate that steep dipping primary magnetizations (mean=72.7±6.2°, equating to a paleo-latitude of 58.1°) are recorded in the formation. These are only consistent with the unit being of Permian age, possibly Artinskian based on a magnetostratigraphic argument. Plate tectonic models for this time consistently show the NE corner of the sub-continent >50°S; in the Early Eocene it was just north of the equator, which would have resulted in the unit recording shallow directions. The mean declination is counter-clockwise rotated by ∼94°, around half of which can be related to the motion of the Indian block; the remainder is likely due local Himalayan-age thrusting in the Eastern Syntaxis. Several workers have correlated the Abor Volcanics with broadly coeval mafic volcanic suites in Oman, NE Pakistan–NW India and southern Tibet–Nepal, which developed in response to the Cimmerian block peeling-off eastern Gondwana in the Early-Middle Permian, but we believe there are problems with this model. Instead, we suggest that the Abor basalts relate to India–Antarctica/India–Australia extension that was happening at about the same time. Such an explanation best accommodates the relevant stratigraphical and structural data (present-day position within the Himalayan thrust stack), as well as the plate tectonic model for Permian eastern Gondwana.

NEW VARIABLE JET MODELS FOR HH 34

We consider newly derived proper motions of the HH 34 jet to reconstruct the evolution of this outflow. We first extrapolate ballistic trajectories for the knots (starting from their present-day positions and velocities) and find that at ~1000 yr in the future most of them will merge to form a larger-mass structure. This mass structure will be formed close to the present-day position of the HH 34S bow shock. We then carry out a fit to the ejection velocity versus time reconstructed from the observed proper motions (assuming that the past motion of the knots was ballistic) and use this fit to compute axisymmetric jet simulations. We find that the intensity maps predicted from these simulations do indeed match reasonably well the [S II] structure of HH 34 observed in Hubble Space Telescope images.

Productivity of the benguella upwelling in the pleistocene: Evidence from benthic foraminifers

The taxonomic composition of the benthic foraminifers studied in 36 samples of Pleistocene sediments from DSDP Hole 362 drilled in Leg 40 of the D/V Glomar Challenger made it possible to divide the section into the lower and upper Pleistocene units. The variations in the relative proportions of the planktonic and benthic foraminifers and the abundance, diversity, and taxonomic composition of their benthic taxa imply that the Benguella Current in the Walvis Ridge area was located in the early Pleistocene westward of its present-day position, and the upwelling was more intense and its productivity was higher as compared with these parameters in the late Pleistocene. At the onset of the late Pleistocene, the Benguella Current occupied its present-day position with the intermediate waters ascending to the surface, which resulted in the decline of the upwelling’s productivity.