Showing papers in "International Journal of Earth Sciences in 2000"

Transport of terrestrial organic carbon to the oceans by rivers: re-estimating flux- and burial rates

Abstract: This study re-estimates one important component in the global carbon cycle: the modern global fluviatile organic carbon discharge- and burial rates. According to these results, approximately 430×1012 g of terrestrial organic carbon are transported to the ocean in modern times. This amount is higher than the latest estimates but takes into account new data from Oceania not previously considered in global flux studies. However, only the minor amount of 10% or approximately 43×1012 gC year–1 is most likely buried in marine sediments. This amount is similar to the burial of marine organic carbon in the coastal ocean (55×1012 gC year–1). Adding both estimates gives approximately 100×1012 gC year–1, which is the value calculated by Berner (1982) for "terrestrial" deltaic-shelf sediments. However, the results in this study suggest that on a global scale the organic carbon content in coastal ocean sediments is not solely of terrestrial origin but a mixture of nearly equal amounts of marine and terrestrial organic carbon. The major part of the terrestrial organic carbon that enters the ocean by rivers (approximately 400×1012 gC year–1) seems to be either (a) remineralised in the ocean, whereas the mechanism by which the terrestrial organic carbon is oxidised in the ocean are unknown; or (b) is dispersed throughout the oceans and accumulates in pelagic sediments.

Zircon ages and Nd isotopic and chemical compositions of orthogneisses from the Black Forest, Germany: evidence for a Cambrian magmatic arc

Abstract: Single zircon U–Pb dating combined with 207Pb/206Pb ages obtained by the evaporation method constrains the emplacement of tonalitic, trondhjemitic, and granodioritic orthogneisses of the Moldanubian zone in the Black Forest between 500 and 510 Ma Two detrital zircon populations of 19 and 16 Ga indicate Early-Middle Proterozoic material in the former setting of the basement The initial eNd values range from –01 to –34 and mean crustal residence ages of 10–14 Ga are consistent with involvement of Early-Middle Proterozoic crust, and a subordinate juvenile component probably originating from subduction-related melting of the mantle The orthogneisses have fractionated REE patterns and slightly higher K2O/Na2O ratios than typical low-K tonalite–trondhjemite–granite suites The chemical data are interpreted as evidence for melting of amphibolite and contributions from evolved crust The emplacement of the orthogneisses was superceded by a high-temperature metamorphic event at ∼480 Ma which we interpret as a result of lithospheric thinning in a marginal basin behind a Cambrian magmatic arc

Physical weathering of marbles caused by anisotropic thermal expansion

Abstract: Marbles as building stones as well as in their natural environments show complex weathering phenomena. The most important damage scenario is based on the highly anisotropic thermal expansion coefficient α of calcite, i.e. extreme expansion parallel and contraction normal to the crystallographic c-axis. Therefore, the rock fabric and especially the lattice-preferred orientation (texture) of calcite and/or dolomite as the predominant mineral phases in marbles have a significant influence on the mechanical weathering. The textures of marbles from five different locations vary from a more or less perfect prolate to moderate oblate shape of the [006] pole figure tensor. Accordingly, the texture-derived bulk thermal dilatation anisotropy covers a broad range from –0.048 to 0.680. The modelled thermal dilatations correlate with those obtained from experimental measurements. The difference in magnitude is basically explained by the microcrack fabrics which was not considered in the computations. All samples show a deterioration due to thermal treatment regardless of the strength of texture. The directional dependence of (a) the total magnitude of the thermal dilatation coefficient and (b) of the residual strain is highest in marbles with a strong texture, whereas the Carrara marble with a weak texture exhibits a uniform crack formation. The progressive loss of cohesion along grain boundaries due to dilatancy may serve as an example for the initial stage of physical weathering.

Coastal erosion vs riverine sediment discharge in the Arctic Shelf seas

Abstract: This article presents a comparison of sediment input by rivers and by coastal erosion into both the Laptev Sea and the Canadian Beaufort Sea (CBS). New data on coastal erosion in the Laptev Sea, which are based on field measurements and remote sensing information, and existing data on coastal erosion in the CBS as well as riverine sediment discharge into both the Laptev Sea and the CBS are included. Strong regional differences in the percentages of coastal erosion and riverine sediment supply are observed. The CBS is dominated by the riverine sediment discharge (64.45×106 t a–1) mainly of the Mackenzie River, which is the largest single source of sediments in the Arctic. Riverine sediment discharge into the Laptev Sea amounts to 24.10×106 t a–1, more than 70% of which are related to the Lena River. In comparison with the CBS, the Laptev Sea coast on average delivers approximately twice as much sediment mass per kilometer, a result of higher erosion rates due to higher cliffs and seasonal ice melting. In the Laptev Sea sediment input by coastal erosion (58.4×106 t a–1) is therefore more important than in the CBS and the ratio between riverine and coastal sediment input amounts to 0.4. Coastal erosion supplying 5.6×106 t a–1 is less significant for the sediment budget of the CBS where riverine sediment discharge exceeds coastal sediment input by a factor of ca. 10.

Organic carbon isotope ratios (δ 13 C) of Arctic Amerasian Continental shelf sediments

Abstract: Organic matter origins are inferred from carbon isotope ratios (δ13C) in recent continental shelf sediments and major rivers from 465 locations from the north Bering-Chukchi-East Siberian-Beaufort Sea, Arctic Amerasia. Generally, there is a cross-shelf increase in δ13C, which is due to progressive increased contribution seaward of marine-derived organic carbon to surface sediments. This conclusion is supported by the correlations between sediment δ13C, OC/N, and δ15N. The sources of total organic carbon (TOC) to the Amerasian margin sediments are primarily from marine water-column phytoplankton and terrigenous C3 plants constituted of tundra taiga and angiosperms. In contrast to more temperate regions, the source of TOC from terrigenous C4 and CAM plants to the study area is probably insignificant because these plants do not exist in the northern high latitudes. The input of carbon to the northern Alaskan shelf sediments from nearshore kelp community (Laminaria solidungula) is generally insignificant as indicated by the absence of high sediment δ13C values (–16.5 to –13.6‰) which are typical of the macrophytes. Our study suggests that the isotopic composition of sediment TOC has potential application in reconstructing temporal changes in delivery and accumulation of organic matter resulting from glacial–interglacial changes in sea level and environments. Furthermore, recycling and advection of the extensive deposits of terrestrially derived organic matter from land, or the wide Amerasian margin, could be a mechanism for elevating total CO2 and pCO2 in the Arctic Basin halocline.

Stratigraphic constraints for explosive activity in the past 100 ka at Etna Volcano, Italy

Abstract: The pyroclastic deposits of Etna have been correlated over the whole volcanic edifice for the first time, allowing the construction of a continuous record of tephra-producing events, which extends from approximately 100 ka to the Present. In this interval, five main periods of explosive activity have been identified: (a) ∼100-ka strombolian to subplinian activity; (b) 80- to 100-ka plinian benmoreitic activity; (c) 16- to 80-ka strombolian to subplinian from basaltic to mugearitic activity; (d) 15.5- to 15-ka plinian benmoreitic activity accompanying the caldera-forming eruptions of the Ellittico Volcano; and (e) the most recent 13-ka basaltic explosive activity of strombolian and subplinian type of the present edifice that also includes the 122-B.C. plinian eruption. This study results in a semi-quantitative and in some cases quantitative definition of the intensity and chronology of the explosive activity at Etna. Moreover, this work gives a new significance to the volcanic hazards of Etna, a volcano generally considered to be the site of gentle effusive eruptions.

Metamorphic Discontinuities In Orogenic Belts: Example Of The Garnet–Biotite–Albite Zone In The Otago Schist, New Zealand

Abstract: A regional petrographic reconnaissance of psammitic and pelitic rocks in the Otago Schist, New Zealand, has revealed the presence of garnet (“grossalspite” with typical rim composition almandine41, spessartine25, grossular33, pyrope1) and biotite in 37 new samples, more than doubling the previously known number. A new garnet–biotite–albite zone can now be defined in the greenschist facies Otago Schist that is distinct from the better-known biotite, garnet and oligoclase zones in the along-strike Alpine Schist. The garnet–biotite–albite zone is in part metamorphically discontinuous with adjacent schists and does not support models of simple, continuous, progressive Jurassic regional metamorphism in Otago. The structurally higher (lower grade) boundary of the zone coincides in at least three places with previously mapped regional shear zones. The structurally lower (expected higher grade) boundary of the zone appears to be obliterated by a chlorite zone overprint which can be spatially related to Alpine Schist recrystallisation of ?Cretaceous age. The Otago situation serves as an example of the subtle metamorphic discontinuities that probably pervade many orogenic belts.

Freshwater chlorophycean algae in recent marine sediments of the Beaufort, Laptev and Kara Seas (Arctic Ocean) as indicators of river runoff

Abstract: Freshwater chlorophycean algae are characteristic organic-walled microfossils in recent coastal and shelf sediments from the Beaufort, Laptev and Kara seas (Arctic Ocean). The persistent occurrence of the chlorophycean algae Pediastrum spp. and Botryococcus cf. braunii in marine palynomorph assemblages is related to the discharge of freshwater and suspended matter from the large Siberian and North American rivers into the Arctic shelf seas. The distribution patterns of these algae in the marine environments reflect the predominant deposition of riverine sediments and organic matter along the salinity gradient from the outer estuaries and prodeltas to the shelf break. Sedimentary processes overprint the primary distribution of these algae. Resuspension of sediments by waves and bottom currents may transport sediments in the bottom nepheloid layer along the submarine channels to the shelf break. Bottom sediments and microfossils may be incorporated into sea ice during freeze-up in autumn and winter leading to an export from the shelves into the deep sea. The presence of these freshwater algae in sea-ice and bottom sediments in the central Arctic Ocean confirm that transport in sea ice is an important process which leads to a redistribution of shallow water microfossils.

Complex structural pattern of the Alpine-Dinaridic-Pannonian triple junction

Abstract: Due to the political boundaries between the Central European countries, on one hand, and the thick Tertiary cover in the Pannonian Basin, on the other, the eastward continuation of the Alpine and Dinaridic units has been ambiguous and poorly documented. Based on comparative analyses, the aim of the present paper is to define the pre-Tertiary structural units in the junction area of the Alpine, Dinaridic, and Pannonian regions, in the SW part of the Pannonian Basin, and to draw conclusions on the continuation of the Alpine and Dinaridic units. According to diagnostic characteristics of the Periadriatic Lineament system, the Balaton Lineament system may be considered as its direct eastern continuation. North of the Periadriatic–Balaton Lineament system, the Transdanubian Range Unit, due to its pre-Tertiary paleogeographic setting, shows mainly South Alpine facies relations; however, its present structural position is identical to that of the Upper Austroalpine nappes. Between the Periadriatic–Balaton and Zagreb–Zemplin Lineament systems heterogeneous structural units are juxtaposed, forming the Sava Composite Unit. In the northern part of this composite unit non-metamorphosed nappes occur which can be considered the eastern continuation of the South Alpine units. These nappes are overthrust onto Internal Dinaridic units in the Tertiary. The Zagreb–Zemplin (Mid-Hungarian) Lineament separates the Sava Unit from the Tisza Unit showing close affinity to the Tethyan margin of the Eurasian plate during the early stage of the Alpine evolution.

Geochemistry, petrogenesis and tectonic setting of late Neoproterozoic Dokhan-type volcanic rocks in the Fatira area, eastern Egypt

Abstract: The Neoproterozoic Dokhan volcanics of the Fatira area in eastern Egypt comprise two main rock suites: (a) an intermediate volcanic suite, consisting of basaltic andesite, andesite, dacite, and their associated pyroclastic rocks; and (b) a felsic volcanic suite composed of rhyolite and rhyolitic tuffs. The two suites display well-defined major and trace element trends and a continuum in composition with wide ranges in SiO2 (54–76%), CaO (8.19–0.14%), MgO (6.96–0.04%), Sr (983–7 ppm), Zr (328–95 ppm), Cr (297–1 ppm), and Ni (72–1 ppm). They are enriched in LILEs (Rb, Ba, K, Th, Ce) relative to high field strength elements (Nb, Zr, P, Ti) and show strong affinity to calc-alkaline subduction-related rocks. However, their undeformed character, their emplacement temporally and spatially with post-orogenic A-type granite, and their high Zr/Y values suggest that their emplacement follow the cessation of subduction in eastern Egypt in an extensional-related within-plate setting. Major and trace element variations in the intermediate volcanics are consistent with their formation via partial melting of an enriched subcontinental lithospheric mantle source followed by a limited low-pressure fractional crystallization of olivine and pyroxene before emplacement. The LILE enrichment relative to HFSE is attributed to the inheritance of a subduction component from mantle material which constituted the mantle wedge during previous subduction events in eastern Egypt. The evolution of the whole volcanic spectrum was governed mainly by crystal/melt fractionation of amphibole, plagioclase, titanomagnetite, and apatite in the intermediate varieties and plagioclase, amphibole, biotite, Fe–Ti oxides, apatite, and zircon in the felsic varieties. At each stage of evolution, crystal fractionation was accompanied by variable degrees of crustal contamination.

Origins and development of Holocene coral reefs: a revisited model based on reef boreholes in the Seychelles, Indian Ocean

Abstract: Until recently, concepts of coral reef growth and accumulation have been predominantly based on a Darwinian model. In this, the upwards and outwards growth of a reef core (a coral framework) takes place over a foreslope consisting of reef talus, with the simultaneous filling of the back-reef lagoon by reef-derived debris. The principal adaptations of this pattern relate to the influence of relative changes in sea level and commonly ignore oceanographic factors such as storm frequency and severity. Boreholes through the outer edge of a fringing reef in the Seychelles, western Indian Ocean, reveal a record of Holocene sediment accumulation first established approximately 8 ka ago. Faunal and floral associations show that growth of this body began in relatively deep water but that this shallowed to <5 m within 1 ka. Subsequent accumulation was of “keep-up” style but, as the rate of sea-level rise slowed, shoaling became more frequent and aggradation was limited by reducing accommodation space. Constructional facies are characterised either by massive corals, including Leptastrea, Porites and faviids, or by branching corals, typically Acropora of the danai-robusta group. Coral surfaces may be encrusted by red algae, foraminifera and vermetids, and are commonly bored by filamentous algae, clionids and molluscs. However, detrital facies are volumetrically dominant, and the paucity of a constructional framework requires a re-evaluation of models of reef accretion. New models relate the geometry of accretion to the interplay between extreme storm events and fairweather hydrodynamic conditions. These suggest that a contiguous framework forms in areas of moderate fairweather energy without extreme storm events. Severe storms destroy the continuity of reef structures and generate increasing volumes of coarse detritus. Low storm severity, coupled with low fairweather hydrodynamic energy, may promote the accumulation of fine-grained reef-derived sediments that inhibit framework growth. While ecology reflects year-by-year sea conditions, lithology and structure are controlled by exceptional storms, with the effects of changing sea level superimposed.

U-Pb geochronology of the Southern Black Forest Batholith (Central Variscan Belt): timing of exhumation and granite emplacement

Abstract: Some granites, granitoid dykes and volcanic rocks of the Southern Black Forest were dated by U–Pb techniques using zircon and monazite. An effusive rhyolite, which is interbedded in upper Visean sedimentary sequences of the Badenweiler-Lenzkirch zone, was dated at 340 ±2 Ma. This weakly metamorphic zone of supracrustal rocks borders high-grade gneiss terrains in the north and the south, which are intruded by a series of granitoid intrusions: the strongly sheared Schlachtenhaus granite is dated by monazite at 334±2 Ma and the hypothesis of a Devonian emplacement is therefore discarded. The emplacement of all other granites, crosscutting dykes and of an ignimbrite were all within analytical uncertainty: St. Blasien granite 333±2 Ma; Barhalde granite 332±3 Ma; Albtal granite 334±3 Ma; and a porphyry dyke at Prag 332+2/-4 Ma. Deformation and thrusting of the basement units near the Badenweiler-Lenzkirch zone occurred after the emplacement of the Schlachtenhaus granite, but before the intrusion of the other granitoids, and may therefore be constrained to the time period unresolved between 334±2 and 333±2 Ma. The ignimbritic rhyolite of Scharfenstein was deposited in a caldera 333±3 Ma ago. This age coincides within error limits with published U–Pb monazite and Rb–Sr small slab ages of mimatitic gneisses, Ar–Ar hornblende ages of metabasites and Sm–Nd mineral isochron ages of eclogitic rocks in the underlying basement. This suggests that exhumation and cooling of this basement unit must have been active at rates of approximately 20 km and a few 100°C per million years. The silicic melts are interpreted to be of hybrid crust/mantle origin and their formation was most likely linked to these exhumation tectonics. A phase of mantle upwelling and heat advection into the crust is proposed to be the reason for this short-episodic magmatic pulse.

Stratigraphic and structural evolution of the Selenga Delta Accommodation Zone, Lake Baikal Rift, Siberia

Abstract: Seismic reflection profiles from the Lake Baikal Rift reveal extensive details about the sediment thickness, structural geometry and history of extensional deformation and syn-rift sedimentation in this classic continental rift. The Selenga River is the largest single source of terrigenous input into Lake Baikal, and its large delta sits astride the major accommodation zone between the Central and South basins of the lake. Incorporating one of the world's largest lacustrine deltas, this depositional system is a classic example of the influence of rift basin structural segmentation on a major continental drainage. More than 3700 km of deep basin-scale multi-channel seismic reflection (MCS) data were acquired during the 1989 Russian and the 1992 Russian–American field programs. The seismic data image most of the sedimentary section, including pre-rift basement in several localities. The MCS data reveal that the broad bathymetric saddle between these two major half-graben basins is underlain by a complex of severely deformed basement blocks, and is not simply a consequence of long-term deltaic deposition. Maximum sediment thickness is estimated to be more than 9 km in some areas around the Selenga Delta. Detailed stratigraphic analyses of the Selenga area MCS data suggest that modes of deposition have shifted markedly during the history of the delta. The present mode of gravity- and mass-flow sedimentation that dominates the northern and southern parts of the modern delta, as well as the pronounced bathymetric relief in the area, are relatively recent developments in the history of the Lake Baikal Rift. Several episodes of major delta progradation, each extending far across the modern rift, can be documented in the MCS data. The stratigraphic framework defined by these prograding deltaic sequences can be used to constrain the structural as well as depositional evolution of this part of the Baikal Rift. An age model has been established for this stratigraphy, by tying the delta sequences to the site of the Baikal Drilling Project 1993 Drill Hole. Although the drill hole is only 100 m deep, and the base of the cores is only ∼670 ka in age, ages were extrapolated to deeper stratigraphic intervals using the Reflection-Seismic-Radiocarbon method of Cohen et al. (1993). The deep prograding delta sequences now observed in the MCS data probably formed in response to major fluctuations in sediment supply, rather than in response to shifts in lake level. This stratigraphic framework and age model suggest that the deep delta packages developed at intervals of approximately 400 ka and may have formed as a consequence of climate changes affiliated with the northern hemisphere glaciations. The stratigraphic analysis also suggests that the Selenga Basin and Syncline developed as a distinct depocentre only during the past ∼2–3 Ma.

Sea-ice transport of riverine particles from the Laptev Sea to Fram Strait based on clay mineral studies

Abstract: The aim of this study was to identify pathways and processes of modern sediment transport from the Siberian hinterland to the Laptev Sea and further to the Arctic Ocean. Clay mineral analyses were performed on riverine suspended particulate material (SPM), surface sediments of the Laptev Sea shelf, and sea-ice sediments (SIS). Material collected during seven expeditions was included in this study. Clay mineral assemblages are used to decipher the distribution of riverine sediments on the shallow Laptev Sea shelf, the entrainment of fine particles into newly forming ice, and the transport of SIS from the Laptev Sea towards the ablation areas. A cluster analysis of our data set shows that the clay mineral assemblages of Laptev Sea shelf sediments and SIS are controlled mainly by the input of riverine SPM supplied by the Khatanga, Lena, and Yana Rivers. Whereas the western shelf clay-mineral province is characterized by enhanced smectite concentrations supplied by the Khatanga River, the eastern Laptev Sea is dominated by illite discharged through the Lena and Yana Rivers. The SIS smectite concentration serves as an indicator for sediment source areas on the circum-Arctic shelves. Subsequently, the Transpolar Drift can be distinguished into a Siberian Branch fed from the eastern Kara Sea and the western Laptev Sea, and a Polar Branch originating from the eastern Laptev Sea.

Monitoring climate variability over the past 116 years in coral oxygen isotopes from Ningaloo Reef, Western Australia

Abstract: 116-year record of coral skeletal δ18O is presented from a colony of Porites lutea from Ningaloo Reef, western Australia. Interannual variability of sea-surface temperatures (SST) inferred from skeletal δ18O is dominated by a 9.5-year period, and may constitute a characteristic signal of the Leeuwin Current. On long-terms coral skeletal δ18O indicates a near-continuous increase of SST at Ningaloo Reef over one century. The skeletal δ18O time series was checked for the presence of seasonal cooling events resulting from major volcanic eruptions. An ∼1 °C cooling is evident following the eruption of Pinatubo in 1991, which reproduces the results of previous investigations. However, only weak or no signals can be related to the eruptions of Krakatau (1883) and Agung (1963).

What factors control mechanical erosion rates

Abstract: Mechanical erosion rates are important factors in understanding how continents evolve. Mechanical erosion is much faster than chemical erosion, especially for highly elevated regions of the Earth's surface. It is a principal way in which mountain ranges are removed, exposing deep metamorphic roots, which comprise much of the older portions of the continental masses. In addition, there has to be a long-term balance between erosion and mountain building. A new data set allows us to explore in greater detail some of the many factors which control mechanical erosion rates. The most important factors are some expression of the average slope of a drainage basin, some measurement of the amount of water available for erosion, some environmental measurements, and also a measurement of basin length, for which we have no good explanation. The estimate of global mechanical erosion rate obtained here is considerably lower than those obtained by some other workers, some of whom have concentrated on the fact that smaller river basins tend to get eroded faster than larger basins, and it is mainly smaller basins which have not been measured and which are therefore not allowed for by simple arithmetic averaging of observed erosion rates. It is shown here that although smaller basins are eroded faster, this is mainly because they are steeper than larger basins. We also show that extrapolation of current data to smaller basins does not work because the observed continental area which is draining to the ocean cannot be attained by the simplest extrapolation scheme.

Upper Neoproterozoic–Lower Cambrian sedimentary successions in the Central Iberian Zone (Spain): sequence stratigraphy, petrology and chemostratigraphy. Implications for other European zones

Abstract: O3, TiO2, Zr, and Nb abundances in shales from all the units, strongly suggest a gradual compositional change within this sedimentary succession. Together with the petrological data, the chemical results do not reveal any obvious coeval volcanic contribution to the sediments. On the basis of the chemical data, a comparison is made with other European zones containing detrital sediments composed of reworked crustal components.

Evolution of the Academician Ridge Accommodation Zone in the central part of the Baikal Rift, from high-resolution reflection seismic profiling and geological field investigations

Abstract: New high-resolution seismic reflection data from the central part of Lake Baikal provide new insight into the structure and stratigraphy of Academician Ridge, a large intra-rift accommodation zone separating the Central and North Baikal basins. Four seismic packages are distinguished above the basement: a thin top-of-basement unit; seismic-stratigraphic unit X; seismic-stratigraphic unit A; and seismic-stratigraphic unit B. Units A and B were cored on selected key locations. The four packages are correlated with a series of deposits exposed on the nearby western shores: the Ularyar Sequence (Oligocene); the Tagay Sequence (Lower to Middle Miocene); the Sasa Sequence (Upper Miocene to Lower Pliocene); the Kharantsy Sequence (Upper Pliocene); and the Nyurga Sequence (Lower Pleistocene). Based on stratal relationships, sedimentary geometries, distribution patterns and principal morphostructural elements – both onshore and offshore – we propose a new palaeogeographic evolution model for the area. In this model progressive tectonic subsidence of the Baikal basins and successive pulses of uplift of various segments of the rift margins lead to: (a) formation of the ridge as a structural and morphological feature separating the Central and North Baikal basins during the Middle to Late Miocene; (b) gradual flooding of the main parts of the ridge and establishment of a lacustrine connection between the two rift basins during the Late Miocene; and (c) total submergence of the top parts of the crest of the ridge during the latest Pleistocene. This new model helps to better constrain numerous phases in the structural evolution of the Baikal Rift, in which the Academician Ridge as an accommodation zone plays a crucial role.

An exhumation model of the south Peloponnesus, Greece

Abstract: An exhumation model comprising forward and backward thrusting and late orogenic collapse is proposed in order to explain the kinematics of the tectonic windows in the south Peloponnesus. The model is based on mapping, mesoscopic structural data and strain analysis. Syn-compressional thickening took place throughout the Oligocene and Early Miocene which includes the subduction of the Pindos Ocean at the western margin of the Pelagonian microcontinent and the intracontinental subduction of the Phyllite–Quartzite and the Plattenkalk series. The latter subduction was associated with blueschist metamorphism, westward-directed ductile thrusting, and folding. The exhumation history of the deeper parts of the orogen began at the Oligocene–Miocene boundary with the progressive entrance of the low-density crust and the Plattenkalk carbonates in the subduction zone. Increased buoyancy caused: (a) the initiation of the Phyllite–Quartzite series extrusion; (b) vertical coaxial stretching; and (c) the evolution of two pop-up structures, i.e. the Parnon and Taygetos anticlines. This syn-compressional exhumation was taking place in the lower Miocene with decreasing rates from 7 to 1.5 mm/year. The change in the local stress field from compression to extension began in the middle Miocene with the formation of hinterland-dipping normal faults. The exhumation/denudation rate caused by the footwall uplift along these faults does not exceed 0.2 mm/year.

Modern river discharge and pathways of supplied material in the Eurasian Arctic Ocean: evidence from mineral assemblages and major and minor element distribution

Abstract: Clay-mineral, heavy-mineral, and elemental distributions in sediments from the Arctic Ocean and the adjacent Laptev and Kara seas can be attributed to the geology of the hinterland and the transport of terrigenous material by rivers onto the shelves. Kara Sea sediments are characterized by increased contents of smectite and elevated Ni/Al-, Ti/Al-, and Cr/Al ratios. In the western Laptev Sea sediments are enriched in smectite and clinopyroxene and increased in Ti/Al-, Cr/Al-, and Ca/Al ratios. The composition of the sediments reflects suspended matter input from the large trap basalt of the Putoran Mountains. The eastern Laptev Sea sediments display increased illite and amphibole contents as well as a chemical composition similar to average shale. This composition is due to the discharge from the Lena and Yana rivers, which drain a large catchment area consisting of sedimentary Mesozoic and Paleozoic rocks. Material from the eastern Laptev Sea is transported by ocean currents and sediment-laden sea ice along the Transpolar Drift into the central Arctic Ocean. This is indicated by similar values of Ti/Al-, Cr/Al-, Rb/Al-, and K/Al ratios as well as increased concentrations of amphibole and illite, determined in sediments from the Lomonosov Ridge. A minor input from the Beaufort Sea into the central Arctic Ocean is suggested from increased Ca/Al ratios and increased contents of opaque minerals.

Changes in the deposition of terrestrial organic matter on the Laptev Sea shelf during the Holocene: evidence from stable carbon isotopes

Abstract: Stable carbon isotope ratios in the organic fraction of surface sediments from the Laptev Sea shelf were analyzed in order to study the modern distribution pattern of terrestrial organic matter. The delta C-13 signature of the surface sediments range from -26.6 parts per thousand near the coastal margin to -22.8 parts per thousand in the north towards the outer shelf. Characterizing the possible sources of organic matter by their delta C-13(org) signature reveals that the terrestrial influence reaches further north in the eastern than in the western Laptev Sea. Downcore records of the delta C-13(org), measured on three AMS C-14-dated cores from water depths between 46 and 77 m, specify the spatial and temporal changes in the deposition of terrestrial organic matter on the Laptev Sea shelf during the past 12.7 ka. The major depositional changes of terrestrial organic matter occurred between 11 and 7 ka and comprised the main phase of the southward retreat of the coastline and of the river depocenters due to the postglacial sea level rise.

Pan-African granulite formation in the Kabye Massif of northern Togo (West Africa): Pb-Pb zircon ages

Abstract: Metamorphic zircons from seven granulite facies orthogneisses of tonalitic composition (ender- bites), collected from four different nappes of the Kabye Massif in the Dahomeyide belt of northern Togo, were dated by the Pb-Pb evaporation method. They yielded consistent Neoproterozoic ages with a mean of 612.5  0.8 Ma, interpreted to reflect the peak of regional granulite facies metamorphism following Pan-African continental collision between the West African and Benin-Nigerian plates. These results sup- port previous ages obtained by various chronometers on high-grade rocks from the same suture zone and from surrounding units in Togo, Benin and Ghana. They are also similar to zircon ages from granulites in the Mozambique belt of souteastern Africa. These Pan-African metamorphic ages reflect continental amalgamation resulting in the formation of the Gond- wana supercontinent towards the end of the Neopro- terozoic.

Sedimentation processes and new age constraints on rifting stages in Lake Baikal: results of deep-water drilling

Abstract: With this paper we present a first attempt to combine the direct results on lithology, composition and age dating in the boreholes BDP-93, BDP-96 and BDP-97 with geological and seismic data from the areas where those sections were drilled. The sedimentary environments represented by the BDP boreholes are markedly different and possess characteristic lithological features. The results of the deep drilling provide the essential means for testing numerous age models used in geological reconstructions of the Lake Baikal rifting dynamics. Neither the basin-wide unconformity interpreted from seismic data, nor the interpreted change from shallow-water to deep-water facies at the boundary of the seismic stratigraphic complexes were found in the BDP-96 boreholes on Academician Ridge. Also, lithology does not support the proposed reconstructions of intense lake level fluctuations and transgressions during the Pliocene at Academician Ridge. The continuous deep-water hemipelagic sedimentation at Academician Ridge has existed for the past 5 Ma. The beginning of an intense rifting phase of the Neobaikalian sub-stage and related drastic changes in sedimentation processes were interpreted on seismic sections as the basin-wide unconformity B10. Different age estimates for this boundary ranged from Late Pliocene (3.5 Ma) to Plio-Pleistocene boundary. As shown by BDP-96 borehole, B10 is associated with a lithological change from diatomaceous ooze to dense silty clay and not with an erosional contact. The new age for this boundary in BDP-96 is approximately 2.5 Ma. This new age constraint suggests that the upper sedimentary strata of Northern Baikal (1.5–1.7 km thick) have formed during the past 2.5 Ma with average sedimentation rates of 60–70 cm/ka. The BDP-93 boreholes at Buguldeika suggest that uplift in Primorsky Range took place prior to 1.07–1.31 Ma, a date which exceeds the age of previous geological models.

High-pressure/temperature metamorphism in the St. Leonhard Granulite Massif, Austria: evidence from intermediate pyroxene-bearing granulites

Abstract: The St Leonhard Granulite Massif, Lower Austria, is one of the small occurrences of high-pressure granulite found in the Gfohl unit at the highest tectono-stratigraphic level of the Moldanubian zone Although predominantly composed of extremely deformed acidic, garnet+kyanite-bearing rocks, thin conformable layers of intermediate garnet+clinopyroxene-bearing granulites are seen Pressure–temperature estimates for the peak metamorphic assemblage of garnet+clinopyroxene+ternary feldspar+quartz in these rocks are 15–19 kbar, 950–1050°C A close coherence between results obtained from a combination of independent geothermobarometers and those derived from an internally consistent thermobarometric method indicate the retention of high-pressure/temperature equilibrium mineral compositions, even though there is a wealth of petrographic evidence for significant post-peak metamorphic decompression Pressure–temperature estimates for the orthopyroxene-bearing, intermediate-pressure decompression stage, obtained from discrete reaction textures, are 8–12 kbar and 800–900°C Post-decompressive cooling from 800 to 500°C, at ca 5–8 kbar, is recorded by the final amphibolite-facies, biotite-bearing assemblage, together with petrological constraints from the enclosing acid granulites

The impact of glaciation, river-discharge and sea-level change on Late Quaternary environments in the southwestern Kara Sea

Abstract: Sedimentary records from the southwestern Kara Sea were investigated to better understand the extent of the last glaciation on the Eurasian Arctic shelf, sea-level change, and history of the Ob' and Yenisey river discharge. Sediment-core and seismic-reflection data indicate that the Quaternary depositional sequence in the southwestern Kara Sea consists of glacial, glaciomarine, and marine sedimentary units. Glaciogenic sediments in the deep Novaya Zemlya Trough are presumably related to the Last Glacial Maximum (LGM), whereas further east they may represent an earlier glaciation. Thus, it is inferred that the southeastern margin of the LGM Barents-Kara ice sheet was contained in the southwestern Kara Sea east of the Novaya Zemlya Trough. Changes in mineralogical, foraminiferal, and stable-isotopic composition of sediment cores indicate that riverine discharge strongly influenced sedimentary and biotic environments in the study area during the Late Weichselian and early Holocene until ca. 9 ka, consistent with lowered sea levels. Subsequent proxy records reflect minor changes in the Holocene hydrographic regime, generally characterized by reduced riverine inputs.

Aragonite preservation in late Quaternary sediment cores on the Brazilian Continental Slope: implications for intermediate water circulation

Abstract: We present late Quaternary records of aragonite preservation determined for sediment cores recovered on the Brazilian Continental Slope (1790–2585 m water depth) where North Atlantic Deep Water (NADW) dominates at present. We have used various indirect dissolution proxies (carbonate content, aragonite/calcite contents, and sand percentages) as well as gastropodal abundances and fragmentation of Limacina inflata to determine the state of aragonite preservation. In addition, microscopic investigations of the dissolution susceptibility of three Limacina species yielded the Limacina Dissolution Index which correlates well with most of the other proxies. Excellent preservation of aragonite was found in the Holocene section, whereas aragonite dissolution gradually increases downcore. This general pattern is attributed to an overall increase in aragonite corrosiveness of pore waters. Overprinted on this early diagenetic trend are high-frequency fluctuations of aragonite preservation, which may be related to climatically induced variations of intermediate water masses.

Style and evolution of salt pillows and related structures in the northern part of the Northeast German Basin

Abstract: The northern part of the Northeast German Basin contains a large number of Late Permian (Zechstein) salt pillows, whereas diapiric structures are almost completely absent. This lack of diapirs facilitated the study of early stages of salt movement in the basin. Salt pillows and related structures were investigated in terms of distribution, geometry and time of initiation of salt flow within the regional geological context. The primary Zechstein thickness in the study area was reconstructed to gain more insight into the relationship between the geometry of the salt layer and the style of the salt-related structures. In this study, no clear spatial relationship between the salt structures and basement faults has been found and the location of the salt structures in this area appears to be highly independent of the underlying structural grain. The overburden is affected by minor faulting. We propose that buckling of the overburden due to regional compression significantly contributed to the initiation of the Late Jurassic to Early Cretaceous salt structures in the basin. Reverse faulting of the Gardelegen and Haldensleben Faults is related to inversion tectonics and exerted a compression on the basin fill. During the deformation, the Late Permian salt layer acted as an efficient detachment and led to a marked decoupling of the Mesozoic overburden from the underlying pre-Zechstein rocks.

The Paleocene–Eocene transition in the marginal northeastern Tethys (Kazakhstan and Uzbekistan)

Abstract: We studied two sections that accumulated during the Paleocene–Eocene transition in shelf waters in the northeastern Tethys. Stable carbon isotopic compositions of marine and terrestrial biomarkers are consistent with a 13C depletion in the oceanic and atmospheric carbon dioxide pools during the Late Paleocene Thermal Maximum (LPTM; Subzone P5b). The 2–3‰ negative δ 13C excursion in planktic foraminifera coincides with minimum δ 18O values, an incursion of transient subtropical planktic foraminiferal fauna, and the occurrence of an organic-rich sapropelite unit in Uzbekistan, which accumulated at the onset of a transgressive event. Biomarker distributions and hydrogen indices indicate that marine algae and bacteria were the major organic matter sources. During the Late Paleocene (Subzones P4 and P5a), the marginal northeastern Tethys experienced a temperate to warm climate with wet and arid seasons. Most likely, warm and humid climate initiated during the LPTM (Subzone P5b) and subsequently extended during the Eocene (Zone P6) onto adjacent land areas of the marginal northeastern Tethys.

Record of seismotectonic events in siliceous cyanobacterial sediments (Magadi cherts), Lake Magadi, Kenya

Abstract: The Quaternary sediments of Lake Magadi in the Kenya Rift include large volumes of chert. Before their lithification, these siliceous sediments underwent very strong deformation, as recorded by diapirism with pillow-chert mounds, extrusion along dykes and fault ramps, horizontal liquefaction slides with brecciation, slumping, as well as petee-, flow- and shear structures. Eugster (1969) considered these structures to have resulted from desiccation of Na-silicate precipitates (magadiite) that were deposited over wide areas and were several metres thick. Magadiite can remain soft over long periods; therefore, these “Magadi-type cherts” are called the type examples of inorganic cherts. However, field observations and microbiological studies of the cherts show that real inorganic cherts are rare at the type locality of Magadi-type cherts. Most of the cherts are older than the High Magadi Beds and developed from flat-topped calcareous bioherms of Pleurocapsa, Gloecocapsa, and other coccoid cyanobacteria, thinly bedded filamentous microbial mats, stromatolites, bacterial slimes, diatoms, Dascladiacea colonies and other organic matter. Silicification occurred from a silicasol via opal-A to opal-C with final recrystallisation to a chert of quartzine composition. The metabolic processes of cyanobacteria controlled the pH and influenced the dissolution–precipitation mechanism. Collapse, liquefaction and extrusion of the pre-lithified siliceous matrix was caused by seismotectonic rift activity, which activated fault scarplets and large-scale dyke systems. It led to liquefaction and other earthquake-induced structures along the fault ramps and on tilted blocks. Concentrated silicasols were generated by the interaction of alkaline waters with volcanic detritus, coupled with biochemical processes. After liquefaction and extrusion, the material solidified by spontaneous crystallisation in an environment that was characterised by highly variable pH and salinity. The Lake Magadi basin is a remarkable example of sedimentation in a continuously seismotectonically active basin. This paper presents a first description of the micro-organisms in Magadi-type cherts, the silicification process, and the deformation that occurred still in the putty-like state before lithification.