U. Bleil

Geological history of the polar oceans : Arctic versus Antarctic

The Geological History of Cenozoic Polar Oceans: Arctic Versus Antarctic - An Introduction.- Physiography and Plate Tectonics of the Polar Deep-Sea Basins and Their Continental Margins.- Morphology and Plate Tectonics: The Modern Polar Oceans.- The Opening of the Arctic Ocean.- On the Tectonic Evolution and Paleoceanographic Significance of the Fram Strait Gateway.- The Evolution of the Svalbard Margins: Synthesis and New Results.- The Western Barents Sea During the Cenozoic.- Structures in Rift-Basin Sediments on the Conjugate Margins of Western Tasmania, South Tasman Rise, and Ross Sea, Antarctica.- A Fine-Scale Seismic Stratigraphy of the Eastern Margin of the Weddell Sea.- Some Speculations Regarding the Nature of the Explora-Andenes Escarpment, Weddell Sea.- Polar Ice - Covers as Geological Agents.- A Comparison of Arctic and Antarctic Sea Ice and the Effects of Different Properties on Sea Ice Biota.- Sea Ice Characteristics and the Role of Sediment Inclusions in Deep-Sea Deposition: Arctic - Antarctic Comparisons.- Cycles, Rhythms, and Events in Quaternary Arctic and Antarctic Glaciomarine Deposits.- Cenozoic Glacier Fluctuations in Polar Regions - Terrestrial Records From Antarctica and the North Atlantic Sector of the Arctic.- Past Changes in Precipitation Rate and Ice Thickness as Derived From Age - Depth Profiles in Ice-Sheets Application to Greenland and Canadian Arctic Ice Core Records.- Stability of the Arctic Ocean Ice-Cover and Pleistocene Warming Events: Outlining the Problem.- Late Weichselian Ice Recession in the Central Barents Sea.- Modern Depositional Environments of Polar Oceans.- Distribution Patterns of Calcareous Foraminifers in Arctic Ocean Sediments.- Physiographic and Biologic Factors Controlling Surface Sediment Distribution in the Fram Strait.- Norwegian - Iceland Seas: Transfer Functions Between Marine Planktic Diatoms and Surface Water Temperature.- Particle Sedimentation and Productivity in Antarctic Waters of the Atlantic Sector.- Sediment Patterns in the Southern Weddell Sea: Filchner Shelf and Filchner Depression.- Quaternary History and Paleoceanography of Polar Oceans.- The Last Deglaciation in the Southern and Northern Hemispheres: A Comparison Based on Oxygen Isotope, Sea Surface Temperature Estimates, and Accelerator 14C Dating From Deep-Sea Sediments.- Synthesis of Arctic and Sub-Arctic Coccolith Biochronology and History of North Atlantic Drift Water Influx During the Last 500.000 Years.- Coccoliths in Sediments of the Eastern Arctic Basin.- Foraminiferal Assemblages in Sediments From Mendeleev Ridge, Arctic Ocean.- Physical and Acoustic Properties of Arctic Ocean Deep-Sea Sediments: Paleoclimatic Implications.- High Resolution 10Be and 230Th Stratigraphy of Late Quaternary Sediments From the Fram Strait (Core 23235).- The Enigma of Oxygen Isotope Stage 5 in the Central Fram Strait.- Dropstones in the Norwegian-Greenland Sea - Indications of Late Quaternary Circulation Patterns?.- Glacial to Interglacial Changes in the Isotopic Gradients of Southern Ocean Surface Water.- Stable Isotope Record and Late Quaternary Sedimentation Rates at the Antarctic Continental Margin.- Pre-Quaternary Records of Polar Ocean History.- Evolution of Biosiliceous Sedimentation Patterns - Eocene through Quaternary: Paleoceanographic Response to Polar Cooling.- Neogene to Recent Palynostratigraphy of Circum-Arctic Basins: Results of ODP Leg 104, Norwegian Sea, Leg 105, Baffin Bay, and DSDP Site 611, Irminger Sea.- Miocene to Quaternary Paleoceanography in the Northern North Atlantic: Variability in Carbonate and Biogenic Opal Accumulation.- Neogene and Pleistocene Glaciations in the Northern Hemisphere and Late Miocene - Pliocene Global Ice Volume Fluctuations: Evidence From the Norwegian Sea.- Southern Ocean Response to the Intensification of Northern Hemisphere Glaciation at 2.4 Ma.- Pliocene - Pleistocene Paleoceanography in the Weddell Sea - Siliceous Microfossil Evidence.- Polar Front Fluctuations and the Upper Gauss to Brunhes Paleoceanographic Record in the Southeast Atlantic Ocean.- Authors Index.

Geological record and reconstruction of the late Pliocene impact of the Eltanin asteroid in the Southern Ocean.

In 1995, an expedition on board the research vessel FS Polarstern explored the impact site of the Eltanin asteroid in the Southern Ocean, the only known asteroidimpact into a deep ocean basin. Analyses of the geological record of the impact region place the event in the late Pliocene (∼2.15 Myr) and constrain thesize of the asteroid to be >1 km. The explosive force inferred for this event places it at the threshold of impacts believed to have global consequences, and its studyshould therefore provide a baseline for the reconstruction and modelling of similar events, which are common on geological timescales.

The Mid-Pleistocene climate transition as documented in the deep South Atlantic Ocean: initiation, interim state and terminal event

Abstract The Mid-Pleistocene transition (MPT) of the global climate system, initiated by a shift towards much larger northern hemisphere ice shields at around 920 ka and ending with predominance of 100 kyr ice age cyclicity since about 640 ka, is one of the fundamental enigmas in Quaternary climate evolution. Climate proxy records not exclusively linked to global ice volume are necessary to advance understanding of the MPT. Here we present a high-resolution Pleistocene magnetic susceptibility time series of 12 sediment cores from the subtropical South Atlantic essentially reflecting dissolution driven variations in carbonate accumulation controlled by changes in deep water circulation. In addition to characteristics known from δ18O records, the data sets reveal three remarkable features intimately related to the MPT: (1) an all-Pleistocene minimum of carbonate accumulation in the South Atlantic at 920 ka, (2) a MPT interim state of reduced carbonate deposition, indicating that the MPT period may have been a discrete state of the Pleistocene deep water circulation and climate system and (3) a terminal MPT event at around 540–530 ka documented in several peculiarities such as thick laminated layers of the giant diatom Ethmodiscus rex.

Detection of live magnetotactic bacteria in South Atlantic deep-sea sediments

Abstract Since they were first descrbed in 1975 [1], magnetotactic bacteria have been observed in a wide variety of environments, including soils [2], rivers, freshwater lakes and coastal marine sediments [3,4], and in one case at a water depth of about 600 m in a hemipelagic sediment [5]. Here we report the first systematic findings of magnetotactic bacteria in pelagic and hemipelagic sediments of the eastern South Atlantic. Different morphologies were identified (cocci, spirilla, vibrionic and rod-shaped forms) at water depths to about 3000 m on the African continental margin between the equator and 30°S, and on the Walvis Ridge in a pelagic environment about 1400 km off the coast. No actively swimming magnetotactic bacteria could be detected in deeper waters. Magnetotactic bacteria were observed only in the upper 10 cm of the sediments, with the highest concentrations always found in a well-defined narrow subsurface layer. The depth of this layer is dependent on the input and turnover rates of organic matter in the sediment. Measurements of oxygen and nitrate concentrations [6,7] provide an assignment of depth distributions of magnetotactic bacteria to the stratification of terminal electron acceptors in the sediment. The data show clearly that the majority of marine magnetotactic bacteria live in the anaerobic zone, in contrast to former assumptions that they may inhabit microaerobic layers [8–11].

Neogene and Pleistocene Glaciations in the Northern Hemisphere and Late Miocene — Pliocene Global Ice Volume Fluctuations: Evidence from the Norwegian Sea

Studies of ice-rafted detritus in ODP holes from the Norwegian Sea document a series of glacial episodes in the surroundings of the Norwegian — Greenland Sea from the late Miocene (5.45 Ma) through the Pliocene. These glacial events were of smaller magnitude than those of the period postdating the major onset of large scale northern hemisphere glacial cyclicity at 2.57 Ma. A further amplification of the glaciations took place after 1.2 Ma.

Magnetic characterization of Holocene sedimentation in the South Atlantic

Surface sediment samples representative for the tropical and subtropical South Atlantic (15°N to 40°S) were investigated by isothermal magnetic methods to delineate magnetic mineral distribution patterns and to identify their predominant Holocene climatic and oceanographic controls. Individual parameters reveal distinct, yet frequently overlapping, regional sedimentation characteristics. A probabilistic (“fuzzy c-means”) cluster analysis was applied to five concentration independent magnetic properties assessing magnetite to hematite ratios and diagnostic of bulk and fine-particle magnetite grain size and coercivity spectra. The resultant 10 cluster structures establish an oceanwide magnetic sediment classification scheme tracing the major terrigenous eolian and fluvial fluxes, authigenic biogenic magnetite accumulation in high-productivity areas, transport by ocean current systems, and effects of bottom water velocity on depositional regimes. Distinct dissimilarities in magnetic mineral inventories between the eastern and western basins of the South Atlantic reflect prominent contrasts of both oceanic and continental influences.

Geomagnetic Events and Relative Paleointensity Records — Clues to High-Resolution Paleomagnetic Chronostratigraphies of Late Quaternary Marine Sediments?

Magnetochronostratigraphies of marine sediment series are generally determined by correlating the polarity pattern derived from their natural remanent magnetization (NRM) to a geomagnetic polarity time scale (GPTS) inferred from dated marine magnetic anomaly lineations. With regard to the Quaternary, this conventional method can only provide a poor resolution as even the latest available GPTS comprises but a few reversals for this time interval. Numerous recent paleomagnetic studies have revealed supplementary prominent NRM features in marine sedimentary deposits, namely (1) recurrent abrupt changes in stable remanent directions documenting a succession of several geomagnetic events within the Brunhes Chron and (2) conspicuous variations in NRM intensity normalized to the concentration of magnetic minerals (the so-called relative paleointensity) which were attributed to pronounced fluctuations in Earth’s paleofield strength. Detailed records of both phenomena are presented here for late Quaternary sediments recovered on the Ceara Rise in the western equatorial Atlantic. Based on an oxygen isotope age model, that was further refined by orbital tuning and sub-Milankovitch correlations of continuous magnetic susceptibility logs, the polarity and normalized intensity time series indicate repeated full reversals of the Earth’s magnetic field configuration during the last about 380 kyr associated with distinct changes in relative paleointensity. The data sets are discussed as primary proxies to unravel the geomagnetic field history and reviewed in their perspectives and deficiencies to develop a high-resolution magnetostratigraphic framework for the late Quaternary. They also substantially contribute to the basic understanding of NRM acquisition processes in marine sediments summarized in a model outlining their temporal and spatial variability as function of sediment lithology.

The Magnetic View on the Marine Paleoenvironment: Parameters, Techniques and Potentials of Rock Magnetic Studies as a Key to Paleoclimatic and Paleoceanographic Changes

The eminent potential of Environmental Magnetism analytical techniques to delineate depositional regimes and climatic changes in the marine realm is reviewed and illustrated with results of three individual studies of sediment series from the South Atlantic Ocean. Rock magnetic properties related to the mineralogy, concentration, domain state and hence grain-size of the magnetic mineral assemblage are explained on grounds of physical principles and discussed as proxy parameters for terrigenous particle fluxes, bioproductivity and diagenetic redox conditions. With cluster analysis of rock magnetic parameters determined for a large collection of surface samples, the regional characteristics of recent depositional environments in the equatorial South Atlantic are established. Notably, the different input mechanisms of terrigenous material via fluvial transport by the Amazon and Congo Rivers at the African and South American continental margins are as clearly identified as the eolian transport from the Sahara and Sahel Zone into the central and eastern equatorial South Atlantic. Based on a detailed susceptibility log and measurements of various laboratory remanences, high-coercive hematite components and different magnetite grain-size fractions could quantitatively be discriminated in a late Quaternary sediment sequence from the central equatorial Atlantic. The data sets allow to assess variations in eolian influx from the Saharan dust plume and several redox events during the last 400 kyr can be recognized. While biogenic magnetite is generally of minor importance in pelagic deposits, it may completely dominate the sediment magnetic properties in high productive areas. An intense primary biologic productivity in surface waters of the Benguela upwelling center supplies a high flux of organic matter to the sea floor at the continental slope off Namibia and causes reducing conditions in the sediment column. Resulting strong diagenetic effects on the biomagnetic mineral component are traced in detail by high-resolution rock magnetic analyses and transmission electron micrographs.

A seismic reconnaissance survey of the northern Congo Fan

Abstract To study the time-varying influence of the Congo River and the Benguela Current on the deposition at the Angola Continental Margin, a high-resolution reflection seismic survey was carried out on the northern Congo Fan. Four seismostratigraphic units have been defined for the upper 800 m (1000 ms TWT) of the data. The units record different depositional environments, ranging from pre-establishment of the Congo River drainage system to the influence of the Benguela Current. An indication of a general change in the turbidite system is provided by a shift in channel distribution and a relocation of the depocentre of coarse material. The ascent of salt is recorded up to the Pliocene. Gas that has migrated out of Lower Cretaceous shales and that was produced from large quantities of organic matter in the younger sediments can be found on the flanks and on top of the salt domes. In a few places, this gas even ascends to the ocean floor along structural pathways through the topmost unit.

An experimental study of the titanomagnetite solid solution series

SummaryAnalysis of saturation magnetization measurements and data from the literature indicate that the cation distribution in the titanomagnetite solid solution series is temperature dependent. The ionic configuration of ferric and ferrous ions on both lattice sites of their spinel structure can be described by a modified Boltzmann relation in agreement with theoretical consideratios. Thermodynamic equilibrium isotherms for the cation distribution and the resulting variation of saturation magnetization are calculated on the basis of the experimental values. These results should be especially valuable for the interpretation of magnetic properties of rapidly cooled igneous rock units where a respective high-temperature metastable state may exist in the magnetic ore component.Further measurements of the Curie temperature and lattice constant did not confirm a similar effect. Both these parameters should therefore be qualified for the identification of naturally occurring titanomagnetites.