Showing papers by "University of Bremen published in 1994"

Moisture supply for northern ice-sheet growth during the Last Glacial Maximum

Abstract: DURING the last ice age, the Barents Sea ice sheet began to grow 22 kyr ago1, only 8 kyr before it began to disintegrate2. This implies that the ice must have grown very rapidly from the coast to the edge of the continental shelf. Such rapid growth of a large ice sheet requires significant amounts of moisture3, but the origin of this moisture has been unclear, particularly as the CLIMAP climate reconstruction suggests4,5 that the Greenland–Iceland–Norwegian (GIN) seas were perennially ice-covered during this period. Here we present data from deep-sea sediment cores from the Fram Strait, which suggest that relatively warm water from the North Atlantic Ocean was advected into the GIN seas in two short-term events (27–22.5 and 19.5–14.5 kyr ago). We suggest that the resulting seasonally ice-free waters were an important regional moisture source for the Barents Sea ice sheet, and that the GIN seas played a much more active role in climate during the last glaciation than has previously been supposed.

Diffusive and total oxygen uptake of deep-sea sediments in the eastern South Atlantic Ocean: in situ and laboratory measurements

Abstract: Total O 2 uptake rates were measured by the benthic flux chamber lander ELINOR, and O 2 microprofiles were measured by the profiling lander PROFILUR in the eastern South Atlantic. Diffusive O 2 fluxes through the diffusive boundary layer and the depth distribution of O 2 consumption rates within the sediment were calculated from the obtained microprofiles. The depth integrated O 2 consumption rate agreed closely with the diffusive O 2 uptake at all stations. Total O 2 uptake was 1.2–4.2 times the diffusive O 2 uptake, and the difference correlated with the abundance of macrofauna in the sediment. Diffusive O 2 uptake and O 2 -penetration depths correlated with the organic content of the sediments and exhibited an inverse correlation with water depth. Total and diffusive rates of in situ O 2 uptake were higher than previously published data for shelf and abyssal sediments in the Atlantic, but were comparable to rates from upwelling areas in the eastern Pacific. Laboratory measurements on recovered sediment cores showed lower O 2 penetration depths and higher diffusive uptake rates than in situ measurements. The differences increased with increasing water depth. We primarily ascribe this compression of O 2 profiles to a transiently increased temperature during recovery and enhanced microbial activity in decompressed sediment cores. Total O 2 uptake rates measured in the laboratory on macrofauna-rich stations were, in contrast, lower than those measured in situ because of underrepresentation and disturbance of the macrofauna.

Role of DNA repair inhibition in lead- and cadmium-induced genotoxicity: a review.

Abstract: Compounds of lead and cadmium have been shown to be carcinogenic to humans and experimental animals. However, the underlying mechanisms are still not understood. In mammalian cells in culture, lead(II) is weakly mutagenic after long incubation times and generates DNA strand breaks only after treatment with high, toxic doses. Cadmium(II) induces DNA strand breaks and chromosomal aberrations, but its mutagenic potential is rather weak. However, both metals exert pronounced indirect genotoxic effects. Lead(II) is comutagenic towards UV and N-methyl-N-nitro-N-nitrosoguanidine (MNNG) and enhances the number of UV-induced sister chromatid exchanges in V79 Chinese hamster cells. With regard to DNA repair, lead(II) causes an accumulation of DNA strand breaks after UV-irradiation in HeLa cells, indicating an interference with the polymerization or ligation step in excision repair. Cadmium(II) enhances the mutagenicity of UV light in V79 Chinese hamster cells and an increased sensitivity toward UV light is observed in various rodent and human cell lines. Furthermore, an inhibition of unscheduled DNA synthesis after UV-irradiation and a partial inhibition of the removal of UV-induced DNA lesions has been shown. For both metals, the indirect genotoxic effects are observed at low, nontoxic concentrations, suggesting that an interference with DNA repair processes may be predominant at biologically relevant concentrations. This might also explain the conflicting results of epidemiological studies obtained for both metals. Possible mechanisms of repair inhibition are discussed.

Long-term uncoupling of chloride secretion from intracellular calcium levels by Ins(3,4,5,6)P4.

Abstract: Osmoregulation, inhibitory neurotransmission and pH balance depend on chloride ion (Cl-) flux. In intestinal epithelial cells, apical Cl- channels control salt and fluid secretion and are, in turn, regulated by agonists acting through cyclic nucleotides and internal calcium ion concentration ([Ca2+]i). Recently, we found that muscarinic pretreatment prevents [Ca2+]i increases from eliciting Cl- secretion in T84 colonic epithelial cells. By studying concomitant inositol phosphate metabolism, we have now identified D-myo-inositol 3,4,5,6-tetrakisphosphate (Ins(3,4,5,6)P4), as the inositol phosphate most likely to mediate this uncoupling. A novel, membrane-permeant ester prepared by total synthesis delivers Ins(3,4,5,6)P4 intracellularly and confirms that this emerging messenger does inhibit Cl- flux resulting from thapsigargin- or histamine-induced [Ca2+]i elevations.

Consideration of Risk in Reinforcement Learning

Abstract: Most Reinforcement Learning (RL) work supposes policies for sequential decision tasks to be optimal that minimize the expected total discounted cost (e.g. Q-Learning; AHC architecture). On the other hand, it is well known that it is not always reliable and can be treacherous to use the expected value as a decision criterion. A lot of alternative decision criteria have been suggested in decision theory to get a more sophisticated consideration of risk but most RL researchers have not concerned themselves with this subject until now. The purpose of this paper is to draw the reader's attention to the problems of the expected value criterion in Markov decision processes and to give Dynamic Programming algorithms for an alternative criterion, namely the minimax criterion. A counterpart to Watkins' Q-Learning with regard to the minimax criterion is presented. The new algorithm, called Qˆ-learning, finds policies that minimize the worst-case total discounted cost.

Reefal coralline algal build-ups within the Arctic Circle: morphology and sedimentary dynamics under extreme environmental seasonality

Abstract: Carbonate frameworks secreted by phototrophic organisms within the Arctic Circle are not well documented. Underwater surveys of the inner-shelf off Troms, northern Norway (70"N), reveal extended fixed algal build-ups which are fringed by rhodolith belts affected by storms. Reefd growth by coralline algae under temperature and light regimes of extreme seasonality is made possible because of a decoupling of carbon fixation during summer and utilization of stored carbon during the period of winter darkness. Although the annual growth of the framework constructing algae is comparatively low, the annual carbonate production rate is similar to subtropical-tropical counterparts because of a remarkably high standing stock. Early diagenetic alteration is restricted to intraparticle cementation processes which start in vivo. Bioerosional destruction is the dominant control on the preservation of high latitude build-ups. Preservation of Holocene autochthonous coralline algal biostromes is enhanced by rapid burial during storm events. Redeposition during storms is the most important process in forming a distinct sedimentary facies zonation.

On reconstructive legal and political theory

Abstract: L'A. presente la theorie soutenue par Habermas dans «Facticite et validite» comme une tentative de «reconstruction» du droit. Il montre qu'il existe plusieurs approches du droit qui sont relatives a la structure symbolique ou non de la societe et a la legitimite de ce droit. L'A., s'il montre les apports de cette theorie, souleve egalement quelques critiques

Healthy and infarcted brain tissues studied at short diffusion times: the origins of apparent restriction and the reduction in apparent diffusion coefficient

Abstract: The significance of NMR water diffusion measurements performed at short diffusion times (<10 ms) for brain tissue is examined. An apparent restriction to diffusion for both healthy and cytotoxically edematous tissue is shown: cytotoxic edema lengthens the diffusion time at which this phenomenon is visible. The dramatic reduction in apparent diffusion coefficient (ADC) observed in the core of cytotoxic edema is explained in terms of the enclosure of extracellular water in non-contiguous pockets in conjunction with the shift of water from the extra-to the intracellular space. The model presented provides an explanation for the ADC reduction without recourse to changes in the cell membrane permeability to water, or unrealistic values for the extra- and intracellular diffusion coefficients.

Quaternary time scale for the Ontong Java Plateau: Milankovitch template for Ocean Drilling Program Site 806

Abstract: A 2 m.y. oxygen isotope record of Globigerinoides sacculifer from the Ontong Java Plateau, based on cores from Ocean Drilling Program Leg 130, is dated by matching variations to an orbital template. The procedure allows us to present the most complete Quaternary record available for the western equatorial Pacific. The template-generating algorithm describes a balance between growth and melting of ice. Following basic Milankovitch theory, ice growth is taken as constant, while melting is taken to depend on summer insolation, current ice mass, and average past ice mass. Template settings must be changed once, between 1 and 1.2 Ma, to reflect a major shift in climate. Template fits are strikingly good over much of the record and can be used to detect and fill gaps from core breaks and other disturbances. One result of template dating is an exact age for the Brunhes-Matuyama magnetic reversal boundary, at 790 ±5 ka, as well as several other precise dates (900 ka for the middle Pleistocene climate shift; 1070, 1240, and 1450 ka for isotope stages 31, 37, and 47, respectively). Sedi- mentation rates fluctuate between 18 and 28 m/m.y., a ca. 400 ka cycle being the most prominent. Major anomalies arise within the transitional regime (1.2 to 1 Ma). The origin of the cycles is unknown; we propose productivity variations in the western equatorial Pacific.

Magnetostratigraphic Data From Late Quaternary Sediments From the Yermak Plateau, Arctic Ocean: Evidence For Four Geomagnetic Polarity Events Within the Last 170 Ka of the Brunhes Chron

Abstract: SUMMARY Palaeomagnetic investigations of two sediment cores recovered from RV Polarstern near the eastern slope of the Yermak Plateau (sites PS 1533 and PS 2212) reveal convincing evidence for four polarity events of the Earth's magnetic field during the last 170 Ka. A comprehensive rock magnetic study of the sediments proved that fine-grained magnetite is the principal carrier of the remanent magnetization. No changes in magneto-mineralogy across the polarity transitions in the sediments investigated were found. Calcareous nannofossil biostratigraphy, AMS-14C (accelerated mass spectrometry) and oxygen isotope data, and 10Be and 230Th stratigraphies yielded age ranges of 24–29 Ka for the Mono Lake event, 34–43 Ka for the Laschamp event, 72–86 Ka for the Norwegian-Greenland Sea event and 118–128 Ka for the Blake event. Two reverse polarity samples at the base of core PS 2212–3 KAL are interpreted as the termination of the Biwa I event (171-181 Ka). the events exhibit full inversion of inclination in both cores. the data suggest that the transition process of the Earth's magnetic field during such polarity events requires some 1 Ka.

Late Quaternary PCO2 Variations in the Angola Current: Evidence from Organic Carbon δ13C and Alkenone Temperatures

Abstract: The δ13C record for organic carbon in Late Quaternary sediments of the Eastern Angola Basin shows a pronounced cyclicity with high values (-18.3 to -20‰) in glacial isotopic stages 2 to 4 and stage 6, and lower values (-20 to -21.2‰) in interglacial stages 1 and 5. Seasurface temperatures, derived from the alkenone Uk 37 index, vary in the opposite sense, ranging from 20–22°C in glacial stages up to 24–26°C in interglacial stages. The inverse relationship between δ13Corg and SST values suggests that the isotopic variations are not due to temperature-dependent isotopic fractionation during photosynthesis, in which case δ13Corg values would be expected to increase with increasing temperature. Relationships between sedimentary δ13Corg and C/N ratios indicate that differing proportions of marine and terrigenous organic matter can also be ruled out as a cause for the δ13Corg variability. Instead we conclude that changes in surface water CO2 concentrations were responsible for the observed glacial to interglacial isotopic variations.

Cell size predicts morphological complexity in the brains of frogs and salamanders

Abstract: The morphological organization of the brain of frogs and salamanders varies greatly in the degree to which it is subdivided and differentiated. Members of these taxa are visually oriented predators, but the morphological complexity of the visual centers in the brain varies interspecifically. We give evidence that the morphological complexity of the amphibian tectum mesencephali, the main visual center, can be predicted from knowledge of cell size, which varies greatly among these taxa. Further, cell size is highly correlated with genome size. Frogs with small cells have more complex morphologies of the tectum than do those with large cells independent of body and brain size. In contrast, in salamanders brain-body size relationships also are correlated with morphological complexity of the brain. Small salamanders with large cells have the simplest tecta, whereas large salamanders with small cells exhibit the most complex tectal morphologies. Increases in genome, and consequently cell size, are associated with a decrease in the differentiation rate of nervous tissue, which leads to the observed differences in brain morphology. On the basis of these findings we hypothesize that important features of the structure of the brain can arise independently of functional demands, from changes at a lower level of organismal organization--in this case increase in genome size, which induces simplification of brain morphology.

Demonstration of the Interactive Graph-Visualization System da Vinci

Abstract: We present the graph visualization system daVinci, an interactive tool that can be used by arbitrary application programs as a user interface for graph data structures. Beside a novel automatic layout algorithm for graphs, da Vinci offers many interactive facilities such as finetuning of a layout, abstractions and scaling operations. A bidirectional application interface is used for tool communication with a program that controls the graph structure.

Protist herbivory in the Oregon upwelling system

Abstract: We investigated the role of phagotrophic protists in the Oregon upwelhng system over a 2 yr period by determining protist abundance, and by measuring grazing and phytoplankton growth rates with the seawater-dilution method. Off Oregon (USA), upwelling occurs predominantly during the summer months as episodic upwelling events which give rise to phytoplankton blooms. Abundance of phagotrophc protists, grouped into ciliates (mostly choreotrich forms), thecate and gymnodinoid dinoflagellates and nonpigmented nanoflagellates, was lowest in freshly upwelled water (3 pg C 1-l) and hghest during the late stages of the phytoplankton blooms (61 pg C 1 l ) . Overall rates of herbivory ranged from 0 to 0.6 d' during the upwelling season which represented 0 to 120 % of the phytoplankton production. Highest phytoplankton growth rates were measured during bloom periods in the upwelling season (0.67 to 1.58 d-l) when phytoplankton biomass was highest (10.7 to 55.3 pg chl a l-l). During bloom periods, phagotrophic protists utilized 16 to 52% of phytoplankton production, or 612 to 1413 pg C 1-' d-'. Gyrnnodinoid dinoflagellates were most abundant during the upwelling season. often exceeding the biovolume of choreotrich ciliates and of thecate dinoflagellates. Rates of herbivory were closely h k e d to the abundance of this group of dinoflagellates. The abundance of large-celled gymnodinoids (>20 pm) and large phytoplankton (>20 pm, mostly &atoms) covaried, indicating that the bloom-forming diatoms were being utilized as a food source. Thus, the microbial food web is an integral part of the trophic interactions in coastal upwelling systems including large phytoplankton cells as prey of protist grazers.

Photooxidation of 2‐mercaptoethanol by various water‐soluble phthalocyanines in aqueous alkaline solution under irradiation with visible light

Abstract: Abstract A selection of charged water‐soluble phthalocyanines was employed for the photocatalytic oxidation of 2‐mercaptoethanol in aqueous alkaline solution in the presence of oppositely charged detergents. Most efficient are Zn(II) and Al(III) phthalocyaninetetrasulfonic acids, which oxidize the thiolate to the sulfonic acid and sulfate, whereas Co(II)phthalocyaninetetrasulfonic acid does not exhibit a photoeffect. During the photooxidation reactions, decomposition (photobleaching) of the zinc phthalocyanine derivatives occurs, whereas the analogous alumina chelates are more stable. Covalent binding of Zn(II)phthalocyanine derivatives to silica carriers results in photocatalytic activities even in the absence of detergents, since the immobilization of the complexes preserves the required monomeric state. Moreover, the photodecomposition (bleaching) of the heterogenized complexes is strongly retarded. For the photo‐oxidations mainly the pathway via the sensitized formation of singlet oxygen with subsequent oxidation reactions is valid. Hydrogen peroxide as one reduction product of oxygen was found.

Nickel(II) Interferes with the Incision Step in Nucleotide Excision Repair in Mammalian Cells

Abstract: Nickel compounds are carcinogenic to humans and experimental animals. However, the mechanisms leading to tumor formation are still not understood since the mutagenic potential is rather weak. In contrast, nickel(II) enhances the cytotoxicity and genotoxicity in combination with several other DNA-damaging agents. To elucidate possible interactions with DNA repair processes, the effect of nickel(II) on the nucleotide excision repair pathway has been investigated after UV irradiation in HeLa cells. Nickel(II) blocks the removal of cyclobutane pyrimidine dimers as determined by T4 endonuclease V-sensitive sites. When the alkaline unwinding technique was applied, significantly less transient DNA strand breaks after UV irradiation were detected in the presence of nickel(II) compared to UV alone, suggesting an inhibition of the incision step of nucleotide excision repair. Once incisions are made, the ligation of repair patches is delayed as well in nickel-treated cells, as observed by the alkaline unwinding and nucleoid sedimentation techniques. This inhibition of DNA repair is partly reversible by the addition of magnesium(II), indicating that the competition between Ni2+ and Mg2+ may provide an important mechanism for the disturbance of DNA-protein interactions involved in the repair process. Since the repair inhibition is observed at noncytotoxic concentrations of nickel(II), it may well be relevant for its carcinogenic action.

How to define a Foreigner? The symbolic politics of immigration in German partisan discourse, 1978–1992

Abstract: This article tries to answer two questions. First, how did Germany, a de facto country of immigration, manage to espouse a counter‐factual ideology in the 1980s and early 1990s? Second, what have been the political consequences of upholding a political discourse that denied the reality of immigration? In a polity that officially denies migration and the development of a multiethnic society, issues such as immigration regulation and the settlement of the regulation of labour migrants’ immigration have not been directly addressed in partisan discourse. An ethno‐cultural conception of citizenship has facilitated a politics of exclusion of ‘guestworkers’ from voting rights, but inclusion of ethnic Germans, and a redefinition of asylum as labour migration. This has reinforced the symbolic uses of politics by Christian Democratic and populist parties and politicians: immigration, asylum and the multiethnic polity have come to be meta‐issues that can be referred to as causes of manifold problems in a context of ...

Clinical phase-I study of Na2B12H11SH (BSH) in patients with malignant glioma as precondition for boron neutron capture therapy (BNCT).

Abstract: Purpose : Within the European collaboration on boron neutron capture therapy (BNCT), a clinical Phase I study is being carried out to establish BNCT as an alternative treatment modality for malignant glioma (WHO III/IV). Data about the pharmacokinetics, biodistribution and toxicity of the boron compound Na2B12H11SH (BSH) are of great importance to avoid radiation damage of healthy tissue and to deliver a sufficient radiation dose. Methods and Materials : Twenty four patients suffering from a glioblastoma multiforme entered the study to date, infused with a maximum concentration of up to 50 mg BSH/kg. Boron concentration measurements in tissues, urine, and blood were carried out, using inductively coupled plasma-atomic spectroscopy (ICP-AES) and quantitative neutron capture radiography (QNCR). A cross-calibration of these boron determination techniques was carried out. Results : In tumor tissue, confirmed by histopathology of small biopsies, we found a consistently high but heterogeneous boron uptake. Necrotic parts contain much lower amounts of boron; normal brain tissue has shown no significant uptake. In skin, bone, muscle, and dura mater only small amounts of boron were found. In blood samples, we found biphasic kinetics, but with variations of the half-lives from patient to patient. The compound is mainly excreted through the urine, but an additional entero-hepatic pathway can be demonstrated. Systematic investigations revealed no toxic side effect of the intravenously administered BSH. Comparable data were obtained by using ICP-AES and QNCR for boron concentration measurements. Conclusion : Taking into account the radiobiological considerations of the neutron beam source, we found promising facts that BNCT could be a useful irradiation method for highly malignant brain tumors. Favorable amounts of the boron compound BSH were found in tumor tissue, whereas healthy brain tissue has shown no significant uptake.

Conditioning (updating) non-additive measures

Abstract: Several update rules for non-additive probabilities, among them the Dempster-Shafer rule for belief functions and certain update rules in the spirit of Bayesian statistics with multiple prior probabilities, are reviewed, investigated and compared with each other. This is done within the unifying framework of general, non-additive measure and integration theory. The methods exposed here are capable of generalizing conditional expectation of random variables to the sub-modular or supermodular case at least if the given algebra is finite.

Algal toxicity of binary combinations of pesticides.

Abstract: Pollution of surface waters is rarely a matter of a single toxicant but aquatic organisms are typically exposed to numerous chemicals simultaneously or in sequence. Consequently, hazard assessment in aquatic toxicology cannot be restricted to considerations on individual compounds. Thereby the question arises whether there are rational approaches for a calculation of the toxicities of mixtures from the concentration response relationships of their components. This paper deals with the predictive value of concentration addition, a concept used for the analysis of combination effects (see Boedeker et al. 1992 for a review). It is given by the formula: Cl/ECx I + C2/ECx 2 = 1

Flux and stable isotope composition of Neogloboquadrina pachyderma and other planktonic foraminifers in the Southern Ocean(Atlantic sector).

Abstract: Planktonic foraminifers were collected with time-series sediment-traps in Bransfield Strait, Northern Weddell Sea, and near Maud Rise Seasonal flux patterns of planktonic foraminifers are similar to total flux patterns The dominant species was the left-coiling Neogloboquadrina pachyderma At the Maud Rise, in an open-ocean polynya,Turborotalita quinqueloba, Globigerinita glutinata and Globigerinoides ruber (white) also were abundant δ18O values of N pachyderma ranged from 3 to 36%, corresponding to an apparent temperature range of about 3°C The smaller size classes showed the lower δ18O values Oxygen isotope variability between size classes and during the year are explained by seasonal changes in temperature and vital effect δ13C values of the foraminifer shells ranged from 01 to 11% The largest sizes had the highest and the smallest sizes the lowest values Changes in the deviation from thermodynamic equilibrium during bio-mineralization seem to be involved in this pattern of 13C/12C ratios