University of Bremen

About: University of Bremen is a education organization based out in Bremen, Germany. It is known for research contribution in the topics: Population & Context (language use). The organization has 14563 authors who have published 37279 publications receiving 970381 citations. The organization is also known as: Universität Bremen.

Very large release of mostly volcanic carbon during the Palaeocene–Eocene Thermal Maximum

Abstract: The Palaeocene–Eocene Thermal Maximum1,2 (PETM) was a global warming event that occurred about 56 million years ago, and is commonly thought to have been driven primarily by the destabilization of carbon from surface sedimentary reservoirs such as methane hydrates3. However, it remains controversial whether such reservoirs were indeed the source of the carbon that drove the warming1,3,4,5. Resolving this issue is key to understanding the proximal cause of the warming, and to quantifying the roles of triggers versus feedbacks. Here we present boron isotope data—a proxy for seawater pH—that show that the ocean surface pH was persistently low during the PETM. We combine our pH data with a paired carbon isotope record in an Earth system model in order to reconstruct the unfolding carbon-cycle dynamics during the event6,7. We find strong evidence for a much larger (more than 10,000 petagrams)—and, on average, isotopically heavier—carbon source than considered previously8,9. This leads us to identify volcanism associated with the North Atlantic Igneous Province10,11, rather than carbon from a surface reservoir, as the main driver of the PETM. This finding implies that climate-driven amplification of organic carbon feedbacks probably played only a minor part in driving the event. However, we find that enhanced burial of organic matter seems to have been important in eventually sequestering the released carbon and accelerating the recovery of the Earth system12.

Angiogenetic Signaling through Hypoxia: HMGB1: An Angiogenetic Switch Molecule

Abstract: The initiation of angiogenesis, called the angiogenetic switch, is a crucial early step in tumor progression and propagation, ensuring an adequate oxygen supply. The rapid growth of tumors is accompanied by a reduced microvessel density, resulting in chronic hypoxia that often leads to necrotic areas within the tumor. These hypoxic and necrotic regions exhibit increased expression of angiogenetic growth factors, eg, vascular endothelial growth factor, and may also attract macrophages, which are known to produce a number of potent angiogenetic cytokines and growth factors. A group of molecules that may act as mediators of angiogenesis are the so-called high-mobility group proteins. Recent studies showed that HMGB1, known as an architectural chromatin-binding protein, can be extracellularly released by passive diffusion from necrotic cells and activated macrophages. To examine the angiogenetic effects of HMGB1 on endothelial cells an in vitro spheroid model was used. The results of the endothelial-sprouting assay clearly show that exogenous HMGB1 induced endothelial cell migration and sprouting in vitro in a dose-dependent manner. Thus, this is the first report showing strong evidence for HMGB1-induced sprouting of endothelial cells.

Occupation and bladder cancer among men in Western Europe.

Abstract: Objectives: We examined which occupations and industries are currently at high risk for bladder cancer in men. Methods: We combined data from 11 case–control studies conducted between 1976–1996 in six European countries. The study comprised 3346 incident cases and 6840 controls, aged 30–79 years. Lifetime occupational and smoking histories were examined using common coding. Results: Odds ratios for eight a priori defined high-risk occupations were low, and with the exception of metal workers and machinists (OR = 1.16, 95% CI = 1.02–1.32), were not statistically significant. Higher risks were observed for specific categories of painters, metal, textile and electrical workers, for miners, transport operators, excavating-machine operators, and also for non-industrial workers such as concierges and janitors. Industries entailing a high risk included salt mining, manufacture of carpets, paints, plastics and industrial chemicals. An increased risk was found for exposure to PAHs (OR for highest exposure tertile = 1.23, 95% CI = 1.07–1.4). The risk attributable to occupation ranged from 4.2 to 7.4%, with an estimated 4.3% for exposure to PAHs. Conclusions: Metal workers, machinists, transport equipment operators and miners are among the major occupations contributing to occupational bladder cancer in men in Western Europe. In this population one in 10 to one in 20 cancers of the bladder can be attributed to occupation.

Free-carrier and phonon properties of n- and p-type hexagonal GaN films measured by infrared ellipsometry

Abstract: Infrared spectroscopic ellipsometry (IRSE) over the wave-number range from 300 to 1200 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ is used to determine the anisotropic room-temperature optical properties of highly resistive, Si-doped n-type and Mg-doped p-type \ensuremath{\alpha}-GaN. The approximately 1-\ensuremath{\mu}m-thick films were deposited on c-plane sapphire by molecular beam epitaxy without a buffer layer. The free-carrier concentrations are obtained from Hall measurements. The IRSE data are analyzed through model calculations of the infrared optical dielectric functions parallel (\ensuremath{\parallel}) and perpendicular (\ensuremath{\perp}) to the c axis of the \ensuremath{\alpha}-GaN films. We obtain the thin-film phonon frequencies and broadening values and the optical mobility and effective-mass parameters for n- and p-type \ensuremath{\alpha}-GaN. In agreement with Perlin et al. [Appl. Phys. Lett. 68, 1114 (1996)] we determine the effective electron masses as ${m}_{e,\ensuremath{\perp}}{/m}_{0}=0.237\ifmmode\pm\else\textpm\fi{}0.006$ and ${m}_{e,\ensuremath{\parallel}}{/m}_{0}=0.228\ifmmode\pm\else\textpm\fi{}0.008.$ For p-type GaN with hole concentration ${N}_{h}=8\ifmmode\times\else\texttimes\fi{}{10}^{17}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ we find ${m}_{h}{/m}_{0}=1.40\ifmmode\pm\else\textpm\fi{}0.33,$ which agrees with recent theoretical studies of the Rashba-Sheka-Pikus parameters in wurtzite GaN. However, no substantial anisotropy of the effective hole mass is obtained to within 25%. The ellipsometry data also allow for derivation of the model quantities ${\ensuremath{\epsilon}}_{\ensuremath{\infty},j}$ $(j=\ensuremath{\perp},\ensuremath{\parallel}),$ which are almost isotropic but may vary between 4.92 and 5.37 depending on whether the films are undoped or doped. In heavily-Si-doped n-type \ensuremath{\alpha}-GaN we observe a thin carrier-depleted surface layer and additional infrared-active vibrational modes at 574, 746, and 851 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. Raman measurements of the GaN films are also performed, and the results are compared to those obtained from the IRSE investigations.

Temporal constraints on hydrate-controlled methane seepage off Svalbard.

Abstract: Methane hydrate is an icelike substance that is stable at high pressure and low temperature in continental margin sediments. Since the discovery of a large number of gas flares at the landward termination of the gas hydrate stability zone off Svalbard, there has been concern that warming bottom waters have started to dissociate large amounts of gas hydrate and that the resulting methane release may possibly accelerate global warming. Here, we corroborate that hydrates play a role in the observed seepage of gas, but we present evidence that seepage off Svalbard has been ongoing for at least 3000 years and that seasonal fluctuations of 1° to 2°C in the bottom-water temperature cause periodic gas hydrate formation and dissociation, which focus seepage at the observed sites.