Black hole entropy associated with the supersymmetric sigma model

By means of an identity that equates the elliptic genus partition function of a supersymmetric sigma model on the N-fold symmetric product (SX)-X-N of X ((SX)-X-N=X-N/S-N, where S-N is the symmetric group of N elements) to the partition function of a second-quantized string theory, we derive the asymptotic expansion of the partition function as well as the asymptotic for the degeneracy of spectrum in string theory. The asymptotic expansion for the state counting reproduces the logarithmic correction to the black hole entropy.

Quantum Hall Effect near the Charge Neutrality Point in a Two-Dimensional Electron-Hole System

We study the transport properties of HgTe-based quantum wells containing simultaneously electrons and holes in a magnetic field B. At the charge neutrality point (CNP) with nearly equal electron and hole densities, the resistance is found to increase very strongly with B while the Hall resistivity turns to zero. This behavior results in a wide plateau in the Hall conductivity sigma(xy) approximate to 0 and in a minimum of diagonal conductivity sigma(xx) at nu = nu(p) - nu(n) = 0, where nu(n) and nu(p) are the electron and hole Landau level filling factors. We suggest that the transport at the CNP point is determined by electron-hole ""snake states'' propagating along the nu = 0 lines. Our observations are qualitatively similar to the quantum Hall effect in graphene as well as to the transport in a random magnetic field with a zero mean value.

sigma- and pi-defects at graphene nanoribbon edges: Building spin filters

The presence of certain kinds of defects at the edges of monohydrogenated zigzag graphene nanoribbons changes dramatically the charge transport properties inducing a spin-polarized conductance. Using an approach based on density functional theory and nonequilibrium Green`s function formalism to calculate the transmittance, we classify the defects in different classes depending on their distinct transport properties: (i) sigma-defects, which do not affect the transmittance close to the Fermi energy (E(F)); and (ii) pi-defects, which cause a spin polarization of the transmittance and that can be further divided into either electron or hole defects if the spin transport polarization results in larger transmittance for the up or down spin channel, respectively.

Black hole partition function using the hybrid formalism of superstrings

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Unconventional Hall effect near charge neutrality point in a two-dimensional electron-hole system

The transport properties of the two-dimensional system in HgTe-based quantum wells containing simultaneously electrons and holes of low densities are examined. The Hall resistance, as a function of perpendicular magnetic field, reveals an unconventional behavior, different from the classical N-shaped dependence typical for bipolar systems with electron-hole asymmetry. The quantum features of magnetotransport are explained by means of numerical calculation of the Landau level spectrum based on the Kane Hamiltonian. The origin of the quantum Hall plateau sigma(xy) = 0 near the charge neutrality point is attributed to special features of Landau quantization in our system.

Zircons, isotope ratios and element analyses from tonalite and oxide gabbro of the mid-ocean ridge from ODP Hole 176-735B

A limiting factor in the accuracy and precision of U/Pb zircon dates is accurate correction for initial disequilibrium in the 238U and 235U decay chains. The longest-lived-and therefore most abundant-intermediate daughter product in the 235U isotopic decay chain is 231Pa (T1/2 = 32.71 ka), and the partitioning behavior of Pa in zircon is not well constrained. Here we report high-precision thermal ionization mass spectrometry (TIMS) U-Pb zircon data from two samples from Ocean Drilling Program (ODP) Hole 735B, which show evidence for incorporation of excess 231Pa during zircon crystallization. The most precise analyses from the two samples have consistent Th-corrected 206Pb/238U dates with weighted means of 11.9325 ± 0.0039 Ma (n = 9) and 11.920 ± 0.011 Ma (n = 4), but distinctly older 207Pb/235U dates that vary from 12.330 ± 0.048 Ma to 12.140 ± 0.044 Ma and 12.03 ± 0.24 to 12.40 ± 0.27 Ma, respectively. If the excess 207Pb is due to variable initial excess 231Pa, calculated initial (231Pa)/(235U) activity ratios for the two samples range from 5.6 ± 1.0 to 9.6 ± 1.1 and 3.5 ± 5.2 to 11.4 ± 5.8. The data from the more precisely dated sample yields estimated DPazircon/DUzircon from 2.2-3.8 and 5.6-9.6, assuming (231Pa)/(235U) of the melt equal to the global average of recently erupted mid-ocean ridge basaltic glasses or secular equilibrium, respectively. High precision ID-TIMS analyses from nine additional samples from Hole 735B and nearby Hole 1105A suggest similar partitioning. The lower range of DPazircon/DUzircon is consistent with ion microprobe measurements of 231Pa in zircons from Holocene and Pleistocene rhyolitic eruptions (Schmitt (2007; doi:10.2138/am.2007.2449) and Schmitt (2011; doi:10.1146/annurev-earth-040610-133330)). The data suggest that 231Pa is preferentially incorporated during zircon crystallization over a range of magmatic compositions, and excess initial 231Pa may be more common in zircons than acknowledged. The degree of initial disequilibrium in the 235U decay chain suggested by the data from this study, and other recent high precision datasets, leads to resolvable discordance in high precision dates of Cenozoic to Mesozoic zircons. Minor discordance in zircons of this age may therefore reflect initial excess 231Pa and does not require either inheritance or Pb loss.

In-situ Iron isotope measurements and mineral compositions from ODP Hole 206-1256D and IODP Hole 335-U1256D

In-situ Fe isotope measurements have been carried out to estimate the impact of the hydrothermal metamorphic overprint on the Fe isotopic composition of Fe-Ti-oxides and Fe-sulfides of the different lithologies of the drilled rocks from IODP Hole 1256D (eastern equatorial Pacific; 15 Ma crust formed at the East Pacific Rise). Most igneous rocks normally have a very restricted range in their 56Fe/54Fe ratio. In contrast, Fe isotope compositions of hot fluids (> 300 °C) from mid-ocean-ridge spreading centers define a narrow range that is shifted to lower delta 56Fe values by 0.2 per mil - 0.5 per mil as compared to igneous rocks. Therefore, it is expected that mineral phases that contain large amounts of Fe are especially affected by the interaction with a fluid that fractionates Fe isotopes during exsolution/precipitation of those minerals. We have used a femtosecond UV-Laser ablation system to determine mineral 56Fe/54Fe ratios of selected samples with a precision of < 0.1 per mil (2 sigma level) at micrometer-scale. We have found significant variations of the delta 56Fe (IRMM-014) values in the minerals between different samples as well as within samples and mineral grains. The overall observed scale of delta 56Fe (magnetite) in 1256D rocks ranges from - 0.12 to + 0.64 per mil, and of delta 56Fe (ilmenite) from - 0.77 to + 0.01 per mil. Pyrite in the lowermost sheeted dike section is clearly distinguishable from the other investigated lithological units, having positive delta 56Fe values between + 0.29 and + 0.56 per mil, whereas pyrite in the other samples has generally negative delta 56Fe values from - 1.10 to - 0.59 permil. One key observation is that the temperature dependent inter-mineral fractionations of Fe isotopes between magnetite and ilmenite are systematically shifted towards higher values when compared to theoretically expected values, while synthesized, well equilibrated magnetite-ilmenite pairs are compatible with the theoretical predictions. Theoretical considerations including beta-factors of different aqueous Fe-chlorides and Rayleigh-type fractionations in the presence of a hydrous, chlorine-bearing fluid can explain this observation. The disagreement between observed and theoretical equilibrium fractionation, the fact that magnetite, in contrast to ilmenite shows a slight downhole trend in the delta 56Fe values, and the observation of small scale heterogeneities within single mineral grains imply that a general re-equilibration of the magnetite-ilmenite pairs is overprinted by kinetic fractionation effects, caused by the interaction of magnetite/ilmenite with hydrothermal fluids penetrating the upper oceanic crust during cooling, or incomplete re-equilibration at low temperatures. Furthermore, the observation of significant small-scale variations in the 56Fe/54Fe ratios of single minerals in this study highlights the importance of high spatial-resolution-analyses of stable isotope ratios for further investigations.

Element abundances and Nd isotopic composition in boninite and tholeiite from DSDP Hole 60-458

Boninites are unusual high MgO-high SiO2 volcanic rocks found in several western Pacific island arcs. Their high Mg/(Mg + total Fe) (0.55-0.83) and compatible element contents (Ni = 70-450 ppm, Cr = 200-1800 ppm) indicate equilibration with mantle peridotite, but their low TiO2 contents (0.1-0.5%) indicate severe depletion of this source. K, Rb, Sr and Ba abundances in boninites are typical of primitive arc basalts, but ratios such as Ti/Zr and La/Yb are variable (Ti/Zr = 23-67, (La/Yb)e.f. = 0.6-4.7). Evidence for both enrichment and depletion of incompatible elements suggests that boninites are derived from refractory peridotite which has been metasomatically enriched in LREE, Zr, Sr, Ba and alkalis. Wide variations in 143Nd/144Nd (0.51262-0.51296) are correlated with La/Sm, Sm/Nd and Ti/Zr, which enables identification of components in the boninite source. Possible LREE depleted components have relative REE and Ti abundances like those in depleted peridotites and high 143Nd/144Nd ratios which reach MORB-like values. Possible LREE enriched components have relative REE abundances similar to those in metasomatized mantle peridotite nodules, and low 143Nd/144Nd ratios which indicate either sedimentary sources or mantle sources with recent to ancient LREE enrichment. Relative abundances of Ba and Sr in boninites decrease with increasing LREE enrichment and suggest a non-sedimentary source for the LREE enriched material. Enrichment in Ba, Sr and alkalis may result from a third component derived from subducted oceanic crust. Two models can account for the successive generation of boninites and arc tholeiites within a single area: 1) boninites can be derived from the peridotite residue of earlier arc tholeiite generation which is metasomatically enriched in LREE before boninite volcanism, or 2) arc tholeiites and boninites can be derived from a variably depleted peridotite source which has been pervasively enriched in LREE. Areas of fertile peridotite would yield tholeiites while refractory areas would yield boninites.

(Table DR1) Trace element contents in basalts from ODP Hole 33-317A

Numerous large igneous provinces formed in the Pacific Ocean during Early Cretaceous time, but their origins and relations are poorly understood. We present new geochronological and geochemical data on rocks from the Manihiki Plateau and compare these results to those for other Cretaceous Pacific plateaus. A dredged Manihiki basalt gives an 40Ar-39Ar age of 117.9+/-3.5 Ma (2 sigma), essentially contemporaneous with the Ontong Java Plateau ~2500 km to the west, and the possibly related Hikurangi Plateau ~3000 km to the south. Drilled Manihiki lavas are tholeiitic with incompatible trace element abundances similar to those of Ontong Java basalts. These lavas may result from high degrees of partial melting during the main eruptive phase of plateau formation. There are two categories of dredged lavas from the Danger Islands Troughs, which bisect the plateau. The first is alkalic lavas having strong enrichments in light rare earth and large-ion lithophile elements; these lavas may represent late-stage activity, as one sample yields an 40Ar-39Ar age of 99.5+/-0.7 Ma. The second category consists of tholeiitic basalts with U-shaped incompatible element patterns and unusually low abundances of several elements; these basalts record a mantle component not previously observed in Manihiki, Ontong Java, or Hikurangi lavas. Their trace element characteristics may result from extensive melting of depleted mantle wedge material mixed with small amounts of volcaniclastic sediment. We are unaware of comparable basalts elsewhere.

Index properties, shear strenghts and consolidation characteristics of ODP Hole 164-995A sediments

Geotechnical properties of sediment from Ocean Drilling Program Leg 164 are presented as: (1) normalized shipboard strength ratios from the Cape Fear Diapir, the Blake Ridge Diapir, and the Blake Ridge; and (2) Atterberg limit, vane shear strength, pocket-penetrometer strength, and constant-rate-of-strain consolidation results from Hole 995A, located on the Blake Ridge. This study was conducted to understand the stress history in a region characterized by high sedimentation rates and the presence of gas hydrates. Collectively, the results indicate that sediment from the Blake Ridge exhibits significant underconsolidated behavior, except near the seafloor. At least 10 m of additional overburden was removed by erosion or mass wasting at Hole 993A on the Cape Fear Diapir, compared to nearby sites.

Fission track analyses of ODP Hole 129-800A samples

Four models of fission track annealing in apatite are compared with measured fission track lengths in samples from Site 800 in the East Mariana Basin, Ocean Drilling Program Leg 129, given an independently determined temperature history. The temperature history of Site 800 was calculated using a one-dimensional, compactive, conductive heat flow model assuming two end-member thermal cases: one for cooling of Jurassic ocean crust that has experienced no subsequent heating, and one for cooling of Cretaceous ocean crust. Because the samples analyzed were only shallowly buried and because the tectonic history of the area since sample deposition is simple, resolution of the temperature history is high. The maximum temperature experienced by the sampled bed is between 16°-21°C and occurs at 96 Ma; temperatures since the Cretaceous have dropped in spite of continued pelagic sediment deposition because heat flow has continued to decay exponentially and bottom-water temperatures have dropped. Fission tracks observed within apatite grains from the sampled bed are 14.6 +/- 0.1 µm (1 sigma) long. Given the proposed temperature history of the samples, one unpublished and three published models of fission track annealing predict mean track lengths from 14.8 to 15.9 µm. These models require temperatures as much as 40°C higher than the calculated paleotemperature maximum of the sampled bed to produce the same degree of track annealing. Measured and predicted values are different because annealing models are based on extrapolation of high temperature laboratory data to geologic times. The model that makes the closest prediction is based on the greatest number of experiments performed at low temperature and on an apatite having composition closest to that of the core samples.

Age determination of ODP Hole 145-887

Atmospheric carbon dioxide concentrations were significantly lower during glacial periods than during intervening interglacial periods, but the mechanisms responsible for this difference remain uncertain. Many recent explanations call on greater carbon storage in a poorly ventilated deep ocean during glacial periods (Trancois et al., 1997, doi:10.1038/40073; Toggweiler, 1999, doi:10.1029/1999PA900033; Stephens and Keeling, 2000, doi:10.1038/35004556; Marchitto et al., 2007, doi:10.1126/science.1138679; Sigman and Boyle, 2000, doi:10.1038/35038000), but direct evidence regarding the ventilation and respired carbon content of the glacial deep ocean is sparse and often equivocal (Broecker et al., 2004, doi:10.1126/science.1102293). Here we present sedimentary geochemical records from sites spanning the deep subarctic Pacific that -together with previously published results (Keigwin, 1998, doi:10.1029/98PA00874)- show that a poorly ventilated water mass containing a high concentration of respired carbon dioxide occupied the North Pacific abyss during the Last Glacial Maximum. Despite an inferred increase in deep Southern Ocean ventilation during the first step of the deglaciation (18,000-15,000 years ago) (Marchitto et al., 2007, doi:10.1126/science.1138679; Monnin et al., 2001, doi:10.1126/science.291.5501.112), we find no evidence for improved ventilation in the abyssal subarctic Pacific until a rapid transition ~14,600 years ago: this change was accompanied by an acceleration of export production from the surface waters above but only a small increase in atmospheric carbon dioxide concentration (Monnin et al., 2001, doi:10.1126/science.291.5501.112). We speculate that these changes were mechanistically linked to a roughly coeval increase in deep water formation in the North Atlantic (Robinson et al., 2005, doi:10.1126/science.1114832; Skinner nd Shackleton, 2004, doi:10.1029/2003PA000983; McManus et al., 2004, doi:10.1038/nature02494), which flushed respired carbon dioxide from northern abyssal waters, but also increased the supply of nutrients to the upper ocean, leading to greater carbon dioxide sequestration at mid-depths and stalling the rise of atmospheric carbon dioxide concentrations. Our findings are qualitatively consistent with hypotheses invoking a deglacial flushing of respired carbon dioxide from an isolated, deep ocean reservoir periods (Trancois et al., 1997, doi:10.1038/40073; Toggweiler, 1999, doi:10.1029/1999PA900033; Stephens and Keeling, 2000, doi:10.1038/35004556; Marchitto et al., 2007, doi:10.1126/science.1138679; Sigman and Boyle, 2000, doi:10.1038/35038000; Boyle, 1988, doi:10.1038/331055a0), but suggest that the reservoir may have been released in stages, as vigorous deep water ventilation switched between North Atlantic and Southern Ocean source regions.

Calcium concentrations and isotopic composition in carbonates and porefluids from ODP Hole 130-807A

The calcium isotopic compositions (d44Ca) of 30 high-purity nannofossil ooze and chalk and 7 pore fluid samples from ODP Site 807A (Ontong Java Plateau) are used in conjunction with numerical models to determine the equilibrium calcium isotope fractionation factor (a_s-f) between calcite and dissolved Ca2+ and the rates of post-depositional recrystallization in deep sea carbonate ooze. The value of a_s-f at equilibrium in the marine sedimentary section is 1.0000+/-0.0001, which is significantly different from the value (0.9987+/-0.0002) found in laboratory experiments of calcite precipitation and in the formation of biogenic calcite in the surface ocean. We hypothesize that this fractionation factor is relevant to calcite precipitation in any system at equilibrium and that this equilibrium fractionation factor has implications for the mechanisms responsible for Ca isotope fractionation during calcite precipitation. We describe a steady state model that offers a unified framework for explaining Ca isotope fractionation across the observed precipitation rate range of ~14 orders of magnitude. The model attributes Ca isotope fractionation to the relative balance between the attachment and detachment fluxes at the calcite crystal surface. This model represents our hypothesis for the mechanism responsible for isotope fractionation during calcite precipitation. The Ca isotope data provide evidence that the bulk rate of calcite recrystallization in freshly-deposited carbonate ooze is 30-40%/Myr, and decreases with age to about 2%/Myr in 2-3 million year old sediment. The recrystallization rates determined from Ca isotopes for Pleistocene sediments are higher than those previously inferred from pore fluid Sr concentration and are consistent with rates derived for Late Pleistocene siliciclastic sediments using uranium isotopes. Combining our results for the equilibrium fractionation factor and recrystallization rates, we evaluate the effect of diagenesis on the Ca isotopic composition of marine carbonates at Site 807A. Since calcite precipitation rates in the sedimentary column are many orders of magnitude slower than laboratory experiments and the pore fluids are only slightly oversaturated with respect to calcite, the isotopic composition of diagenetic calcite is likely to reflect equilibrium precipitation. Accordingly, diagenesis produces a maximum shift in d44Ca of +0.15? for Site 807A sediments but will have a larger impact where sedimentation rates are low, seawater circulates through the sediment pile, or there are prolonged depositional hiatuses.

Permeability, overburden stress and friction coefficients of ODP Hole 190-1173A and 190-1174B

Permeability measured on three samples in a triaxial cell under effective confining pressure from 0.2 to 2.5 MPa ranges from 10**-18 to 10**-19 m**2. Overall, results indicate that permeability decreases with effective confining pressure up to 1.5 MPa; however, measurements at low effective pressure are too dispersed to yield a precise general relationship between permeability and pressure. When the effective pressure is increased from 1.5 to 2.5 MPa, permeability is roughly constant (~1-4 x 10**-19 m**2). Samples deformed in the triaxial cell developed slickenlined fractures, and permeability measurements were performed before and after failure. A permeability increase is observed when the sample fails under low effective confining pressure (0.2 MPa), but not under effective pressure corresponding to the overburden stress. Under isotropic stress conditions, permeability decrease related to fracture closure occurs at a relatively high effective pressure of ~1.5 MPa. Coefficients of friction on the fractures formed in the triaxial cell are ~0.4.

Measurements of velocity-dependent and slip-dependent frictional strength in laboratory friction experiments on natural samples from IODP Expedition 316 and ODP Hole 190-1174B

Slowslip forms part of the spectrum of fault behaviour between stable creep and destructive earthquakes. Slow slip occurs near the boundaries of large earthquake rupture zones and may sometimes trigger fast earthquakes. It is thought to occur in faults comprised of rocks that strengthen under fast slip rates, preventing rupture as a normal earthquake, or on faults that have elevated pore-fluid pressures. However, the processes that control slow rupture and the relationship between slow and normal earthquakes are enigmatic. Here we use laboratory experiments to simulate faulting in natural rock samples taken from shallow parts of the Nankai subduction zone, Japan, where very low-frequency earthquakes - a form of slow slip - have been observed.We find that the fault rocks exhibit decreasing strength over millimetre-scale slip distances rather than weakening due to increasing velocity. However, the sizes of the slip nucleation patches in our laboratory simulations are similar to those expected for the very lowfrequency earthquakes observed in Nankai. We therefore suggest that this type of fault-weakening behaviour may generate slow earthquakes. Owing to the similarity between the expected behaviour of slow earthquakes based on our data, and that of normal earthquakes during nucleation, we suggest that some types of slow slip may represent prematurely arrested earthquakes.