Expedition 306 summary

The overall aim of the North Atlantic paleoceanography study of Integrated Ocean Drilling Program Expedition 306 is to place late Neogene–Quaternary climate proxies in the North Atlantic into a chronology based on a combination of geomagnetic paleointensity, stable isotope, and detrital layer stratigraphies, and in so doing generate integrated North Atlantic millennial-scale stratigraphies for the last few million years. To reach this aim, complete sedimentary sections were drilled by multiple advanced piston coring directly south of the central Atlantic “ice-rafted debris belt” and on the southern Gardar Drift. In addition to the North Atlantic paleoceanography study, a borehole observatory was successfully installed in a new ~180 m deep hole close to Ocean Drilling Program Site 642, consisting of a circulation obviation retrofit kit to seal the borehole from the overlying ocean, a thermistor string, and a data logger to document and monitor bottom water temperature variations through time

The Quiescent Intracluster Medium in the Core of the Perseus Cluster

Clusters of galaxies are the most massive gravitationally-bound objects in the Universe and are still forming. They are thus important probes of cosmological parameters and a host of astrophysical processes. Knowledge of the dynamics of the pervasive hot gas, which dominates in mass over stars in a cluster, is a crucial missing ingredient. It can enable new insights into mechanical energy injection by the central supermassive black hole and the use of hydrostatic equilibrium for the determination of cluster masses. X-rays from the core of the Perseus cluster are emitted by the 50 million K diffuse hot plasma filling its gravitational potential well. The Active Galactic Nucleus of the central galaxy NGC1275 is pumping jetted energy into the surrounding intracluster medium, creating buoyant bubbles filled with relativistic plasma. These likely induce motions in the intracluster medium and heat the inner gas preventing runaway radiative cooling; a process known as Active Galactic Nucleus Feedback. Here we report on Hitomi X-ray observations of the Perseus cluster core, which reveal a remarkably quiescent atmosphere where the gas has a line-of-sight velocity dispersion of 164+/-10 km/s in a region 30-60 kpc from the central nucleus. A gradient in the line-of-sight velocity of 150+/-70 km/s is found across the 60 kpc image of the cluster core. Turbulent pressure support in the gas is 4% or less of the thermodynamic pressure, with large scale shear at most doubling that estimate. We infer that total cluster masses determined from hydrostatic equilibrium in the central regions need little correction for turbulent pressure.Comment: 31 pages, 11 Figs, published in Nature July

Hitomi (ASTRO-H) X-ray Astronomy Satellite

The Hitomi (ASTRO-H) mission is the sixth Japanese x-ray astronomy satellite developed by a large international collaboration, including Japan, USA, Canada, and Europe. The mission aimed to provide the highest energy resolution ever achieved at E  >  2  keV, using a microcalorimeter instrument, and to cover a wide energy range spanning four decades in energy from soft x-rays to gamma rays. After a successful launch on February 17, 2016, the spacecraft lost its function on March 26, 2016, but the commissioning phase for about a month provided valuable information on the onboard instruments and the spacecraft system, including astrophysical results obtained from first light observations. The paper describes the Hitomi (ASTRO-H) mission, its capabilities, the initial operation, and the instruments/spacecraft performances confirmed during the commissioning operations for about a month

Hitomi X-Ray Studies of Giant Radio Pulses from the Crab Pulsar

To search for giant X-ray pulses correlated with the giant radio pulses (GRPs) from the Crab pulsar, we performed a simultaneous observation of the Crab pulsar with the X-ray satellite Hitomi in the 2300 keV band and the Kashima NICT radio telescope in the 1.41.7 GHz band with a net exposure of about 2 ks on 2016 March 25, just before the loss of the Hitomi mission. The timing performance of the Hitomi instruments was confirmed to meet the timing requirement and about 1000 and 100 GRPs were simultaneously observed at the main pulse and inter-pulse phases, respectively, and we found no apparent correlation between the giant radio pulses and the X-ray emission in either the main pulse or inter-pulse phase. All variations are within the 2 fluctuations of the X-ray fluxes at the pulse peaks, and the 3 upper limits of variations of main pulse or inter-pulse GRPs are 22% or 80% of the peak flux in a 0.20 phase width, respectively, in the 2300 keV band. The values for main pulse or inter-pulse GRPs become 25% or 110%, respectively, when the phase width is restricted to the 0.03 phase. Among the upper limits from the Hitomi satellite, those in the 4.510 keV and 70300 keV bands are obtained for the first time, and those in other bands are consistent with previous reports. Numerically, the upper limits of the main pulse and inter-pulse GRPs in the 0.20 phase width are about (2.4 and 9.3) 10(exp 11) erg cm(exp 2), respectively. No significant variability in pulse profiles implies that the GRPs originated from a local place within the magnetosphere. Although the number of photon-emitting particles should temporarily increase to account for the brightening of the radio emission, the results do not statistically rule out variations correlated with the GRPs, because the possible X-ray enhancement may appear due to a >0.02% brightening of the pulse-peak flux under such conditions

Intra- and infra-specific morphological variation in selected coccolithophore species in the equatorial and subequatorial Pacific Ocean

The ecological preferences of morphological groups within major coccolithophore taxa were studied in surface water samples from the equatorial and subequatorial Pacific Ocean. Emiliania huxleyi was subdivided into three morphological groups: Type A, Type C, and variety corona. The most probable factors limiting the occurrence of E. huxleyi Types A and C were high temperatures and low nutrient concentrations, respectively. E. huxleyi var. corona had an affinity for oligotrophic conditions. Calcidiscus leptoporus ssp. small was adapted to fertile waters. Umbilicosphaera foliosa and Umbilicosphaera sibogae preferred mesotrophic upwelling waters and stratified marginal waters surrounding the upwelling front, respectively. Among the three Umbellosphaera tenuis morphotypes observed in this study (Types I, III, and IV), only Type I was found in very warm tropical surface. Both Types III and IV were found in subtropical waters, and Type III differed from Type IV in that its distribution was constrained to hemi-pelagic waters. Habitat segregation among the morphotypes of major taxa indicates that the observed global distributions of these major taxa are, in fact, combinations of discrete morphological groups

Pseudo-cryptic speciation in Braarudosphaera bigelowii (Gran and Braarud) Deflandre

Partial SSU rDNA sequences were obtained from 13 naturally collected cells of Braarudosphaera bigelowii obtained from various parts of seas surrounding Japan. Together with the two previously reported sequences, 15 specimens were classified into five SSU rDNA Genotypes I-V. Based on the side length of the pentaliths forming the coccosphere, these specimens were also classified into three size-morphotypes; Intermediate form-A, Intermediate form-B, and Large form. Genotypes of B. bigelowii were well correlated with size-morphotypes but not with sampling area. This result indicates that size differences in B. bigelowii are the results of speciation and not of intra-population variety. Therefore, Genotypes I and II (Intermediate form-A) and Genotypes IV and V (Large form) are regarded as pseudo-cryptic sibling species of typical B. bigelowii (Genotype III, Intermediate form-B), which corresponds to the original description of the species. From the SSU rDNA sequences, it is evident that Genotype V arose from Genotype IV, and Genotype IV originated from Genotype III. The specimens of Genotypes III-V showed size increase of pentaliths in accordance with their branching order. The consistency in the relationship between genotypes and size-morphotypes of living B. bigelowii-complex observed in this study suggests that inconsistency of size range of 'B. bigelowii' pentaliths among different geological ages reported by palaeontological studies stems from additional pseudo-cryptic speciation in the lineage of 'B. bigelowii' in the geological past

Coccolithophore assemblages and morphotypes of Emiliania huxleyi in the boundary zone between the cold Oyashio and warm Kuroshio currents off the coast of Japan

Living coccolithophore assemblages were studied off the Japanese coast in the boundary zone between the cold Oyashio and the warm Kuroshio Currents as well as in adjacent water masses. A total of 64 coccolithophore taxa were identified. Ten taxa that comprised more than 10% of at least one sample were designated major taxa. Morphological observations and morphometric measurements enabled us to sub-divide the Emiliania huxleyi into four morphotypes: Type A, Type B, Type B/C, and Type B/C-2. Based on the composition of major taxa and morphotypes of Emiliania huxleyi, four coccolithophore assemblages were recognized, corresponding to distinct water masses: Oyashio Current, Tsugaru Warm Current, Kuroshio Current, and Western North Pacific Central Water (WNPCW). In the Oyashio Current regime, the standing crop of coccolithophores was high, although diversity was low. The floral assemblage was dominated by E. huxleyi Type B and consistently contained Coccolithus pelagicus ssp. pelagicus HOL, Calciopappus caudatus, and Syracosphaera orbiculus. In the Tsugaru Warm Current regime, the flora was characterized by abundant E. huxleyi Type B/C-2 and Gephyrocapsa oceanica as well as by relatively common Braarudosphaera bigelowii. In the Kuroshio Current regime, E. huxleyi Type A, Gephyrocapsa ericsonii, or G. oceanica were abundant. The coccolithophore flora in the WNPCW was characterized by abundant E. huxleyi Type A, Discosphaera tubifera, and Umbellosphaera tenuis Type IV. Large Emiliania huxleyi specimens (≥4.2 μm in length) only occurred north of the Oyashio Front, while to the south, E. huxleyi were consistently ≤4.1 μm long, regardless of the morphotype. Large E. huxleyi can be a useful paleoenvironmental indicator for reconstructing past migrations of the Kuroshio and Oyashio Currents in the North Pacific Ocean