Coastal upwelling along the north coast of Papua New Guinea and El Niño events during 1981–2005

We investigate the relationship between sea surface temperature (SST) cooling and upwelling along Papua New Guinea’s (PNG) north coast before the onset of El Niño events using a hindcast experiment with a high-resolution ocean general circulation model. Coastal upwelling and related SST cooling appear along PNG north coast during the boreal winter before the onsets of six El Niño events occurring during 1981–2005. Relatively cool SSTs appear along PNG north coast during that time, when anomalous northwesterly surface wind stress, which can cause coastal upwelling by offshore Ekman transport appearing over the region. In addition, anomalous cooling tendencies of SST are observed, accompanying anomalous upward velocities at the base of the mixed layer and shallow anomalies of 27°C isotherm depth. It is also shown that entrainment cooling plays an important role in the cooling of the mixed layer temperature in this region.     

The record of Himalayan erosion preserved in the sedimentary rocks of the Hatia Trough of the Bengal Basin and the Chittagong Hill Tracts,Bangladesh

The Cenozoic sedimentary succession of Bangladesh provides an archive of Himalayan erosion. However, its potential as an archive is currently hampered by a poor lithostratigaphic framework with limited age control. We focus on the Hatia Trough of the Bengal Basin and the adjacent fold belt of the Chittagong Hill Tracts which forms the outermost part of the west‐propagating Indo‐Burmese wedge. We present a basin‐wide seismic stratigraphic framework for the Neogene rocks, calibrated by biostratigraphy, which divides the succession into three seismically distinct and regionally correlatable Megasequences (MS). MS1 extends to NN15‐NN16 (ca. 2.5–3.9 Ma), MS2 to NN19‐NN20 (ca. 0.4–1.9 Ma) and MS3 to present day. Our seismic mapping, thermochronological analyses of detrital mineral grains, isotopic analyses of bulk rock, heavy mineral and petrographic data, show that the Neogene rocks of the Hatia Trough and Chittagong Hill Tracts are predominantly Himalayan‐derived, with a subordinate arc‐derived input possibly from the Paleogene IndoBurman Ranges as well as the Trans‐Himalaya. Our seismic data allow us to concur with previous work that suggests folding of the outer part of the west‐propagating wedge only commenced recently, within the last few million years. We suggest that it could have been the westward encroachment and final abutment of the Chittagong Hill Tracts fold belt onto the already‐uplifted Shillong Plateau that caused diversion of the palaeo‐Brahmaputra to the west of the plateau as the north‐east drainage route closed.     

Instability of a thin magnetic tube in the solar atmosphere

Abstract

The static equilibrium of a thin vertical magnetic tube embedded in the solar atmosphere is shown to be dynamically unstable against the fundamental mode of perturbation having no nodes in the vertical displacement. The instability has its origin in the convection zone, and the eigenfunction is extended further up in the stable upper layers by the magnetic field which guides the displacement mainly in the longitudinal direction. It is suggested that the downdraft observed in the solar network structure is a finite amplitude consequence of this instability. The overtone modes are found to be stable.     

In-situ diurnal sea surface temperature variations and near-surface thermal structure in the tropical hot event of the Indo-Pacific warm pool

Diurnal Sea Surface Temperature (SST) variations and the near-surface thermal structure of the tropical hot event (HE) have been investigated using advanced in-situ equatorial observations with hourly temporal resolution. The information on the HE area defined by the satellite cloud-free SSTs is used to sample the in-situ observations. The in-situ SSTs sampled for the HE conditions show that a maximum (minimum) SST has a histogram mode at 30.8°C (29.0°C), and frequently appears at 15:00 (07:00) local time. The amplitude of the diurnal SST variation (DSST) is defined by the difference between the maximum and minimum SSTs. The mean DSST during HEs is greater than 0.5°C, and has a maximum of about 0.75°C at the HE peak. The time series of mean DSST gradually increases (rapidly decreases) before (after) the peak. The satellite SST has a systematic positive bias against the corresponding daytime SST measured by the Triangle Trans-Ocean buoy Network. This bias is enhanced under conditions of large in-situ DSST. One-dimensional numerical model simulation suggests that the systematic bias is caused by the sharp vertical temperature gradient in the surface layer of HE. The near-surface thermal structure is generated by conditions of high insolation and low wind speed, which is the typical HE condition.     

Faulting in the Mikawa earthquake of 1945

This paper discusses the fault parameters of the Mikawa earthquake of January 12, 1945 on the basis of a simple dislocation model. Basically, the model assumes a rectangular shape of the fault plane striking N-S, so that it may fit the observed surface fault trace. Several sets of the fault parameters are tested to interpret the vertical and horizontal ground movements as observed geodetically. The fault model which is finally accepted is as follows: total length: 12 km; width: 11 km; dip angle: 30°; reverse dip-slip: 2 m; right-lateral strike-slip: 1 m. Geometry and slip in the present model seem to harmonize with the other sorts of evidence such as the seismological, tsunami genetic and reconnaissance data. From the tectonic point of view, this earthquake may be attributed to the secondary fault activity associated with the right lateral movement of the Median Tectonic Line.     

Influence of submarine fumaroles on the distribution of mercury in the sediment of Kagoshima Bay,Japan

To estimate the influence of mercury emitted from submarine fumaroles, the horizontal and vertical distribution of mercury in sediment of Kagoshima Bay was studied. The fumaroles are located in the northern bay head area, and the sediment samples had been taken from 52 points throughout the bay with a gravity core sampler. The core samples obtained were cut at a thickness of 1–2 cm and used for measurements. The total concentration of mercury in surface sediment in the northern and central areas of the bay was 51–679 μg kg^− 1 (average 199 μg kg^− 1, n = 22) and 23–100 μg kg^− 1 (average 55 μg kg^− 1, n = 30), respectively. The highest value was obtained in the vicinity of the fumaroles. The mercury concentration in sediment near the fumaroles varied with depth, which may reflect the variation in fumarolic activity. A successive extraction method was applied to the speciation of mercury in the sediment. The results showed that sediment taken in the vicinity of submarine fumaroles contained a higher percentage of mercury bound with organic matter.     

The phase boundary between wadsleyite and ringwoodite in Mg2SiO4 determined by in situ X-ray diffraction

The phase boundary between wadsleyite and ringwoodite in Mg₂SiO₄ has been determined in situ using a multi-anvil apparatus and synchrotron X-rays radiation at SPring-8. In spite of the similar X-ray diffraction profiles of these high-pressure phases with closely related structures, we were able to identify the occurrence of the mutual phase transformations based on the change in the difference profile by utilizing a newly introduced press-oscillation system. The boundary was located at ~18.9 GPa and 1,400°C when we used Shim’s gold pressure scale (Shim et al. in Earth Planet Sci Lett 203:729–739, 2002), which was slightly (~0.8 GPa) lower than the pressure as determined from the quench experiments of Katsura and Ito (J Geophys Res 94:15663–15670, 1989). Although it was difficult to constrain the Clapeyron slope based solely on the present data due to the kinetic problem, the phase boundary [P (GPa)=13.1+4.11×10⁻³×T (K)] calculated by a combination of a P–T position well constrained by the present experiment and the calorimetric data of Akaogi et al. (J Geophys Res 94:15671–15685, 1989) reasonably explains all the present data within the experimental error. When we used Anderson’s gold pressure scale (Anderson et al. in J Appl Phys 65:1535–1543, 1989), our phase boundary was located in ~18.1 GPa and 1,400°C, and the extrapolation boundary was consistent with that of Kuroda et al. (Phys Chem Miner 27:523–532, 2000), which was determined at high temperature (1,800–2,000°C) using a calibration based on the same pressure scale. Our new phase boundary is marginally consistent with that of Suzuki et al. (Geophys Res Lett 27:803–806, 2000) based on in situ X-ray experiments at lower temperatures (<1,000°C) using Brown’s and Decker’s NaCl pressure scales.     

Critical-level crossing of rossby-gravity wave-packets in a linear-shear,stratified flow

Abstract

The propagation of Rossby-gravity waves, which are of astronomical interest near the critical level (or corotation point), is considered in a linear-shear, exponentially stratified flow by means of a ray tracing method. It is shown, using the analytic solutions for the stream function, that for a Richardson number J > 1/4 a wave-packet can cross the critical level in a finite time. In the unstable stratified case (J < 0), it cannot cross the critical level for large |J| (J < 0), but may do so for some intermediate |J| values.

Based on the above results, the possible existence of the regular normal mode with a discrete point eigenvalue in the continuous spectrum is discussed for bounded systems.     

Fourth‐ to third‐order cycles in the Hakobuchi Formation: Shallow‐marine Campanian final deposition of the Yezo Group,Nakagawa area,northern Hokkaido,Japan

The Yezo Group has a wide longitudinal distribution across Hokkaido, northern Japan. It represents a Cretaceous (Early Aptian–Late Maastrichtian) and Late Paleocene forearc basin‐fill along the eastern margin of the paleo‐Asian continent. In the Nakagawa area of northern Hokkaido, the uppermost part of the Yezo Group consists of the Hakobuchi Formation. Along the western margin of the Yezo basin, 24 sedimentary facies (F) represent 6 facies associations (FA), suggesting prevailing storm‐dominated inner shelf to shoreface environments, subordinately associated with shoreface sand ridges, outer shelf, estuary and fluvial environments. The stacking patterns, thickness and facies trends of these associations allow the discrimination of six depositional sequences (DS). Inoceramids Sphenoceramus schmidti and Inoceramus balticus, and the ammonite Metaplacenticeras subtilistriatum, provide late Early to Late Campanian age constraints to this approximately 370‐m thick final stage of deposition and uplift of the Yezo forearc basin. Six shallow‐marine to subordinately non‐marine sandstone‐dominated depositional sequences include four 10 to 110‐m thick upward‐coarsening regressive successions (FS1), occasionally associated with thin, less than 10‐m thick, upward‐fining transgressive successions (FS2). The lower DS1–3, middle DS4–5 and upper DS6 represent three depositional sequential sets (DSS1–3). These eastward prograding and westward retrograding recurring shallow‐marine depositional systems may reflect third‐ and fourth‐order relative sealevel changes, in terms of sequence stratigraphy.