Relative influence of biotic and abiotic processes on travertine fabrics,Satono‐yu hot spring,Japan

The relative influences of biotic and abiotic processes on travertine fabrics are still not well understood, despite increasing interest in the last decade to better understand the record of ancient microbial life and sedimentary fabrics in microbial hydrocarbon reservoirs. This study examines travertines at Satono‐yu hot spring in Japan (the temperature of water flowing over the travertine was ca 35°C), to better understand the interaction between depositional, hydrochemical and microbial parameters at different flow settings. Characteristics of the bulk hydrochemistry, mineralogy (exclusively aragonite) and the driving force for precipitation (primarily abiotic CO₂ degassing with some photosynthetic microbial contribution) were similar among all of the flow settings. Conversely, the increase in flow velocity suppressed the influence of photosynthesis and enhanced the abiotic precipitation due to the thinner diffusive boundary layer at the travertine surface–water interface. Additionally, the increase in flow velocity changed the microbial composition and decreased the bacterial diversity by reflecting their adhesion efficiency on the travertine substrate. The acidity of the cyanobacterial sheaths controls the aragonite nucleation rate and the resulting calcification, even at significantly high equilibrium CO₂ partial pressure (ca 22 to 28 matm), high dissolved inorganic carbon concentration (ca 35 to 38 mmol l^?1), and elevated aragonite saturation state (ca 20‐fold to 34‐fold). Therefore, the increase in flow velocity suppresses the microbial influence with respect to the increase in the saturation state, the nucleation site supply and pore space generation. Overall, this results in the predominance of abiotic precipitation under high flow velocities. Consequently, a sparse‐micritic fabric with abundant interlamina porosity forms under lower flow velocity where the microbial influence is effective, while a dense‐sparitic fabric with little inter‐crystalline porosity forms under higher flow velocity where abiotic precipitation prevails. These findings provide an essential base for assessing the formation processes of ancient travertines and comparable deposits from petrological fabrics.     

Impact fracture patterns on phobos ellipsoids

Phobos-ellipsoid models made of clay were fragmented by the impact of high-velocity projectiles to examine the idea proposed by P. Thomas, J. Veverka, and T. Duxbury ((1978) Nature273, 282–284) that the grooves on Phobos are the manifestation of fractures produced by the Stickney-forming impact. The fracture lines on the models consist of two sets. One is concentric around the impact site and along E lines, which are defined as the intersecting lines of the ellipsoid surface and a set of spherical surfaces with the center of the spheres at the impact site. The other runs radially from the impact site and along P lines, which are defined as the lines crossing E lines perpendicularly on the ellipsoid surface. Some patterns of the grooves originating radially from the crater Stickney on Phobos are very similar to the P lines. The gridded topography, hummocky groove sections, and smooth topography on Phobos could have been formed by the fracture or associated surface disturbances due to the wave induced by the Stickney-forming impact, because they are distributed along the E lines surrounding the converging point of the P lines. All the models except one showed that the density of the fractures east of the impact site is greater than that of those to the west. Fracture patterns similar to one of the most prominent groove sets, which converge and diminish into the region of about (270°, 0°) were not produced by the impact on the ellipsoid of uniform constituent. These grooves would have been produced by the opening of preexisting cracks by the Stickney-forming impact. Other grooves also seem to be affected by such latent cracks.     

Anomalously large seismic amplifications in the seafloor area off the Kii peninsula

Seismic wave amplifications were investigated using strong-motion data obtained from the ground’s surface (K-net) on the Kii peninsula (southwestern Japan) and from the network of twenty seismic stations on the seafloor (DONET) located off the peninsula near the Nankai trough. Observed seismograms show that seismic signals at DONET stations are significantly larger than those at K-net stations, independent of epicentral distances. In order to investigate the cause of such amplifications, seismic wavefields for local events were simulated using the finite-difference method, in which a realistic 3D velocity structure in and around the peninsula was incorporated. Our simulation results demonstrate that seismic waves are significantly amplified at DONET stations in relation to the presence of underlying low-velocity sediment layers with a total thickness of up to 10 km. Our simulations also show considerable variations in the degree of amplification among DONET stations, which is attributed to differences in the thickness of the sediment layers. The degree of amplification is relatively low at stations above thin sediment layers near the trough axis, but seismic signals are much more amplified at stations closer to the Kii peninsula, where sediment layers are thicker than those at the trough axis. Simulation results are consistent with observations. This study, based on seafloor observations and simulations, indicates that because seismic signals are amplified due to the ocean-specific structures, the magnitude of earthquakes would be overestimated if procedures applied to data observed at land stations are used without corrections.