PDF orientations in shocked quartz grains around the Chicxulub crater

Abstract— We measured 852 sets of planar deformation features (PDFs) in shocked quartz grains in impactite samples of the Yaxcopoil (YAX‐1) core and from 4 Cretaceous/Tertiary (K/T) boundary deposits: the Monaca, the Cacarajícara, and the Peñalver formations in Cuba, and DSDP site 536, within 800 km of the Chicxulub crater, in order to investigate variations of PDF orientations in the proximity of the crater. Orientations of PDFs show a broad distribution with peaks at ω {101¯3}, π {101¯2}, and ω {111¯2}, plus r, z {1011¯} orientations with minor c(0001), s{112¯1}, t{224¯1} plus x{516¯1}, and m{101¯0} plus a{112¯0} orientations. Planar deformation features with c(0001) orientation are relatively more abundant in the proximity of the Chicxulub crater than in distal sites such as North America, the Pacific Ocean, and Europe. This feature indicates that in the proximity of the crater, part of the shocked quartz grains in the K/T boundary deposits were derived from the low shock pressure zones. Moreover, the orientations of PDFs with ω {112¯2} plus r, z {101¯1} are high in our studied sites, and frequencies of these orientations decrease with increasing distance from the crater. On the other hand, absence of c(0001) and the rare occurrence of PDFs with ?ω {112¯2} plus r, z {101¯1} orientations in the sample from the YAX‐1 core that was taken at the top of the impactite layer of the Chicxulub crater suggests that the sampling horizon that reflects a certain cratering stage is also an important factor for variations in shocked quartz.     

Sedimentary effects on the expansion of a Himalayan supraglacial lake

Supraglacial Tsho Rolpa Lake in the Nepal Himalaya has been increasing rapidly in size since the 1950s, corresponding to the mountain-glacier shrinkage after the Little Ice Age. The lake basin expansion results from the subsidence by dead-ice melt below the bottom of the lake, and the retreat of the glacier terminus. Field observations of Tsho Rolpa in 1996 revealed that the retreat of glacier terminus is connected to a wind-induced vertical circulation of surface water heated by solar radiation. In order to clarify the mechanism of the lake expansion associated with sedimentary processes, we measured bottom sedimentation rate with some sediment traps, and vertical suspended sediment concentration (SSC) and water temperature, and analyzed the grain size of suspended and trapped sediments. The sediments, mostly composed of clay-sized grains, are dominantly supplied by glacier-melt water inflow at the glacier terminus. Sedimentary processes of such fine sediment comprise: (1) suspended-sediment fallout from intrusion of horizontal currents; (2) sediment sorting by sediment-laden underflows; and (3) the debris supply from the ice collapse at the glacier terminus. The (1) and (2) processes produce the density stratification of the lake, accompanied by a pycnocline at a depth of about 27 m. The existence of the pycnocline builds up the vertical water circulation in the surface layer to enhance the glacier-melt at the terminus. With respect to the subsidence of the lake bottom, nearly molecular thermal diffusion is probably dominant near the bottom of the deepest point, which results from the kinetic-energy dissipation of sediment-laden underflows. The stable existence of the bottom turbid water throughout the year could cause continuous dead-ice melt below the lake bottom.     

Role of synoptic-scale forcing in cyclogenesis over the Bay of Bengal

The cyclone frequency distribution over the Bay of Bengal during 1990–2009 was distinctly bimodal, with a primary post-monsoon peak and a secondary pre-monsoon peak, despite the very high convective available potential energy (CAPE) during the pre-monsoon. The location of the monsoon trough over the bay is a primary factor in tropical cyclogenesis. Because the trough was in the northernmost bay during the pre-monsoon season, cyclogenesis was inactive in the southern bay, where a strong southwesterly wind shear was found. In this season, moreover, a hot, dry air mass extending vertically from 950 to 600 hPa was advected from northwestern India toward the bay. Moist, warm southwesterly winds penetrating below the deep, dry air mass caused a prominent dryline to form aloft on the northwestern side of the bay. The synoptic-scale hot, dry air forcing to the bay suppressed the active convection necessary for cyclogenesis. The strength of the stable environmental layer, represented by convective inhibition (CIN), was extremely large, and acted as a cap over the northern and northwestern bay. Conversely, during the post-monsoon, there were no horizontal temperature or moisture gradients, and CAPE and CIN were fairly modest. The entire bay was covered by a very deep, moist layer from the surface to 700 hPa transported from the east. The monsoon trough position and the environmental CIN in combination can explain the lower frequency of cyclogenesis during the pre-monsoon compared with the post-monsoon season.     

The 2011 Tohoku-oki Earthquake Tsunami: Similarities and Differences to the 869 Jogan Tsunami on the Sendai Plain

A post-tsunami field survey following the 2011 Tohoku-oki Earthquake Tsunami was carried out to asses inundated area in Sendai Plain, Northeast Japan. The type of inundation was classified into two categories (major and minor) according to the amount of accumulated debris, garbage and sediment. Major and minor inundations were identified up to 4 and 5 km from the coastline, respectively. Many artificial geomorphological features, such as roadway embankments and canals, were believed to have affected the run-up process of the tsunami. The inundation area of the 2011 tsunami on the Sendai Plain is compared with that of the 869 Jogan tsunami, which was reconstructed using numerical modeling based on available historical and geological records. The inundation area of the 2011 Tohoku-oki tsunami is comparable to that of the 869 Jogan tsunami, although a direct comparison is difficult due to differences in geomorphological contexts between the paleo period and the present.     

Evidence for ocean water invasion into the Chicxulub crater at the Cretaceous/Tertiary boundary

Abstract— The possibility of ocean water invasion into the Chicxulub crater following the impact at the Cretaceous/Tertiary boundary was investigated based on examination of an impactite between approximately 794.63 and 894.94 m in the Yaxcopoil‐1 (Yax‐1) core. The presence of cross lamination in the uppermost part of the impactite suggests the influence of an ocean current at least during the sedimentation of this interval. Abundant occurrence of nannofossils of late Campanian to early Maastrichtian age in the matrices of samples from the upper part of the impactite suggests that the carbonate sediments deposited on the inner rim margin and outside the crater were eroded and transported into the crater most likely by ocean water that invaded the crater after its formation. The maximum grain size of limestone lithics and vesicular melt fragments, and grain and bulk chemical compositions show a cyclic variation in the upper part of the impactite. The upward fining grain size and the absence of erosional contact at the base of each cycle suggest that the sediments were derived from resuspension of units elsewhere in the crater, most likely by high energy currents association with ocean water invasion.