Recent macroids on the Kikai‐jima shelf,Central Ryukyu Islands,Japan

Sandy and gravelly carbonate sediments found off Kikai‐jima, southern Japan, a coral reef‐related island shelf, represent the northernmost sub‐tropical, carbonate deposits in the Central Ryukyu Islands (Ryukyus). On the Kikai‐jima shelf, at water depths of 61 to 105 m, these sediments are characterized by macroid pavements. Since the abundance of very small and of exceptionally large macroids may indicate specific hydrodynamic controls regarding constraints on growth and taphonomy, the detailed analysis of recent and fossil macroid pavements is meaningful ecologically and environmentally. Macroids, ranging in size from ca 25 to 130 mm in diameter, are spheroidal and sub‐spheroidal in shape and consist mainly of the encrusting foraminifer Acervulina inhaerens and subordinate thin encrusting and lumpy coralline algae. Accessory components include bryozoans, serpulids and, to a lesser extent, encrusting arborescent foraminifera (Homotrema and Miniacina). Low sedimentation rates and occasional movement due to current action are indicated by sizes, shapes and growth‐forms of the studied macroids, the Entobia–Gastrochaenolites–Trypanites–Maeandropolydora ichnocoenosis and the ‘Bioerosion Index’ for coated grains (introduced herein). The deep‐water tidally induced current energy was sufficient to maintain multi‐directional growth (spheroidal shapes) of the larger macroids and to initiate macroid growth using the diverse biogenic remnants as nuclei. The asymmetrical inner arrangement suggests possible periods of stability for the macroids. The residence time of the coated grain in its original environment determines the size and morphology of the macroid and the selection of coating organisms. The composition of the coating community is mainly a consequence of component growth rates in relation to turnover time and residence time. Long‐term studies are needed to assess the spatial and temporal resolution of present‐day encrusting communities across biogeographic provinces and shelf to slope regions.     

南疆尼雅地区4000a来的地化元素分布特征与古气候环境演化的初步研究

通过应用因子分析法对尼雅剖面中的多种地化元素和氧化物的分析,在R、Q型因子分析的基础上,提取出古气候信息。初步的研究表明:近4000a来,尼雅地区气候环境仍以干旱化为主,但此期间出现过多次不同程度的干湿变化,大致可分为5个气候期:4.0~3.0kaB P.相对暖干期;3.0~2.2kaB.P.相对冷湿期;2.2~1.6kaB.P.相对暖干期;1.6~1.0kaB.P.相对冷湿期;1.0kaB.P.左右之后的相对暖干期。     

Unravelling the conundrum of river response to rising sea‐level from laboratory to field. Part II. The Fly–Strickland River system,Papua New Guinea

The most recent deglaciation resulted in a global sea‐level rise of some 120 m over ca 12 000 years. A moving boundary numerical model is developed to predict the response of rivers to this rise. The model was motivated by experiments at small scale, which have identified two modes describing the transgression of a river mouth: (i) autoretreat without abandonment of the river delta (no sediment starvation at the topset–foreset break); and (ii) sediment‐starved autoretreat with abandonment of the delta. In the latter case, transgression is far more rapid, and its effects are felt much further upstream of the river mouth. A moving boundary numerical model that captures these features in experimental deltas is adapted to describe the response of the Fly–Strickland River system, Papua New Guinea. In the absence of better information, the model is applied to the case of sea‐level rise without local climate change in New Guinea. The model suggests that: (i) sea‐level rise has forced the river mouth to transgress over 700 km since the last glacial maximum; (ii) sediment‐starved autoretreat has forced enough bed aggradation to block a tributary with a low sediment load and create the present‐day Lake Murray; (iii) the resulting aggradation was sufficient to move the gravel–sand transition on the Strickland River upstream; (iv) the present‐day Fly Estuary may be, in part, a relict river valley drowned by sea‐level rise and partially filled by tidal effects; and (v) the Fly River is presently reforming its bankfull geometry and prograding into the Fly Estuary. A parametric study with the model indicates that sediment concentration during floods plays a key role in determining whether or not, and to what extent, transgression is expressed in terms of sediment‐starved autoretreat. A sufficiently high sediment concentration can prevent sediment‐starved autoretreat during the entire sea‐level cycle. This observation may explain why some present‐day river mouths are expressed in terms of deltas protruding into the sea, and others are wholly contained within embayments or estuaries in which water has invaded landward.     

SLIDING MODE CONTROL OF BUILDINGS WITH ATMD

In the present paper, the application of the sliding mode control (SMC) scheme is discussed in a systematic manner for controlling the vibration of tall buildings with an Active Tuned Mass Damper (ATMD) installed at the top floor. It is shown that the application of the SMC theory for buildings with ATMD may lead to large responses in the building due to the interaction effect from the ATMD caused by the comparatively large response of the ATMD. Based on the theory of compensators, a method is proposed which eliminates the interaction effect from the ATMD to the building and thus prevents large response in the building. The results are demonstrated through simple numerical examples of building–ATMD system subjected to initial condition loading as well as two different types of external excitations. © 1997 by John Wiley & Sons, Ltd.     

Unravelling the conundrum of river response to rising sea‐level from laboratory to field. Part I: Laboratory experiments

The most recent deglaciation resulted in a global sea‐level rise of some 120 m over approximately 12 000 years. In this Part I of two parts, a moving boundary numerical model is developed to predict the response of rivers to this rise. The model was motivated by experiments at small scale, which have identified two modes describing the transgression of a river mouth: autoretreat without abandonment of the river delta (no sediment starvation at the topset–foreset break) and sediment‐starved autoretreat with abandonment of the delta. In the latter case, transgression is far more rapid and its effects are felt much further upstream of the river mouth. The moving boundary numerical model is checked against experiments. The generally favourable results of the check motivate adaptation of the model to describe the response of the much larger Fly‐Strickland River system, Papua New Guinea to Holocene sea‐level rise; this is done in the companion paper, Part II.     

Submerged reefal deposits near a present‐day northern limit of coral reef formation in the northern Ryukyu Island Arc,northwestern Pacific Ocean

High‐resolution single‐channel seismic reflection surveys were carried out in the northern Ryukyu Island Arc during an NT05‐14 cruise of the R/V Natsushima. The survey area is located northeast of Amami Oshima and southwest of Kikai Shima immediately south of the current northern limit of coral reef formation in the northwestern Pacific Ocean. The main purpose of the surveys was to ascertain whether coral reefs formed during glacial periods and, if so, to determine their three‐dimensional distribution. We collected 784 km of high‐resolution single‐channel seismic data during the cruise. The acoustic basement is clearly observed under the Amami Spur, off Amami Oshima. The seismic profiles show two anticlines, each with an axis trending northeast to southwest. Thin Quaternary stratified sediment overlies the acoustic basement in the northwestern and southeastern slope areas, as well as between the anticlinal axes. The stratified sediment can be divided into several sedimentary units. The mound‐shaped reflections are found within the stratified sedimentary units in the central to eastern parts of the spur. Because the mound‐shaped reflections are characterized by strong reflections and chaotic internal structures in profiles, they are considered biogenic ‘reefs’ or banks consisting of coarse‐grained bioclasts. They overlie the acoustic basement or stratified sediment and reach 15 m in thickness and 400 m in width. In contrast, irregularly shaped topographic highs were detected at the shelf edge southwest off Kikai Shima, which are likely to be remnants of coral reefs formed during the last glacial period. Our seismic data indicate probable coral reef formation at low stands during glacial stages, such as the Last Glacial Maximum, even in the northern Central Ryukyus. The occurrence of drowned reefs may indicate that their growth rate was not sufficient to keep up with a rapid rise in sealevel after a glacial period.     

Wavy lamination in a mixed sand and gravel foreshore facies of the Pleistocene Hosoya Sandstone, Aichi, central Japan

Although sandy foreshore facies are generally characterized by parallel lamination, wavy lamination is predominant in the mixed sand and gravel foreshore facies of the Pleistocene Hosoya Sandstone, which crops out along the Pacific coast of the Atsumi Peninsula, Aichi, central Japan. The foreshore facies consists of three sedimentary subfacies; interbeds of gravel and parallel laminated sand of the lower foreshore facies, parallel laminated fine to medium sand beds containing scattered pebbles and cobbles of the middle foreshore facies, and wavy laminated fine to medium sand beds containing scattered pebbles and cobbles of the upper foreshore facies. A lack of erosional surfaces in the middle foreshore facies indicates the continuous accumulation of sand in flat beds under upper plane bed flow. The wavy laminated sands of the upper foreshore facies exhibit erosional surfaces indicative of repeated deposition and erosion. The erosional surfaces are undulatory, with depressions (10 cm wide and 3 cm deep) that contain scattered pebbles and cobbles. These depressions reflect backwash erosion of sand around and below the pebbles and cobbles. Sand draping over the undulating erosional surfaces forms the wavy lamination. The wavy laminated sand with scattered pebbles and cobbles is a key facies of an upper foreshore or swash zone, and is a good sea-level marker.     

Historical climate changes in southern Xinjiang

Based on the synthetic researches of multi-index geologic records of Niya section, which are of high resolution in southern margin of the Tarim Basin, together with other geologic records in southern Xinjiang, this paper has reconstructed the history of paleoclimatic changes in this region since about 4.00 ka BP. During the last 4.00 ka, the region of southern Xinjiang has experienced alternations of relative cold-wet and relative warm-dry periods. Three remarkable cold-wet periods (4.00-3.45 ka BP., 2.50-1.90 ka BP., ca.1.40-1.00 ka BP.) and three warm-dry periods (3.45-2.50 ka BP., 1.90-1.40 ka BP., 1.00 ka BP.-present) are identified. It is shown that human activities have an intimate relation with the evolution of paleoclimate in southern Xinjiang.