Analbite → monalbite transition in a heat treated twinned amelia albite

Amelia albite annealed at > 1080 °C for 3200 hrs by Duba and Piwinskii (1974) shows very fine twin lamellae (～1 μm) after the albite law, suggesting that it once underwent transformation into monalbite. A fragment of this specimen was investigated at 27 °C, 300 °C, 550 °C, 800 °C and 930 °C using the high-temperature precession technique. As the temperature increases, the splitting angle of c ^*-axes (likewise c ^*-axes) of two twin individuals continues to decrease. The photographs taken at 930 °C show that these two splitting angles have converged to 0^o, indicating completion of the transformation into monalbite. The transition point we observe supports the results of MacKenzie (1952) (920±20 °C) and Grundy et al. (1967) (930 °C) rather than those of Sueno et al. (1973) and Prewitt et al. (1974) (> 1080 °C); the discrepancy is most likely due to the differences in the degree of Al-Si disorder of the samples used in the experiments.     

Water masses and currents of deep circulation southwest of the Shatsky Rise in the western North Pacific

We conducted full-depth hydrographic observations in the southwestern region of the Northwest Pacific Basin in September 2004 and November 2005. Deep-circulation currents crossed the observation line between the East Mariana Ridge and the Shatsky Rise, carrying Lower Circumpolar Deep Water westward in the lower deep layer (θ<1.2 °C) and Upper Circumpolar Deep Water (UCDW) and North Pacific Deep Water (NPDW) eastward in the upper deep layer (1.3–2.2 °C). In the lower deep layer at depths greater than approximately 3500 m, the eastern branch current of the deep circulation was located south of the Shatsky Rise at 30°24′–30°59′N with volume transport of 3.9 Sv (1 Sv=10⁶ m³ s⁻¹) in 2004 and at 30°06′–31°15′N with 1.6 Sv in 2005. The western branch current of the deep circulation was located north of the Ogasawara Plateau at 26°27′–27°03′N with almost 2.1 Sv in 2004 and at 26°27′–26°45′N with 2.7 Sv in 2005. Integrating past and present results, volume transport southwest of the Shatsky Rise is concluded to be a little less than 4 Sv for the eastern branch current and a little more than 2 Sv for the western branch current. In the upper deep layer at depths of approximately 2000–3500 m, UCDW and NPDW, characterized by high and low dissolved oxygen, respectively, were carried eastward at the observation line by the return flow of the deep circulation composing meridional overturning circulation. UCDW was confined between the East Mariana Ridge and the Ogasawara Plateau (22°03′–25°33′N) in 2004, whereas it extended to 26°45′N north of the Ogasawara Plateau in 2005. NPDW existed over the foot and slope of the Shatsky Rise from 29°48′N in 2004 and 30°06′N in 2005 to at least 32°30′N at the top of the Shatsky Rise. Volume transport of UCDW was estimated to be 4.6 Sv in 2004, whereas that of NPDW was 1.4 Sv in 2004 and 2.6 Sv in 2005, although the values for NPDW may be slightly underestimated, because they do not include the component north of the top of the Shatsky Rise. Volume transport of UCDW and NPDW southwest of the Shatsky Rise is concluded to be approximately 5 and 3 Sv, respectively. The pathways of UCDW and NPDW are new findings and suggest a correction for the past view of the deep circulation in the Pacific Ocean.     

Gravelly spit deposits in a transgressive systems tract: the Pleistocene Higashikanbe Gravel, central Japan

The Pleistocene Higashikanbe Gravel, which crops out along the Pacific coast of the Atsumi Peninsula, central Japan, consists of well‐sorted, pebble‐ to cobble‐size gravel beds with minor sand beds. The gravel includes large‐scale foreset beds (5–10 m high) and overlying subhorizontal beds (0·5–3 m thick), showing foreset and topset structure, from which the gravel has previously been interpreted as deposits of a Gilbert‐type delta. However, (1) the gravel beds lack evidence of fluvial activity, such as channels in the subhorizontal beds; (2) the foresets incline palaeolandwards; (3) the gravels fill a fluvially incised valley; and (4) the gravels overlie low‐energy deposits of a restricted environment, such as a bay or an estuary. The foresets generally dip towards the inferred palaeoshoreline, indicating landward accretion of gravel. Reconstruction of the palaeogeography of the peninsula indicates that the Higashikanbe Gravel was deposited as a spit similar to that developed at the western tip of the present Atsumi Peninsula, rather than as a delta. According to the new interpretation, the large‐scale foreset beds are deposits on the slopes of spit platforms and accreted in part to the sides of small islets that are fragments of the submerging spit during relative sea‐level rise. The subhorizontal beds include nearshore deposits on the spit platform topsets and deposits of gravel shoals or bars, which are reworked sediments of the spit beach gravels during a transgression. The lack of spit beach facies in the subhorizontal beds results from truncation by shoreface erosion. Dome structure, which is a cross‐sectional profile of a recurved gravel spit at its extreme point, and sandy tidal channel deposits deposited between the small islets were also identified in the Higashikanbe Gravel. The Higashikanbe Gravel fills a fluvially incised valley and occupies a significant part of a transgressive systems tract, suggesting that gravelly spits are likely to be well developed during transgressions. The large‐scale foreset beds and subhorizontal beds of gravelly spits in transgressive systems tracts contrast with the foreset and topset beds of deltas, characteristic of highstand, lowstand and shelf‐margin systems tracts.     

Late Precambrian and Early Cambrian in the East European platform

Carbon suboxide polymers reacted with hydroxylamine and ammonia under UV irradiation in aqueous medium to form amino acids such as glycine, alanine, serine, threonine, aspartic acid and glutamic acid. This finding suggests that carbon suboxide is a new candidate as starting material for the synthesis of biomolecules on the primitive earth.     

Estuarine, barrier-island to strand-plain sequence and related ravinement surface developed during the last interglacial in the Paleo-Tokyo Bay, Japan

Recognition of sequence boundaries and transgressive surfaces (i.e. ravinement surfaces, RS) is now known to be of great importance in stratigraphy. The sedimentary features of deposits immediately above a transgressive surface are well exposed in the Upper Pleistocene Kioroshi Formation of the Kanto Plain in central Japan. The formation comprises mainly coastal and shallow-marine deposits (estuarine, barrier-island and the strand-plain systems) which accumulated along a wavedominated coast in the Late Pleistocene, i.e., the last interglacial to last glacial period. The Kioroshi Formation is bounded above and below by sequence boundaries that formed in the lowstand periods correlative to the glacial periods of oxygen isotope stages 4 and 6, respectively. A significant transgressive surface that was formed by landward migration of barrier islands during the transgressive interval, the ravinement surface (RS), is found within the deposits of the upper shelf environment.

This ravinement surface is characterized by the exotic nature of the overlying sediment veneer (pebbles, shells and scattered mud clasts) which is poorly sorted. The RS shows a very flattened erosional surface in the shore-parallel sense, and the gradient of the surface in shore-normal sense is calculated as 0.0021, where the syndepositional tectonic movement is revised. The RS commonly cuts through the lower sequence boundary. However, in the places where the river or tidal channel valleys incised, the valley-filling sediment shows a deepening-upward sequence recognized as a transgressive systems tract and the RS can be clearly distinguished from the lower sequence boundary.     

Generation of tidal bedding in a circular flume experiment: formation process and preservation potential of mud drapes

Flume experiments aimed to produce flaser bedding were conducted using fine sand and clay in a circular flume. The formation process of mud drapes during the slack-water stage was revealed in detail. When the tidal current declines, a uniform mobile mud layer initially settles from suspension and drapes the entire rippled sand bed (type A mud). When the remaining flow velocity is very low, a more fluid mud begins to settle out (type B mud) that preferentially fills the ripple troughs, the ripples and mud together forming a flat surface. At slack tide, the two-phase mud drape is temporarily stationary. After the onset of the reversed flow phase, most of the type B mud is resuspended, while the type A mud is eroded from the crests, leaving behind a remnant mud drape (flaser) in the troughs that is subsequently buried by migrating ripples. Type B mud generally contains variable amounts of sand derived from eroded ripple crests, but does not show any visible internal sedimentary structures. Type A mud represents the ‘mud drapes’ commonly described in the literature, the temporary existence of type B mud having gone unnoticed because of its low preservation potential. When present, it can be recognized by its sand content and the occurrence of flame structures in ripple troughs. Tidal deposits reflecting the existence and depositional characteristics of both type A and type B mud are found in, for example, the macrotidal Oligocene Ashiya Group, Japan.     

Use of flood chronology for detailed environmental analysis: a case study of Lake Kizaki in the northern Japanese Alps,central Japan

This paper presents a study of the usefulness of flood layers as a time marker in sediments and a report of a case study of Lake Kizaki in central Japan. A flood layer can be identified as a layer having a higher density, coarser grain size, lower TN content, and higher C/N ratio than those of the upper and lower horizons. It can also be characterized by a hyperpycnal sequence composed of a basal coarsening-upward unit and a top fining-upward unit. When flood layers can be correlated with heavy rains in meteorological records, detailed age markers are well established in the sediment. Five flood layers were identified in the surface sediment of Lake Kizaki, and they could be attributed to the historical heavy rainfalls that took place on July 12, 1995; September 28, 1983; August 25, 1974; September 26, 1959; and September 1, 1949 under the constraint of an age model. A precise age model is essential to clarify the environmental changes such as the pollutant history in detail.     

Direct velocity measurements of deep circulation southwest of the Shatsky Rise in the western North Pacific

Direct velocity measurements undertaken using a nine-system mooring array (M1–M9) from 2004 to 2005 and two additional moorings (M7p and M8p) from 2003 to 2004 reveal the spatial and temporal properties of the deep-circulation currents southwest of the Shatsky Rise in the western North Pacific. The western branch of the deep-circulation current flowing northwestward (270–10° T) is detected almost exclusively at M2 (26°15′N), northeast of the Ogasawara Plateau. It has a width less than the 190 km distance between M1 (25°42′N) and M3 (26°48′N). The mean current speed near the bottom at M2 is 3.6±1.3 cm s^?1. The eastern branch of the deep-circulation current is located at the southwestern slope of the Shatsky Rise, flowing northwestward mainly at M8 (30°48′N) on the lower part of the slope of the Shatsky Rise with a mean near-bottom speed of 5.3±1.4 cm s^?1. The eastern branch often expands to M7 (30°19′N) at the foot of the rise with a mean near-bottom speed of 2.8±0.7 cm s^?1 and to M9 (31°13′N) on the middle of the slope of the rise with a speed of 2.5±0.7 cm s^?1 (nearly 4000 m depth); it infrequently expands furthermore to M6 (29°33′N). The width of the eastern branch is 201±70 km on average, exceeding that of the western branch. Temporal variations of the volume transports of the western and eastern branches consist of dominant variations with periods of 3 months and 1 month, varying between almost zero and significant amount; the 3-month-period variations are significantly coherent to each other with a phase lag of about 1 month for the western branch. The almost zero volume transport occurs at intervals of 2–4 months. In the eastern branch, volume transport increases with not only cross-sectional average current velocity but also current width. Because the current meters were too widely spaced to enable accurate estimates of volume transport, mean volume transport is overestimated by a factor of nearly two, yielding values of 4.1±1.2 and 9.8±1.8 Sv (1 Sv=10⁶ m³ s^?1) for the western and eastern branches, respectively. In addition, a northwestward current near the bottom at M4 (27°55′N) shows a marked variation in speed between 0 and 20 cm s^?1 with a period of 45 days. This current may be part of a clockwise eddy around a seamount located immediately east of M4.     

Pathway and variability of deep circulation around 40°N in the northwest Pacific Ocean

To clarify the global deep-water circulation in the northwest Pacific, we conducted current observations with seven moorings at 40°N east of Japan from May 2007 to October 2008, together with hydrographic observations. By analyzing the data, while taking into consideration that the deep circulation has a northward component in this region and carries low-silica, high-dissolved-oxygen water, we clarified that the deep circulation flows within the region between 144°30′ and 146°10′E at 40°N on and east of the eastern slope of the Japan Trench with marked variability; the deep circulation flows partly on the eastern slope of the trench and mainly to the east during P1 (10 May–24 November 2007), is confined to the eastern slope of the trench during P2 (25 November 2007–20 May 2008), and flows on and to the immediate east of the eastern slope of the trench during P3 (21 May–15 October 2008). Previous studies have identified two branches of the deep circulation at lower latitudes in the western North Pacific; one flows off the western trenches and the other detours near the Shatsky Rise. It was thus concluded that the eastern branch flows westward at 38°N and then northward to the east of the trench, finally joining the western branch around 40°N during P1 and P3, whereas the eastern branch passes westward south of 38°N, joins the western branch around 38°N, and flows northward on the eastern slope of the trench during P2.