Numerical study of tide-induced mixing over rough bathymetry in the abyssal ocean

Locally enhanced turbulent mixing over rough bottom bathymetry is one of the candidates that might make up for the lack of diapycnal diffusivity in maintaining the global overturning circulation. In the present study, using a two-dimensional vertical numerical model for the Brazil Basin, we numerically examine the intensity and vertical structure of tide-induced mixing over multi-beam bottom bathymetry via the comparison with those over somewhat smoothed bottom bathymetry. Note that even this smoothed bottom bathymetry is finer than in commonly used datasets. In comparison to the response over the smoothed bottom bathymetry, energy dissipation rates are enhanced within a few hundred meters over the multi-beam bottom bathymetry. In spite of several limitations of the two-dimensional vertical numerical model, the magnitude and vertical distribution of the calculated dissipation rates agree well with those from microstructure measurements. We find that tidal interaction with fine-scale (≤2 km) bottom bathymetry efficiently generates high wavenumber internal waves, which are subject to local energy dissipation and hence strongly control the abyssal mixing; the most important finding is that the intensity and vertical decay scale of abyssal mixing are in a trade-off relationship with each other, which is not taken into account in the existing parameterizations.     

Assessment of turbulence closure models for resonant inertial response in the oceanic mixed layer using a large eddy simulation model

Large eddy simulation (LES) of the resonant inertial response of the upper ocean to strong wind forcing is carried out; the results are used to evaluate the performance of each of the two second-order turbulence closure models presented by Mellor and Yamada (Rev Geophys Space Phys 20:851–875, 1982) (MY) and by Nakanishi and Niino (J Meteorol Soc Jpn 87:895–912, 2009) (NN). The major difference between MY and NN is in the formulation of the stability functions and the turbulent length scale, both strongly linked with turbulent fluxes; in particular, the turbulent length scale in NN, unlike that in MY, is allowed to decrease with increasing density stratification. We find that MY underestimates and NN overestimates the development of mixed layer features, for example, the strong entrainment at the base of the oceanic mixed layer and the accompanying decrease of sea surface temperature. Considering that the stability functions in NN perform better than those in MY in reproducing the vertical structure of turbulent heat flux, we slightly modify NN to find that the discrepancy between LES and NN can be reduced by more strongly restricting the turbulent length scale with increasing density stratification.     

Strong tidal currents observed near the bottom in the Suruga Trough,central Japan

Current measurements carried out at the depth of 4 m above the sea bottom near the northern edge of the Suruga Trough in the early fall of 1985 indicated the existence of strong semidiurnal tidal currents, which were considered to be associated with internal tides. In order to examine the spatial structure of the bottom intensified tidal flow, more detailed current observations were carried out at three or four depths at two stations along the main axis of the Suruga Trough during about 70 days from August to October 1988. We obtained the following results: (1) the variations of the current velocity caused by the semidiurnal and diurnal internal tides are evident in all of the records, and the orientation of the major axis of each tidal ellipse nearly coincides with that of the main axis of the trough; (2) the semidiurnal internal tide is dominant over the diurnal internal tide at 4 m above the sea bottom at both stations; (3) at the northern station the semidiurnal internal tide is dominant over the diurnal internal tide, whereas they are nearly equal at the southern station except at 4 m above the sea bottom; (4) the biharmonic internal tides with 1/3 day and 1/4 day periods, are found near the sea bottom and the major axis of the tidal ellipse is perpendicular to the orientation of the main axis of the Suruga Trough.     

The processes of semi-enclosed basin–ocean water exchange across a tidal mixing zone

To assess each of the two possible mechanisms responsible for the fortnightly modulation of semi-enclosed basin–ocean water exchange (‘density tides’), a set of numerical experiments is carried out using a vertically two-dimensional numerical model with realistic situations in Puget Sound in mind. It is found that, although the localized vertical viscosity (or the localized vertical diffusivity) enhanced in the entrance sill region primarily controls the bottom-water transport (or the bottom-water density) during spring tides, it does not lead to any appreciable variations of bottom-water density (or bottom-water transport). This indicates that the fortnightly modulation of vertical viscosity and that of vertical diffusivity both play important roles in creating density tides. In the real ocean, the vertical viscosity and diffusivity are enhanced simultaneously during spring tides, so that it is difficult to discriminate between both effects on density tides. This causes the widespread misunderstanding that density tides are mainly caused by the decreased advection of dense bottom-water from outside due to the enhanced vertical viscosity during spring tides.     

Upper Silurian and Devonian pelagic deep-water radiolarian chert from the Khangai–Khentei belt of Central Mongolia: Evidence for Middle Paleozoic subduction–accretion activity in the Central Asian Orogenic Belt

Recent mapping projects undertaken in Central Mongolia have revealed the widespread occurrence of radiolarian chert within a Paleozoic accretionary complex. We present the results of the first detailed tectonostratigraphic and radiolarian biostratigraphic investigations of the Gorkhi Formation in the Khangai–Khentei belt of the Central Asian Orogenic Belt.The Gorkhi Formation consists of sandstone shale, alternating sandstone and shale of turbidite affinity and chert with small amounts of siliceous shale, basalt, limestone, and clast-bearing mudstone. Radiolarian chert that is completely devoid of terrigenous clastic material is commonly associated with underlying basalt (sedimentary contact) and with conformably overlying siliceous shale and turbidite deposits. The tectonic stacking of basalt–chert and chert–turbidite successions is the most remarkable structural feature of the formation.The recovery of moderately well-preserved radiolarians and conodonts from red chert led to the recognition of four radiolarian assemblages that have a combined age range from the latest Silurian (Pridolian) to the Late Devonian (Frasnian). No age control exists for the siliceous shale, shale, and sandstone, although they are considered to be latest Devonian or slightly younger on the basis of stratigraphic relationships with underlying chert.The Gorkhi Formation has previously been interpreted as a thick sedimentary basin deposit overlying an unexposed Archean–Neoproterozoic basement; however, the stratigraphy within individual tectonic slices clearly corresponds to that of an ocean plate stratigraphy of an accretionary complex generated by the trenchward movement of an oceanic plate. From the lowermost to uppermost units, the stratigraphy comprises ocean floor basalt, pelagic deep-water radiolarian chert, hemipelagic siliceous shale, and terrigenous turbidite deposits. The biostratigraphic data obtained in the present study provide corroborating evidence for the existence of an extensive deep-water ocean that enabled the continuous sedimentation of pelagic chert over a period of nearly 50 million years. These data, together with structural data characterized by tectonic repetition of the stratigraphy, indicate that these rocks formed as an accretionary wedge along an active continental margin, possibly that of the Angara Craton. The mid-oceanic chert was probably deposited in the Northern Hemisphere portion of the Paleo–Pacific Ocean that faced the Angara Craton and the North China–Tarim blocks. Thus, we propose that subduction–accretion processes along the Paleo–Pacific rim played an important role in the accretionary growth of the active continental margin of the Angara Craton, directly influencing the evolution of the Central Asian Orogenic Belt.     

Internal Wave Generation by Tidal Flow over a Continental Shelf Slope

The generation process of internal waves by strong tidal flow over a continental shelf slope is reproduced using a multi-level numerical model. On the basis of the numerical results, the crucial role of the tidal advection effect in the generation process of internal waves is demonstrated. The close relation between the resulting internal waveform and the strength of the tidal advection effect is also examined. The barotropic forcing on the internal wave actually works within a relatively small horizontal scale over the top of the continental shelf slope. When the maximum internal Froude number at the shelf break (Fr_m) is less than about 0.6, the amplitude of the resulting internal wave is almost proportional to F_rm. When F_rm is more than about 0.6, however, the amplitude of the resulting internal wave becomes larger than predicted by linear theory. In particular, when F_rm is more than unity, the time period during which the shoreward propagating internal wave stays in the barotropic forcing region becomes much longer. Consequently, the internal wave is significantly amplified with the horizontal scale approaching that of the barotropic forcing, which concentrates in a relatively small region over the top of the continental shelf slope. This revised version was published online in July 2006 with corrections to the Cover Date.     

Biological productivity, terrigenous influence and noncrustal elements supply to the Central Indian Ocean Basin: Paleoceanography during the past ∼ 1 Ma

A 2m-long sediment core from the siliceous ooze domain in the Central Indian Ocean Basin (CIOB; 13‡03′S: 74‡44′E; water depth 5099m) is studied for calcium carbonate, total organic carbon, total nitrogen, biogenic opal, major and few trace elements (Al, Ti, Fe, K, Mg, Zr, Sc,V, Mn, Cu, Ni, Zn, Co, and Ba) to understand the productivity and intensity of terrigenous supply. The age model of the sediment core is based on U-Th dating, occurrence of Youngest Toba Tuff of ∼ 74 ka and Australasian microtektites of ∼ 770 ka. Low carbonate content (< 1%) of sediment core indicates deposition below the carbonate compensation depth. Organic carbon content is also very low, almost uniform (mean 0.2 wt%) and is of marine origin. This suggests a well-oxygenated bottom water environment during the past ∼ 1100ka. Our data suggest that during ∼ 1100 ka and ∼ 400 ka siliceous productivity was lower, complimented by higher supply of terrigenous material mostly derived from the metasedimentary rocks of High Himalayan crystalline. However, during the last ∼ 400 ka, siliceous productivity increased with substantial reduction in the terrigenous sediment supply. The results suggest that intensity of Himalayan weathering, erosion associated with monsoons was comparatively higher prior to 400 ka. Manganese, Ba, Cu, Ni, Zn, and Co have around 90% of their supply from noncrustal (excess) source and their burial to seafloor remained unaffected throughout the past ∼ 1100 ka.     

Horizontal and vertical distributions of <Superscript>137</Superscript>Cs in seabed sediments around the river mouth near Fukushima Daiichi Nuclear Power Plant

Investigations including a bathymetric survey, sonic prospecting, and vibrocoring were performed to understand the horizontal and vertical distribution of ¹³⁷Cs in seabed sediments in shallow seas with depths less than 30 m near the Fukushima Daiichi Nuclear Power Plant. Especially, features of ¹³⁷Cs distributions in deeper sections of the seabed sediments were studied to evaluate the vertical heterogeneity of ¹³⁷Cs distribution in the seabed sediments in shallow seas. The distribution area of the seabed sediments was less than half of the investigation area, and the locations of the seabed sediments were divided into flat and terrace-like seafloors based on their topographical features. The thicknesses of the seabed sediment layers were mostly <2 m. The ¹³⁷Cs inventories in the seabed sediments varied from 13 ± 1 to 3,510 ± 26 kBq m^?2, and continuous distributions of ¹³⁷Cs at depths greater than 81 cm were observed. The ¹³⁷Cs distributions were not uniform; however, the ¹³⁷Cs inventories tended to be larger near the base of the steeper ascending slopes than in the terrace-like seafloors themselves. Based on the relationship between the ¹³⁷Cs inventories and mean shear stress, features of the seafloor topography were inferred to be significant control factors governing the horizontal and vertical distribution of ¹³⁷Cs in the seabed sediments. Rapid changes and multiple peaks in the vertical profile of the ¹³⁷Cs distributions suggest that they are related to pulse input caused by heavy-rain events. Change in the ¹³⁷Cs inventories with depth in this study are larger than those reported in previous studies, indicating earlier results of ¹³⁷Cs inventories per unit in seabed sediments in shallow seas, especially near the river mouth, which drains a radiologically highly-contaminated basin, were underestimated.