Abyssal current along the northern periphery of Shikoku Basin

Abyssal currents along the northern periphery of the Shikoku Basin south of Japan were measured by current meters moored off Cape Daio-zaki, Cape Shiono-misaki and Cape Ashizuri-misaki and on the eastern foot of the northernmost part of the Kyushu-Palau Ridge. Total length of observation off Cape Shiono-misaki was about five years including the periods of the Kuroshio large meander and no meander. Analyses of current data show:

1. Mean currents with a magnitude of 5–10 cm sec^?1 were observed during the whole observation period at all of current meters which were set 400 m above the sea bottom that was deeper than 4,500 m. The mean current for each current meter was parallel to the local bottom contour arond each station and was toward a direction looking the Nankai Trough (a trough located along the northern end of the Shikoku Basin) to the left.
2. At each station located above the shelf toe off Cape Daio-zaki and off Cape Shiono-misaki and on the foot of the Kyushu-Palau Ridge, the mean current increases with depth (a bottomward intensification of the mean current), and the vertical extent of the mean current is estimated to be about 2,000 m above the sea bottom.
3. At a station located at 2,600 m depth on the continental slope off Cape Shiono-misaki, no bottom-ward intensification of currents was observed.

These results strongly suggest that a steady abyssal flow exists in the depths deeper than about 3,000 m along the northern and northwestern peripheries of the Shikoku Basin. Existence of the abyssal circulation system is also suggested, at least, in the north of the Shikoku Basin.     

A Preliminary Study of Oceanic Bomb Radiocarbon Inventory in the North Pacific during the Last Two Decades

Radiocarbon and total carbonate data were obtained near the 1973 GEOSECS stations in the North Pacific along 30°N and along 175°E between 1993 and 1994. In these stations, we estimated radiocarbon originating from atomic bomb tests using tritium, trichlorofluoromethane and silicate contents. The average penetration depth of bomb radiocarbon during the two decades has deepened from 900 m to 1300 m. Bomb radiocarbon inventories above the average value for the whole North Pacific were found widely in the western subtropical region around 30°N both in the 1970s and 1990s, and its area in the 1990s was broader than that in the 1970s. In most of the North Pacific, while the bomb radiocarbon has decreased above 25.4, the bomb radiocarbon flux below 25.4 was over 1 × 10¹² atom m^-2yr^-1 in the subtropical region around 30°N. In the tropical area south of 20°N, the bomb radiocarbon inventory below 25.4 increased from zero to over 10 × 10¹² atom m^-2 during the last three decades. These distributions suggest that the bomb radiocarbon removed from the surface is currently accumulated with bomb ¹⁴C flux of over 1 × 10¹² atom m^-2yr^-1 below 25.4 in the subtropical region, mainly by advection from the higher latitude, and that part of the accumulated bomb ¹⁴C gradually spread southward with about 30 years.     

Hydrographic structure in association with deep boundary current in the north of the Shikoku Basin

High quality CTD data were collected in the north of the Shikoku Basin where an abyssal boundary current has been observed through direct current measurements. Analyses of hydrographic data showed:

1. Colder and saltier water (heavier water) compared to surrounding waters is found above the continental shelf-toe and the eastern flank of the Kyushu-Palau Ridge where the existence of the abyssal boundary current has been expected. The heavier water has a horizontal extent of about 50 km.
2. The heavier water has the vertical scale of 2000 m from the sea bottom, and is associated with a thermal wind shear which enhances a component of the flow toward a direction looking the Nankai Trough (a trough located along the northern end of the Shikoku Basin) to the left in the abyss. The assumed “level of no motion” at about 2500 m depth gives the geostrophically estimated current in a good agreement with the directly measured current.

A volume transport associated with the colder and higher salinity water is estimated to be about 2 Sv off Cape Shiono-misaki which may include a recirculation above the Nankai Trough. This is about twice of the transport estimated in the interior of the Shikoku Basin through a vorticity balance between the stretching term and latitudinal variation of the planetary vorticity.     

On the distribution of isothermal layers in the vicinity of the shoal,Kokushô-sone

Data from XBT observations in the vicinity of the shoal, Kokushô-sone (30°00′N, 128°30′E) which is located in the current zone of the Kuroshio in the East China Sea, are analysed to show the distribution of isothermal layers. Isothermal layers are found in abundance in and near a cold water region along the slope of the shoal where the existence of upwelling is suggested. It is found that there is a good spatial correlation between the distribution of isothermal layers and that of cold waters along the slope, and that the distribution of isothermal layers is a helpful indicator for elucidating the nature of oceanic structures in the vicinity of the shoal.     

Diamond-bearing and diamond-free metacarbonate rocks from Kumdy–Kol in the Kokchetav Massif, northern Kazakhstan

Abstract Dolomite marble from the Kumdy–Kol area of the Kokchetav Massif contains abundant microdiamond, mainly in garnet and a few in diopside. The mineral assemblage at peak metamorphic condition consists of dolomite + diopside + garnet (+ aragonite) ± diamond. Inclusions of very low MgCO₃ calcite and almost pure calcite occur in diopside and are interpreted as aragonite and/or aragonite + dolomite. Single-phase Mg–calcite in diopside with a very high MgCO₃ component (up to 21.7 mol%) was also found in diamond-free dolomitic marble, and is interpreted as a retrograde product from aragonite + dolomite to Mg–calcite. The dolomite stability constrains the maximum pressure (P) at < 7 GPa using previous experimental data, whereas the occurrence of diamond yields the minimum peak pressure–temperature (P–T) condition at 4.2 GPa and 980 °C at X co ₂ = 0.1. The highest MgCO₃ in Mg–calcite constrains the minimum P–T condition higher than 2.5 GPa and 800 °C for the exhumation stage. As these marbles were subjected to nearly identical P–T metamorphic conditions, the appearance of diamond in some carbonate rocks was explained by high X co ₂. A low X co ₂ condition refers to high oxidized conditions and diamond (and/or graphite) becomes unstable. Difference in X co ₂ for marble from the same area suggests local heterogeneity of fluid compositions during ultrahigh-pressure metamorphism.     

Characteristics of deep currents off Cape Shiono-misaki before and after formation of the large meander of the Kuroshio in 1981

Deep currents measured by moored current meters over the shelf-slope off Cape Shiono-misaki, Kii Peninsula during the period from 28 April, 1981 to 4 May, 1982 are analyzed to determine characteristics of the deep current before and after the large meander of the Kuroshio formed. The observed deep currents show some different characteristics between the periods before and after the formation of the large meander of the Kuroshio,i.e.:

1. The mean current direction over the shelf slope changed to westward after the meander was formed, though it was eastward at two offshore stations before the meander was formed.
2. The eddy kinetic energy, \(ke((\overline {u'^2 } + \overline {\upsilon '^2 } )/2)\) became large at all stations after the meander formed.
3. It appears that there were current variations in the period band shorter than 10 days which propagated offshore before the meander formed but inshore after the meander formed.
4. After the meander formed, the current variations with a period of O(25 days) were amplified at two of the three stations. The current variations in this period band showed high coherence among the three stations.

Data from tidal stations showed that sea level variations with a period of O(30 days) were also amplified along the south coast of Japan after the meander was formed. But sea level variations were not coherent with current variations in this period band.     

Cold water areas in the vicinity of the shoal,Kokushô-sone

By using data obtained at about 120 XBT stations, cold water regions in the vicinity of the shoal, Kokushô-sone (30°00N, 128°30E), which is located in the current zone of the Kuroshio in the East China Sea, were investigated.The temperature cross-sections obtained were compared with corresponding cross-sections obtained from the four former cruises which were already reported. On the present cruise forced upwelling area was found along the south slope of the shoal, instead of the north slope as was found on the former cruises.The area of the cold water region found along the south slope tends to decrease with decrease in depth, and at depths shallower than 250 m the cold water region extends northward passing the shoal. The area at a depth of 400 m is comparable to that of the shoal itself, and is about 35 km².Physical parameters and their scales which seem to be related to the dynamics near the shoal are given in the Appendix.     

The latest batch-to-batch difference table of standard seawater and its application to the WOCE onetime sections

An updated batch-to-batch difference table of IAPSO standard seawater (SSW) up to P145 is proposed. The batch-to-batch difference table is based on several recent SSW comparison experiments, including the experiments conducted independently at the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) and Woods Hole Institute of Oceanography (WHOI) at about the same time using the same procedure. Proposed batch-to-batch differences range from 1.2 × 10⁻³ to −1.9 × 10⁻³ with reference to the average of those from P91 to P102. Batch-to-batch differences from P29 to P145 with reference to the recent batches and this average over every 5 years since 1960 are also presented, together with standard deviation. This reveals that inconsistency among batches has improved since 1980s. In particular, the standard deviation was 0.3 × 10⁻³ in this decade, which is about one-half the value reported previously and almost equal to the modern measurement precision (0.2 × 10⁻³) and is within-batch difference (<0.3 × 10⁻³). Proposed batch-to-batch differences were applied to the observational results of the WOCE hydrographic onetime section (WHP onetime) in the Indian Ocean. Average absolute salinity differences at 14 crossover points in the Indian Ocean were slightly larger, from 1.2 × 10⁻³ to 1.5 × 10⁻³, when the batch-to-batch difference table was applied; however, when results from the Indian, Pacific, and Atlantic Oceans were combined, application of the batch-to-batch difference table yielded statistically acceptable salinity differences. The table was also applied to WHP sections P1 and P17 (revisited about 10 years after the original observations during the WOCE period) and sections I1, I7, and I8 (visited twice by different research vessels in the same year). In all cases, the table corrected unrealistically large salinity changes in space and time. The results suggest that the application of the batch-to-batch table to well-controlled salinity data such as WOCE datasets would be effective in making the datasets more consistent in space and time.