Masaaki Wakatsuchi

Reconstruction of paleoproductivity in the Sea of Okhotsk over the last 30 kyr

[1] Marine- and terrestrial-derived biomarkers (alkenones, brassicasterol, dinosterol, and long-chain n-alkanes), as well as carbonate, biogenic opal, and ice-rafted debris (IRD), were measured in two sediment cores in the Sea of Okhotsk, which is located in the northwestern Pacific rim and characterized by high primary productivity. Down-core profiles of phytoplankton markers suggest that primary productivity abruptly increased during the global Meltwater Pulse events 1A (about 14 ka) and 1B (about 11 ka) and stayed high in the Holocene. Spatial and temporal distributions of the phytoplankton productivity were found to be consistent with changes in the reconstructed sea ice distribution on the basis of the IRD. This demonstrates that the progress and retreat of sea ice regulated primary productivity in the Sea of Okhotsk with minimum productivity during the glacial period. The mass accumulation rates of alkenones, CaCO3, and biogenic opal indicate that the dominant phytoplankton species during deglaciation was the coccolithophorid, Emiliania huxleyi, which was replaced by diatoms in the late Holocene. Such a phytoplankton succession was probably caused by an increase in silicate supply to the euphotic layer, possibly associated with a change in surface hydrography and/or linked to enhanced upwelling of North Pacific Deep Water. INDEX TERMS: 1050 Geochemistry: Marine geochemistry (4835, 4850); 1055 Geochemistry: Organic geochemistry; 4267 Oceanography: General: Paleoceanography; KEYWORDS: Okhotsk Sea, paleoproductivity, sediment Citation: Seki, O., M. Ikehara, K. Kawamura, T. Nakatsuka, K. Ohnishi, M. Wakatsuchi, H. Narita, and T. Sakamoto (2004), Reconstruction of paleoproductivity in the Sea of Okhotsk over the last 30 kyr, Paleoceanography, 19, PA1016,

Formation of 26.8–26.9 σθ water in the Kuril Basin of the Sea of Okhotsk as a possible origin of North Pacific Intermediate Water

By using all Japanese hydrographic data obtained south of 55°N in latitude in the Sea of Okhotsk, we compile a data set for this study. Our investigations with these data show that a large quantity of relatively cold, fresh, oxygen-rich homogeneous intermediate water with a density range of 26.8 to 26.9 σθ exists in the Kuril Basin; we name this intermediate water the Kuril Basin Intermediate Water (KBIW). Results of isosteric analyses show that a possible origin of KBIW is located in a region off the Hokkaido coast in the southwestern Sea of Okhotsk. We suggest that KBIW is produced by the isopycnal mixing of relatively dense Soya Warm Water, which appears only in early spring, with the colder, fresher water which originates from the northwestern Okhotsk. Finally, we demonstrate that KBIW may be a source water of the North Pacific Intermediate Water (NPIW) because it has the same density range and water properties.

Relationship between sea‐ice motion and geostrophic wind in the northern hemisphere

This paper presents maps showing the relationship between sea-ice motion derived from Defense Meteorological Satellite Program (DMSP) Special Sensor Microwave Imager (SSM/I) imagery and the geostrophic wind in the Northern Hemisphere for seven winters (1991/92–1997/98). The sea-ice motion is highly correlated with the geostrophic wind except for some coastal regions. Overall the direction of the ice motion is nearly parallel to the wind. An obvious contrast between seasonal ice zones and the interior of the Arctic is observed in the speed reduction factor (ratio of ice speed to wind speed), which is probably caused by the spatial variation of internal ice stress. Surface ocean currents are also derived by subtracting the wind effect from the ice motion.

Processes controlling the advance and retreat of sea ice in the Sea of Okhotsk

Using Defense Meteorological Satellite Program (DMSP) special sensor microwave imager (SSM/I) sea-ice data and meteorological data, we investigate processes controlling the advance and retreat of sea ice in the Sea of Okhotsk. For this investigation we apply a new algorithm to the SSM/I data, which is capable of calculating ice concentration and distinguishing sea-ice types; this algorithm is confirmed by comparison with National Oceanic and Atmospheric Administration (NOAA) advanced very high resolution radiometer (AVHRR) imagery. Our investigations show that there exists a locality for the areas of new-ice formation, and the ice formation near the ice edge always occurs as a recovery after sea ice has retreated suddenly. We also reveal that the speed of ice-edge advance fairly coincides with that predicted by 2% of the geostrophic wind. Furthermore, there exists a good correspondence between the times when the overall advance and retreat of ice cover begin and the decrease and increase of air temperature begin. We conclude that the advance of the ice cover in the Sea of Okhotsk occurs primarily through ice advection by wind and that the interannual variation in the maximum sea-ice extent can be interpreted by that in wind and air temperature conditions.

Radiolarians under the seasonally sea-ice covered conditions in the Okhotsk Sea: flux and their implications for paleoceanography

Abstract Four time-series sediment traps at two stations and three piston cores from the Okhotsk Sea were quantitatively examined for coarse-sized radiolarian shells (>63 μm). Traps were deployed at 300 m and 1550 m at Station M4 (53°01′N, 145°30′E) and at 300 m and 700 m at Station M6 (49°30′N, 146°28′E) during August 1998 through May 2000. The chronologies of the piston cores were established applying δ 18 O and paleomagnetic intensity variations; they provide records extending back to marine isotope stage (MIS) 5.51. The modern and past changes in radiolarian assemblages are associated with environmental and productivity changes. Radiolarian fluxes in the sediment traps exhibited significant summer to autumn flux peaks with suppressed values during the winter when sea-ice covered the sea surface at the trap sites. Total radiolarian accumulation rate (RAR) variations in each core tended to correspond to glacial–interglacial cycles and increased during the last deglaciation. In particular, the temporal RAR variation in Core XP98-PC1 (off Kamchatka) showed a similar trend with the climatic changes expressed by δ 18 O values during the glacial–interglacial cycles for the last 125 kyr. Regional differences were also apparent. RARs showed extremely low values during 12–40 kyr and 63–70 kyr in Core XP98-PC2 (central Okhotsk Sea), indicating the enhanced sea-ice impact. The sea-ice coverage might have continued for a significant part of the year during the intervals since radiolarians did not appear to increase even during the summer to autumn. RAR variations in Core XP98-PC4 (off Sakhalin) showed higher values than XP98-PC2 during MIS 2 and 4. Thus, in eastern Sakhalin around Site XP98-PC4, sea-ice coverage might have been seasonal, but not perennial even during MIS 2 and 4. Among the radiolarian taxa, Cycladophora davisiana was the most abundant species at the lower traps and in all cores. The fluxes of C. davisiana at lower traps showed much higher values than those of upper traps, and also showed similar temporal patterns with the fluxes of aluminum and terrigenous materials. Therefore, C. davisiana fluxes may be associated with terrigenous organic materials. Increased nutrient supply from the continental shelves, presumably transported by seasonal sea-ice and released by sea-ice melting, might be mainly responsible for the high production of C. davisiana in the Okhotsk Sea, especially during the last deglaciation.

Sverdrup Balance and the Cyclonic Gyre in the Sea of Okhotsk

Abstract It is proposed that the cyclonic gyre over the northern half-basin of the Okhotsk Sea is driven by the wind stress curl and that a major part of the East Sakhalin Current (ESC) can be regarded as its western boundary current. Both from the high-resolution ECMWF and Comprehensive Ocean–Atmosphere Dataset (COADS) data, the annual mean wind stress curl is positive over the sea. When the Sverdrup streamfunction is calculated by excluding the shallow shelves, the streamfunction shows a cyclonic pattern over the central basin, which is roughly consistent with the geopotential anomaly distribution from all the available hydrographic data. Profiling floats suggest that the cyclonic gyre extends to at least a depth of 500 m: a relatively intense southward flow (ESC) with an average speed of approximately 10 cm s−1 near the western boundary and slow northward flow with an average speed of approximately 2 cm s−1 in the east. Climatological data show that along zonal sections at 50°–53°N isopycnal surfaces g...

Sediment core profiles of long‐chain n‐alkanes in the Sea of Okhotsk: Enhanced transport of terrestrial organic matter from the last deglaciation to the early Holocene

[1] Terrestrial plant-derived n-alkanes (C25–C35 )w ere measured in three piston cores (PC1, PC2 and PC4) in the Sea of Okhotsk covering the last 30 kyrs. Down core profiles of the n-alkane concentrations and mass accumulation rates (MAR) were characterized by deglacial maxima. In particular, cores PC2 and PC4, which were collected from the central and western Sea of Okhotsk, respectively, show a two-step increase around the Meltwater Pulse events (MWP) 1A (14.5–13.5 kyr BP) and 1B (about 10 kyr BP). This finding was interpreted by the outflow of terrestrial organic matter from the submerged land shelf to the Sea of Okhotsk through the East Sakhalin Current. This study demonstrated that the sea level rise forced by global warming in the deglaciation period may have caused the enhanced transport of terrestrial organic matter in marginal seas. INDEX TERMS: 3022 Marine Geology and Geophysics: Marine sediments—processes and transport; 4267 Oceanography: General: Paleoceanography; 1055 Geochemistry: Organic geochemistry. Citation: Seki, O., K. Kawamura, T. Nakatsuka, K. Ohnishi, M. Ikehara, and M. Wakatsuchi, Sediment core profiles of long-chain n-alkanes in the Sea of Okhotsk: Enhanced transport of terrestrial organic matter from the last deglaciation to the early Holocene, Geophys. Res. Lett. , 30 (1), 1001, doi:10.1029/ 2001GL014464, 2003.

Observations of a street of cyclonic eddies in the Indian Ocean sector of the Antarctic Divergence

Hydrographic and drifting buoy data from Japanese cruises show that the Antarctic Divergence in the Indian Ocean sector is composed of a street of cyclonic eddies. These eddies measure about 500 km in the zonal direction and 200 km in the meridional. Part of the eastward flowing Antarctic Circumpolar Current (ACC) meanders southward in the regions between the eddies. In the eddy regions, warm, saline Circumpolar Deep Water is upwelled into the shallow layers, while cold, dense coastal water advects into the deep layers; the advection occurs along the isobaths of ridges which extend north from the coast. The combination of the advection with the upwelling produces a water column denser than the surrounding water and leads to the formation and maintenance of the cyclonic eddies. Presence of the northward extending ridges approximately governs the location of eddy formation. The eddy formation recurs year after year, although eddy locations can vary somewhat. A polynya was observed to persistently occur and corresponded with one of the eddies in location, size, and form. The oceanographic observations also suggest that the primary meridional exchanges of heat and salt in the Antarctic are caused through the eddies and ACC meanders within the Antarctic Divergence.

Winter oceanographic conditions in the southwestern part of the Okhotsk Sea and their relation to sea ice

In the southwestern part of the Okhotsk Sea, oceanographic and sea-ice observations on board the icebreaker Soya were carried out in February 1997. A mixed layer of uniform temperature nearly at the freezing point extending down to a depth of about 300 m was observed. This is much deeper than has previously been reported. It is suggested that this deep mixed layer originated from the north (off East Sakhalin), being advected along the shelf slope via the East Sakhalin Current, accompanied with the thick first-year ice (average thickness 0.6 m). This vertically uniform winter water, through mixing with the surrounding water, makes the surface water more saline (losing a characteristic of East Sakhalin Current Water) and the water in the 100–300 m depth zone less saline, colder, and richer in oxygen (a characteristic of the intermediate Okhotsk Sea water). The oceanographic structure and a heat budget analysis suggest that new ice zone, which often appears at ice edges, can be formed through preconditioning of thick ice advection and subsequent cooling by the latent heat release due to its melting.

Mechanisms for the variation of sea ice extent in the Northern Hemisphere

Using daily sea ice data derived from satellite-borne sensors and atmospheric data, we examined processes controlling the variation of sea ice extent in the Northern Hemisphere. The daily ice motion field was computed from imagery of the Defense Meteorological Satellite Program (DMSP) Special Sensor Microwave Imager (SSM/I) for seven winters (December to April) from 1991/1992 to 1997/1998, by employing the maximum cross-correlation method. In order to examine mechanisms of the temporal variation of the ice extent, we analyzed 50 specified lines across the daily ice edge. Although a high correlation between the ice motion and the geostrophic wind speed was observed in all the ice edge areas, the degree for correlation between the speed of the ice edge displacement and the wind speed varied with region. The degree for response of the ice edge speed to the wind speed largely depended upon that of the ice edge speed to the ice motion. The following mechanisms controlling the variation of ice extent for regions in the Northern Hemisphere were anticipated. In the Barents Sea, Bering Sea, and the Sea of Okhotsk the ice extent advances by wind-driven ice advection and the daily scale variation of the ice extent were also controlled by the variation in wind speed. In contrast, the ice extent in the Labrador Sea and the Greenland Sea seemed to be considerably affected by oceanographic factors such as the location of the thermal front and was not related to the variation in wind speed. The regional difference of the variation mechanism was also reflected in the interannual variation in maximum ice extent.