Takeshi Nakatsuka

Seasonal phytodetritus deposition and responses of bathyal benthic foraminiferal populations in Sagami Bay, Japan: preliminary results from “Project Sagami 1996–1999”

The seasonal carbon cycle was studied in the bathyal environment of Sagami Bay, Japan, to determine whether “benthic–pelagic coupling” takes place in this eutrophic marginal oceanic setting. Both Japanese sea color observation satellite (ADEOS) photography and sediment trap moorings have been used since 1996 for monitoring sea surface primary production. Video records at a real time deep-sea floor observatory off Hatsushima Island in Sagami Bay were also used to monitor the deposition of phytodetritus on the sea floor. At this location, a spring bloom starts in mid-February and ends in mid-April. About 2 weeks after the start of the spring bloom, phytodetrital material is deposited on the sea floor. Video records clearly show that phytodetritus deposition has taken place in the spring of every year since 1994, even though the exact timing is different from year to year. The population size of benthic foraminifera is highly correlated to this phytodetritus deposition. The phytodetritus triggers rapid, opportunistic reproduction of the shallow infaunal taxa, Bolivina pacifica, Stainforthia apertura and Textularia kattegatensis. Shallow infaunal species mainly occur in the phytodetrital layer or just below this layer during the spring. This indicates that such opportunistic species are key indicators of phytodetrital deposition. The deep infaunal taxa Globobulimina affinis and Chilostomella ovoidea show less pronounced seasonal fluctuations in population size, but nevertheless exhibit some response to phytodetrital deposition. Thus the seasonal flux of organic matter is the most important determinant of population size, microhabitats and reproduction of benthic foraminifera in Sagami Bay.

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,

A biomarker approach for assessing marine and terrigenous inputs to the sediments of Sea of Okhotsk for the last 27,000 years

Abstract A sediment core taken from the Sea of Okhotsk, which is located in the western North Pacific rim with an extensive seasonal sea-ice cover, was investigated for lipid class compounds, total organic carbon, total nitrogen, and δ 13 C of bulk organic matter to reconstruct changes in sedimentary regimes of marine and terrestrial organic matter over the last 27 kyrs. Higher molecular weight n -alkanes, fatty acids, and fatty alcohols showed an increase in terrigenous inputs in glacial and, to a greater extent, in early deglacial periods. The enhanced terrestrial input in the deglaciation stages has, for the first time, been found and never been reported in the open ocean. Biomarker profiles, together with the coarse-grain size distribution in nearby sediment cores, suggested that sea-ice rafting was the main transport mechanism of continental material to the Okhotsk Sea sediments. However, the ice-rafting inputs have been weakened from 14 kyrs BP to the Holocene probably due to a rapid reduction of the sea-ice cover and continental shelf erosion. Profiles of C 37 alkenones, phytol, and brassicasterol, as well as temporal variation in calcium carbonate and organic carbon contents, indicated that the marine inputs were enhanced during the deglaciation. The enhanced marine inputs were likely caused by an intensification of the primary production induced by a possibly increased nutrient supply from the continental shelves in early deglaciation, and by the deglacial retreat of the sea-ice cover at 14 kyrs BP. This study demonstrates that biomarkers in the marginal marine sediments had recorded changes in the biogeochemical processes in the Sea of Okhotsk over the last 27 kyrs, which may be significantly affected by changes in the Okhotsk Water exchange processes such as sea-ice activity and sea-ice transport of terrigenous materials.

Tree-ring reconstructed summer temperature anomalies for temperate East Asia since 800 C.E.

We develop a summer temperature reconstruction for temperate East Asia based on a network of annual tree-ring chronologies covering the period 800–1989 C.E. The East Asia reconstruction is the regional average of 585 individual grid point summer temperature reconstructions produced using an ensemble version of point-by-point regression. Statistical calibration and validation tests indicate that the regional average possesses sufficient overall skill to allow it to be used to study the causes of temperature variability and change over the region. The reconstruction suggests a moderately warm early medieval epoch (ca. 850–1050 C.E.), followed by generally cooler ‘Little Ice Age’ conditions (ca. 1350–1880 C.E.) and 20th century warming up to the present time. Since 1990, average temperature has exceeded past warm epochs of comparable duration, but it is not statistically unprecedented. Superposed epoch analysis reveals a volcanic forcing signal in the East Asia summer temperature reconstruction, resulting in pulses of cooler summer conditions that may persist for several years. Substantial uncertainties remain, however, particularly at lower frequencies, thus requiring caution and scientific prudence in the interpretation of this record.

Glacial to interglacial surface nutrient variations of Bering Deep Basins recorded by δ13C and δ15N of sedimentary organic matter

Stable carbon and nitrogen isotopic ratios (δ13C and δ15N) of organic matter were measured in three sediment cores from deep basins of the Bering Sea to investigate past changes in surface nutrient conditions. For surface water reconstructions, hemipelagic layers in the cores were distinguished from turbidite layers (on the basis of their sedimentary structures and 14C ages) and analyzed for isotopic studies. Although δ13C profiles may have been affected by diagenesis, both δ15N and δ13C values showed common positive anomalies during the last deglaciation. We explain these anomalies as reflecting suppressed vertical mixing and low nutrient concentrations in surface waters caused by injection of meltwater from alpine glaciers around the Bering Sea. Appendix tables are available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington , DC 20009. Document P95-003;

Origin and decomposition of sinking particulate organic matter in the deep water column inferred from the vertical distributions of its δ15N, δ13 and δ14

2.50. Payment must accompany order.

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

Sinking particles were analyzed for their nitrogen isotopic ratio δ15N) of total particulate nitrogen (PN), stable carbon isotopic ratio (δ13C) and radioactive isotopic ratio (δ14C) of total particulate organic carbon (POC), at three different latitudinal (temperate, subpolar and equatorial) and geomorphological (trench, proximal abyssal plain and distal abyssal plain) sites in the western North Pacific Ocean using year-long time series sediment trap systems, to clarify the common vertical trends of the isotopic signals in deep water columns. Although the δ15N and δ13C values of sinking particulate organic matter (POM) were partly affected by the resuspension of sedimentary POM from the sea floor, especially in the trench, the changes in δ15N and δ13C values owing to the resuspension could be corrected by calculation of the isotopic mass balance from δ14C of sinking POC. After this correction, common downward decreasing trends in δ15N and δ13C values were obtained in the deep water columns, irrespective of the latitudes and depths. These coincidental isotopic signals between δ15N and δ13C values provide new constraints for the decomposition process of sinking POM, such as the preferential degradation of 15N- and 13C-rich compounds and the successive re-formation of the sinking particles by higher trophic level organisms in the deep water column.

Production and transport of long-chain alkenones and alkyl alkenoates in a sea water column in the northwestern Pacific off central Japan

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.

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

Abstract Sinking particles collected from year-long time-series sediment traps at 1674, 4180, 5687 and 8688 m depths, the underlying bottom sediment at 9200 m depth, and suspended particles from surface and subsurface waters in the northwestern North Pacific off Japan were analyzed for long-chain alkenones and alkyl alkenoates (A&A) which are derived mainly from Gephyrocapsacean algae, especially Emiliania huxleyi and Gephyrocapsa oceanica. Alkenone temperature records in sediment trap samples at 1674 m were almost similar to observed sea surface temperatures (SST) with a time delay of one half to one full month. However, alkenone temperatures in trap samples were about slightly lower than measured SST in late spring to early fall. The lowering might be caused by formation of the seasonal thermocline. Nevertheless, these temperature drops observed in trap samples were smaller than those actually observed in a subsurface layer off central Japan. Vertical profiles of A&A concentrations and alkenone temperatures in suspended particles collected from the subsurface waters in early fall indicated that these compounds were produced mostly in a surface mixed layer above the depth of the chlorophyll maximum even in warm seasons. These results suggested that alkenone temperatures strongly reflected SST rather than the temperatures of thermocline waters in these study areas even in such a warm season. Pronounced maxima in A&A fluxes found in sediment trap samples at 1674 m in late spring to summer showed that A&A productions were highest during the periods of spring bloom, according to a time delay between alkenone temperatures and observed SST. Seasonal patterns of alkenone records in trap samples at 4180 and 5687 m could also preserve SST signals well, suggesting that A&A in deep sea waters were mainly derived from primary products in the surface layer. A&A fluxes tended to decrease with water depth, and the ratios of A&A to particulate organic carbon (POC) rapidly decreased in underlying bottom sediment. This clearly indicates that A&A were decomposed and diluted by other refractory organic materials in either the water column or the sediment–water interface. However, A&A compositions were consistently uniform between the trap samples and the underlying bottom sediments, so that A&A could not qualitatively alter during early diagenetic processes.

FRESH WATER SEEPAGE AND PORE WATER RECYCLING ON THE SEAFLOOR : SAGAMI TROUGH SUBDUCTION ZONE, JAPAN

[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.