Itaru Koizumi

Paleoenvironmental Changes in the Japan Sea During the Last 85,000 Years

Five distinct changes in the paleoenvironment of the Japan Sea within the last 85,000 years are revealed from the sedimentary record of a piston core recovered from the Oki Ridge. Changes in both surface and deepwater conditions are registered by changes in lithology, calcium carbonate content, organic carbon content, oxygen and carbon isotope ratios, and microfossil assemblages including calcareous nannoplankton, diatoms, radiolaria, and foraminifera. Between 85 and 27 ka the warm Tsushima Current did not flow into the Japan Sea, and cold surface water conditions prevailed. Environments at the seafloor fluctuated between dysaerobic to weakly oxic conditions. Between 27 and 20 ka, freshwater input to the Japan Sea, probably from the Huang Ho River in China, stratified the water column, and the severe anoxic conditions eliminated most benthic fauna. Between 20 and 10 ka the cold Oyashio Current flowed into the Japan Sea through the Tsugaru Strait, reestablishing deepwater ventilation. Shallow water benthic assemblages of the North Pacific Ocean subsequently colonized the Japan Sea and occupied the vacant niches of the deep basins. Between 10 and 8 ka the foraminifer compensation level (FCL) gradually rose to a depth shallower than 1000 m, and bottom conditions changed from dysaerobic to oxic. At 10 ka the warm Tsushima Current started to flow into the Japan Sea through the Tsushima Strait to establish the modern oceanographic regime which has existed since 8 ka. The eustatic sea level during the last glacial maximum was above the sill depths (130 m) of the Tsushima and Tsugaru straits, assuming that tectonic movements at these straits were negligible for the last 20 ka.

Land-ocean linkages over orbital and millennial timescales recorded in Late Quaternary sediments of the Japan Sea

Late Quaternary sediments of the Japan Sea are characterized by centimeter- to meter-scale alternations of dark and light layers which are synchronous basinwide. High-resolution analyses of the sediments from Ocean Drilling Program site 797 reveal that deposition of the meter-scale alternations reflect variations in paleoceanographic conditions which were closely associated with glacio-eustatic sea level changes through the modulation of the volume and character of the influx to the sea through the Tsushima Strait. The centimeter- to decimeter-scale alternations reflect millennial-scale variations which are possibly associated with Dansgaard-Oeschger (D-O) cycles, with each dark layer appearing to correspond to an interstadial. This variability is attributed to the development of a humid climate in central to eastern Asia and the consequent increase in discharge from the Huanghe and Changjiang Rivers during interstadials. This caused expansion of the East China Sea coastal water (ECSCW), which penetrated more strongly into the Japan Sea. The increased influence of the lower-salinity, nutrient-enriched ECSCW reduced deep water ventilation and enhanced the surface productivity, leading to the development of anoxic bottom waters and deposition of the dark layers. Thus the centimeter- to decimeter-scale alternations of the dark and light layers record wet and dry cycles in central to eastern Asia possibly associated with D-O cycles.

Eastern Mediterranean surface water temperatures and d18O composition during deposition of sapropels in the late Quaternary

0indices) and d 18 O of planktonic foraminifer calcite (d 18 Ofc) across late Pleistocene sapropel intervals show that d 18 Ofc decreased (between 1 and 4.6%) and SST increased (between 0.7� and 6.7� C). Maximal d 18 Oseawater depletion of eastern Mediterranean surface waters at the transition is between 0.5 and 3.0%, and in all but one case exceeded the depletion seen in a western Mediterranean core. The depletion in d 18 Oseawater is most pronounced at sapropel bases, in agreement with an initial sudden input of monsoon-derived freshwater. Most sapropels coincide with warming trends of SST. The density decrease by initial freshwater input and continued warming of the sea surface pooled fresh water in the surface layer and prohibited deep convection down to ageing deep water emplaced during cold and arid glacial conditions. An exception to this pattern is ‘‘glacial’’ sapropel S6; its largest d 18 Oseawater depletion (3%) is almost matched by the depletion in the western Mediterranean Sea, and it is accompanied by surface water cooling following an initially rapid warming phase. A second period of significant isotopic depletion is in isotope stage 6 at the 150 kyr insolation maximum. While not expressed as a sapropel due to cold SST, it is in accord with a strengthened monsoon in the southern catchment. INDEX TERMS: 1055 Geochemistry: Organic geochemistry; 1620 Global Change: Climate dynamics (3309); 4267 Oceanography: General: Paleoceanography; 9604 Information Related to Geologic Time: Cenozoic; KEYWORDS: Mediterranean Sea, sapropels, sea surface temperatures, oxygen isotopes, Quaternary Citation: Emeis, K.-C., et al., Eastern Mediterranean surface water temperatures and d 18 O composition during deposition of sapropels

Glacial Holocene environment of the southeastern Okhotsk Sea: evidence from geochemical and palaeontological data

Abstract Environmental conditions and productivity changes in the southeastern Okhotsk Sea have been reconstructed for the last 20 ka using planktonic and benthic foraminiferal oxygen isotope records and calcium carbonate, organic carbon and opal content data from two sediment cores. Species variability in benthic foraminiferal and diatom assemblages provides additional palaeoceanographic evidence. AMS radiocarbon dating of the sediments and oxygen isotope stratigraphy serve as the basis for the age models of the cores for the last 20 14 C kyr and for correlation between environmental variations in the Okhotsk Sea, and regional and global climate changes. Benthic foraminiferal assemblages in the two cores (depth 1590 and 1175 m) varied with time, so that we could recognise seven zones with different species composition. Changes in the benthic foraminiferal assemblages parallel major environmental and productivity variations. During the last glaciation, fluxes of organic matter to the sea floor showed strong seasonal variations, indicated by the presence of abundant A. weddellensis and infaunal Uvigerina spp. Benthic foraminiferal assemblages changed with warming at 12.5–11 and 10–8 14 C kyr BP, when productivity blooms and high organic fluxes were coeval with global meltwater pulses 1A and 1B. Younger Dryas cooling caused a decline in productivity (11–10 kyr BP) affecting the benthic faunal community. Subsequent warming triggered intensive diatom production, opal accumulation and a strong oxygen deficiency, causing significant changes in benthic fauna assemblages from 5.26–4.4 kyr BP to present time.

The Pliocene to recent history of the Kuroshio and Tsushima Currents: a multi-proxy approach

The Kuroshio Current is a major western boundary current controlled by the North Pacific Gyre. It brings warm subtropical waters from the Indo-Pacific Warm Pool to Japan exerting a major control on Asian climate. The Tsushima Current is a Kuroshio offshoot transporting warm water into the Japan Sea. Various proxies are used to determine the paleohistory of these currents. Sedimentological proxies such as reefs, bedforms, sediment source and sorting reveal paleocurrent strength and latitude. Proxies such as coral and mollusc assemblages reveal past shelfal current activity. Microfossil assemblages and organic/inorganic geochemical analyses determine paleo- sea surface temperature and salinity histories. Transportation of tropical palynomorphs and migrations of Indo-Pacific species to Japanese waters also reveal paleocurrent activity. The stratigraphic distribution of these proxies suggests the Kuroshio Current reached its present latitude (35 °N) by ~3 Ma when temperatures were 1 to 2 °C lower than present. At this time a weak Tsushima Current broke through Tsushima Strait entering the Japan Sea. Similar oceanic conditions persisted until ~2 Ma when crustal stretching deepened the Tsushima Strait allowing inflow during every interglacial. The onset of stronger interglacial/glacial cycles ~1 Ma was associated with increased North Pacific Gyre and Kuroshio Current intensity. This triggered Ryukyu Reef expansion when reefs reached their present latitude (~31 °N), thereafter the reef front advanced (~31 °N) and retreated (~25 °N) with each cycle. Foraminiferal proxy data suggests eastward deflection of the Kuroshio Current from its present path at 24 °N into the Pacific Ocean due to East Taiwan Channel restriction during the Last Glacial Maximum. Subsequently Kuroshio flow resumed its present trajectory during the Holocene. Ocean modeling and geochemical proxies show that the Kuroshio Current path may have been similar during glacials and interglacials, however the glacial mode of this current remains controversial. Paleohistorical studies form important analogues for current behavior with future climate change, however, there are insufficient studies at present in the region that may be used for this purpose. Modeling of the response of the Kuroshio Current to future global warming reveals that current velocity may increase by up to 0.3 m/sec associated with a northward migration of the Kuroshio Extension.

Diatom record of the late Holocene in the Okhotsk Sea

Abstract The environmental conditions during the past 7000 cal yr BP in the Okhotsk Sea can be subdivided into three intervals based on diatom assemblages: (1) 6600–4650 cal yr BP open-ocean sea-ice free conditions alternated with sea-ice cover, (2) 4650–1300 cal yr BP open-ocean conditions and sea-ice cover declined, fluctuating with short duration, and cold open-ocean conditions gradually developed, and (3) 1300 cal yr BP–Present when cold open-ocean conditions predominated with occasional and rapid warming. Chronological comparisons between climatic fluctuations and cultural history in Hokkaido and Okhotsk suggest that deteriorating climate could have affected local culture.

Pliocene carbonate accumulation along the California Margin

Recent modeling studies call on increased ocean heat transport to explain high-latitude warming observed for intervals throughout the middle Pliocene. Possible vehicles for ocean heat transport are the poleward arms of the subtropical gyres. Sites from the California margin (Ocean Drilling Program Leg 167) provide monitors of wind field within the eastern arm of the gyre which may be an indication of basin-wide subtropical gyral strength. At most sites (water depths from 1106 to 4212 m) CaCO3 mass accumulation rate (MAR) was highest in the middle Pliocene (3.5–2.0 Ma). This high CaCO3 MAR “event” is attributed primarily to higher CaCO3 production due to higher offshore upwelling associated with the zone of the greatest wind stress curl. Thus, in the middle Pliocene, there was enhanced wind stress curl along the California margin, and possibly enhanced North Pacific sub-tropical gyral circulation and meridional ocean heat advection.

TAXONOMY OF THE THALASSIOSIRA TRIFULTA GROUP IN LATE NEOGENE SEDIMENTS FROM THE NORTHWEST PACIFIC OCEAN

Several lower Miocene, Pliocene, and Pleistocene samples from the northwest Pacific Ocean were used to study the taxonomy of the Thalassiosira trifulta group, which is characterized by the inward tubes of the fultoportulae and a rimoportula located away from the valve margin. T. ferelineata, T. frenguelliopsis, T. tetraoestrupii, and Thalassiosira sp. A (referred to as the T. frenguelliopsis subgroup) were found to have operculate fultoportulae with an operculate basal structure covering the 4 satellite pores in the basal siliceous membrane. A second subgroup, consisting of T. oestrupii, T. praeoestrupii, and T. trifulta (referred to as the T. oestrupii subgroup), have a trifultate fultoportula with 3 columnar supports and 3 satellite pores. In T. praeoestrupii sensu stricto the trifultate fultoportula is intermediate in structure with 3 columnar supports and an operculate basal structure covering the 3 satellite pores.

Spectral analysis of the diatom paleotemperature records at DSDP sites 579 and 580 near the subarctic front in the western North Pacific

Abstract Maximum entropy spectral analyses and a fitting test to find the best suitable curve for the modified time series based on the non-linear least squares method for Td (diatom temperature) values were performed for the Quaternary portion of the DSDP Sites 579 and 580 in the western North Pacific. The sampling interval averages 13.7 kyr in the Brunhes Chron (0–780 ka) and 16.5 kyr in the later portion of the Matuyama Chron (780–1800 ka) at Site 580, but increases to 17.3 kyr and 23.2 kyr, respectively, at Site 579. Among dominant cycles during the Brunhes Chron, there are 411.5 kyr and 126.0 kyr at Site 579, and 467.0 kyr and 136.7 kyr at Site 580 correspond to 413 kyr and 95 to 124 kyr of the orbital eccentricity. Minor cycles of 41.2 kyr at Site 579 and 41.7 kyr at Site 580 are near to 41 kyr of the obliquity (tilt). During the Matuyama Chron at Site 580, cycles of 49.7 kyr and 43.6 kyr are dominant. The surface-water temperature estimated from diatoms at the western North Pacific DSDP Sites 579 and 580 shows correlation with the fundamental Earths orbital parameters during Quaternary time.

TAXONOMY OF THE AZPEITIA NODULIFERA GROUP IN LATE NEOGENE SEDIMENTS FROM THE NORTHWEST PACIFIC OCEAN

Upper Miocene, Pliocene, and Pleistocene samples from DSDP Holes 579A, 580, and 581 in the northwest Pacific Ocean were examined in order to identify Azpeitia species in LM and SEM. Azpeitia species which do not possess a marginal hyaline region are here defined as the Azpeitia nodulifera group. The following taxa which belong to this group are discussed and illustrated here: A. barronii, A. nodulifera f. nodulifera, A. nodulifera f. cyclopus, A. nodulifera type 1, Azpeitia sp. A, and one new form, A. nodulifera f. variantia Shiono f. nov. The first report of a pseudonodulus in Azpeitia confirms the earlier familial placement of this genus by Fryxell et al. (1986). A pseudonodulus was regularly found in A. nodulifera f. nodulifera and Azpeitia sp. A, but only rarely found in A. nodulifera f. variantia.