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Accurate prediction of future climate requires an understanding of the mechanisms of the Holocene climate; however, the driving forces, mechanisms, and processes of climate change in the Holocene associated with different time scales remain unclear. We investigated the drivers of Holocene sea surface temperature (SST) and sea ice extent in the North Pacific Ocean, and the Okhotsk and Bering Seas, as inferred from sediment core records, by using the alkenone unsaturation index as a biomarker of SST and abundances of sea ice-related diatoms (F. cylindrus and F. oceanica) as an indicator of sea ice extent to explore controlling mechanisms in the high-latitude Pacific. Temporal changes in alkenone content suggest that alkenone production was relatively high during the middle Holocene in the Okhotsk Sea and the western North Pacific, but highest in the late Holocene in the eastern Bering Sea and the eastern North Pacific. The Holocene variations of alkenone-SSTs at sites near Kamchatka in the Northwest Pacific, as well as in the western and eastern regions of the Bering Sea, and in the eastern North Pacific track the changes of Holocene summer insolation at 50°N, but at other sites in the western North Pacific, in the southern Okhotsk Sea, and the eastern Bering Sea they do not. In addition to insolation, other atmosphere and ocean climate drivers, such as sea ice distribution and changes in the position and activity of the Aleutian Low, may have systematically influenced the timing and magnitude of warming and cooling during the Holocene within the subarctic North Pacific. Periods of high sea ice extent in both the Okhotsk and Bering Seas may correspond to some periods of frequent or strong winter-spring dust storms in the Mongolian Gobi Desert, particularly one centered at ~4-3 thousand years before present (kyr BP). Variation in storm activity in the Mongolian Gobi Desert region may reflect changes in the strength and positions of the Aleutian Low and Siberian High. We suggest that periods of eastward displacement or increased intensity of the Aleutian Low correspond with times of increased extent of sea ice in the western Okhotsk Sea and eastern Bering Sea. [ABSTRACT FROM AUTHOR]

Abstract: Diatom fossils from core sediments and living diatoms from water samples of Notoro Lagoon in northern Japan were examined to evaluate natural climate effects on lagoon environmental changes. In 1974, the artificial inlet was excavated. Immediately after, the anoxic bottom water in Notoro Lagoon began to disappear due to an increasing water exchange rate. However, chemical oxygen demand (COD) in the bottom water of Notoro Lagoon gradually increased, with fluctuations, during the last 30 years. In addition, the dominant diatom assemblages in Notoro Lagoon shifted to ice-related and spring bloom taxa after the excavation. The dominant taxa of each year in the sediment core were also strongly related to the timing of lagoon ice melting. This is because the COD in Notoro Lagoon was affected by the deposited volume of blooming diatoms, which was controlled by the duration of ice cover and the timing of ice discharge to the Okhotsk Sea likely due to an air pressure pattern change over the northern North Pacific like the Pacific Decadal Oscillation (PDO). [Copyright &y& Elsevier]

Abstract: Diatom assemblages in suspended particulate matter (SPM), surface sediments, and piston core samples in the southern Bering Sea and the western subarctic Pacific were examined. The distribution of diatom assemblages in the southern Bering Sea shows not only differences in species composition between the SPM and surface sediment samples, but also preferential dissolution of different species in the upper water column or surface sediments. Furthermore, we present the downcore profiles of selected diatom taxa for four piston cores to indicate changes in paleoceanographic conditions, e.g., seasonal productivity, sea-ice and surface-water circulation, of the glacial and deglaciation periods during the late Quaternary. In particular, we propose five models for the annual cycling of the water masses in the western subarctic Pacific and the southern Bering Sea during the late Quaternary. Significant regional differences in diatom accumulation rates and taxonomic composition between the southern Bering Sea and the western subarctic Pacific reflect spatial shifts in water masses caused by the variable extent of sea-ice cover and cold low-salinity surface waters in these regions. [Copyright &y& Elsevier]

• Benthic foraminifera in the NRD contain the Marine, Bay, and Opportunistic taxa. • Precipitation and resultant river discharge increased in Korean Peninsula at ∼ 4 ka. • Two small maxima of low oxygen-resistant taxon represent the fluctuated TWC. • Replacement of two species implies the enhanced East Asian winter monsoon at 4.6 ka. Fossil benthic foraminifera from five cores in the Nakdong River Delta (southeast Korea) were investigated to reveal paleoenvironmental changes across the middle/late Holocene transition, particularly focusing on ∼4 ka climatic event. Based on multi-dimensional scaling, benthic foraminifera are divided into (1) Marine taxa, (2) Bay taxa, and (3) Opportunistic taxa. Planktonic/total (benthic and planktonic) foraminiferal ratio decreased abruptly during 4.6–3.3 ka, and Nonionella stella was found commonly across ∼4 ka in core ND-01 (intermediate site among the five cores). These results imply that the ∼4 ka climatic event led to the stratification of less saline epilimnetic water underlain by more saline oxygen-poor hypolimnetic water in the Nakdong River Delta. This indicates temporal increase in precipitation in the Nakdong River drainage, which can be attributed to the intensified rainfall with the second Changma peak, associated with typhoons toward the Korean Peninsula. The small twin maxima of N. stella across ∼4 ka in core ND-01 were similar to those of the total sulfur contents in both Yeja Bay (southern Korea) and Lake Togo-ike (southwestern Japan). Such similarity in the stratigraphic patterns among the remote locations can be explained by the small-scale sea-level variations with the fluctuation of the Tsushima Warm Current, in addition to changes in precipitation. The common species of Haynesina sp. A was replaced by Elphidium somaense after ∼4.6 ka, which is likely due to the shift of passive dispersal as a result of the enhanced East Asian winter monsoon and/or the change in the westerly jet path. [ABSTRACT FROM AUTHOR]

Abstract: High-resolution analyses of geochemical parameters (biogenic opal, calcium carbonate, organic carbon, and nitrogen) and microfossil assemblages (diatoms and radiolarians) on Core MD01-2412 clarified detailed paleoceanographic changes such as sea-ice cover and biological production in the southwestern Okhotsk Sea during the last 115kyr. An age model of Core MD01-2412 was established based on δ18O stratigraphy, accelerator mass spectrometer (AMS) 14C, and tephrochronology. Sea-ice history reconstructed by siliceous microplankton records indicated that the present sea-ice condition was formed during the last 8kyr. Only during Marine Isotope Stage (MIS) 2 was the duration of sea-ice cover in this region much longer than that of today (4–5 months a year). Two diatom species, Thalassionema nitzschioides and Fragilariopsis doliolus, revealed that the Soya Warm Current Water (SWCW) flowed into the Okhotsk Sea near the site of Core MD01-2412 during the last 12–14kyr and during MIS 5a, and was associated with sea-level rise. Biological productivity rapidly increased during MIS 1, associated with sea-ice retreat. Two major increases of organic carbon (OC) contents (wt%) and Corg/N ratios were observed, and the timings of these events were 15.8–16.7ka (Event 1) and 13.1–13.6ka (Event 2). Corresponding to these events, the abundance of Cycladophora davisiana, an intermediate water dwelling radiolarian species, increased. This high C. davisiana abundance can be correlated to the input of terrestrial organic matter from the submerged shelf to the intermediate water. Apart from the radiolarians, the production of diatoms in the surface waters was suppressed by the development of well-stratified surface water along with sea-ice melting during the early Holocene. Diatom production increased gradually during the last 10kyr with enhanced vertical mixing. [Copyright &y& Elsevier]

Fossil benthic and planktonic foraminifera from a part of core ND-02 and the published data of core ND-01 from the Nakdong River delta, southeast Korea were analyzed to evaluate the impact of the Tsushima Warm Current on coastal environments in the Korea/Tsushima Strait between Korea and Japan during the early to middle Holocene. Based on cluster analysis, four sample clusters (A, B, C, and D), suggestive of a landward–seaward gradient of environmental conditions, can be defined. According to the spatio-temporal and cluster distributions of benthic foraminifera in faunal compositions, the landward occurrence of benthic foraminifera was delimited in the inner part of the Nakdong River delta at ∼8–7 ka. The common occurrence of warm-water planktonic foraminifera suggests that this maximum landward expansion of benthic foraminifera within the delta was caused by the intensification of the Tsushima Warm Current in addition to sea level rise during the early to middle Holocene. Changes in both marine biota and sedimentation were reported in the coastal areas of southwestern Japan in that period. They were also caused by a precursor of the enhanced Tsushima Warm Current at ∼8 ka. Thus, the evolution of the Tsushima Warm Current during the early to middle Holocene is important for understanding various phenomena in the coastal environments of the Korea/Tsushima Strait. [ABSTRACT FROM AUTHOR]

We investigated fossil benthic and planktonic foraminifera in sediment cores FV10-05 and FV10-06-2, off Fukuoka, southwestern Japan to evaluate the impact of the Tsushima Warm Current on coastal environments around the strait between southeastern Korea and southwestern Japan during the early to middle Holocene. A cluster analysis of benthic foraminifera in these cores established four sample clusters (A, B, C, and D), which are interpreted as indicators of relatively high energy condition due to wave/current processes. Clusters A and C, characterized by attached taxa, became dominant at ∼6.6 ka. Such faunal changes are consistent with the current-influenced sedimentation, confirming that the Tsushima Warm Current reached its present-day distribution at that time. A hiatus at ∼8–7 ka has been reported in cores from deeper water off Fukuoka, whereas our shallow core shows deposition under high energy conditions was continuous even at ∼8–7 ka near the coast off river mouth. Across the strait, the maximum landward extent of benthic foraminifera occurred in the Nakdong River delta (southeastern Korea) in ∼8–7 ka, and an outer bay fauna of benthic foraminifera with seagrass beds occurred transiently in the restricted bay of the San-in district (southwestern Japan) at that time. The evolution of the Tsushima Warm Current during the early to middle Holocene thus appears to play an important role in coastal environments not only in the strait areas between southeastern Korea and southwestern Japan, but also in the San-in district. [ABSTRACT FROM AUTHOR]

The fluvial to marine transition zone (FMTZ) is the area of coastal rivers in which sedimentation is controlled by the interaction of fluvial and marine processes. This study examines the FMTZ of the Mekong River delta, along a total channel length of ~660 km. Methods consist of collection and analysis of channel bed sediment samples, measurements of channel morphological parameters, and recognition of mangrove, molluscan, and diatom species. The process regime, salinity, morphological, and sedimentary trends recognized were used to define two main tracts for this FMTZ: an upstream, fluvial-dominated tract and a downstream, tide-dominated tract. In more detail, they allow the identification of four subzones, from upstream to downstream: 1) fluvial-dominated, tide-affected; 2) fluvial-dominated, tide-influenced; 3) tide-dominated, fluvial-influenced; and 4) tide-dominated, fluvial-affected. Tide-induced water-level changes affect the entire study area and extend into Cambodia. Measured salinity intrusion extends ~15 km upstream of the river mouth during wet season, and ~50 km during dry season. Brackish water species of mangroves, mollusks, and diatoms, however, occur landward of these limits, suggesting that highly diluted brackish water may reach ~160 km upstream of the river mouth during the dry season. In the fluvial-dominated tract, channels are sinuous and show a seaward-deepening trend, whereas width is relatively constant. In the tide-dominated tract, channels are straight, and show seaward-widening and seaward-shallowing trends. Natural levees are present in the fluvial-dominated, tide-affected subzone, but are replaced by mangroves elsewhere along the FMTZ. In the fluvial-dominated tract, mud content is low, sand grain size fines seaward, and gravelly sand and sand are the dominant facies. In the tide-dominated tract, mud content is high, sand grain size is constant, recycled sand is common, and tidal rhythmites are the dominant facies. Mud pebbles are common in sediments throughout a large part of the FMTZ. These trends characterizing the FMTZ of the Mekong River delta seem to be present in other systems and likely represent a general FMTZ pattern. Nonetheless, minor differences may be observed between different types of systems, or because of differences in local conditions. The comprehensive description of trends and their mutual relationships along the FMTZ presented herein provides critical information that can form the basis of a general conceptual model and can help to better understand these complex zones. [ABSTRACT FROM AUTHOR]

To detect long-term change in the reservoir effect of the eastern coast of Korea, we measured the radiocarbon ages of terrestrial plant and marine shell pairs from the same horizons from approximately 12 m of Holocene sediments under reclaimed land around the Mae-ho Lagoon. The lagoon sediment was examined for lithology, and sedimentary structures, as well as mollusk and diatom assemblages. From bottom to top the sediment comprised three units: transgressive lagoon, restricted lagoon, and delta. The offset in the radiocarbon age of nine pairs were also divided into three stages: 300 ± 90 to 410 ± 120 years (transgressive lagoon sediment in 7350–5800 cal BP), 640 ± 150 to 800 ± 150 years (restricted lagoon sediment in 4320–1600 cal BP), and 120 ± 140 to 330 ± 100 years (deltaic sediment in 1200–390 cal BP). The reservoir ages (R) in the sediments of transgressive lagoon and the restricted lagoon include the variation associated with dietary of shells, herbivorous and deposit-feeding, respectively. The R values in the deltaic lagoon sediment were the most representative of those in the Mae-ho Lagoon because they were obtained from the shells of a filter-feeding taxon, Potamocorbula amurensis , in the most recent sediment. These values from the deltaic lagoon sediment are also consistent with previously reported modern reservoir ages from the Korean Peninsula and Peter the Great Gulf around Vladivostok. [ABSTRACT FROM AUTHOR]

Abstract: Changes in biological productivity of the Bering Sea have been evaluated for the late Quaternary based on biogenic opal, calcium carbonate, and microfossils in two piston cores. Biological productivity increased during Marine Isotope Stage (MIS) 1 after the last glaciation. During the last deglaciation, two pronounced peaks of calcium carbonate content were observed at ca. 2400m water depth, which can be explained by the “CaCO3 compensation hypothesis” and coccolithophore blooms. Simultaneous with the CaCO3 peaks, oxygen-poor deep water was apparently distributed in the Aleutian Basin based on benthic foraminiferal assemblages. The low-O2 events seem to be related to deep-water circulation and/or elevated productivity in the northwest Pacific. After the CaCO3 peak events, biogenic opal contents and diatoms increased gradually, associated with enhanced vertical mixing with an inflow of the Alaskan Stream through the eastern Aleutian Arc passes. [Copyright &y& Elsevier]