Article

High-resolution reconstructions of Holocene Sea-surface conditions from dinoflagellate cyst assemblages in the northern South China Sea

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Abstract

A high-resolution dinoflagellate cyst analysis on a sediment core GLW1D from the northern South China Sea (SCS) was performed to reconstruct paleoceanographic conditions over the last 12,500 years through qualitative, semi-quantitative, and quantitative methods. A modern dataset with 398 reference sites in the northern Pacific was assembled and used to identify the relationship between dinoflagellate cyst assemblages and sea-surface temperature (SST), sea-surface salinity (SSS) and primary productivity (PP). Modern analog technique (MAT) was applied to offer first dinoflagellate-cyst-based quantitative estimates of Holocene sea-surface conditions in the western North Pacific. The downcore reconstructions show that SST, SSS and PP were predominantly controlled by the changes in coastal and oceanic currents due to the changes of sea level and monsoon systems. Our results indicate that SST increased while SSS and PP decreased from 12,500 to ~6800 cal yr BP, reaching the maximum SST and the minimum SSS and PP during ~6800–5000 cal yr BP, and followed by a slight decline in SST with minor increases in SSS and PP. The three intervals correspond to the regional onshore sea-level stages of rising, stabilization in a highstand and slight drop, respectively. The Kuroshio Current strongly influenced the core site before ~9900 cal yr BP, reflected by the highest abundances of oceanic Impagidinium spp. and high reconstructed SSS values. This can be explained by a lack of water input from the East China Sea before the opening of the Taiwan Strait. The warmest period, from ~6800 cal yr BP to ~5500 cal yr BP, is recorded by the highest Dapsilidinium pastielsii abundances. Two short-term high-PP events of ~2700–2400 cal yr BP and ~1000–600 cal yr BP, which were characterized by opposite climatic conditions, coincided with two notable societal (dynasty) collapses of China. Enhanced anthropogenic activities since the Late Bronze Age most likely partially affected the high PP through influencing river inputs to the northern SCS.

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The 4.2 ka event was one of Holocene abrupt-change events and characterized by dry and cool climatic conditions in many parts of the world. The event was reported to have played important roles in the collapses of those major ancient civilizations. This paper first attempts to portray a global picture of this “4.2 ka Event” and then reviews those reliably-dated sequences containing “4.2 ka Event” retrieved from the domains of four major civilizations. This review further confirms the proposition advocated by the International Stratigraphy Committee that 4.2 ka was the chronological divider between the warmer-wetter middle Holocene and the cooler-drier late Holocene. This review also lends a further support to early proposals that the declines or disruptions of Ancient Egyptian Culture, Mesopotamian Culture, and Ancient Indian Culture at ∼4.2 ka BP were directly or indirectly associated with the “4.2 ka Event”. In Chinese cultural domain, the derailment of nearly all of the well-developed Neolithic cultures at ∼4.2 ka BP was also chronologically correlated with the proxy data-recorded abrupt climatic change at ∼4.2 ka BP. But, it should be admitted that either drying or cooling or drying and cooling at ∼4.2 ka BP might not be sufficient to collapse these well-developed cultures in presently warm and wet southern China. Surely, further studies are needed for constraining the climatic and environmental variables at ∼4.2 ka BP not only in southern China but also in the adjacent Central Plains of China.
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The South China Sea (SCS), characterized by a large continental shelf, is located at the edge of the Asian monsoon domain. In this study, two marine sediment cores from the northern SCS (NSCS) continental slope were investigated to construct composite vegetation and precipitation isotopic composition records based on the d 13 C and dD values of plant-wax n-alkanes throughout the Holocene (last 11,200 years; i.e. 11.2 ka). The composite d 13 C wax record indicates an overall predominance of C 3 vegetation over the last 11.2 ka. Before 8 ka BP, higher d 13 C wax values are attributed to preferential wax input from grassland and wetland biomes on the exposed continental shelf. After the inundation of the shelf by eustatic sea level rise until ca. 8 ka BP grassland and wetland biomes suffered a major size reduction and arboreal vegetation became better represented in the d 13 C wax record. The composite temperature corrected dD wax-T record suggests that moisture source variability drove precipitation isotopic composition changes during the Holocene. Lower dD wax-T values before 8.3 ka BP are interpreted as a larger moisture contribution by Pacific Ocean tropical cyclones, whereas higher dD wax-T values after 8.5 ka BP are interpreted as a larger moisture contribution from the Indian Ocean summer monsoon. Higher incidence of tropical cyclones in the NSCS during the Early Holocene was related to a temporary westward shift of the Western Pacific Warm Pool and enhanced insolation over the Northern Hemisphere. Both external and internal forcing mechanisms regulated moisture source changes in East Asia during the Holocene.
Article
We present a multiproxy record using pollen, magnetic susceptibility, carbon isotopic composition, carbon/nitrogen ratio, and particle size of mid- to late Holocene environmental changes based on a sediment core from the Pomaeho lagoon on the east coast of Korea. The records indicate that climate deteriorations around 6400 cal yr BP and 4000 cal yr BP caused rapid vegetation changes in the study area, which were presumably attributable to low sunspot activity and strong El Niño–like conditions, respectively. These two cooling events were likely modulated by different climate mechanisms, as El Niño–Southern Oscillation activity began to strengthen around 5000 cal yr BP. These events may have had a substantial impact on ancient societies in the study area. Combining our results with archaeological findings indicated that climate deterioration led to drastic declines in local populations around 6400 cal yr BP, 4400 cal yr BP, and 4000 cal yr BP. Because of its high population, coastal East Asia (e.g., eastern China, Japan, and Korea) is particularly vulnerable to potential cooling events in the future. Therefore, there is a strong need for detailed paleoclimate information in this region.
Article
The taxonomy of the dinoflagellate cyst-based genus Votadinium has been in need of revision. This was accomplished here by integrating morphology, LSU rDNA sequences, and cyst wall composition analyses through micro-Fourier transform infrared spectroscopy. Three new species, Votadinium pontifossatum, V. rhomboideum and V. reidii, were described as cyst stages of Protoperidinium paraoblongum, P. quadrioblongum, and P. steidingerae, respectively. A previously undescribed type of ornamentation was reported for V. pontifossatum. A fourth new species was assigned to V. concavum. Furthermore, Lejeunecysta psilodora Benedek (=Lejeunecysta psuchra Matsuoka), was re-described as V. psilodora and the holotype re-illustrated, as was done for the holotype of V. spinosum Reid. New illustrations were provided for V. elongatum and V. nanhaiense. LSU rDNA sequences of V. concavum revealed its placement in the Oceanica section of the genus Protoperidinium, supporting earlier evidence that the Oceanica section and the cyst based genus Votadinium form a monophyletic clade. Thus, Votadinium could be considered the cyst equivalent of the motile-defined Oceanica section. The cyst walls of Votadinium spinosum, V. calvum and V. pontifossatum are built from carbohydrate-based macromolecules with N-containing functional groups in agreement with the heterotrophic nature of their motile stage. Comparison with related genera Trinovantedinium and Quinquecuspis reveals that the species and genera can be separated based on cyst wall composition, supporting the phylogeny based on molecular data and/or ecological differences. Furthermore, Trinovantedinium applanatum spectra unexpectedly show an additional absorption at 1510 cm⁻¹ indicating an aromatic group that is commonly reported for sporopollenin and not seen in dinoflagellate cysts so far. This study stabilizes the taxonomy of the cyst genus Votadinium, clarifies its relationship to the Oceanica section of Protoperidinium, and further supports the application of whole cyst wall chemistry in helping address problems in dinoflagellate cyst taxonomy and ecology.
Article
Here we present an almost complete and integrated Holocene record of marine and terrestrial palaeoenvironmental change from the NE shelf of the Black Sea. A dinoflagellate cyst record used to reconstruct Holocene sea-surface conditions highlights that the NE shelf was a brackish water environment, with a minimum salinity of 7 psu in the early-Holocene before changing at a gradual rate to a more saline environment with maximum salinities of ~18 psu being reached around 3 cal. ka. A warming phase was detected from 6 cal. ka BP, with warmest conditions between 3 and 2.5 cal. ka BP. A pollen record is used to examine the major climate and land-use changes in the eastern Black Sea region. Biome reconstructions show that the temperate deciduous forest dominates throughout the record, although with an overall decline. From early-Holocene to the first hiatus around ~9 cal. ka BP, Pinus pollen dominates, while taxa representing a mixed oak-hornbeam-beech forest are less abundant, indicating relatively cool and dry conditions. Between ~7.9 and ~6.1 cal. ka BP, a thermophilous deciduous forest established, suggesting an overall warming trend and humid conditions. From 4 cal. ka BP, Pinus dominates the pollen record, accompanied by an increase of herbs, implying an opening of the landscape, which would coincide with the beginning of the Meghalayan Age. The integrated record of the marine and terrestrial climate indicators supports the notion that this change in landscape may have been triggered by a combination of warmer and drier conditions and human activities in this region.
Article
The South China Sea (SCS), influenced by the tropical East Asian Monsoon (EAM), experiences winter monsoon and summer monsoon shifts in near-surface wind, air temperature, sea-surface temperature, salinity, primary productivity, and other oceanographic conditions. To understand how monsoon seasons influence palynological dynamics and which palynological index could be a reliable indicator of winter or summer monsoons, we studied palynological records of sediment trap samples collected in March–April (winter monsoon season) and July–August (summer monsoon season). Fluxes and assemblages of terrestrial pollen and spores, as well as marine dinoflagellate cysts, were investigated using sediment traps in the southwest Taiwan waters of the SCS. The pollen and spores of 109 taxa and dinoflagellate cysts of 53 taxa were identified in 24 sediment trap samples that were collected at 3-day intervals. The average abundance of Pinus pollen was notably higher in March–April at ~ 40%, which was double that in July–August. This trend was associated with transport by the northwest wind in March–April when Pinus pollen are produced by the coniferous vegetation in the South China and Taiwan Island. The pollen abundances of Chenopodiaceae/Amaranthaceae and Compositae seemed to be greatly reduced in July–August, with an opposite pattern observed for Poaceae, Artemisia, Cyperaceae, Typha, and fern spores. Fluvial transport is likely the controlling factor since river runoff intensifies in summer. High relative abundances and fluxes of Poaceae pollen are not indicators of summer monsoons but related to cultivated activities. Dinoflagellate cyst assemblages were dominated by cysts produced by heterotrophic taxa, especially Brigantedinium spp. at 68–91%. Proportionally higher increases than those of cysts produced by autotrophic dinoflagellates resulted in lower heterotrophic to autotrophic ratios in July–August. The highest total cyst fluxes occurred in late July of ~ 20,000 cysts m⁻² d⁻¹ together with the highest fluxes of autotrophic Operculodinium centrocarpum, Spiniferites spp., and Lingulodinium hemicystum. The increased fluxes of almost all dinoflagellate cyst taxa during July–August were related to the decreased sea-surface salinity (SSS) due to the greater river water inputs and nutrients.
Article
This study has examined how the sedimentary processes in the prevalent non-deltaic coastal marine environments along the southeast coast of China have responded to sea-level change and other environmental drivers during the Holocene. Three sediment cores collected from a shallow marine embayment of the Lantau Island, Hong Kong, on the northern coast of the South China Sea, were analyzed. The results of particle size, total organic carbon content, organic carbon and nitrogen ratios, organic carbon stable isotope ratios, abundance of alkenones and radiocarbon dating of these cores have helped reconstruct the sedimentary history of this site. This history shares the similarity in sedimentary processes within the deltaic environment for the early Holocene, because palaeo-incised channels exist in both environments. The first 1500 years of deposition involved rapid channel infill caused by the rapidly rising sea level and strong monsoon-driven discharge. This was followed by a period of decelerated sedimentation under strong marine influence as the sea flooded the wide accommodation space in the subsequent two millennia. Major differences between the two environmental settings occur in the past 7000 years: vertical accretion was seen in the non-deltaic coastal environment, whilst lateral shoreline advance was the dominant process in the deltaic basin. The progressively slowing down in sedimentation rate in the non-deltaic site during the middle and late Holocene was caused by the reduced monsoon discharge, intensified disturbance from tropical cyclone activity and coastal currents.
Article
This study has collected and analyzed seven sediment cores from the Pearl River delta, from which 16 new and high-quality sea-level index points are generated using a new approach. This study has also re-checked and re-calibrated the previously published sea-level data from China's southeast coast with corrections made for tectonic subsidence and sediment compaction factors. These sea-level data indicate a rise of relative sea level from −49.3 ± 0.8 m to the present height between 10,500 and 7000 cal. a BP. This sea-level history is similar to those recorded from other far-field locations and ice-volume equivalent sea-level models. The early to early-middle Holocene sea-level history in the study area shows a phase of accelerated rise at a rate increasing rapidly from 16.4 ± 6.1 mm/a at 10,500 cal. a BP to 33.0 ± 7.1 mm/a at 9500 cal. a BP. This phase was followed by a period of rapid decrease in the rate of sea-level rise to 8.8 ± 1.9 mm/a at 8500 cal. a BP and 1.7 ± 1.3 mm/a at 7500 cal. a BP. During the past 7000 years, the relative sea level in the study area changed very little. This new and complete history of Holocene sea-level change supports the following findings: (1) no obvious higher-than-present sea-level highstand in the Holocene is found from the northern South China Sea; (2) certain proportion of the effects of the predicted glacial isostatic adjustment were cancelled out by the effects of the weak upper mantle viscosity in the study area; (3) meltwater pulse 1b likely exists spanning into the early Holocene; (4) there are significant misfit between sea-level data and glacial isostatic adjustment models, and a revision to the existing ice melting history for the early Holocene is possibly needed.
Article
Dinoflagellate cyst assemblages in a marine sediment core from the Iceland Basin were used to carry out qualitative and quantitative assessments of upper ocean conditions in the northern branch of the Subpolar Gyre (SPG) during the Holocene. The data revealed that the early Holocene, i.e. prior to ~6.8 kyr B.P., experienced elevated summer sea surface temperatures (SST). In contrast, winter SST was lower compared to modern conditions. This stronger-than-present seasonality revealed by our data thus shows the influence of the early Holocene insolation forcing. Higher seasonality also dominated the reconstructed sea surface salinity (SSS) in the same period, with lower salinity during both summers and winters. The lower SSS suggests freshwater advection into the SPG, and the dinoflagellate cyst content from this interval indicates a dominant sourcing from the Canadian Labrador coast, and thus the eastern margin of the Laurentide Ice Sheet. Within this interval of low SSS, the onset of the Holocene Climate Optimum stands out by its distinct “marine” assemblage components and associated high summer SST and relative SSS increase between 9.3 and 7.8 kyr B.P. A prominent shift in the overall assemblage composition occurs at ~6.8 kyr B.P., with lower seasonality in sea surface conditions from that time onwards. Increased summer and winter SSS suggest that this change is linked to the halted influence of meltwater from the Laurentide Ice Sheet, a westward shift of the subpolar front and a contracted SPG allowing an easier northward movement of southern-sourced North Atlantic waters, contributing to and consequently sustained by the spin-up of the overturning circulation. The subpolar front appears to gradually have moved slightly eastward again between 2.8 and 1.9 kyr B.P. towards its modern-day position, with a subtle increase of elements characterized by a typical East Greenland Current signature in the Iceland Basin. Finally, spectral analyses indicate that a range of periodicities may be present in the new SSS and SST records, most notably a 2000-yr periodicity in SSS changes, although none of the periodicities are dominant in both records or throughout the time interval covered by the records.
Article
The climatic event between 4.2 and 3.9 ka BP known as the “4.2 ka event” is commonly considered to be a synchronous global drought that happened as one pulse. However, careful comparison of records from around the world shows that synchrony is possible only if the published chronologies of the various records are shifted to the extent allowed by the uncertainties of their age data, that several records suggest a two-pulsed event, and that some records suggest a wet rather than dry event. The radiometric ages constraining those records have uncertainties of several decades if not hundreds of years, and in some records the event is represented by only one or two analyses. This paper reports a new record from Stalagmite DP1 from northeastern Namibia in which high ²³⁰Th/²³²Th activity ratios allow small age uncertainties ranging between only 10–28 years, and the event is documented by more than 35 isotopic analyses and by petrographic observation of a surface of dissolution. The ages from Stalagmite DP1 combine with results from 11 other records from around the world to suggest an event centered at about 4.07 ka BP with bracketing ages of 4.15 to 3.93 ka BP. The isotopic and petrographic results suggest a two-pulsed wet event in northeastern Namibia, which is in the Southern Hemisphere's summer rainfall zone where more rain presumably fell with southward migration of the Inter-Tropical Convergence Zone as the result of cooling in the Northern Hemisphere. Comparison with other records from outside the region of dryness from the Mediterranean to eastern Asia suggests that multiple climatic zones similarly moved southward during the event, in some cases bringing wetter conditions that contradict the notion of global drought.
Article
Relic archeological settlement is used to indicate the development of agriculture. We extracted 8865 relic archeological settlements from the Atlas of Chinese Cultural Relics to analyze how the spatiotemporal distribution of archaeological settlements was influenced by temperature changes and social factors during the last millennium. During the Liao dynasty (AD 916–1125) and Jin dynasty (AD 1115–1234) in the Medieval Warm Period (MWP), a large number of settlements indicated the development of agriculture as far north as 47°N. The warm climate of the MWP provided sufficient heat resources to promote the implication of positive policies of the Liao and Jin dynasties to develop agriculture and settlements. By contrast, during the dynasties of Yuan (AD 1279–1368), Ming (AD 1368–1644), and Qing (AD 1644–1911) in the Little Ice Age (LIA), the number of settlements declined drastically, and the northern boundary of the settlement distribution retreated by 3–4 degrees of latitude to modern Liaoning Province. Although the southward retreat of the settlements and related agriculture occurred in the cold climate of the LIA, it could not be completely explained by the drop in temperature. Social factors including nomadic customs, ethnic policies, and postal road systems played more important roles to the northern boundaries of the settlement distributions during the Yuan, Ming, and Qing dynasties.
Article
Sediment grain size is an important proxy to detect sedimentary process and record paleoenvironmental variations in continental slopes. Grain size measurements were performed for 298 samples of the 5.96-m-long core PC338 spanning the past 36 kyr recovered from the northern South China Sea (SCS). Results show that sea level and climatic influences on grain size distributions are crucial. A turbidite formed during the Last Glacial Maximum (LGM) is ascribed to the favorable topography close to the Red River estuary induced by the low sea level. Four end-member (EM) components for the normal sediment layers were unmixed using a lognormal parametric end member modeling (EMM). With the sea level decreasing, more EM4 (∼15-100 μm) and less EM1 (∼0-2 μm) supplied the SCS slope. Because EM2 (∼2-10 μm) consisted of fluvial suspension is dominated by the East Asian summer monsoon (EASM) precipitation, it has the potential to substitute for the EASM intensity. EM2 varied in-phase with many south China paleoclimate records and the 30°N July insolation over the past 36 kyr, and cooling climate events (including the LGM, Heinrich events H1 and H3, Younger Dryas, 8.2 ka and 10.3 ka) lowered the EM2 content. Our results further confirm the synchronous EASM evolution in the South China continent mainly forced by the Northern Hemisphere summer insolation.
Article
Our knowledge about the Holocene evolution of the East Asian summer monsoon (EASM) and its relation to human activities remains incomplete. A detailed palynological investigation of two sediment sections from the Daiyun Mountain Nature Reserve (ca. 25°38′–25°44′N, 118°05′–118°21′E, Fujian Province) suggests EASM-controlled subtropical climate conditions that were wetter and warmer than present over the early and middle Holocene. After 5700 cal. yr BP, steadily increasing non-arboreal pollen and fern spore percentages imply an attenuation of the EASM. The general climate trend is interrupted by several century-scale changes in composition and concentration of arboreal pollen taxa around 8200, 7200, 6300, and 4400 cal. yr BP, coinciding with weaker precipitation (and lower temperature) as suggested by the EASM oxygen isotope record. Our results further support the hypothesis that the natural vegetation cover of southern China was not markedly affected by human activities until 3000 cal. yr BP. The pollen records suggest a quick decrease in the forest cover ca. 3000–2450 cal. yr BP followed by a generally open (agricultural) landscape and spread of secondary pine forests. This pattern is in line with a major population growth promoted by the southward expansion of rice-based agriculture across the region that postdates the widely accepted ‘Holocene climate optimum’ by several millennia. This highlights the multidimensional significance of this term, which is commonly used in palaeoenvironmental and archaeological studies and should be consequently applied with respect to the specific issue under consideration.
Article
The late Quaternary paleoceanography and paleoenvironment of the South China Sea (SCS) have been well reconstructed over the last decade. In contrast, the provenance of the terrigenous sediments that have accumulated in the northwestern continental shelf mud area remains enigmatic. This study investigated the provenance of these sediments and the paleoenvironmental changes archived in Core X2 via the analysis of geochemical elements, grain size, and accelerator mass spectrometry (AMS) ¹⁴C ages. Based on the upper continental crust (UCC)-normalized REE patterns and REE fractionation parameters, southwestern and western Taiwanese rivers and the Pearl River were identified as the main sources of the fine-grained sediment deposited in the northwestern shelf mud area off Hainan Island. This finding further confirms the long-distance transport (>1,000 km) of fine-grained sediment from Taiwanese rivers to the northern SCS shelf and slope. Obvious changes in the grain size and Chemical Index of Alteration (CIA) record occurred at approximately 4.0 cal kyr BP and were likely caused by increased Hainan Island inputs due to sea level changes.
Article
For the first time, palynological records of terrestrial palynomorphs and dinoflagellate cysts are investigated in a sediment core from the northern South China Sea (SCS) covering the last 12,500 years. Both terrestrial and marine palynomorph records show strong signals of the sea-level change during the studied interval. The highest herb pollen content was associated with extensive grasslands on the exposed shelf at the low sea-level stand during the Younger Dryas and early Holocene. The increase in fern spores and decrease in concentrations of dinoflagellate cysts and terrestrial palynomorphs was observed during the sea-level rise interval from 12,500 to ~ 6800 (or 6000) cal yr BP. Then, the sea level became stabilized and consistently low dinoflagellate cyst abundances and high abundances of fern spores were recorded.
Article
In order to reconstruct the Holocene environmental history of a coastal site in the northern South China Sea, this study analysed the organic carbon isotope ratios (δ¹³Corg) and alkenone unsaturation ratios (UK’37) from a 36.5 m-long sediment core drilled at seabed in the mouth region of the Pearl River estuary and generated a coupled hydrological and temperature record. This record reveals changes of monsoon-induced sediment discharge and sea surface temperature of the Holocene in four stages. In Stage I, the site was under fluvial conditions prior to postglacial marine transgression. Stage II saw an increase of sea surface temperature from c. 23.0 °C to 27.0 °C, associated with a strengthened summer monsoon from c. 10,350 to 8900 cal. years BP. This was also a period of rapid sea-level rise and marine transgression, during which the sea inundated the palaeo-incised channel, i.e. the lower part of the T-shape accommodation space created by the rising sea. In these 1500 years, fluvial discharge was strong and concentrated within the channel, and the high sedimentation rate (11.8 mm/yr) was very close to the rate of sea-level rise. In the subsequent 2000 years (Stage III) sea level continued to rise and the sea flooded the broad seabed above the palaeo-incised channel, resulted in fluvial discharge spreading thinly across the wide accommodation space and a much reduced sedimentation rate (1.8 mm/yr). Sea surface temperature in this stage reached 27.3 °C initially, but dropped sharply to 26.1 °C towards c. 8200 cal. years BP. The final stage covers the last 7000 years, and the site was under a stable sea level. Sedimentation in this stage varied a little, but averaged at 1.8 mm/yr. While fluvial discharge and sea surface temperature didn’t change much, two short periods of hydrological and temperature change were observed, which are related to the climatic cooling events of c. 4200 cal. years ago and the Little Ice Age.
Article
The number of diatom valves and fragments per gram of surface sediment in the Atlantic Ocean accurately reflects diatom abundance in the overlying waters, without any evidence of significant lateral drift during settling to the sea bottom. The distribution pattern of the number of resting spores per gram is similar to that for the whole and fragmented diatom valves. Fresh water diatoms and opal phytoliths are abundant in the sediments off the west coast of Africa where they have been deposited by the Trade Winds. Despite the relatively small number of core tops analyzed, the abundance and distribution patterns of diatoms in the sediments exhibit striking similarities to the patterns of primary productivity, phosphates, and annual production of silica in suspension in surface waters. Areas with high phosphate values and primary productivity and, therefore, areas of upwelling can be inferred from the quantitative distribution of diatoms in the sediments. Q-mode factor analysis, based on the abundance of forty-two species in thirty-seven core tops, produced six diatom species assemblages whose distributions provide additional information on the positions of certain water masses and major currents. Since the data on the quantitative distribution of diatom valves as well as on the diatom assemblages in the sediments of the Atlantic Ocean today allow prediction of certain water mass characteristics and circulation patterns of the overlying waters, they therefore, permit the reconstruction of paleoceanographic circulation patterns in ancient Atlantic Oceans, using the diatom distribution in sediments from dated horizons.
Article
We present continuous bi-weekly to bi-monthly dinoflagellate cyst, tintinnid loricae and tintinnid cyst fluxes at two mooring sites in Hudson Bay (subarctic Canada) from October 2005 to September 2006. The total dinoflagellate cyst fluxes at the site on the western side of the bay ranged from 4600 to 53,600 cysts m− 2 day− 1 (average 20,000 cysts m− 2 day− 1), while on average three times higher fluxes (average 62,300 cysts m− 2 day− 1) were recorded at the site on the eastern side of the bay with a range from 2700 to 394,800 cysts m− 2 day− 1. These values are equivalent to the average fluxes calculated from the top 1-cm sediment layer of 210Pb-dated box cores at corresponding locations, and hence lend support to the use of sediment dinoflagellate cysts in palaeoceanography. Tintinnid fluxes ranged from 1200 to 80,000 specimens m− 2 day− 1 (average 32,100 tintinnids m− 2 day− 1) in the west, and 1600 to 1,240,800 specimens m− 2 day− 1 (average 106,800 tintinnids m− 2 day− 1) in the east, with the highest Salpingella sp. fluxes recorded during the sea-ice cover season.
Chapter
Factor analyses of relative abundance counts of coccoliths from selected Pacific Ocean core tops yield six assemblages that coincide with the following surface water masses and regions: (1) the Kuroshio system and Pacific Equatorial Water Mass; (2) the southern and northern central Pacific regions (both occupied by two assemblages), (3) the Western South Pacific Central Water Mass, and (4) the Subarctic Pacific and Intermediate Water Masses. The sixth assemblage represents a flora that is transitional between the subpolar and central Pacific regions in both hemispheres and that occurs as well within the Peru Current system. Transfer functions relating the assemblages to sea-surface temperatures were applied to coccolith populations that existed over the past 200,000 yr and that were taken from cores in the western equatorial Pacific (cores V28-238 and V28-239) and the eastern Pacific (core V19-55). The values produced are floral indices calibrated on seabed samples to be unbiased estimates of summer sea-surface temperatures (Ts), winter sea-surface temperatures (Tw), and seasonality (Ts - Tw). Ts in the western equatorial Pacific varies within relatively narrow limits, with a maximum range of 5.7° and 3.9°C. Tw shows relatively greater fluctuation, with a maximum range of 8.9° and 6.3°C. Ts and Tw in the eastern Pacific show much less variation, with a maximum range of 1.9° and 3.3°C, respectively. There appears to be no correlation between Ts and glacial events. A seasonality (Ts - Tw) maximum occurs in midcycle just before glaciation in "glacial cycles" (Kukla, 1961) B, D, and E. The seasonality change is greater in the western equatorial cores. The foregoing adds evidence to support the theory that sea-surface temperature over the past 200,000 yr has not globally varied sinusoidally but is subject to complex regional dynamics.
Article
Fifty species of dinoflagellate cysts, among them two new species, have been identified in oceanic sediments of the Izu-Bonin region, south of Japan. All these species have a paleoenvironmental significance. The Quaternary climatic oscillation and environment evolution in the NW Pacific have been demonstrated by the variation of dinoflagellate cyst assemblages and the contents pollen grains in the arc sediments of the Izu-Bonin region. The variation of species ratios: temperate-warm to temperate-cold dinoflagellate cysts reveals the cyclic temperature changes of sea surface waters. The data are used to reconstruct the climatic evolution since 1,79 Ma ago. The results of the present study and a consideration of relevant sedimentological factors allow us to draw the following conclusions concerning the environment of the lzu-Bonin region. It appears that the carbonate concentration in sediments was higher in the temperate-cold than in the temperate-warm phases. In comparison with the ridge of outer arc, the axial region of the inner arc rift is characterized by greater proportion of volcaniclastic materials as well as a higher sedimentation rate and the palynological contents by high percentages of coniferous pollen. These particularity suggest a possible effect of the volcanic activity on the palynological contents in the intra-arc basin since 1 Ma ago. Thus the axial region of the inner arc has maintained outside of the influence of warm current Kuroshio since 0.46 Ma ago; this is reflected by the very high abundance of Impagidinium patulum to Impagidinium aculeatum.
Article
Surface collection, exposed sections and the use of irrigation wells and channels enabled the authors to map the settlement pattern of the elusive Dian kingdom before it became a subsidiary of the Han empire. The pattern showed that the Dian were already hierarchical, with settlements of different sizes and a political centre in which ritual bronzes featured. The empire redrew the landscape, with settlement migrating away from the wetlands into the hills where it could oversee the routes of communication into Southeast Asia.
Article
An analysis of historical oxygen data provides evidence on the water exchange between the South China Sea (SCS) and the Pacific Ocean (PO). In the vicinity of the Luzon Strait (LS), the dissolved oxygen concentration of sea water is found to be lower on the Pacific side than on the SCS side at depths between 700 and 1500 m (intermediate layer), while the situation is reversed above 700 m (upper layer) and below 1 500 m (deep layer). The evidence suggests that water exits the SCS in the intermediate layer but enters it from the Pacific in both the upper and the deep layers, supporting the earlier speculation that the Luzon Strait transport has a sandwiched structure in the vertical. Within the SCS basin, the oxygen distribution indicates widespread vertical movement, including the upwelling in the intermediate layer and the downwelling in the deep layer.