Glacial–interglacial variability in lower North Atlantic deep water: inference from silt grain-size analysis and carbonate preservation in the western equatorial Atlantic

Grain-size records of the terrigenous and calcareous silt fraction, preservation of planktic foraminifera, and benthic foraminiferal stable-isotope data (δ¹³C, δ¹⁸O values of C. wuellerstorfi) at ODP Site 927 on the Ceará Rise (5°27.7′N, 44°28.8′W), are used to reconstruct variations in the history of bottom current strength, ventilation, and carbonate corrosiveness of deep waters during the time interval from 0.8 to 0.3 Ma. Glacial periods are characterized by generally smaller mean sizes of the terrigenous sortable silt fraction (mean_(SS)), lower δ¹³C values, and poorer preservation of planktic foraminifera compared to interglacials. This indicates lower bottom current speeds, larger nutrient contents and more corrosive deep water. By contrast, larger mean_(SS) sizes, higher δ¹³C values, and well preserved planktic foraminifera indicate strong circulation and a well ventilated deep-water mass during interglacials. The observed changes are most likely related to the weakening and strengthening of circulation of Lower North Atlantic Deep Water (LNADW). Cross-spectral analysis between the mean_(SS) and benthic δ¹⁸O records reveals that minima in mean_(SS) occur about 7.6 k.y. after the maximum in ice volume. This indicates a considerable lag time between ice-shield induced changes in LNADW production and subsequent changes in the velocity of LNADW flow in the western equatorial Atlantic. Striking changes in bottom current speed occur regularly during glacial to interglacial transitions. Extremely fine mean_(SS) minima point to an almost complete shutdown of bottom current vigor in response to a cessation of LNADW production caused by an enhanced melt water release during the initial phases of deglaciation. However, each of the fine minima extremes is followed by a rapid shift to very high mean_(SS) values that indicate strong bottom currents, and hence, vigorous LNADW flow during the early interglacials. After the onset of glacial Stage 12, generally poorer carbonate preservation and higher variability is registered. This coincides with a global decrease in carbonate preservation during the mid-Brunhes (mid-Brunhes dissolution event). Detailed grain-size analysis of the calcareous fine fraction (<63 μm) revealed a considerable reduction of particles in the fraction from 7 to 63 μm during periods of enhanced dissolution. This indicates a preferential dissolution of larger planktic foraminiferal fragments which leads to an enrichment of coccoliths in the calcareous fine fraction.     

Reconstruction of the postglacial environments in the southwestern Barents Sea based on foraminiferal assemblages

Environmental changes in the surface and bottom water layers of the Ingøydjupet Basin and the history of the Atlantic Water inflow to the southwestern Barents Sea during the last 16 ka are reconstructed based on planktic and benthic foraminiferal assemblages. The multiproxy study of sediment cores PSh-5159R and PSh-5159N, including AMS ¹⁴C dating, provides a time resolution of about 200 years for the deglaciation, 100 years for the Holocene, and 25–50 years for the last 400 years. Stable polar conditions with the sea ice at the surface were typical for the Early Deglaciation period. Unstable bottom settings and the onset of ice rafting marked the Oldest Dryas. The cold Atlantic Water inflow increased notably during the Bölling-Alleröd interstadial nearby the site location and then decreased during the Younger Dryas. The initial Holocene was characterized by abrupt warming in bottom and surface water layers, especially ～9.7–7.6 ka BP. Stable conditions prevailed during the Middle Holocene. Remarkable changes in the sea-surface temperature and bottom environments occurred during the last 2.5 cal. ka BP.     

Foraminifera from the continental slope off Cape Hatteras, North Carolina

The recent benthic meiofaunal foraminiferal assemblage from the continental slope (590-2 003 m) off Cape Hatteras, North Carolina exhibits high species richness and evenness, moderate diversity values, and lacks numerically dominant species. The preserved planktic assemblage has relatively low species richness, high evenness, low diversity, and a few numerically dominant species. Approximately 9% of the benthic species are those that typically live within continental shelf depth ranges. The benthic assemblage abundances and diversities do not follow depth patterns or geophysical characteristics. No biogeographic boundary can be described within the study area for meiofaunal foraminifera. Oxygen limitation does not appear to be a factor affecting the benthos of the North Carolina continental slope based upon the community structure of the benthic foraminifera, if total assemblage is assumed to reflect the recently living community. The high carbonate content of sediments in the area may be explained by foraminiferal tests. Within the study area, the foraminiferal assemblages are uniform, and probably reflect relative consistency of primary environmental variables as well as dynamic downslope transport and high influx of material from the water column in the vicinity where the Gulf Stream and the Western Boundary Undercurrent cross.     

Abrupt changes of intermediate-water oxygen in the northwestern Pacific during the last 27 kyr

An oxygen minimum zone (OMZ) currently exists at intermediate water depths on the northern Japanese margin in the northwestern Pacific. The OMZ results largely from a combination of high surface–water productivity and poor ventilation of intermediate waters. We investigated the late Quaternary history (last 27 kyr) of the intensity of this OMZ using changes in benthic foraminiferal carbon isotopes and assemblages in a sediment core taken on the continental slope off Shimokita Peninsula, northern Japan, at a water depth of 975 m. The core was located well within the region of the present-day OMZ and high surface–water productivity. The benthic foraminiferal δ¹³C values, which indicate millennial-scale fluctuations of nutrient contents at the sediment–water interface, were 0.48‰ lower during the last glacial maximum (LGM) than during the late Holocene. These results do not indicate the formation of glacial intermediate waters of subarctic Pacific origin, but rather the large contribution of high-nutrient water masses such as the Antarctic Intermediate Water, implying that the regional circulation pattern during the LGM was similar to that of modern times. Benthic foraminiferal assemblages underwent major changes in response to changes in dissolved oxygen concentrations in ocean floor sediments. The lowest oxygen and highest nutrient conditions, marked by dysoxic taxa and negative values of benthic foraminiferal δ¹³C, occurred during the Bølling/Allerød (B/A) and Pre-Boreal warming events. Dysoxic conditions in this region during these intervals were possibly caused by high surface–water productivity at times of reduced intermediate–water ventilation in the northwestern Pacific. The benthic assemblages show dysoxic events on approx. 100- to 200-year cycles during the B/A, reflecting centennial-scale productivity changes related to freshwater cycles and surface–water circulation in the North Pacific.     

Regional primary productivity differences off Morocco (NW-Africa) recorded by modern benthic foraminifera and their stable carbon isotopic composition

The influence of different primary productivity regimes on live (Rose Bengal stained) and dead benthic foraminiferal distribution, as well as on the stable carbon isotopic composition of foraminiferal tests, was investigated in sediment surface samples (0–1 cm) from the upwelling region off Morocco between Cape Ghir (31°N) and Cape Yubi (27°N). A combination of factor analysis, detrended correspondence analysis (DCA) and canonical correspondence analysis (CCA) was applied to the benthic foraminiferal data sets. Five major assemblages for both the live and dead fauna were revealed by factor analysis. In the cape regions organic matter fluxes are enhanced by high chlorophyll-α concentrations in the overlying surface waters. Here, benthic foraminiferal faunas are characterized by identical live and dead assemblages, high standing stocks, and low species δ¹³C values, indicating constant year-round high productivity. Bulimina marginata dominates the unique fauna at the shallowest station off Cape Ghir indicating highest chlorophyll-a concentrations. Off both capes, the succession of the Bulimina aculeata/Uvigerina mediterranea assemblage, the Sphaeroidina bulloides/Gavelinopsis translucens assemblage, and the Hoeglundina elegans assemblage from the shelf to the deep sea reflects the decrease in chlorophyll-a concentrations, hence the export flux. In contrast, the area between the capes is characterized by differently composed live and dead assemblages, low standing stocks, and less depleted δ¹³C values, thus reflecting low primary productivity. High foraminiferal numbers of Epistominella exigua, Eponides pusillus, and Globocassidulina subglobosa in the dead fauna indicate a seasonally varying primary productivity signal. Significantly lower mean δ¹³C values were recorded in Bulimina mexicana, Cibicidoides kullenbergi, H. elegans, U. mediterranea and Uvigerina peregrina. Cibicidoides wuellerstorfi is a faithful recorder of bottom water δ¹³C in the Canary Islands regions. The mean δ¹³C signal of this species is not significantly influenced by constant high organic matter fluxes. The species-specific offset between live and dead specimens is the same.     

NotesBenthic foraminiferal distribution in surface sediments along continental slope of the southern Okinawa Trough: dependance on water masses and food supply

Benthic foraminiferal analysis of 29 samples in surface sediments from the southern Okinawa Trough is carried out. The results indicate that benthic foraminiferal abundance decreases rapidly with increasing water depth. Percentage frequencies of agglutinated foraminifera further confirm the modern shallow carbonate lysocline in the southern Okinawa Trough. From continental shelf edge to the bottom of Okinawa Trough, benthic foraminiferal fauna in the surface sediments can be divided into 5 assemblages: (1) Continental shelf break assemblage, dominated by Cibicides pseudoungerianus, corresponds to subsurface water mass of the Kuroshio Current; (2) upper continental slope assemblage, dominated by Cassidulina carinata , Globocassidulina subglobosa, corresponds to intermediate water mass of the Kuroshio Current; (3) intermediate continental slope assemblage, dominated by Uvigerina hispi-da, corresponds to the Okinawa Trough deep water mass above the carbonate lysocline; (4) lower continental slope- trough b     

Benthic foraminiferal distribution in surface sediments along continental slope of the southern Okinawa Trough: dependance on water masses and food supply

Benthic foraminiferal analysis of 29 samples in surface sediments from the southern Okinawa Trough is carried out.The results indicate that benthic foraminiferal abundance decreases rapidly with increasing water depth.Percentage frequencies of agglutinated foraminifera further confirm the modern shallow carbonate lysocline in the southern Okinawa Trough.From continental shelf edge to the bottom of Okinawa Trough,benthic foraminiferal fauna in the surface sediments can be divided into 5 assemblages:(1) Continental shelf break assemblage,dominated by Cibicides pseudoungerianus,corresponds to subsurface water mass of the Kuroshio Current;(2) upper continental slope assemblage,dominated by Cassidulina carinata,Globocassidulina subglobosa,corresponds to intermediate water mass of the Kuroshio Current;(3) intermediate continental slope assemblage,dominated by Uvigerina hispida,corresponds to the Okinawa Trough deep water mass above the carbonate lysocline.     

Original oxygen isotopic composition of planktic foraminifers preserved in diagenetically altered Pleistocene shallow-marine carbonates

The original stable isotopic composition of low-Mg calcitic planktic foraminifer tests is preserved in Pleistocene shallow-marine carbonates (in the Ryukyu Group; Okinawa, Japan) that have been altered by meteoric diagenesis. Whole-rock analyses indicate depleted isotopic values for both δ¹³C (−1.9 to −5.4‰) and δ¹⁸O (−2.9 to −5.2‰), as well as carbonate mineralogy exclusively composed of low-Mg calcite. However, analysis of carefully-extracted planktic foraminifer tests (Globigerinoides sacculifer) that were separated from these whole-rock samples yield heavier δ¹³C values (−0.4 to 1.9‰) and δ¹⁸O values (−3.2 to −1.0‰). The foraminiferal values themselves and comparison of values of various components suggest that the low-Mg calcite tests preserve the original stable isotopic values. Subsequently, the downcore δ¹⁸O change of planktic foraminifers recorded in the Ryukyu Group results from middle Pleistocene glacial–interglacial change. By comparison, isotopic measurements based on whole-rock samples can be obtained diagenetic environmental signals, but misleading with regard to paleoclimatic inferences.     

西北冰洋表层沉积物中的底栖有孔虫组合及其古环境意义

通过对中国第1~4次北极考察在西北冰洋采集的表层沉积物中底栖有孔虫丰度及其优势种分布特征与环境因素关系的综合研究发现,楚科奇海区低的底栖有孔虫丰度主要受较高的陆源物质输入的稀释作用影响;楚科奇海台和阿尔法脊较高的底栖有孔虫丰度主要受到暖而咸的大西洋中层水的影响;受碳酸钙溶解作用影响的门捷列夫深海平原和加拿大海盆底栖有孔虫丰度较低,并且水深3 597 m的站位出现了似瓷质壳的Pyrgo williamsoni和Quinqueloculina orientalis,说明该区的CCD深度大于3 600 m。根据底栖有孔虫7个优势属种的百分含量分布特征可以划分出5个区域组合:南楚科奇海陆架-白令海峡组合以优势种Elphidium excavatum和Buccella frigida为特征,可能反映受白令海陆架水影响的浅水环境;阿拉斯加沿岸-波弗特海组合以优势种Florilus scaphus和Elphidium albiumbilicatum为特征,可能反映受季节性海冰融化,低盐的阿拉斯加沿岸流以及河流淡水输入的低盐环境;大西洋中层水组合以优势种Cassidulina laevigata为特征,可能反映高温高盐的大西洋中层水影响的环境;北极深层水组合以优势种Cibicides wuellerstorfi为特征,可能反映水深大于1 500 m低温高盐的北极深层水环境;门捷列夫深海平原组合以优势种Oridorsalis umbonatus为特征,可能反映低营养的底层水环境。     

Living (Stained) Benthic Foraminifera and their Response to the Seasonal Hydrographic Cycle, Periodic Hypoxia and to Primary Production in Havstens Fjord on the Swedish West Coast

Havstens Fjord on the Swedish west coast is a silled fjord that is characterized by strong stratification and stagnant bottom-water with periodically occurring hypoxic (0–2 ml l⁻¹) or anoxic (≤0 ml l⁻¹) conditions. Renewal of Havstens Fjord's deep-water occurs every year, mostly during winter or spring. The aim of this study was to discover how living benthic Foraminifera respond to hydrographic variations, periodic oxygen deficiency and variations in primary production. A long series of monthly hydrographic measurements combined with sediment sampling were performed from August 1993 to December 1994. Sampling was carried out at four different sites along a depth transect (12, 20, 30 and 40 m). Monthly values of chlorophyll a from the water surface down to 15 m were obtained. With increasing water depth, the foraminiferal fauna changed from a low diverse brackish fauna, through a diverse and abundant fauna, to a low diversity and sparse fauna characterized by species tolerant of oxygen depletion. At the deepest site (40 m) Elphidium magellanicum and Stainforthia fusiformis survived five months of anoxic or near anoxic conditions. At 30 m the periodic hypoxic conditions were severe enough to prevent a rich benthic macrofauna establishing, but there was enough oxygen for an abundant and diverse foraminiferal fauna to thrive. Under oxic conditions, freshly sedimented phytoplankton seem to be an important food source for the Foraminifera.     

南海东沙海域HD319岩心富甲烷环境底栖有孔虫群落结构

对南海东北部东沙海域HD319岩心富甲烷环境和非富甲烷环境底栖有孔虫进行对比研究,旨在探讨富甲烷环境对底栖有孔虫群落结构的影响。结果表明,该岩心富甲烷环境和非富甲烷环境的底栖有孔虫组成有所差异:富甲烷缺氧环境底栖有孔虫群落中耐低氧类内生种群占绝对优势,Uvigerina(U.peregerina,U.hispido-costata,U.rugosa,U.probscidea,U.spp.),Bulimina(B.aculeata,B.mexicana),Bolivina(B.bradyi,B.quad-rilatera),Globocassidulina subglobosa,Brizalina superba,Hoglundina elegans等冷泉甲烷相关种大量出现;非富甲烷一般低氧环境下底栖有孔虫以内生种含量占优势,表生种和旋向种零星出现;富氧环境下底栖有孔虫以表生种和旋向种占优势,受有机碳含量变化控制明显。推测底栖有孔虫这种群落结构的差异是适应高甲烷含量的结果,底栖有孔虫对富甲烷环境的响应通过群落结构的差异表现出来。     

Upwelling and productivity along the Peru–Chile Current derived from faunal and isotopic compositions of planktic foraminifera in surface sediments

We report on the spatial distribution of isotopic compositions of the two planktic foraminifera species Globigerina bulloides and Neogloboquadrina pachyderma (dex.), and the faunal assemblages of planktic foraminifera in 91 surface sediment samples along the Chilean continental slope between 23°S and 44°S. Both δ¹³C and δ¹⁸O data of N. pachyderma (dex.) show little variability in the study area. North of 39°S, the isotopic values of N. pachyderma (dex.) are heavier than those of G. bulloides, whereas south of 39°S, this relation inverses. This is indicative for a change from a well-mixed, deep thermocline caused by coastal upwelling north of 39°S to well-stratified water masses in a non-upwelling environment south of 39°S. In addition, the faunal composition of planktic foraminifera marks this change by transition from an upwelling assemblage north of 39°S to a high-nutrient-non-upwelling assemblage south of 39°S, which is characterized by decreased contributions of upwelling indicators such as G. bulloides, N. pachyderma (sin.), and Globigerinita glutinata. In general, we can conclude that food and light rather than temperature are the primary control of the planktic foraminiferal assemblage between 23°S and 44°S off Chile. Our data point to higher marine productivity at upwelling centers north of 25°S and at 30–33°S. South of 39°S, significant supply of nutrients by fluvial input most likely boosts the productivity.     

Distribution of benthic foraminifera in Upper Quaternary sediments of the Deryugin Basin (Sea of Okhotsk)

Benthic foraminifera are investigated in sediment core LV28-34-2 (53°51.971′N, 146°47.499′E, sea depth 1431 m, core length 965 cm). The distribution of foraminifera is studied in coarse-grained (>0.125 mm) sediment fractions of 191 samples taken with a step of 5 cm. The core covers the interval from oxygen isotope stage (OIS) 6 up to the Holocene. The foraminiferal assemblages of the penultimate (OIS 6) and last (OIS 5d-2) glaciations are characterized by low abundances and prevalence of Uvigerina auberiana. The specific structure of the OIS 6 assemblages differs from the last glaciation ones by the mass presence of Cassidulina teretis, which characterizes low temperatures and a high influx of organic matter to the sea floor. The major factor responsible for the weak development of benthic foraminifers during the glacial time is the deficiency of food resources. The foraminiferal assemblage of the interglacial optimum (OIS 5e) is composed of both calcareous and agglutinated species (Martinottiella communis).The inflow of Pacific waters was probably more intensive, the bioproductivity was higher, and the critical carbonate compensation depth was shallower at that time than during the glaciations. During the deglaciation, the quantity and structure of the foraminiferal assemblages reflected two pulses of warming (terminations 1B and 1A) with an intervenient cooling event (Younger Dryassic). The assemblages of warm periods were characterized by exclusively high foraminiferal abundances, which sharply decreased during the Younger Dryassic cooling. The specific structure of the foraminiferal assemblage during deglaciation was relatively uniform, being composed of only calcareous taxa typical of highly productive areas of the ocean. The paleoenvironmental conditions were similar to the conditions of the interglacial optimum (OIS 5e), although the preservation of calcareous tests is better. In the Holocene sediments, the concentration of benthic foraminifera is substantially lower than during the deglaciation, which is explainable by their dilution in the sediments by diatoms; the composition of the foraminiferal assemblages is similar to that of their glacial counterparts.     

Vertical distribution of living mangrove foraminifera from KwaZulu-Natal,South Africa

Modern foraminiferal assemblage zones can be used to reconstruct palaeo sea levels when applied to fossil foraminifera down a sediment core. Previous intertidal foraminiferal studies have predominantly focused on assemblages in surface sediments (0–1 cm), with the rationale that surface assemblages reflect the modern-day environment. Foraminifera live infaunally and therefore there is a need to document the infaunal vertical distribution of living foraminifera to fully capture the modern environment. Infaunal foraminiferal populations may compositionally differ from or be similar to those in the uppermost 1 cm of a core sample, but abundance is variable vertically, making it very complex to reconstruct and interpret past sea levels. This can have implications for the choice of assemblages to use as modern analogues for past sea-level reconstructions. This study documents the vertical infaunal distribution of living foraminifera, to allow for more informed interpretations of palaeo-reconstructions in mangrove environments. The down-core vertical distribution and abundance of living foraminifera, along with grain size and organic content, were documented using sediment cores along an elevational transect. Nine taxa were recorded as living at the time of collection, six of which were restricted to the top 4 cm. The majority of these were calcareous and found in the cores situated closer to the intertidal channel. Therefore, we argue that the diversity of living calcareous and agglutinated foraminifera could be restricted by grain size, with coarser grain sizes associated with lower species diversity. The findings suggest that foraminiferal species inhabiting the top 4 cm represent deeper living foraminiferal populations. Therefore, the top 4-cm interval can be used to establish a modern training set upon which reconstructions can be based. The findings from this study will provide guidance on the use of South African mangrove environments for future sea-level reconstructions.     

Foraminiferal ecozones,a high resolution proxy for the late Quaternary biochronology in the central Mediterranean Sea

The planktic foraminiferal distribution identified in 60 cores collected in different basins of the Mediterranean Sea allowed to establish an ecostratigraphical scheme which provides a very important tool for the biochronological subdivision of the uppermost Quaternary. We identified a succession of ten ecozones during the last 23 ka in the Tyrrhenian basin and eight ecozones in the Adriatic Sea during the last 15 ka. The ecozones boundaries have been calibrated by ¹⁴C AMS radiometric data and by the stable oxygen isotope record. The chronological framework defined by the successive bioevents shows a very high resolution (millenary scale) and evidences that the changes in the planktic microfauna occurred more or less synchronously throughout the central Mediterranean Sea. Differences due to different oceanographic settings of the basins do not affect the general distributional pattern of planktic foraminifera.     

Foraminifers in late Pleistocene-Holocene sediments of the Deryugin Basin of the Sea of Okhotsk

The study of foraminifers from sediments accumulated during the last 30 ky revealed their similar distribution in the central and marginal parts of the Deryugin Basin, which implies similar environmental changes throughout this morphological structure. The lack or extreme taxonomical impoverishment of the benthic foraminiferal assemblages during the Last Glacial Maximum indicates a combination of several factors: the significant deterioration of the parameters controlling their bioproductivity in response to the general cooling; the development of anoxic bottom conditions; and, probably, the unfavorable influence of cold barite-methane seeps on the geochemical parameters of the bottom waters. The weaker activity of these seeps in the central part of the basin during the Holocene is evident from some deviations in the structure of the benthic foraminiferal assemblages against the background of their general taxonomic similarity.     

Messinian pre-evaporite sapropels and precession-induced oscillations in western Mediterranean climate

Cyclical fluctuations in planktic foraminiferal assemblages have been recognized in the pre-evaporitic Messinian in a marginal basin of the western Mediterranean. The fluctuations coincide with a dominantly precession-controlled sedimentary cyclicity (sapropels). During sapropel deposition, high planktic foraminiferal diversities are indicative of relatively stable marine conditions, while during homogeneous marl deposition low diversities seem to indicate the presence of unfavourable, more saline surface water conditions. The dominance of a precession-related signal indicates that regional climate oscillations rather than (obliquity-related) glacio-eustatically controlled influxes of Atlantic and/or Mediterranean waters are responsible for the faunal fluctuations and sedimentary cyclicity. Our scenario links the persistence of normal marine conditions during sapropel formation with increased rainfall and run-off along the western Mediterranean at times that perihelion occurred in Northern Hemisphere summer. Less favourable, highly saline surface water conditions prevailed during periods of drier climate induced by opposite precessional extremes. The cyclical oceanographic fluctuations could also have governed periodic reef growth along the margins.     

Benthic response of Munnopsurus atlanticus (Crustacea Isopoda) to the carbon content of the near-bottom sedimentary environment on the southern margin of the Cap-Ferret Canyon (Bay of Biscay,northeastern Atlantic Ocean)

The response of benthic organisms to organic carbon fluxes in a continental margin region was studied by investigating the diet of the suprabenthic isopod Munnopsurus atlanticus, which is well represented on the southern margin of the Cap-Ferret Canyon (Bay of Biscay). The grain-size distribution, foraminiferal assemblages, particulate organic carbon and pigments found in the sediment and in the gut of the isopods were analyzed. These results suggest that M. atlanticus feeds on benthic agglutinated foraminifers which are in a high “nourishment state” and represent a link between primary and secondary producers.     

Distribution patterns of living benthic foraminifera from Cap Breton canyon,Bay of Biscay: Faunal response to sediment instability

Analysis of living (Rose Bengal-stained) benthic foraminifera in 13 multicorer samples taken along the Cap Breton canyon (Bay of Biscay) revealed that the combination of organic-rich material and sediment instability provides very specific benthic ecosystem conditions. The active canyon hosts different foraminiferal assemblages that appear to be determined by different types and frequencies of environmental disturbance at the sites. Most of them are strongly dominated by shallow-infaunal living taxa that combine a tolerance for low-quality organic matter with a high reproductive potential. Foraminiferal assemblages characterized by high densities, very superficially living taxa and strong dominance of bolivinids and buliminids, follow a poor pioneer fauna dominated by Technitella melo. These assemblages are observed in the narrow canyon axis, where frequent sediment resuspension occurs and affects habitat stability. Assemblages studied from sites outside the canyon axis are still dominated by shallow-infaunal species but show lower foraminiferal densities and higher diversities. Deep-infaunal taxa are only present in some inner meanders and more distal stations. These assemblages are typical for ecological niches that are relatively stable and unaffected by re-sedimentation processes. They have attained a more advanced stage of ecosystem stability. They are influenced by neither lateral sediment nor enriched organic matter input.     

东海中陆架晚第四纪底栖有孔虫定量分析

利用东海中陆架EY02-1孔的柱状样品对底栖有孔虫动物群进行了定量分析研究,结合Q型因子分析及AMS“C测年数据,对东海中陆架古水团演化进行了初步探讨。Q型因子分析得到5个主因子．方差累积贡献为84．9％,这5个有孔虫组合在钻孔相关层位上反映特定的古水团特征。主因子1为Elphidium magellanicum组合,代表低温低盐沿岸水团(5～10m);主因子2为Bolivina robusta组合,代表中陆架水团(50～100m);主因子3为Protelphidium tuberculatum组合,代表外延的古黄海冷水团;主因子4为Florilus decorus组合,代表内陆架水团(0～50m);主因子5为Epistominella naraensis组合,代表水体较凉的水深大于30m的内陆架水团。结果表明,冰后期以古长江冲淡水为主体的低温低盐沿岸水团、中陆架水团及内陆架水团交替影响EY02-1孔附近中陆架。在氧同位素3期以古长江冲淡水为主体的东海沿岸流与外延的古黄海冷水团控制着有孔虫组合。在氧同位素5a-5c期间,EY02-1孔附近中陆架可能已受到古黄海冷水团的影响。该孔顶部11．4m含大量贝壳的细砂沉积可能对应氧同位素1期U2层潮流沙脊脊部沉积,细砂平均沉积速率高达0.877mm／a。