首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 593 毫秒
1.
A compilation of phytoplankton species abundance data from the spring bloom along the northern Norwegian coast and in the Barents Sea shows that the quantitatively most important species are the prymnesiophyte Phaeocystis pouchetii and the common cold water to temperate diatoms Chaetoceros socialis, Skeletonema costatum sensu lato, Fragilariopsis oceanica, Thalassiosira spp., Chaetoceros furcellatus, Chaetoceros compressus, Chaetoceros debilis and Bacterosira bathyomphala. The relative abundance of diatoms and Phaeocystis varied highly and apparently stochastically between years. P. pouchetii occurred during all stages of the spring bloom and sometimes completely dominated the phytoplankton community. Along the Norwegian coast, the importance of P. pouchetii increased northwards. The species composition in coastal fjords at 70° N is surprisingly similar to that of shelf waters in the Barents Sea (up to 80° N). An exception is S. costatum sensu lato which was seldom observed in Arctic waters. Small flagellates (<10 μm) other than dinoflagellates and P. pouchetii are also important among the Barents Sea spring phytoplankton. Associations of species seem rigid over time and are dominated by C. socialis and P. pouchetii in northern waters. Biogeographical categories of spring bloom species in relation to environmental conditions are discussed.  相似文献   

2.
Diatoms from surface sediment samples collected from Enderby Basin of Indian Sector of Southern Ocean were analyzed to determine the relative abundance and distribution of seven key indicator diatom species viz. Sea ice related species Fragilariopsis rhombica, F. separanda, F. curta, F. ritscheri, Thalassiosira tumida and Actinocyclus actinochilus and Open Ocean species F. kerguelensis on the basis of modern physico-chemical parameters. The relative abundances of different species observed viz. F. rhombica — 6.25%; F. separanda — 12.5%; F. curta — 10.53–13.33%; F. ritscheri — 4.55–12.5%; F. kerguelensis — 6.25–63.64%; T. tumida — 3.13% and A. actinochilus — 9.38–13.33%. The increasing abundance of F. kerguelensis consecutively suggests the effect of Antarctic bottom water in the study area which is further substantiated by the presence and increasing abundance of F. ritscheri. The gradual decrease in abundance or absence of sea ice related species from the sampled stations indicates the summer and winter sea ice extent concentration in the study area. The nutrient concentration correlates with the distribution and abundances of diatom species.  相似文献   

3.
Eight diatom and two silicoflagellate datum levels from the Pliocene and Pleistocene of the equatorial Pacific are ranked and related to the paleomagnetic reversal record, calcium carbonate fluctuations, and the oxygen isotope record. The datum levels and their position with respect to the paleomagnetic reversal record are as follows: (1) last occurrence of Thalassiosira convexa midway between the top of the Gauss and the base of the Olduvai; (2) transition from Rhizosolenia praebergonii to Rhizosolenia praebergonii var. A just below the base of the Olduvai; (3) first appearance of Pseudoeunotia doliolus in the lower part of the Olduvai; (4) last occurrence of Rhizosolenia praebergonii just above the Olduvai; (5) first appearance of Asteromphalus hiltonianus midway between the top of the Olduvai and the base of the Jaramillo; (6) first appearance of Mesocena elliptica just below the base of the Jaramillo; (7) last occurrence of Mesocena elliptica midway between the top of the Olduvai and the base of the Brunhes; (8) upward increase in abundance of Thalassiosira oestrupii just below the base of the Olduvai; (9) last occurrence of Nitzschia reinholdii just above the base of the Olduvai; (10) peak occurrence of Roperia tesselata var. ovata just above Datum Level 9. By second-order correlation it is possible to relate these datum levels to the oxygen isotope record. Such a correlation suggests that some datum levels may be climate related.  相似文献   

4.
Ice-distal Upper Miocene marine strata from inland Antarctica   总被引：2，自引：0，他引：2  
ABSTRACT Glacimarine strata of the Battye Glacier Formation (≈ 130 m thick), Pagodroma Group, exposed in the Amery Oasis of East Antarctica, provide a record of Late Miocene palaeoenvironmental conditions in an ice‐distal setting. The formation overlies the Amery Erosion Surface (≈ 300 m to ≈ 270 m above sea level) that formed during an advance of the Lambert Glacier into Prydz Bay (ODP Site 739), at least 750 km further north than today. Two lithological members: a grey and muddier Lower Member and a brown, sand‐rich Upper Member, reflect variation in proximity to the terminus of the Lambert Glacier. Ice‐distal, glacimarine, diatom‐bearing mud (up to 12% biogenic silica) and in situ articulated molluscs occur in the Lower Member. The Battye Glacier Formation is significant because of its inland location, which indicates that ice‐distal marine conditions existed 250 km inland from the current Amery Ice Shelf edge. Similar Neogene strata are known on land only from the Pliocene Sørsdal Formation in the Vestfold Hills, near the Antarctic coast. Three stratigraphic intervals of diatom‐bearing mud are recognized from glacially reworked clasts and from in situ strata informally referred to as the McLeod Beds and ‘Bed A’. The diatom‐bearing mud also contains sponge spicules and minor silicoflagellates and ebridians. Marine diatom biostratigraphy constrains the age of the beds to between 10·7 and 9·0 Ma (late Miocene). Abundant benthic diatoms suggest deposition within shallow euphotic waters. The high abundance of intercalary valves of Eucampia antarctica from an interval of the McLeod Beds suggests that there was less winter sea‐ice than in Prydz Bay today. It is unlikely that sea‐ice was perennial because the presence of Thalassionema spp. and Stellarima stellaris (Roper) Hasle et Sims suggests that summer sea‐surface temperatures were too warm (> 0°C and > 3°C respectively). The palaeoclimate at the time of deposition appears to have been analogous to that in modern fjords of East Greenland (e.g. Kangerdlugssuaq Fjord), which is consistent with the depositional model proposed previously for the Pagodroma Group. The three diatom‐bearing mud intervals were deposited in the Amery Oasis, ≈ 250 km inland of the current Amery Ice Shelf edge, when the East Antarctic Ice Sheet was reduced in size relative to today.  相似文献   

5.
Fossil diatoms from the Milky River Formation of the Sandy Ridge section, Alaska Peninsula are analyzed. Data on stratigraphic distribution of diatoms and silicoflagellates are presented. Based on presence of stratigraphically important forms (Neodenticula kamtschatica, Thalassiosira oestrupii, Cosmiodiscus insignis), the assemblages studied are correlated with the North Pacific Cenozoic diatom zones. An assemblage similar to that of subzone “b” of the Neodenticula kamtschatica Zone (Barron and Gladenkov, 1995) indicates the latest Miocene-initial early Pliocene (5.5-4.8 Ma) age of its host deposits. Additional species of biostratigraphic importance (Thalassiosira temperei, Th. latimarginata, and others) refine ages of different parts of the section. In particular, age of stratigraphically lowest beds bearing Astarte (mollusks valuable for establishing the Bering Strait opening) were dated at 5.5-5.4 Ma. Some paleogeographic inferences are made from analysis of composition of the diatom assemblages.  相似文献   

6.
Diatom assemblages from the marine Paleogene of Western Kamchatka (the Kovachina, Viventek, and Kuluven formations) are studied. The shallow-water neritic assemblage from the Kovachina Formation of the Pyatibratskii Cape section is tentatively attributed to the middle Eocene, as it is lacking zonal index species. A high abundance and great taxonomic diversity are characteristic of diatoms from the upper part of the Viventek Formation in the Kovran River section. The zonal index species Cavitatus rectus identified here among the diatoms suggests that the respective assemblage is macerated from the synonymous diatom zone of the middle Oligocene (29.6?28.2 Ma). According to its taxonomic composition and structure, this assemblage is indicative of transgression environments and the presumable influence of the middle Oligocene climatic optimum. The Rocella gelida (upper part) and Thalassiosira praefraga diatom zones of the Oligocene-Miocene transition are established in the Kuluven Formation sediments of the Mt. Uvuch section in Kovachina Bay.  相似文献   

7.
Very little research has been conducted on mid-Atlantic estuarine populations of the diatom Pseudo-nitzschia despite recent evidence of toxicity in regional isolates. We collected field samples from the Chesapeake Bay region from 2002 to 2007 for Pseudo-nitzschia enumeration and toxin analysis. Abundances of Pseudo-nitzschia were highest in the winter and spring at ∼103 cells ml−1. Domoic acid (DA) was detectable in 42% of samples tested, but concentrations were generally low, ranging from 0 to 1,037 pg DA ml−1 (mean 176 pg DA ml−1), and DA cell quota ranged from <0.1 to 49.4 pg DA per cell (mean 1.4 pg DA per cell). Although Pseudo-nitzschia populations were observed year round when salinity was ≥5, populations were highest from February to May when temperatures were low (2–15°C) and salinity relatively high (≥10). Six species of Pseudo-nitzschia were identified via transmission electron microscopy of the samples: P. pungens (Grunow ex Cleve) Hasle, P. calliantha Lundholm, Moestrup et Hasle, P. subpacifica (Hasle) Hasle, P. cuspidata (Hasle) Hasle emend. Lundholm, Moestrup et Hasle, P. fraudulenta (Cleve) Hasle and P. multiseries (Hasle) Hasle. P. calliantha was the most common and not previously reported from the Chesapeake Bay. Of these species, P. pungens, P. calliantha, P. cuspidata, P. fraudulenta and P. multiseries are known to produce domoic acid.  相似文献   

8.
南海北部陆坡ODP 1144站位第四纪硅藻及其古环境演变   总被引：9，自引：0，他引：9  
李家英 《地质论评》2002,48(5):542-551
中国南海北部陆坡ODP 1144站位硅藻植物群的研究，建立了西太平洋边缘海一个新的中更新世晚期以来的硅藻生物地层图式，根据硅藻化石中具有指示意义的硅藻种的分布和生态变化（暖水种和冷水种），划分了8个硅藻组合带，其硅藻组合带分别在不同的高低海面环境下形成的，根据ODP1144站位氧同位素（OIS）测定结果，8个硅藻组合带与OIS 1－8期相对应。1、3、5、7硅藻组合带相当于OIS1、3、5、7期，间冰期是以热带和亚热带硅藻占优势，其中冷期出现大量的沿岸硅藻为特征，反映高海平面温暖的气候条件；2、4、6、8硅藻组合带相当于OIS2、4、6、8期，冰期是以亚热带，热带和出现较多冷水硅藻为特征，反映低海平面较冷气候条件，硅藻丰度值的变化与冰期和间冰期有关，可以证实间冰期时期高的海平面和较低的生物生产力以及冰期时低的海平面和高的生物生产力，而生物生产力的变化又与沉积时期沿岸流或上升流的强弱及水团活动有密切关系，进而揭示该区古海洋环境的演化与季风强弱之间的内在关系。  相似文献   

9.
A total of 85 samples, collected from the UBGH1-9 core taken from the Ulleung Basin, East Sea, Korea, were analyzed using diatom assemblages. 111 diatom species belonging to 46 genera were identified, and three diatom assemblage zones were established on the basis of occurrence and distribution pattern of diatoms. Diatom assemblage zone I(134.10–174 m) is characterized by a relatively high abundance of marine species, while the increased number of the brackish species is recorded in diatom assemblage zone II(75–125 m). The assemblage zones IIIa became drastic drop of valve abundances and brackish planktons, whereas it became increase during the IIIb. High Td values which indicate an influence of warm current are recorded both in diatom assemblage zone I and III, and low Td values in diatom assemblage zone II. Analysis of diatom assemblages indicating that the depositional condition moved from oceanic to littoral-neritic environments and that paleotemperature underwent a shift from warm to cold condition at the middle interval, and from cold to warm condition in the upper interval of the UBGH1-9 core. This suggests that the lower(130–162 m) and upper intervals(0–20 m) of the UBGH1-9 core were deposited in the warm current condition(Tsushima Warm Current).  相似文献   

10.
Quantitative diatom abundance and species composition was studied in the longest 81-m Lake Hovsgol drill core, HDP-04. Lake Hovsgol diatom biostratigraphy of the past 1 Ma includes 14 local diatom assemblage zones (LDAZ). Planktonic diatom flora of Lake Hovsgol is dominated by members of Stephanodiscus and Cyclotella. A large-scale shift from Stephanodiscus-dominated to Cyclotella-dominated flora occurred in the mid-Brunhes chron. Significant morphological variability of Cyclotella ocellata complex and Cyclotella cf. minuta, as well as the presence of new extinct endemic taxa of genus Stephanodiscus, suggest long speciation and/or adaptation history of diatom flora. The recognition of taxa morphologically similar to the extinct endemic species of Lake Baikal suggests potential correlation ties between Lake Hovsgol and Lake Baikal diatom biostratigraphies around the MIS 17, MIS 25, and, possibly, MIS 15 interglacial intervals. Narrow peaks of diatom abundance in the Lake Hovsgol sedimentary record are episodic and short-lived. These peaks may represent humid optima of past interglacials and/or interstadials, characterized by elevated surface runoff in the Hovsgol basin and relative highstands of the lake.  相似文献   

11.
Understanding phytoplankton species-specific responses to multiple biotic and abiotic stressors is fundamental to assess phenological and structural shifts at the community level. Here, we present the case of Thalassiosira curviseriata, a winter-blooming diatom in the Bahía Blanca Estuary, Argentina, which displayed a noticeable decrease in the past decade along with conspicuous changes in phenology. We compiled interannual field data to assess compound effects of environmental variations and grazing by the invasive copepod Eurytemora americana. The two species displayed opposite trends over the period examined. The diatom decreased toward the last years, mainly during the winters, and remained relatively constant over the other seasons, while the copepod increased toward the last years, with an occurrence restricted to winter and early spring. A quantitative assessment by structural equation modeling unveiled that the observed long-term trend of T. curviseriata resulted from the synergistic effects of environmental changes driven by water temperature, salinity, and grazing. These results suggest that the shift in the abundance distribution of T. curviseriata toward higher annual ranges of temperature and salinity—as displayed by habitat association curves—constitutes a functional response to avoid seasonal overlapping with its predator in late winters. The observed changes in the timing and abundance of the blooming species resulted in conspicuous shifts in primary production pulses. Our results provide insights on mechanistic processes shaping the phenology and structure of phytoplankton blooms.  相似文献   

12.
Throughout the last 1.1 million years repeated glaciations have modified the southern Fennoscandian landscape and the neighbouring continental shelf into their present form. The glacigenic erosion products derived from the Fennoscandian landmasses were transported to the northern North Sea and the SE Nordic Seas continental margin. The prominent sub‐marine Norwegian Channel trough, along the south coast of Norway, was the main transport route for the erosion products between 1.1 and 0.0 Ma. Most of these erosion products were deposited in the North Sea Fan, which reaches a maximum thickness of 1500 m and has nearly 40 000 km3 of sediments. About 90% of the North Sea Fan sediments have been deposited during the last 500 000 years, in a time period when fast‐moving ice streams occupied the Norwegian Channel during each glacial stage. Back‐stripping the sediment volumes in the northern North Sea and SE Nordic Seas sink areas, including the North Sea Fan, to their assumed Fennoscandian source area gives an average vertical erosion of 164 m for the 1.1–0.0 Ma time period. The average 1.1–0.0 Ma erosion rate in the Fennoscandian source area is estimated to be 0.15 mm a?1. We suggest, however, that large variations in erosion rates have existed through time and that the most intense Fennoscandian landscape denudation occurred during the time period of repeated shelf edge ice advances, namely from Marine Isotope Stage 12 (c. 0.5 Ma) onwards.  相似文献   

13.
Bulk carbonate content, planktic and benthic foraminiferal assemblages, stable isotope compositions of bulk carbonate and Nuttallides truempyi (benthic foraminifera), and non-carbonate mineralogy were examined across ∼30 m of carbonate-rich Paleogene sediment at Deep Sea Drilling Project (DSDP) Site 259, on Perth Abyssal Plain off Western Australia. Carbonate content, mostly reflecting nannofossil abundance, ranges from 3 to 80% and generally exceeds 50% between 35 and 57 mbsf. A clay-rich horizon with a carbonate content of about 37% occurs between 55.17 and 55.37 mbsf. The carbonate-rich interval spans planktic foraminiferal zones P4c to P6b (∼57–52 Ma), with the clay-rich horizon near the base of our Zone P5 (upper)—P6b. Throughout the studied interval, benthic species dominate foraminiferal assemblages, with scarce planktic foraminifera usually of poor preservation and limited species diversity. A prominent Benthic Foraminiferal Extinction Event (BFEE) occurs across the clay-rich horizon, with an influx of large Acarinina immediately above. The δ13C records of bulk carbonate and N. truempyi exhibit trends similar to those observed in upper Paleocene–lower Eocene (∼57–52 Ma) sediment from other locations. Two successive decreases in bulk carbonate and N. truempyi δ13C of 0.5 and 1.0‰ characterize the interval at and immediately above the BFEE. Despite major changes in carbonate content, foraminiferal assemblages and carbon isotopes, the mineralogy of the non-carbonate fraction consistently comprises expanding clay, heulandite (zeolite), quartz, feldspar (sodic or calcic), minor mica, and pyrolusite (MnO2). The uniformity of this mineral assemblage suggests that Site 259 received similar non-carbonate sediment before, during and after pelagic carbonate deposition. The carbonate plug at Site 259 probably represents a drop in the CCD from ∼57 to 52–51 Ma, as also recognized at other locations.  相似文献   

14.
南海渐新世以来粘土矿物的演变特征及意义   总被引：14，自引：1，他引：13  
唐松  邵磊  赵泉鸿 《沉积学报》2004,22(2):337-342
通过对南海北部ODP184航次1148站32.8Ma以来深海沉积物中粘土矿物组成及其变化特征的分析,发现以15.5Ma为界,下部沉积物粘土矿物组成中蒙脱石含量高达70%,上部则以伊利石和绿泥石逐步增高、蒙脱石含量明显下降为特征,揭示出该区深海沉积物中粘土矿物演变可以分为32.8～15.5Ma的构造控制期和15.5Ma以来的气候控制期。在构造控制期,粘土矿物成分记录了南海扩张演变的历史;而在气候控制期,粘土矿物特征除反映出南海北部周边总体降温的过程外,还显示了3Ma的突然降温,与该站底栖有孔虫氧同位素资料反映的全球气候变化具有很好的对比性。  相似文献   

15.
本文对“第33次南极科考—海洋六号航次”在乔治王岛南侧、象岛西侧海域内通过箱式取样获得的表层沉积物样品进行了硅藻组成和分布特征的研究。主要统计了硅藻丰度、简单分异度、属种组成等, 并结合沉积物粒度数据进行分析, 结果表明乔治王岛南侧站位丰度和分异度高于象岛西侧站位, 最高丰度达到1.96×106壳/g。丰度的差异分布可能主要与洋流有关, 别林斯高晋海表层洋流带入充足营养盐导致乔治王岛南侧站位硅藻繁盛, 而威德尔海涡流底层水影响下的象岛西侧站位则难以沉降保存包括硅藻壳体在内的细小沉积颗粒。各站位硅藻组成的差异主要体现在海冰相关种分布不均和半咸水种类繁盛情况等, 前者可能与漂浮海冰控制的时间长短有关, 而后者与淡水输入等综合环境因素有关。  相似文献   

16.
上新世最重要的古气候事件就是北极冰盖的形成和扩张（3.2—2.5Ma）。根据北极冰盖的形成过程,文中将上新世—早更新世（4.5—1.6Ma）西太平洋暖池核心区ODP807A孔2个浮游有孔虫属种Globigerinoides ruber和Pulleniatina obliquiloculata的δ18O（氧同位素）、δ13C（碳同位素）记录数据细分为3个阶段：形成前（4.5—3.2Ma）、形成中（3.2—2.5Ma）和形成后（2.5—1.6Ma）。研究结果表明,北极冰盖形成以来,G.ruber和P.obliquiloculata的 δ18O值均呈增大趋势;北极冰盖形成中,G.ruber 的δ18O值基本不变,P.obliquiloculata的 δ18O值略增大,反映出西太平洋暖池次表层海水受北极冰盖形成的影响更甚;Δδ18O（P-G）（G.ruber和P.obliquiloculata的 δ18O 的差值）反映出温跃层无明显变化,受北极冰盖形成影响微弱。根据用Ma/Ca值得到的ODP806站SST值（Wara et al.,2005）以及用UK′37得到的东太平洋ODP847站SST值数据（Herbert and Laura,2010）,对北极冰盖形成以来东西太平洋古海洋学演化进行比较发现：冰盖形成以来东西赤道太平洋海水表层温度均有所下降,且东太平洋降温幅度更大;对比同时期的西太平洋ODP807A孔Δδ18O（P-G）与东太平洋ODP847站Δδ18O（T-S）的变化趋势,发现东太平洋温跃层变浅,表明受北极冰盖形成和扩张的影响,东西太平洋温跃层深度梯度增大。对比ODP807A孔与南海南部1143站、南海北部1148站G.ruber的 δ18O 和δ13C记录,揭示西太平洋边缘海如南海北部和南部受北极冰盖形成的影响有所不同,北部的表层海水温度和生产力水平受冰盖形成的影响更加明显。  相似文献   

17.
《Quaternary Science Reviews》2007,26(5-6):808-827
During the Late Pliocene–Middle Pleistocene, 56 species and 15 genera of elongate, cylindrical benthic foraminifera disappeared from the deep sea in the South China Sea (ODP Sites 1143 and 1146) as part of the last global extinction in the deep sea. This extinction group (Ext. Gp) exhibited a pulsed decline in abundance and species richness mostly during glacials, and often associated with periods of expansion of polar ice that resulted from increased cooling of the Earth's climate since ∼2.5 Ma, particularly during the Mid-Pleistocene Climate Transition (MPT, 1.2–0.6 Ma). We infer that the Ext. Gp decline in abundance and disappearance was a result of the increased glacial cooling and consequent increased ventilation of the deep-sea water masses. The detailed record of withdrawal of the Ext. Gp differs between the two sites, with far more disappearances occurring prior to the MPT in the deeper Site 1143 (2772 m) than in the shallower Site 1146 (2092 m). The Late Pliocene and Early Pleistocene declines in deeper parts of the South China Sea (Site 1143) may have resulted from enhanced glacial production of deep, southern-sourced water passing over the sill into the basin from the North-west Pacific. During the MPT however, the Ext. Gp declines and disappearances were of similar timing and magnitude in both sites, implying that both were influenced by the same deep-water mass during glacials at this time. This could have been North Pacific Deep Water, which many workers infer was formed in the northern Pacific during the last glacial, and may have begun forming during MPT glacials, in association with the progressively enhanced cooling of the Northern Hemisphere.  相似文献   

18.
为了多角度理解海洋纹层沉积物的形成过程,利用在西菲律宾海采集到的含纹层硅藻席的XT47孔岩心样品,进行多囊虫类放射虫的组合变化与环境意义分析.研究表明XT47孔岩心上段260 cm厚硅藻席沉积中含有丰富的放射虫、丰度高于2×104个/克;中段260~460 cm出现5次放射虫丰度高峰与红粘土的交替旋回沉积;下段460~630 cm红粘土沉积层中放射虫稀少.依据放射虫地层种的初现生物事件,470 cm以深的年龄大于0.34 Ma,表明西菲律宾海硅藻质沉积形成于中更新世以来.此外,受暖水影响控制的Tetrapyle group和Didymocyrtis tetrathalamus tetrathalamus相对丰度在末次冰盛期（LGM）呈现显著的低值期,推测在LGM期西菲律宾海北赤道暖流的影响强度是变弱的.值得注意的是,多囊虫类放射虫的丰度与生物硅含量的变化趋势出现明显的不同步现象,推测4次生物硅含量高峰是由大型硅藻（盘筛藻）的急剧增加导致的,即西菲律宾海LGM期盘筛藻的勃发呈现明显的阶段性强弱变化;进一步发现盘筛藻的4次勃发强盛期对应着胶体虫相对丰度变化的低谷期和中深层冷水种相对丰度变化的高峰期,前者指示上层水体成层化减弱、后者指示中深层水体中营养物增多.据此,认为西菲律宾海LGM期盘筛藻勃发的4次强盛期是由于上层水体成层化减弱、下层水体中丰富的营养物上涌引起.研究结果可为菲律宾海盘筛藻勃发在LGM长时间持续并最终在海底形成厚达近3 m的硅藻席沉积提供了一种可能的解释.   相似文献   

19.
安徽伏川蛇绿岩套的Nd-Sr-O同位素研究   总被引：13，自引：1，他引：13       下载免费PDF全文
安徽歙县伏川的蛇绿岩套形成于中-晚元古宙,其Nd、Sr和O同位素组成是:εNd(T)=+0.7-+3.8,εSr(T)=+30.7-+53.9,δ18O=3.2-11.0‰。据地质学和同位素地球化学特征,该岩套位于杨子板块南缘、江南古岛弧的弧后小洋盆地轴部。εNd(T)值的变化是由于蛇绿岩形成过程中受到下伏不成熟硅铝质基底地壳的混染引起的;εSr(T)和δ18O的变化,是在蛇绿岩形成时或形成后不久遭受海水热液蚀变的结果。  相似文献   

20.
To detect climatic linkages between the Baltic Sea, the Skagerrak and the Nordic Seas, we present multi‐proxy reconstructions covering the last 4500 years from three sediment cores taken in the Skagerrak and along the SW Norwegian margin. Foraminiferal assemblages at all three sites show a distinct change at c. 1700 years BP, associated with a transition from absence and rare occurrence of Brizalina skagerrakensis during c. 4500–2300 years BP to its subsequent abundance increase, suggesting a stronger influence of nutrient‐rich water‐masses during the last c. 1700 years. Increased nutrient availability, which probably stimulated higher primary productivity, is further supported by an increase in diatoms, total organic carbon and benthic foraminiferal species indicative of high productivity and carbon fluxes during the last c. 1700 years as compared to c. 4500–2300 years BP. The amplitude of the B. skagerrakensis signal is largest in the central Skagerrak and gradually becomes smaller towards the Norwegian Sea suggesting that the dominant source of the nutrient‐rich water was the brackish outflow from the Baltic Sea. The generally lower abundances of planktonic foraminifera since c. 1700 years BP support the hypothesis of less saline surface water conditions in the Skagerrak. These results agree with other studies, which suggest a stronger Baltic outflow over the last 1700 years coinciding with a general cooling, increased wintertime westerlies bringing more winter precipitation to northern Europe, increased river runoff and higher frequency of floods. The increase in outflow also occurs during deposition of laminated sediments in the deep Baltic Sea. Leakage of dissolved inorganic phosphorus from anoxic sediments, as well as enhanced erosion due to deforestation in combination with higher runoff from Norway, coastal upwelling and more vigorous frontal dynamics may all have contributed to higher nutrient availability within the adjacent Skagerrak during the last 1700 years BP as compared to c. 4500–2300 years BP, when low productivity prevailed in the study area.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号