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1.
Much uncertainty exists in the phosphorus (P) cycle in the marshes of the intertidal zone. This study explored the P cycling in the two Suaeda salsa marshes [middle S. salsa marsh (MSM) and low S. salsa marsh (LSM)] of the Yellow River estuary during April 2008 to November 2009. Results showed seasonal fluctuations and vertical distributions of P in different S. salsa marsh soils, and variations in P content in different parts of plants due to water and salinity status. The N/P ratios of the different S. salsa were 9.87 ± 1.23 and 15.73 ± 1.77, respectively, indicating that plant growth in MSM was limited by N, while that in LSM was limited by both N and P. The S. salsa litter in MSM released P to the environment throughout the year, while that in LSM immobilized P from the environment at all times. The P absorption coefficients of S. salsa in MSM and LSM were very low (0.0010 and 0.0001, respectively), while the biological cycle coefficients were high (0.739 and 0.812, respectively). The P turnovers among compartments of MSM and LSM showed that the uptake amounts of roots were 0.4275 and 0.0469 g m?2 year?1 and the values of aboveground parts were 1.1702 and 0.1833 g m?2 year?1, the re-translocation quantities from aboveground parts to roots were 0.8544 and 0.1452 g m?2 year?1, the translocation amounts from roots to soil were 0.0137 and 0.0012 g m?2 year?1, the translocation quantities from aboveground living bodies to litter were 0.3157 and 0.0381 g m?2 year?1, and the annual return quantities from litter to soil were less than 0.0626 and ?0.0728 g m?2 year?1 (minus represented immobilization), respectively. P was an important limiting factor in S. salsa marshes, especially in LSM. S. salsa was seemingly well adapted to the low-nutrient condition and the vulnerable habitat, and the nutrient enrichment due to the import of N and P from the Yellow River estuary would be a potential threat to the S. salsa marshes.  相似文献   

2.
The Late Cretaceous (Maastrichtian) to early Paleocene (Thanetian) shallow water (<100 m) agglutinated foraminifera from a section at Dakhla Oasis (Western Desert, Egypt) were analyzed for their assemblage, species and genera distribution, diversity, depositional environment, community structure and palaeobathymetry with respect to regional tectonics, climate and global eustasy. Data suggest an equitable benthic environment with low species dominance deposited in a brackish littoral and/or marsh setting. Sea level curves using characteristic benthic foraminiferal species, genera and assemblages corroborate quantitatively generated estimate and statistical analysis. Data suggests that in the absence of or of an impoverished benthic foraminiferal fauna, a high resolution agglutinated foraminiferal dataset can be as good a predictor of the benthic community structure and environment, as its calcareous counterpart, at least for shallow settings (<100 m). Present data also provides a good window in better understanding the distribution and interrelationship between the three dominant genera, Haplophragmoides, Trochammina and Ammobaculites. Faunal changes at boundaries (Cretaceous/Paleogene, Danian/Selandian and Selandian/Thanetian) are also evaluated.  相似文献   

3.
A new centennial scale benthic foraminiferal record of late Holocene climate variability and oceanographic changes off West Greenland (Disko Bugt) highlights substantial subsurface water mass changes (e.g. temperature and salinity) of the West Greenland Current (WGC) over the past 3.6 ka BP. Benthic foraminifera reveal a long-term late Holocene cooling trend, which may be attributed to increased advection of cold, low-salinity water masses derived from the East Greenland Current (EGC). Cooling becomes most pronounced from c. 1.7 ka BP onwards. At this point the calcareous Atlantic benthic foraminiferal fauna decrease significantly and is replaced by an agglutinated Arctic fauna. Superimposed on this cooling trend, centennial scale variability in the WGC reveals a marked cold phase at c. 2.5 ka BP, which may correspond to the 2.7 ka BP cooling-event recorded in marine and terrestrial archives elsewhere in the North Atlantic region. A warm phase recognized at c. 1.8 ka BP is likely to correspond to the ‘Roman Warm Period’ and represents the warmest bottom water conditions. During the time period of the ‘Medieval Climate Anomaly’ we observe only a slight warming of the WGC. A progressively more dominant cold water contribution from the EGC on the WGC is documented by the prominent rise in abundance of agglutinated Arctic water species from 0.9 ka BP onwards. This cooling event culminates at c. 0.3 ka BP and represents the coldest episode of the ‘Little Ice Age’.Gradually increased influence of cold, low-salinity water masses derived from the EGC may be linked to enhanced advection of Polar and Arctic water by the EGC. These changes are possibly associated with a reported shift in the large-scale North Atlantic Oscillation atmospheric circulation pattern towards a more frequent negative North Atlantic Oscillation mode during the late Holocene.  相似文献   

4.
1975年我们报道了泥河湾盆地怀来及蔚县境内发现半咸水有孔虫化石,当时认为它们的层位可能属第四系上部,后来进一步工作证实应是泥河湾组上部沉积。1977年我们对华北平原第四纪海进海退现象作了初步报道,认为华北平原在早更新世时发生过海进(渤海海进),并引用陈方吉在北京东南发现海相介形虫资料,说明早更新世渤海海进时海水曾经到达这一地区。1978年李鼎容等在北京市顺义县早更新世地层中发现大量有孔虫化石。进一步证实北京地区早更新世确曾发生过海进,使我们解释怀来及蔚县所发现的半咸水有孔虫来源,有更多证据。这一发现,对于北京地区第四纪地层划分对比、新构造运动判别和古地理恢复有重大意义。  相似文献   

5.
Assemblages of benthic foraminifera from one clastic succession in the Afales Basin (Ithaki Island, western Greece) were investigated to reconstruct palaeoenvironmental conditions during the Oligocene. The section consists of alternating hemipelagic marls and detrital deposits, designated as flysch-like beds, attributed to biostratigraphic Zones P20 and P21. Planktic percentages are mostly high (66–80%). Benthic foraminiferal assemblages comprise calcareous and agglutinated taxa (up to 15%). The prevalence of epifaunal foraminifera indicates good ventilation of the bottom water resulting from basin morphology, which enabled the undisturbed flow of water throughout the basin. Palaeodepth estimates imply bathyal deposition, from about 800 to 1200 m deep. The benthic foraminiferal fauna is of high diversity along the section, as is expected in deep marine environments. The abundances of the most common foraminiferal taxa (Cibicidoides spp., Oridorsalis umbonatus, Gyroidinoides spp., Stilostomella spp., Nodosariidae, Nuttallides umbonifera) are quite variable and imply generally oligotrophic to mesotrophic environmental conditions with variable organic flux.  相似文献   

6.
Diatoms, silicoflagellates, and biogenic silica (BSi) were analyzed from two piston cores recovered from Effingham Inlet, British Columbia. Relatively productive marine conditions from 4850 to 4000 cal yr BP were followed by a transition to the modern ocean-climate regime marked by a decreased siliceous microfossil production since 2800 cal yr BP. This change in the northeast Pacific climate was characterized by an apparent cooling associated with higher rainfall and lower light levels. The reduced abundance of most spring-summer bloom diatom taxa (Skeletonema-Thalassiosira-Chaetoceros) was coupled with a decreased abundance of diatoms normally associated with incursions of offshore water into coastal inlets. This pattern reflected a weaker summer upwelling along Vancouver Island associated with the insolation-related increase in the strength of the Aleutian Low and a weakened North Pacific High. After ca. 2800 cal yr BP, diatom assemblages also indicated more frequent periods of relatively low spring-summer surface water salinity and a disruption of the typical bloom sequence, indicative of increased climatic variability. A period of warmer and drier climate conditions and possibly increased coastal upwelling offshore occurred ca. 1450-1050 cal yr BP. The most recent 500 yr are marked by reduced diatom production and the appearance of three distinct diatom biomarkers in the stratigraphic record (Rhizosolenia setigera ca. AD 1940; Minidiscus chilensis ca. AD 1860; Thalassionema nitzschioides morphotype A, ca. AD 1550). The oceanographic changes recorded in Effingham Inlet are correlative with other marine and terrestrial paleoenvironmental records in the northeast Pacific Ocean.  相似文献   

7.
Benthic macroinvertebrate abundance, taxonomic composition, and surface flooding dynamics were compared among high and low elevation stands of narrow-leaved cattail (Typha angustifolia) and invasive common reed (Phragmites australis) at Iona Island Marsh, an oligohaline wetland, and Piermont Marsh, a mesohaline wetland, within the Hudson River National Estuarine Research Reserve during 1999 and 2000. Overall, the benthic macroinvertebrate community at both sites was similar in composition and abundance to those documented from other low-salinity systems. Macroinvertebrate taxa richness was lowest in mesohaline common reed, but similar among common reed and cattail habitats in oligohaline wetlands. Total macroinvertebrate densities were greater at high-elevation compared to low-elevation reed stands at the mesohaline site during summer 1999 and spring 2000. Total macroinvertebrate densities were similar among both oligohaline vegetation types during all seasons, except for spring 2000, when lower densities were observed in low-elevation common reed. A weak positive relationship between macroinvertebrate density and depth of flooding suggests that surface hydrology may be influencing the observed patterns of macroinvertebrate density among the vegetation stands. These results suggest that benthic macroinvertebrate abundance and diversity may not necessarily be impaired in low-salinity wetlands experiencing invasion by common reed unless the change in vegetation is accompanied by a measurable alteration to physical conditions on the marsh surface (i.e., elevation and flooding dynamics).  相似文献   

8.
Sundarban is a largest mangrove forest delta developed along the NE-SW direction covering parts of India and Bangladesh. Little work has been done on Indian part of Sundarban in respect of heterogeneity in channel morphology which could be mostly due to the effect of tilting and basin subsidence. These changes might have played a major role on development of high marshes, which offers a congenial environment for survival of Haplophragmoides wilberti, Jadammina macrescens, Trochammina inflata, and Miliammina fusca. These marsh benthonic foraminiferal assemblages provide a direct evidence of recent past sea level changes. To establish the depositional pattern and their effects rendered by merciless changing environment, 11 pit sections have been excavated along three E-W transects from Indian Sundarban. Depth of these pit sections varies from 2 to 3 m. Generally, top 20 cm sediment (in pit section W-1 to W-11) deposited under the intertidal environment, as indicated by the presence of Ammonia tepida. However, sediment below 20 cm in some of the pit sections (W-3 and W-5) exhibits fresh water signatures as indicated by the presence of Charophytes algae. In other pit sections (W-1, W-2, W-6, W-7, W-8, W-9 and W-11), the intertidal assemblage is noticed just above the upper marshes assemblage and vice versa, signifying that depositional environment is in proximity to the mangrove dominated area as indicated by the presence of marsh benthonic foraminiferal assemblage containing T. inflata, H. wilberti, Haplophragmoides sp., J. macrescens and M. fusca. Bottom sediment in most of the pit sections from south to north have different depositional environment with alternate presence of intertidal to subtidal faunal assemblages. The peculiar presence of intertidal assemblage above the upper marshes assemblages in recent sediment points towards the theory of submergence due to relative rise in the sea level. But the effect of relative sea level rise is not uniform throughout the area because of differential subsidence due to varied rate of sediment supply (0.5 to 3.3 cm/year) and eastward tilting of the basin. Based on the upper marshes benthonic foraminiferal assemblage and radiocarbon age (in W-1 at 100 cm ~?150 years age), the average subsidence rate as recorded is approximately 0.3 to 0.5 cm/year. Hence, such depositional sequence conjectures that the Indian part of Sundarban is undergoing a phase of submergence concomitant to basin subsidence.  相似文献   

9.
August–July precipitation has been reconstructed back to AD 1724 for the Mohe region in the northern Greater Higgnan Mountains, China, using Pinus sylvestris var. mongolica tree-ring width. The reconstruction explains 39% of the variance in the precipitation observed from AD 1960–2008. Some droughts noted in historical documents are precisely captured in our reconstruction. Wet periods occurred during the periods of AD 1734–1785, AD 1805–1830, AD 1863–1880, AD 1922–1961, and AD 1983–1998; while the periods of AD 1786–1804, AD 1831–1862, AD 1881–1921, and AD 1962–1982 were relatively dry. Power spectral and wavelet analyses demonstrated the existence of significant 24-yr, 12-yr, and 2-yr cycles of variability. The results of the spatial correlations suggest that our reconstruction contains climatic signals for the southern Stanovoy Range and the northern Greater Higgnan Mountains. The positive correlations between the new reconstructed precipitation series and two precipitation reconstructions indicate that our precipitation reconstruction captures broad-scale regional climatic variations. A comparison between the weakening tendency of summer monsoon and the dry period of our reconstruction reveals that the annual precipitation in the Mohe region is partly influenced by the East Asian Summer Monsoon.  相似文献   

10.
Composition and abundance of benthic and planktonic foraminifera in surface sediments of the brine-filled Shaban and Kebrit Deeps and some bathyal-slope environments in the northern Red Sea were examined for correlation with environmental conditions (e.g., bathymetry, sediment grain-size, organic matter, and carbonates) of the brine-filled deeps and normal Red Sea water. About 67 benthic foraminiferal species were recorded in these sediments. The lowest faunal density and diversity were recorded in the Shaban and Kebrit Deeps, whereas the highest density and diversity were recorded in the bathyal-slope sediments. Cluster analysis divided the benthic foraminiferal species into three major faunal assemblages. Buccella granulata–Gyroidinoides soldanii–Bolivina persiensis assemblage dominated the 650–1,300 m depth due to predominance of oligotrophic, highly oxygenated bottom waters. The Melonis novozealandicum–Spirophthalmidium acutimargo assemblage was recorded in the deep and bathyal-slope sediments indicating its tolerance for wider ranges of environmental conditions. The deeps were only dominated by the Brizalina spathulata assemblage indicating existence of un-totally anoxic conditions. The deeps yielded also very low planktonic foraminiferal density that may be attributed to occurrence of the seawater–brine interface which not only minimized the deposition of high buoyancy, large-test species (Globigerinoides sacculifer, Globigerinella siphonifera, and Orbulina universa), but also overestimated the small-test species (Globigerinoides ruber, Globoturborotalita rubescens, and Globigerinita glutinata) in the sediments. These findings should be taken into consideration when reconstructing paleoceanographic conditions of the Red Sea using core sediments from the brine-filled deeps.  相似文献   

11.
In the salt marshes of Tomales Bay, California, where grazing by cattle increases the input of nitrogen to the marsh (either directly or indirectly as runoff from within the salt marsh watershed), high salt marsh vegetation is dominated byDistichlis spicata and is less diverse than marshes without excess nutrients. Using a field experiment, I investigated the role of soil fertility on the plant community of the high salt marsh. I hypothesized that when soil fertility is increased by nitrogen addition plant productivity will increase, as indicated by height, biomass, and cover, and competitive exclusion, byD. spicata, will lead to a reduction in species richness and evenness, especially where the initial density ofDistichlis is high (from transplanting). After two growing seasons, biweekly nitrogen addition to the high salt marsh led to increased plant biomass and cover. Diversity was not reduced, and space preemption byDistichlis-transplants did not confer a competitive advantage. Although the dominant species thrived (e.g.,Salicornia virginica, D. spicata, Triglochin concinna) they did not displace subdominant species and decrease diversity. The vegetation response in this high salt marsh system does not support the hypothesis that as biomass and cover (indicators of productivity) increase in response to increased nitrogen, competitive exclusion will occur and diversity will decrease.  相似文献   

12.
With the decrease of gloal temperature,glacial epoch came over the earth and global climate fluctuated over a great range since the beginning of Quatenary .Paleoclimotologists of various countries have focussed attention to the periodic characteristics and dynamics of climation flucturation in the past many years(Berger,1977;Imbrie and Hays,1984:ding zhongli et al.,1990;Yu zhiwei et al.,1992:Liu Youmei et al.,1996),Although some of the workers have paid their attention to the nonlinear characteristics of the global Quaternary environmental evolution(Nicolis and Nicolis,1984,Lu Houyuan et al.,1993)_,it is worth while to do this kind of work in some special areas in the world,for example the Qinghai-Tibet Plateau.Using R/S analysis,the authors calculated the Hurst indexes H of some geochemical proxies,including organic carbon,FeO,Fe2O3 and FeO/Fe2O3,from the Tianshuihai Lake core in West Kunlun Mountain of the Qinghai-Tibet Plateau,The proxies satisfy the Hurst law with Honrg.carbon=0.735,HFe2O3=0.757,HFeO=0.848and HFeO/Fe2O3=0./646,All the indexs are greater than 0.5,meaning that from 240 to 15ka B.P.,there were some long-run dependencies-persistence in the climatic and environmental evolution around the Tianshuihai Lake area.This is in accordance with the climate there from 240 to 15ka B.P.(Yu Suhua et al.,1996),The paleo-climate and paleo-environment evolution around the Tianshuihai Lake area is of persistence as well as of fluctuation and is a combination of these two components,There are some differences between the four Hurst indexes,Which Probably resulted from the differnet intensitites of Persistence of the four proxies,organic carbon.FeO,Fe2O3 and FeO/Fe2O3,or from the change of drainage system around the Tianshuihai Lake area from opemness to closeness(Li Bingyuan et al.,1991;Sun Honglie,1996:Shi Yafeng et al.1998_).The Qinghai-Tibet plateau was the starter and sensor of the climatic and environmental variation of the surrounding areas(Yao Tandong et al.,1991;Feng Song et al., 1998) and some other scientists even regarded it as the driver and amplifier of global climatic variations (Pan Baotian and Li Jijun,1996).The persistence in which the climate and environment around the Tianshuihai Lake area evolved from 240 to 15ka B.P.is probably a function of the continuous uplift of the palteau in the same period of time.  相似文献   

13.
Two hydrobiid gastropods are important components of the salt marsh pool invertebrate community at the Parker River National Wildlife Refuge near Rowley, Massachusetts.Hydrobia truncata was found to be the dominant snail in the two higher salinity pools, withSpurwinkia salsa becoming the more important at lower salinities.H. truncata began to reproduce between May 19 and June 9. Cohorts lasted about 14 months, and 99% of production occurred during the first 12 months. Shell height inH. truncata was greater in pool 2 which had a much lower density than pool 1 on all dates of the 1980 growing season. Snail densities were positively correlated with the presence of floating algal mats of the genusCladophora, but the snails apparently do not graze on this alga.H. truncata production, calculated by four methods, ranged from 41.0 to 78.1 and averaged 60.5 g m?2 cohort?1 for pool 1, but were estimated to be less than 10% of these values in pool 2. In pool 1,H. truncata was estimated to assimilate 55% of net primary production or 28% of total pool energy income when detritus income was included. When found in the same pool asH. truncata, Spurwinkia salsa ordinarily comprised less than 0.5% of the total snail population.S. salsa veligers did not metamorphose into juveniles until July and were partitioned fromH. truncata by size durig their first growing season. The main separation between the two species, however, appeared to be spatial, on a salinity gradient. TheS. salsa cohort lasted about 19 months with a few individuals surviving to 27 months in their third summer.  相似文献   

14.
This study of the upper Maastrichtian to Danian sedimentary succession from the northern part of the Romanian Eastern Carpathians (Varniţa section) aims to establish an integrated biostratigraphy based on calcareous nannofossils, organic-walled dinoflagellate cysts (dinocysts) and foraminiferal assemblages, and to reconstruct the depositional environments of the interval. The stratigraphic record across the studied section is incomplete, considering that an approximately 16 m thick strata interval from the top of the Maastrichtian to lowermost Danian cannot be analyzed due to a landslide covering the outcrop. The upper Maastrichtian is marked by a succession of biostratigraphic events, such as the First Appearance Datum (FAD) of the nannoplankton taxon Nephrolithus frequens and FAD of the dinocyst species Deflandrea galeata and Disphaerogena carposphaeropsis, and the Last Appearance Datum (LAD) of Isabelidinium cooksoniae in the lower part of the section. These bioevents are followed by the LAD of the Dinogymnium spp. and Palynodinium grallator dinocyst markers in the top of the Maastrichtian deposits analyzed. In terms of foraminiferal biostratigraphy, the upper Maastrichtian Abathomphalus mayaroensis Zone is documented in the lower part of the studied section. Some bioevents, such as the bloom of the calcareous dinoflagellate genus Thoracosphaera and the FAD of the organic-walled dinocysts Damassadinium californicum, Senoniasphaera inornata, Xenicodinium lubricum and X. reticulatum suggest an early Danian age for the middle part of the section. From the Danian deposits in the Varniţa section, we describe a new organic-walled dinocyst species, Pentadinium darmirae sp. nov., which is until now the only species of the Pentadinium genus discovered in the Paleocene. The occurrence of the global Danian dinocyst marker Senoniasphaera inornata in the top of the section, suggests an age not younger than middle Danian (62.6 Ma) for the analyzed deposits.The palynofacies constituents, as well as the agglutinated foraminiferal morphogroups, used to reconstruct the depositional environments, show that the late Maastrichtian sediments were deposited in an outer shelf to distal (bathyal) environment, followed by a marine transgression during the Danian.  相似文献   

15.
Li/Ca ratios were measured in planktonic and benthic foraminifera from a variety of hydrographic settings to investigate the factors influencing lithium incorporation into foraminiferal tests including temperature, dissolution, pressure, and interspecies differences. Down-core measurements of planktonic (Orbulina universa, Globigerinoides ruber, and Globigerinoides sacculifer) and benthic foraminifera (calcitic Cibicides wuellerstorfi and aragonitic Hoeglandina elegans) show a systematic variation in Li/Ca with δ18O through the last glacial-interglacial transition. All species examined exhibit an increase in Li/Ca between 14 to 50% from the Holocene to the last glacial maximum. Li/Ca generally increases with decreasing temperature as seen in a latitudinal transect of planktonic O. universa and down-slope benthic species along the Bahama Bank margins. Postdepositional dissolution possibly causes a decrease in planktonic foraminiferal Li/Ca along the Sierra Leone Rise, and increased water depth causes a decrease in benthic foraminiferal Li/Ca in the deep Caribbean. However, none of these effects are sufficient to account for the observed glacial-interglacial changes. Physiological factors such as calcification rate may affect the Li/Ca content of foraminiferal calcite. The calcification rate in turn may be a function of carbonate ion concentration of ambient ocean water. This work shows that incorporation of lithium by foraminifera appears to be influenced by factors other than seawater composition and does not appear to be dominated by changes in temperature, dissolution, or pressure. We hypothesize that the consistent increase in foraminiferal Li/Ca during the last glacial maximum may be linked to changes in seawater carbonate ion concentration. Important parameters to be tested include calcification rate and foraminiferal test size and weight. If foraminiferal Li/Ca is dominantly controlled by calcification rate as a function of seawater carbonate ion concentration, then Li/Ca may act as a proxy of past atmospheric CO2.  相似文献   

16.
Dansgaard–Oeschger (D–O) cycles had far-reaching effects on Northern Hemisphere and tropical climate systems during the last glacial period, yet the climatic response to D–O cycles in western North America is controversial, especially prior to 55 ka. We document changes in precipitation along the western slope of the central Sierra Nevada during early Marine Oxygen Isotope Stages (MIS) 3 and 4 (55–67 ka) from a U-series dated speleothem record from McLean's Cave. The timing of our multi-proxy geochemical dataset is coeval with D–O interstadials (15–18) and stadials, including Heinrich Event 6. The McLean's Cave stalagmite indicates warmer and drier conditions during Greenland interstadials (GISs 15–18), signified by elevated δ18O, δ13C, reflectance, and trace element concentrations, and less radiogenic 87Sr/86Sr. Our record extends evidence of a strong linkage between high-latitude warming and reduced precipitation in western North America to early MIS 3 and MIS 4. This record shows that the linkage persists in diverse global climate states, and documents the nature of the climatic response in central California to Heinrich Event 6.  相似文献   

17.
The microfossil assemblages of subsurface Carboniferous rocks from Faghur-1x were examined and identified. Their biostratigraphical and paleoenvironmental importance were investigated. The assemblage included well-preserved foraminifera like Omphalotis omphalota, Omphalotis sp. 2, Omphalotis sp. 3, Paraarchaediscus stilus, Paraarchaediscus koktjubensis, Archaediscus krestovnikovi, Archaediscus complanatus, Archaediscus inflatus, Archaediscus karreri, Diplosphearina inequalis, Eotubertina sp., Tetrataxis conica, Cribrostomum lecomptei, Palaeotextularia angulata, and Palaeotextularia longiseptata. This foraminiferal association indicates the late Viséan-early Serpukhovian. The other microfossils are gastropods, brachiopods, ostracods, crinoidal ossicles, frond-like fenestrate bryozoan types and stick-like colonies, echinoderms, microproblematica like Draffania biloba and algal Calcisphaera and the dasyclad Koninckopora. This microfossils assemblage points to the deposition in a restricted to open platform in a lagoonal framework environment. The Tehenu Basin is the eastern segment of northern African Sahara basins that provided refuge for the foraminiferal genera through the mass extinction events during the glacial Viséan-Serpukhovian times. However, its foraminiferal associations have lower diversities than the western basins, which indicate that it was more readily affected by the mass extinction event.  相似文献   

18.
Climatic reconstruction of glacial to interglacial episodes from oxygen isotopes in sediment cores from the Nordic seas is complicated by strong local meltwater contributions to the oxygen isotope changes. Combination of benthic and planktic foraminiferal isotope data with foraminiferal abundances and ice-rafted debris (IRD) allows separation of local and global effects and subdivision of the marine oxygen isotope events 6.2–5.4, which include the last interglaciation, into: (1) a meltwater phase after glacial stage 6, recorded by large amounts of IRD and low foraminiferal abundance, indicating surface water warming; (2) an IRD-free period with high deposition rates of subpolar foraminifera and other CaCO3pelagic components, recognized here as the “full” interglaciation; and (3) a phase with the recurrence of IRD and the demise of subpolar species. Comparison of ice-core records and marine data implies that the global climate during the last full interglaciation and that during the postdeglacial Holocene were similar. The records show no significantly different variations in the proxy data. In contrast, the oxygen isotopes of planktic foraminifera and ice cores indicate significant differences during each of the deglacial transitions (Terminations I and II) that preceded these two interglaciations. These suggest that during Termination II the climatic evolution in the Nordic seas was less affected by abrupt changes in ocean–atmosphere circulation than during the last glacial to interglacial transition.  相似文献   

19.
This work presents two novel climate‐related time series for the northwest of Portugal. The first is an AD 1626–1820 triennial‐resolved wine production series, based on the Benedictine accounts from six monasteries of the Entre‐Douro‐e‐Minho (EDM) region. The second, an AD 1654–2010 benthic foraminiferal record from the Caminha salt marsh, located in the lower estuary of the Minho River. The series were analysed together for the common period to outline how both palaeoclimatic proxies respond to the most likely natural environmental drivers of temporal variability, solar forcing included. Singular spectral analysis revealed a common significant multidecadal periodicity agreeing with recognized long‐term changes in solar activity, i.e. the Lower Gleissberg cycle (50–80 years). The application of wavelet analysis allowed the detection of high coherence at this time scale (centred at c. 64 years) between marsh foraminifera and both total solar irradiance and the North Atlantic Oscillation index. This relationship persists throughout the c. AD 1730–1875 period. The continuous wavelet transform results for wine production were inconclusive. As the time‐span analysed is recognized as one of high socio‐economic and political distress, the main human‐driven impacts on wine production, particularly in the two periods of greatly reduced solar activity – the Maunder and Dalton Minima – are reviewed in the light of the available historical records. In addition to a documented climate‐related agricultural crisis in Portugal, damage and losses to wine production may have been triggered by several local and international conflicts in which the country was involved. But to what extent the two influences contributed to the wine production variations observed in the EDM region during both periods remains an open question.  相似文献   

20.
Holocene paleoenvironmental changes have been interpreted on the basis of benthic foraminifera and calcareous nannofossils recovered in samples from Napostá Grande Stream, Bahía Blanca estuary, southern Buenos Aires Province. Samples are fine sands and clay sediments from a Holocene outcrop and were studied with quantitative techniques. The benthic foraminiferal assemblage is dominated by Ammonia parkinsoniana, Ammonia tepida, Bolivina pseudoplicata, Bolivina striatula, Bolivina sp., Buccella peruviana, and Elphidium spp. The calcareous nannofossil assemblage recovered is a typical cold-water association, dominated by Calcidiscus leptoporus, Coccolithus pelagicus, Emiliana huxleyi and Gephyrocapsa oceanica. A dendrogram classification by cluster analysis was made for each microfossil group. The results of these analyses were coincident, showing a liaison between changes in the assemblages of benthic foraminifera and calcareous nannofossils. Those results, jointly with the sedimentological information, lead to the identification of three different paleoenvironments along the Napostá N1 site. The lower part of the succession represents an estuarine environment with larger marine connection. The middle part represents a gradual passage to a more restricted estuarine environment, and the upper part represents the establishment of the modern continental fresh-water environment.  相似文献   

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