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1.
The Gongga Mountain is the largest area of modern glaciation in the Hengduan Mountains and,with a summit elevation of 7556 m,is the highest mountain on the eastern margin of the Tibetan Plateau.During the Quaternary glacial-interglacial cycles the Gongga Mountain was extensively and repeatedly glaciated,and glacial landforms and outwash deposits from multiple glaciations are well-preserved in valleys,in basins,and on piedmonts.To constrain the glacial chronology of the eastern slope of Gongga Mountain,sample sites were selected based on the distribution and weathering of glacial tills,relationships among glacial deposits,and soil development on moraines.Dating of the tills and glaciofluvial deposits was undertaken with electron spin resonance(ESR) and optically stimulated luminescence(OSL).The ages of the glacial deposits can be divided into four clusters:2.2±0.5,11.9±0.6,35.9±2.7-58.0±6.3 and 119.2±15.9-194.2±32.8 ka.Five glacial advances in this region have been identified,which are equivalent in age to the Little Ice Age(LIA),Neoglaciation,marine oxygen isotope stage(MIS) 2,mid-MIS3,and MIS6.The largest local last glacial maximum(LGML) occurred on Gongga Mountain during mid-MIS3,characterized by a cold-humid climate,rather than the global Last Glacial Maximum(LGMG) of MIS2.The Gongga,Nanmenguangou(NMGG) and Yajiageng Glaciations occurred during the late part of the last glacial cycle,the middle of the last glacial cycle and the penultimate glacial cycle,respectively.On the basis of geomorphological,sedimentological,and compositional characteristics,landforms of the Moxi Platform and terraces can be grouped by facies and geochronology.In combination with the dating results,this analysis indicates the basal part of the Moxi Platform between Xinxin and the Moxi Hotel is correlative with the till of the Nanguanmen Glaciation(mid-MIS3).This basal unit has occasional lenses of glaciofluvial sandy gravel and lacustrine sediments.The remainder of the Moxi Platform and the terraces beside the platform are glaciofluvial deposits occasionally mixed with debris flow deposits and range in age from MIS3 to Holocene.  相似文献   

2.
Pollen analysis was carried out on the Core MD982194 of 29.78 m retrieved from the Middle Okinawa Trough which was dated as old as~200 ka BP.The results revealed that pollen assemblages mainly presented an alternation of coniferous and herb pollen.The coniferous saccate pollen,principally Pinus and Tsuga,predominated in most parts of the core,especially highlighted in the interstadial stages including MIS 1,3,5 and MIS7,whereas the herb pollen significantly increased in the glacial periods.Thus the pollen flora and their percentages showed the sensitive changes under the influence of ice volume during the glacial and interglacial periods.Our record from this core has first documented that the percentage of Cyperaceae was extremely high in the glacial stages with a notable increase in Artemisia,Gramineae,Asteraceae,Chenopodiaceae,and freshwater algae,which can be used as a proxy for sea-level change at the study site because of their close negative correlation of the orbital-scale changes in sea level.The distance between the continental coastline and the Okinawa Trough has deeply shortened due to the sea-level drop in the LGM.As a result,the sediment materials from Yangtze River were extensively deposited on the flat,exposed continental shelf owing to the rapid decline of river flow speed,leading to that pollen grains from Okinawa Trough are derived mainly from the flat coastal vegetation of exposed continental shelf at glacial stages.Changes of pollen assemblage were consistent with the variation of temperature and humidity,which showed that the percentage of arboreal pollen was highly augmented at MIS 7,5 and MIS 1,corresponding to the strengthening of the East Asian summer monsoon and increasing of rainfall.Moreover,the peak of Pinus percentage in MIS 5.3,5.1 and MIS 3.3 may be closely linked by orbital and sub-orbital cycles of solar radiation and monsoonal variability.The present study of core MD981294 implied lower temperature and precipitation during the lowest sea-level stage(LGM),and more visibly testified that the vegetation of the flat plain on the exposed continental shelf was dominated by intrazonal communities such as halophyte grasslands and freshwater wetlands instead of zonal steppe or semi-arid desert.All above evidence demonstrated that the fundamental changes of pollen assemblage and their origins in Okinawa Trough since~200 ka BP were affected by combine factors including the coastline position and climate fluctuation.Moreover,the substantial shortening of distance between shoreline and the Okinawa Trough driven by orbital insolation cycles was clearly indicated by the pollen spectra,whereas the source-area climate signal of the pollen record was largely weakened.  相似文献   

3.
The Pochengzi Glaciation is a typical glaciation in Quaternary in the Tianshan Mountains. The glacial landforms comprise several integrated end moraines, like a fan spreading from the north to the south at the mouth of the Muzhaerte River valley and on the piedmont on the southeastern slope of the Tumur Peak, the largest center of modern glaciation in the Tianshan Mountains. The landforms recorded a complex history of the ancient glacier change and contained considerable information of the glacial landscape evolution, and dating these landforms helps us understand the temporal and spatial shifts of the past cryosphere in this valley and reconstruct the paleoenvironment in this region. Electron spin resonance (ESR) dating of the glacial tills in the upper stratum from a well-exposed section, end moraines, and associated outwashes was carried out using Ge centers in quartz grains, which are sensitive to the sunlight and grinding. The results could be divided into three clusters, 13.6–25.3, 39.5–40.4 and 64.2–71.7 ka. Based on the principle of geomorphology and stratigraphy and the available paleoen- vironmental data from northwestern China, the end moraines were determined to deposit in the Last Glaciation. The landforms and the three clusters of ages demonstrate that at least three large glacial advances occurred during the Pochengzi Glaciation, which are corresponding to marine oxygen isotope stage 4 (MIS4), MIS3b and MIS2. The landforms also indicate that the gla- ciers were compound valley glacier in MIS2 and MIS3b and piedmont glacier in MIS4, and the ancient Muzhaerte glacier were 94, 95 and 99 km at their maximum extensions in these three glacial advances.  相似文献   

4.
Chronologies of glacial advances during the last glacial period in the Nyainqentanglha mountain range may provide constraints on the past climate in a transition zone of the Asian monsoon.We present 15 new10Be exposure ages from two moraines in the Payuwang valley,on the north slope of the range.The inner moraine has exposure ages ranging from 18.0±1.7 to 30.6±2.8 ka(n=10),with a mean age of 23.8±4.0 ka,corresponding to the global Last Glacial Maximum(LGM).The outer moraine yields exposure ages ranging from 18.0±1.6 to 39.9±3.7 ka(n=5).Evidence for weathering leads us to view the oldest age as a minimum age,placing moraine formation during MIS3.Chronologies from the last glacial period from south slope of the Nyainqentanglha support this interpretation.Thus,there appears to have been a local LGM(LLGM)during MIS3 and a more limited glacial advance during the global LGM.Glacial advances during MIS3 in the Nyainqentanglha may correlate with millennial-scale climate change(Heinrich events).  相似文献   

5.
Mississippi Valley-type(MVT)Pb-Zn deposits serve as the world’s major supply of Pb-Zn resources.However,the age constraint of MVT Pb-Zn deposits has long been a big challenge,due to the lack of minerals that are unequivocally related to ore deposition and that can be used for radioisotopic dating.Here we show sporopollens can provide useful chronological information on the Changdong MVT Pb-Zn deposit in the Simao basin,Sanjiang belt,West China.The Pb-Zn ores in the Changdong deposit are hosted by internal sediments in paleo-karst caves of meteoric origin.Sphalerite and galena occur as replacements of carbonate minerals and void infillings in the internal sediments.The relations suggest that the Pb-Zn mineralization occurred after the deposition of the internal sediments.A palynological assemblage mainly composed of angiosperm pollen dominated by Castanea,Quercus,and Carya and fern spores dominated by Polypodiaceae,Pteris,and Athyriaceae was identified.These pollen and spores place the ore-hosting internal sediments and the Changdong paleo-karst at early to middle Oligocene.Consequently,the Changdong Pb-Zn deposit must have formed after the early Oligocene(~34 Ma).These age constraints,together with the geological characteristics,indicate that the Changdong Pb-Zn deposit is a paleo-karst-controlled MVT deposit related to fold-thrust systems in the Sanjiang belt.The Changdong deposit is similar to other MVT Pb-Zn deposits in the northern part of the Sanjiang belt,making it possible to extend this Pb-Zn belt 500 km further to the South.Results presented here highlights the potential of sporopollens in dating the age of MVT deposits related to paleo-karst formation in young orogenic belts.  相似文献   

6.
With rapid development of microbiology on ex-treme environment, many microorganisms have been found on glacial surface and in deep glacier around the world. The extensive 16S rRNA sequence analysis of bacteria isolated from glacial samples indicates that the bacteria in ice are highly similar to those from ocean, lake, terrestrial sediment and agricultural soil on the earth[1,2]. This suggests a close relationship be-tween the microorganisms in glacial region and those in other environments s…  相似文献   

7.
The Neogene sediments of Gansu Group from northwestern China contain eolian and fluvial deposits.The origins of these sediments are very important for exploring the onset of Asian inland aridification,the pattern of paleo-atmospheric circulation,and the regional tectonics and geomorphic evolution during the Miocene.Here we present detrital-zircon age spectra of typical eolian and fluvial deposits from highlands and subsidence basin,and compare them with those of surrounding eroded mountain(such as the West Qinling Mountains and Liupan Mountains)materials and Quaternary loess derived from the Asian inlands.The results reveal that(1)the detrital-zircon age spectrum of the Miocene eolian sample is remarkably different from the eroded materials of the West Qinling Mountains and the Miocene fluvial deposits from Tianshui region,but very similar to the Quaternary loess deposits.This indicates that the provenance of Miocene eolian sediments is similar with the Quaternary loess,and thus further confirms the previous conclusions that the distribution of Asian arid lands and the pattern of atmospheric circulation during the Miocene are broadly similar with the Quaternary.(2)The detrital-zircon age spectrum of the fluvial deposits(with age about 11.5 Ma)from Tianshui region is different from the eroded materials of West Qinling Mountains,but similar with that of the Liupan Mountains to the east,which may suggest that the Liupan Mountains have already been exhumed by11.5 Ma.  相似文献   

8.
Neogene eolian successions are one of the most important terrestrial palaeoenvironmental archives in East Asia.However, they have received far less attention than Quaternary loess deposits, especially in the case of lipid biomarker analysis.In order to obtain a better insight into the early-middle Miocene palaeoenvironment, we conducted a study of n-alkane biomarkers from sediments of the QA-I section(Qinan) in the western Chinese Loess Plateau, and compared the results with those of previous n-alkane analyses of eolian and aquatic sediments of varying age. Our principal results are as follows:(1) All QA-I samples contain n-alkanes ranging from C_(14) to C_(35), among which the relative content of short-chain n-alkanes(C_(14)–C_(20)) from microorganisms is significantly greater than that of long-chain n-alkanes(C_(26)–C_(35)) from the waxes of terrestrial higher plants;the main peak is at C_(16)–C_(18). All samples have a relatively lower abundance of medium-chain n-alkanes(C_(21)–C_(25)) than that of long-and short-chain n-alkanes, similar to strongly weathered palaeosols in Quaternary loess and Late Miocene-Pliocene Hipparion Red-Earth; however, this distribution is significantly different from that in weakly-weathered loess of Quaternary loess and Late Miocene-Pliocene Hipparion Red-Earth, as well as from aquatic sediments.(2) Despite some odd-over-even carbon predominance of long-chain n-alkanes in the QA-I samples, the carbon preference index(CPI) values are significantly lower than those of most of the weakly-weathered sediments. Our results show that strong weathering and microbial processes have significantly altered the n-alkanes in the Miocene eolian deposits in Qinan, and led to a significant oxidation and degradation of long-chain n-alkanes and the predominance of short-chain n-alkanes from bacteria. Therefore, the contribution of microorganism to total organic carbon(TOC) and its resulting in carbon isotopic composition should be carefully assessed in future studies.  相似文献   

9.
It is generally considered that four-times ice age happened during the Quaternary epoch on the Tibetan Plateau. However, the research on the chronology of the four-times ice age is far from enough. The Shaluli Mountain on the Southeastern Tibetan Plateau is an ideal place for plaeo-glacier study, because there are abundant Quaternary glacial remains there. This paper discusses the ages of the Quaternary glaciations, based on the exposure dating of roche moutonnée, moraines and gla- cial erosion surfaces using in situ cosmogenic isotopes 10Be. It is found that the exposure age of the roche moutonnée at Tuershan is 15 ka, corresponding to Stage 2 of the deep-sea oxygen isotope, suggesting that the roche moutonnée at Tuershan is formed in the last glacial maximum. The expo- sure age of glacial erosion surface at Laolinkou is 130―160 ka, corresponding to Stage 6 of the deep-sea oxygen isotope. The oldest end moraine at Kuzhaori may form at 421―766 kaBP, corre- sponding to Stages 12―18 of the deep-sea oxygen isotope. In accordance with the climate charac- teristic of stages 12,14,16 and 18 reflected by the deep-sea oxygen isotope, polar ice cores and loess sequence, the oldest end moraine at Kuzhaori may form at stage 12 or stage 16, the latter is more possible.  相似文献   

10.
Particulate fluxes investigated in the central South China Sea (SCS) during 1993―1996 indicate that opal flux can be used to show primary productivity change, which provides a foundation for tracing the evolutionary relationship between the surface productivity and East Asian monsoon in the SCS during the late Quaternary glacial and interglacial periods. Based on the studies of opal % and their mass accumulation rates (MAR) at the six sites recovered from the SCS during the “Resolution” ODP Leg 184 and “Sonne” 95 cruise of the Sino-Germany cooperation, opal % and their MARs increased evidently in the northern sites since 470―900 ka, and they enhanced and reduced, respectively, during the glacial and interglacial periods. Whereas they increased obviously in the southern sites since 420―450 ka, and they augmented and declined, respectively, during the interglacial and glacial periods. The vari- ability in opal % and their MARs in the late Quaternary glacial cyclicity indicate the “seesaw” pattern of surface productivity in the SCS. The winter monsoon intensified during the glacial periods, surface productivity increased and decreased, respectively, in the northern and southern SCS. The summer monsoon strengthened during the interglacial periods, surface productivity increased and decreased, respectively, in the southern and northern SCS. The cross spectral analyses between the opal % in the northern and southern SCS during the Quaternary and global ice volume (δ 18O) and orbital forcing (ETP) indicate that the East Asian winter and summer monsoons could be ascribed to the different drive mechanisms. On the orbital time scale, the global ice volume change could be a dominant factor for the winter monsoon intension and temporal variations. As compared with the winter monsoon, the correlative summer solar radiation with the obliquity and precession in the Northern Hemisphere could be a mostly controlling factor for the summer monsoon intension and temporal variations.  相似文献   

11.
This paper inspects the relationship betweenHomo erectus from Beijing, Nanjing and Chenjiawo on the viewpoint of environmental variations. Pieces of evidence show that Nanjing and ChenjiawoHomo erectus lived in glacial age at the Marine Oxygen Isotope Stage (MIS) 16, whileHomo erectus pekinensis in major interglacial age at MISs 15-13. It is also recovered that the cave deposits in Tangshan, Nanjing, and Zhoukoudian, Beijing, as well as the deposits at theHomo erectus site in Chenjiawo, have undergone some similar development process. All of them originated from early-middle Middle Pleistocene, and have gone through the test of the major glacial age at MIS 16 and the major interglacial period at the paleosol stage S5. At the same time, they were also closely connected with some environment events, such as the uplift of the Qinghai-Xizang Plateau, the formation of glaciations and the loess accumulations.  相似文献   

12.
Two new records of glacial stratigraphy obtained from borecores collected in southcentral Saskatchewan are compared to previously published records from Wellsch Valley and Swift Current Creek in southern Saskatchewan. The comparisons are based on magnetostratigraphy and tephrochronology, and describe preglacial, glacial, and interglacial deposits. The new stratigraphy is compared to the composite global marine oxygen isotope record. Although the ages and extents of late Pleistocene continental glaciations are relatively well constrained, they are less well defined for the middle and early Pleistocene. Data presented here highlight recent studies carried out from borecores and outcrops of extensive pre-Illinoian (pre-Saalian) glacial deposits. Based on this new data, at least seven Laurentide (continental) glaciations are recognized in Saskatchewan, and these records are the most extensive to date in the Northern Interior Plains of Canada. The magnetostratigraphic records from the Sutherland Overpass and Wascana Creek sites provide the first evidence of reversely magnetized glacial deposits in the Canadian Prairies. These deposits can be assigned to the latest Matuyama (MIS 20) and indicate that Laurentide (continental) glaciations did not impact southern Saskatchewan until the late Early Pleistocene.  相似文献   

13.
A geochronological framework for the sequential development of coastal barrier aeolianite complexes in the mouth region of the River Murray, Australia's largest river system is presented based on amino acid racemization and thermoluminescence dating. The sedimentary successions represent a foreshortened and condensed sequence of coastal barriers compared with those of the Coorong Coastal Plain in southern South Australia where the barrier complexes are more widely separated in response to tectonic uplift. The barriers have formed during interglacial sea-level highstands and are correlatives of genetically equivalent landforms of the Coorong Coastal Plain. Thermoluminescence dating and the extent of amino acid racemization in aeolianite ‘whole-rock’ sediment samples, reveal a general increase in age of the barriers landwards from the modern coastline. In detail, however, the individual barriers represent composite structures having formed in more than one interglaciation, due to the reoccupation of Pleistocene shoreline positions during sea-level highstands of similar amplitude, in a zone of gradual basin subsidence. The most seaward Pleistocene aeolianite at Surfer Beach is of interstadial age (Marine Isotope Stage 5c, 105 ± 5 ka; MIS 5c), and correlates with the Robe Range of the Coorong Coastal Plain. The last interglacial shoreline (130 ± 15 ka; MIS 5e) is particularly well-defined in the River Murray mouth region. It is represented by a complex association of coastal parabolic dunes superimposed on a transverse dune system, which runs parallel with the former coastline, and also includes associated estuarine, lagoonal and open ocean beach facies. Landward of the last interglacial succession are distinct barriers relating to the penultimate interglaciation (215 ± 35 ka; MIS 7), as well as earlier interglaciations (350 ± 65 ka; MIS 9 or 11 and 470 ± 70 ka; MIS 11 or 13). The coastal barriers have been successively breached by the ancestral River Murray at times of lower sea level during glacial cycles. Former mouths of the River Murray during interglacial sea-level highstands are likely to have existed near Tauwitchere Island during MIS 7, and between Goolwa and Hindmarsh Island and near the southern-most part of Lake Albert during the last interglacial (MIS 5e). The River Murray mouth region represents a failed delta as the limited sediment brought to this area since late middle Pleistocene time has been either rapidly incorporated within aeolian deposits during sea-level highstands, or transported to the edge of the Lacepede Shelf during glacial maxima. The Holocene and modern River Murray has not established a marine delta, but deposits its load in the settling basins of the terminal lakes. Only a small digitate delta has formed where the river enters Lake Alexandrina.  相似文献   

14.
Environmental history of the northern continental shelf of the South China Sea during the last 280 ka BP, e.g. Marine Isotope Stages 1–8 (MIS 1–8) was reconstructed based on pollen record from the top 225m of ODP 1144 Site. During the interglacial periods, pollen assemblages are predominated by pine similar to those of the present day indicating that the environment of the interglacial periods was more or less close to that of today. Nevertheless, those from glacial periods are characterized by a large amount of herbaceous pollen, e.g.Artemisia, Gramineae, Cyperaceae, etc. inferring that grassland covered the merged continental shelf when the sea level lowered and the continental shelf was exposed. The exposed areas of the shelf were insignificant before MIS 5, but enlarged since MIS 4 and reached its maximum during MIS 2 according to ratios of pollen percentages between pine and herbs. The history of different exposure of the shelf can be compared with transgression records of the coastal areas of China and might result from neotectonic movement of Chinese continent. Some changes also took place in the components of grassland growing on the shelf during glaciations. Gramineae is the main element at MIS 8. ThenArtemisia increased upwards the profile and at last became the main component at the Last Glacial Maximum (MIS 2). Such changes in vegetation might be in response to cooler and drier climate.  相似文献   

15.
大荔人遗址黄土-古土壤剖面岩石磁学性质研究   总被引：1，自引：0，他引：1       下载免费PDF全文
自从大荔人化石被发现以来,其遗址剖面便成为研究热点.先前的研究主要集中在地层的对比划分与头盖骨年龄的推断方面,然而对于载磁矿物的鉴定及其古气候含义方面的研究却相对较少.鉴于此,本文运用热磁分析、饱和等温剩磁和剩磁矫顽力谱分析、磁滞回线分析、频率磁化率分析、热退磁分析等方法,对大荔人遗址剖面进行了系统的岩石磁学性质研究,鉴定出其主要载磁矿物为磁铁矿,赤铁矿,磁赤铁矿,磁畴状态主要是似单畴.磁铁矿,赤铁矿是样品中特征剩磁的携带者;主要起源于成土作用的超顺磁颗粒和新生成的亚铁磁性矿物,是古土壤样品磁化率增强的主要贡献者;古土壤中软磁性矿物的含量高于黄土.样品中磁赤铁矿的含量并不高.黄土-古土壤样品的频率磁化率曲线,古里雅冰芯氧同位素、细微粒浓度曲线,岐山五里铺剖面有机质含量曲线在古气候记录方面具有一致性,都展示出至少从MIS5以来,气候从冰期到间冰期的变化是渐变的,反之则表现了突变特征.上述岩石磁学研究丰富了大荔剖面的磁学领域研究内容,为相关课题的深入研究提供了依据.  相似文献   

16.
This study presents new dating results from the Verkhoyansk Mountains in northeastern Siberia. Pleistocene sediments of aeolian and glacial origin have been studied and dated by infrared optically stimulated luminescence (IRSL). The chronostratigraphy of this vast area is mainly based on radiocarbon dating up to now. Aeolian sediments are widespread in the foreland of the Verkhoyansk Mountains. IRSL-dating results indicate two major periods of increased accumulation between 33 and 24 ka and between 13 and 9 ka. A new stratigraphy of Pleistocene glaciations in this area has been developed. Up to five end moraines have been identified in two catchments areas. IRSL was applied to date the sandy and silty sediments covering the glacial deposits in order to get minimum ages. Furthermore, glacial and glacio-fluvial deposits were also sampled from a few locations. According to the IRSL dating results, the uppermost end moraine was deposited prior to 50 ka. The three outermost moraines might have been formed during Early Weicheslian to Saalian times. It is very likely that no major glaciers reached the foreland of the Verkhoyansk Mountains during the Last Glacial Maximum (LGM, MIS 2).  相似文献   

17.
A south to north gradient of increasing marine isotope substage (MIS) 5a (∼80 ka BP) sea level has been recorded across the Caribbean and surrounding region. Relative to present, MIS-5a sea levels range from −19 m to more than +3 m between Barbados, Haiti, the Bahamas, Florida, Bermuda and the US Atlantic Coast. In contrast, no gradient in sea level is observed for the last interglacial period MIS-5e (∼128–118 ka BP) at tectonically stable localities in the same region, with deposits generally lying several metres above present. We demonstrate here that these controversial observations are reconciled by taking into account the isostatic response of the Earth to glacial loading and unloading – a fundamental effect that is commonly overlooked in the interpretation of sea-level observations from different locations to define a ‘global sea-level curve’. Furthermore, the observed gradient can be used to place constraints on Earth rheology and is an important indicator of the behaviour of the North American ice sheets during the last glacial cycle.  相似文献   

18.
Quartz optically stimulated luminescence (OSL) forms the basis for the chronology of Weichselian ice advances in Arctic Eurasia developed over the last few years. There is almost no age control on this chronology before 40 ka, except for some marine sediments correlated with marine isotope stage (MIS) 5e on the basis of their palaeofauna. Results from more southern latitudes have shown that dose estimates based on quartz OSL and the single aliquot regenerative (SAR) dose procedure may underestimate the age of MIS 5e deposits. Here we use the same method to date well-described marine sediments, thought to have been deposited during the very beginning of the Eemian interglacial at 130 ka, and exposed in two sections on the river Sula in northern Russia. Various quality-control checks are used to show that the OSL behaviour is satisfactory; the mean of 16 ages is 112±2 ka (σ=9 ka). This represents an underestimate of 14% compared to the expected age, a discrepancy similar to that reported elsewhere. In contrast to SAR, the single aliquot regeneration and added (SARA) dose procedure corrects for any change in sensitivity during the first OSL measurement. The SARA results are shown to be 10% older than those from SAR, confirming the geological age estimate and suggesting that SAR ages may underestimate older ages (larger doses), despite their good performance in the younger age range.  相似文献   

19.
In Southern Ocean sediments south of the Antarctic Polar Front, the scarcity of calcareous microfossils hampers the development of sediment chronologies based on radiocarbon dating and oxygen isotope stratigraphy established from carbonate. In this study, radiometric dating, magnetic susceptibility (MS), biogenic opal content, diatom abundance fluctuation, and paleomagnetic information were investigated on a north–south transect of central Scotia Sea sediment cores to verify their reliability as stratigraphic tools in the study area. Radiocarbon dating on organic carbon humic acid fraction can be used to establish the stratigraphy of upper core sections, but regional comparison and correlation are needed to verify a possible bias by fossil carbon contamination. For the long-term stratigraphy, MS, which can be correlated to the Antarctic ice core dust/climate signal, represents the most valuable parameter. Fine-grained single domain magnetite, probably of biogenic origin, makes a significant contribution to the interglacial MS signal, while major contributions from detrital material affect the glacial MS record. The core from the southern Scotia Sea contains significant proportions of biogenic magnetite also in glacial sediments, suggesting depositional environments different from those of the northern Scotia Sea. Our data suggest low contributions of high-coercive minerals to the overall magnetic intensity of glacial and interglacial Scotia Sea sediments, which excludes dust as a main source of the magnetic signal. Opal content can be used to distinguish between cold and warm intervals for the past 300 thousand years. Abundance fluctuation patterns of diatom species Fragilariopsis kerguelensis and Eucampia antarctica are useful stratigraphic tools for periods back to Marine Isotope Stage (MIS) 6. The Mono Lake geomagnetic excursion is identified in Scotia Sea sediments for the first time. Possible correlations of ash layers are suggested between Scotia Sea sediments and East Antarctic ice cores. They have potential to serve as additional age markers for further studies in this area.  相似文献   

20.
Despite the nearly 600 named species of the land snail Cerion, studies of the geological and paleontological framework of modern species are few. To address this deficiency, the biostratigraphic succession of Cerion was investigated at several areas on Long Island, Bahamas. A chronostratigraphic framework was developed through whole-rock and Cerion land snail aminostratigraphies. About 175 individual Cerion shells from last interglacial and Holocene deposits were age-ranked using stratigraphic position and amino acid racemization (AAR) geochronology. AAR ages were generated using an existing AAR-14C age model for Cerion from the central Bahamas. The age structure of Cerion fossils in sediments was determined with AAR ages, and the magnitude of “dead carbon” anomalies was evaluated using this chronological approach.Temporal changes in gross shell morphology were examined from four study areas. The last interglacial, marine isotope stage/substage (MIS) 5e (Aminozone E) is characterized by generally large shells and in some cases, bimodal sets of very small (α shells) and very large forms (β shells) coexisting in the same stratigraphic levels (primarily soils), which may encompass the transition from between MIS 5e and 5d/c. Similar bimodality of nearly identical α and β shell forms and sizes is observed at other late MIS 5e sites from the furthest reaches of Great Bahama Bank (including Long, Exumas, Eleuthera, and New Providence Islands). The widespread distribution of α and β forms in soils capping MIS 5e marine and eolian deposits implies that there may have been a synchronous, regional morphological convergence on Great Bahama Bank. None of these forms are observed in Holocene deposits of Aminozone A.The earliest MIS 1 Cerion appear in a oolite deposited 6500 a BP, and are of intermediate size compared to the Pleistocene α and β forms. As MIS 1 progressed, the diversity of shell sizes and shapes increased into modern times. The greater variety of shell forms over the past 1000–2000 a suggests that humans may have played a role in the introduction and redistribution of Cerion across the region. The potential for frequent and widespread human introductions, combined with the propensity of Cerion to hybridise freely may explain the farrago of shell sizes and shapes in the recent snail faunas of Long Island and other Bahama islands.  相似文献   

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