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1.
Un‐fragmented stratigraphic records of late Quaternary multiple incised valley systems are rarely preserved in the subsurface of alluvial‐delta plains due to older valley reoccupation. The identification of a well‐preserved incised valley fill succession beneath the southern interfluve of the Last Glacial Maximum Arno palaeovalley (northern Italy) represents an exceptional opportunity to examine in detail evolutionary trends of a Mediterranean system over multiple glacial–interglacial cycles. Through sedimentological and quantitative meiofauna (benthic foraminifera and ostracods) analyses of two reference cores (80 m and 100 m long) and stratigraphic correlations, a mid‐Pleistocene palaeovalley, 5 km wide and 50 m deep, was reconstructed. Whereas valley filling is chronologically constrained to the penultimate interglacial (Marine Isotope Stage 7) by four electron spin resonance ages on bivalve shells (Cerastoderma glaucum), its incision is tentatively correlated with the Marine Isotope Stage 8 sea‐level fall. Above basal fluvial‐channel gravels, the incised valley fill is formed by a mud‐prone succession, up to 44 m thick, formed by a lower floodplain unit and an upper unit with brackish meiofauna that reflects the development of a wave‐dominated estuary. Subtle meiofauna changes towards less confined conditions record two marine flooding episodes, chronologically linked to the internal Marine Isotope Stage 7 climate‐eustatic variability. After the maximum transgressive phase, recorded by coastal sands, the interfluves were flooded around 200 ka (latest Marine Isotope Stage 7). The subsequent shift in river incision patterns, possibly driven by neotectonic activity, prevented valley reoccupation guiding the northward formation of the Last Glacial Maximum palaeovalley. The applied multivariate approach allowed the sedimentological characterization of the Marine Isotope Stage 7 and Marine Isotope Stage 1 palaeovalley fills, including shape, size and facies architecture, which revealed a consistent river‐coastal system response over two non‐consecutive glacial–interglacial cycles (Marine Isotope Stages 8 to 7 and Marine Isotope Stages 2 to 1). The recurring stacking pattern of facies documents a predominant control exerted on stratigraphy by Milankovitch and sub‐Milankovitch glacio‐eustatic oscillations across the late Quaternary period.  相似文献   

2.
Maar lake Laguna Potrok Aike is located north of the Strait of Magellan (south‐eastern Patagonia). Seismic reflection profiles revealed a highly dynamic palaeoclimate history. Dunes were identified in the eastern part of the lake at approximately 30 to 80 m below the lake floor, overlying older lacustrine strata, and suggest that the region experienced dry conditions probably combined with strong westerly winds. It is quite likely that this can be linked to a major dust event recorded in the Antarctic ice cores during Marine Isotope Stage 4. The dunes are overlain by a series of palaeo‐shorelines indicating a stepwise water‐level evolution of a new lake established after this dry period, and thus a change towards wetter conditions. After the initial, rapid and stepwise lake‐level rise, the basin became deeper and wider, and sediments deposited on the lake shoulder at approximately 33 m below present‐day lake level point towards a long period of lake‐level highstand between roughly 53·5 ka cal. bp and 30 ka cal. bp with a maximum lake level some 200 m higher than the desiccation horizon. This highstand was then followed by a regressional phase of uncertain age, although it must have happened some time between approximately 30 ka cal. bp and 6750 yrs cal. bp . Dryer conditions during the Mid‐Holocene are evidenced by a dropping lake level, resulting in a basin‐wide erosional unconformity on the lake shoulder. A second stepwise transgression between ca 5·8 to 5·4 ka cal. bp and ca 4·7 to 4 ka cal. bp with palaeo‐shorelines deposited on the lake shoulder unconformity again indicates a change towards wetter conditions.  相似文献   

3.
Travertine deposits in western Turkey are very well‐exposed in the area of Kocaba?, in the eastern part of the Denizli Basin. The palaeoclimatic significance of these travertines is discussed using U/Th dates, stable isotope data and palynological evidence. The Kocaba? travertine occurrences are characterized by successions of depositional terraces associated with palaeosols and karstic features. The travertines have been classified into eight lithotypes and one erosional horizon, namely: laminated, coated bubble, reed, paper‐thin raft, intraclasts, micritic travertine with gastropods, extra‐formational pebbles and a palaeosol layer. The analysed travertines mostly formed between 181 ka and 80 ka (Middle to Late Pleistocene) during a series of climatic changes including glacial and interglacial intervals; their δ13C and δ18O values indicate that the depositional waters were mainly of basinal thermal origin, occasionally mixed with surficial meteoric water. Palynological results obtained from the palaeosols showed an abundance of non‐arboreal percentage and xerophytic plants (Oleaceae and Quercus evergreen type) indicating that a drought occurred. Marine Isotope Stage 6 is represented by grassland species but Marine Isotope Stage 5 is represented by Pinaceae–Pinus and Abies, Quercus and Oleaceae. Uranium/thorium analyses of the Kocaba? travertines show that deposition began in Marine Isotope Stage 6 (glacial) and continued to Marine Isotope Stage 4 (glacial), but mostly occurred in Marine Isotope Stage 5 (interglacial). The travertine deposition continued to ca 80 ka in the south‐west of the study area, in one particular depression depositional system. Palaeoenvironmental indicators suggest that the travertine depositional evolution was probably controlled by fault‐related movements that influenced groundwater flow. Good correlation of the stable isotope values and dates of deposition of the travertines and palynological data of palaeosols in the Kocaba? travertines serve as a starting point for further palaeoclimate studies in south‐west Turkey. Additionally, the study can be compared with other regional palaeoclimate archives.  相似文献   

4.
New stratigraphical, palynological and dating evidence is presented for pre‐Late Devensian/Weichselian sediments at Fugla Ness and Sel Ayre, Shetland. The Fugla Ness Peat rests on till and formed during an interglacial that saw the development of maritime heaths, with scattered trees and shrubs, including Pinus and possibly Ilex. A decline into stadial conditions is marked by overlying periglacial breccia and till. The Sel Ayre Organic Sands and Gravels lie between periglacial breccias and beneath till and appear to record a changing interstadial environment in which trees were absent and the vegetation comprised largely heaths, with Bruckenthalia, and grasslands. The Fugla Ness Peat is dated to 110+40/?35 ka by uranium series disequilibrium, suggesting that it formed during the Ipswichian/Eemian Interglacial (Marine Isotope Substage 5e). Luminescence ages of ca. 98–105 ka on intercalated sands within the Sel Ayre Organic Sands and Gravels place these deposits in Marine Isotope Substage 5c (Brørup Interstadial). The two sites provide the first detailed record of Marine Isotope Stage 5 environments on Shetland. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

5.
Late Quaternary glacier fluctuations in the Macha Khola valley (Gorkha Himal, Nepal) were reconstructed using relative and absolute dating techniques. Our results indicate that younger moraine complexes were left by Late Holocene (<1.7 cal. ka BP), mid-Holocene (ca 3 cal. ka BP), and Lateglacial (ca 13 cal. ka BP) ice advances. Older Late Quaternary glacier advances occurred during Marine Oxygen Isotope Stages (MIS) 2 and 3–4. No relics of Middle or Early Pleistocene glaciations could be found. During MIS 3–4, glaciers advanced down to an altitude of at least 2150 m a.s.l., corresponding to an ELA depression of approximately 1300 m. At about 3500 m a.s.l., the MIS 2 Macha Khola glacier reached almost the thickness of the former MIS 3–4 glacier and retreated some time before 17.9 cal. ka BP. The Lateglacial glacier advanced again several times to altitudes between 2450 and 3400 m a.s.l. The mid-Holocene glaciers extended much farther down-valley than the Late Holocene ones. Dendrochronological data of Abies spectabilis suggested several periods of unfavourable growth conditions especially at the beginning of the 19th (1820) and 20th (1905) centuries.  相似文献   

6.
Pleistocene fluvial sediments of the Northmoor Member of the Upper Thames Formation exposed at Latton, Wiltshire, record episodic deposition close to the Churn–Thames confluence possibly spanning the interval from Marine Isotope Stages (MIS) 7 to 2. The sequence is dominated by gravel facies, indicating deposition by a high‐energy, gravel‐bed river. A number of fine‐grained organic sediment bodies within the sequence have yielded palaeoenvironmental and biostratigraphical data from Mollusca, Coleoptera, vertebrates, pollen and plant macrofossils. The basal deposit (Facies Association A) contains faunal material indicating temperate conditions. Most of the palaeontological evidence including a distinctive small form of mammoth (Mammuthus cf. trogontherii), together with the U‐series age estimate of >147.4 ± 20 kyr suggest correlation with MIS 7. The overlying deposits (Facies Associations B and C) represent deposition under a range of climatic conditions. Two fine‐grained organic deposits occurred within Association B; one (Association Ba) in the northern part of the pit as a channel fill and the other (Association Bb) in its southern part as a scour‐fill deposit. The coleopteran assemblages from Ba, indicate that it accumulated under temperate oceanic conditions, while Bb, which also yielded a radiocarbon age estimate of 39 560 ± 780 14C yr BP, was formed under much colder and more continental climatic conditions. The sequence is considered to represent deposition within an alluvial fan formed at the Churn–Thames confluence; a depositional scenario which may account for the juxtaposition of sediments and fossils of widely differing age within the same altitudinal range. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

7.
To provide insights into the long‐term evolution of aquatic ecosystems without human interference, we here evaluate a decadal‐ to centennial‐scale‐resolution diatom record spanning about 12 ka of the Holsteinian interglacial (Marine Isotope Stage 11c). Using a partially varved sediment core from the Dethlingen palaeolake (northern Germany), which has previously been studied for palynological and microfacies signals, we document the co‐evolution of the aquatic and surrounding terrestrial environment. The diatom record is dominated by the genera Stephanodiscus, Aulacoseira, Ulnaria and Fragilaria. Based on the diatom assemblages and physical sediment properties, the evolution of the Dethlingen palaeolake can be subdivided into three major phases. During the oldest phase (lasting ～1900 varve years), the lake was ～10–15 m deep and characterized by anoxic bottom‐water conditions and a high nutrient content. The following ～5600 years exhibited water depths >20 m, maximum diatom and Pediastrum productivity, and a peak in allochtonous nutrient input. During this phase, water‐column mixing became more vigorous, resulting in a breakdown of anoxia. The youngest lake phase (～4000–5000 years) was characterized by decreasing water depth, turbulent water conditions and decreased nutrient loading. Based on our palaeolimnological data, we conclude that the evolution of the Dethlingen palaeolake during the Holsteinian interglacial responded closely to (i) changes within the catchment area (as documented by vegetation and sedimentation) related to the transition from closed forests growing on nutrient‐rich soils (mesocratic forest phase) to open forests developing on poor soils (oligocratic forest phase), and (ii) short‐term climate variability as reflected in centennial‐scale climate perturbations.  相似文献   

8.
The late Quaternary glacial history of the Nun‐Kun massif, located on the boundary between the Greater Himalaya and the Zanskar range in northwestern India, was reconstructed. On the basis of morphostratigraphy and 10Be dating of glacial landforms (moraines and glacial trimlines), five glacial stages were recognized and defined, namely: (i) the Achambur glacial stage dated to Marine Oxygen Isotope Stage 3 to 4 (38.7–62.7 ka); (ii) the Tongul glacial stage dated to the early part of the Lateglacial (16.7–17.4 ka); (iii) the Amantick glacial stage dated to the later part of the Lateglacial (14.3 ka, 11.7–12.4 ka); (iv) the Lomp glacial stage dated to the Little Ice Age; and (v) the Tanak glacial stage, which has the youngest moraines, probably dating to the last few decades or so. Present and former equilibrium‐line altitudes (ELAs) were calculated using the standard area accumulation ratio method. The average present‐day ELA of ～4790 m above sea level in the Greater Himalaya is lower than those in the Ladakh and Zanskar ranges, namely 5380 and ～5900 m a.s.l., respectively. The ELA in the Zanskar range is higher than in the Ladakh range, possibly due to the higher peaks in the Ladakh range that are able to more effectively capture and store snow and ice. ELA depressions decrease towards the Ladakh range (i.e. inner Plateau). Peat beds interbedded with aeolian deposits that cap the terminal moraine of Tarangoz Glacier suggest millennial‐time‐scale climate change throughout the Holocene, with soil formation times at c. 1.5, c. 3.4 and c. 5.2 ka, probably coinciding with Holocene abrupt climate change events. Given the style and timing of glaciation in the study area, it is likely that climate in the Nun‐Kun region is linked to Northern Hemisphere climate oscillations with teleconnections via the mid‐latitude westerlies.  相似文献   

9.
The extensive aeolian deposits of the Tibetan Plateau (TP) represent important environmental archives, recording information about the past interplay between the Asian monsoon and Westerlies and the link between dust accumulation and Quaternary glaciations. In the northeast TP, mantles of sandy loess form a distinct belt lying between 3500 and 4500 m a.s.l. on the east‐facing slopes of the Anyemaqen Mountains. However, there is little chronological information about the loess deposits in this region. This study provides a detailed chronology for loess formation in the region using luminescence dating. A total of 29 samples were collected from an 8‐m‐thick homogeneous loess section at Hebei (HB) in order to date sand‐sized (63–90 μm) quartz and K‐feldspar fractions using optically stimulated luminescence (OSL) and infrared stimulated luminescence (IRSL and pIRIR) signals, respectively. The resulting quartz and feldspar ages are in good agreement over the last 40 ka; beyond this (i.e. De >120 Gy), the quartz age is underestimated, and the pIRIR170 feldspar ages are considered more reliable. The HB loess section records continuous environmental information from c. 50 to c. 30 ka, i.e. throughout Marine Isotope Stage (MIS) 3. Mass accumulation rates (MARs) varied considerably over this period with increased dust accumulation around c. 38 ka and after c. 32 ka; in between, and at the beginning of MIS 3 (50–40 ka), the dust accumulation rate was ~50% lower. Finally, the HB section also records a MIS 2 hiatus of c. 17 ka duration, probably resulting from deflation. This study implies that loess deposition on the TP is predominantly an interglacial/interstadial phenomenon and the TP may be deflating at the same time as the Chinese Loess Plateau is accumulating, at least during MIS 2.  相似文献   

10.
Helmens, K. F. & Engels, S. 2010: Ice‐free conditions in eastern Fennoscandia during early Marine Isotope Stage 3: lacustrine records. Boreas, 10.1111/j.1502‐3885.2010.00142.x. ISSN 0300‐9483. The traditional notion that Fennoscandia was glaciated throughout Marine Isotope Stages (MIS) 4–2, from c. 70 kyr BP to the deglaciation 15–10 kyr BP ago, has been challenged during the last decade. Recent studies have shown that climate and environmental settings during MIS 3 were more dynamic than previously assumed, and lacustrine sediment bodies indicate open‐water conditions for several sites in eastern Fennoscandia. In this study, three sediment sequences from western, eastern and northeast Finland are compared in detail with respect to their chronology, vegetation reconstruction and climatic inferences. OSL‐dating places the sediments in early MIS 3. Pollen evidence suggests the presence of isolated birch trees and open birch forest close to the retreating ice margin, in contrast to vegetation reconstructions from central Europe, which indicate tree‐less vegetation. Furthermore, reconstructions of climate using transfer functions have yielded surprising results, indicating present‐day summer temperatures in northeast Finland. The combined results suggest ice‐free and warm conditions in major parts of eastern Fennoscandia in early MIS 3, possibly during Greenland Interstadial (GIS) 14 around 53 kyr BP ago.  相似文献   

11.
A multi‐disciplinary approach was followed to investigate two thick palaeosol strata that alternate with wind‐blown dominated deposits developed along the Alghero coast (North‐west Sardinia, Italy). Optically stimulated luminescence ages reveal that both palaeosols were developed during cooler drier periods: the first one at around 70 ka Marine Isotope Stage 4 and the latter around 50 ka (Marine Isotope Stage 3). In contrast, the pedological features indicate that the palaeosols underwent heavy weathering processes under warm humid to sub‐humid conditions, characteristic of the Sardinian climate during the last interglacial stage (Marine Isotope Stage 5e). To reconcile this apparent data discrepancy, a range of sedimentological and pedological analyses were conducted. These analyses reveal that the palaeosols possess a complex history, with accumulation and weathering occurring during Marine Isotope Stage 5e, and erosion, colluviation and final deposition taking place during the following cold stages. Thus, even if these reddish palaeosols were last formed during the glacial period, the sediments building up these strata probably record the climate of the last interglacial stage (Marine Isotope Stage 5e). Trace element and X‐ray diffraction analyses, together with scanning electron microscope images, reveal the presence of Saharan dust in the parent material of the palaeosols. However, no evidence of any far‐travelled African dust has been observed in the Marine Isotope Stage 4–3 aeolian deposits. It is possible to conclude that in the West Mediterranean islands, Saharan dust input, even if of modest magnitude, is preserved preferentially in soils accumulated and weathered during interglacial stages.  相似文献   

12.
We examined 14 subaerially deposited speleothems retrieved from submerged caves in the northeastern Yucatán Peninsula (Mexico). These speleothems grew during the Middle to Late Quaternary and were dated by 230Th-U techniques to provide upper depth limits for past sea levels. We report the first relative sea-level limits for Marine Isotope Stages (MIS) 11 and 6, and present new evidence for sea-level oscillations during MIS 5 and early MIS 1. For the latter periods, the origin of growth interruptions is evaluated by combining petrographic methods with trace element analyses. The MIS 5c sea-level highstand probably occurred between 103.94 ± 0.58 ka and 96.82 ± 0.42 ka and must have exceeded -10.8 m (relative to present-day local sea level). The minimum average rate of sea-level fall over a 9.4 ka-long period during the MIS 5e/5d transition is calculated from stalagmite and published coral data at 1.74 ± 0.37 m/ka. For the early Holocene, previous discrepancies with respect to a potential multimetre oscillation of local sea level were found to be challenging to reconcile with the existing speleothem data from the area.  相似文献   

13.
Very few high‐resolution and directly dated terrestrial archives of the last glacial period exist for the western Mediterranean region, yet this is a key locality for recording sub‐millennial North Atlantic and Mediterranean climate change. Here, we present evidence of effective precipitation changes based on growth history and δ13C of calcite in a Mallorcan stalagmite that grew between 112 and 48 ka. Effective precipitation in Mallorca appears to have been sensitive to proximal sea surface temperature variations and at certain times, ca. 76 ka for example, changed rapidly from moist to arid conditions in only a few centuries. A sea‐level highstand during Marine Isotope Stage (MIS) 5a interrupted growth. Regrowth started promptly after this, but effective precipitation decreased markedly for much of the later part of MIS 5a, and also for shorter periods correlative with Heinrich events H8 (ca. 90 ka) and H6 (ca. 65 ka), with growth ceasing during H5 (ca. 48 ka). Arid episodes in Mallorca appear to be expressions of extremely cold periods recorded further north in Europe and occur contemporaneously with rapid decreases in Greenland temperature. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

14.
Two sand wedge structures and their host sediments, from Jonzac in SW France, were successfully dated using Optically Stimulated Luminescence (OSL) measurements on both small aliquots and single grains of quartz from the 180–212 μm size fraction. One of the sand wedges clearly contains primary infilling. However, grain‐size analysis and field observations do not clearly indicate whether the other feature represents a primary sand wedge or a composite sand wedge with primary and secondary infilling. OSL results and the geological setting justify using the Central Age Model (CAM) for the calculation of age estimates. Grain‐size analysis and detailed investigations of OSL results revealed the contamination of one sand wedge sample with host sediment. However, age calculation using the Finite Mixture Model (FMM) provided what is considered to be a reliable age estimate for the contaminated sample. The age estimates for all samples correspond to Marine Isotope Stage (MIS) 3. While fine‐grained sediments were deposited in the middle of MIS 3 (c. 43–55 ka), the sand wedges unexpectedly correspond to the end of this period (c. 33 ka) or the onset of MIS 2 (c. 27 ka). The sand wedges were probably formed during intense but short cold periods, possibly correlated with a Heinrich event (H2 and/or H3). The results help us to assess how effective luminescence dating is on sand wedges and the limitations involved in correlating sand wedge ages with Heinrich events, and contribute to the debate on the timing of cryogenic formation processes and the permafrost distribution in SW France.  相似文献   

15.
Coastal dune systems consisting of allochemical grains are important sedimentary archives of Pleistocene age in both of the hemispheres between the latitudes of 20° to 40°. The south Saurashtra coast in western India exhibits a large section of Middle Pleistocene aeolianites in the form of coastal cliffs, which is famous as ‘Miliolite’. Miliolites of Gopnath in south‐east Saurashtra are the oldest known coastal aeolianite deposits (age >156 ka which corresponds to Marine Isotope Stage 6) in western India. Aeolian deposits of similar ages have also been reported from the Thar Desert in north‐west India and from Southern Arabia which were largely controlled by the south‐west monsoon wind system that affects the entire belt corresponding to Sahara–Sahel, the Arabian Peninsula and north‐western India. Miliolite deposits in Gopnath are characterized by grainfall, grainflow and wind ripple laminations. At least three types of aeolian bounding surfaces have been identified. Five major facies have been identified which represent the dune and interdune relationship within the coastal aeolian system. The major dune bodies are identified as transverse dune types. The Gopnath aeolianites were deposited under dominantly dry aeolian conditions. Facies association reveals two different phases of aeolian accumulation, namely initiation of aeolian sedimentation after a prolonged hiatus and the establishment of a regularized aeolian sedimentation system. While initiation of aeolian sedimentation is marked by vast stretches of sheet sand with occasional dune bodies, the overlying thick, tabular, laterally extensive cross‐stratified units manifest regular aeolian sedimentation. However, the dune building events in Gopnath were interrupted by development of laterally extensive palaeosol horizons. Eustasy and climate exerted the major allogenic controls on the aeolian sedimentation by affecting the sediment budget as well as influencing the sedimentation pattern.  相似文献   

16.
《第四纪科学杂志》2017,32(1):48-62
The southernmost terrestrial extent of the Irish Sea Ice Stream (ISIS), which drained a large proportion of the last British–Irish Ice Sheet, impinged on to the Isles of Scilly during Marine Isotope Stage 2. However, the age of this ice limit has been contested and the interpretation that this occurred during the Last Glacial Maximum (LGM) remains controversial. This study reports new ages using optically stimulated luminescence (OSL) dating of outwash sediments at Battery, Tresco (25.5 ± 1.5 ka), and terrestrial cosmogenic nuclide exposure dating of boulders overlying till on Scilly Rock (25.9 ± 1.6 ka), which confirm that the ISIS reached the Isles of Scilly during the LGM. The ages demonstrate this ice advance on to the northern Isles of Scilly occurred at ∼26 ka around the time of increased ice‐rafted debris in the adjacent marine record from the continental margin, which coincided with Heinrich Event 2 at ∼24 ka. OSL dating (19.6 ± 1.5 ka) of the post‐glacial Hell Bay Gravel at Battery suggests there was then an ∼5‐ka delay between primary deposition and aeolian reworking of the glacigenic sediment, during a time when the ISIS ice front was oscillating on and around the Llŷn Peninsula, ∼390 km to the north. Copyright © 2017 The Authors. Journal of Quaternary Science Published by John Wiley & Sons, Ltd.
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17.
In 2007, we demonstrated that radiolarians are proxies for a wide range of oceanic physico-chemical properties from the surface to depths of up to 500 m below sea level. In this study, our results are refined and Correspondence Analysis (CA) scores derived from census counts of radiolarian subfossils from southern Indian Ocean core-tops are correlated with the physico-chemical properties of the region obtained from the 2005 World Ocean Database.Calibration and regression techniques are employed to reconstruct palaeoenvironmental conditions spanning the last 40 ka for four Indian Ocean cores MD88-769 [46°04′S 90°06′E], MD88-770 [46°01′S 96°27′E], MD94-102 [43°30′S 79°50′E], and MD94-103 [45°35′S 86°31′E], all from close to the Southeast Indian Ridge. For the first time, reconstructions of temperature, salinity, dissolved oxygen, and the silicate, nitrate, and phosphate concentrations for a range of water depths are proved possible.Changes of the oceanic environment and the movement of water masses over the last 40 ka, as suggested by these reconstructions, are discussed. During Marine Isotope Stages 2 and 3 (MIS-2 and MIS-3), the water column at some of the core sites has similar characteristics to the waters south of the Polar Front today. At the MIS-1/MIS-2 transition, the development of the Subantarctic Mode Water is apparent. Temperature reconstructions include evidence of the Antarctic Cold Reversal and the Holocene Optimum.  相似文献   

18.
The North West Shelf is an ocean‐facing carbonate ramp that lies in a warm‐water setting adjacent to an arid hinterland of moderate to low relief. The sea floor is strongly affected by cyclonic storms, long‐period swells and large internal tides, resulting in preferentially accumulating coarse‐grained sediments. Circulation is dominated by the south‐flowing, low‐salinity Leeuwin Current, upwelling associated with the Indian Ocean Gyre, seaward‐flowing saline bottom waters generated by seasonal evaporation, and flashy fluvial discharge. Sediments are palimpsest, a variable mixture of relict, stranded and Holocene grains. Relict intraclasts, both skeletal and lithic, interpreted as having formed during sea‐level highstands of Marine Isotope Stages (MIS) 3 and 4, are now localized to the mid‐ramp. The most conspicuous stranded particles are ooids and peloids, which 14C dating shows formed at 15·4–12·7 Ka, in somewhat saline waters during initial stages of post‐Last Glacial Maximum (LGM) sea‐level rise. It appears that initiation of Leeuwin Current flow with its relatively less saline, but oceanic waters arrested ooid formation such that subsequent benthic Holocene sediment is principally biofragmental, with sedimentation localized to the inner ramp and a ridge of planktic foraminifera offshore. Inner‐ramp deposits are a mixture of heterozoan and photozoan elements. Depositional facies reflect episodic environmental perturbation by riverine‐derived sediments and nutrients, resulting in a mixed habitat of oligotrophic (coral reefs and large benthic foraminifera) and mesotrophic (macroalgae and bryozoans) indicators. Holocene mid‐ramp sediment is heterozoan in character, but sparse, most probably because of the periodic seaward flow of saline bottom waters generated by coastal evaporation. Holocene outer‐ramp sediment is mainly pelagic, veneering shallow‐water sediments of Marine Isotope Stage 2, including LGM deposits. Phosphate accumulations at ≈ 200 m water depth suggest periodic upwelling or Fe‐redox pumping, whereas enhanced near‐surface productivity, probably associated with the interaction between the Leeuwin Current and Indian Ocean surface water, results in a linear ridge of pelagic sediment at ≈ 140 m water depth. This ramp depositional system in an arid climate has important applications for the geological record: inner‐ramp sediments can contain important heterozoan elements, mid‐ramp sediments with bedforms created by internal tides can form in water depths exceeding 50 m, saline outflow can arrest or dramatically slow mid‐ramp sedimentation mimicking maximum flooding intervals, and outer‐ramp planktic productivity can generate locally important fine‐grained carbonate sediment bodies. Changing oceanography during sea‐level rise can profoundly affect sediment composition, sedimentation rate and packaging.  相似文献   

19.
Elmejdoub, N., Mauz, B. & Jedoui, Y. 2010: Sea‐level and climatic controls on Late Pleistocene coastal aeolianites in the Cap Bon peninsula, northeastern Tunisia. Boreas, 10.1111/j.1502‐3885.2010.00162.x. ISSN 0300‐9483. The chronology of coastal dunes (aeolianites) along the western littoral of the Cap Bon peninsula (northeastern Tunisia) was investigated using an optical dating technique to examine their tentative correlation with the Marine Isotope Stage (MIS) record. These dunes, formed under a northwesterly wind regime and supplied by sand from the shore, are an indicator of sea‐level and climate changes. We obtained optically stimulated luminescence ages for these aeolianites ranging from 112±10 to 53±2 ka and clustering around the last interglacial period (～125–75 ka), implying that the former stratigraphic allocation of these dunes is inaccurate. The optical chronology suggests dune formation during MIS 5 in association with a sea level lower than today but higher than the glacial sea‐level lowstand.  相似文献   

20.
The current detailed chronostratigraphic framework of the last 1 Ma of an eastern Mediterranean sequence (Haifa Bay, Israel) aims to examine the relative roles of sea‐level changes, climate and tectonics. Seven continuous marine cores, up to ～120 m long, were recovered from shallow water depths. The cores were dated by optically stimulated luminescence, 14C, magnetostratigraphy, 230Th/234U, 26Al/10Be, occurrence of index fossils and correlated to the global sea‐level curve and Marine Isotope Stages (MIS). The sedimentary sequence accumulated during the last ca. 1.0 Ma consists of 21 transgression–regression units with hiatuses between them. Five marine/terrestrial cycles, which occur in the lower part of the sequence, are attributed to the Jaramillo subchron and the Brunhes–Matuyama boundary, and correspond to MIS 29–21. The top ～50 m includes three sedimentary cycles deposited in the last ca. 400 ka. The regressive phases during this interval correspond to Glacial MIS 8, 6 and 2, while the transgressions correspond to Interglacial MIS 11, 7, 5 and 1. Thus, for the first time, this study documents the longest Quaternary succession dated so far in a key area of the Levant, sensitive to global history of sea‐level changes and glacial/interglacial fluctuations. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

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