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1.
High sedimentation rates in Pleistocene active margin basins can provide a very detailed record of tectonic and climatic controls on sediment preservation. A 500 m thick, Pleistocene rock section exposed in northeastern North Island of New Zealand (Kidnappers Group), provides the opportunity to discuss these controls. The section is composed of conglomerate, sandstones, siltstones and minor shales, interbedded with tephra layers. The sediments were deposited in alluvial to shallow marine environments and preserved in stacks of depositional units decimetres to hundreds of metres thick as a result of base‐level changes through time. The correlation of base‐level changes in the section with the deep sea oxygen isotope stratigraphy shows that the sequences at 10 m and 80 m scales can correlate, respectively, to the 20 and 100 kyr changes in eustatic sea‐level, but that the 80‐m‐thick sequences correlate also to changes in tectonic uplift rates. A major change in the stratigraphical architecture occurs at the Mid‐Pleistocene Transition (MPT) when the 40 kyr ice volume variations shifted to a dominant 100 kyr variation. This change includes an increase in the amplitude of the shifts in depositional environments and an overall simplification of the stacking pattern of the depositional units through the MPT. This study illustrates that active margin basins can record orbitally forced sedimentary cycles and points to a possible leading influence of eustasy on the pattern of sediment preservation in tectonically active areas. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

2.
Lake Estanya is a small (19 ha), freshwater to brackish, monomictic lake formed by the coalescence of two karstic sinkholes with maximum water depths of 12 and 20 m, located in the Pre‐Pyrenean Ranges (North‐eastern Spain). The lake is hydrologically closed and the water balance is controlled mostly by groundwater input and evaporation. Three main modern depositional sub‐environments can be recognized as: (i) a carbonate‐producing ‘littoral platform’; (ii) a steep ‘talus’ dominated by reworking of littoral sediments and mass‐wasting processes; and (iii) an ‘offshore, distal area’, seasonally affected by anoxia with fine‐grained, clastic sediment deposition. A seismic survey identified up to 15 m thick sedimentary infill comprising: (i) a ‘basal unit’, seismically transparent and restricted to the depocentres of both sub‐basins; (ii) an ‘intermediate unit’ characterized by continuous high‐amplitude reflections; and (iii) an ‘upper unit’ with strong parallel reflectors. Several mass‐wasting deposits occur in both sub‐basins. Five sediment cores were analysed using sedimentological, microscopic, geochemical and physical techniques. The chronological model for the sediment sequence is based on 17 accelerator mass spectrometry 14C dates. Five depositional environments were characterized by their respective sedimentary facies associations. The depositional history of Lake Estanya during the last ca 21 kyr comprises five stages: (i) a brackish, shallow, calcite‐producing lake during full glacial times (21 to 17·3 kyr bp ); (ii) a saline, permanent, relatively deep lake during the late glacial (17·3 to 11·6 kyr bp ); (iii) an ephemeral, saline lake and saline mudflat complex during the transition to the Holocene (11·6 to 9·4 kyr bp ); (iv) a saline lake with gypsum‐rich, laminated facies and abundant microbial mats punctuated by periods of more frequent flooding episodes and clastic‐dominated deposition during the Holocene (9·4 to 0·8 kyr bp ); and (v) a deep, freshwater to brackish lake with high clastic input during the last 800 years. Climate‐driven hydrological fluctuations are the main internal control in the evolution of the lake during the last 21 kyr, affecting water salinity, lake‐level changes and water stratification. However, external factors, such as karstic processes, clastic input and the occurrence of mass‐flows, are also significant. The facies model defined for Lake Estanya is an essential tool for deciphering the main factors influencing lake deposition and to evaluate the most suitable proxies for lake level, climate and environmental reconstructions, and it is applicable to modern karstic lakes and to ancient lacustrine formations.  相似文献   

3.
Cryoturbated organic beds and channel fills, intercalated with sandy and gravelly fluvial units, have been studied in an opencast brown‐coal mine near Nochten (Niederlausitz), eastern Germany. The fluvial–aeolian sequence covers parts of the Early, Pleni‐ and Late‐glacial. The detailed chronology is based on 11 radiocarbon and 12 OSL dates, covering the period between ca. 100 kyr and 11 kyr BP. Basal peat deposits are correlated with an Early Weichselian interstadial. During this period boreal forests were present and minimum mean summer temperatures were > 13°C. Early Pleniglacial deposits are absent. The Middle and Late Pleniglacial environments were treeless and different types of tundra vegetation can be recognised. Minimum mean summer temperatures varied between 10 and 15°C. Vegetation and climate is reconstructed in detail for the periods around 34–38 kyr BP and 24–25 kyr BP. Around 34–38 ka, a mixture between a low shrub tundra and a cottongrass tussock–subshrub tundra was present. The botanical and sedimentological data suggest that from the Middle to the Late Pleniglacial, the climate became more continental, aridity and wind strength increased, and the role of a protecting winter snow cover decreased. A sedge–grass–moss tundra dominated around 24 and 25 kyr BP. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献   

4.
The evolution of incised valleys is an important area of research due to the invaluable data it provides on sea‐level variations and depositional environments. In this article the sedimentary evolution of the Ría de Ferrol (north‐west Spain) from the Last Glacial Maximum to the present is reconstructed using a multidisciplinary approach, combining seismic and sedimentary facies, and supported by radiocarbon data and geochemical proxies to distinguish the elements of sedimentary architecture within the ria infill. The main objectives are: (i) to analyse the ria environment as a type of incised valley to evaluate the response of the system to the different drivers; (ii) to investigate the major controlling factors; and (iii) to explore the differentiation between rias and estuaries. As a consequence of the sea‐level rise subsequent to the Last Glacial Maximum (ca 20 kyr bp ), an extensive basin, drained by a braided palaeoriver, evolved into a tide‐dominated estuary and finally into a ria environment. Late Pleistocene and Holocene high‐frequency sea‐level variations were major factors that modulated the type of depositional environments and their evolution. Another major modulating factor was the antecedent morphology of the ria, with a rock‐incised narrow channel in the middle of the basin (the Ferrol Strait), which influenced the evolution of the ria as it became flooded during Holocene transgression. The strait acted as a rock‐bounded ‘tidal inlet’ enhancing the tidal erosion and deposition at both ends, i.e. with an ebb‐tidal delta in the outer sector and tidal sandbanks in the inner sector. The final step in the evolution of the incised valley into the modern‐defined ria system was driven by the last relative sea‐level rise (after 4 kyr bp ) when the river mouths retreated landward and a single palaeoriver was converted into minor rivers and streams with scattered mouths in an extensive coastal area.  相似文献   

5.
The lacustrine record of Oberwinkler Maar (Eifel, Germany) is the northernmost continuous record documenting the Weichselian Pleniglacial in central Europe – a period characterized by multiple abrupt climate oscillations known as the Dansgaard/Oeschger cycles. Here, the results of a high-resolution study of chironomid remains are presented, with a focus on the earlier part of Oxygen Isotope Stage (OIS) 3 (60–50 kyr BP) covering four stadial/interstadial cycles. During the stadials, the chironomid fauna of the former lake was dominated by many cold-stenothermic chironomid taxa, indicating a cold, oligotrophic lake. The concentrations of chironomid remains were lower during the interstadials, and featured a higher number of warm-indicating taxa. This could have been the result of a higher summer temperature at the study site, but also of bottom-water anoxia, an increase in trophic state or a combination of all these factors. During the stadial intervals, a taxon that is restricted to (sub-)arctic environments is present in our record, suggesting a change in the temperature regime rather than in-lake processes as the driving mechanism for the changes in the chironomid record. Although, consistently, there was a response of the lake ecosystem to climate changes, the amplitude of this response was not constant. This study provides unique and independent evidence of the impact of Dansgaard/Oeschger climate variability on the European continent during the earlier part of OIS-3.  相似文献   

6.
Stanford, S. D. 2009: Onshore record of Hudson River drainage to the continental shelf from the late Miocene through the late Wisconsinan deglaciation, USA: synthesis and revision. Boreas, 10.1111/j.1502‐3885.2009.00106.x. ISSN 0300‐9483. Fluvial and glacial deposits in New Jersey, Long Island, and the Hudson valley provide a record of Hudson River drainage since the late Miocene. Late Miocene fluvial deposits record southerly flow across the emerged inner New Jersey shelf. In the late Miocene–early Pliocene this drainage incised, shifted southwesterly, and discharged to the shelf south of New Jersey. During late Pliocene or Early Pleistocene glaciation, discharge to the shelf in the New York City area was established. This drainage incised and stabilized in the Early and Middle Pleistocene and remained open during pre‐Wisconsinan (Oxygen Isotope Stage 6? (OIS‐6?)) and late Wisconsinan (OIS‐2) glacial advances. During late Wisconsinan retreat, moraine deposits dammed the valley at the Narrows to form Lake Albany. From 19 to 15.5 kyr BP (all dates in 14C yr), Hudson drainage was directed eastward into the Long Island Sound lowland. Drainage of Lake Wallkill into Lake Albany at 15.5 kyr BP breached the Narrows dam and initiated the unstable phase of Lake Albany, which was controlled by eroding spillways, first on the moraine dam, then on emerged lake‐bottom in the mid‐Hudson valley. Marine incursion between 12 and 11 kyr BP limited fluvial incision of the lake bottom, stabilizing the Quaker Springs, Coveville, and upper Fort Ann spillways. Lowering sea level between 11 and 10 kyr BP allowed incision from the upper to lower Fort Ann threshold. Sediment eroded by lake outflows between 15 and 10.5 kyr BP was trapped in the glacially deepened lower valley. Little inland sediment reached the shelf after 20 kyr BP.  相似文献   

7.
The timing of the extinction of the Australian Megafauna and whether it was simultaneous and widespread has been a much researched topic in Quaternary geoscience. The Black Creek Swamp Megafauna site on Kangaroo Island was thought to be a refugium for Megafauna; however, recent and multidiscipline age determinations have established that the fossils are considerably older than the well-quoted extinction age of 45 kyr. Further radiocarbon age determinations, δ13C isotope analysis and 13C-NMR spectroscopy of the fossil containing organic matter demonstrates that it is highly soluble and accumulated as recently as 31–18 cal. kyr BP. These radiocarbon ages are much younger than the 100–50 kyr age bracket ascertained for the fossil material itself, implying separate episodes of death, deposition and burial. The soluble nature of the organic matter and increasing radiocarbon ages with depth suggests lateral accumulation, probably transported by subsurface waters from elevated areas proximal to the low-lying swamp. Such depositional conditions and 14C age range implies that the site may have experienced an unusually wet Last Glacial Maximum, due maybe to its proximity to the continental shelf and thus to maritime conditions. C3 vegetation dominates the Black Creek Swamp and its organic matter (δ13C; −30‰ and −23‰); however, variations in δ13C may indicate climatic shifts. 13C-enrichment and an abundance of salt-tolerant gastropods within the site's final phase of sediment accumulation (<6 cal. kyr BP) suggest that conditions during this most recent period were not as wet as those of the Last Glacial Maximum.  相似文献   

8.
Two of the most important factors that control the accumulation rate of material in carbonate platform environments on geological time scales are climate and eustasy. Accurately assessing the importance of these inter‐related factors through the study of both modern and ancient carbonate facies, however, is problematic. These difficulties arise from both the complexities inherent in carbonate depositional systems and the demonstrable incompleteness of the stratigraphic record. Here, a new compilation of more than 19 000 global Phanerozoic shallow marine carbonate accumulation rates derived from nearly 300 individual stratigraphic sections is presented. These data provide the first global holistic view of changes in shallow marine carbonate production in response to climate and eustasy on geological time scales. Notably, a clear latitudinal dependence on carbonate accumulation rates is recognized in the data. Moreover, it can also be demonstrated that rates calculated across the last glacial maximum and Holocene track changes in sea‐level. In detail, the data show that globally averaged changes in carbonate accumulation rates lagged changes in sea‐level by ca 3 kyr, reflecting the commonly observed delay in the response of individual carbonate successions to sea‐level rise. Differences between the rates of carbonate accumulation and sea‐level change over the past 25 kyr ostensibly reflect changing accumulation mode, with platform drowning (give‐up mode) pervasive during peak Early Holocene sea‐level rise, followed by a switch to catch‐up mode accumulation from ca 9 ka to the present. Carbonate accumulation rates older than the Quaternary are typically calculated over time spans much greater than 100 kyr, and at these time spans, rates primarily reflect long‐term tectonically mediated accommodation space changes rather than shorter term changes in climate/eustasy. This finding, coupled with issues of stratigraphic incompleteness and data abundance, tempers the utility of this and other compilations for assessing accurately the role of climate and eustasy in mediating carbonate accumulation rates through geological time.  相似文献   

9.
A well-preserved moraine on the northern coast of County Donegal, Ireland, has played a critical role in our understanding of the glacial history of this sector of the Irish Ice Sheet (IIS). Because of a lack of numerical dating of the moraine, however, previous interpretations of its age and significance to the glacial history of this region have varied widely. Here we report eight in situ cosmogenic 10Be ages on boulders sampled from the moraine. Two of these ages are outliers, with the remaining six ranging from 18.8±1.0 10Be kyr to 20.9±1.3 10Be kyr, with an uncertainty-weighted mean age of 19.4±0.3 10Be kyr (19.4±1.2 kyr accounting for production rate uncertainty). Our results confirm one previous 10Be age obtained from the moraine, with the combined data ( n =7) constraining the age of initial deglaciation of the IIS from its LGM position on the continental shelf to be 19.3±0.3 10Be kyr (19.3±1.2 kyr accounting for production rate uncertainty). These ages are in excellent agreement with calibrated 14C ages that constrain retreat of the IIS margin from the continental shelf elsewhere in northwestern and western Ireland and the Irish Sea Basin associated with the start of the Cooley Point Interstadial (≥20–≤18.2 cal. kyr BP), suggesting widespread deglaciation of the IIS ∼19.5–20 kyr ago.  相似文献   

10.
Recent results indicate contrasting Holocene moisture histories at different elevations in arid central Asia (ACA). However, relatively little is known about Holocene temperature changes at different elevations. Here we report an independently dated peat brGDGTs-based MBT'5ME record from the Narenxia peatland (NRX) in the southern Altai Mountains. The record suggests a long-term warming trend since ~7.7 cal. kyr bp , with a warmer stage during ~7–5.5 cal. kyr bp , a cold stage during ~5.5–4 cal. kyr bp , and a warming trend over the last ~4 kyr. The long-term warming trend indicated by the NRX MBT'5ME record is largely consistent with Holocene temperature records from nearby sites covering an altitudinal range of ~1700–4100 m above sea level. This consistent long-term warming trend at different elevations differs from the long-term Holocene drying/wetting trends at high/low elevations of the Altai Mountains. We propose that the warming trend and consequent permafrost thawing at high elevations could have resulted in increased meltwater runoff, which would have contributed to the long-term wetting trend at low elevations. Our findings potentially provide an improved understanding of regional climate change and associated water resource availability, with implications for their possible future status.  相似文献   

11.
The Song Hong (Red River) delta occurs on the northwest coast of the South China Sea. Its evolution in response to Holocene sea-level changes was clarified on the basis of sedimentary facies and 14 radiocarbon dates from the 40 m long Duy Tien core from the delta plain, and using previously reported geological, geomorphological, and archaeological data. The delta prograded into the drowned valley as a result of early Holocene inundation from 9 to 6 cal. kyr BP, as sea-level rise decelerated. The sea-level highstand at +2–3 m from 6 to 4 cal. kyr BP allowed widespread mangrove development on the delta plain and the formation of marine notches in the Ha Long Bay and Ninh Binh areas. During sea-level lowering after 4 cal. kyr BP, the former delta plain emerged as a marine terrace, and the delta changed into the present tide- and wave-influenced delta with accompanying beach ridges. Delta morphology, depositional pattern, and sedimentary facies are closely related to Holocene sea-level changes. In particular, falling sea level at 4 cal. kyr BP had a major impact on the evolution of the Song Hong delta, and is considered to be linked to climate changes.  相似文献   

12.
Nemec  & Kazanci 《Sedimentology》1999,46(1):139-170
The Quaternary colluvial aprons in Lake Eğirdir area, Taurus Mountains, consist of steep coalescent fans, up to 17–20 m thick and 350–450 m in plan-view radius, and the sedimentary succession comprises four lithostratigraphic divisions. The basal red–brown colluvium consists of a chaotic, bouldery fan-core rubble covered with bedded, openwork to matrix-rich gravel, whose deposition is attributed to rockfalls and cohesive debrisflows, with minor grainflows and sheetwash processes. The middle part of this division contains interbeds of early Pleistocene tephra. The overlying light-grey colluvium consists of bedded gravel with numerous palaeogullies and its deposition is attributed to waterflow, rockfalls and cohesive debrisflows. The younger, medium-grey colluvium consists of stratified pebbly sand interspersed with cobble/boulder gravel and its deposition is attributed to sheetwash processes accompanied by rockfalls and wet snowflows/slushflows. A bulk-pollen radiocarbon date indicates Late Würm age. The youngest, yellow–grey colluvium consists of bedded, mainly pebbly and openwork gravel, whose deposition is attributed to dry grainflows, rockfalls and minor cohesive debrisflows. Based on the sedimentary facies assemblages and available isotopic dates, the four colluvial divisions are correlated with the following stages of the region's climatic history: (1) the latest Pliocene to Early Pleistocene stage of warm–humid climate with pronounced phases of drier conditions; (2) the Late Pleistocene stage of colder climate, with alternating phases of higher and lower humidity; (3) the last glacial (Würm) stage of coldest climate; and (4) the Holocene stage of warm semi-arid climate. It is concluded that colluvial depositional systems bear a valuable proxy record of climatic changes and regional geoclimatic differences.  相似文献   

13.
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14.
In order to contribute to our understanding of the linkage between climate and ocean circulation we have studied benthic foraminifera from near the northern end of the Faeroe–Shetland Channel covering isotope stages 6 to lower stage 3 (∼150–55 ka). Our records demonstrate shifts between recurring assemblages, which on millennial timescales monitor the outflow history of Norwegian Sea Deep Water. The records show that the outflow is closely linked to the climate of the region as documented in the Greenland ice cores. Outflow was relatively strong during all major warmer interstadials whereas there was no outflow during the colder stadials. During isotope substage 5e outflow was stable for ∼10–12 kyr with significant changes at the beginning and end only.  相似文献   

15.
Two parallel drilling cores, BDP96-1 (200 m), and BDP96-2 (100 m), have been taken from top of the underwater Akademichesky Ridge in Lake Baikal and dated by paleomagnetic techniques. These cores are part of the Baikal Drilling Project (Kuzmin et al., 1997a, b). Diatom analysis of the first 100 m of these sediments reveals that they store a 2.5 Myr high-resolution continuous record of the palaeoclimates of East Siberia. Sediments belonging to warmer climates have a high content of diatom algae frustules, whereas those belonging to global glaciations are diatom-barren. The record of Upper Pleistocene (500 kyr BP) strongly correlates with the oceanic SPECMAP curve and contains distinct 100, 41, 23, and 19 kyr periods, as revealed by Fourier analysis. Diversity of diatom species was small in Pliocene. Abrupt and frequent changes of the climate in Pleistocene resulted in frequent dramatic changes in the diatom communities. The extant dominating endemics, Cyclotella minuta and Aulacoseira baicalensis, became important in Lake Baikal 760 kyr and less than 120 kyr BP, respectively.  相似文献   

16.
The passive margin Texas Gulf of Mexico Coastal Plain consists of coalescing late Pleistocene to Holocene alluvial–deltaic plains constructed by a series of medium to large fluvial systems. Alluvial–deltaic plains consist of the Pleistocene Beaumont Formation, and post-Beaumont coastal plain incised valleys. A variety of mapping, outcrop, core, and geochronological data from the extrabasinal Colorado River and the basin-fringe Trinity River show that Beaumont and post-Beaumont strata consist of a series of coastal plain incised valley fills that represent 100 kyr climatic and glacio-eustatic cycles.

Valley fills contain a complex alluvial architecture. Falling stage to lowstand systems tracts consist of multiple laterally amalgamated sandy channelbelts that reflect deposition within a valley that was incised below highstand alluvial plains, and extended across a subaerially-exposed shelf. The lower boundary to falling stage and lowstand units comprises a composite valley fill unconformity that is time-transgressive in both cross- and down-valley directions. Coastal plain incised valleys began to fill with transgression and highstand, and landward translation of the shoreline: paleosols that define the top of falling stage and lowstand channelbelts were progressively onlapped and buried by heterolithic sandy channelbelt, sandy and silty crevasse channel and splay, and muddy floodbasin strata. Transgressive to highstand facies-scale architecture reflects changes through time in dominant styles of avulsion, and follows a predictable succession through different stages of valley filling. Complete valley filling promoted avulsion and the large-scale relocation of valley axes before the next sea-level fall, such that successive 100 kyr valley fills show a distributary pattern.

Basic elements within coastal plain valleys can be correlated with the record offshore, where cross-shelf valleys have been described from seismic data. Falling stage to lowstand channelbelts within coastal plain valleys were feeder systems for shelf-phase and shelf-margin deltas, respectively, and demonstrate that falling stage fluvial deposits are important valley fill components. Signatures of both upstream climate change vs. downstream sea-level controls are therefore interpreted to be present within incised valley fills. Signatures of climate change consist of the downstream continuity of major stratigraphic units and component facies, which extends from the mixed bedrock–alluvial valley of the eroding continental interior to the distal reaches, wherever that may be at the time. This continuity suggests the development of stratigraphic units and facies is strongly coupled to upstream controls on sediment supply and climate conditions within hinterland source regions. Signatures of sea-level change are critical as well: sea-level fall below the elevation of highstand depositional shoreline breaks results in channel incision and extension across the newly emergent shelf, which in turn results in partitioning of the 100 kyr coastal plain valleys. Moreover, deposits and key surfaces can be traced from continental interiors to the coastal plain, but there are downstream changes in geometric relations that correspond to the transition between the mixed bedrock–alluvial valley and the coastal plain incised valley. Channel incision and extension during sea-level fall and lowstand, with channel shortening and delta backstepping during transgression, controls the architecture of coastal plain and cross-shelf incised valley fills.  相似文献   


17.
Based on a well-established bio- and sequence-stratigraphic framework, a narrow time window in the Bimammatum ammonite zone (Late Oxfordian) is investigated in six Swiss Jura sections representing a shallow-water carbonate platform. From the detailed facies and microfacies analysis of oncoid-rich (Hauptmumienbank Member) and ooid-rich (Steinebach Member) limestones, a microfacies classification is established, depositional environments are interpreted, and a depositional model for the Swiss Jura platform is proposed. The sequence- and cyclostratigraphic interpretation has been performed for the transgressive part of the medium-scale sequence Ox6+, independently for each section, with a very high time resolution at the scale of elementary depositional sequences. The good correlation of the elementary and small-scale sequences between the six studied sections and the similar number of elementary sequences in all sections strongly suggest that allocyclic processes were involved in their formation. The hierarchically stacked depositional sequences (small-scale and elementary sequences) result from orbitally controlled sea-level changes with periodicities of 100 and 20 kyr, respectively. Thickness variations in the correlated small-scale and elementary sequences imply variable sedimentation rates, probably resulting from differential subsidence due to the activity of tectonic blocks. The tectonically controlled platform morphology contributed significantly to the general pattern of depositional environments and, combined with high-frequency sea-level fluctuations, created a complex facies distribution in time and space on the Swiss Jura carbonate platform.  相似文献   

18.
The Gulf of Papua inner mid-shelf clinothem and lowstand deposits in Pandora Trough record sediment source and routing through the last sea-level cycle on 20 kyr cycles. Clay mineralogy tracked dispersal of sediment from the two types of rivers (wide versus narrow floodplains) to constrain the contributions of river systems to the Gulf of Papua clinothem and Pandora Trough deposits. Fly River sediment has higher illite:smectite than clays from the small mountainous rivers (Bamu, Turama, Kikori and Purari rivers) that drain regions with more limestones. X-ray diffraction shows high illite:smectite proximal to the Fly River delta that decrease towards the north-east. Downcore mineralogy of inner mid-shelf cores reveals that the largest shifts in illite:smectite correspond to changes in sediment units. The relict clinothem emplaced on the Gulf of Papua shelf during Marine Isotope Stage 3 has lower illite:smectite than the Holocene clinothem that has been building since 2 ka and the Marine Isotope Stage 5a relict clinothem. In the inner mid-shelf, downcore decreases in illite:smectite during Marine Isotope Stage 3 suggest that this region received less clay from the Fly River and more contributions from small mountainous rivers. During Marine Isotope Stage 3, the exposed physiography and narrower shelf in this region may have deflected Fly River sediment more south-eastward, where it bypassed the inner shelf via the Kiwai, Purutu and Umuda valleys and was deposited in the Pandora Trough. The Fly River may have been more susceptible to valley incision because of its limited shelf accommodation and higher ratio of water to sediment discharge. Such bypass of the inner mid-shelf by Fly River sediment during the Marine Isotope Stage 2 sea-level lowstand is recorded in Pandora Trough deposits with high illite:smectite ratios. Inner mid-shelf clinothems with compositional shifts on the order of 20 kyr may be influenced by shelf physiography, accommodation and the variable incision by small and large rivers.  相似文献   

19.
A detailed cyclostratigraphy is present in the rhythmically bedded Cenomanian hemipelagic chalks of the northern Anglo-Paris Basin, which can be correlated over 100,000 km2. Individual decimetre-scale couplets probably represent the precession cycle (modes at 18 or 23 kyr), and can be readily grouped in sets averaging five, indicative of the short cycle of eccentricity (100 kyr). A composite cyclostratigraphy for the basin is presented here, and is used as the basis for a Cenomanian time-scale (20 and 100 kyr units). The total duration of the Cenomanian thus obtained (4.4 Myr) compares well with recent radiometric dates of 4.0–4.5 Myr. The time-scale is used to estimate the length of ammonite, inoceramid, and planktonic foraminiferal zones, the rates of formation of authigenic minerals (glauconite), sedimentation rates and the duration of transgressions and anoxic events.  相似文献   

20.
Two coeval stalagmites from Katerloch Cave show pronounced intervals of low δ18O values around 8.2, 9.1, and 10.0 kyr (all ages are reported before the year 2000 AD) and represent an inorganic U–Th dated climate archive from the southeast of the European Alps, a region where only very few well-dated climate records exist. The O isotope curves, providing near-annual resolution, allow a direct comparison to the Greenland ice core records, as temperature was the primary factor controlling the O isotopic composition of Katerloch speleothems.The 8.2 kyr climate anomaly lasted about one century, from 8196 to 8100 yr, with a maximum amplitude of 1.1‰ at 8175 yr. The event is characterized by a rapid onset and a more gradual demise and U–Th data as well as annual lamina counting support a rapid climate change towards cooler conditions within 10–20 yr. There is no strong evidence that the 8.2 kyr anomaly was superimposed on a pronounced longer-term cooling episode, nor do the new data support two separate cooling events within the 8.2 kyr event as reported by other studies. Our record also shows a distinct climate anomaly around 9.1 kyr, which lasted 70–110 yr and showed a maximum amplitude of 1.0‰, i.e. it had a similar duration and amplitude as the (central) 8.2 kyr event. Compared to the 8.2 kyr event, the 9.1 kyr anomaly shows a more symmetrical structure, but onset and demise still occurred within a few decades only. The different progression of the 8.2 (asymmetrical) and 9.1 kyr anomaly (symmetrical) suggests a fundamental difference in the trigger and/or the response of the climate system. Moreover, both stalagmites show evidence of a climate anomaly around 10.0 kyr, which was of comparable magnitude to the two subsequent events.Using a well constrained modern calibration between air temperature and δ18O of precipitation for the study area and cave monitoring data (confirming speleothem deposition in Katerloch reflecting cave air temperature), a maximum cooling by ca 3 °C can be inferred at 8.2 and 9.1 kyr, which is similar to other estimates, e.g., from Lake Ammersee north of the Alps. The O isotopic composition of meteoric precipitation, however, is a complex tracer of the hydrological cycle and these temperature estimates do not take into account additional effects such as changes in the source area or synoptic shifts. Apart from that, the relative thickness of the seasonally controlled lamina types in the Katerloch stalagmites remains rather constant across the intervals comprising the isotopic anomalies, i.e. the stalagmite petrography argues against major shifts in seasonality during the early Holocene climate excursions.  相似文献   

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