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1.
《Quaternary Science Reviews》2003,22(8-9):943-947
We present 21 radiocarbon dates on 19 charcoal samples from the sedimentary sequence preserved in Border Cave, South Africa. The background radiocarbon activity for charcoal from the cave was determined to be 0.050±0.018 percent modern carbon, from the analysis of a radiocarbon-dead sample from unit 5WA. Radiocarbon ages for individual samples ranged from 25.2 to >58.2 ka BP.The error-weighted mean ages for successively older strata are 38.5+0.85/−0.95 ka BP for unit 1WA, 50.2+1.1/−1.0 ka BP for units 2BS.LR.A and 2BS.LR.B, 56.5+2.7/−2.0 ka BP for unit 2BS.LR.C and 59.2+3.4/−2.4 ka BP for unit 2WA. This radiocarbon chronology is consistent with independent chronologies derived from electron spin resonance and amino acid racemization dating. The results therefore provide further evidence that radiocarbon dating of charcoal by the ABOX-SC technique can yield reliable radiocarbon ages beyond 40 ka BP. They also imply that Border Cave 5, a modern human mandible, predates >58.2 ka BP and that the Middle Stone Age (Mode 3)—Later Stone Age (Mode 5) transition of Border Cave was largely effected between ∼56.5 and ∼41.6 ka ago.  相似文献   

2.
《Quaternary Science Reviews》2007,26(17-18):2090-2112
The geomorphology and morphostratigraphy of numerous worldwide sites reveal the relative movements of sea level during the peak of the Last Interglaciation (Marine Isotope Stage (MIS) 5e, assumed average duration between 130±2 and 119±2 ka). Because sea level was higher than present, deposits are emergent, exposed, and widespread on many stable coastlines. Correlation with MIS 5e is facilitated by similar morphostratigraphic relationships, a low degree of diagenesis, uranium–thorium (U/Th) ages, and a global set of amino-acid racemization (AAR) data. This study integrates information from a large number of sites from tectonically stable areas including Bermuda, Bahamas, and Western Australia, and some that have experienced minor uplift (∼2.5 m/100 ka), including selected sites from the Mediterranean and Hawaii. Significant fluctuations during the highstand are evident at many MIS 5e sites, revealed from morphological, stratigraphic, and sedimentological evidence. Rounded and flat-topped curves derived only from reef tracts are incomplete and not representative of the entire interglacial story. Despite predictions of much different sea-level histories in Bermuda, the Bahamas, and Western Australia due to glacio- and hydro-isostatic effects, the rocks from these sites reveal a nearly identical record during the Last Interglaciation.The Last Interglacial highstand is characterized by several defined sea-level intervals (SLIs) that include: (SLI#1) post-glacial (MIS 6/5e Termination II) rise to above present before 130 ka; (SLI#2) stability at +2 to +3 m for the initial several thousand years (∼130 to ∼125 ka) during which fringing reefs were established and terrace morphology was imprinted along the coastlines; (SLI#3) a brief fall to near or below present around 125 ka; (SLI#4) a secondary rise to and through ∼+3–4 m (∼124 to ∼122 ka); followed by (SLI#5) a brief period of instability (∼120 ka) characterized by a rapid rise to between +6 to +9 m during which multiple notches and benches were developed; and (SLI#6) an apparently rapid descent of sea level into MIS 5d after 119 ka. U/Th ages are used to confirm the Last Interglacial age of the deposits, but unfortunately, in only two cases was it possible to corroborate the highstand subdivisions using radiometric ages.Sea levels above or at present were relatively stable during much of early MIS 5e and the last 6–7 ka of MIS 1, encouraging a comparison between them. The geological evidence suggests that significant oceanographic and climatic changes occurred thereafter, midway through, and continuing through the end of MIS 5e. Fluctuating sea levels and a catastrophic termination of MIS 5e are linked to the instability of grounded and marine-based ice sheets, with the Greenland (GIS) and West Antarctic (WAIS) ice sheets being the most likely contributors. Late MIS 5e ice volume changes were accompanied by oceanographic reorganization and global ecological shifts, and provide one ominous scenario for a greenhouse world.  相似文献   

3.
Curaçao has reef terraces with the potential to provide sea-level histories of interglacial periods. Ages of the Hato (upper) unit of the “Lower Terrace” indicate that this reef dates to the last interglacial period, Marine Isotope Stage (MIS) 5.5. On Curaçao, this high sea stand lasted at least 8000 yr (~ 126 to ~ 118 ka). Elevations and age of this reef show that late Quaternary uplift rates on Curaçao are low, 0.026–0.054 m/ka, consistent with its tectonic setting. Ages of ~ 200 ka for corals from the older Cortalein unit of the Lower Terrace correlate this reef to MIS 7, with paleo-sea level estimates ranging from ? 3.3 m to + 2.3 m. The estimates are in agreement with those for MIS 7 made from other localities and indicate that the penultimate interglacial period was a time of significant warmth, on a par with the present interglacial period. The ~ 400 ka (MIS 11) Middle Terrace I on Curaçao, dated by others, may have formed from a paleo-sea level of + 8.3 to + 10.0 m, or (less likely) + 17 m to + 20 m. The lower estimates are conservative compared to previous studies, but still require major ice sheet loss from Greenland and Antarctica.  相似文献   

4.
Coral terrace surveys and U-series ages of coral yield a surface uplift rate of ∼0.5 m/ka for Kisar Island, which is an emergent island in the hinterland of the active Banda arc–continent collision. Based on this rate, Kisar first emerged from the ocean as recently as ∼450 ka. These uplifted terraces are gently warped in a pattern of east–west striking folds. These folds are strike parallel to more developed thrust-related folds of similar wavelength imaged by a seismic reflection profile just offshore. This deformation shows that the emergence of Kisar is influenced by forearc closure along the south-dipping Kisar Thrust. However, the pinnacle shape of Kisar and the protrusion of its metamorphic rocks through the forearc basin sediments also suggest a component of extrusion along shear zones or active doming.Coral encrusts the island coast in many locations over 100 m above sea level. Terrace morphology and coral ages are best explained by recognizing major surfaces as mostly growth terraces and minor terraces as mostly erosional into older terraces. All reliable and referable coral U-series ages determined by MC-ICP-MS correlate with marine isotope stage (MIS) 5e (118–128 ka). The only unaltered coral samples are found below 6 m elevation; however an unaltered Tridacna (giant clam) shell in growth position at 95 m elevation yields a U-series age of 195 ± 31 ka, which corresponds to MIS 7. This age agrees with the best-fit uplift model for the island. Loose deposits of unaltered coral fragments found at elevations between 8 and 20 m yield U-series ages of <100 years and may represent paleotsunami deposits from previously undocumented tectonic activity in the region.  相似文献   

5.
《Quaternary Science Reviews》2007,26(3-4):386-404
This study provides an interpretation of interrelated Quaternary fluvial and aeolian activity related to climate change on Cooper Creek in the Lake Eyre Basin in southwestern Queensland, central Australia. The extensive muddy floodplain is characterised by buried sandy palaeochannels now almost entirely invisible but stratigraphically connected to source-bordering dunes that emerge as distinctive sandy islands through the floodplain surface. Luminescence dating has identified pronounced periods of fluvial activity represented by abundant sandy alluvium from Marine Isotope Stages (MIS) 8–3. While all these sandy fluvial episodes on Cooper Creek were much more powerful than anything subsequent, they appear to be ranked in order of declining activity. MIS 8–6 saw reworking of almost the entire floodplain whereas subsequent phases of reworking were far less extensive. Source-bordering dunes were derived from active sandy channels in late MIS 5 (∼85–80 ka) and mid MIS 3 (50–40 ka). After ∼40 ka sand-channel activity largely ceased and the floodplains and channels were inundated with mud, isolating the dunes as emergent features. Although aeolian reworking of the upper parts of some dunes has continued to the present, they show remarkable resilience, having survived without appreciable migration for at least 40 ka. Whilst the channels once determined the location of source-bordering dunes, in an interesting role reversal the remnant dunes now determine the position of many contemporary flood-channels and waterholes by deflection and confinement of overbank flows.  相似文献   

6.
《Quaternary Science Reviews》2007,26(5-6):759-772
Quantitative reconstruction of the climatic history of the Chinese Loess Plateau is important for understanding present and past environment and climate changes in the Northern Hemisphere. Here, we reconstructed mean annual temperature (MAT) and mean annual precipitation (MAP) trends during the last 136 ka based on the analysis of phytoliths from the Weinan loess section (34°24′N, 109°30′E) near the southern part of the Loess Plateau in northern China. The reconstructions have been carried out using a Chinese phytolith–climate calibration model based on weighted averaging partial least-squares regression. A series of cold and dry events, as indicated by the reconstructed MAT and MAP, are documented in the loess during the last glacial periods, which can be temporally correlated with the North Atlantic Heinrich events. Our MAT and MAP estimations show that the coldest and/or driest period occurred at the upper part of L2 unit (Late MIS 6), where MAT dropped to ca 4.4 °C and MAP to ca 100 mm. Two other prominent cold-dry periods occurred at lower Ll-5 (ca 77–62 ka) and L1-1 (ca 23–10.5 ka) where the MAT and MAP decreased to about 6.1–6.5 °C and 150–370 mm, respectively, ca 6.6–6.2 °C and 400–200 mm lower than today. However, the highest MAT (average 14.6 °C, max. 18.1 °C) and MAP (average 757 mm, max. 1000 mm) occurred at Sl interval (MIS 5). During the interstadial of L1-4–L1-2 (MIS 3) and during the Holocene warm-wet period, the MAT was about 1–2 °C and MAP 100–150 mm higher than today in the Weinan region. The well-dated MAT and MAP reconstructions from the Chinese Loess Plateau presented in this paper are the first quantitatively reconstructed proxy record of climatic changes at the glacial–interglacial timescale that is based on phytolith data. This study also reveals a causal link between climatic instability in the Atlantic Ocean and climate variability in the Chinese Loess Plateau.  相似文献   

7.
Sedimentological, geomorphic, and ground penetrating radar (GPR) data are combined with optically stimulated luminescence data to define the Holocene evolution of a coastal system in peninsular Malaysia. The Setiu coastal region of northeast Malaysia comprises five geological and geomorphic units representing distinct evolutionary phases of this coastline. Estimated marine limiting point elevations indicate deposition of an early aggradational shoreline associated with a sea-level elevation of −0.1 to +1.7 m (MSLPMVGD datum) between ∼6.8 ka and 5.7 ka, in agreement with previous sea-level studies from the Malay–Thai peninsula. A hiatus occurs in the record between ∼5.7 ka and 3.0 ka, possibly due to a relative sea-level oscillation and shoreline erosion. Long-term relative sea-level fall and possible still-stands created strandplains that are interrupted by aggradational to transgressive paleo-barrier and estuary formation corresponding with brief episodes of RSL rise. Analyses of GPR facies and OSL ages suggest annual clinoform deposition, with geometries dictated by variations in ENSO. These data demonstrate the utility of high resolution studies of coastal facies as useful proxy indicators for paleoclimate studies at subdecadal to millennial time-scales.  相似文献   

8.
《Quaternary Science Reviews》2007,26(9-10):1212-1222
How fast and how much climate can change has significant implications for concerns about future climate changes and their potential impacts on society. An abrupt climate change 8200 years ago (8.2 ka event) provides a test case to understand possible future climatic variability. Here, methane concentration (taken as an indicator for terrestrial hydrology) and nitrogen isotopes (Greenland temperature) in trapped air in a Greenland ice core (GISP2) are employed to scrutinize the evolution of the 8.2 ka event. The synchronous change in methane and nitrogen implies that the 8.2 ka event was a synchronous event (within ±4 years) at a hemispheric scale, as indicated by recent climate model results [Legrande, A. N., Schmidt, G. A., Shindell, D. T., Field, C. V., Miller, R. L., Koch, D. M., Faluvegi, G., Hoffmann, G., 2006. Consistent simulations of multiple proxy responses to an abrupt climate change event. Proceedings of the National Academy of Sciences 103, 837–842]. The event began with a large-scale general cooling and drying around ∼8175±30 years BP (Before Present, where Present is 1950 AD). Greenland temperature cooled by 3.3±1.1 °C (decadal average) in less than ∼20 years, and atmospheric methane concentration decreased by ∼80±25 ppb over ∼40 years, corresponding to a 15±5% emission reduction. Hemispheric scale cooling and drying, inferred from many paleoclimate proxies, likely contributed to this emission reduction. In central Greenland, the coldest period lasted for ∼60 years, interrupted by a milder interval of a few decades, and temperature subsequently warmed in several steps over ∼70 years. The total duration of the 8.2 ka event was roughly 150 years.  相似文献   

9.
Lake Ngami, a 3000 km2 basin at the distal end of the Okavango Delta, Botswana, has a complex suite of shorelines ranging from the lake sump at 919 m asl up to 945 m asl, linked to topographic thresholds and inflows, current and fossil, in the Okavango system. The lake has been ephemeral throughout the 20th century and completely dry since 1982, yet held a substantial body of water up to a level of 930 m asl when visited by European travellers in the mid-nineteenth century. The historical decline in the lake has been well documented and used as evidence for regional desiccation, but the Holocene record, based on archaeological studies and radiocarbon dating of inorganic carbonates, is more speculative.A study of a 30 km2 diatomite at the eastern end of the basin indicates that a substantial freshwater lake at a level of at least 932 m asl was present throughout the Holocene, linked to a now abandoned inflow of the Kunyere River. A programme of 18 luminescence age determinations of the peripheral Dautsa and Magotlwanen shoreline sequences suggests that they are old features that have been constructed on a lacustrine substrate dating back to 90–100 ka or earlier, and have been modified by Holocene fluctuations in lake level and subsequent aeolian reworking. The water level peaked at 936 m asl between 4 and 3 ka, returned to 934 m at 2.8–2 ka, ponding water into the Phatane Gap, and fell rapidly thereafter to 932 m and thence to historical levels. The 936 m level can be correlated with increased throughflow in the Okavango Delta, hence increased rainfall in the catchment. The subsequent decline can be attributed to episodic closure and rerouting of Okavango distributaries, in particular the Thaoghe, perhaps aided by anthropogenic activities. As such the desiccation of the lake is not indicative of regional climatic change.  相似文献   

10.
Late Pleistocene paleoclimatic variability on the northeastern Qinghai–Tibetan Plateau (NE QTP) was reconstructed using a chronology based on AMS 14C and 230Th dating results and a stable oxygen isotopic record. These are derived from lake carbonates in a 102-m-long Qarhan sediment core (ISL1A) collected from the eastern Qaidam Basin. Previous research indicates that the δ18O values of lacustrine carbonates are mainly controlled by the isotopic composition of lake water, which in turn is a function of regional P/E balance and the proportion of precipitation that is monsoon-derived on the NE QTP. Modern isotopic observations indicate that the δ18O values of lake carbonates in hyper-arid Qaidam Basin are more positive during the warm and wet period. Due to strong evaporation and continental effect in this basin, the positive δ18O values in the arid region indicate drier climatic conditions. Based on this interpretation and the δ18O record of fine-grained lake carbonates and dating results in ISL1A, the results imply that drier climatic conditions in the Qarhan region occurred in three intervals, around 90–80 ka, 52–38 ka and 10–9 ka, which could correspond to late MIS 5, middle MIS 3 and early Holocene, respectively. These three phases were almost coincided with low lake level periods of Gahai, Toson and Qinghai Lakes (to the east of Qarhan Lake) influenced by ASM on the orbital timescales. Meanwhile, there was an episode of relatively high δ18O value during late MIS 3, suggesting that relatively dry climatic condition in this period, rather than “a uniform Qarhan mega-paleolake” spanning the ∼44 to 22 ka period. These results insight into the understanding of “the Greatest Lake Period” on the QTP.  相似文献   

11.
《Quaternary Science Reviews》2005,24(1-2):195-210
Low-field magnetic susceptibility has been widely used to determine the pedostratigraphy of the Chinese loess/paleosol sequences. However, uncertainties remain in correlating between the loess magnetic susceptibility and the marine oxygen isotope records because susceptibility variations are affected by both global and local paleoclimatic changes. To provide a more sound paleoclimatic interpretation of magnetic susceptibility variations, age models across Marine Oxygen Isotope Stage (MIS) 5 for the Jiuzhoutai (JZT) and Yuanbao (YB) sections, western Chinese Loess Plateau, were constructed through an integrated approach by linking the major pedostratigraphic boundaries of the loess profiles to the SPECMAP oxygen isotope curve, and by correlating relative magnetic paleointensity records with both the SINT800 global paleointensity stack from marine sediments and 36Cl records from the GRIP ice core. Results indicate good correlation of SIRM60 mT (a residual remanence of saturation isothermal remanent magnetization after a 60 mT alternating field demagnetization) variations between these two sites, which agree well with fluctuations in subtropical Atlantic sea surface temperatures. All cooling events recorded by ice-core and Atlantic marine sediments within MIS5 have counterparts in SIRM60 mT. SIRM60 mT is partially controlled by the degree of low-temperature oxidation, which is strongly temperature dependent. However, strong pedogenesis can decrease SIRM60 mT due to further oxidation of partially oxidized magnetites above some critical points. Therefore, we propose that SIRM60 mT is best suited to record paleotemperature changes in loess profiles from the western Chinese Loess Plateau, where pedogenesis is the weakest. Furthermore, by inter-profile correlation between the YB and JZT sections, we note that the seemingly uniform sub-paleosol unit with a broad susceptibility peak (previously assigned to MIS5c) between ∼34.4 and ∼37.4 m in the YB profile actually consists of two independent units (lower part of S1L1/MIS5b and S1S2/MIS5c). This indicates that susceptibility values can be strongly affected by local factors (e.g., mainly precipitation). Therefore, beside the simplistic traditional paleoclimatic interpretation of variations in loess susceptibility involving only cold/dry and warm/humid scenarios, cold/humid and warm/dry scenarios should also be considered.  相似文献   

12.
The extensive shoreline deposits of Lake Chilwa, southern Malawi, a shallow water body today covering 600 km2 of a basin of 7500 km2, are investigated for their record of late Quaternary highstands. OSL dating, applied to 36 samples from five sediment cores from the northern and western marginal sand ridges, reveal a highstand record spanning 44 ka. Using two different grouping methods, highstand phases are identified at 43.7–33.3 ka, 26.2–21.0 ka and 17.9–12.0 ka (total error method) or 38.4–35.5 ka, 24.3–22.3 ka, 16.2–15.1 ka and 13.5–12.7 ka (Finite Mixture Model age components) with two further discrete events recorded at 11.01 ± 0.76 ka and 8.52 ± 0.56 ka. Highstands are comparable to the timing of wet phases from other basins in East and southern Africa, demonstrating wet conditions in the region before the LGM, which was dry, and a wet Lateglacial, which commenced earlier in the southern compared to northern hemisphere in East Africa. We find no evidence that wet phases are insolation driven, but analysis of the dataset and GCM modelling experiments suggest that Heinrich events may be associated with enhanced monsoon activity in East Africa in both timing and as a possible causal mechanism.  相似文献   

13.
Ancient cave systems in the Northern Calcareous Alps, today located well above the timberline at altitudes of 2400–2500 m, host U-rich speleothems that preserved growth layers on the microscopic scale of presumably annual origin. Two flowstone samples were dated to 2.019 + 0.037/?0.069 Ma and 1.730 + 0.032/?0.068 Ma, respectively, using U–Pb isochron techniques. These ages are corroborated by the Late Pliocene to Early Pleistocene pollen spectrum extracted from one of the samples. We use a multiproxy approach and exploit laminated speleothem sequences to tie high-resolution stable isotope data to a floating lamina-counted chronology. O isotope values of growth intervals when calcite deposition was close to isotopic equilibrium are low compared to modern and Holocene speleothems from other alpine caves and are inconsistent with the current altitudinal setting of the caves. A vegetated but geomorphologically stable alpine catchment (i.e. ～2000 m asl., no (peri)glacial processes) combined with a deep-seated cave (the thickness of the vadose zone might have exceeded 1000 m) is required in order to reconcile the isotopic data with the pollen record and the petrographic evidence. Furthermore, the data can be used to constrain the rate for Quaternary rock-uplift to ≤0.8 mm/annum for this frontal part of the European Alps. Collectively, the data suggest that these speleothems formed both during interglacials (MIS 59 or 61) and interglacial–glacial transitions (MIS 75/74 or 77/76), but the seasonal precipitation pattern was arguably markedly different from today's. Provided that the highly regular microscopic laminae are indeed annual, lamina counts suggest a minimum length of ca 6 ka for interglacials during the earliest Pleistocene.  相似文献   

14.
A core, recovered from a water depth of 53 m in Loch Assynt, North-West Scotland, has yielded a 9 m sequence comprising two distinct units, an upper, organic-rich unit (Unit I, ca. 6 m) overlying a sequence of laminated clays, silts and sands (Unit II, ca. 3 m). The upper unit is essentially Holocene in age based upon three bulk AMS radiocarbon dates while a fourth radiocarbon date from Unit II confirms a late-glacial age for that interval and supports a broadly linear age–depth relationship. Distinct variations in the magnetic susceptibility record of the lower unit can be visually correlated to major changes in the Greenland ice core (GISP2), this together with pollen evidence supports the radiocarbon dating suggesting an age of approximately 11,000 to around 17,000 cal. BP for Unit II, with evidence for the Younger Dryas (Loch Lomond) stadial and the Bolling–Allerød climatic phases. Variations in the magnetic susceptibility record of the late-glacial sediments are thought to relate to climatically driven changes in soil cover and erosion rates. The multiproxy record from Loch Assynt indicates relatively continuous, sub-aqueous sedimentation during the last ～17,000 years, providing an approximate age for the initiation of modern Loch Assynt and supporting recent dates of moraine retreat lines in the Loanan Valley from about 14–15 ka BP. Pollen and chironomid sampling provides further insights to the history of this relatively deep water body and compliment existing high-resolution palaeo-precipitation records for the mid to late Holocene interval from speleothem archives within the loch catchment.  相似文献   

15.
Irene Zembo 《Sedimentary Geology》2010,223(3-4):206-234
The sedimentary record of the Val d'Agri basin is of great importance for understanding the Quaternary tectonic activity and climatic variability in the Southern Apennines. Changes in tectonic controls, sediment supply and climatic input have been identified. The interval from ～ 56 to ～ 43 ka was associated with asymmetric subsidence restricted to the north-eastern actively faulted margin of the basin and development of axial braided river and transverse alluvial fan systems. Short-lasting Mediterranean-type pedogenesis between ～ 43 and ～ 32 ka (MIS Stage 3) coexisted with progradation–aggradation of the southern alluvial fan deposits and southwards tilting of the basin floor. Aggradation ended with consumption of accommodation space after 32 ka. During a subsequent stage of decline of vegetation cover, possibly as a consequence of climatic cooling (probably MIS Stage 2), active progradation of alluvial fans occurred. Breakthrough of the basin threshold and entrenchment of the drainage network must therefore be attributed to a latest Pleistocene to Holocene age. The first stages of basin opening and fill, predating ～ 56 ka have only been inferred by stratigraphic considerations: the earliest lacustrine sedimentation should be middle Pleistocene or older in age. The following south-eastward basin widening allowed progradation of alluvial fan systems, which completely filled the lacustrine area (tentatively late middle Pleistocene). Pedogenesis in “Mediterranean-like” climate conditions caused the final development of a highly mature fersiallitic paleosol at the top of the fan surfaces, in areas of morpho-tectonic stability, plausibly during MIS Stage 5. The study results demonstrate the potential of applying a multidisciplinary approach in an intermontane continental settings marked by a relative rapid and constant tectonic subsidence and a high rate of sediment supply during the Pleistocene glacial–interglacial cycles.  相似文献   

16.
Dun structures are common in the Sub-Himalayan zone of the Himalaya bounded by the Main Boundary Thrust (MBT) and the Himalayan Frontal Thrust (HFT). They are broad synclinal longitudinal valleys formed as a consequence of the exhumation of the range front of the Himalaya. In the Garhwal Sub-Himalaya, these structures have grown since 0.5 Ma, with the peak activity postdating ∼100 ka. A series of out-of-sequence deformation structures have been identified within the MBT-HFT-bounded Dun structures. They are identified on the basis of geomorphic, post-100 ka stratigraphic, and structural expressions, with activity as young as the early Holocene. To the south of the range front of the Himalaya, uplift has been observed in the Piedmont Zone, with peculiar active tectonic geomorphic expressions. Piedmont sediments of 15–5 ka, determined by Optically Simulated Luminescence (OSL), have been affected by the above uplift. The complete tectonic scenario has been analyzed and an attempt has been made to delineate the sequential evolution of these structures during the post-100 ka period (Late Quaternary–Holocene) in the Himalayan range front.  相似文献   

17.
An abrupt climatic change during the MIS 5a/4 transition is evident in the loess records of China (S1/L1). Proxies including geochemical elements, grain size, soil color, magnetic susceptibility and carbonate (CaCO3) content indicate a warming interval, which lasted approximately 3 ka, during the MIS 5a/4 transition in both the Wangguan and Shagou loess sections, located in Sanmenxia (Henan Province) and Wuwei (Gansu Province), respectively. Both the winter and summer monsoon proxies demonstrate that this warming interval occurred at the same time in both sections (nearly 70.5–73.6 ka BP), with maximum warming from 71.4 to 72.0 ka BP. This study suggests a universal abrupt warming interval in the East Asia monsoon region at this time. Comparisons with marine, terrestrial and ice-core records indicate this event was very likely an abrupt global warming interval during the last glacial–interglacial transition.  相似文献   

18.
《Quaternary Science Reviews》2007,26(17-18):2281-2300
We review Late Cenozoic climate and environment changes in the western interior of China with an emphasis on lacustrine records from Lake Qinghai. Widespread deposition of red clay in the marginal basins of the Tibetan Plateau indicates that the Asian monsoon system was initially established by ∼8 Ma, when the plateau reached a threshold altitude. Subsequent strengthening of the winter monsoon, along with the establishment of the Northern Hemisphere ice sheets, reflects a long-term trend of global cooling. The few cores from the Tibetan Plateau that reach back a million years suggest that they record the mid-Pleistocene transition from glacial cycles dominated by 41 ka cycles to those dominated by 100 ka cycles.During Terminations I and II, strengthening of the summer monsoon in China's interior was delayed compared with sea level and insolation records, and it did not reach the western Tibetan Plateau and the Tarim Basin. Lacustrine carbonate δ18O records reveal no climatic anomaly during MIS3, so that high terraces interpreted as evidence for extremely high lake levels during MIS3 remain an enigma. Following the Last Glacial Maximum (LSM), several lines of evidence from Lake Qinghai and elsewhere point to an initial warming of regional climate about 14 500 cal yr BP, which was followed by a brief cold reversal, possibly corresponding to the Younger Dryas event in the North Atlantic region. Maximum warming occurred about 10 000 cal yr BP, accompanied by increased monsoon precipitation in the eastern Tibetan Plateau. Superimposed on this general pattern are small-amplitude, centennial-scale oscillations during the Holocene. Warmer than present climate conditions terminated about 4000 cal yr BP. Progressive lowering of the water level in Lake Qinghai during the last half century is mainly a result of negative precipitation–evaporation balance within the context of global warming.  相似文献   

19.
《Quaternary Science Reviews》2004,23(16-17):1733-1756
This study shows that successions of Pleistocene carbonate aeolian deposits can be placed successfully in a geochronologic framework using magnetostratigraphic and susceptibility stratigraphic analysis supplemented by luminescence dating, studies of wave-cut platforms, and biostratigraphic evidence. The investigated aeolian system covers a significant part of southernmost Mallorca and is exposed in impressive coastal cliff sections.At the study site at Els Bancals the aeolian system has a maximum thickness of 16 m and is composed of alternating dark red colluvial deposits and greyish red aeolian dune and sand-sheet deposits forming seven cyclostratigraphic units. Each cyclostratigraphic unit represents landscape stabilisation, colluviation, and soil formation followed by dunefield development, when marine carbonate sand was transported far inland by westerly or north-westerly winds. The aeolian system is located on top of a wave-cut marine platform 12–14 m a.s.l. This platform probably formed during a sea-level highstand in Marine Isotope Stage (MIS) 11 (427–364 ka), and renewed marine activity probably later in MIS 11 is indicated by the formation of beach deposits.Two sections at Els Bancals were sampled for a paleomagnetic study; additional samples were taken to detect variations in magnetic susceptibility (MS). The characteristic remanent magnetisation has been recovered for the most part of the succession in spite of diagenetic overprinting. There is evidence for two probably three reversal polarity excursions, possible connected to the Levantine, CR1 and CR0/Biwa III episodes. If this correlation is correct, the sampled succession represents a time interval in the Middle Pleistocene between ca 410 and ca 260 ka. This age estimate is supported by the MS study and by luminescence dates of 333±70 ka (aeolianite from lower part of the succession) and 275±23 ka (aeolianite from the top of the succession).The nature of the succession suggests deposition during alternating warm and moist (colluvial deposition; soil formation) and cold, dry and windy conditions (dunefield formation). The susceptibility signal can be correlated with the insolation signal at 65°N suggesting that environmental variation on Mallorca was linked to orbitally forced climate change, and it seems that aeolian activity and dunefield formation were linked to glacial or stadial periods.  相似文献   

20.
Suture zones often archive complex geologic histories underscored by episodes of varying style of deformation associated with intercontinental collision. In the Lopukangri area of south-central Tibet (29°54′N, 84°24′E) field relationships between tectonic units juxtaposed by the India–Asia suture are well exposed, including Indian passive margin rocks (Tethyan Sedimentary Sequence), forearc deposits (Xigaze Group), magmatic arc rocks (Gangdese batholith and Linzizong Formation) and syncollision deposits (Eocene–Miocene conglomerates). To better understand the structural history of this area, we integrated geologic mapping with biotite 40Ar/39Ar thermochronology and zircon U–Pb geochronology. The first-order structure is a system of north-directed thrusts which are part of the Great Counter thrust (GCT) that places Indian passive margin rocks and forearc deposits on top of magmatic arc rocks and syn-tectonic conglomerates. We infer the south-directed Late Oligocene Gangdese Thrust (GT) exists at unexposed structural levels based on field mapping, cross sections, and regional correlations as it has been documented immediately to the east. A granite in the footwall has a U–Pb zircon age of 38.4 ± 0.4 Ma, interpreted to be the age of emplacement of the granite, and a younger 40Ar/39Ar biotite age of 19.7 ± 0.1 Ma. As the granite sample is situated immediately below a nonconformity with low grade greenschist facies rocks, we interpret the younger age to reflect Miocene resetting of the biotite Ar system. Syn-tectonic deposits in the Lopukangri area consist of three conglomerate units with a total thickness of ∼1.5 km. The lower two units consist of cobble gravel pebble conglomerates rich in volcanic and plutonic clasts, transitioning to conglomerates with only sedimentary clasts in the upper unit. We correlate the syncollision deposits to the Eocene–Oligocene Qiuwu Formation based on field relationships, stratigraphy and petrology. Petrology and clast composition suggest the lower two units of the Qiuwu Formation had a northern provenance (Lhasa block and magmatic arc) and the upper unit had a southern provenance (Tethyan Sedimentary Sequence). Our observations are consistent with paleocurrent data from other studies which suggest a predominant south-directed paleoflow for this formation. We propose a model in which: (1) granites intrude at 38.4 ± 0.4 Ma; (2) are exhumed by erosion; (3) and buried due to regional subsidence and initial deposition of a conglomerate unit; (4) exposed by the GT at ∼27–24 Ma to provide detritus; (5) buried a second time by hanging wall-derived sedimentary deposits and the GCT, then (6) exposed from a depth of ∼12–10 km by a blind thrust at ∼19 Ma. An alternate model describes: (1) intrusion of the granites at 38.4 ± 0.4 Ma, followed by (2) exhumation of the granites via normal faulting to provide detritus; (3) then burial by the GCT at ∼24 Ma, followed by (4) exhumation via regional erosional denudation at ∼19 Ma. Exposure of the GT west of Xigaze has not been confirmed. We suggest that shallower structural levels of the India-Asia suture zone are exposed to the west of the study area, compared to the east, where the GT has been previously documented. The GCT in the area is short-lived, as it is cut and offset by a Middle Miocene ∼N-striking W-dipping oblique normal fault system.  相似文献   

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