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1.
Development of an accurate chronology for glacial deposits in the Sierra Nevada has long been problematic given the lack of suitable organic material for radiocarbon dating. Lichenometry initially appeared promising as ages showed an increase from cirque headwalls to down-canyon moraines. However, while Recess Peak lichen age estimates range from 2 to 3 ka, recent work shows these deposits to be at least 10 ka older. Here, we present evidence for a late Holocene reset of Recess Peak lichen ages by significant post-depositional climate change. Following late-Pleistocene deposition of Recess Peak moraines, warming through the mid-Holocene allowed forests to advance into shallow basins eliminating local inverted tree lines. This produced a partial canopy where shading killed the original post-Pleistocene crustose lichen colonies. Late-Holocene cooling resulted in forest retreat from these basins as alpine tree line fell. Lichens then recolonized the re-exposed Recess Peak deposits. We conclude that while Recess Peak lichen ages are accurate to within the dating uncertainty of the technique, existing lichen ages actually date the timing of post-mid-Holocene cooling and recolonization, and not the original emplacement of these deposits. Thus, applications of Lichenometry should consider post-depositional environmental change when interpreting the meaning of these dates.  相似文献   

2.
《Quaternary Science Reviews》2007,26(17-18):2185-2200
Taiwan, located at the junction of the Pacific Ocean, the Eurasian continent, and the South China marginal Sea, is of particular interest for reconstructing paleoclimate periods in Eastern Asia. This study reports the first cosmic ray exposure dating (CRE) of glacial features in Taiwan. Among the areas where glacial relicts have been described in Taiwan, the Nanhuta Shan range is probably the place where glacial landforms are best preserved. We consequently focused on this area combining glacial geomorphology observations together with CRE dating using in situ produced 10Be of erratic boulders and ice-sculpted surfaces. When combined with the geomorphic characteristics of the sampled areas, the obtained minimum CRE ages suggest that the glacial retreat in the Nanhuta Shan commenced about 10±3 ka ago and retreat was complete by 7±1 ka ago. This is consistent with the Holocene warming trend deduced from other biological and physico-chemical paleoclimatic records for the region. Estimates of local bedrock surface denudation rates either directly from in situ produced 10Be measurements or from geomorphic considerations are employed to determine the preservation of such glacial features within the highly dynamic setting of Taiwan.  相似文献   

3.
The Burhan Budai Shan in NE Tibet represents a key location for examining the variable influence of the mid‐latitude westerly and monsoonal circulations on late Quaternary glaciations in this sector of the Tibetan Plateau. Our study investigates the glacial history of mountains near Lake Donggi Cona (35°17′N, 98°33′E) using field mapping in combination with 10Be surface exposure dating and numerical reconstructions of former glacial equilibrium line altitudes (palaeo‐ELA). A set of 23 new exposure ages, collected from moraines in four glacial valleys, ranges from 45 to 190 ka, indicating ice expansion during the early and middle part of the last glacial cycle, and during the penultimate and possibly an earlier Mid‐Pleistocene glaciation. Ice advances reaching 12–15 km in length occurred at around 190–180 ka (≥MIS 6), between 140–100 ka (late MIS 6/MIS 5), and 90–65 ka (late MIS 5/early MIS 4), with a maximum ELA depression of 400–500 m below the estimated modern snowline. Exposure ages from the valley headwaters further indicate a small glaciation between c. 60–50 ka (late MIS 4/early MIS 3), which was essentially restricted to the cirque areas. Significantly, we find no evidence for any subsequent glaciation in the area during MIS 2 or the Holocene period. These results indicate a diminishing trend of glaciation in the region since at least MIS 4, and corroborate the case of a ‘missing LGM’ in the more interior parts of the northeastern Tibetan Plateau. The emerging pattern suggests that the most favourable conditions for glaciation during the Late Pleistocene correspond to periods of relatively moderate cooling combined with an intermediate or rising East Asian monsoon strength.  相似文献   

4.
Our knowledge about the glaciation history in the Russian Arctic has to a large extent been based on geomorphological mapping supplemented by studies of short stratigraphical sequences found in exposed sections. Here we present new geochronological data from the Polar Ural Mountains along with a high‐resolution sediment record from Bolshoye Shchuchye, the largest and deepest lake in the mountain range. Seismic profiles show that the lake contains a 160‐m‐thick sequence of unconsolidated lacustrine sediments. A well‐dated 24‐m‐long core from the southern end of the lake spans the last 24 cal. ka. From downward extrapolation of sedimentation rates we estimate that sedimentation started about 50–60 ka ago, most likely just after a large glacier had eroded older sediments from the basin. Terrestrial cosmogenic nuclide (TCN) exposure dating (10Be) of boulders and Optically Stimulated Luminescence (OSL) dating of sediments indicate that this part of the Ural Mountains was last covered by a coherent ice‐field complex during Marine Isotope Stage (MIS) 4. A regrowth of the glaciers took place during a late stage of MIS 3, but the central valleys remained ice free until the present. The presence of small‐ and medium‐sized glaciers during MIS 2 is reflected by a sequence of glacial varves and a high sedimentation rate in the lake basin and likewise from 10Be dating of glacial boulders. The maximum extent of the mountain glaciers during MIS 2 was attained prior to 24 cal. ka BP. Some small present‐day glaciers, which are now disappearing completely due to climate warming, were only slightly larger during the Last Glacial Maximum (LGM) as compared to AD 1953. A marked decrease in sedimentation rate around 18–17 cal. ka BP indicates that the glaciers then became smaller and probably disappeared altogether around 15–14 cal. ka BP.  相似文献   

5.
Improved chronological control on the penultimate advance of the Cordilleran Ice Sheet in northwest Canada (the Reid glaciation) is required for a better understanding of late Quaternary palaeoclimatic and palaeoenvironmental change in eastern Beringia. However, reliable dating of glaciation events beyond the last glacial maximum is commonly hindered by a lack of directly dateable material. In this study we (i) provide the first combined minimum and maximum age constraint on the Reid glaciation at Ash Bend, its reference locale in the Stewart River valley, northwestern Canadian Cordillera, using single-grain optically stimulated luminescence dating of quartz; and (ii) compare the timing of the Reid glaciation with other penultimate ice sheet advances in the region with the aim of establishing improved glacial reconstructions in eastern Beringia. We obtain ages of 158 ± 18 ka and 132 ± 18 ka for glaciofluvial sands overlying and underlying the Reid till, respectively. These ages indicate that the Reid advance, at its reference locale, occurred during MIS 6. This precludes an earlier MIS 8 age, and suggests that the Reid advance may have been synchronous with the Delta glaciation of central Alaska, and is likely correlative with the Mirror Creek glaciation in southern Yukon.  相似文献   

6.
Recent estimates of the timing of the last glaciation in the southern and western Uinta Mountains of northeastern Utah suggest that the start of ice retreat and the climate-driven regression of pluvial Lake Bonneville both occurred at approximately 16 cal. ka. To further explore the possible climatic relationship of Uinta Mountain glaciers and the lake, and to add to the glacial chronology of the Rocky Mountains, we assembled a range-wide chronology of latest Pleistocene terminal moraines based on seventy-four cosmogenic 10Be surface-exposure ages from seven glacial valleys. New cosmogenic-exposure ages from moraines in three northern and eastern valleys of the Uinta Mountains indicate that glaciers in these parts of the range began retreating at 22–20 ka, whereas previously reported cosmogenic-exposure ages from four southern and western valleys indicate that ice retreat began there between 18 and 16.5 ka. This spatial asynchrony in the start of the last deglaciation was accompanied by a 400-m east-to-west decline in glacier equilibrium-line altitudes across the Uinta Mountains. When considered together, these two lines of evidence support the hypothesis that Lake Bonneville influenced the mass balance of glaciers in southern and western valleys of the range, but had a lesser impact on glaciers located farther east. Regional-scale variability in the timing of latest Pleistocene deglaciation in the Rocky Mountains may also reflect changing precipitation patterns, thereby highlighting the importance of precipitation controls on the mass balance of Pleistocene mountain glaciers.  相似文献   

7.
This study reports the nature and timing of Quaternary fluvial activity in the Fitzroy River basin, which drains a diverse 143,000 km2 area in northeastern Queensland, before discharging into the Great Barrier Reef Marine Park. The catchment consists of an extensive array of channel and floodplain types that we show have undergone large-scale fluvial adjustment in-channel planform, geometry and sinuosity. Optically stimulated luminescence (OSL) dating of quartz sediments from fifteen (3–18 m) floodplain cores throughout the basin indicates several discrete phases of active bedload activity: at ～105–85 ka in Marine Isotope Stage (MIS) 5, at ～50–40 ka (MIS 3), and at ～30–10 ka (MIS 3/2). The overall timing of late Quaternary fluvial activity correlates well with previous accounts from across Australia with rivers being primarily active during interstadials. Fluvial activity, however, does not appear to have been synchronous throughout the basin’s major sub-catchments. Fluvial activity throughout MIS 2 (i.e. across the Last Glacial Maximum) in the meandering channels of the Fitzroy correlates well with regional data in tropical northeastern Queensland, and casts new light on the river response to reduced rainfall and vegetation cover suggested by regional palaeoclimate indicators. Moreover, the absence of a strong Holocene signal is at odds with previous accounts from elsewhere throughout Australia. The latitudinal position of the Fitzroy across the Tropic of Capricorn places this catchment at a key location for elucidating the main hydrological drivers of Quaternary fluvial activity in northeastern Australia, and especially for determining tropical moisture sources feeding into the headwaters of Cooper Creek, a major river system of the continental interior.  相似文献   

8.
Mongolian glaciers have been the subject of relatively little research, resulting in less geochronological constraint than other parts of Central Asia. The Khentey Mountains (latitude 47–51°N, longitude 105–112°E) are a typical landlocked mountain range exhibiting clear geomorphic evidence of late Quaternary glaciation. Yet, compared to western parts of Mongolia such as the Mongolian Altay, Gobi Altay, Khangay, and Khovsgol, glacial history of the Khentey Mountains is poorly understood. To address this, and permit comparison of the Khentey glacier–climate record with other alpine regions in Mongolia, we performed geomorphological mapping and cosmogenic 10Be surface‐exposure dating in two glaciated regions of the Khentey Mountains: Yestii and Khagiin Khar. Specifically, we measured 10Be in 34 samples collected from five moraine sequences, which, together with morphostratigraphy, correspond to four main glacial stages: (i) The My1 terminal moraine sequence for Yestii (21.0±4.9 ka) and the Mk1 moraine for Khagiin Khar (19.6±2.6 ka), both of which represent the Last Glacial Maximum; (ii) the Lateglacial Mk2 moraine, dated to 16.0±3.5 ka; (iii) the Mk3 moraine, dated to either 17.6±7.0 ka (Lateglacial) or 12.1±1.1 ka (Younger Dryas); and (iv) the currently undated Mk4 moraine (~2200 m a.s.l.), to which we assign a Holocene age. Our results suggest that the timing of maximum glacier extent in Mongolia was regionally variable. In the Khentey Mountains, maximum glaciation occurred during Marine Isotope Stage (MIS) 2, whereas the maximum occurred during MIS 3 in Khangay and Khovsgol and during MIS 4 in the Altay. The MIS 2 glacial maximum in the Khentey Mountains coincided with the global sea level minimum during the Last Glacial Maximum, and was followed by at least three glacial re‐advances during the Lateglacial to possibly the Holocene.  相似文献   

9.
Continuous high-resolution mass accumulation rates (MAR) and X-ray fluorescence (XRF) measurements from marine sediment records in the Bay of Biscay (NE Atlantic) have allowed the determination of the timing and the amplitude of the ‘Fleuve Manche’ (Channel River) discharges during glacial stages MIS 10, MIS 8, MIS 6 and MIS 4–2. These results have yielded detailed insight into the Middle and Late Pleistocene glaciations in Europe and the drainage network of the western and central European rivers over the last 350 kyr. This study provides clear evidence that the ‘Fleuve Manche’ connected the southern North Sea basin with the Bay of Biscay during each glacial period and reveals that ‘Fleuve Manche’ activity during the glaciations MIS 10 and MIS 8 was significantly less than during MIS 6 and MIS 2. We correlate the significant ‘Fleuve Manche’ activity, detected during MIS 6 and MIS 2, with the extensive Saalian (Drenthe Substage) and the Weichselian glaciations, respectively, confirming that the major Elsterian glaciation precedes the glacial MIS 10. In detail, massive ‘Fleuve Manche’ discharges occurred at ca 155 ka (mid-MIS 6) and during Termination I, while no significant discharges are found during Termination II. It is assumed that a substantial retreat of the European ice sheet at ca 155 kyr, followed by the formation of ice-free conditions between the British Isles and Scandinavia until Termination II, allowed meltwater to flow northwards through the North Sea basin during the second part of the MIS 6. We assume that this glacial pattern corresponds to the Warthe Substage glacial maximum, therefore indicating that the data presented here equates to the Drenthe and the Warthe glacial advances at ca 175–160 ka and ca 150–140 ka, respectively. Finally, the correlation of our records with ODP site 980 reveals that massive ‘Fleuve Manche’ discharges, related to partial or complete melting of the European ice masses, were synchronous with strong decreases in both the rate of deep-water formation and the strength of the Atlantic thermohaline circulation. ‘Fleuve Manche’ discharges over the last 350 kyr probably participated, with other meltwater sources, in the collapse of the thermohaline circulation by freshening the northern Atlantic surface water.  相似文献   

10.
Surface exposure dating has become a helpful tool for establishing numeric glacial chronologies, particularly in arid high-mountain regions where radiocarbon dating is challenging due to limited availability of organic material. This study presents 13 new 10Be surface exposure ages from the Kitschi-Kurumdu Valley in the At Bashi Range, Tien Shan. Three moraines were dated to ~ 15, 21 and > 56 ka, respectively, and corroborate previous findings that glacial extents in the Tien Shan during Marine Oxygen Isotope Stage (MIS) 2 were limited compared to MIS 4. This likely documents increasingly arid conditions in Central Asia during the last glacial cycle. Morphological evidence in the Kitschi-Kurumdu Valley and a detailed review of existing numeric glacial chronologies from the Tien Shan indicate that remnants of the penultimate glaciation (MIS 6) are preserved, whereas evidence for MIS 5 glacier advances remains equivocal. Reviewed and recalculated exposure ages from the Pamir mountains, on the other hand, reveal extensive MIS 5 glacial extents that may indicate increased monsoonal precipitation. The preservation of MIS 3 moraines in the Tien Shan and the southern Pamir does not require any monsoonal influence and can be explained alternatively with increased precipitation via the westerlies.  相似文献   

11.
Abstract

Quaternary alluvial and colluvial sediments infill major river valleys and form alluvial fans and colluvium-filled bedrock depressions on the range fronts and within the Mount Lofty Ranges of southern Australia. A complex association of alluvial successions occurs in the Sellicks Creek drainage basin, as revealed from lithostratigraphy, physical landscape setting and optically stimulated luminescence (OSL) ages. Correlation of OSL ages with the Marine Oxygen Isotope record reveals that the alluvial successions represent multiple episodes of alluvial sedimentation since the penultimate glaciation (Marine Isotope Stage 6; MIS 6). The successions include a penultimate glacial maximum alluvium (Taringa Formation; 160?±?15?ka; MIS 6), an unnamed alluvial succession (42?±?3.2?ka; MIS 3), a late last glacial colluvial succession within bedrock depressions (ca 15?ka; MIS 2) and a late last glacial alluvium (ca 15?ka; MIS 2) in the lowest, distal portion of Sellicks Creek. In addition, the Waldeila Formation, a Holocene alluvium (3.5?±?0.3?ka; MIS 1), and sediments deposited during a phase of Post-European Settlement Aggradation (PESA) are also identified. The age and spatial distribution of the red/brown successions, mapped as the Upper Pleistocene Pooraka Formation, directly relate to different topographic and tectonic settings. Neotectonic uplift locally enhanced erosion and sedimentation, while differences in drainage basin sizes along the margin of the ranges have influenced the timing and delivery of sediment in downstream locations. Close to the Willunga Fault Scarp at Sellicks Creek, sediments resembling the Pooraka Formation have yielded a pooled mean OSL age of 83.9?±?7?ka (MIS 5a) corroborating the previously identified extended time range for deposition of the formation. Elsewhere, within major river valleys, the Pooraka Formation was deposited during the last interglacial maximum (128–118?ka; MIS 5e). In general, alluviation occurred during interglacial and interstadial pluvial events, while erosion predominated during drier glacial episodes. In both cases, contemporaneous erosion and sedimentation continued to affect the landscape. For example, in the Sellicks Creek drainage basin, which lies across an actively uplifting fault zone, late glacial age sediments (MIS 2) occur within the ranges and near the distal margin of the alluvial fan complex. OSL dating of the alluvial successions reported in this paper highlights linkages between the terrestrial and marine environments in association with sea-level (base-level) and climatic perturbations. While the alluvial successions relate largely to climatically driven changes, especially in major river valleys, tectonics, eustasy, geomorphic setting and topography have influenced erosion and sedimentation, especially on steep-sloped alluvial fan environments.
  1. KEY POINTS

  2. Luminescence dating of the Sellicks Creek alluvial fan complex reveals that sedimentation occurred predominantly during the later stages of glacial cycles accompanying lower sea-levels than present.

  3. Luminescence dating confirms that the stratigraphically lower portions of the Pooraka Formation are beyond the range of radiocarbon dating.

  4. Upper Pleistocene alluvial fan sedimentation at Sellicks Creek correlates with pluvial events in southeastern Australia.

  相似文献   

12.
We present a new U-series dated speleothem record (PC-1) from the Great Basin that documents deglacial climate variability between ca 20.1 and 15.6 ka. Our data show an abrupt 18.6 ka cold event preceding Heinrich event 1 that is consistent with expansion of the Laurentide Ice sheet during the ‘binge’ phase of ice growth. This event coincided with dessication of pluvial Lake Mojave suggesting cold and dry conditions in the southern Great Basin at this time. PC-1 δ18O values before and during Heinrich event 1 are similar, but an increase in stalagmite growth rates suggests wetter conditions that coincided with deposition of spring deposits in southern Nevada. The time interval of our record is consistent with the timing of pluvial conditions in the Great Basin as evident from a comparison to regional wetness proxies. Our new speleothem record, recovered from the recharge area for Devils Hole, does not show a δ18O increase coincident with the abrupt increase in Devils Hole δ18O at c. 18 ka, challenging the view that the Great Basin experienced an early Termination I. This hypothesis is supported by two other southwest speleothem records that demonstrate deglaciation was synchronous with forcing from the North Atlantic Ocean. We suggest that Devils Hole speleothem δ18O values may partly reflect source water changes in the regional aquifer. Further, Devils Hole δ13C minima coincide with peak global glacial conditions and weak Asian monsoon periods, suggesting that they constrain better the timing of pluvial conditions in the Great Basin.  相似文献   

13.
Loess accumulated on a Bull Lake outwash terrace of Marine Oxygen Isotope Stage 6 (MIS 6) age in southern Jackson Hole, Wyoming. The 9 m section displays eight intervals of loess deposition (Loess 1 to Loess 8, oldest), each followed by soil development. Our age-depth model is constrained by thermoluminescence, meteoric 10Be accumulation in soils, and cosmogenic 10Be surface exposure ages. We use particle size, geochemical, mineral-magnetic, and clay mineralogical data to interpret loess sources and pedogenesis. Deposition of MIS 6 loess was followed by a tripartite soil/thin loess complex (Soils 8, 7, and 6) apparently reflecting the large climatic oscillations of MIS 5. Soil 8 (MIS 5e) shows the strongest development. Loess 5 accumulated during a glacial interval (~ 76-69 ka; MIS 4) followed by soil development under conditions wetter and probably colder than present. Deposition of thick Loess 3 (~ 43-51 ka, MIS 3) was followed by soil development comparable with that observed in Soil 1. Loess 1 (MIS 2) accumulated during the Pinedale glaciation and was followed by development of Soil 1 under a semiarid climate. This record of alternating loess deposition and soil development is compatible with the history of Yellowstone vegetation and the glacial flour record from the Sierra Nevada.  相似文献   

14.
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   

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

16.
A high-resolution record of Holocene deglacial and climate history was obtained from a 77 m sediment core from the Firth of Tay, Antarctic Peninsula, as part of the SHALDRIL initiative. This study provides a detailed sedimentological record of Holocene paleoclimate and glacial advance and retreat from the eastern side of the peninsula. A robust chronostratigraphy was derived from thirty-three radiocarbon dates on carbonate material. This chronostratigraphic framework was used to establish the timing of glacial and climate events derived from multiple proxies including: magnetic susceptibility, electric resistivity, porosity, ice-rafted debris content, organic carbon content, nitrogen content, biogenic silica content, and diatom and foraminiferal assemblages. The core bottomed-out in a stiff diamicton interpreted as till. Gravelly and sandy mud above the till is interpreted as proximal glaciomarine sediment that represents decoupling of the glacier from the seafloor circa 9400 cal. yr BP and its subsequent landward retreat. This was approximately 5000 yr later than in the Bransfield Basin and South Shetland Islands, on the western side of the peninsula. The Firth of Tay core site remained in a proximal glaciomarine setting until 8300 cal. yr BP, at which time significant glacial retreat took place. Deposition of diatomaceous glaciomarine sediments after 8300 cal. yr BP indicates that an ice shelf has not existed in the area since this time.The onset of seasonally open marine conditions between 7800 and 6000 cal. yr BP followed the deglacial period and is interpreted as the mid-Holocene Climatic Optimum. Open marine conditions lasted until present, with a minor cooling having occurred between 6000 and 4500 cal. yr BP and a period of minor glacial retreat and/or decreased sea ice coverage between 4500 and 3500 cal. yr BP. Finally, climatic cooling and variable sea ice cover occurred from 3500 cal. yr BP to near present and it is interpreted as being part of the Neoglacial. The onset of the Neoglacial appears to have occurred earlier in the Firth of Tay than on the western side of the Antarctic Peninsula. The Medieval Warm Period and Little Ice Age were not pronounced in the Firth of Tay. The breadth and synchroneity of the rapid regional warming and glacial retreat observed in the Antarctic Peninsula during the last century appear to be unprecedented during the Holocene epoch.  相似文献   

17.
High-resolution pollen analysis of Alborán Sea core MD95-2043 provides a 48-ka continuous vegetation record that can be directly correlated with sea surface and deep-water changes. The reliability of this record is supported by comparison with that of Padul (Sierra Nevada, Spain). Marine Isotope Stage (MIS) 3 was characterised by fluctuations in Quercus forest cover in response to Dansgaard-Oeschger climate variability. MIS 2 was characterised by the dominance of semi-desert vegetation. Despite overall dry and cold conditions during MIS 2, Heinrich events (HEs) 2 and 1 were distinguished from the last glacial maximum by more intensely arid conditions. Taxon-specific vegetation responses to a tripartite climatic structure within the HEs are observed. In MIS 1, the Bölling-Allerød was marked by rapid afforestation, while a re-expansion of semi-desert environments occurred during the Younger Dryas. The maximum development of mixed Quercus forest occurred between 11.7 and 5.4 cal ka BP, with forest decline since 5.4 cal ka BP. On orbital timescales, a long-term expansion of semi-desert vegetation from MIS 3 into MIS 2 reflects global ice-volume trends, while Holocene arboreal decline reflects summer insolation decrease. The influence of precession on the amplitude of forest development and vegetation composition is also detected.  相似文献   

18.
Pleistocene fluvial landforms and riparian ecosystems in central California responded to climate changes in the Sierra Nevada, yet the glacial history of the western Sierra remains largely unknown. Three glacial stages in the northwestern Sierra Nevada are documented by field mapping and cosmogenic radionuclide surface-exposure (CRSE) ages. Two CRSE ages of erratic boulders on an isolated till above Bear Valley provide a limiting minimum age of 76,400±3800 10Be yr. Another boulder age provides a limiting minimum age of 48,800±3200 10Be yr for a broad-crested moraine ridge within Bear Valley. Three CRSE ages producing an average age of 18,600±1180 yr were drawn from two boulders near a sharp-crested bouldery lateral moraine that represents an extensive Tioga glaciation in Bear Valley. Nine CRSE ages from striated bedrock along a steep valley transect average 14,100±1500 yr and suggest rapid late-glacial ice retreat from lower Fordyce Canyon with no subsequent extensive glaciations. These ages are generally consistent with glacial and pluvial records in east-central California and Nevada.  相似文献   

19.
A sensitivity-corrected Multiple Aliquot Regenerative-dose protocol has been developed for fine-grained quartz OSL dating of Chinese loess. Its reliability has been assessed on the basis of the methodology and by dating reference samples of known age close to the transition from the last interglacial paleosol (S1) to the last glacial loess (L1), which corresponds to the Marine Oxygen Isotope Stage (MIS) 5/4 transition. On the basis of the fine-grained quartz OSL-age estimates for 33 loess samples from the upper part of the Luochuan profile, a detailed chronostratigraphy of continuous dust accumulation in the past 130 ka has been proposed. Changes in the accumulation rate occurred during the last glacial period (MIS 4 to MIS 2); unexpectedly, high accumulation rates were found in the weakly developed L1-2(S) paleosol of the last interstadial (MIS 3), rather than in the classic L1-1 and L1-3 loess of the cold-dry glacial condition (MIS 2 and 4). The OSL ages show some disagreement with the previous numerical chronology for the loess-paleosol sequence based on correlation of variations in grain size with sedimentation rate; the latter method resulted in an almost constant accumulation rate from 72 to 12 ka.  相似文献   

20.
Modern global warming is likely to cause future melting of Earth's polar ice sheets that may result in dramatic sea-level rise. A possible collapse of the West Antarctic Ice Sheet (WAIS) alone, which is considered highly vulnerable as it is mainly based below sea level, may raise global sea level by up to 5–6 m. Despite the importance of the WAIS for changes in global sea level, its response to the glacial–interglacial cycles of the Quaternary is poorly constrained. Moreover, the geological evidence for the disintegration of the WAIS at some time within the last ca. 750 kyr, possibly during Marine Isotope Stage (MIS) 11 (424–374 ka), is ambiguous. Here we present physical properties, palaeomagnetic, geochemical and clay mineralogical data from a glaciomarine sedimentary sequence that was recovered from the West Antarctic continental margin in the Amundsen Sea and spans more than the last 1 Myr. Within the sedimentary sequence, proxies for biological productivity (such as biogenic opal and the barium/aluminum ratio) and the supply of lithogenic detritus from the West Antarctic hinterland (such as ice-rafted debris and clay minerals) exhibit cyclic fluctuations in accordance with the glacial–interglacial cycles of the Quaternary. A prominent depositional anomaly spans MIS 15–MIS 13 (621–478 ka). The proxies for biological productivity and lithogenic sediment supply indicate that this interval has the characteristics of a single, prolonged interglacial period. Even though no proxy suggests environmental conditions much different from today, we conclude that, if the WAIS collapsed during the last 800 kyr, then MIS 15–MIS 13 was the most likely time period. Apparently, the duration rather than the strength of interglacial conditions was the crucial factor for the WAIS drawdown. A comparison with various marine and terrestrial climate archives from around the world corroborates that unusual environmental conditions prevailed throughout MIS 15–MIS 13. Some of these anomalies are observed in the pelagic Southern Ocean and the South Atlantic and might originate in major ice-sheet drawdown in Antarctica, but further research is required to test this hypothesis.  相似文献   

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