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1.
Paleoclimatic records from the climatically sensitive Canadian prairies are relatively rare due to the scarcity of study sites with continuous Holocene stratigraphic sequences. Oro Lake, a meromictic lake in the dry grasslands of Saskatchewan (Canada), contains a continuous Holocene diatom record spanning the last 10,000 years. Here we present analyses at three different time scales and resolution: (1) 1–3 yr resolution of the past 80 years, (2) century-scale analysis of the Holocene, and (3) decadal-scale analysis of the past 7000 years. Recent changes in the diatom assemblages and their respective salinity inferences were significantly related to measured effective moisture (precipitation minus evaporation, P−ET). The droughts of the 1930s, and a wet period during the 1950s are clearly evident in the diatom record, suggesting the Oro Lake record contains a sensitive archive of past climatic conditions. Century-scale analysis of the diatom record during the Holocene is consistent with a cool and moist climate in the early Holocene (prior to ca 9700 cal yr BP, 8600 14C yr BP). An abrupt increase in diatom-inferred salinity at 9600 cal yr BP (8500 14C yr BP) indicates the onset of an arid climate, with continuing arid conditions throughout the mid-Holocene. Decadal-scale analysis of the past 7000 years suggests that the mid-Holocene was more complex, with extended periods of increased variability in precipitation, particularly between ca 5800–3600 cal yr BP (5000–3200 14C yr BP) which is characterized by intervals of increased effective moisture. The past 2000 years is characterized by reduced salinities and generally wetter conditions in comparison to the mid-Holocene. The combination of the different scales of analyses in this study provides a detailed account of the dynamic nature of climate from sub-decadal to millennial scale in the Oro Lake region within the Palliser Triangle. Climate model predictions suggest that the Canadian prairie region may see a higher frequency of extreme droughts under projected global warming, potentially similar to the most arid periods seen during the mid-Holocene when many lake basins completely dried out.  相似文献   

2.
Melting glaciers and ice caps on Baffin Island contribute roughly half of the sea-level rise from all ice in Arctic Canada, although they comprise only one-fourth of the total ice in the region. The uncertain future response of arctic glaciers and ice caps to climate change motivates the use of paleodata to evaluate the sensitivity of glaciers to past warm intervals and to constrain mechanisms that drive glacier change. We review the key patterns and chronologies of latest Pleistocene and Holocene glaciation on Baffin Island. The deglaciation by the Laurentide Ice Sheet occurred generally slowly and steadily throughout the Holocene to its present margin (Barnes Ice Cap) except for two periods of rapid retreat: An early interval 12 to 10 ka when outlet glaciers retreated rapidly through deep fiords and sounds, and a later interval 7 ka when ice over Foxe Basin collapsed. In coastal settings, alpine glaciers were smaller during the Younger Dryas period than during the Little Ice Age. At least some alpine glaciers apparently survived the early Holocene thermal maximum, which was several degrees warmer than today, although data on glacier extent during the early Holocene is extremely sparse. Following the early Holocene thermal maximum, glaciers advanced during Neoglaciation, beginning in some places as early as 6 ka, although most sites do not record near-Little Ice Age positions until 3.5 to 2.5 ka. Alpine glaciers reached their largest Holocene extents during the Little Ice Age, when temperatures were 1–1.5 °C cooler than during the late 20th century. Synchronous advances across Baffin Island throughout Neoglaciation indicate sub-Milankovitch controls on glaciation that could involve major volcanic eruptions and solar variability. Future work should further elucidate the state of glaciers and ice caps during the early Holocene thermal maximum and glacier response to climate forcing mechanisms.  相似文献   

3.
This paper documents a continuous  44,000-yr pollen record derived from the Mfabeni Peatland on the Maputaland Coastal Plain. A detailed fossil pollen analysis indicates the existence of extensive Podocarpus-abundant coastal forests before  33,000 cal yr BP. The onset of wetter local conditions after this time is inferred from forest retreat and the development of swampy conditions. Conditions during the last glacial maximum ( 21,000 cal yr BP) are inferred to have been colder and drier than the present, as evidenced by forest retreat and replacement of swampy reed/sedge communities by dry grassland. Forest growth and expansion during the Holocene Altithermal ( 8000–6000 cal yr BP) indicates warm, relatively moist conditions. Previous records from Maputaland have suggested a northward migration of Podocarpus forest during the late Holocene. However, we interpret a mid-Holocene decline in Podocarpus at Mfabeni as evidence of deforestation. Forest clearance during the mid-Holocene is supported by the appearance of Morella serrata, suggesting a shift towards more open grassland/savanna, possibly due to burning. These signals of human impact are coupled with an increase in Acacia, indicative of the development of secondary forest and hence disturbance.  相似文献   

4.
Sedimentology, carbon isotope and sequence stratigraphic analysis of subsurface sediments from western part of Ganges–Brahmaputra (GB) delta plain shows that a Late Quaternary marine clay and fluvial channel-overbank sediments of MIS 5 and 3 highstands are traceable below the Holocene strata. During the Last Glacial Maximum (LGM) sea-level lowering of >100 m produced a regional unconformity (type 1), represented by palaeosols and incised valley. C4 vegetation expanded on exposed lowstand surface in an ambient dry glacial climate. At 9 ka transgression inundated the lowstand surface pushing the coastline and mangrove front 100 km inland. Simultaneous intensification of monsoon and very high sediment discharge (4–8 times than modern) caused a rapid aggradation of both floodplain and estuarine valley fill deposits between 8 and 7 ka. The Hoogli River remaining along its present drainage possibly acted as the main conduit for transgression and sediment discharge that was subsequently abandoned. C3 vegetation dominated the delta plain during this time. From 7 ka onward progradation of delta plain started and continued till recent. This period experienced a mixed C3–C4 vegetation with localized mangroves in the mid-Holocene to dominant return of C4 vegetation in the late Holocene period. The study indicates that while the initiation of western part of GB delta occurred at least 1 ka earlier than the global mean delta formation age, the progradation started at 7 ka, at least 2 ka earlier than thought before. The terrestrial vegetation change was modulated by changes in depositional environment, specific ecological niches and climate rather than pCO2.  相似文献   

5.
A high-resolution pollen and Pediastrum record, spanning 12,500 yr, is presented for Lake Bayanchagan (115.21°E, 41.65°N, and 1355 m a.s.l.), southern Inner Mongolia. Individual pollen taxa (PT-MAT) and the PFT affinity scores (PFT-MAT) were used for quantitative climatic reconstruction from pollen and algal data. Both techniques indicate that a cold and dry climate, similar to that of today, prevailed before 10,500 cal yr B.P. The wettest climate occurred between 10,500 and 6500 cal yr B.P., at which time annual precipitation was up to 30–60% higher than today. The early Holocene increases in temperature and precipitation occurred simultaneously, but mid-Holocene cooling started at approximately 8000 cal yr B.P., 1500 yr earlier than the drying. Vegetation reconstruction was based on the objective assignment of pollen taxa to the plant functional type. The results suggest that this region was dominated by steppe vegetation throughout the Holocene, except for the period 9200 to 6700 cal yr B.P., when forest patches were relatively common. Inner Mongolia is situated at the limit of the present East Asian monsoon and patterns of vegetation and climate changes in that region during the Holocene probably reflect fluctuations in the monsoon's response to solar insolation variations. The early to middle Holocene monsoon undoubtedly extended to more northern latitudes than at present.  相似文献   

6.
The Indo-Pacific Warm Pool (IPWP) is thought to play a key role in the propagation and amplification of climate changes through its influence on the global distribution of heat and water vapour. However, little is known about past changes in the size and position of the IPWP. In this study, we use a total of 48 modern and fossil coral records from the Mentawai Islands (Sumatra, Indonesia) and Muschu/Koil Islands (Papua New Guinea) to reconstruct oscillations in the extent of the IPWP since the mid-Holocene. We show that reliable estimates of mean sea surface temperature (SST) can be obtained from fossil corals by using low-resolution Sr/Ca analysis of a suite of corals to overcome the large uncertainties associated with mean Sr/Ca-SST estimates from individual coral colonies. The coral records indicate that the southeastern and southwestern margins of the IPWP were cooler than at present between 5500 and 4300 years BP (1.2 °C ± 0.3 °C) and were similarly cool before 6800 years BP. This mid-Holocene cooling was punctuated by an abrupt, short-lived shift to mean SSTs that were warmer than at present between 6600 and 6300 years BP (1.3 °C ± 0.3 °C), while similarly warm conditions may have also existed after 4300 years BP. We suggest that mid-Holocene cooling at our study sites was related to contractions of the southeastern and southwestern margins of the IPWP, associated with the more northerly position of the Inter-tropical Convergence Zone (ITCZ) that accompanied mid-Holocene strengthening of the Asian summer monsoon. Conversely, intervals of abrupt warming appear to correspond with widespread episodes of monsoon weakening and accompanying southward migrations of the ITCZ that caused the IPWP to expand beyond our coral sites. Intervals of a strengthened Asian monsoon and cooling in the southwestern IPWP during the mid-Holocene appear to correspond with a more positive Indian Ocean Dipole (IOD)-like mean configuration across the tropical Indian Ocean, suggesting that the Asian monsoon–IOD interaction that exists at interannual time scales also persists over centennial to millennial scales. Associated mean changes in the Pacific ENSO modes may have also occurred during the mid-Holocene. The dynamic and inter-connected behaviour of the IPWP with tropical climate systems during the mid-Holocene highlights the fundamental importance of the warm pool region for understanding climate change throughout the tropics and beyond.  相似文献   

7.
The late Quaternary paleoclimate of eastern Beringia has primarily been studied by drawing qualitative inferences from vegetation shifts. To quantitatively reconstruct summer temperatures, we analyzed lake sediments for fossil chironomids, and additionally we analyzed the sediments for fossil pollen and organic carbon content. A comparison with the δ18O record from Greenland indicates that the general climatic development of the region throughout the last glaciation–Holocene transition differed from that of the North Atlantic region. Between  17 and 15 ka, mean July air temperature was on average 5°C colder than modern, albeit a period of near-modern temperature at  16.5 ka. Total pollen accumulation rates ranged between  180 and 1200 grains cm− 2 yr− 1. At  15 ka, approximately coeval with the Bølling interstadial, temperatures again reached modern values. At  14 ka, nearly 1000 yr after warming began, Betula pollen percentages increased substantially and mark the transition to shrub-dominated pollen contributors. Chironomid-based inferences suggest no evidence of the Younger Dryas stade and only subtle evidence of an early Holocene thermal maximum, as temperatures from  15 ka to the late Holocene were relatively stable. The most recognizable climatic oscillation of the Holocene occurred from  4.5 to 2 ka.  相似文献   

8.
Integration of on-land and offshore geomorphological and structural investigations coupled to extensive radiometric dating of co-seismically uplifted Holocene beaches allows characterization of the geometry, kinematics and seismotectonics of the Scilla Fault, which borders the eastern side of the Messina Strait in Calabria, Southern Italy. This region has been struck by destructive historical earthquakes, but knowledge of geologically-based source parameters for active faults is relatively poor, particularly for those running mostly offshore, as the Scilla Fault does. The  30 km-long normal fault may be divided into three segments of  10 km individual length, with the central and southern segments split in at least two strands. The central and northern segments are submerged, and in this area marine geophysical data indicate a youthful morphology and locally evidence for active faulting. The on-land strand of the western segment displaces marine terraces of the last interglacial (124 to 83 ka), but seismic reflection profiles suggest a full Quaternary activity. Structural data collected on bedrock faults exposed along the on-land segment provide evidence for normal slip and  NW-SE extension, which is consistent with focal mechanisms of large earthquakes and GPS velocity fields in the region. Detailed mapping of raised Holocene marine deposits exposed at the coastline straddling of the northern and central segments supplies evidence for two co-seismic displacements at  1.9 and  3.5 ka, and a possible previous event at  5 ka. Co-seismic displacements show a consistent site value and pattern of along-strike variation, suggestive of characteristic-type behaviour for the fault. The  1.5–2.0 m average co-seismic slips during these events document Me  6.9–7.0 earthquakes with  1.6–1.7 ka recurrence time. Because hanging-wall subsidence cannot be included into slip magnitude computation, these slips reflect footwall uplift, and represent minimum average estimates. The palaeoseismological record based on the palaeo-shorelines suggests that the last rupture on the Scilla Fault during the February 6, 1783 Mw = 5.9–6.3 earthquake was at the expected time but it may have not entirely released the loaded stress since the last great event at  1.9 ka. Comparison of the estimated co-seismic extension rate based on the Holocene shoreline record with available GPS velocities indicates that the Scilla Fault accounts for at least  15–20% of the contemporary geodetic extension across the Messina Strait.  相似文献   

9.
Approximately 70 km of new decimeter-resolution seismic reflection profile data from Owasco Lake, New York define a middle Holocene (4600 14C yr B.P.) erosion surface in the north end of the lake at water depths as great as 26 m. Beneath the lake, post-glacial sediments are up to 9 m thick and represent about 10% of the total sediment fill. Early to middle Holocene sediments, 6 m thick, contain biogenic gas at the south end of the basin and a large (4 km×300 m×15 m) subaqueous slide deposit along the east-central portion of the lake. Late Holocene sediments are thinner or absent, particularly at the north end of the lake. The middle Holocene erosion surface may have been produced by a drop in lake level. Alternatively, it may represent a change in climate during the transition between the relatively warm Holocene hypsithermal and cool neoglacial. At this time (4600 14C yr B.P.) circulation in Owasco Lake appears to have evolved from sluggish to active. The increased circulation, which persists today, probably resulted from atmospheric cold fronts with strong southwesterly winds that piled up water at the north end of the lake. The increased water circulation may have been ultimately driven by decreasing insolation, which produced an increased pole-to-equator thermal gradient and, thus, stronger global winds that began at the transition between the hypsithermal and neoglacial.  相似文献   

10.
A comprehensive record of lake level changes in the Dead Sea has been reconstructed using multiple, well dated sediment cores recovered from the Dead Sea shore. Interpreting the lake level changes as monitors of precipitation in the Dead Sea drainage area and the regional eastern Mediterranean palaeoclimate, we document the presence of two major wet phases ( 10–8.6 and  5.6–3.5 cal kyr BP) and multiple abrupt arid events during the Holocene. The arid events in the Holocene Dead Sea appear to coincide with major breaks in the Near East cultural evolution (at  8.6, 8.2, 4.2, 3.5 cal kyr BP). Wetter periods are marked by the enlargement of smaller settlements and growth of farming communities in desert regions, suggesting a parallelism between climate and Near East cultural development.  相似文献   

11.
This paper explores the environmental conditions that faced the people of ancient Jawa during the Holocene, as well as previous prehistoric periods of the mid-late Pleistocene. Calcite speleothems in a lava tube are dated using the U-Th method, to marine oxygen isotope stage 7 from  250 to 240 ka and from  230 to  220 ka; and the stage 5/4 transition between  80 and 70 ka. The available evidence indicates general aridity of the Black Desert during most of the mid-late Quaternary, punctuated by short wetter periods, when the Mediterranean cyclonic systems intensified and penetrated the north Arabian Desert. These Mediterranean systems had a longer and more intense effect on the desert fringe closer to the Mediterranean and only rarely penetrated the Black Desert of Jawa. The results do not exclude some increase of rainfall which did not change water availability dramatically during the warm Holocene. The ancient Jawa city appears to have depended on technological ability to build elaborate runoff-collection systems, which became the prime condition for success.  相似文献   

12.
A detailed study of uplifted Middle–Late Pleistocene marine terraces on the eastern side of northern Calabria, southern Italy, provides insights into the temporal and spatial scale variability of vertical displacement rates over a time span of 400 ka. Calabria is located in the frontal orogen of southern Italy above the westerly-plunging Ionian slab, and a combination of lithospheric, crustal, and surface processes concurred to rapid Late Quaternary uplift. Eleven terrace orders and a raised Holocene beach were mapped up to 480 m a.s.l., and were correlated between the coastal slopes of Pollino and Sila mountain ranges across the Sibari Plain, facing the Ionian Sea side of northeastern Calabria. Precise corrections were applied to the measured shoreline angles in order to account for uncertainty in measurement, erosion of marine deposits, recent debris shedding, and bathymetric range of markers. Radiometric (ESR and 14C) dating of shells provides a crono-stratigraphic scheme, although many samples were found to be resedimented in younger terraces. Terrace T4, whose inner margin stands at elevations of 94–130 m, is assigned to MIS 5.5 (124 ka), based on new ESR dating and previous amino acid racemization estimations. The underlying terraces T3, T2 and T1 are attributed to MIS 5.3 (100 ka), 5.1 (80 ka) and 3 (60 ka), as inferred from their relative position supplemented by ESR and 14C age determinations. The age of higher terraces is poorly constrained, but conceivably is tracked back to MIS 11 (400 ka). The reconstructed depositional sequence of terraces attributed to MIS 5.5 and 7 reveals two regressive marine cycles separated by an alluvial fanglomerate, which, given the steady uplift regime, points to minor sub-orbital sea-level changes during interstadial highstands. Based on the terrace chronology, uplift in the last 400 ka occurred at an average rate of 1 mm/a, but was characterized by the alternation of more rapid (up to 3.5 mm/a) and slower (down to 0.5 mm/a) periods of displacement. Spatial variability in uplift rates is recorded by the deformation profile of terraces parallel to the coast, which document the growth of local fold structures.  相似文献   

13.
Brenda L. Hall   《Quaternary Science Reviews》2009,28(21-22):2213-2230
A history of Holocene glaciation in the Antarctic and sub-Antarctic affords insight into questions concerning present and future ice-sheet and mountain-glacier behavior and global climate and sea-level change. Existing records permit broad correlation of Holocene ice fluctuations within the region. In several areas, ice extent was less than at present in mid-Holocene time. An important exception to this is the West Antarctic Ice Sheet, which has undergone continued recession throughout the Holocene, probably in response to internal dynamics. The first Neoglacial ice advances occurred at 5.0 ka, although some sites (e.g., western Ross Sea) lack firm evidence for glacial expansion at that time. Glaciers in all areas underwent renewed growth in the past millennium, and most have subsequently undergone recession in the past 50 years, ranging from near-catastrophic in parts of the Antarctic Peninsula to minor in the western Ross Sea region and sections of East Antarctica. This magnitude difference likely reflects the much greater warming that is taking place in the Antarctic Peninsula region today as compared to East Antarctica.  相似文献   

14.
We present marine sedimentologic and radiocarbon data for the timing of retreat of the largely marine-based Antarctic Peninsula Ice Sheet since the Last Glacial Maximum (LGM). Our findings indicate minimum estimates of deglaciation between 18,000 and 9000 calibrated years before present (cal yr BP), roughly in phase with the Northern Hemisphere deglaciation and eustatic sea-level rise. Our findings show this retreat occurred progressively from the outer, middle, and inner continental shelf regions, as well as progressively from the north to the south. Retreat initiated on the outer shelf of the northern Peninsula by 18,000 cal yr BP and continued southward by 14,000 cal yr BP on the outer shelf of Marguerite Bay, several thousand years earlier than estimated by numeric models. While individual cores yield estimates of glacial retreat that may vary up to ±1100 years, we note steps in the data occur at 14,000 and possibly 11,000 cal yr BP, coincidental to rapidly rising (eustatic) sea level, including the well documented melt water pulses (MWP 1a and 1b). These data support the hypothesis that rapidly rising sea level is associated with marine ice sheet destabilization, although additional dates are necessary to substantiate this finding. This study highlights problems with radiocarbon dating acid insoluble organic (AIO) matter in proximal Lateglacial sediments as well as the need for more accurate dating techniques.  相似文献   

15.
Pollen analysis of sediments from a high-altitude (4215 m), Neotropical (9°N) Andean lake was conducted in order to reconstruct local and regional vegetation dynamics since deglaciation. Although deglaciation commenced 15,500 cal yr B.P., the area around the Laguna Verde Alta (LVA) remained a periglacial desert, practically unvegetated, until about 11,000 cal yr B.P. At this time, a lycopod assemblage bearing no modern analog colonized the superpáramo. Although this community persisted until 6000 cal yr B.P., it began to decline somewhat earlier, in synchrony with cooling following the Holocene thermal maximum of the Northern Hemisphere. At this time, the pioneer assemblage was replaced by a low-diversity superpáramo community that became established 9000 cal yr B.P. This replacement coincides with regional declines in temperature and/or available moisture. Modern, more diverse superpáramo assemblages were not established until 4600 cal yr B.P., and were accompanied by a dramatic decline in Alnus, probably the result of factors associated with climate, humans, or both. Pollen influx from upper Andean forests is remarkably higher than expected during the Late Glacial and early to middle Holocene, especially between 14,000 and 12,600 cal yr B.P., when unparalleled high values are recorded. We propose that intensification of upslope orographic winds transported lower elevation forest pollen to the superpáramo, causing the apparent increase in tree pollen at high altitude. The association between increased forest pollen and summer insolation at this time suggests a causal link; however, further work is needed to clarify this relationship.  相似文献   

16.
The small granite plutons occurring at the contact of the Singhbhum-Orissa Iron Ore craton (IOC) to the north and the Eastern Ghat Granulite Belt (EGGB) to the south in eastern Indian shield are characterised by the presence of enclaves of the granulites of EGGB and the greenschist facies rocks of IOC. These granites also bear the imprints of later cataclastic deformation which is present at the contact of the IOC and the EGGB. In situ Pb-Pb zircon dating of these granites gives minimum age of their formation 2.80 Ga. A whole-rock three point Rb-Sr isochron age of this rock is found to be 2.90 Ga. Therefore, the true age of formation of these granites will be around 2.90–2.80 Ga. These granitic rocks also contain xenocrystic zircon components of 3.50 Ga and show a later metasomatic or metamorphic effect 2.48 Ga obtained from the analyses on overgrowths developed on 2.80 Ga old zircon cores. The presence of granulitic enclaves within these contact zone granite indicates that the granulite facies metamorphism of the EGGB is 2.80 Ga or still older in age. The cataclastic deformations observed at the contact zone of the two adjacent cratons is definitely younger than 2.80 Ga and possibly related to 2.48 Ga event observed from the overgrowths. As 2.80 Ga granite plutons of small dimensions are also observed at the western margin of the IOC; it can be concluded that a geologic event occurred 2.80 Ga over the IOC when small granite bodies evolved at the marginal part of this craton after its stabilisation at 3.09 Ga.  相似文献   

17.
The lack of paleoecological records from the montane Atlantic Rainforest of coastal Brazil, a hotspot of biological diversity, has been a major obstacle to our understanding of the vegetational changes since the last glacial cycle. We present carbon isotope and pollen records to assess the impact of the glaciation on the native vegetation of the Serra do Mar rainforest in São Paulo, Brazil. From ca. 28,000 to  22,000 14C yr BP, a subtropical forest with conifer trees is indicative of cool and humid conditions. In agreement carbon isotopic data on soil organic matter suggest the presence of C3 plants and perhaps C4 plants from  28,000 to  19,000 14C yr BP. The significant increase in the sedimentation rate and algal spores from  19,450 to  19,000 14C yr BP indicates increasing humidity, associated to an erosion process between  19,000 and  15,600 14C yr BP. From  15,600 14C yr BP to present there is a substantial increase in arboreal elements and herbs, indicating more humid and warmer climate. From  19,000 to  1000 14C yr BP, δ13C values indicated the predominance of C3 plants. These results are in agreement with studies in speleothems of caves, which suggest humid conditions during the last glacial maximum.  相似文献   

18.
It has long been recognised that sea levels along the shores of Atlantic Canada have been rising rapidly during the Holocene in response to isostatic crustal movements. New sea-level data for the Bay of Fundy coast of southern New Brunswick (Little Dipper Harbour) and the Atlantic coast of Nova Scotia (Chezzetcook Inlet) show that late Holocene average rates of sea-level rise in these areas have been 1.0 and 2.5 m per 1000 yr, respectively. Numerical model calculations suggest that the high rates of sea-level rise are due to crustal subsidence produced by the combined effects of Laurentide ice loading (forebulge collapse) and ocean loading of the Scotian shelf. Although ice loading is the dominant contributor to the regional sea-level pattern, ocean loading is also important, contributing up to 40% of the total crustal subsidence in some areas. Tide gauges record rates of sea-level rise during the 20th century that are 0.7–1.9 mm/yr higher than late Holocene trends, with the highest residuals occurring in the Bay of Fundy.  相似文献   

19.
A bottomland flora that prevailed between 9900 and 6000 cal yr B.P. in a North Carolina stream valley may not reflect a regionally much wetter Atlantic climate, coeval with record drought in the Great Plains region and assumed dry Gulf coastal conditions. Such conditions were inferred for 6000 ± 1000 yr ago when the Bermuda High may have consistently occupied summer positions far to the NE. Arid episodes coeval with the Little River local wet interval are known from eolian sediments and pollen spectra in the Atlantic and the Gulf coastal plain. For multiple reasons, the regional extent, intensity, and duration of coastal aridity and alternating wet phases and the Bermuda High positions are not yet adequately constrained. The climate and edaphic causes for the steadily growing predominance of southern pines over hardwoods, achieved between 8900 and 4200 cal yr B.P. at different sites at different times are similarly still unresolved. New data from Shelby Lake, AL, reconfirms that no credible field or other proxy evidence exists for a previously postulated “catastrophic Gulf hurricane phase” in the late Holocene.  相似文献   

20.
A 61-m-long sediment core (HB-1) and 690 km of high-resolution seismic profiles from offshore of the Yellow River delta, Bohai Sea, were analyzed to document the stratigraphy and sea-level changes during the Late Pleistocene and Holocene. Accelerator mass spectrometry 14C dating and analyses of benthic foraminifera, ostracods, the mineral composition, and sedimentary characteristics were performed for core HB-1, and seven depositional units (DU 1–DU 7 in descending order) were identified. The seismic reflection data were interpreted in light of the sedimentological data of the core and correlated with other well-studied cores obtained previously in the Bohai Sea area. Seven seismic units (SU 1 to SU 7 in descending order) were distinguished and interpreted as follows: SU 7 corresponds to marine facies in Marine Isotopic Stage (MIS) 5; SU 6, to terrestrial facies in MIS 4; SU 5 and SU 4, to alternating terrestrial and marine facies (DU 7–DU 5) in MIS 3; SU 3, to terrestrial facies (DU 4) in MIS 2; SU 2, to Holocene marine facies (DU 3 and DU 2); and SU 1, to modern Yellow River delta sediments deposited since 1855 (DU 1).The sedimentary facies from DU 7 to DU 5 reflect sea-level fluctuations during MIS 3, and the boundary between DU 5 and DU 6, which coincides with that between SU 4 and SU 5, is a distinctive, laterally continuous, undulating erosion surface, with up to 20 m of relief. This surface reflects subaerial exposure between transgressions during MIS 3. Estimated sea levels during MIS 3 ranged from −35 ± 5 to −60 ± 5 m or lower, with short-term fluctuations of 20 m. Sedimentary environments in the Bohai Sea area were governed mainly by eustatic sea-level changes and the Bohai Strait topography, which controls the entry of sea water into the Bohai Sea area.The mineral composition of the sediment core suggests that the Yellow River did not discharge into the Bohai Sea, or at least did not influence the study area significantly, during parts of MIS 3 and MIS 2 to the early Holocene (11–8.5 cal kyr BP).  相似文献   

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