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1.
Despite growing evidence for environmental oscillations during the last glacial–interglacial transition from high latitude, terrestrial sites of the North Pacific rim, oxygen-isotopic records of these oscillations remain sparse. The lack of data is due partially to the paucity of lakes that contain carbonate sediment suitable for oxygen-isotopic analysis. We report here the first record of oxygen-isotopic composition in diatom silica (δ18OSi) from a lake in that region. δ18OSi increases gradually from 19.0 to 23.5‰ between 12,340 and 11,000 14C yr B.P., reflecting marked climatic warming at the end of the last glaciation. Around 11,000 14C yr B.P., δ18OSi decreases by 1.7‰, suggesting a temperature decrease of 3.5–8.9 °C at the onset of the Younger Dryas (YD) in southwestern Alaska. Climatic recovery began ca. 10,740 14C yr B.P., as inferred from the increase of δ18OSi to a maximum of 23.9‰ near the end of the YD. Our data reveal that a YD climatic reversal in southwestern coastal areas of Alaska occurred, but the YD climate did not return to full-glacial conditions.  相似文献   

2.
GISP2 Oxygen Isotope Ratios   总被引：3，自引：0，他引：3  
The GISP2 oxygen isotope record, with its high-resolution detail, yields crucial information on past climate change. The glacial δ18O oscillations of the GISP2 core, with their very fast onsets, are templates of a prototype oscillation of variable duration with an amplitude of 3.9‰. The halfway mark of the cold–warm transition is reached in 2 years; the top is reached in 50 years. The δ18O–time gradient of the leading front is about 7.8‰ per 100 yr. After reaching the top, δ18O slowly declines by −0.14‰ per 100 yr. The duration of δ18O decline varies from a couple of centuries for fast oscillations to about 4000 yr for slower ones. The subsequent δ18O downturn during the warm–cold transition has a δ18O–time gradient of −3.2‰ per 100 yr and lasts about 80 yr.  相似文献   

3.
Isotopic and pollen results from a marl lake (White Lake) in the Mid-Atlantic region of USA indicate the coupling of climate and vegetation changes. Oxygen isotopes of calcite from this site show multiple oscillations at millennial and centennial scales, including the Younger Dryas with 3‰ negative shifts in δ18O at 12.4-11.4 ka (1 ka = 1000 cal yr BP) and three cold events of magnitude 1-2‰ shifts during the Bølling-Allerød warm period (BOA) at 14.3-12.4 ka. Pollen data from the same core show nearly synchronous, close correspondence with isotope-inferred climate shifts, indicating rapid forest response to deglacial climate oscillations in southern New England. A plateau-like BOA is similar to other records around the North Atlantic Ocean.  相似文献   

4.
Full‐glacial pollen assemblages from four radiocarbon‐dated interstadial deposits in southwestern Ohio and southeastern Indiana imply the presence of herbaceous vegetation (tundra or muskeg with subarctic indicator Selaginella selaginoides) on the southern margin of the Miami lobe of the Laurentide Ice Sheet ca. 20 000 14C yr BP. Scattered Picea (spruce) and possibly Pinus (pine) may have developed regionally ca. 19 000 14C yr BP, and ca. 18 000 14C yr BP, respectively. Spruce stumps in growth position support a local source of pollen. Prior to the ca. 14 000 14C yr BP glacial advance, small amounts of Quercus (oak) and other deciduous pollen suggest development of regional boreal (conifer–hardwood) forests. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

5.
A high-resolution calcite oxygen stable isotopic (δ18O) record, covering the past 4000 years, was obtained from Kaite Cave, northern Spain. The record has a mean δ18O value of -6.25‰ VPDB and a range of 2‰. Spectral analysis of the δ18O data shows significant periodicities of 2400–1900, 600, 150, 27, and 22 years. The amplitudes during these periods range from 0.2‰ to 2‰. Factors controlling the isotopic ratio in the speleothem were evaluated. The calcite is most likely precipitated under equilibrium conditions, with the cave calcite δ18O interpreted as a proxy of oxygen isotopic composition in local rainwater. Other factors such as temperature or fractionation in the karst system prior to calcite precipitation are considered of negligible or of minor importance. Mechanisms affecting rainfall isotopic composition were also investigated on different time scales. Precipitation amount is the primary factor controlling the high-frequency δ18O oscillations. Other climate parameters, such as changes of storm tracks may have significant contributions on centennial and millennial time scales.  相似文献   

6.
A comparison of a 6450 14C yr δ18O and δ13C record of authigenic calcite from Lake Awassa, Ethiopia, with other proxy climate records in the area suggests that the lake records long-term regional climate changes. Co-varying and increasing δ18O and δ13C values from 4800 BP suggest an aridification of climate after the early Holocene insolation maximum. After 4000 BP, humid conditions return until after 2800 BP when δ18O increases again, reflecting more arid conditions recorded elsewhere in Ethiopia. In addition to these long-term changes, there are abrupt decreases in both δ18Ocalcite and δ13Ccalcite immediately after tephra layers. The likeliest explanation for these abrupt decreases in isotopes is the effect of tephra on the lake's catchment vegetation. δ18O, δ13C and lake-level measurements from Lake Awassa since the 1970s suggest that the lake is currently isotopically sensitive to short-term (annual–decadal) climate change. However, during this period, the catchment has undergone progressive deforestation that may have caused an increase in runoff. Caution is therefore required when reconstructing palaeoclimates as a contemporary lake may not always be a good analogue for lake hydrology in the past.  相似文献   

7.
A sediment core 7.2 m long from Lake Mirabad, Iran, was examined for loss-on-ignition, mineralogy, oxygen-isotopic composition of authigenic calcite, and trace-element composition of ostracodes to complement earlier pollen and ostracode-assemblage studies. Pollen, ostracode-inferred lake level, and high Sr/Ca ratios indicate that the early Holocene (10000 to 6500 cal yr BP) was drier than the late Holocene. Low δ18O values during this interval are interpreted as resulting from winter-dominated precipitation, characteristic of a Mediterranean climate. Increasing δ18O values after 6500 cal yr BP signal a gradual increase in spring rains, which are present today. A severe 600-yr drought occurred at ca. 5500 cal yr BP, shortly after the transition from pistachio-almond to oak forest. During the late Holocene, two milder droughts occurred at about 1500 and 500 cal yr BP. Within the resolution of the record, no drought is evident during the collapse of the Akkadian empire (4200–3900 cal yr BP). Rather, a decrease in δ18O values to early-Holocene levels may indicate the return to a Mediterranean precipitation regime.  相似文献   

8.
Twenty two samples of calcretes from seven depth-profiles in the Menindee catchment, Broken Hill region, Australia were analysed for their inorganic and organic carbon contents and inorganic carbon and oxygen isotopes. The organic carbon content is very low (from 0.06 to 0.31 wt.%) while inorganic carbon (carbonate) is up to 3.9 wt.%. Both δ13C and δ18O become more positive closer to the surface. Carbon isotopes vary from − 8.5‰ to −5.5‰ PDB. Oxygen isotopes vary from − 6‰ to − 1.8‰ V-PDB. Depth-related δ13C and δ18O variations correlate over at least 15 km and show no significant variation along the flow path. δ13C values increase by 3‰ and δ18O values increase by 4‰ with decreasing depth in a 1.40 m thick soil profile. The variation is interpreted to indicate an increasingly elevated air temperature, greater water stress and subsequently an aridification of the area through time. The Broken Hill calcrete data confirm that climatic evolution can be deduced from isotopic series and be applied successfully to the Broken Hill region.  相似文献   

9.
Black and white dolomite crystals (mm to cm width) of different isotopic composition are associated with Triassic diapirism in central Tunisia, as well as with evaporite minerals and clays. The white dolomites occur mostly in the Jabal Hadifa diapir near the contact with Cretaceous limestones, whereas the smaller black dolomites occur in the Jabal Hamra diapir. The former dolomite has a narrow range of δ18O and δ13C values (− 3.83‰ to − 6.60‰ VPDB for δ18O; − 2.11‰ to − 2.83‰ VPDB for δ13C), whereas the latter dolomite has a wider range and more depleted values (− 4.92‰ to − 9.97‰ for δ18O; − 0.55‰ to − 6.08‰ for δ13C). However, the 87Sr / 86Sr ratios of most of the samples are near Triassic seawater values. Dolomite formation is due to at least two different fluids. The main fluid originated from deeper hydrothermal or basinal sources related to the Triassic saliferous rocks and ascended through faults during the diapiric intrusion. The second, less important fluid source is related to meteoric water originating from Cretaceous rocks.  相似文献   

10.
Holocene environmental changes in the northern Fertile Crescent remain poorly understood because of the scarcity of local proxy records in the region. In this study we investigated pedogenic (soil-formed) carbonate coatings on stones at the Pre-Pottery Neolithic site Göbekli Tepe as an indicator of local early-mid Holocene environmental changes. The 14C ages and stable isotopic composition of carbon and oxygen in thin (0.2–0.3 mm thick) pedogenic carbonate lamina indicate two main periods of coating formation: the early-Holocene (ca. 10000–6000 cal yr BP) and the mid-Holocene (ca. 6000–4000 cal yr BP). During the first period, there was an inverse relationship between δ13C and δ18O curves: a decrease in δ13C values coincide with an increase in δ18O values. For this period a trend towards higher temperatures is suggested. In the mid-Holocene, the mean rate of coating growth was 2–3 times higher than in the early Holocene. Both δ13C and δ18O reached their maximum values during this time and the direction of changes of the δ13C and δ18O curves became similar. The combination of data suggests that this period was the most humid in the Holocene and on average warmer than the early Holocene. At ca. 4000 cal yr BP secondary accumulation of carbonate ceased, presumably reflecting a shift to a more arid climate.  相似文献   

11.
Krüger, L. C., Paus, A., Svendsen, J. I. & Bjune, A. E. 2011: Lateglacial vegetation and palaeoenvironment in W Norway, with new pollen data from the Sunnmøre region. Boreas, 10.1111/j.1502‐3885.2011.00213.x. ISSN 0300‐9483. Two sediment sequences from Sunnmøre, northern W Norway, were pollen‐analytically studied to reconstruct the Lateglacial vegetation history and climate. The coastal Dimnamyra was deglaciated around 15.3 ka BP, whereas Løkjingsmyra, further inland, became ice‐free around 14 ka BP. The pioneer vegetation dominated by snow‐bed communities was gradually replaced by grassland and sparse heath vegetation. A pronounced peak in Poaceae around 12.9 ka BP may reflect warmer and/or drier conditions. The Younger Dryas (YD) cooling phase shows increasing snow‐bed vegetation and the local establishment of Artemisia norvegica. A subsequent vegetation closure from grassland to heath signals the Holocene warming. Birch forests were established 500–600 years after the YD–Holocene transition. This development follows the pattern of the Sunnmøre region, which is clearly different from the Empetrum dominance in the Lateglacial interstadial further south in W Norway. The Lateglacial oscillations GI‐1d (Older Dryas) and GI‐1b (Gerzensee) are hardly traceable in the north, in contrast to southern W Norway. The southern vegetation was probably closer to an ecotone and more susceptible to climate changes.  相似文献   

12.
Late Quaternary terrestrial climate records from the semi-arid zone of the Western Cape of South Africa are rare. However, palaeoenvironmental information may be inferred from ancient termite mounds of the region. Calcrete lenses in these mounds have δ13C and δ18O values that show systematic changes with radiocarbon dates, which range from 33,629–36,709 to 21,676–23,256 cal yr BP. These dates confirm that these heuweltjies had been present in the landscape since the last glacial period. The decrease in δ13C and δ18O from 33,629–36,709 to 21,676–23,256 cal yr BP indicates that climate information is recorded by the calcretes. It is suggested that a progressive decline in air temperature and an increase in moisture availability, and a decline in abundance of C4 or CAM plants, occurred in the region during the time heuweltjie calcite precipitated.  相似文献   

13.
Dolomitic concretions in diatomaceous hemipelagic sediments of the Miocene Pohang Basin in the southwestern East Sea (Sea of Japan) preserve distinct signals of two independent sedimentary processes, which controlled the extents of isotopic compositions. Variable δ18O (−9.1‰ to +0.7‰) and high δ13C (+3.1‰ to +17.9‰) values suggest that the concretions formed in the methanogenic zone with alteration of the residual mid-Miocene seawater by volcanogenic sediments. Remarkable δ18O and δ13C values show a strong linear relationship, indicating that distinctly independent depositional processes operated during the formation of the concretions. The degree of methanogenesis was enhanced during rapid hemipelagic sedimentation of organic-rich particles, resulting in higher δ13C values, and the effect of volcaniclastics was diluted, maintaining the original properties of ambient mid-Miocene seawater. In contrast, lower δ18O and 87Sr/86Sr values characterize the effect of volcaniclastic sediments that were transported by intermittent gravity flows and interacted with mid-Miocene seawater. The input of volcaniclastic sediment probably degraded the role of methanogenesis by lowering the contents of organic matter and thereby decreased the δ13C values within the concretions. Isotopic signals recorded within the concretions highlight understanding of the depositional environment and evolution of the pore-water chemistry.  相似文献   

14.
By using heavy coring equipment in two high-altitudinal lakes (1253 and 1316 m a.s.l.) at Dovre, Central Norway, 1–1.5 m of unsorted coarsely minerogenic sediments were retrieved below the Holocene organic sediments. The minerogenic sequence contained well-preserved pollen and chironomid remains, revealing new and detailed palaeoenvironmental knowledge of the mountains in Central Norway during the last 5–6000 years of the Lateglacial (LG) period. However, the LG chronology is based on biostratigraphical correlations and not on 14C-dates, due to low organic content in the minerogenic sediments. The emerging LG nunataks, probably indicating a thin and multi-domed Scandinavian ice-sheet, was rapidly inhabited by immigrating species which could explain the present centric distributions of certain arctic-alpine plants. The LG vegetation development included a pre-interstadial dominated by mineral-soil pioneers, an interstadial dominated by shrubs and dwarf-shrubs, and the Younger Dryas cold period with recurring dominance of pioneers. Pollen and stomata of Pinus and Picea indicate their local LG presence at Dovre. LG climate oscillations are indicated by pollen stratigraphy and for the later part of LG also by chironomids. These oscillations could correspond to Heinrich event 1, GI-1d, GI-1b, and the Younger Dryas cold events. The LG interstadial reached July mean temperatures of more than 7–8 °C, similar to the present. Chironomids colonized the lake already during the onset of the interstadial, albeit at very low richness and abundances. Starting from YD, there are sufficient chironomid head capsules to perform a temperature reconstruction. The Holocene warming of about 2 °C initiated a vegetation closure from snow beds and dwarf-shrub tundra to shrubs and forests. Birch-forests established about 10 ka cal BP, slightly earlier than pine forests. Alnus expanded ca 9.2 ka cal BP and a thinning of the local forests occurred from ca 7 ka cal BP. Two short-lasting climate deteriorations found in the pollen record and the chironomid record may represent the Preboreal Oscillation and the 8.2 event. The Holocene Thermal Maximum is indicated around ca 7.8–7.3 ka cal BP showing a chironomid-inferred July mean of at least 11 °C. This is ca 3 °C warmer than today.  相似文献   

15.
Foraminiferal stable isotopes and assemblages from Franz Victoria and St. Anna troughs provide a valuable record of freshwater and Atlantic Water flows to the northern Barents and Kara seas from deglaciation to present. The δ18O and δ13C of planktonic Neogloboquadrina pachyderma (s) and benthic Elphidium excavatum were up to 1.4‰ lower than present at ca 13, 11.5, and 10 14C ka (global sea-level corrected), mostly reflecting substantial freshwater inputs coincident with glacial–marine sediment deposition. Cassidulina teretis exceeded 40% of benthic foraminifera ca 13 and 10 14C ka, indicating subsurface penetrations of Atlantic Water. The transition to postglacial marine conditions is marked by a 1‰ rise in foraminiferal δ18O and a sharp fall in % C. teretis soon after 10 14C ka. These changes imply reduced inputs of freshwater and Atlantic Water. Subsequent isotopic and foraminiferal assemblage variations reflect changing Atlantic Water conditions “upstream” in the Nordic Seas and shifts between the warm Fram Strait and cold Barents Sea branches of Atlantic Water. We hypothesize that glacial-isostatically induced deepening by up to 150 m influenced Atlantic Water inflows to the northern Barents Sea during deglaciation and the Holocene. Thus, effects of isostatic recovery have to be factored into paleoceanographic reconstructions.  相似文献   

16.
We report here a 30 W CO2 laser heating protocol for analyzing oxygen isotope composition (δ18O in ‰ vs. V-SMOW) of quartz and amorphous silica grains lower than 50 and 2 μm with a good external precision (1σ < 0.15‰). This technique is used to investigate δ18O composition of macro-, micro- and crypto-crystalline quartz cements of quartzite levels occurring in a sand sequence from the South of France (Apt Series), after a physical separation of the quartz cements. δ18O data obtained from this technique are compared with δ18O data obtained from in situ ion microprobe analyses. This study also presents promising results on δ18O analysis of phytoliths obtained with the laser heating protocol (1σ < 0.1‰).  相似文献   

17.
A carbon and oxygen isotope survey based on 42 samples from the Amba Dongar carbonatite complex of Gujarat, India, indicates that the magmatic differentiation series sövite → alvikite → ankeritic carbonatite is beset with a distinct isotope trend characterized by a moderate rise in 13C coupled with a sizeable increase in 18O. From an average of −4.6 ± 0.4 ‰ [PDB] for the least differentiated (coarse) sövite member, δ13C values slowly increase in the alvikite (−3.7 ± 0.6 ‰) and ankeritic fractions (−3.0 ± 1.1 ‰), whereas δ18O rises from 10.3 ± 1.7 ‰ [SMOW] to 17.5 ± 5.8 ‰ over the same sequence, reaching extremes between 20 and 28 ‰ in the latest generation of ankeritic carbonatite. While an apparent correlation between δ13C and δ18O over the δ18O range of 7–13 ‰ conforms with similar findings from other carbonatite complexes and probably reflects a Rayleigh fractionation process, the observed upsurge of 18O notably in the ankeritic member is demonstrably related to a late phase of low-temperature hydrothermal activity involving large-scale participation of 18O-depleted groundwaters. As a whole, the Amba Dongar carbonatite province displays the characteristic 13C/12C label of deep-seated (primordial) carbon, reflecting the carbon isotope composition of the subcontinental upper mantle below the Narmada Rift Zone of the Indian subcontinent.  相似文献   

18.
The δ18O and δ13C values of the calcites associated with E-W and NE-SW transverse faults in the Negev, Israel, indicate that calcite was deposited from meteoric water. A regional change in the δ18O and δ13C values was observed. The 18O content in the calcite increases, from the southwestern (δ18O = −17.8‰) to the northeastern (δ18O = −2.9‰) part of the region. The δ13C values show the opposite trend of the 13C content decrease: from +2‰ in the south to −10‰ in the northeast. These trends had to reflect changes in regional paleoclimate, suggesting a change in the isotopic composition of the solution from which the calcite was deposited in different periods. The variations in the δ18O values reflect shifts in the δ18O values of precipitation and are associated with a change in the source of moist air masses which came from the equatorial Atlantic in the early Pleistocene and from the Mediterranean during a later period. Variations in δ13C values reflect changes from humid to arid conditions. Two modes of calcite deposition are suggested: (1) precipitation of calcite minerals in the unsaturated zone following the dissolution in the soil or (2) calcite deposition that occurred as CO2 was lost during emergence of paleogroundwater from Lower Cretaceous and Jurassic aquifers.  相似文献   

19.
Large-scale atmospheric circulation patterns determine the quantity and seasonality of precipitation, the major source of water in most terrestrial ecosystems. Oxygen isotope (δ18O) dynamics of the present-day hydrologic system in the Palouse region of the northwestern U.S.A. indicate a seasonal correlation between the δ18O values of precipitation and temperature, but no seasonal trends of δ18O records in soil water and shallow groundwater. Their isotope values are close to those of winter precipitation because the Palouse receives  75% of its precipitation during winter. Palouse Loess deposits contain late Pleistocene pedogenic carbonate having ca. 2 to 3‰ higher δ18O values and up to 5‰ higher carbon isotope (δ13C) values than Holocene and modern carbonates. The late Pleistocene δ18O values are best explained by a decrease in isotopically light winter precipitation relative to the modern winter-dominated infiltration. The δ13C values are attributed to a proportional increase of atmospheric CO2 in soil CO2 due to a decrease in soil respiration rate and 13C discrimination in plants under much drier paleoclimate conditions than today. The regional climate difference was likely related to anticyclonic circulation over the Pleistocene Laurentide and Ice Sheet.  相似文献   

20.
The presence of oil shows associated with fractures provides a significant opportunity to a) unravel the type, origin and evolution of fluids involved in fracture-fills, and b) examine how they relate to oil migration. Two stages of calcite cement (C1 and C2) were distinguished in the fractures of the Eocene Armàncies platform carbonates; C1 is characterised by fence-like crystals, exhibits dull red luminescence and contains abundant twin planes, inclusions and δ18O values that range from − 6.2‰ to − 4.8‰ VPDB. C2 consists of blocky clean crystals, is characterized by dark brown-red luminescence that alternates with yellow bands, and contains hydrocarbon fluid inclusions with homogenisation temperatures of approximately 120 °C. δ18O values range from − 9.6‰ to − 8.9‰ VPDB. The remaining porosity after C2 precipitation is filled with liquid oil that reached 115 °C. This would seem to indicate that free oil and fluid inclusions oil probably come from the same migration pulse. Oil migration timing was coeval with C2 and continued after calcite cementation was completed.  相似文献   

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