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1.
Analyses of sediment cores from Jellybean Lake, a small, evaporation-insensitive groundwater-fed lake, provide a record of changes in North Pacific atmospheric circulation for the last ∼7500 yr at 5- to 30-yr resolution. Isotope hydrology data from the southern Yukon indicate that the oxygen isotope composition of water from Jellybean Lake reflects the composition of mean-annual precipitation, δ18Op. Recent changes in the δ18O of Jellybean sedimentary calcite (δ18Oca) correspond to changes in the North Pacific Index (NPI), a measure of the intensity and position of the Aleutian Low (AL) pressure system. This suggests that δ18Op variability was related to the degree of fractionation during moisture transport from the Gulf of Alaska across the St. Elias Mountains and that Holocene shifts were controlled by the intensity and position of the AL. Following this model, between ∼7500 and 4500 cal yr B.P., long-term trends suggest a predominantly weaker and/or westward AL. Between ∼4500 and 3000 cal yr B.P. the AL shifted eastward or intensified before shifting westward or weakening between ∼3000 and 2000 cal yr B.P. Rapid shifts eastward and/or intensification occurred ∼1200 and 300 cal yr B.P. Holocene changes in North Pacific atmospheric circulation inferred from Jellybean Lake oxygen isotopes correspond with late Holocene glacial advances in the St. Elias Mountains, changes in North Pacific salmon abundance, and shifts in atmospheric circulation over the Beaufort Sea.  相似文献   

2.
Continuous pollen and sediment records from two ∼8.5-m-long cores document late Pleistocene and Holocene sedimentation and vegetation change in the Ballston Lake basin, eastern New York State. Pebbles at the base of both cores and the geomorphology of the watershed reflect the presence of the Mohawk River in the basin prior to ∼12,900 ± 70 cal yr B.P. Ballston Lake formed at the onset of the Younger Dryas (YD) by an avulsion of the Mohawk River. The transition from clay to gyttja with low magnetic susceptibility (MS), low bulk density, and high organic carbon indicates rapid warming and increased lake productivity beginning 11,020 cal yr B.P. MS measurements reveal that the influx of magnetic particles, associated with pre-Holocene clastic sedimentation, ceased after ∼10,780 cal yr B.P. The pollen record is subdivided into six zones: BL1 (12,920 to 11,020 cal yr B.P.) is dominated by boreal forest pollen; BL2 (11,020 to 10,780 cal yr B.P.) by pine (Pinus) forest pollen; BL3 (10,780 to 5290 cal yr B.P.) by hemlock (Tsuga) and mixed hardwood pollen; BL4 (5290 to 2680 cal yr B.P.) by mixed hardwood pollen; BL5a (2680 cal yr B.P. to 1030 cal yr B.P.) by conifer and mixed hardwood pollen; and BL5b (1030 cal B.P. to present) by increasing ragweed (Ambrosia) pollen. A 62% decrease in spruce (Picea) pollen in <320 cal years during BL1 reflects rapid warming at the end of the YD. Holocene pollen zones record more subtle climatic shifts than occurred at the end of the YD. One of the largest changes in the Holocene pollen spectra began ∼5300 cal yr B.P., and is characterized by a marked decline in hemlock pollen. This has been noted in other pollen records from the region and may record preferential selection of hemlock by a pathogen or parasites.  相似文献   

3.
Lithology, pollen, macrofossils, and stable carbon isotopes from an intermontane basin bog site in southern New Zealand provide a detailed late-glacial and early Holocene vegetation and climate record. Glacial retreat occurred before 17,000 cal yr B.P., and tundra-like grassland–shrubland occupied the basin shortly after. Between 16,500 and 14,600 cal yr B.P., a minor regional expansion of forest patches occurred in response to warming, but the basin remained in shrubland. Forest retreated between 14,600 and 13,600 cal yr B.P., at about the time of the Antarctic Cold Reversal. At 13,600 cal yr B.P., a steady progression from shrubland to tall podocarp forest began as the climate ameliorated. Tall, temperate podocarp trees replaced stress-tolerant shrubs and trees between 12,800 and 11,300 cal yr B.P., indicating sustained warming during the Younger Dryas Chronozone (YDC). Stable isotopes suggest increasing atmospheric humidity from 11,800 to 9300 cal yr B.P. Mild (annual temperatures at least 1°C higher than present), and moist conditions prevailed from 11,000 to 10,350 cal yr B.P. Cooler, more variable conditions followed, and podocarp forest was completely replaced by montane Nothofagus forest at around 7500 cal yr B.P. with the onset of the modern climate regime. The Cass Basin late-glacial climate record closely matches the Antarctic ice core records and is in approximate antiphase with the North Atlantic.  相似文献   

4.
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5.
Analysis of oxygen isotope ratios (δ18O) by ion microprobe resolves a sub-annual climate record for the Eastern Mediterranean from a Soreq Cave stalagmite that grew between 2.2 and 0.9 ka. In contrast to conventional drill-sampling methods that yield a total variation of 1.0‰ in δ18Ocalcite values across our sample, the methods described here reveal up to 2.15‰ variation within single annual growth bands. Values of δ18O measured by ion microprobe vary in a regular saw-tooth pattern that correlates with annual, fluorescent growth banding where calcite grades from light to dark fluorescence. Modern records of precipitation and of cave dripwater indicate that variable δ18Ocalcite values record regular seasonal differences in δ18Orainfall modified by mixing in the vadose zone. Large differences in δ18O values measured across a single band (i.e., between the dark and light fluorescent calcite, or Δ18Odark-light) are interpreted to indicate wetter years, while smaller differences represent drier years. Oxygen isotopes record: 1) month-scale growth increments, 2) changes in Δ18Odark-light that represent seasonality, 3) a systematic, long-term decrease in maximum Δ18Odark-light values, and 4) an overall increase in average δ18Ocalcite values through time. These results suggest a drying of regional climate that coincides with the decline of the Roman and Byzantine Empires in the Levant region.  相似文献   

6.
We present a record of monsoon variations for the early and middle Holocene that is inferred from the geochemistry of sediment cores from Ahung Co, a lake in central Tibet. The resolution of this record is better than 50 yr and the age model is derived from radiocarbon ages of terrestrial charcoal, which eliminates errors associated with the lake hard-water effect. We made down-core geochemical measurements of % carbonate, % organic carbon, C/N and δ13C of bulk organic matter, δ13C and δ18O of carbonate, and % dolomite. Proxy calibration and modern water-balance reconstruction show that these are proxies for lake depth and the amount of monsoon precipitation. We find that lake level and monsoon precipitation have been decreasing at Ahung Co since the early Holocene (∼7500 cal yr B.P.). Superimposed on this trend are rapid declines in monsoon rainfall at 7000-7500 and 4700 cal yr B.P. and seven century-scale wet-dry oscillations. The cores do not contain sediment from the last ∼4000 yr. Surface sediments from the lake accumulated during the 20th century, however. From this, we argue that lake levels have risen again recently following a late Holocene dry period.  相似文献   

7.
A well-dated δ18O record in a stalagmite from a cave in the Klamath Mountains, Oregon, with a sampling interval of 50 yr, indicates that the climate of this region cooled essentially synchronously with Younger Dryas climate change elsewhere in the Northern Hemisphere. The δ18O record also indicates significant century-scale temperature variability during the early Holocene. The δ13C record suggests increasing biomass over the cave through the last deglaciation, with century-scale variability but with little detectable response of vegetation to Younger Dryas cooling.  相似文献   

8.
Measured 18O/16O ratios from the Greenland Ice Sheet Project 2 (GISP2) ice core extending back to 16,500 cal yr B.P. provide a continuous record of climate change since the last glaciation. High-resolution annual 18O/16O results were obtained for most of the current millennium (A.D. 818-1985) and record the Medieval Warm Period, the Little Ice Age, and a distinct 11-yr 18O/16O cycle. Volcanic aerosols depress central Greenland annual temperature (1.5°C maximally) and annual 18O/16O for about 4 yr after each major eruptive event. On a bidecadal to millennial time scale, the contribution of solar variability to Holocene Greenlandic temperature change is 0.4°C. The role of thermohaline circulation change on climate, problematic during the Holocene, is more distinct for the 16,500-10,000 cal yr B.P. interval. (Analogous to 14C age calibration terminology, we express time in calibrated (cal) yr B.P. (A.D. 1950 = 0 cal yr B.P.)). The Oldest Dryas/Bølling/Older Dryas/Allerød/Younger Dryas sequence appears in great detail. Bidecadal variance in 18O/16O, but not necessarily in temperature, is enhanced during the last phase of lateglacial time and the Younger Dryas interval, suggesting switches of air mass transport between jet stream branches. The branched system is nearly instantaneously replaced at the beginning of the Bølling and Holocene (at 14,670 and 11,650 cal yr B.P., respectively) by an atmospheric circulation system in which 18O/16O and annual accumulation initially track each other closely. Thermodynamic considerations of the accumulation rate-temperature relationship can be used to evaluate the 18 O/16O-temperature relationship. The GISP2 ice-layer-count years of major GISP2 climate transitions also support the use of coral 14C ages for age calibration.  相似文献   

9.
Variability of atmospheric 14C content often complicates radiocarbon-based chronologies; however, specific features such as periods of constant 14C age or steep changes in radiocarbon ages can be useful stratigraphic markers. The Younger Dryas event in the Northern Hemisphere is one of those periods, showing conspicuous 14C wiggles. Although the origin of those variations is not fully understood, we can make practical use of them and determine: (i) whether the Younger Dryas was global in extent; if so, (ii) were the initial cooling and the final warming synchronous worldwide; and (iii) what are the implications of these similarities/differences? Here we report high-resolution AMS 14C chronologies from the mid-latitudes of South America that pinpoint a cool episode between 11,400 and 10,20014C yr B.P. The onset of the final cool episode of the Late Glacial in the southern mid-latitudes, i.e., the Huelmo/Mascardi Cold Reversal, preceded the onset of the Younger Dryas cold event by ∼550 calendar years. Both events ended during a radiocarbon-age plateau at ∼10,20014C yr B.P. Thus, the Huelmo/Mascardi Cold Reversal encompasses the Younger Dryas, as well as a couple of short-term cool/warm oscillations that immediately preceded its onset in the North Atlantic region.  相似文献   

10.
Pollen analysis from a peat core 7.0 m in length, taken from a bog near Bisoca, in a mid-altitude area of the Buzăului Subcarpathian mountains, is used to reconstruct the postglacial vegetation history of the region. The vegetation record, which is supported by twelve 14C dates, starts at the end of the Late Glacial period. At the Late Glacial/Holocene transition, open vegetation was replaced by forest, suggesting a fast response to climatic warming. The Holocene began with the expansion of Betula, Pinus and Ulmus, followed, after 11,000 cal yr BP, by Fraxinus, Quercus, Tilia and Picea. The rapid expansion of these taxa may be due to their existence in the area during the Late Glacial period. At ca. 9200 cal yr BP, Corylus expanded, reaching a maximum after 7600 cal yr BP. The establishment of Carpinus occurred at ca. 7200 cal yr BP, with a maximum at ca. 5700 cal yr BP. Fagus pollen is regularly recorded after 7800 cal yr BP and became dominant at ca. 2000 cal yr BP. The first indications of human activities appear around 3800 cal yr BP.  相似文献   

11.
Pollen records from two sites in western Oregon provide information on late-glacial variations in vegetation and climate and on the extent and character of Younger Dryas cooling in the Pacific Northwest. A subalpine forest was present at Little Lake, central Coast Range, between 15,700 and 14,850 cal yr B.P. A warm period between 14,850 and 14,500 cal yr B.P. is suggested by an increase inPseudotsugapollen and charcoal. The recurrence of subalpine forest at 14,500 cal yr B.P. implies a return to cool conditions. Another warming trend is evidenced by the reestablishment ofPseudotsugaforest at 14,250 cal yr B.P. Increased haploxylonPinuspollen between 12,400 and 11,000 cal yr B.P. indicates cooler winters than before. After 11,000 cal yr B.P. warm dry conditions are implied by the expansion ofPseudotsuga.A subalpine parkland occupied Gordon Lake, western Cascade Range, until 14,500 cal yr B.P., when it was replaced during a warming trend by a montane forest. A rise inPinuspollen from 12,800 to 11,000 cal yr B.P. suggests increased summer aridity.Pseudotsugadominated the vegetation after 11,000 cal yr B.P. Other records from the Pacific Northwest show an expansion ofPinusfrom ca. 13,000 to 11,000 cal yr B.P. This expansion may be a response either to submillennial climate changes of Younger Dryas age or to millennial-scale climatic variations.  相似文献   

12.
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   

13.
The mid-to-late Pleistocene Devils Hole δ18O record has been extended from 60,000 to 4500 yr ago. The new δ18O time series, in conjunction with the one previously published, is shown to be a proxy of Pacific Ocean sea surface temperature (SST) off the coast of California. During marine oxygen isotope stages (MIS) 2 and 6, the Devil Hole and SST time series exhibit a steady warming that began 5000 to > 10,000 yr prior to the last and penultimate deglaciations. Several possible proximate causes for this early warming are evaluated. The magnitude of the peak δ18O or SST during the last interglacial (LIG) is significantly greater (1 per mill and 2 to 3°C, respectively) than the peak value of these parameters for the Holocene; in contrast, benthic δ18O records of ice volume show only a few tenths per mill difference in the peak value for these interglacials. Statistical analysis provides an estimate of the large shared information (variation) between the Devils Hole and Eastern Pacific SST time series from ∼ 41 to ∼ 2°N and enforces the concept of a common forcing among all of these records. The extended Devils Hole record adds to evidence of the importance of uplands bordering the eastern Pacific as a source of archives for reconstructing Pacific climate variability.  相似文献   

14.
The Niers valley was part of the Rhine system that came into existence during the maximum Saalian glaciation and was abandoned at the end of the Weichselian. The aim of the study was to explain the Late Pleniglacial and Late Glacial fluvial dynamics and to explore the external forcing factors: climate change, tectonics and sea level. The sedimentary units have been investigated by large‐scale coring transects and detailed cross‐sections over abandoned channels. The temporal fluvial development has been reconstructed by means of geomorphological relationships, pollen analysis and 14C dating. The Niers‐Rhine experienced a channel pattern change from braided, via a transformational phase, to meandering in the early Late Glacial. This change in fluvial style is explained by climate amelioration at the Late Pleniglacial to Late Glacial transition (at ca. 12.5 k 14C yr BP) and climate‐related hydrological, lithological and vegetation changes. A delayed fluvial response of ca. 400 14C yr (transitional phase) was established. The channel transformations are not related to tectonic effects and sea‐level changes. Successive river systems have similar gradients of ca. 35–40 cm km?1. A meandering river system dominated the Allerød and Younger Dryas periods. The threshold towards braiding was not crossed during the Younger Dryas, but increased aeolian activity has been observed on the Younger Dryas point bars. The final abandonment of the Niers‐Rhine was dated shortly after the Younger Dryas to Holocene transition. Traces of Laacher See pumice have been found in the Niers valley, indicating that the Niers‐Rhine was still in use during the Younger Dryas. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

15.
Previous research has shown that speleothems from the northern rim of the European Alps captured submillennial-scale climate change during the last glacial period with exceptional sensitivity and resolution, mimicking Greenland ice-core records. Here we extend this so-called NALPS19 record across the Late Glacial using two stalagmites which grew continuously into the Holocene. Both specimens show the same high-amplitude δ18O signal as Greenland ice cores down to decadal resolution. The start of the warming at the onset of the equivalent of Greenland Interstadial (GI) GI-1e at 14.66 ± 0.18 ka agrees with the North Greenland Ice Core Project (NGRIP) (14.64 ± 0.28 ka) and comprised a temperature rise of about 5–6 °C. The transition from the equivalent of GI-1a into the equivalent of Greenland Stadial (GS) GS-1 (broadly equivalent to the Younger Dryas) commenced at 13.02 ± 0.13 ka which is consistent with NGRIP (12.80 ± 0.26 ka) within errors. The onset of the Holocene started at 11.78 ± 0.14 ka (11.65 ± 0.10 ka at NGRIP) and involved a warming of about 4–5 °C. In contrast to δ18O, δ13C values show no response to (sub)millennial climate shifts due to strong rock-buffering and only record a long-term trend of soil development starting with the rapid warming at 14.7 ka.  相似文献   

16.
Oxygen isotope profiles along the growth axis of fossil bivalve shells of Macoma calcarea were established to reconstruct hydrographical changes in the eastern Laptev Sea since 8400 cal yr B.P.. The variability of the oxygen isotopes (δ18O) in the individual records is mainly attributed to variations in the salinity of bottom waters in the Laptev Sea with a modern ratio of 0.50‰/salinity. The high-resolution δ18O profiles exhibit distinct and annual cycles from which the seasonal and annual salinity variations at the investigated site can be reconstructed. Based on the modern analogue approach oxygen isotope profiles of radiocarbon-dated bivalve shells from a sediment core located northeast of the Lena Delta provide seasonal and subdecadal insights into past hydrological conditions and their relation to the Holocene transgressional history of the Laptev Sea shelf. Under the assumption that the modern relationship between δ18Ow and salinity has been constant throughout the time, the δ18O of an 8400-cal-yr-old bivalves would suggest that bottom-water salinity was reduced and the temperature was slightly warmer, both suggesting a stronger mixture of riverine water to the bottom water. Reconstruction of the inundation history of the Laptev Sea shelf indicates local sea level ∼27 m below present at this time and a closer proximity of the site to the coastline and the Lena River mouth. Due to continuing sea level rise and a southward retreat of the river mouth, bottom-water salinity increased at 7200 cal yr B.P. along with an increase in seasonal variability. Conditions comparable to the modern hydrography were achieved by 3800 cal yr B.P.  相似文献   

17.
This article presents a new comprehensive assessment of the Holocene hydrological variability of Lake Ladoga, northwest Russia. The reconstruction is based on oxygen isotopes of lacustrine diatom silica (δ18Odiatom) preserved in sediment core Co 1309, and is complemented by a diatom assemblage analysis and a survey of modern isotope hydrology. The data indicate that Lake Ladoga has existed as a freshwater reservoir since at least 10.8 cal. ka BP. The δ18Odiatom values range from +29.8 to +35.0‰, and relatively higher δ18Odiatom values around +34.7‰ between c. 7.1 and 5.7 cal. ka BP are considered to reflect the Holocene Thermal Maximum. A continuous depletion in δ18Odiatom since c. 6.1 cal. ka BP accelerates after c. 4 cal. ka BP, indicating Middle to Late Holocene cooling that culminates during the interval 0.8–0.2 cal. ka BP, corresponding to the Little Ice Age. Lake‐level rises result in lower δ18Odiatom values, whereas lower lake levels cause higher δ18Odiatom values. The diatom isotope record gives an indication for a rather early opening of the Neva River outflow at c. 4.4–4.0 cal. ka BP. Generally, overall high δ18Odiatom values around +33.5‰ characterize a persistent evaporative lake system throughout the Holocene. As the Lake Ladoga δ18Odiatom record is roughly in line with the 60°N summer insolation, a linkage to broader‐scale climate change is likely.  相似文献   

18.
Holocene high-resolution cores from the margin of the Arctic Ocean are rare. Core P189AR-P45 collected in 405-m water depth on the Beaufort Sea slope, west of the Mackenzie River delta (70°33.03′N and 141°52.08′W), is in close vertical proximity to the present-day upper limit of modified Atlantic water. The 5.11-m core spans the interval between ∼6800 and 10,400 14C yr B.P. (with an 800-yr ocean reservoir correction). The sediment is primarily silty clay with an average grain-size of 9 φ. The chronology is constrained by seven radiocarbon dates. The rate of sediment accumulation averaged 1.35 mm/yr. Stable isotopic data (δ18O and δ13C) were obtained on the polar planktonic foraminifera Neogloboquadrina pachyderma (s) and the benthic infaunal species Cassidulina neoteretis. A distinct low-δ18O event is captured in both the benthic and planktonic data at ∼10,000 14C yr B.P.—probably recording the glacial Lake Agassiz outburst flood associated with the North Atlantic preboreal cold event. The benthic foraminifera are dominated in the earliest Holocene by C. neoteretis, a species associated with modified Atlantic water masses. This species decreases toward the core top with a marked environmental reversal occurring ∼7800 14C yr B.P., possibly coincident with the northern hemisphere 8200 cal yr B.P. cold event.  相似文献   

19.
《Quaternary Science Reviews》2005,24(1-2):105-121
Pollen analysis of radiocarbon-dated lake sediment from northern Vancouver Island, southwest British Columbia reveals regional changes in forest vegetation over the last 12,200 14C yr (14,900 cal yr). Between at least 12,200 and 11,700 14C yr BP (14,900–13,930 cal yr BP), open woodlands were dominated by Pinus contorta, Alnus crispa, and various ferns. As P. contorta decreased in abundance, Alnus rubra and more shade-tolerant conifers (i.e., Picea and Tsuga mertensiana) increased. Increases in T. mertensiana, P. contorta, and A. crispa pollen accumulation rates (PARs) between 10,600 and 10,400 14C yr BP (11,660–11,480 cal yr BP) reflect a cool and moist climate during the Younger Dryas chronozone. Orbitally induced warming around 10,000 14C yr BP (11,090 cal yr BP) allowed the northward extension of Pseudotsuga menziesii, although Picea, Tsuga heterophylla, and A. rubra dominated early Holocene forests. By 7500 14C yr BP (8215 cal yr BP), shade-tolerant T. heterophylla was the dominant forest tree. Cupressaceae (Thuja plicata and Chamaecyparis nootkatensis) was present by 7500 14C yr BP but reached its maximum after 3500 14C yr BP (3600 cal yr BP), when a cooler and wetter regional climate facilitated the development of temperate rainforest. The highest rates of vegetation change are associated with Lateglacial climate change and species with rapid growth rates and short life spans.  相似文献   

20.
We investigated the oxygen isotope composition (δ18O) of shell striae from juvenile Comptopallium radula (Mollusca; Pectinidae) specimens collected live in New Caledonia. Bottom-water temperature and salinity were monitored in-situ throughout the study period. External shell striae form with a 2-day periodicity in this scallop, making it possible to estimate the date of precipitation for each calcite sample collected along a growth transect. The oxygen isotope composition of shell calcite (δ18Oshell calcite) measured at almost weekly resolution on calcite accreted between August 2002 and July 2003 accurately tracks bottom-water temperatures. A new empirical paleotemperature equation for this scallop species relates temperature and δ18Oshell calcite:
t(°C)=20.00(±0.61)-3.66(±0.39)×(δ18Oshell calcite VPDB-δ18Owater VSMOW)  相似文献   

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