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31.
Mid-Holocene ocean feedback on global monsoon area, global monsoon precipitation(GMP), and GMP intensity(GMPI) was investigated by a set of numerical experiments performed with the Community Climate System Model version 4. Results showed that ocean feedback induced an increase in land monsoon area for northern Africa but a decrease for Asia and North and South America, and led to an increase in ocean monsoon area for the western Indian Ocean and southeastern Pacific but a decrease for the eastern Indian Ocean, tropical western Pacific, and tropical West Atlantic between the mid-Holocene and pre-industrial period. Dynamic oceaninduced changes in GMP and GMPI were 10.9×109 m3 d-1and-0.11 mm d-1 between the two periods, respectively.Ocean feedback induced consistent change between the Northern and Southern Hemisphere land or ocean but opposite change between land and ocean in the Northern or Southern Hemisphere monsoon areas for monsoon precipitation. The mid-Holocene summer changes in meridional temperature gradient and land-sea thermal contrast were the underlying mechanisms. 相似文献
32.
Robert A. Dull 《Quaternary Research》2004,61(2):159-167
An ∼8000-cal-yr stratigraphic record of vegetation change from the Sierra de Apaneca, El Salvador, documents a mid-Holocene warm phase, followed by late Holocene cooling. Pollen evidence reveals that during the mid-Holocene (∼8000-5500 cal yr B.P.) lowland tropical plant taxa were growing at elevations ∼200-250 m higher than at present, suggesting conditions about 1.0°C warmer than those prevailing today. Cloud forest genera (Liquidambar, Juglans, Alnus, Ulmus) were also more abundant in the mid-Holocene, indicating greater cloud cover during the dry season. A gradual cooling and drying trend began by ∼5500 cal yr B.P., culminating in the modern forest composition by ∼3500 cal yr B.P. A rise in pollen from weedy plant taxa associated with agriculture occurred ∼5000 cal yr B.P., and pollen from Zea first appeared in the record at ∼4440 cal yr B.P. Human impacts on local vegetation remained high throughout the late Holocene, but decreased abruptly following the Tierra Blanca Joven (TBJ) eruption of Volcán Ilopango at ∼1520 cal yr B.P. The past 1500 years are marked by higher lake levels and periodic depositions of exogenous inorganic sediments, perhaps indicating increased climatic variability. 相似文献
33.
In order to improve the reliability of climate reconstruction, especially the climatologies outside the modern observed climate
space, an improved inverse vegetation model using a recent version of BIOME4 has been designed to quantitatively reconstruct
past climates, based on pollen biome scores from the BIOME6000 project. The method has been validated with surface pollen
spectra from Eurasia and Africa, and applied to palaeoclimate reconstruction. At 6 cal ka BP (calendar years), the climate
was generally wetter than today in southern Europe and northern Africa, especially in the summer. Winter temperatures were
higher (1–5°C) than present in southern Scandinavia, northeastern Europe, and southern Africa, but cooler in southern Eurasia
and in tropical Africa, especially in Mediterranean regions. Summer temperatures were generally higher than today in most
of Eurasia and Africa, with a significant warming from ∼3 to 5°C over northwestern and southern Europe, southern Africa, and
eastern Africa. In contrast, summers were 1–3°C cooler than present in the Mediterranean lowlands and in a band from the eastern
Black Sea to Siberia. At 21 cal ka BP, a marked hydrological change can be seen in the tropical zone, where annual precipitation
was ∼200–1,000 mm/year lower than today in equatorial East Africa compared to the present. A robust inverse relationship is
shown between precipitation change and elevation in Africa. This relationship indicates that precipitation likely had an important
role in controlling equilibrium-line altitudes (ELA) changes in the tropics during the LGM period. In Eurasia, hydrological
decreases follow a longitudinal gradient from Europe to Siberia. Winter temperatures were ∼10–17°C lower than today in Eurasia
with a more significant decrease in northern regions. In Africa, winter temperature was ∼10–15°C lower than present in the
south, while it was only reduced by ∼0–3°C in the tropical zone. Comparison of palaeoclimate reconstructions using LGM and
modern CO2 concentrations reveals that the effect of CO2 on pollen-based LGM reconstructions differs by vegetation type. Reconstructions for pollen sites in steppic vegetation in
Europe show warmer winter temperatures under LGM CO2 concentrations than under modern concentrations, and reconstructions for sites in xerophytic woods/scrub in tropical high
altitude regions of Africa are wetter for LGM CO2 concentrations than for modern concentrations, because our reconstructions account for decreased plant water use efficiency. 相似文献
34.
南海北部陆架中全新世沉积记录及古环境意义 总被引：2，自引：1，他引：1
通过分析珠江口外陆架C069孔沉积柱样的粒度、黏土矿物、主量元素及底栖有孔虫,来研究南海北部中全新世晚期古环境演化。4.2 ka前为全新世大暖期后期,气候湿润,珠江口外陆架受大量的珠江淡水影响,导致C069孔位置的水动力条件较强,形成下段粒度较粗的碎屑沉积物,高岭石含量较高,低的MgO/Al2O3比值,以及受冲淡水影响形成的底栖有孔虫。由于受4.2 ka的极冷事件的影响,C069孔上段的沉积环境受珠江冲淡水影响减小,水动力条件减弱,沉积物粒度变细,高岭石含量降低,MgO/Al2O3比值增大,形成正常浅水环境的底栖有孔虫。4.2 ka以后,台湾来源物质的贡献增多,导致沉积物中伊利石和绿泥石的含量增多,伊利石结晶度和化学指数值都明显变小。 相似文献
35.
We report multiproxy analyses of a sediment core obtained from Laguna Aculeo that spans the past 7500 years. Laguna Aculeo (33°50′S, 70°55′W) is one of the few natural inland lakes located in the Mediterranean zone of Central Chile, near the northern margin of the influence of the southern westerlies. The record shows elevated pollen counts of halophytes and seasonally drying of the lake basin prior to 5700 cal yr B.P., indicating severe aridity and warmer-than-present conditions. This was followed by the establishment of a fresh-water lake, along with an increase in arboreal and herbaceous plant diversity between 5700 and 3200 cal yr B.P. An intensification of this trend started at 3200 cal yr B.P., along with the abrupt decrease of halophytes until 100 cal yr B.P. Within this humid period, pollen accumulation rates show large-amplitude fluctuations, coeval with numerous turbidite layers, suggesting a highly variable and torrential rainfall pattern. This intense and variable precipitation regime is probably associated with the El Niño–Southern Oscillation (ENSO) phenomenon. We suggest that the modern Mediterranean climate of Central Chile was established at 3200 cal yr B.P. Paleovegetation and paleolimnological changes starting at 100 cal yr B.P. correlate with documented human activity surrounding the lake. 相似文献
36.
Floodplain soil-paleosol successions are valuable archives for reconstructing Pleistocene-Holocene climate changes but have been relatively unstudied in the southern Appalachian region. Two soil profiles on a small floodplain in southeastern Tennessee, USA were described and sampled in detail using both pedological and geological approaches, including stable carbon isotope analysis of soil organic matter (SOM). Correlation between the 2 profiles was constrained by uncalibrated AMS 14C ages of bulk humates, and using SOM δ13C values, both mobile and immobile elements. Four distinct 2.5-4‰ shifts towards less negative δ13C values for SOM suggest ∼ 300-yr cyclicity and transient warmer and drier climate events, with either water-stressed C3 vegetation or as much as 35% C4 plants present during the mid-Holocene. These postulated multi-episodic drier climate conditions have never before been documented in the southern Appalachian region and are tentatively correlated with mid-Holocene warming and drying in the eastern US, the nearly time-equivalent mid-Holocene events documented in Texas, the US High Plains and in the Gulf of Mexico. High rates of floodplain sediment accumulation (0.5-3 mm/yr), high clay content and maintenance of poorly drained soil conditions favor preservation of high-resolution climate archives in floodplain deposits by inhibiting oxidation and translocation of organic C. 相似文献
37.
The Seasonal Climate and Low Frequency Oscillation in the Simulated Mid-Holocene Megathermal Climate 总被引：7，自引：2，他引：5
Wang Huijun 《大气科学进展》2000,17(3):445-457
l.IntroductionThegoalofPMIP(PaleoclimateModellingIntercomparisonProject)istocomparethestate-ofthe-artclimatemodels'simulationsforthepastclimateonthebasisofthepaleoclimatedata.Oneofthemostinterestingperiodsisthemid-Holoceneduring8.5-3.OkaBPwiththemaximumin7.2-6.OkaBP(beforepresent)inChina(Shietal.,l992).Itwasre-vealedthatthetemPeraturewasl-4'Chigherandtheprecipitationwas4O%-loO%largercomparedtothepresentoverChina(Shietal.,l992;Anetal..l99l;Kongetal.,l99O,l99l).Therehavebeensomesimula… 相似文献
38.
中全新世河南南阳石笋记录的百年季风干旱事件 总被引：3，自引：1，他引：2
基于河南南阳洞1支石笋的5个230 Th 年龄和186个氧碳同位素数据,建立了中全新世持续1 570a( 6 115～ 4 544a B. P. )较高分辨率东亚季风降水序列。在整个研究时段,石笋氧碳同位素曲线记录了3次弱季风事件,分别发生在5 830～5 720a B. P. 、5 350～ 5 080a B. P.、4 770～ 4 640a B. P. ,其中5 350～ 5 080a B. P.时段持续干旱,历时270a。在测年误差范围内,平均分辨率达4～ 5 a 的氧同位素曲线与相应时段树轮14 C残差曲线基本对应。功率谱分析表明: 南阳洞石笋具有显著的130a 周期旋回,与树轮Δ14 C( 132a )以及低纬地区其它石笋记录的周期变化基本一致,说明中全新世东亚季风降水在百年尺度上可能主要受太阳辐射驱动。 相似文献