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121.
Records of stable carbon isotopes (δ13C) are presented from cores collected from four San Francisco Bay marshes and used as a proxy for changes in estuary salinity. The δ13C value of organic marsh sediments are a reflection of the relative proportion of C3 vs. C4 plants occupying the surface, and can thus be used as a proxy for vegetation change on the marsh surface. The four marshes included in this study are located along a natural salinity gradient that exists in the San Francisco Bay, and records of vegetation change at all four sites can be used to infer changes in overall estuary paleosalinity. The δ13C values complement pollen data from the same marsh sites producing a paleoclimate record for the late Holocene period in the San Francisco Bay estuary. The data indicate that there have been periods of higher-than-average salinity in the Bay estuary (reduced fresh water inflow), including 1600-1300 cal yr B.P., 1000-800 cal yr B.P., 300-200 cal yr B.P., and ca. A.D. 1950 to the present. Periods of lower-than-average salinity (increased fresh water inflow) occurred before 2000 cal yr B.P., from 1300 to 1200 cal yr B.P. and ca. 150 cal yr B.P. to A.D. 1950. A comparison of the timing of these events with records from the California coast, watershed, and beyond the larger drainage of the Bay reveals that the paleosalinity variations reflected regional precipitation. 相似文献
122.
青海湖南岸全新世黄土地球化学特征及气候环境意义 总被引:13,自引:6,他引:13
位于青藏高原东北部的青海湖是我国最大的内陆湖泊,通过湖相沉积进行湖区全新世环境演变研究已取得诸多成果。作者选择青海湖南岸黑马河有绝对年代控制的全新世黄土剖面,利用CaCO2、CaO、MgO和Sr元素组分以及氧、碳同位素等较为可靠的地球化学指标,尝试进行了湖区全新世黄土的分层,探讨了黄土堆积过程中的气候环境演变。结果表明,青海湖南岸的黄土按成土作用强度的不同可分为两层:上部为成土化较弱的黄土层;下部为成土化较高的古土壤层,其仍可进一步分为两个亚层。全新世湖区气候环境的波动在地球化学分布特征中有较强表现。 相似文献
123.
中国兴凯湖北岸平原晚全新世花粉记录及泥炭沼泽形成 总被引:7,自引:1,他引:7
通过兴凯湖北岸平原泥炭剖面高分辩率花粉分析研究,对晚全新世花粉划分4个组合带。XKH-4组合时期(1857~1746aB.P.)为沼泽发育前期,这一时期花粉浓度小,陆生草本植物占优势,气候干冷。XKH-3组合时期(1746~1287aB.P.)为沼泽发育早期,这一时期花粉浓度较大,且水生植物花粉含量为剖面最高,喜温落叶阔叶植物大发展时期,气候温和湿润。XKH-2组合时期(1287~602aB.P.)为沼泽发育中期,这一时期花粉浓度最小,但以陆生草本植物为主,木本植物为辅,水生植物急剧减少,针叶植物出现两次高峰,气候向冷干方向发展。XKH-1组合时期(602aB.P.至今)为沼泽发育盛期,这一时期花粉浓度最大,陆生草本植物大发展时期,气候波动较大。 相似文献
124.
F. García-Rodríguez D. Metzeltin P. Sprechmann R. Trettin G. Stams L.F. Beltrán-Morales 《Journal of Paleolimnology》2004,32(2):117-135
Paleolimnological data are presented on trophic development in relation to sea level variation in Rocha Lagoon, a 72 km2 coastal lagoon in southern Uruguay. Using sediment cores that extended to ∼20,000 yr BP, analyses of grain size, organic matter, carbonate, total carbon, nutrients, and diatoms allowed us to infer changes in trophic state and paleosalinities, which were closely related to Holocene relative sea level variation. Higher trophic states were observed during regressive events, most probably due to increases in runoff and erosion as regressions progressed. Diatom Association Zones (DAZ) were identified in both cores. Those DAZ corresponding to transgressive events were dominated by marine/brackish taxa and relatively low organic matter and nutrient values, while those DAZ corresponding to regressive events showed increases in brackish/freshwater diatoms and both organic matter and nutrients. Although the lagoon formed after the first Holocene marine transgression, our data indicate the existence of a marine/brackish aquatic system during upper Pleistocene (i.e., before 15,000 yr BP), but by ∼20,000 yr BP, the system was still likely to be a semi-arid terrestrial system. 相似文献
125.
M. Teresa Ramírez-Herrera Vladimir Kostoglodov Jaime Urrutia-Fucugauchi 《Geomorphology》2004,58(1-4):291-304
This paper presents the preliminary results from a study of Holocene-emerged shorelines, marine notches, and their tectonic implications along the Jalisco coast. The Pacific coast of Jalisco, SW Mexico, is an active tectonic margin. This coast has been the site of two of the largest earthquakes to occur in Mexico this century: the 1932 (Mw 8.2) Jalisco earthquake and the 1995 (Mw 8.0) Colima earthquake. Measurement and preliminary radiocarbon dating of emergent paleoshorelines along the Jalisco coast provide the first constraints upon the timing for tectonic uplift. Along this coastline, uplifted Holocene marine notches and wave-cut platforms occur at elevations ranging from ca. 1 to 4.5 m amsl. In situ intertidal organisms dated with radiocarbon, the first ever reported for the Jalisco area, provide preliminary results that record tectonic uplift during at least the past 1300 years BP at an average rate of about 3 mm/year. We propose a model in which coseismic subsidence produced by offshore earthquakes is rapidly recovered during the postseismic and interseismic periods. The long-term period is characterized by slow tectonic uplift of the Jalisco coast. We found no evidence of coastal interseismic and long-term subsidence along the Jalisco coast. 相似文献
126.
Introduction East Kunlun active fault is one of the largest sinistral slip fault zones in northern Tibetan Pla-teau. The fault tails primarily after the ancient eastern Kunlun suture zone, which was reactivatedby the northward subduction of the Indian plate beneath the Eurasian plate. The western end of thefault starts near the western flank of the Buxedaban peak in Qinghai Province. The fault then ex-tends eastwards through the Kusai Lake, Xidatan, Dongdatan, Alag Lake, Tuosuo Lak… 相似文献
127.
长江三角洲全新世环境演变与人地关系研究综述 总被引:9,自引:1,他引:9
综合分析了近20年来长江三角洲全新世环境演变与人地关系研究的进展,着重对长江三角洲的形成与演变、环境演变与人类活动、人地关系的历史演变、古环境研究方法等进行了综述。指出今后研究的主要内容,包括多种环境替代指标的相互验证,高分辨率研究,定量研究,加强环境质量的时空演变研究,加强综合研究,寻求人地关系协调发展的途径。 相似文献
128.
This paper describes palynological evidence for what appears to be comparatively large-scale human impact in the catchment of the Sungai Niah in the wet tropical lowland swamp forests of Sarawak, Malaysian Borneo close to the Great Cave of Niah. Pollen associated with cleared landscapes and rice cultivation is evident in the sedimentary record from before 6000 cal yr B.P. Human activity seems to have been associated with changes in sedimentary regime, with peat-dominated environments being replaced diachronously by clay-dominated deposition. This may reflect anthropogenic soil erosion in the catchment of the Sungai Niah. 相似文献
129.
The Holocene Period for the province of West New Britain, Papua New Guinea, is characterised by periodic catastrophic volcanism. The region is mantled in dense wet tropical rainforest, and has been occupied by people since the Pleistocene. Analyses of peat from two nearby sites within a lowland rainforest environment provide us with a macro-level landscape account of the periodic destruction and recovery of the coastal forests during seven periods of volcanic activity in the latter part (2900 yr ago to present) of the Holocene. Radiocarbon dating shows the very close correlation of the peat and tephra layers at both sites, yet the pollen analysis reveals different vegetation communities. These initial results allow us to begin identifying the processes of recovery, and to recognise different ecological pressures placed on vegetation at these neighbouring sites. Evidence of hydrological changes are observed beginning with a marine incursion recorded at Garu Site 3 1360 14C yr B.P. The distinct differences in the vegetation re-establishment and community regeneration rates suggest the greater level of disturbance at Garu Site 1 could be related to the depth of the ashfall, although the proximity of a known human settlement may also be a contributing factor. Of note, palynologically, we found that the fern spore flora is particularly rich and believe it will be useful for ecological interpretation. 相似文献
130.
Laurence Le Callonnec Alain Person Maurice Renard René Létolle Nathalie Nebout Leila Ben Khelifa Ivan Rubanov 《Comptes Rendus Geoscience》2005,337(12):1035-1044
The withdrawal of the Aral Sea tributaries (Amu and Syr Daria) for cultures has led to significant falls of its level and an important increase in its salinity. During the Holocene, a succession of low and high water inputs occurred. Silty deposits correspond to the high levels and carbonates to the low levels. This study makes a distinction between the Syr Daria and the Amu Daria water inputs during low-level periods by using mineralogical and chemical compositions of the carbonates deposits. Waters from the Syr Daria are more sulphatic and have a low iron content in comparison with that of the Amu Daria. The Syr Daria was the major tributary around 7500, 4956 and 970 yr?BP, whereas around 6200 and 3610 yr?BP, inflow also from the Amu Daria is observed. To cite this article: L. Le Callonnec et al., C. R. Geoscience 337 (2005). 相似文献