Holocone paleovegetation and paleohydrology of a prairie pothold in southern Saskatchewan, Canada     

Catherine H. Yansa 《Journal of Paleolimnology》1998,19(4):429-441

The Andrews site represents one of countless prairie potholes found in areas of hummocky moraine on the northern Great Plains. Sediments from a depth of 5.8 to 3.1 m at this 'kettle-fill' site in the Missouri Coteau upland of southern Saskatchewan, Canada, provides a record of vegetation, climate, and hydrologic changes within a small, ca 30 m diameter, closed-drainage basin from ca 10.2 to 5.8 ka. Plant macrofossil analyses of 67 samples, 6 ¹⁴C ages, and stratigraphy were used to identify 5 zones, representing the paleohydrological changes that followed deglaciation in southern Saskatchewan.Results of this study indicate that with the melting of residual stagnant ice a pond (>2 m deep) with abundant aquatic, emergent, and shoreline plants developed in the basin at ca 10.2 ka and persisted until at least ca 8.8 ka. During this time there was a shift in upland vegetation from a white spruce forest (Zone II) to a deciduous parkland at ca 10 ka (Zone III). As climate warmed, brackish and alkaline conditions developed coincident with shallowing of the pond at the end of Zone III. The perennial water phase ended at ca 8.8 ka and was followed by a low-water stand lasting ca 1100 years. Prairie fires and slopewash from unstable slopes were dominant sedimentological processes until ca 7.7 ka (Zone IV). Water levels began to rise and between ca 7.7 and 5.8 ka a semi-permanent pond was established in a grassland setting (Zone V). After ca 5.8 ka this prairie pothole wetland became ephemeral, to the point that plant macrofossils could not be preserved, a situation continuing today. Interactions between climate change, variability in local groundwater supply, and sedimentological processes likely account for the paleohydrologic events reconstructed at the Andrews site.  相似文献   

Accelerator mass spectrometry radiocarbon dating of 1994 Lake Winnipeg cores   总被引：1，自引：1，他引：0  

R. E. Vance  A. M. Telka 《Journal of Paleolimnology》1998,19(3):329-334

Conventional radiocarbon dating of Lake Winnipeg core samples has produced erroneously old ages due to the incorporation of pre-Quaternary carbon derived from carbonaceous rocks, soils and sediments in the watershed, as well as a hard-water effect resulting from leaching of calcareous rocks and soils. To circumvent these problems and develop a reliable chronology for the Lake Winnipeg core series, a total of 64 samples from the Lake Winnipeg core series were processed to isolate well preserved macrofossils suitable for Accelerator Mass Spectrometry (AMS) radiocarbon dating. Here we report six radiocarbon ages derived from plant macrofossils and ostracodes, and reconstruct aspects of the depositional environment of each sample based on the associated macrofossil assemblage.  相似文献   

早白垩世Ruffordia goepperti的原位孢子及与分散孢子的关系   总被引：2，自引：0，他引：2  

邓胜徽 《地质论评》1998,44(3):243-248

海金沙特的Ｒｕｆｆｏｒｄｉａ ｇｏｅｐｐｅｒｔｉ（Ｄｕｎｋｅｒ）Ｓｅｗａｒｄ是世界上早白垩世极为重要的蕨类植物。通过对比发现,Ｒｕｆｆｏｒｄｉａ ｇｏｅｐｐｅｒｔｉ原位孢子的形态和细微纹饰与地层中分散孢子Ｃｉｃａｔｒｉｃｏｓｉｓｐｏｒｉｔｅｓ ａｕｓｔｒａｌｉｅｎｓｉｓ（Ｃｏｏｋｓｏｎ）Ｐｏｔｏｎｉｅ和Ｃ．ｍｉｎｏｒ（Ｂｏｌｃｈ．）Ｐｏｃｏｃｋ一致,而且大化石与分散孢子的地史、地理分布范围也相同,  相似文献   

A model that helps explain Sr-isotope disequilibrium between feldspar phenocrysts and melt in large-volume silicic magma systems     

Wendell A. Duffield  Joaquin Ruiz   《Journal of Volcanology and Geothermal Research》1998,87(1-4)

Feldspar phenocrysts of silicic volcanic rocks are commonly in Sr-isotopic disequilibrium with groundmass. In some cases the feldspar is more radiogenic, and in others it is less radiogenic. Several explanations have been published previously, but none of these is able to accommodate both senses of disequilibrium. We present a model by which either more- or less-radiogenic feldspar (or even both within a single eruptive unit) can originate. The model requires a magma body open to interaction with biotite- and feldspar-bearing wall rock. Magma is incrementally contaminated as wall rock melts incongruently. Biotite preferentially melts first, followed by feldspar. Such melting behavior, which is supported by both field and experimental studies, first contaminates magma with a relatively radiogenic addition, followed by a less-radiogenic addition. Feldspar phenocrysts lag behind melt (groundmass of volcanic rock) in incorporating the influx of contaminant, thus resulting in Sr-isotopic disequilibrium between the crystals and melt. The sense of disequilibrium recorded in a volcanic rock depends on when eruption quenches the contamination process. This model is testable by isotopic fingerprinting of individual feldspar crystals. For a given set of geologic boundary conditions, specific core-to-rim Sr-isotopic profiles are expectable. Moreover, phenocrysts that nucleate at different times during the contamination process should record different and predictable parts of the history. Initial results of Sr-isotopic fingerprinting of sanidine phenocrysts from the Taylor Creek Rhyolite are consistent with the model. More tests of the model are desirable.  相似文献   

Volcanic plume simulation on large scales     

Josef M. Oberhuber  Michael Herzog  Hans-F. Graf  Karsten Schwanke 《Journal of Volcanology and Geothermal Research》1998,87(1-4)

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The role of magma composition and water content in explosive eruptions: 1. Conduit ascent dynamics     

Paolo Papale  Augusto Neri  Giovanni Macedonio   《Journal of Volcanology and Geothermal Research》1998,87(1-4)

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Chromite in the Paricutin lava flows (1943–1952)     

Victoria Bannister  Peter Roeder  Alexei Poustovetov 《Journal of Volcanology and Geothermal Research》1998,87(1-4)

Small euhedral chromite crystals are found in olivine macrophenocrysts (Fo_80–84) from the basaltic andesites (150 ppm Cr) erupted in 1943–1947, and in orthopyroxene macrophenocrysts of the andesites (75 ppm Cr) erupted in 1947–1952. The majority of the chromite octahedra are 5–20 μm in diameter, and some are found in clusters and linear chains of three or more oriented chromite crystals. The composition of the majority of the chromite grains within olivine and orthopyroxene macrophenocrysts is Fe²⁺/(Fe²⁺+Mg)=0.5–0.6, Cr/(Cr+Al)=0.5–0.6 and Fe³⁺/(Fe³⁺+Al+Cr)=0.2–0.3. The chromite crystals in contact with the groundmass are larger, subhedral, and grade in composition from chromite cores to magnetite rims. Comparison of the composition of chromite with those of other volcanic rocks shows that the most primitive Paricutin chromite is richer in total iron and higher in Fe³⁺/(Fe³⁺+Al+Cr) than primary chromite in most lavas. The linear chains of oriented chromite octahedra are found in olivine and orthopyroxene macrophenocrysts, and in the groundmass. These chromite chains are thought to result from diffusion-controlled crystallization because of the very high partition coefficient (1000) of Cr between chromite and melt. We conclude that chromite was a primary phase in the lavas at the time of extrusion and that magnetite only crystallized after extrusion during cooling of the lava flows. The presence of chromite microphenocrysts in andesitic lavas containing as little as 70 ppm Cr can be explained by dissolved H₂O in the melt depressing the liquidus temperature for orthopyroxene such that chromite becomes a liquidus phase. The influence of dissolved H₂O can also explain the lack of plagioclase macrophenocrysts in most of the lavas and the relatively high partition coefficient (20) of Ni between olivine and melt and the high partition coefficient (40) of Cr between orthopyroxene and melt. The liquidus temperature of the basaltic andesite is estimated to have been less than 1140°C, assuming H₂O>1 wt.%, and the log f_O2 to have been above that of the QFM buffer. The chromite and orthopyroxene liquidus temperature of the andesites, assuming H₂O>1 wt.%, is estimated to have been 1100°C or less. The derivation of the later andesites from the earlier basaltic andesites has been explained by a combination of fractional crystallization of olivine, orthopyroxene and plagioclase, and assimilation of xenoliths. The significantly lower Cr, Ni and Mg of the andesites may have been in part due to the separation of olivine macrophenocrysts plus enclosed chromite crystals from the earlier basaltic andesites.  相似文献   

Topographically moderated soil water seasons impact vegetation dynamics in semiarid mountain catchments: Illustrations from the Dry Creek Experimental Watershed,Idaho, USA     

Michael J. Poulos  Toni J. Smith  Shawn G. Benner  Jennifer L. Pierce  Alejandro N. Flores  Mark S. Seyfried  James P. McNamara 《水文研究》2021,35(12):e14421

Water stored in soils, in part, controls vegetation productivity and the duration of growing seasons in wildland ecosystems. Soil water is the dynamic product of precipitation, evapotranspiration and soil properties, all of which vary across complex terrain making it challenging to decipher the specific controls that soil water has on growing season dynamics. We assess how soil water use by plants varies across elevations and aspects in the Dry Creek Experimental Watershed in southwest Idaho, USA, a mountainous, semiarid catchment that spans low elevation rain to high elevation snow regimes. We compare trends in soil water and soil temperature with corresponding trends in insolation, precipitation and vegetation productivity, and we observe trends in the timing, rate and duration of soil water extraction by plants across ranges in elevation and aspect. The initiation of growth-supporting conditions, indicated by soil warming, occurs 58 days earlier at lower, compared with higher, elevations. However, growth-supporting conditions also end earlier at lower elevations due to the onset of soil water depletion 29 days earlier than at higher elevations. A corresponding shift in peak NDVI timing occurs 61 days earlier at lower elevations. Differences in timing also occur with aspect, with most threshold timings varying by 14–30 days for paired north- and south-facing sites at similar elevations. While net primary productivity nearly doubles at higher elevations, the duration of the warm-wet period of active water use does not vary systematically with elevation. Instead, the greater ecosystem productivity is related to increased soil water storage capacity, which supports faster soil water use and growth rates near the summer solstice and peak insolation. Larger soil water storage does not appear to extend the duration of the growing season, but rather supports higher growing season intensity when wet-warm soil conditions align with high insolation. These observations highlight the influence of soil water storage capacity in dictating ecological function in these semiarid steppe climatic regimes.  相似文献   

盐度调控对花鲈(Lateolabrax maculatus)生长相关基因表达的影响     

张沛  迟美丽  温海深  钱焜  倪蒙  张亚晨  黄政举  宋志飞  柴森浩 《海洋与湖沼》2015,46(2):446-453

生长激素受体(GHR)作为GH/IGF轴的中心环节,在内分泌调控中发挥重要作用。本实验采用c DNA末端快速扩增法(RACE)技术克隆出花鲈GHR1和GHR2的c DNA全长序列。急性低盐度调控实验设为海水组、半海水组和淡水组。测定了急性低盐度调控24、48、96、144、192h后,花鲈肝脏中GHRs、IGF-1及垂体GH表达情况。结果表明,GHR1 c DNA全长序列2436bp,编码637个氨基酸;GHR2 c DNA全长序列2940bp,编码582个氨基酸。GHR1与GHR2由信号肽、胞外区、跨膜区、胞内区组成,且结构存在差异。脑、肾、鳃中GHR1表达明显高于GHR2;而在肌肉、垂体、肝脏、盲肠、胸腺、心脏中,GHR2表达明显高于GHR1。24h时,各组GHR1表达不变,GHR2、GH、IGF-1显著下降。之后,相对于海水组,淡水组和半海水组GHRs和IGF-1表达升高,而GH下降,GH与GHR负相关。据结果推测,花鲈GHR2可能为SL受体,GH/IGF轴参与低渗调控可能是通过增加GHRs,进而激活下游IGF-1表达而实现。  相似文献   

脊尾白虾蜕皮抑制激素基因全长cDNA的克隆及其对环境胁迫的响应     

张美  刘萍  李吉涛  李健 《海洋与湖沼》2015,46(4):764-775

为研究脊尾白虾蜕皮抑制激素基因(MIH)在环境适应中的作用,本文采用RACE技术从蜕皮间期脊尾白虾眼柄中克隆获得MIH-like基因全长c DNA序列,命名为Ec MIH。该基因全长1218bp,包括360bp的开放阅读框,420bp的5'端非编码区和394bp的3'端非编码区;开放阅读框编码120个氨基酸,包括41个氨基酸组成的信号肽和79个氨基酸组成的成熟肽,预测蛋白分子量为13.71k Da,理论等电点p I为8.02。同源性比较分析发现,脊尾白虾MIH-like基因与日本沼虾和罗氏沼虾同源性最高,分别为87%和86%。荧光定量PCR分析结果表明,脊尾白虾MIH-like基因在眼柄中的表达量最高,在卵巢中的表达量最少,而在肝脏、肌肉、鳃和血细胞中不表达。环境胁迫后该基因具有明显的时序性。p H6.5胁迫后24h和48h,眼柄和腹神经节中MIH-like基因的表达量较对照组均显著增加(P0.05);4mg/L氨氮浓度组胁迫后12h和24h,眼柄和腹神经节中MIH-like基因的表达量较对照组均显著增加(P0.05);盐度39胁迫后12h和24h,眼柄和腹神经节中MIH-like基因的表达量较对照组均显著增加(P0.05)。本研究结果表明脊尾白虾mih基因参与了环境胁迫后的机体应激反应。  相似文献