Loss on ignition vs. thermogravimetric analysis: a comparative study to determine organic matter and carbonate content in sediments     

Bensharada  Mohammed  Telford  Richard  Stern  Ben  Gaffney  Vince 《Journal of Paleolimnology》2022,67(2):191-197

Journal of Paleolimnology - Mass loss at specified temperatures has been used widely to determine amounts of organic matter and carbonate in sediment samples. The loss on ignition (LOI) method is...  相似文献   

Persistence of clear-water,shallow-lake ecosystems: the role of protected areas and stable aquatic food webs     

William O. Hobbs  Kevin M. Theissen  Sean M. Hagen  Charles W. Bruchu  Ben C. Czeck  Joy M. Ramstack Hobbs  Kyle D. Zimmer 《Journal of Paleolimnology》2014,51(3):405-420

The roles of both landscape alteration and in-lake processes need to be considered in conservation strategies for shallow lakes in the prairie regions of North America. Here we focus on shallow lakes in west-central Minnesota, USA, highlighting the long-term ecological history and response to known landscape changes of a clear-water, macrophyte-dominated, shallow lake. Contemporary limnological data suggest the aquatic ecosystem has been very stable and fishless for the last ~15 years. Sediment proxies for primary production and ecological change confirm that a stable ecosystem likely prevailed for the last ~200 years. However, sedimentary indicators of catchment erosion detail a distinct response to land-use change during the conversion of native grassland to agricultural land, and following establishment of a protected waterfowl production area (WPA) around the lake. Post-WPA, the rate of sediment accrual decreased dramatically within 5–10 years and sources of organic matter were similar to those of the pre-settlement period. The aquatic ecosystem has been able to withstand nutrient enrichment and allochthonous inputs because stable trophic interactions have likely been in place for more than 200 years. We conclude that lack of hydrologic connectivity and isolated, small catchments are important factors in the promotion of clear-water shallow lake ecosystems, mainly because they prevent colonization by fish and associated ecological consequences. This study highlights the importance of managing both the landscape and in-lake processes to maintain stable, clear-water, shallow lakes.  相似文献   

Dynamics of the coupled human–climate system resulting from closed-loop control of solar geoengineering     

Douglas G. MacMartin  Ben Kravitz  David W. Keith  Andrew Jarvis 《Climate Dynamics》2014,43(1-2):243-258

If solar radiation management (SRM) were ever implemented, feedback of the observed climate state might be used to adjust the radiative forcing of SRM in order to compensate for uncertainty in either the forcing or the climate response. Feedback might also compensate for unexpected changes in the system, e.g. a nonlinear change in climate sensitivity. However, in addition to the intended response to greenhouse-gas induced changes, the use of feedback would also result in a geoengineering response to natural climate variability. We use a box-diffusion dynamic model of the climate system to understand how changing the properties of the feedback control affect the emergent dynamics of this coupled human–climate system, and evaluate these predictions using the HadCM3L general circulation model. In particular, some amplification of natural variability is unavoidable; any time delay (e.g., to average out natural variability, or due to decision-making) exacerbates this amplification, with oscillatory behavior possible if there is a desire for rapid correction (high feedback gain). This is a challenge for policy as a delayed response is needed for decision making. Conversely, the need for feedback to compensate for uncertainty, combined with a desire to avoid excessive amplification of natural variability, results in a limit on how rapidly SRM could respond to changes in the observed state of the climate system.  相似文献   

Metamorphic re-equilibration and metasomatism of highly chromian,garnet-rich peridotitic xenoliths from South African kimberlites     

Tim?J.?IvanicEmail author  Ben?Harte  John?J.?Gurney 《Contributions to Mineralogy and Petrology》2012,164(3):505-520

A history of decompression and metasomatism is preserved in a suite of highly chromian, garnet-rich peridotitic xenoliths from the diamondiferous Newlands and Bobbejaan kimberlites, South Africa. A high proportion of the garnets and chromites in these rocks plot in the diamond-facies fields on Cr₂O₃–CaO and Cr₂O₃–MgO wt% plots, and Cr-rich compositions are found in both the harzburgitic and lherzolitic fields. Petrographic evidence suggests that the earliest known mineralogies were those of olivine-bearing, garnet-rich rocks. These were modified by a decompression event that caused recrystallization of garnets and led to orientated spinel and pyroxene inclusions in garnet. Chemical zonation within garnet is divided into (1) external re-equilibration between garnet and matrix; (2) internal re-equilibration between garnet and its inclusions; and (3) metasomatically induced zoning between garnet core and a metasomatic rim. The compositional trajectories associated with zonations (1) and (2) in Ca–Cr plots may be closely modelled by means of sliding, garnet–spinel transition reactions whose slopes vary with bulk Ca composition; at intermediate Ca compositions, the trajectories closely match the slope of the lherzolite line or harzburgite/lherzolite boundary. The decreasing Cr/(Cr + Al) of the garnet in these zonations is in agreement with the evidence for decompression given by the petrographic recrystallization features, and overall decompression of probably 10–20 kb is indicated. We speculate on the age of these events, and consider the possibility of their association with major orogenic events documented by South African crustal rocks at 2.9–2.7 Ga, and events evidenced by peridotite-xenolith Re–Os model ages at 2.8–2.7 Ga.  相似文献   

Spatial patterns of soil δC reveal grassland-to-woodland successional processes     

Edith Bai  Thomas W. BouttonFeng Liu  X. Ben WuSteven R. Archer 《Organic Geochemistry》2012,42(12):1512-1518

Many grasslands and savannas around the world have experienced woody plant encroachment in recent history. In the Rio Grande Plains of southern Texas, subtropical woodlands dominated by C₃ trees and shrubs have become significant components of landscapes once dominated almost exclusively by C₄ grasslands. In this study, spatial variation of soil δ¹³C to was used to investigate patterns of transformation. Previous research has shown that grassland-to-shrubland transitions are initiated when discrete, multi-species shrub clusters organized around a honey mesquite (Prosopis glandulosa) tree nucleus established in grassland. It is inferred from space-for-time substitution and modeling studies that as new shrub clusters are initiated and existing clusters enlarge, coalescence will occur, leading to the formation of groves; and that groves will eventually merge to form woodlands. The hypothesis that present-day mesquite groves represent areas where individual discrete shrub clusters have proliferated and coalesced was evaluated by comparing patterns of soil δ¹³C within isolated shrub clusters (n = 6) to those in nearby groves (n = 3). Mean soil δ¹³C within discrete clusters was lowest in the center (−23.3‰), increased exponentially toward the dripline (−20.1‰), and stabilized at a relatively high value approximately 15 cm beyond the dripline (−18.9‰). The spatial structure of soil δ¹³C in groves was consistent with that which would be expected to occur if present-day grove communities were a collection of what once were individual discrete clusters that had fused. As such, it provides direct evidence in support of conceptual and mathematical models derived from indirect assessments. However, spatial patterns of soil δ¹³C suggest that groves are not simply a collection of clusters with respect to primary production and SOC turnover. This study illustrates how soil δ¹³C values can be used to reconstruct successional processes accompanying vegetation compositional change, and its consequences for ecosystem function.  相似文献   

Straddling the tholeiitic/calc-alkaline transition: the effects of modest amounts of water on magmatic differentiation at Newberry Volcano,Oregon     

Ben?E.?MandlerEmail author  Julie?M.?Donnelly-Nolan  Timothy?L.?Grove 《Contributions to Mineralogy and Petrology》2014,168(4):1066

Melting experiments have been performed at 1 bar (anhydrous) and 1- and 2-kbar H₂O-saturated conditions to study the effect of water on the differentiation of a basaltic andesite. The starting material was a mafic pumice from the compositionally zoned tuff deposited during the ~75 ka caldera-forming eruption of Newberry Volcano, a rear-arc volcanic center in the central Oregon Cascades. Pumices in the tuff of Newberry caldera (TNC) span a continuous silica range from 53 to 74 wt% and feature an unusually high-Na₂O content of 6.5 wt% at 67 wt% SiO₂. This wide range of magmatic compositions erupted in a single event makes the TNC an excellent natural laboratory in which to study the conditions of magmatic differentiation. Our experimental results and mineral–melt hygrometers/thermometers yield similar estimates of pre-eruptive H₂O contents and temperatures of the TNC liquids. The most primitive (mafic) basaltic andesites record a pre-eruptive H₂O content of 1.5 wt% and a liquidus temperature of 1,060–1,070 °C at upper crustal pressure. This modest H₂O content produces a distinctive fractionation trend that is much more enriched in Na, Fe, and Ti than the calc-alkaline trend typical of wetter arc magmas, but slightly less enriched in Fe and Ti than the tholeiitic trend of dry magmas. Modest H₂O contents might be expected at Newberry Volcano given its location in the Cascade rear arc, and the same fractionation trend is also observed in the rim andesites of the rear-arc Medicine Lake volcano in the southern Cascades. However, the Na–Fe–Ti enrichment characteristic of modest H₂O (1–2 wt%) is also observed to the west of Newberry in magmas erupted from the arc axis, such as the Shevlin Park Tuff and several lava flows from the Three Sisters. This shows that modest-H₂O magmas are being generated directly beneath the arc axis as well as in the rear arc. Because liquid lines of descent are particularly sensitive to water content in the range of 0–3 wt% H₂O, they provide a quantitative and reliable tool for precisely determining pre-eruptive H₂O content using major-element data from pumices or lava flows. Coupled enrichment in Na, Fe, and Ti relative to the calc-alkaline trend is a general feature of fractional crystallization in the presence of modest amounts of H₂O, which may be used to look for “damp” fractionation sequences elsewhere.  相似文献   

Hydrogeological and mixing process of waters in deep aquifers in arid regions: south east Tunisia   总被引：1，自引：0，他引：1  

Jalila Makni  Fatma Ben Brahim  Saoussan Hassine  Salem Bouri  Hamed Ben Dhia 《Arabian Journal of Geosciences》2014,7(2):799-809

This study addresses the hydrogeochemistry of thermal and cold waters from south east Tunisia. Temperature intervals are 38.5–68 °C and 22–27.8 °C for thermal water and cold water, respectively. Three distinct hydrogeological systems supply water either for irrigation or for drinking; they are: (1) the Continental Intercalaire geothermal aquifer (CI), (2) the Turonian aquifer and (3) the Senonian aquifer. A synthetic study including hydrochemical, hydrogeological and geothermal approaches have been applied in order to evaluate the inter-aquifers water transfer in south east of Tunisia. By using silica geothermometers and saturation indices for different solid phases, estimated thermal reservoir temperature varies between 52 and 87 °C and between 75 and 110 °C, respectively. Based on chemical and thermal data, mixing, which occurs between the ascending deep geothermal water and shallow cold water, is about 57 % cold water.  相似文献   

First microbialites associated to organic-rich facies of the Oceanic Anoxic Event 2 (Northern Tunisia,Cenomanian–Turonian transition)     

Mohsen Layeb  Moez Ben Fadhel  Yosra Layeb-Tounsi  Mohamed Ben Youssef 《Arabian Journal of Geosciences》2014,7(8):3349-3363

From northern Tunisia, small-scale well-preserved microbialites, contemporaneous to the global oceanic anoxic event 2 (OAE 2) are first reported on the southern Tethyan Margin. These microbialites are encased within the pelagic organic-rich black shales of the Bahloul Formation (Cenomanian–Turonian transition). Biostratigraphic, petrographic, and geochemical investigations carried out to constrain their biogenicity and genesis character led to consider them as thrombolites and stromatolites occurring in lenticular bioherms/biostromes and columnar bodies co-relatable to the global ‘filament event’ of the authors, close to the base of the Watinoceras ammonite zone. Abundant clotted micrite, cyanobacterial filaments, and algal tissues point to the key process of microbial carbonate precipitation and to a major role played by microbes in the stabilisation and subsequent lithification, which in turn favoured the preservation of the original structure of the microbialites. These microbially induced carbonate formations are considered as favoured by chemosynthetic fauna of bivalve molluscs and lithistid sponges which were able to host symbiotic microbial communities. The latter contributed to the precipitation of authigenic calcite and non-carbonate minerals (e.g. pyrite) fuelled by microbial activity under sulphate-reducing conditions. The carbonate body onset is considered to be initiated by seafloor instability due to syndepositional fault acting that induced the appraisal of uplifted tilted blocks within oxygenated waters but near the anoxic water masses. Generated depressions allowed the development of chemosynthetic-based communities. Deep faults related to Triassic salt domes acted as conducts for hydrocarbon and salt brine expulsion to the seafloor and the microbialite growth was enhanced by an abrupt uprising sea level under warmer conditions.  相似文献   

贵州晚三叠世卡尼期周氏黔鱼龙(Qianichthyosaurus zhoui)性双形研究          下载免费PDF全文

何犇  马知恒  付丽红 《贵州地质》2023,40(3):249-258, 320

本文对产于贵州兴义乌沙地区上三叠统卡尼阶法郎组瓦窑段中的17件鱼龙标本进行研究,认为本次研究标本均为Qianichthyosaurus zhoui。通过对其肢骨的定性及定量分析认为,Qianichthyosaurus zhoui在后肢形态上存在性双形现象。暨雄性后肢外形窄长,雌性后肢则相对宽短;雄性的股骨比肱骨长度略长,而雌性的股骨与肱骨长度相当;雄性股骨近端和远端相较于雌性更为膨大股骨远端宽度为中轴宽度的253倍,股骨长与股骨中轴的比值较大,通常大于35;雌性股骨近端和远端相较于雄性较小股骨远端宽度一般为中轴宽度的2倍左右,股骨长与股骨中轴的比例较小,比值约为3左右雌性第一跖骨面积较小且具有凹缺,雄性第一跖骨面积较大不具凹缺。  相似文献   

基于Landsat-8的城市建成区生态环境质量研究     

郭豫宾  张船红  范本 《测绘与空间地理信息》2022,(2):29-33,36

城市化与人类生存环境之间的矛盾日益突出,准确、快速、定量的监测城市生态环境质量已成为客观了解城市生态状况的一个研究重点.利用遥感影像反演土地覆被指数、地表温度、地表裸露度和湿度等自然生态参数,构建城市生态环境质量评价的指数体系,并将之运用于德阳市城市建成区生态环境定量评价.在此基础上,利用均值-标准差对研究区生态环境进...  相似文献