Response of Plant Community Carbon and Nitrogen Stoichiometry to Experimental Warming on the Qinghai-Tibet Plateau

Low temperature is an important limiting factor for alpine ecosystems on the Tibetan Plateau. This study is based on data from on-site experimental warming platforms (open top chambers, OTC) at three elevations (4300 m, 4500 m, 4700 m) on the Qinghai-Tibet Plateau. The carbon and nitrogen stoichiometry characteristics of plant communities, both above-ground and below-ground, were observed in three alpine meadow ecosystems in August and September of 2011 and August of 2012. Experimental warming significantly increased above-ground nitrogen content by 21.4% in September 2011 at 4500 m, and reduced above-ground carbon content by 3.9% in August 2012 at 4300 m. Experimental warming significantly increased below-ground carbon content by 5.5% in August 2011 at 4500 m, and the below-ground ratio of carbon to nitrogen by 28.0% in September 2011 at 4300 m, but reduced below-ground nitrogen content by 15.7% in September 2011 at 4700 m, below-ground carbon content by 34.3% in August 2012 at 4700 m, and the below-ground ratio of carbon to nitrogen by 37.9% in August 2012 at 4700 m. Experimental warming had no significant effect on the characteristics of community carbon and nitrogen stoichiometry under other conditions. Therefore, experimental warming had inconsistent effects on the carbon and nitrogen stoichiometry of plant communities at different elevations and during different months. Soil ammonium nitrogen and nitrate nitrogen content were the main factors affecting plant community carbon and nitrogen stoichiometry.     

Calibration and use of continuous heat-type automated seepage meters for submarine groundwater discharge measurements

Submarine groundwater discharge (SGD) assessments were conducted both in the laboratory and at a field site in the northeastern Gulf of Mexico, using a continuous heat-type automated seepage meter (seepmeter). The functioning of the seepmeter is based on measurements of a temperature gradient in the water between downstream and upstream positions in its flow pipe. The device has the potential of providing long-term, high-resolution measurements of SGD. Using a simple inexpensive laboratory set-up, we have shown that connecting an extension cable to the seepmeter has a negligible effect on its measuring capability. Similarly, the observed influence of very low temperature (≤3 °C) on seepmeter measurements can be accounted for by conducting calibrations at such temperatures prior to field deployments. Compared to manual volumetric measurements, calibration experiments showed that at higher water flow rates (>28 cm day⁻¹ or cm³ cm⁻² day⁻¹) an analog flowmeter overestimated flow rates by ≥7%. This was apparently due to flow resistance, turbulence and formation of air bubbles in the seepmeter water flow tubes. Salinity had no significant effect on the performance of the seepmeter. Calibration results from fresh water and sea water showed close agreement at a 95% confidence level significance between the data sets from the two media (R² = 0.98). Comparatively, the seepmeter SGD measurements provided data that are comparable to manually-operated seepage meters, the radon geochemical tracer approach, and an electromagnetic (EM) seepage meter.     

Injection and arrest of dykes: implications for volcanic hazards

Dykes are the principal channels through which magma reaches the surface in volcanic eruptions. For this reason dykes observed in the field are commonly assumed to be feeders to lava flows. The actual proportion of dykes reaching the surface is, however, poorly known. In order to develop models for the purpose of estimating volcanic hazard, this proportion must be known. This follows because such models should not only consider the probability of dykes being injected from magma chambers during periods of unrest in the associated volcanoes, but also the probability of the injected dykes being arrested. This paper presents field data on several thousand dykes from Iceland and Tenerife (Canary Islands) indicating that many, and probably most, dykes become arrested at various crustal levels and never reach the surface to feed eruptions. Using the results of analytical and numerical models, it is shown that, for common loading conditions, the stress field in the vicinity of a magma chamber may favour the injection and propagation of dykes while the stress field at a certain distance from the chamber favours dyke arrest. This means that many dykes that are injected from the chamber propagate only for a very limited distance from the chamber to the point where they become arrested. The implication is that during periods of unrest in volcanoes, the probability of volcanic eruption is only a small fraction of the probability of dyke injection from the source magma chamber.     

河北武安坦岭多斑斜长斑岩中基质矿物特征及其研究意义

河北武安坦岭斜长斑岩具有多斑斑状结构,基质为显微晶质结构。岩相学观察表明,斜长石斑晶有一个宽广的核部和一个宽度可变的条纹长石反应边,个别核部包含有角闪石、黑云母等矿物。基质矿物主要由蓝透闪石、条纹长石(An0Ab8.4Or91.5～An0.1Ab57.3Or42.6)、石英、钾长石(An0.3Ab5.9Or93.7～An0.3Ab4.7Or95.2)、钠长石(An0.2Ab98.3Or1.5～An0.1Ab99.2Or0.7)、磁铁矿、赤铁矿、钛铁矿、磷灰石、榍石和锆石等11种矿物组成。角闪石温压计计算结果得出,基质角闪石核部的结晶压力高于边部,核部为34.05 MPa,对应的结晶温度为660.35℃,结晶深度为1.29km;边部的结晶压力为24.32MPa,结晶温度为598.32℃,结晶深度为0.92km;而斜长石斑晶中的角闪石形成时压力为159.51～178.19MPa,温度为817.68～819.79℃,对应的形成深度为6.03～6.73km。基质角闪石在Al2O3-TiO2图上落在壳源区,而斑晶中的角闪石和黑云母都落在壳幔混合区。斜长石、条纹长石、磁铁矿和磷灰石的微量和稀土元素测试数据显示,其都具有相对富集LILE、亏损HFSE的特点,暗示了基质矿物的形成有流体参与。ICT三维扫描结果显示,斜长斑岩基质中的孔隙体积含量约为3.428%,铁质体积含量为4.371%,且铁氧化物和孔隙具弱连通性。通过讨论分析,笔者得出:(1)坦岭斜长斑岩中斜长石斑晶具有明显的交代结构,且晶体本身没有明显熔蚀现象,这些特征表明大量的斜长石斑晶快速上升,即"冻结岩浆房"的活化机制与流体密切相关;(2)斜长斑岩中基质矿物有十一种,且矿物类型复杂,不符合平衡系统矿物相律,应属于流体晶矿物组合;(3)坦岭斜长斑岩的基质"岩浆"可能是一种富Fe、K、P、Si、Na等元素的熔体-流体流;(4)多斑斜长斑岩的形成经历了(1)深度6～7km的深部岩浆房形成斜长石堆晶→(2)富Fe、K、P、Si、Na等元素的熔体-流体流加入深部岩浆房,冻结岩浆房活化→(3)由于流体超压,含大量斜长石斑晶的熔体-流体在地壳浅部(0.9～1.2km)呈小岩株状或岩脉状就位。多斑斜长斑岩为深部找矿提供了有力的线索。     

Effects of elevated air temperatures on soil thermal and hydrologic processes in the active layer in an alpine meadow ecosystem of the Qinghai-Tibet Plateau

In this study,effects of elevated air temperatures on thermal and hydrologic process of the shallow soil in the active layer were investigated. Open-top chambers(OTCs)were utilized to increase air temperatures 1-2℃ in OTC-1 and 3-5℃ in OTC-2 in the alpine meadow ecosystem on the Qinghai- Tibetan Plateau.Results show that the annual air temperatures under OTC-1 and OTC-2 were 1.21℃ and 3.62℃ higher than the Control,respectively.The entirely-frozen period of shallow soil in the active layer was shortened and the fully thawed period was prolonged with temperature increase.The maximum penetration depth and duration of the negative isotherm during the entirely-frozen period decreased, and soil freezing was retarded in the local scope of the soil profile when temperature increased.Meanwhile, the positive isotherm during the fully-thawed period increased,and the soil thawing was accelerated.Soil moisture under different manipulations decreased with the temperature increase at the same depth. During the early freezing period and the early fully- thawed period,the maximum soil moisture under the Control manipulation was at 0.2 m deep,whereas under OTC-1 and OTC-2 manipulations,the maximum soil moisture were at 0.4-0.5 m deep. These results indicate that elevated temperatures led to a decrease of the moisture in the surface soil.The coupled relationship between soil temperature and moisture was significantly affected by the temperature increase.During the freezing and thawing processes, the soil temperature and moisture under different manipulations fit the regression model given by the equationθV=a/{1+exp[b(TS+c)]}+d.     

Subduction influence on magma supply at the East Scotia Ridge

Despite a spreading rate of 65–70 km Ma⁻¹, the East Scotia Ridge has, along most of its length, a form typically associated with slower rates of sea floor spreading. This may be a consequence of cooler than normal mantle upwelling, which could be a feature of back-arc spreading. At the northern end of the ridge, recently acquired sonar data show a complex, rapidly evolving pattern of extension within 100 km of the South Sandwich Trench. New ridge segments appear to be nucleating at or near the boundary between the South American and Scotia Sea plates and propagating southwards, supplanting older segments. The most prominent of these, north of 56°30′S, has been propagating at a rate of approximately 60 km Ma⁻¹ for at least 1 Ma, and displays a morphology unique on this plate boundary. A 40 km long axial high exists at the centre of this segment, forming one of the shallowest sections of the East Scotia Ridge. Beneath it, seismic reflection profiles reveal an axial magma chamber, or AMC, reflector, similar to those observed beneath the East Pacific Rise and Valu Fa Ridge. Simple calculations indicate the existence here of a narrow (<1 km wide) body of melt at a depth of approximately 3 km beneath the sea floor. From the topographic and seismic data, we deduce that a localised mantle melting anomaly lies beneath this segment. Rates of spreading in the east Scotia Sea show little variation along axis. Hence, the changes in melt supply are related to the unique tectonic setting, in which the South American plate is tearing to the east, perhaps allowing mantle flow around the end of the subducting slab. Volatiles released from the torn plate edge and entrained in the flow are a potential cause of the anomalous melting observed. A southward mantle flow may have existed beneath the axis of the East Scotia Ridge throughout its history.     

Rates and Timescales of Fractional Crystallization from 238U-230Th-226Ra Disequilibria in Trachyte Lavas from Longonot Volcano, Kenya

A suite of peralkaline trachytes from Longonot volcano, Kenya,which erupted during the last 6000 years, has been analysedfor major and trace elements, Pb and Nd isotopes, and U–Th–Radisequilibria. The lavas are divided into three stratigraphicgroups of trachytes (Lt²a, Lt²b and Lt³), and hybrid lavas,designated LMx¹ and LMx², which, respectively, pre-date andpost-date the Lt² lavas. Major and trace elements are consistent,with up to 37% within-group fractional crystallization of predominantlyalkali feldspar. The parental magma for the different trachytegroups had a more mafic composition—probably hawaiitic.Nd and Pb isotopes show minimal variation, both within and betweenmagma groups, and indicate that up to 10% comendite magma fromthe neighbouring Olkaria volcanic field may have intermixedwith the Longonot magma. (²³⁰Th/²³⁸U) disequilibria indicatethat limited U/Th fractionation occurred during the past 10kyr, whereas (²²⁶Ra/²³⁰Th) disequilibria reflect the effectof alkali feldspar fractionation >8 kyr ago in the Lt²a lavas,between 3 and 7 kyr ago in the Lt²b lavas and in the past 3kyr for the Lt³ lavas. (²²⁶Ra/²³⁰Th) disequilibria in the Lt²blavas are interpreted using a model that combines the equationsof radioactive decay and in-growth with Rayleigh crystallizationto give fractionation rates of about 0·2 x 10^–4/yearfor the evolution of hawaiite to trachyte, but more rapid ratesof up to 3 x 10^–4/year for fractionation within the trachytesequence. (²²⁶Ra/²³⁰Th) from two whole-rock–alkali feldsparpairs are interpreted to show the crystals formed at 5800 yearsBP (Lt²b) and 2800 years BP (Lt³), implying that phenocrystformation continued almost up to the time of eruption. The resultsstrongly indicate that fractionated magmas can be stored forperiods on the order of 1000–2500 years prior to eruption,whereas other magmas were erupted as fractionation was proceeding. KEY WORDS: trachyte; magma chambers; u-series; Kenya     

O3浓度增加对油菜影响的诊断试验研究

利用OTC-1型农田开顶式气室对油菜进行了不同臭氧浓度200×10-9、100×10-9、50×10-9、未过滤（25×10-9～40×10-9）和过滤掉自然大气的O₃后（约为10×10-9）5个处理的长期接触试验，结果表明:目前大气本底（25×10-9～40×10-9）和50×10-9的低浓度臭氧对油菜有慢性伤害作用。臭氧浓度增加到100×10-9、200×10-9时油菜出现退绿、失水等急性伤害症状。臭氧浓度增加可导致植株矮化，株型缩小，叶片数和叶面积减少，光合速率、生物产量和经济产量下降。试验还表明，正常生长的油菜移入浓度为100×10-9、200×10-9的臭氧环境下，首先受影响的是叶肉和表皮，而此环境下的新生叶片其叶脉将首先扭曲变形。     

Construction ages of the Upton Stone Chamber: Preliminary findings and suggestions for future luminescence research

The Upton Chamber in Massachusetts, an earth-covered stone structure 3.4 meters (m) in diameter, with a corbelled stone dome, and a 4.3 m long entrance passageway, is studied with the aim of determining whether optically stimulated luminescence (OSL) dating methods can be used to establish the approximate construction date of the entranceway. Three samples, taken from soil behind the lowest stones in the wall of the entrance passageway, returned OSL ages between 385 and 660 years ago (or from 1625 A.D. to 1350 A.D.; using the year 2011 as the 0 year). One sample, taken below the bottom of the artifact layers in an archeological test pit in front of the chamber entrance, returned OSL ages between 650 and 880 years ago. A modern sample collected from a nearby fluvial channel returned an age between 55 and 175 years. The Upton Chamber OSL sampling results are challenging to interpret because there are mixtures in the samples of both younger and older grains that likely result from human modification, root or soil processes, animal bioturbation (i.e. ants and worms), and/or partial bleaching. The ages were determined using the lowest component of the finite mixture model as applied to a distribution of quartz grains. Further research may enable us to determine whether older components are of anthropomorphic or geological origin.