Long-term ocean simulations in FESOM: evaluation and application in studying the impact of Greenland Ice Sheet melting

The Finite Element Sea-ice Ocean Model (FESOM) is formulated on unstructured meshes and offers geometrical flexibility which is difficult to achieve on traditional structured grids. In this work, the performance of FESOM in the North Atlantic and Arctic Ocean on large time scales is evaluated in a hindcast experiment. A water-hosing experiment is also conducted to study the model sensitivity to increased freshwater input from Greenland Ice Sheet (GrIS) melting in a 0.1-Sv discharge rate scenario. The variability of the Atlantic Meridional Overturning Circulation (AMOC) in the hindcast experiment can be explained by the variability of the thermohaline forcing over deep convection sites. The model also reproduces realistic freshwater content variability and sea ice extent in the Arctic Ocean. The anomalous freshwater in the water-hosing experiment leads to significant changes in the ocean circulation and local dynamical sea level (DSL). The most pronounced DSL rise is in the northwest North Atlantic as shown in previous studies, and also in the Arctic Ocean. The released GrIS freshwater mainly remains in the North Atlantic, Arctic Ocean and the west South Atlantic after 120 model years. The pattern of ocean freshening is similar to that of the GrIS water distribution, but changes in ocean circulation also contribute to the ocean salinity change. The changes in Arctic and sub-Arctic sea level modify exchanges between the Arctic Ocean and subpolar seas, and hence the role of the Arctic Ocean in the global climate. Not only the strength of the AMOC, but also the strength of its decadal variability is notably reduced by the anomalous freshwater input. A comparison of FESOM with results from previous studies shows that FESOM can simulate past ocean state and the impact of increased GrIS melting well.     

Modeled steric and mass-driven sea level change caused by Greenland Ice Sheet melting

Meltwater from the Greenland Ice Sheet (GIS) has been a major contributor to sea level change in the recent past. Global and regional sea level variations caused by melting of the GIS are investigated with the finite element sea-ice ocean model (FESOM). We consider changes of local density (steric effects), mass inflow into the ocean, redistribution of mass, and gravitational effects. Five melting scenarios are simulated, where mass losses of 100, 200, 500, and 1000 Gt/yr are converted to a continuous volume flux that is homogeneously distributed along the coast of Greenland south of 75°N. In addition, a scenario of regional melt rates is calculated from daily ice melt characteristics. The global mean sea level modeled with FESOM increases by about 0.3 mm/yr if 100 Gt/yr of ice melts, which includes eustatic and steric sea level change. In the global mean the steric contribution is one order of magnitude smaller than the eustatic contribution. Regionally, especially in the North Atlantic, the steric contribution leads to strong deviations from the global mean sea level change. The modeled pattern mainly reflects the structure of temperature and salinity change in the upper ocean. Additionally, small steric variations occur due to local variability in the heat exchange between the atmosphere and the ocean. The mass loss has also affects on the gravitational attraction by the ice sheet, causing spatially varying sea level change mainly near the GIS, but also at greater distances. This effect is accounted for by using Green's functions.     

Chemical depletion of sediment under the Greenland Ice Sheet

The chemical composition of sediment sampled from a network of ice boreholes contacting the bed of the ablation zone of the Greenland Ice Sheet is compared to the composition of intact bedrock clasts. This sediment is enriched in silica and depleted in reactive cations compared to the underlying bedrock. In order to test whether these differences result from grain size biases either in sample collection or due to natural sorting, sediment samples were analyzed by grain size aliquots. Enrichment of silicon and depletion of cations is consistent across grain size classes and the compositions of bedrock and subglacial sediment are statistically separate. The difference in composition between subglacial sediment and rock aligns closely with the composition of dissolved solutes in waters sampled from the same field sites and is dissimilar to the composition of the sites' suspended sediment. This implies that chemical weathering rather than disproportionate physical removal of friable minerals is responsible for the compositional differences between rock and sediment. Mass balance analysis implies 3–10% of the sediment's mass is lost to solute dissolution, with approximately double that amount precipitated as clay minerals (a large portion of which may have been physically expelled). This result implies that temperate ice sheet subglacial environments may be more chemically active than previously realized. Copyright © 2016 John Wiley & Sons, Ltd.     

格陵兰冰盖次表面湖多源遥感监测——以格陵兰中西部流域为例

格陵兰冰盖在夏季会发生剧烈融化,融水在低洼处汇集形成冰面湖,储存了大量的融水,在冰盖的水文系统中起着至关重要的作用,是格陵兰冰盖质量平衡的重要组成部分.近期有研究发现大量的冰面湖在冬季不会完全冻结,而是掩埋在雪层或冰层下以液态水形式存在,形成冰盖次表面湖.冰盖次表面湖对格陵兰冰盖稳定性和物质平衡会产生重要的影响.由于次表面湖存在于冰盖表层之下难以通过可见光影像进行提取和分析,次表面湖的监测成为次表面湖研究的难点之一.本研究提出一种利用Landsat8和Sentinel-1数据自动提取次表面湖范围的方法.该方法先利用可见光影像提取夏季冰面湖的范围对SAR影像进行掩膜,再根据冬天水体和冰面的后向散射具有可分性的原理,通过Rosin阈值分割算法,提取得到2018—2019年冬季格陵兰中西部流域的次表面湖并对其分布进行分析.在研究区内选择10个测试区域,利用该方法对区域内的次表面湖进行目视解译,并对算法自动提取次表面湖结果进行精度验证.结果表明,本研究提出的自动提取算法Kappa系数为0.85.基于该方法,本研究在2018年格陵兰中西部流域共提取夏季冰面湖的面积为102.28 km².约43.09%的夏季冰面湖不会完全冻结,从而在冬季形成次表面湖(面积为44.07 km²).

     

Characterizing supraglacial meltwater channel hydraulics on the Greenland Ice Sheet from in situ observations

Supraglacial rivers on the Greenland Ice Sheet (GrIS) transport large volumes of surface meltwater toward the ocean, yet have received relatively little direct research. This study presents field observations of channel width, depth, velocity, and water surface slope for nine supraglacial channels on the south‐western GrIS collected between July 23 and August 20, 2012. Field sites are located up to 74 km inland and span 494–1485 m elevation, and contain measured discharges larger than any previous in situ study: from 0.006 to 23.12 m³/s in channels 0.20 to 20.62 m wide. All channels were deeply incised with near vertical banks, and hydraulic geometry results indicate that supraglacial channels primarily accommodate greater discharges by increasing velocity. Smaller streams had steeper water surface slopes (0.74–8.83%) than typical in terrestrial settings, yielding correspondingly high velocities (0.40–2.60 m/s) and Froude numbers (0.45–3.11) with supercritical flow observed in 54% of measurements. Derived Manning's n values were larger and more variable than anticipated from channels of uniform substrate, ranging from 0.009 to 0.154 with a mean value of 0.035 ± 0.027 despite the absence of sediment, debris, or other roughness elements. Ubiquitous micro‐depressions in shallow sections of the channel bed may explain some of these roughness values. However, we find that other, unobserved sources of flow resistance likely contributed to these elevated Manning's n values: future work should explicitly consider additional sources of flow resistance beyond bed roughness in supraglacial channels. We conclude that hydraulic modeling for these channels must allow for both subcritical and supercritical flow, and most importantly must refrain from assuming that all ice‐substrate channels exhibit similar hydraulic behavior, especially for Froude numbers and Manning's n. Finally, this study highlights that further theoretical and empirical work on supraglacial channel hydraulics is necessary before broad scale understanding of ice sheet hydrology can be achieved. Copyright © 2016 John Wiley & Sons, Ltd.     

利用ICESat数据确定格陵兰冰盖高程和体积变化

两极冰盖消融是造成海平面上升的重要原因,作为世界第二大冰盖,格陵兰冰盖消融速度在进入21世纪以后明显加快,引起了广泛关注.本文利用ICESat卫星激光测高数据,探讨了坡度改正的方法,通过改进平差模型解决了病态问题,并采用重复轨道方法计算了2003年9月至2009年10月间格陵兰冰盖的体积和高程变化趋势,对格陵兰冰盖各冰川流域系统的变化情况进行了详细分析.结果表明,格陵兰冰盖在这6年间平均高程变化趋势为-16.79±0.84 cm·a^-1,体积变化速率为-301.37±15.16 km³·a^-1,体积流失主要发生在冰盖边缘,其中DS1、DS8等流域的体积损失正在加剧,而高程在2000 m以上的冰盖内陆地区表现出高程积聚的状态,但增长速度明显减缓.与现有研究成果的对比表明,算法优化后的本文结果更具可靠性.

     

Assessing the Current Evolution of the Greenland Ice Sheet by Means of Satellite and Ground-Based Observations

The present study utilises different satellite and ground-based geodetic observations in order to assess the current evolution of the Greenland Ice Sheet (GIS). Satellite gravimetry data acquired by the Gravity Recovery and Climate Experiment are used to derive ice-mass changes for the period from 2003 to 2012. The inferred time series are investigated regarding long-term, seasonal and interannual variations. Laser altimetry data acquired by the Ice, Cloud, and land Elevation Satellite (ICESat) are utilised to solve for linear and seasonal changes in the ice-surface height and to infer independent mass-change estimates for the entire GIS and its major drainage basins. We demonstrate that common signals can be identified in the results of both sensors. Moreover, the analysis of a Global Positioning System (GPS) campaign network in West Greenland for the period 1995–2007 allows us to derive crustal deformation caused by glacial isostatic adjustment (GIA) and by present-day ice-mass changes. ICESat-derived elastic crustal deformations are evaluated comparing them with GPS-observed uplift rates which were corrected for the GIA effect inferred by model predictions. Existing differences can be related to the limited resolution of ICESat. Such differences are mostly evident in dynamical regions such as the Disko Bay region including the rapidly changing Jakobshavn Isbræ, which is investigated in more detail. Glacier flow velocities are inferred from satellite imagery yielding an accelerated flow from 1999 to 2012. Since our GPS observations cover a period of more than a decade, changes in the vertical uplift rates can also be investigated. It turns out that the increased mass loss of the glacier is also reflected by an accelerated vertical uplift.     

Observed Mass Balance of Mountain Glaciers and Greenland Ice Sheet in the 20th Century and the Present Trends

Glacier mass balance and secular changes in mountain glaciers and ice caps are evaluated from the annual net balance of 137 glaciers from 17 glacierized regions of the world. Further, the winter and summer balances for 35 glaciers in 11 glacierized regions are analyzed. The global means are calculated by weighting glacier and regional surface areas. The area-weighted global mean net balance for the period 1960?C2000 is ?270 ± 34 mm a^?1 w.e. (water equivalent, in mm per year) or (?149 ± 19 km³ a^?1 w.e.), with a winter balance of 890 ± 24 mm a^?1 w.e. (490 ± 13 km³ a^?1 w.e.) and a summer balance of ?1,175 ± 24 mm a^?1 w.e. (?647 ± 13 km³ a^?1 w.e.). The linear-fitted global net balance is accelerating at a rate of ?9 ± 2.1 mm a^?2. The main driving force behind this change is the summer balance with an acceleration of ?10 ± 2.0 mm a^?2. The decadal balance, however, shows significant fluctuations: summer melt reached its peak around 1945, followed by a decrease. The negative trend in the annual net balance is interrupted by a period of stagnation from 1960s to 1980s. Some regions experienced a period of positive net balance during this time, for example, Europe. The balance has become strongly negative since the early 1990s. These decadal fluctuations correspond to periods of global dimming (for smaller melt) and global brightening (for larger melt). The total radiation at the surface changed as a result of an imbalance between steadily increasing greenhouse gases and fluctuating aerosol emissions. The mass balance of the Greenland ice sheet and the surrounding small glaciers, averaged for the period of 1950?C2000, is negative at ?74 ± 10 mm a^?1 w.e. (?128 ± 18 km³ a^?1 w.e.) with an accumulation of 297 ± 33 mm a^?1 w.e. (519 ± 58 km³ a^?1 w.e.), melt ablation ?169 ± 18 mm a^?1 w.e. (?296 ± 31 km³ a^?1 w.e.), calving ablation ?181 ± 19 mm a^?1 w.e. (?316 ± 33 km³ a^?1 w.e.) and the bottom melt-21 ± 2 mm a^?1 w.e. (?35 ± 4 km³ a^?1 w.e.). Almost half (?60 ± 3 km³ a^?1) of the net mass loss comes from mountain glaciers and ice caps around the ice sheet. At present, it is difficult to detect any statistically significant trends for these components. The total mass balance of the Antarctic ice sheet is considered to be too premature to evaluate. The estimated sea-level contributions in the twentieth Century are 5.7 ± 0.5 cm by mountain glaciers and ice caps outside Antarctica, 1.9 ± 0.5 cm by the Greenland ice sheet, and 2 cm by ocean thermal expansion. The difference of 7 cm between these components and the estimated value with tide-gage networks (17 cm) must result from other sources such as the mass balance of glaciers of Antarctica, especially small glaciers separated from the ice sheet.     

Modelling the impact of ocean warming on melting and water masses of ice shelves in the Eastern Weddell Sea

The Eastern Weddell Ice Shelves (EWIS) are believed to modify the water masses of the coastal current and thus preconditions the water mass formation in the southern and western Weddell Sea. We apply various ocean warming scenarios to investigate the impact on the temperature–salinity distribution and the sub-ice shelf melting in the Eastern Weddell Sea. In our numerical experiments, the warming is imposed homogeneously along the open inflow boundaries of the model domain, leading to a warming of the warm deep water (WDW) further downstream. Our modelling results indicate a weak quadratic dependence of the melt rate at the ice shelf base on the imposed amount of warming, which is consistent with earlier studies. The total melt rate has a strong dependence on the applied ocean warming depth. If the warming is restricted to the upper ocean (above 1,000  m), the water column (aside from the mixed surface layer) in the vicinity of the ice shelves stabilises. Hence, reduced vertical mixing will reduce the potential of Antarctic Bottom Water formation further downstream with consequences on the global thermohaline circulation. If the warming extends to the abyss, the WDW core moves significantly closer to the continental shelf break. This sharpens the Antarctic Slope Front and leads to a reduced density stratification. In contrast to the narrow shelf bathymetry in the EWIS region, a wider continental shelf (like in the southern Weddell Sea) partly protects ice shelves from remote ocean warming. Hence, the freshwater production rate of, e.g., the Filchner–Ronne Ice Shelf increases much less compared with the EWIS for identical warming scenarios. Our study therefore indicates that the ice-ocean interaction has a significant impact on the temperature-salinity distribution and the water column stability in the vicinity of ice shelves located along a narrow continental shelf. The effects of ocean warming and the impact of increased freshwater fluxes on the circulation are of the same order of magnitude and superimposed. Therefore, a consideration of this interaction in large-scale climate studies is essential.     

地震海洋学方法在研究南海东北部海水水体结构中的应用

反射地震方法一般用于地下地质结构的探测.本文采用南海东北部的一条地震测线记录,通过对水体反射信号的处理,获取了该测线上海水的声速剖面,并获得了海水水体结构的直观图像.在反射地震剖面上,可以看出南海东北部上层海水结构较为稳定,没有明显的水体侵入痕迹,但中、深层海水中内波较为发育,这与物理海洋学上的研究结果是一致的.本文通过设计合理的数据模型说明了地震海洋学这种方法的可行性,并对地震海洋学的工作方法和方向上提出了一些意见和建议.     

半干旱地区洪水模拟效果差异性分析及影响因子响应评估

中国北方半干旱地区的降水与下垫面条件具有明显的时空异质性,如何完整准确地描述该类区域的水文过程是当代水文学研究的难点之一.选择半干旱地区水文实验区域——绥德流域和曹坪流域,通过构建不同时空规律的降水场,并结合3种不同产流机制的水文模型,进行大型数值模拟实验,去探究时间、空间、产流机制等因素对半干旱地区洪水模拟的影响,为该类地区水文模型的研制工作提供借鉴.结果 表明:1)半干旱地区中小流域的产流对降雨强度较为敏感,因此降水输入的时间步长对洪水模拟效果的影响程度较大;相比之下,流域雨量站数量的增减,仅体现在降雨分布场的暴雨中心缺失以及面平均降雨量的微小差别,对洪水模拟效果的影响程度较小.2)水文模型能否准确描述主导水文过程是半干旱地区洪水模拟效果优良的关键,流域的尺度效应及其下垫面条件的空间异质性是半干旱地区不同水文模型研制和调整应当优先考虑的问题,无论时间步长、雨量站数量怎么组合,产流结构适宜的模型其模拟效果总是趋于较好的结果.     

Representing Grounding Line Dynamics in Numerical Ice Sheet Models: Recent Advances and Outlook

Recent satellite observations of the Antarctic and Greenland ice sheets show accelerated ice flow and associated ice sheet thinning along coastal outlet glaciers in contact with the ocean. Both processes are the result of grounding line retreat due to melting at the grounding line (the grounding line is the contact of the ice sheet with the ocean, where it starts to float and forms an ice shelf or ice tongue). Such rapid ice loss is not yet included in large-scale ice sheet models used for IPCC projections, as most of the complex processes are poorly understood. Here we report on the state-of-the art of grounding line migration in marine ice sheets and address different ways in which grounding line migration can be attributed and represented in ice sheet models. Using one-dimensional ice flow models of the ice sheet/ice shelf system we carried out a number of sensitivity experiments with different spatial resolutions and stress approximations. These are verified with semi-analytical steady state solutions. Results show that, in large-scale finite-difference models, grounding line migration is dependent on the numerical treatment (e.g. staggered/non-staggered grid) and the level of physics involved (e.g. shallow-ice/shallow-shelf approximation).     

Mass,Volume and Velocity of the Antarctic Ice Sheet: Present-Day Changes and Error Effects

This study examines present-day changes of the Antarctic ice sheet (AIS) by means of different data sets. We make use of monthly gravity field solutions acquired by the Gravity Recovery and Climate Experiment (GRACE) to study mass changes of the AIS for a 10-year period. In addition to ‘standard’ solutions of release 05, solutions based on radial base functions were used. Both solutions reveal an increased mass loss in recent years. For a 6-year period surface-height changes were inferred from laser altimetry data provided by the Ice, Cloud, and land Elevation Satellite (ICESat). The basin-scale volume trends were converted into mass changes and were compared with the GRACE estimates for the same period. Focussing on the Thwaites Glacier, Landsat optical imagery was utilised to determine ice-flow velocities for a period of more than two decades. This data set was extended by means of high-resolution synthetic aperture radar (SAR) data from the TerraSAR-X mission, revealing an accelerated ice flow of all parts of the glacier. ICESat data over the Thwaites Glacier were complemented by digital elevation models inferred from TanDEM-X data. This extended data set exhibits an increased surface lowering in recent times. Passive microwave remote sensing data prove the long-term stability of the accumulation rates in a low accumulation zone in East Antarctica over several decades. Finally, we discuss the main error sources of present-day mass-balance estimates: the glacial isostatic adjustment effect for GRACE as well as the biases between laser operational periods and the volume–mass conversion for ICESat.     

Cascading ocean basins: numerical simulations of the circulation and interbasin exchange in the Azov-Black-Marmara-Mediterranean Seas system

In this paper, we use the unstructured grid model SCHISM to simulate the thermohydrodynamics in a chain of baroclinic, interconnected basins. The model shows a good skill in simulating the horizontal circulation and vertical profiles of temperature, salinity, and currents. The magnitude and phases of the seasonal changes of circulation are consistent with earlier observations. Among the mesoscale and subbasin-scale circulation features that are realistically simulated are the anticyclonic coastal eddies, the Sebastopol and Batumi eddies, the Marmara Sea outflow around the southern coast of the Limnos Island, and the pathway of the cold water originating from the shelf. The superiority of the simulations compared to earlier numerical studies is demonstrated with the example of model capabilities to resolve the strait dynamics, gravity currents originating from the straits, high-salinity bottom layer on the shallow shelf, as well as the multiple intrusions from the Bosporus Strait down to 700 m depth. The warm temperature intrusions from the strait produce the warm water mass in the intermediate layers of the Black Sea. One novel result is that the seasonal intensification of circulation affects the interbasin exchange, thus allowing us to formulate the concept of circulation-controlled interbasin exchange. To the best of our knowledge, the present numerical simulations, for the first time, suggest that the sea level in the interior part of the Black Sea can be lower than the sea level in the Marmara Sea and even in some parts of the Aegean Sea. The comparison with observations shows that the timings and magnitude of exchange flows are also realistically simulated, along with the blocking events. The short-term variability of the strait transports is largely controlled by the anomalies of wind. The simulations demonstrate the crucial role of the narrow and shallow strait of Bosporus in separating the two pairs of basins: Aegean-Marmara Seas from one side and Azov-Black Seas from the other side. The straits of Kerch and Dardanelles provide sufficient interbasin connectivity that prevents large phase lags of the sea levels in the neighboring basins. The two-layer flows in the three straits considered here show different dependencies upon the net transport, and the spatial variability of this dependence is also quite pronounced. We show that the blocking of the surface flow can occur at different net transports, thus casting doubt on a previous approach of using simple relationships to prescribe (steady) outflow and inflow. Specific attention is paid to the role of synoptic atmospheric forcing for the basin-wide circulation and redistribution of mass in the Black Sea. An important controlling process is the propagation of coastal waves. One major conclusion from this research is that modeling the individual basins separately could result in large inaccuracies because of the critical importance of the cascading character of these interconnected basins.     

High-pressure partial melting of garnet pyroxenite: possible mafic lithologies in the source of ocean island basalts

Many ocean island basalts (OIB) that have isotopic ratios indicative of recycled crustal components in their source are silica-undersaturated and unlike silicic liquids produced from partial melting of recycled mid-ocean ridge basalt (MORB). However, experiments on a silica-deficient garnet pyroxenite, MIX1G, at 2.0-2.5 GPa show that some pyroxenite partial melts are strongly silica-undersaturated [M.M. Hirschmann et al., Geology 31 (2003) 481-484]. These low-pressure liquids are plausible parents of alkalic OIB, except that they are too aluminous. We present new partial melting experiments on MIX1G between 3.0 and 7.5 GPa. Partial melts at 5.0 GPa have low SiO₂ (<48 wt%), low Al₂O₃ (<12 wt%) and high CaO (>12 wt%) at moderate MgO (12-16 wt%), and are more similar to primitive OIB compositions than lower-pressure liquids of MIX1G or experimental partial melts of anhydrous or carbonated peridotite. Solidus temperatures at 5.0 and 7.5 GPa are 1625 and 1825°C, respectively, which are less than 50°C cooler than the anhydrous peridotite solidus. The liquidus temperature at 5.0 GPa is 1725°C, indicating a narrow melting interval (∼100°C). These melting relations suggest that OIB magmas can be produced by partial melting of a silica-deficient pyroxenite similar to MIX1G if its melting residue contains significant garnet and lacks olivine. Such silica-deficient pyroxenites could be produced by interaction between recycled subducted oceanic crust and mantle peridotite or could be remnants of ancient oceanic lower crust or delaminated lower continental crust. If such compositions are present in plumes ascending with potential temperatures of 1550°C, they will begin to melt at about 5.0 GPa and produce appropriate partial melts. However, such hot plumes may also generate partial melts of peridotite, which could dilute the pyroxenite-derived partial melts.     

南极冰盖物质平衡变化的卫星遥感监测现状与发展趋势

南极冰盖物质平衡对全球海平面变化影响巨大,提高南极冰盖物质平衡估算精度对实现全球海平面变化趋势准确预测至关重要.卫星遥感技术的发展,特别是极地卫星对南极冰盖系统监测能力的提升,有助于认识、保护和利用南极,解决极地科学国际前沿难题.揭示南极冰盖对全球气候环境的响应及其反馈机制,对深化人类对南极的科学认知也具有重要意义.本文首先系统总结了用于南极冰盖监测卫星的发展现状及现有的卫星遥感数据产品序列,然后讨论了运用遥感卫星监测南极物质平衡的主要方法和研究进展,并对南极卫星遥感技术的最新进展进行了分析.最后在我国极地遥感卫星发展方面提出了建议,旨在进一步提升我国对极地冰盖变化和全球海平面上升的长期观测及预测能力,为相关应对策略的制定提供有力的科学支撑.

     

Response of the Greenland and Antarctic Ice Sheets to Multi-Millennial Greenhouse Warming in the Earth System Model of Intermediate Complexity LOVECLIM

Calculations were performed with the Earth system model of intermediate complexity LOVECLIM to study the response of the Greenland and Antarctic ice sheets to sustained multi-millennial greenhouse warming. Use was made of fully dynamic 3D thermomechanical ice-sheet models bidirectionally coupled to an atmosphere and an ocean model. Two 3,000-year experiments were evaluated following forcing scenarios with atmospheric CO₂ concentration increased to two and four times the pre-industrial value, and held constant thereafter. In the high concentration scenario the model shows a sustained mean annual warming of up to 10°C in both polar regions. This leads to an almost complete disintegration of the Greenland ice sheet after 3,000 years, almost entirely caused by increased surface melting. Significant volume loss of the Antarctic ice sheet takes many centuries to initiate due to the thermal inertia of the Southern Ocean but is equivalent to more than 4 m of global sea-level rise by the end of simulation period. By that time, surface conditions along the East Antarctic ice sheet margin take on characteristics of the present-day Greenland ice sheet. West Antarctic ice shelves have thinned considerably from subshelf melting and grounding lines have retreated over distances of several 100 km, especially for the Ross ice shelf. In the low concentration scenario, corresponding to a local warming of 3?C4°C, polar ice-sheet melting proceeds at a much lower rate. For the first 1,200 years, the Antarctic ice sheet is even slightly larger than today on account of increased accumulation rates but contributes positively to sea-level rise after that. The Greenland ice sheet loses mass at a rate equivalent to 35 cm of global sea level rise during the first 1,000 years increasing to 150 cm during the last 1,000 years. For both scenarios, ice loss from the Antarctic ice sheet is still accelerating after 3,000 years despite a constant greenhouse gas forcing after the first 70?C140 years of the simulation.     

Partitioning of U and Th during garnet pyroxenite partial melting: Constraints on the source of alkaline ocean island basalts

Uranium series disequilibria in ocean island basalts (OIB) provide evidence for the presence of garnet in their source region. It has been suggested that enriched OIB signatures derive from mantle lithologies other than peridotite, such as eclogite or pyroxenite, and, in particular, that silica-poor garnet pyroxenite is the source lithology for alkali basalts. To test the ability of such a source to produce the U–Th disequilibria observed in alkali OIB, we determined experimentally clinopyroxene-melt and garnet-melt partition coefficients for a suite of trace elements, including U and Th, at 2.5 GPa and 1420–1450 °C. The starting composition for the experiments was a 21% partial melt of a silica-poor garnet pyroxenite. Experimentally determined clinopyroxene-melt partition coefficients range from 0.0083 ± 0.0006 to 0.020 ± 0.002 for Th and from 0.0094 ± 0.0006 to 0.024 ±0.002 for U, and garnet-melt partition coefficients are 0.0032 ± 0.0004 for Th and 0.013 ± 0.002 for U. Comparison of our experimental results with partition coefficients from previous experimental studies shows that the relative compatibilities of U and Th in both garnet and clinopyroxene are different for different mineral compositions, leading to varying degrees of U/Th fractionation with changing lithology. For a given melting rate and extent of partial melting, mafic lithologies tend to produce larger ²³⁰Th excesses than peridotite. However, this effect is minimized by the greater overall extents of melting experienced by eclogites and pyroxenites relative to peridotite. Results from chromatographic, batch, and fractional melting calculations with binary mixing between partial melts of pyroxenite and peridotite, carried out using our new partitioning data for the pyroxenite component and taking into account variable productivities and different solidus depths for the two lithologies, suggest that OIB are not the product of progressive melting of a source containing a fixed quantity of garnet pyroxenite. Melting a peridotite with enriched signatures, and mixing those melts with melts of a depleted, “normal” peridotite, is an alternative explanation for the trends seen in Hawaiian, Azores and Samoan lavas.     

Ice cover of the Amur River and its impact on channel processes

Special features of the Amur R. ice regime in its middle and lower reaches are discussed along with the role of river ice in bank erosion, transport of coarse fragmental material, bottom washout, and water flow redistribution among branches. Numerous and various consequences of the ice drift impact on the banks and floodplain, as well as the intensity of erosion processes in the Amur River channel during winter have been revealed. The water flow redistribution among the Amur River branches near Khabarovsk noticeably changed the ice regime, which was rehabilitated as a result of implementation of a complex of special measures.