Deglaciation history of Lake Ladoga (northwestern Russia) based on varved sediments

Lake Ladoga in northwestern Russia is Europe's largest lake. The postglacial history of the Ladoga basin is for the first time documented continuously with high temporal resolution in the upper 13.3 m of a sediment core (Co1309) from the northwestern part of the lake. We applied a multiproxy approach including radiographic imaging, (bio‐)geochemical and granulometric analyses. Age control was established combining radiocarbon dating with varve chronology, the latter anchored to a correlated radiocarbon age from a lake close by. The age‐depth model reveals the onset of glacial varve sedimentation at 13 910±140 cal. a BP, when Lake Ladoga was part of the Baltic Ice Lake. Linear extrapolation of published retreat rates of the Scandinavian Ice Sheet provides a formation age of the Luga moraine close to Lake Ladoga's southern shore of 14.5–15.9 cal. ka BP, older than previously assumed. Varve sedimentation covers the Bølling/Allerød interstadial, the Younger Dryas stadial and the Early Holocene. Varve‐thickness variations, conjoined with grain‐size and geochemical variations, inform about the relative position of the Scandinavian Ice Sheet and the climate during the deglaciation phase. The upper limit of the varved succession marks the change from glaciolacustrine to normal lacustrine sedimentation and post‐dates the drainage of the Baltic Ice Lake as well as the formation of the Salpausselkä II moraine north of Lake Ladoga, by c. 250 years. The Holocene sediment record is divided into three periods in the following order: (i) a lower transition zone between the Holocene boundary and c. 9.5 cal. ka BP, characterized by mostly massive sediments with low organic content, (ii) a phase with increased organic content from c. 9.5 to 4.5 cal. ka BP corresponding to the Holocene Thermal Maximum, and (iii) a phase with relatively stable sedimentation in a lacustrine environment from c. 4.5 cal. ka BP until present.     

Establishment of tree-ring chronology and climatic response of Tibetan juniper (S. tibetica) in south Tibet, western China

A tree-ring width chronology of 442 years(1567-2008) was developed from Tibetan junipers(S.tibetica) derived from south Tibet in western China.Three versions of chronology were produced according to standard dendrochronological techniques.The correlation and response analysis displays a high correlation between the standard tree ring-width chronology and observed annual mean precipitation series during the period 1961-2008.Based on a linear regression model,an annual(prior August to current July) precipitation for the past 229 years was reconstructed.This is the first well-calibrated precipitation reconstruction for the Nanggarze region,south Tibet.The results show that relatively wet years with above-average precipitation occurred in 1780-1807,1854-1866,1886-1898,1904-1949,1967-1981 and 2000-2008,whereas relatively dry years with below-average precipitation prevailed during 1808-1853,1867-1885,1899-1903,1950-1966 and 1982-1999.Common dry/wet periods during 1890s,1910s,1940s-1960s and 1980s were also identified from other moisture reconstructions of nearby regions,indicating a synchronous climatic variation in south Tibet.Abrupt change beginning in 1888 was detected,revealing a transition from wet to dry conditions in south Tibet.Power spectrum analysis reveals significant cycles of 28-year,5.5-5.6-year and 3.3-year during the past 200 years.     

The tidal and wind induced hydrodynamics of the composite system Adriatic Sea/Lagoon of Venice

A shallow water hydrostatic 2D hydrodynamic numerical model, based on the boundary conforming coordinate system, was used to simulate aspects of both general and small scale oceanic features occurring in the composite system constituted by the Adriatic Sea and the Lagoon of Venice (Italy), under the influence of tide and realistic atmospheric forcing. Due to a specific technique for the treatment of movable lateral boundaries, the model is able to simulate efficiently dry up and flooding processes within the lagoon. Firstly, a model calibration was performed by comparing the results of the model, forced using tides and ECMWF atmospheric pressure and wind fields, with observations collected for a set of 33 mareographic stations uniformly distributed in the Adriatic Sea and in the Lagoon of Venice. A second numerical experiment was then carried out by considering only the tidal forcing. Through a comparison between the results obtained in the two experiments it was possible to assess the reliability of the estimated parameter through the composite forcing. Model results were then verified by comparing simulated amplitude and phase of each tidal constituent as well as tidal velocities simulated at the inlets of the lagoon and in the Northern Adriatic Sea with the corresponding observed values. The model accurately reproduces the observed harmonics: mean amplitude differences rarely exceed 1 cm, while phase errors are commonly confined below 15°. Semidiurnal and diurnal currents were correctly reproduced in the northern basin and a good agreement was obtained with measurements carried out at the lagoon inlets. On this basis, the outcomes of the hydrodynamic model were analyzed in order to investigate: (i) small-scale coastal circulation features observed at the interface between the adjoining basins, which consist often of vortical dipoles connected with the tidal flow of Adriatic water entering and leaving the Lagoon of Venice and with along-shore current fields connected with specific wind patterns; (ii) residual oscillations, which are often connected to meteorological forcing over the basin. In particular, it emerges that small-scale vortical features generated near the lagoon inlet can be efficiently transported toward the open sea, thus contributing to the water exchange between the two marine regions, and a realistic representation of observed residual oscillations in the area would require a very detailed knowledge of atmospheric as well as remote oceanic forcing.     

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.     

Observations of internal undular bores on the Japan/East Sea shelf-coastal region

Observations are presented of internal undular bores (IUBs) on the Japan/East Sea coast. We found three types of IUB amplitude modulation functions. The first type (IUB₁) has a leading edge, which is formed by the soliton with maximal amplitude. The second type (IUB₂) has a soliton with maximal amplitude (SMA) located between the leading and trailing edges. The third type (IUB₃) consists of 2–3 solitons, which have identical amplitudes. The evolution of the IUB₂ from the moment of generation to the moment of maximal development was analyzed. The speed of the IUB₂ approached to the phase speed of linear internal waves with a period of 12.4 h. The amplitude modulation function of the IUB₂ is believed to be unique in ocean observations. Observations are discussed in the context of recently raised hypotheses concerning the evolution of IUBs in the coastal environment.     

区域气候模式REMO对中国气温和降水模拟能力的评估

采用泰勒图和偏差分析等统计方法,评估分析了德国区域气候模式（REMO）对中国1989-2008年气温和降水的模拟能力。结果表明：REMO气温模拟值与观测值空间相关系数为0.94,降水空间相关系数较低（0.42）,气温模拟结果明显优于降水;从空间偏差上看,在中国大部分地区,REMO模拟的气温高于观测值,偏差在±4℃以内,青藏高原整体有明显的-4～-2℃的冷偏差;模拟的降水值则高于观测值,空间偏差分布较均匀,中国大部分地区偏差在±300 mm之内;除青藏高原、华南和西南地区外,REMO能较准确地反映出中国气温和降水的空间分布特征,其中华北和东北地区模拟效果最好;REMO对夏季气温和冬季降水的模拟能力相对较好;REMO在地形起伏较大地区的模拟能力有待提高。     

Geochronology and paleomagnetism of mafic igneous rocks in the Olenek Uplift, northern Siberia: Implications for Mesoproterozoic supercontinents and paleogeography

We present a new, reliably dated Mesoproterozoic paleopole for Siberia, based on a combined geochronological and paleomagnetic study of mafic rocks within the Mesoproterozoic Sololi Group of the Olenek Uplift in northern Siberia. Ion microprobe (SHRIMP) U–Pb analysis yields crystallisation ages of 2036 ± 11 Ma for zircon from a basement granite and 1473 ± 24 Ma for baddeleyite from a large dolerite sill within the Kyutingde Formation. The baddeleyite result indicates that the lower Sololi Group is significantly older than was suggested by previous K–Ar results. Paleomagnetic analysis of the dolerite sill and related mafic intrusive rocks yields a paleopole at 33.6°N, 253.1°E, A₉₅ = 10.4°. A positive baked-contact test between the Kyutingde sill and sedimentary country rocks shows that the magnetisation is primary. Comparison of this paleopole with coeval results for Laurentia provides a revised reconstruction between Siberia and Laurentia, and implies that these two continents were parts of a single Mesoproterozoic supercontinent since at least 1473 Ma. We argue that Siberia, Laurentia, and Baltica belonged to the same supercontinent between 1473 Ma and mid-Neoproterozoic time.     

Pressure solution creep under cyclic loading

Pressure solution creep was studied on sodium chloride, calcium carbonate and ammonium nitrate in respective saturated aqueous solutions under static loading and cyclic unloading. Ball indentation and powder compaction curves show that each transition from static to cyclic regime produces an increase—sometimes manifold—in creep rate which lasts over the whole time of cyclic impact. After returning to static regime, the initial creep rate reappears. Over longer-term tests, both in static and cyclic regime, the creep gradually slows down. Increasing impact frequency enhances the effect. Any noticeable changes in strain rate are absent in a pure inert medium (paraffin oil). Possible mechanisms of the cyclic unloading effect are discussed.