The late Pleistocene environment of the Eastern West Beringia based on the principal section at the Main River,Chukotka

Chukotka is a key region for understanding both Quaternary environmental history and transcontinental migrations of flora and fauna during the Pleistocene as it lies at the far eastern edge of Asia bordering the Bering Sea. The now submerged land bridge is the least understood region of Beringia yet the most critical to understanding migrations between the Old and New Worlds. The insect fauna of the Main River Ledovy Obryv (Ice Bluff) section, which is late Pleistocene in age (MIS 3-2), is markedly different from coeval faunas of areas further to the west, as it is characterized by very few thermophilous steppe elements. From the fauna we reconstruct a steppe-tundra environment and relatively cold conditions; the reconstructed environment was moister than that of typical steppe-tundra described from further west. The data from this locality, if typical of the Chukotka Peninsula as a whole, may indicate that a barrier associated with the environments of the land bridge restricted trans-Beringian migrations, particularly the more thermophilous and xeric-adapted elements of the Beringian biota, supporting the hypothesis of a cool but moist land-bridge filter inferred from evidence from several other studies.     

Sea surface temperature variability in the shelf‐slope region of the Northwest Atlantic

Abstract

Sea surface temperature (SST) variability in the shelf‐slope region of the northwest Atlantic is described and then explained in terms of latent and sensible heat exchange with the atmosphere. The basic data are primarily engine‐intake temperature measurements made by merchant ships over the period 1946–80. The data have been grouped by month and area and an empirical orthogonal function analysis has been performed to determine the dominant modes of variation. The first two modes account for 44% of the total variance. The first mode corresponds to in‐phase changes of SST from the Grand Banks to Mid‐Atlantic Bight; the second mode corresponds to opposite changes of SST on the Grand Banks and Mid‐Atlantic Bight. The time‐dependent amplitudes of these large‐scale modes have pronounced low‐frequency components; the associated changes in SST are typically 3°C. It is also shown that winter anomalies last longer than summer anomalies; their typical decay scales are 6 and 3 months, respectively.

The onshore component of geostrophic wind is significantly correlated with the amplitude of the first mode in winter. We note the strong land‐sea contrast of temperature and humidity in this region during winter and explain the wind‐SST correlation in terms of latent and sensible heat exchanges. The second mode (i.e. the difference in SST between the Grand Banks and Mid‐Atlantic Bight) also appears to be related to changes in atmospheric circulation during the winter. A stochastic model for mixed layer temperature is finally used to model the SST autocorrelation functions. Following Ruiz de Elvira and Lemke (1982), it includes a seasonally‐varying feedback coefficient. The model successfully reproduces the extended persistence of winter anomalies with physically realistic parameter values but it cannot account for the summer reinforcement of winter anomalies on the Scotian Shelf. We speculate that this is due to the occasional entrainment of water, cooled the previous winter, into the shallow summer mixed layer.     

Improvement of the solute transfer in a conceptual unsaturated zone scheme: a case study of the Seine River basin

For predicting the evolution of solute concentrations in groundwater and testing the impact of remediation policies, a coupling between the agronomical model STICS and the hydrogeological model MODCOU was implemented. When applied to the Seine River basin, this model accurately represents the temporal evolution of average nitrate concentrations in the aquifer, but with large local errors. We propose an improvement to the simple unsaturated zone (UZ) scheme NonsatSW used in STICS–MODCOU. The modifications are based on a comparison with the mechanistic model Metis considered as a reference as it solves Richards' equation. A more realistic saturation profile and a varying percolation rate are integrated in NonsatSW. This new model, named NonsatVG, is assessed by comparing it with NonsatSW and Metis. In an ideal case, NonsatVG generates a solute transfer and a dispersion closer to that of Metis than of NonsatSW. In real cases, without additional calibration, NonsatVG and Metis simulate better the average transfer velocities of the observed nitrate profiles. Furthermore, modifications in NonsatVG give a direct relationship between the depth of the water table and the saturation profile. We obtain, therefore, as in Metis, an evolution of the solute transfer velocity depending on the piezometric level. These dynamics are not simulated in NonsatSW. Despite a modified water transfer through the UZ, NonsatVG is also as valid as NonsatSW in the modelling of water transfer to the saturated zone. Finally, an application to the Seine basin shows that solute transfer velocities are lower with NonsatVG than with NonsatSW, but are in better agreement with literature. Copyright © 2010 John Wiley & Sons, Ltd.     

Hydrolytic ectoenzyme activity associated with suspended and sinking organic particles within the anoxic Cariaco Basin

Ectohydrolase activities of suspended microbiota were compared to those associated with sinking particles (sed-POM) retrieved from sediment traps deployed in the permanently anoxic Cariaco Basin. In shore-based assays, activities of aminopeptidase, β-glucosidase, chitinase and alkaline phosphatase were measured in samples obtained from oxic and anoxic depths using MUF- and MCA-labeled fluorogenic substrate analogs. Hydrolysis potentials for these enzymes in the seston varied widely over the nine cruises sampled (8 Nov 1996–3 May 2000) and among depths (15–1265 m); from <10 to over 1600 nM d^?1 hydrolysate released, generally co-varying with one another and with suspended particulate organic carbon (POC) and particulate nitrogen (PN). Hydrolytic potentials, prokaryotic abundances and POC/PN concentrations in sinking debris were 400–1.3×10⁷ times higher than in comparable volumes of seawater. However when normalized to PN, hydrolytic potentials in sediment trap samples were not demonstrably higher than in Niskin bottle samples. We estimate that PN pools in sediment trap samples were turned over 2–1400 times (medians=7–26x) slower by hydrolysis than were suspended PN pools. Median prokaryotic growth rates (divisions d^?1) in sinking debris were also ～150 times slower than for bacterioplankton. Hydrolytic potentials in surface oxic waters were generally faster than in underlying anoxic waters on a volumetric basis (nM hydrolysate d^?1), but were not significantly (p>0.05) different when normalized to PN or prokaryote abundances. Alkaline phosphatase was consistently the most active ectohydrolase in both sample types, suggesting that Cariaco Basin assemblages were adapted to decomposing phosphate esters in organic polymers. However, phosphorus limitation was not evident from nutrient inventories in the water column. Results support the hypothesis that efficiencies of polymer hydrolysis in anoxic waters are not inherently lower than in oxic waters.     

Propagators and ridge jumps in a back‐arc basin,the West Philippine Basin

We explore the tectono‐magmatic processes in the western West Philippine Basin, Philippine Sea Plate, using bathymetric data acquired in 2003 and 2004. The northwestern part of the basin formed through a series of northwestward propagating rifts. We identify at least five sequences of propagating rifts, probably triggered by mantle flow away from the mantle thermal anomaly that is responsible for the origin of the Benham and Urdenata plateaus. Gravitational forces caused by along‐axis topographic gradient and a ～30° ridge reorientation appear to also be driving the rift propagations. The along‐axis mantle flow appears to be reduced and deflected along the Luzon‐Okinawa fracture zone, because the spreading system remained stable west of this major fault zone. North‐east of the Benham plateau, a left‐lateral fracture zone has turned into a NE–SW‐trending spreading axis. As a result, a microplate developed at the triple junction.     

The pace of Holocene vegetation change – testing for synchronous developments

Mid to high latitude forest ecosystems have undergone several major compositional changes during the Holocene. The temporal and spatial patterns of these vegetation changes hold potential information to their causes and triggers. Here we test the hypothesis that the timing of vegetation change was synchronous on a sub-continental scale, which implies a common trigger or a step-like change in climate parameters. Pollen diagrams from selected European regions were statistically divided into assemblage zones and the temporal pattern of the zone boundaries analysed. The results show that the temporal pattern of vegetation change was significantly different from random. Times of change cluster around 8.2, 4.8, 3.7, and 1.2 ka, while times of higher than average stability were found around 2.1 and 5.1 ka. Compositional changes linked to the expansion of Corylus avellana and Alnus glutinosa centre around 10.6 and 9.5 ka, respectively. A climatic trigger initiating these changes may have occurred 0.5 to 1 ka earlier, respectively. The synchronous expansion of C. avellana and A. glutinosa exemplify that dispersal is not necessarily followed by population expansion. The partly synchronous, partly random expansion of A. glutinosa in adjacent European regions exemplifies that sudden synchronous population expansions are not species specific traits but vary regionally.     

Switching of a paleo-ice stream in northwest Svalbard

Ice streams are the fast-flowing zones of ice sheets that can discharge a large flux of ice. The glaciated western Svalbard margin consists of several cross-shelf troughs which are the former ice stream drainage pathways during the Pliocene–Pleistocene glaciations. From an integrated analysis of high-resolution multibeam swath-bathymetric data and several high-resolution two-dimensional reflection seismic profiles across the western and northwestern Svalbard margin we infer the ice stream flow directions and the deposition centres of glacial debris that the ice streams deposited on the outer margin. Our results show that the northwestern margin of Svalbard experienced a switching of a major ice stream. Based on correlation with the regional seismic stratigraphy as well as the results from ODP 911 on Yermak Plateau and ODP 986 farther south on the western margin of Spitsbergen, off Van Mijenfjord, we find that first a northwestward flowing ice stream developed during initial northern hemispheric cooling (starting ～2.8–2.6 Ma). A switch in ice stream flow direction to the present-day Kongsfjorden cross-shelf trough took place during a glaciation at ～1.5 Ma or probably later during an intensive major glaciation phase known as the ‘Mid-Pleistocene Revolution’ starting at ～1.0 Ma. The seismic and bathymetric data suggest that the switch was abrupt rather than gradual and we attribute it to the reaching of a tipping point when growth of the Svalbard ice sheet had reached a critical thickness and the ice sheet could overcome a topographic barrier.     

A supercoarsening multigrid method for poroelasticity in 3D coupled flow and geomechanics modeling

The Galerkin finite-element discretization of the force balance equation typically leads to large linear systems for geomechanical problems with realistic dimensions. In iteratively coupled flow and geomechanics modeling, a large linear system is solved at every timestep often multiple times during coupling iterations. The iterative solution of the linear system stemming from the poroelasticity equations constitutes the most time-consuming and memory-intensive component of coupled modeling. Block Jacobi, LSOR, and Incomplete LU factorization are popular preconditioning techniques used for accelerating the iterative solution of the poroelasticity linear systems. However, the need for more effective, efficient, and robust iterative solution techniques still remains especially for large coupled modeling problems requiring the solution of the poroelasticity system for a large number of timesteps. We developed a supercoarsening multigrid method (SCMG) which can be multiplicatively combined with commonly used preconditioning techniques. SCMG has been tested on a variety of coupled flow and geomechanics problems involving single-phase depletion and multiphase displacement of in-situ hydrocarbons, CO₂ injection, and extreme material property contrasts. Our analysis indicates that the SCMG consistently improves the convergence properties of the linear systems arising from the poroelasticity equations, and thus, accelerates the coupled simulations for all cases subject to investigation. The joint utilization of the two-level SCMG with the ILU1 preconditioner emerges as the most optimal preconditioning/iterative solution strategy in a great majority of the problems evaluated in this work. The BiCGSTAB iterative solver converges more rapidly compared to PCG in a number of test cases, in which various SCMG-accelerated preconditioning strategies are applied to both iterators.     

INCREASE IN DUST CONCENTRATION IN THE DASUOPU ICE CORE FROM THE MIDDLE HIMALAYAS AND DRYING TREND IN SOUTH AND WEST ASIA AND NORTH AFRICA OVER THE LAST 500 YEARS

本文重建了近500年来喜马拉雅山中段达索普冰芯中尘埃浓度与尘埃沉积通量的变化历史,谱分析结果表明它们的变化均存在较显著的12.3年、9.8年、7.4年、6.1年和5.3年周期.该冰芯中尘埃含量与净积累量之间不存在任何相关性,说明其尘埃可能主要来源于上风方向的远源地区.基于后向轨迹法,并结合该地区附近已有的气溶胶研究结果,认为西、南亚甚至北非地区可能是该冰芯中尘埃物质的主要源区.近500年来达索普冰芯中尘埃沉积通量的增加趋势,反映了其源区干旱程度的加重趋势.