Cold anticyclonic eddies formed from cold pool water in the southern Middle Atlantic Bight

AVHRR satellite imagery of the southern Mid-Atlantic Bight during May 1993 revealed a large area of cold water over the shelf break and slope that appeared to spin up into a series of southward propagating anticyclonic eddies. The eddies had diameters of 35–45 km at the surface and moved southward at about 20 cm/sec. A radial TOYO CTD (to 50m) and ADCP velocity (to 400m) transect was conducted across the southern-most of these eddies. The upper 50 meters had minimum temperatures of less than 7°C and salinities of about 33 pss, characteristics similar to cold pool waters usually found over the continental shelf. ADCP velocity data from one of the eddies revealed anticyclonic flow extending to a depth of about 250m. The transport of cold pool water by the eddies was estimated to be 0.1 to 0.2 Sv which is of the same order as the annual mean alongshore transport of shelf water in this region. The origin of the deeper water within the eddy is unlikely to be the continental shelf because the shelf break is less than 100 m. The depth and velocity profiles along the TOYO transect were consistent with the constant potential vorticity eddy model of Flierl (1979) although the source of the eddy kinetic energy is uncertain. The cause for the exodus of cold pool water from the shelf, which extended northward to at least 38°N, is unclear but must involve the establishment of an alongshore baroclinic pressure gradient against the usual southwestward shelf flow. It is possible that the intrusion of Gulf Stream waters onto the shelf near Cape Hatteras was a precursor of this off shelf transport. The southern-most eddy was marked by high biological productivity and very high oxygen supersaturation. The phytoplankton bloom detected within the exported cold pool water, located over the continental slope, suggests a mechanism whereby production fueled by nutrients derived from the shelf can be locally exported into deep water.     

Trapped Xe and I-Xe ages in aqueously altered CV3 meteorites

Twenty-two dark inclusions (DIs) from Allende (18), Leoville (2), Vigarano (1) and Efremovka (1) were studied by the I-Xe method. All except two of these DIs (Vigarano 2226 and Leoville LV2) produce well-defined isochrons, and precise I-Xe ages. The Allende DIs formed a tight group about 1.6 Ma older than Shallowater (4.566 ± 0.002 Ga), about 5 Ma older than four previously studied Allende CAIs. Most of the dark inclusions require trapped Xe with less ¹²⁹Xe (or more ¹²⁸Xe) than conventional planetary Xe (well restricted in composition by Q-Xe or OC-Xe). Studies of an irradiated/unirradiated DI pair from Allende demonstrate that the ¹²⁸Xe/¹³²Xe ratio in trapped is normal planetary, so that a ¹²⁹Xe/¹³²Xe ratio below planetary seems to be required. Yet, this is not possible given constraints on ¹²⁹Xe evolution in the early solar system. Trends among all of the Allende DIs suggest that an intimate mixture of partially decayed iodine and Xe formed a pseudo trapped Xe component enriched in both ¹²⁹Xe and ¹²⁷I, and subsequently in ¹²⁸Xe after n-capture during reactor irradiation. Enrichment in radiogenic ¹²⁹Xe, but with a ¹²⁹Xe/¹²⁷I ratio less than that observed in the iodine host phase, places closure of this trapped mixture ≥13 Ma after precipitation of the major iodine-bearing phase. Because the I-Xe isochron is a mixing line between iodine-derived and trapped Xe (pseudo or not), I-Xe ages, given by the slope of this mixing line, are not compromised by the presence of pseudo trapped Xe, and the precision of the I-Xe ages is given by the statistics of the line fit.     

Potential toxicity of resuspended particulate matter and sediments: Environmental samples from the Bay of Banyuls-sur-Mer and Thermaikos Gulf

This contribution attempts to determine whether, and to what degree, environmental samples of resuspended particulate matter and sediments exert a toxicological impact. Further, an attempt is made also to screen the toxic level of potentially hazardous sites, based upon established sediment toxicity criteria. Therefore, a rapid, cost-effective and highly sensitive biotest (bioluminescence assay, based upon marine bacteria) has been applied on: biological fluid extracts; bottom sediments; and sediment trap samples. Samples were taken either from the Bay of Banyuls-sur-Mer (northwestern Mediterranean, France) or Thermaikos Gulf (northeastern Mediterranean, Greece). Biological fluid extracts and sediment trap samples corresponded to periods of resuspension events, or preceding and following such events.The results have revealed that the sampling strategy and biotest implemented in this study might be a useful tool for screening the toxicity of resuspended matter and sediments. Resuspension events appear to be able to exert an influence on the chemical forms of the micro-pollutants; thus on their bioavailability and toxicity. Nevertheless, based upon chemical analysis combined with the bioassay, the toxic level of the sediment samples could be determined, with the level at potentially hazardous sites being characterised.     

Détermination expérimentale des paramètres équivalents d'un milieu poreux hétérogène en écoulements uniforme ou radial

Tracer tests are carried out in a heterogeneous porous medium that has a 3D correlated random distribution of the permeabilities. The fitting of numerical models provides the values of equivalent permeability and macrodispersivity characterizing a 2D homogeneous horizontal medium. Different flow configurations are studied: uniform, radial and pump and treat (doublet). The fitted parameter sets are independent of the flow type, except for the doublet. They are greater than the values predicted by stochastic theories, due to the small number of correlation lengths explored by the tracer and the limited extension of the experimental set-up. To cite this article: C. Danquigny, P. Ackerer, C. R. Geoscience 337 (2005).     

Analysis of pressuremeter geometry effects in clay using critical state models

The conventional interpretation methods of pressuremeter testing effectively approximate pressuremeter membranes as infinitely long. As a result, the effects of the two‐dimensional geometry of pressuremeters are ignored, leading to an overestimation of soil shear strength by pressuremeter testing, as demonstrated in several previous studies. This paper presents results of a numerical study of two‐dimensional geometry effects on self‐boring pressuremeter tests in undrained clay. The results are obtained using critical state soil models with an effective stress formulation. This is in contrast to most (if not all) existing studies on pressuremeter geometry effects, which were based on perfectly plastic soil models (e.g. Yu (Cavity expansion theory and its application to the analysis of pressuremeters. DPhil Thesis, The University of Oxford, 1990), Yeung and Carter (Proc. 3rd Int. Symp. on Pressuremeters, 1990), and Houlsby and Carter (Géotechnique, 1993; 43 (4):567–576)). The present study suggests that the overestimation of soil strength due to the neglect of finite pressuremeter length is significantly affected by the soil model used in the calculations. It is found that for clays with a high overconsolidation ratio (OCR) the strength overestimation predicted using critical state soil models could be considerably smaller than that predicted using perfectly plastic soil models. The main conclusion of this numerical study is that care must be exercised before directly applying any numerically determined pressuremeter geometry correction factors in practice. Copyright © 2005 John Wiley & Sons, Ltd.     

Thallium diagenesis in lacustrine sediments

Dated sediment cores and porewaters from two Canadian Shield lakes, located 40 km from Québec City (L. Tantaré) and 25 km downwind from an important smelter in northwestern Québec (L. Vose), were analyzed for thallium and other geochemical variables. Atmospheric deposition is the only source of anthropogenic Tl to these lakes. The porewater Tl concentration profiles in L. Tantaré display a peak below the sediment-water interface that suggests post-depositional Tl remobilization and transport to the overlying water and deeper sediments; such a dissolved Tl peak occurs just above the sediment-water interface in L. Vose. Modeling the porewater Tl profiles with a diagenetic reaction-transport equation reveals a zone of dissolved Tl production lying above a zone of Tl consumption in L. Tantaré sediments. In contrast, in L. Vose, Tl diffuses across the sediment-water interface from the anoxic hypolimnion and is fixed to the surficial sediments. The localization of the consumption zones, the shape of the dissolved Tl, sulfide and iron profiles, as well as calculations of saturation states are all consistent with the removal of Tl from porewater by co-precipitation with and/or adsorption to Fe sulfides. The concentrations of Tl removed from or added to sediments after their deposition (i.e., diagenetic Tl) amount to up to 36% of measured sediment Tl concentrations. Comparison of the reconstructed historical Tl records with those of polycyclic aromatic hydrocarbons (PAHs) and of Pb originating from coal burning or from smelting activities indicate that coal combustion is the major source of atmospheric Tl to L. Tantaré and that an additional important Tl source to L. Vose is the nearby smelter.     

A multi‐proxy record of changing environments from ca. 30 000 to 9000 cal. a BP: Onepoto maar palaeolake,Auckland, New Zealand

We present a high‐resolution record of lacustrine sedimentation spanning ca. 30 000 to 9000 cal. a BP from Onepoto maar, northern North Island, New Zealand. The multi‐proxy record of environmental change is constrained by tephrochronology and accelerator mass spectrometric ¹⁴C ages and provides evidence for episodes of rapid environmental change during the Last Glacial Coldest Period (LGCP) and Last Glacial–Interglacial Transition (LGIT) from northern New Zealand. The multi‐proxy palaeoenvironmental record from Onepoto indicates that the LGCP was cold, dry and windy in the Auckland region, with vegetation dominated by herb and grass in a beech forest mosaic between ca. 28 500 and 18 000 cal. a BP. The LGCP was accompanied by more frequent fires and influx of clastic sediment indicating increased erosion during the LGCP, with a mid‐LGCP interstadial identified between ca. 25 000 and 23 000 cal. a BP. Rapid climate amelioration at ca. 18 000 cal. a BP was accompanied by increased terrestrial biomass exemplified by the expansion of lowland podocarp forest, especially Dacrydium cupressinum. Increasing biomass production is reversed briefly by LGIT perturbations which are apparent in many of the proxies that span ca. 14 000–10 500 cal. a BP, suggesting generally increased wetness and higher in situ aquatic plant productivity with reduced terrestrial organic matter and terrigenous detrital influx. Furthermore, conditions at that time were probably warmer and frosts rare based on the increasing importance of Ascarina. The subsequent early Holocene is characterised by podocarp conifer forest and moist mild conditions. Postglacial sea‐level rise breached the crater rim and deposited 36 m of estuarine mud after ca. 9000 cal. a BP. Copyright © 2011 John Wiley & Sons, Ltd.     

Early Permian climate change in the Falkland Islands

An Early Permian glacial diamictite forms a distinctive unit within the Falkland Islands sedimentary succession and two aspects of its significance have recently been serendipitously enhanced. Fossil discoveries in exotic limestone clasts bear on palaeogeography, whilst a series of mineral‐exploration borehole cores have allowed a detailed study of the sedimentary record of deglaciation that followed deposition of the diamictite. Statistical analysis of reflectance and XRF core‐scanning data has identified likely Milankovitch periodicities and enabled tentative time‐scale modelling. The ‘icehouse to greenhouse’ transition appears to have spanned approximately 1.2 million years, with waning cycles of re‐advance superimposed on overall glacial retreat. The new results play into a long‐debated geological paradox: although the Falkland Islands are now proximal to the South Atlantic coastline of South America, their geology bears an uncanny resemblance to that of the Cape Fold Belt and Karoo Basin in South Africa. This puzzled the geological pioneers, but became readily explicable when first continental drift and then plate tectonics were invoked to reconstruct the break‐up of the Gondwana supercontinent—although the details remain controversial. One of the key stratigraphical correlation levels throughout the major fragments of southern Gondwana—South Africa, South America, Antarctica and Australia—is the glacigenic deposit left behind by the extensive, Late Carboniferous to Early Permian regional glaciation; in the Falkland Islands it is designated the Fitzroy Tillite Formation.