Late Glacial Sedimentation and History of the Lake Nipigon Basin, Ontario

The Lake Nipigon basin lies north of the Lake Superior basin and was the hydrological link between glacial Lake Agassiz and the Great Lakes during part of the last deglaciation. A sequence of glaciolacustrine sediments, composed mainly of silt-clay rhythmites and sand, was deposited in the offshore waters of glacial Lake Nipigon by overflow from Lake Agassiz and meltwater from the retreating glacier margin. Sections from six long sediment cores and four lake bluff exposures reveal a sandy (early deglacial) lower section that is overlain by 300 to 850 silt-clay rhythmites (varves). Deposition of these varves, as well as coarser sediment along the western shore, began after 9200 BP, as the glacial margin retreated northward along the continental divide that separated the Nipigon basin from the higher Lake Agassiz basin to the west. The absence of ice rafted clasts in the rhythmites suggests that the ice had retreated from the lake by the time they were deposited. On the basis of their elevation in relation to the lowest raised beach at West Bay, which formed about 9000 BP, most rhythmites probably were deposited between 9000 and 8000 BP. Species of arboreal pollen are present in early postglacial sediments of the Nipigon-Superior lowlands, suggesting that the Lake Nipigon region became colonized by coniferous and deciduous forests soon after déglaciation. The presence of non-arboreal pollen species suggest that these forests were interspersed with open meadows and grasslands, similar to today's floral assemblages. Fossil molluscs recovered from glaciolacustrine sand exposed along the eastern side of the basin suggest that the limnological characteristics of late glacial Lake Nipigon were similar to those of today.Le bassin du lac Nipigon situé au nord du bassin du lac Supérieur a assuré le lien hydrologique entre le Lac Agassiz et les Grands Lacs pendant une partie de la dernière glaciation. Une séquence de sédiments glaciolacustres, surtout composés de rythmites silto-argileuses et de sable, a été déposée au large du rivage du Lac glaciaire Nipigon par les eaux de crue du Lac Agassiz et les eaux de fonte du glacier en recul. Les coupes dans six carottes de sédiments et quatre coupes naturelles dans la falaise révèlent la présence de sable dans la partie inférieure (début de la déglaciation) recouverte par 300 à 850 rythmites silto-argileuses (varves). La mise en place de ces varves, comme celle des sédiments plus grossiers le long de la rive ouest, a commencé après 9200 BP, alors que la marge glaciaire reculait vers le nord le long de la ligne de partage des eaux entre le bassin du Lac Nipigon de celui plus élevé du Lac Agassiz. L'absence de fragments glaciels dans les rythmites indique que le glacier s'était déjà retiré. Selon leur altitude par rapport à la plage perchée la moins élevée à West Bay, formée vers 9000 BP, la plupart des rythmites ont été déposées entre 9000 et 8000 BP. Le pollen arboréen présent dans les premiers sédiments postglaciaires des basses terres Nipigon-Supérieur montre que les forêts de conifères et de décidus se sont établies peu après la déglaciation. La présence de pollen non arboréen montre que ces forêts étaient parsemées de prés ouverts et de prairies, comme c'est le cas aujourd'hui. Les mollusques fossiles recueillis dans les sables glaciolacustres le long du côté est du bassin montrent que les propriétés limnologiques du Lac Nipigon sont semblables à celles d'aujourd'hui.Das Becken des Nipigonsees liegt nôrdlich von dem des Oberen Sees, und war das hydrologische Bindeglied zwischen dem glazialen Agassizsee und den groBen Seen wàhrend eines Teils der letzten Enteisung. Eine Sequenz glaziallimnischer Sedimente, hauptsàchlich aus Schlamm-Lehm, Rhythmiten und Sand bestehend, wurde in dem kùstennahen Wasser des glazialen Nipigansees abgelagert durch ùberlauf vom Agassizsee und Schmelzwasser von dem zurùckweichenden Gletscherrand. Abschnitte von sechs langen Sedimentbohrkernen und vier See-Steilhang-Aufschlùsse zeigen einen sandigen (Beginn der Enteisung) unteren Bereich, der von 300 bis 850 Schlamm-Lehm-Rhythmiten (Warwen) ùberlagert ist. Die Ablagerung dieser Warwen sowie grôberer Sedimente entlang der westlichen Kùste begann nach 9200 v.u.Z., als der Eisrand nordwârts zurùckwich, entlang der Wasserscheide, die das Nipiganbecken von dem hôheren Agassizbecken nach Westen hin trennte. Das Fehlen von Eisfragmenten in den Rhythmiten IaBt vermuten daB das Eis zum Zeitpunkt ihrer Ablagerung vom See schon zurùckgewichen dap das Eis zum Zeitpunkt war. Entsprechend ihrer Erhebung in Bezug auf den niedrigsten gehobenen Strand in der West Bay, der sich um 9000 v.u.Z. herausbildete, wurden die meisten Rhythmite wohl zwischen 9000 und 8000 v.u.Z. abgelagert. Baumpollenarten in den frùhen postglazialen Sedimenten der Superior-Nipigan-Ebenen legen nahe, daB im Nipigansee -Gebiet kurz nach der Enteisung Tannen- und Laubwald sich ausbreiteten. Das Vorkommen von baumfremden Pollenarten zeigt, daB die Wâlder von offenen Weiden und Grasland durchsetzt waren, àhnlich der heutigen Bewachsung. Schalentier-Fossile, die aus dem glaziallimnischen Sand gewonnen wurden, entlang der ôstlichen Seite des Beckens, zeigen, daB die limnologischen Charakteristika des spâtglazialen Nipigan-sees den heutigen àhnlich waren

Long-term variability of CO2 and O in the Mars upper atmosphere from MRO radio science data

We estimate the annual variability of CO2 and O partial density using approximately 6years of Mars Reconnaissance Orbiter (MRO) radio science data from August 2006 to January 2012, which cover three full Martian years (from the northern hemisphere summer of 28 to the northern hemisphere summer of 31). These two elements are the dominant species at the MRO periapsis altitude, constituting about 70-80% of the total density. We report the recovered annual cycle of CO2 and the annual and seasonal cycle of O in the upper atmosphere. Although no other observations are available at those altitudes, our results are in good agreement with the density measurements of the Mars Express Spectroscopy for Investigation of Characteristics of the Atmosphere of Mars, which uses stellar occultations between 60 and 130km to determine the CO2 variability, and with the Mars Global Reference Atmospheric Model 2010 for the O annual and seasonal variabilities. Furthermore, the updated model provides more reasonable MRO drag coefficients (CD), which are estimated to absorb mismodeling in the atmospheric density prediction. The higher content of dust in the atmosphere due to dust storms increases the density, so the CDs should compensate for this effect. The correlation between the drag coefficient and the dust optical depth, measured by the Mars Odyssey Thermal Emission Imaging System (THEMIS) instrument, increases from 0.4 to 0.8 with the a priori and adjusted models, respectively. The trend of CDs not only confirms a substantial improvement in the prediction of the atmospheric density with the updated model but also provides useful information for local dust storms, near MRO periapsis, that cannot be measured by the opacity level since THEMIS does not always sample the southern hemisphere evenly

Properties of Faint Distant Galaxies as seen through Gravitational Telescopes

This paper reviews the most recent developments related to the use of lensing clusters of galaxies as Gravitational Telescopes in deep Universe studies. We summarize the state of the art and the most recent results aiming at studying the physical properties of distant galaxies beyond the limits of conventional spectroscopy. The application of photometric redshift techniques in the context of gravitational lensing is emphasized for the study of both lensing structures and the background population of lensed galaxies. A presently ongoing search for the first building blocks of galaxies behind lensing clusters is presented and discussed.Comment: Review lecture given at "Gravitational Lensing: a unique tool for cosmology",Aussois, France, January 2003. To appear in ASP Conf. S., eds. D. Valls-Gabaud & J.-P. Kneib, 26 pages, 8 figure

Trans-Planckian Dark Energy?

It has recently been proposed by Mersini et al. 01, Bastero-Gil and Mersini 02 that the dark energy could be attributed to the cosmological properties of a scalar field with a non-standard dispersion relation that decreases exponentially at wave-numbers larger than Planck scale (k_phys > M_Planck). In this scenario, the energy density stored in the modes of trans-Planckian wave-numbers but sub-Hubble frequencies produced by amplification of the vacuum quantum fluctuations would account naturally for the dark energy. The present article examines this model in detail and shows step by step that it does not work. In particular, we show that this model cannot make definite predictions since there is no well-defined vacuum state in the region of wave-numbers considered, hence the initial data cannot be specified unambiguously. We also show that for most choices of initial data this scenario implies the production of a large amount of energy density (of order M_Planck^4) for modes with momenta of order M_Planck, far in excess of the background energy density. We evaluate the amount of fine-tuning in the initial data necessary to avoid this back-reaction problem and find it is of order H/M_Planck. We also argue that the equation of state of the trans-Planckian modes is not vacuum-like. Therefore this model does not provide a suitable explanation for the dark energy.Comment: RevTeX - 15 pages, 7 figures: final version to appear in PRD, minor changes, 1 figure adde

The optical depth of the Universe to ultrahigh energy cosmic ray scattering in the magnetized large scale structure

This paper provides an analytical description of the transport of ultrahigh energy cosmic rays in an inhomogeneously magnetized intergalactic medium. This latter is modeled as a collection of magnetized scattering centers such as radio cocoons, magnetized galactic winds, clusters or magnetized filaments of large scale structure, with negligible magnetic fields in between. Magnetic deflection is no longer a continuous process, it is rather dominated by scattering events. We study the interaction between high energy cosmic rays and the scattering agents. We then compute the optical depth of the Universe to cosmic ray scattering and discuss the phenomological consequences for various source scenarios. For typical parameters of the scattering centers, the optical depth is greater than unity at 5x10^{19}eV, but the total angular deflection is smaller than unity. One important consequence of this scenario is the possibility that the last scattering center encountered by a cosmic ray be mistaken with the source of this cosmic ray. In particular, we suggest that part of the correlation recently reported by the Pierre Auger Observatory may be affected by such delusion: this experiment may be observing in part the last scattering surface of ultrahigh energy cosmic rays rather than their source population. Since the optical depth falls rapidly with increasing energy, one should probe the arrival directions of the highest energy events beyond 10^{20}eV on an event by event basis to circumvent this effect.Comment: version to appear in PRD; substantial improvements: extended introduction, sections added on angular images and on direction dependent effects with sky maps of optical depth, enlarged discussion of Auger results (conclusions unchanged); 27 pages, 9 figure

Integral field spectroscopy with SINFONI of VVDS galaxies. II. The mass-metallicity relation at 1.2 < z < 1.6

This work aims to provide a first insight into the mass-metallicity (MZ) relation of star-forming galaxies at redshift z~1.4. To reach this goal, we present a first set of nine VVDS galaxies observed with the NIR integral-field spectrograph SINFONI on the VLT. Oxygen abundances are derived from empirical indicators based on the ratio between strong nebular emission-lines (Halpha, [NII]6584 and [SII]6717,6731). Stellar masses are deduced from SED fitting with Charlot & Bruzual (2007) population synthesis models, and star formation rates are derived from [OII]3727 and Halpha emission-line luminosities. We find a typical shift of 0.2-0.4 dex towards lower metallicities for the z~1.4 galaxies, compared to the MZ-relation in the local universe as derived from SDSS data. However, this small sample of eight galaxies does not show any clear correlation between stellar mass and metallicity, unlike other larger samples at different redshift (z~0, z~0.7, and z~2). Indeed, our galaxies lie just under the relation at z~2 and show a small trend for more massive galaxies to be more metallic (~0.1 logarithmic slope). There are two possible explanations to account for these observations. First, the most massive galaxies present higher specific star formation rates when compared to the global VVDS sample which could explain the particularly low metallicity of these galaxies as already shown in the SDSS sample. Second, inflow of metal-poor gas due to tidal interactions could also explain the low metallicity of these galaxies as two of these three galaxies show clear signatures of merging in their velocity fields. Finally, we find that the metallicity of 4 galaxies is lower by ~0.2 to 0.4 dex if we take into account the N/O abundance ratio in their metallicity estimate.Comment: 7 pages, 4 figures, accepted in A&A Comments: Comments: more accurate results with better stellar mass estimate