Potential paleocirculation implications by coupling Pb and Nd isotope analyses on different grain-size fractions from Labrador Sea sediments

The production of deep water in the North Atlantic is one of the puzzle in understanding oceanic influence in climate changes. In this work, Nd and Pb isotopes were measured on the fine fraction of Labrador Sea sediments. Our aim is to record the relative contribution of fine particle supplies carried by the North Atlantic deep components into Labrador Sea. Based on characterization of potential geographical sources of particles, three main sources contribute to sediment mixture at core location: an old Precambrian crustal material from Canada, Greenland and Scandinavia (NAS), a Paleozoic or younger crustal material from East Greenland, Europa, and Scandinavia (YC) and a volcanic source from Iceland, Faeroe and Reykjanes Ridge (MAR). For the last 12 kyr, clay isotope signatures indicate two mixtures of sediment sources. The first mixture is composed of proximal material from Labrador Sea margins and distal deep current-driven crustal source. From 6.5 kyr onward, the mixture is characterized by the crustal and volcanic components. Since the significant decrease in proximal deglacial supplies, the evolution of the relative contributions of sediment sources suggests major changes in relative contributions of the deep water masses carried by the Western Boundary Undercurrent (WBUC) over the past 8.4 kyr. The progressive intensification of WBUC was associated mainly with the transport of North East Atlantic Deep Water mass until 6.5 kyr and with Denmark Strait Overflow Water thereafter. The establishment of the modern circulation at 3 kyr suggests a reduced influence of the Denmark Strait Overflow Water, synchronous with the full appearance of the Labrador Sea Water mass. Our isotopic dataset emphasizes several changes in the relative contribution of the two major components of North Atlantic Deep Water throughout the Holocene. However, if the clay-size fraction gives informations about the inception and presence of a deep current, it does tell nothing about the strength of this current. Moreover, clays are not likely to be deposited in case of high-strength deep current, resulting in an uncomplete and/or biased reconstruction of deep current evolution. To overcome this problem, it is necessary to look also at coarser fractions. Pb and Nd isotope compositions were thus analysed by MC-ICP-MS on different grain-size fractions (clay-size < 2 `µm, cohesive silt 2-10 µm, fine silt 10-30 µm and coarse silt 30-60 µm) on Late Glacial and Holocene sediments. Four grain-size fractions were investigated for a set of 12 samples from core MD99-2227:. Our results show a clear variation of Pb concentration and isotopic signatures according to grain-size (Nd data still in progress). The observed shifts are interpreted in terms of changes in deep current strength. This approach allows to monitor deep current changes through time, whatever the strength of paleocurrent

The effect of the solar rotational irradiance variation on the middle and upper atmosphere calculated by a three-dimensional chemistry-climate model

This paper analyzes the effects of the solar rotational (27-day) irradiance variations on the chemical composition and temperature of the stratosphere, mesosphere and lower thermosphere as simulated by the three-dimensional chemistry-climate model HAMMONIA. Different methods are used to analyze the model results, including high resolution spectral and cross-spectral techniques. To force the simulations, an idealized irradiance variation with a constant period of 27 days (apparent solar rotation period) and with constant amplitude is used. While the calculated thermal and chemical responses are very distinct and permanent in the upper atmosphere, the responses in the stratosphere and mesosphere vary considerably in time despite the constant forcing. The responses produced by the model exhibit a non-linear behavior: in general, the response sensitivities (not amplitudes) decrease with increasing amplitude of the forcing. In the extratropics the responses are, in general, seasonally dependent with frequently stronger sensitivities in winter than in summer. Amplitude and phase lag of the ozone response in the tropical stratosphere and lower mesosphere are in satisfactory agreement with available observations. The agreement between the calculated and observed temperature response is generally worse than in the case of ozone

Obstacles and benefits of the implementation of a reduced-rank smoother with a high resolution model of the tropical Atlantic Ocean

Most of oceanographic operational centers use three-dimensional data assimilation schemes to produce reanalyses. We investigate here the benefits of a smoother, i.e. a four-dimensional formulation of statistical assimilation. A square-root sequential smoother is implemented with a tropical Atlantic Ocean circulation model. A simple twin experiment is performed to investigate its benefits, compared to its corresponding filter. Despite model's non-linearities and the various approximations used for its implementation, the smoother leads to a better estimation of the ocean state, both on statistical (i.e. mean error level) and dynamical points of view, as expected from linear theory. Smoothed states are more in phase with the dynamics of the reference state, an aspect that is nicely illustrated with the chaotic dynamics of the North Brazil Current rings. We also show that the smoother efficiency is strongly related to the filter configuration. One of the main obstacles to implement the smoother is then to accurately estimate the error covariances of the filter. Considering this, benefits of the smoother are also investigated with a configuration close to situations that can be managed by operational center systems, where covariances matrices are fixed (optimal interpolation). We define here a simplified smoother scheme, called half-fixed basis smoother, that could be implemented with current reanalysis schemes. Its main assumption is to neglect the propagation of the error covariances matrix, what leads to strongly reduce the cost of assimilation. Results illustrate the ability of this smoother to provide a solution more consistent with the dynamics, compared to the filter. The smoother is also able to produce analyses independently of the observation frequency, so the smoothed solution appears more continuous in time, especially in case of a low frenquency observation network

Les mécanismes de repliement des protéines solubles

Mechanisms of folding of soluble proteins. The function of a protein is carried out by its three-dimensional structure. The rules for translation of the information encoded by DNAinto an amino acid sequence are well known. On the other hand, the rules governing the folding of an amino acid sequence into an univocal three-dimensional structure are still unknown. The speed at which this phenomenon takes place makes the experimental investigations difficult using biophysical methods. Various theoretical approaches have been set up with more or less success. None, nowadays, describes in a right way, the mechanisms involved. We present here the various hypotheses found in the literature about the early steps of the folding of soluble proteins

A Deep Photometric Look at Two of Andromeda's Dwarf Spheroidals: X and XVII

We use deep wide-field photometry from the Large Binocular Camera to study the stellar and structural properties of the recently discovered Andromeda X and Andromeda XVII (And X and And XVII) dwarf galaxies. Using the mean apparent magnitude of the horizontal branch (HB), we derive distances of 621 +- 20 kpc to And X and 734+- 23 kpc to And XVII, closer by >60 kpc than the previous estimates which were based on red giant branch (RGB) observations. Thus our results warrant against the use of the RGB tip method for determining distances to systems with sparsely populated RGBs, and show how crucial HB observations are in obtaining accurate distances in systems such as these. We find that And X is a relatively faint (MV = -7.36), highly elongated (e = 0.48) system at a distance of 174 +- 62 kpc from Andromeda. And XVII is brighter (MV = -8.61) with an M31-centric distance of 73 kpc which makes it one of the closest satellites to Andromeda. Both galaxies are metal-poor: we derive =-2.2 for And X, while And XVII shows = -2.0, consistent with the relation of higher luminosity dwarfs being more metal- rich. Additionally, both galaxies show considerable intrinsic spreads in metallicity (0.2 and 0.3 dex for And X and And XVII respectively), consistent with multiple stellar populations.Comment: 8 pages, 6 figure

Non-steady wind turbine response to daytime atmospheric turbulence.

Relevant to drivetrain bearing fatigue failures, we analyse non-steady wind turbine responses from interactions between energy-dominant daytime atmospheric turbulence eddies and the rotating blades of a GE 1.5 MW wind turbine using a unique dataset from a GE field experiment and computer simulation. Time-resolved local velocity data were collected at the leading and trailing edges of an instrumented blade together with generator power, revolutions per minute, pitch and yaw. Wind velocity and temperature were measured upwind on a meteorological tower. The stability state and other atmospheric conditions during the field experiment were replicated with a large-eddy simulation in which was embedded a GE 1.5 MW wind turbine rotor modelled with an advanced actuator line method. Both datasets identify three important response time scales: advective passage of energy-dominant eddies (≈25-50 s), blade rotation (once per revolution (1P), ≈3 s) and sub-1P scale (s) response to internal eddy structure. Large-amplitude short-time ramp-like and oscillatory load fluctuations result in response to temporal changes in velocity vector inclination in the aerofoil plane, modulated by eddy passage at longer time scales. Generator power responds strongly to large-eddy wind modulations. We show that internal dynamics of the blade boundary layer near the trailing edge is temporally modulated by the non-steady external flow that was measured at the leading edge, as well as blade-generated turbulence motions.This article is part of the themed issue 'Wind energy in complex terrains'

Comparison of model estimates of the effects of aviation emissions on atmospheric ozone and methane

One of the significant uncertainties in understanding the effects of aviation on climate is the effects of aviation emissions on ozone and atmospheric chemistry. In this study the effects of aviation emissions on atmospheric ozone for 2006 and two projections for 2050 are compared among seven models. The models range in complexity from a two-dimensional coupled model to three-dimensional offline and fully coupled three-dimensional chemistry-climate models. This study is the first step in a critical assessment and comparison among these model results. Changes in tropospheric O3 burdens range from 2.3 Tg-O3/Tg-N to 3.0 Tg-O3/Tg-N, ozone radiative forcings range from 6 to 37 mW/m2, and methane radiative forcings range from -8.3 to -12.5 mW/m2 for the 2006 aviation emissions. As a group, the chemistry transport models tend to have similar responses while the fully coupled models tend to separate from this group and do not show similar responses to each other. ©2013. American Geophysical Union. All Rights Reserved