357 research outputs found
Deccan volcanism at the Cretaceous-Tertiary boundary
The accuracy with which one can claim that Deccan trap volcanism occurred at the Cretaceous-Tertiary boundary (KTB) over a very short time interval is of key importance in deciding whether a volcanic origin of the KTB events should be taken seriously. In the two years since paleomagnetic, paleontological and geodynamic evidence was published, further data have become available and the case now appears to be well constrained. The Ar-40/Ar-39 results from six labs have yielded some 24 reliable plateau ages that narrow the age range to 65 to 69 Ma. Moreover, it appears that a significant part of this range results from inter-lab spread and possible minor alteration. Paleontology demonstrates that volcanism started in the Maestrichtian, more precisely in the A. mayaroensis zone. Paleomagnetism shows that volcanism spanned only 3 chrons and only one correlation remains possible, that of the main central reversed chron with 29R. Therefore, whereas Ar-40/Ar-39 is able only to restrict the duration of volcanism to some 4 Ma, paleomagnetism restricts it to 0.5 Ma. Using some geochemical indicators such as C-13 as proxy, it is suggested that volcanism actually consists of a few shorter events of unequal magnitude. Extrusion rates may be as high as 100 cu km/yr and fissure lengths as long as several 100 km. Such a scenario appears to be at least as successful as others in accounting for most anomalies observed at the KTB. Particularly important are Iridium and other platinum group elements (PGE) profiles, Sr-87/Sr-86, C-13, 0-18, other exotic geochemical signatures, spherules, soot, shocked minerals, selective and stepwise extinctions. The environmental impact of CO2 possibly released during explosive phases of volcanism, and SO2 released during effusive phases, and the ability of volcanism to ensure worldwide distribution of KTB products are now all addressed. In conclusion, the case for a causal link between internal hotspot activity, birth of the Reunion hotspot itself as the Deccan and KTB events appears to rest on an increasingly stronger basis
On the Slow Drift of Solstices: Milankovic Cycles and Mean Global Temperature
The Earth's revolution is modified by changes in inclination of its rotation
axis. Despite the fact that the gravity field is central, the Earth's
trajectory is not closed and the equinoxes drift. Milankovic (1920) argued that
the shortest precession period of solstices is 20,7kyr: the Summer solstice in
one hemisphere takes place alternately every 11kyr at perihelion and at
aphelion. We have submitted the time series for the Earth's pole of rotation,
global mean surface temperature and ephemeris to iterative Singular Spectrum
Analysis. iSSA extracts from each a trend, a 1yr and a 60yr component. Both the
apparent drift of solstices of Earth around the Sun and the global mean
temperature exhibit a strong 60yr oscillation. The "fixed dates" of solstices
actually drift. Comparing the time evolution of the Winter and Summer solstices
positions of the rotation pole and the first iSSA component (trend) of the
temperature allows one to recognize some common features. A basic equation from
Milankovic links the derivative of heat received at a given location on Earth
to solar insolation, known functions of the location coordinates, solar
declination and hour angle, with an inverse square dependence on the Sun-Earth
distance. We have translated the drift of solstices as a function of distance
to the Sun into the geometrical insolation theory of Milankovic. Shifting the
inverse square of the 60yr iSSA drift of solstices by 15 years with respect to
the first derivative of the 60yr iSSA trend of temperature, that is exactly a
quadrature in time, puts the two curves in quasi-exact superimposition. The
probability of a chance coincidence appears very low. Correlation does not
imply causality when there is no accompanying model. Here Milankovic's equation
can be considered as a model that is widely accepted. This paper identifies a
case of agreement between observations and a mathematical formulation
On variations of global mean surface temperature: When Laplace meets Milankovi\'c
In his mathematical theory, Milankovic finds a link between the heat received
by the Earth surface per unit time as a function of the solar ephemerids and
derives a model of climate changes at periods longer than a few thousand years
and more. In this paper, we investigate the potential connections of global
temperature and Earth rotation at much shorter periods, in the complementary
range of one to a few hundred years. For temperature, we select the HadCrut05.
For Earth rotation, defined by pole coordinates and length of day, we use the
IERS data sets. Using iterative Singular Spectrum Analysis (iSSA), we extract
the trend and quasi-periodic components of these time series. The first
quasi-periodic components (period ~80-90 years) are expressions of the
Gleissberg cycle and are identical (at the level of uncertainty of the data).
Taken together, the trend and Gleissberg components allow one to reconstruct
87% of the variance of the data for lod and 48% for temperature. The next four
iSSA components, with periods ~40, 22, 15 and 9 years. The Lagrange and Laplace
theories imply that the derivative of pole motion should be identical to lod
variations: this strong check is passed by the trend + Gleissberg
reconstructions. The annual oscillations of pole motion and lod are linked to
annual variations in Sun-Earth distance, in agreement with an astronomical, but
not a climatic origin. The results obtained in this paper for the observed
temperature/rotation couple add to the growing list of evidence of solar and
planetary forcings of gravitational nature on a number of geophysical processes
(including sea-level, sea-level pressure, sea-ice extent, oceanic climate
indices).Comment: 13 pages, 13 figure
Is the Earth's magnetic field a constant ? a legacy of Poisson
In the report he submitted to the Acad\'emie des Sciences, Poisson imagined a
set of concentric spheres at the origin of the Earth's magnetic field. It may
come as a surprise to many that Poisson as well as Gauss both considered the
magnetic field to be constant. We propose in this study to test this surprising
assertion for the first time evoked by Poisson (1826). First, we will present a
development of Maxwell's equations in the framework of a static electric field
and a static magnetic field in order to draw the necessary consequences for the
Poisson hypothesis. In a second step, we will see if the observations can be in
agreement with Poisson (1826). To do so, we have chosen to compare 1) the polar
motion drift and the secular variation of the Earth's magnetic field, 2) the
seasonal pseudo-cycles of day length together with those of the sea level
recorded by different tide gauges around the globe and those of the Earth's
magnetic field recorded in different magnetic observatories. We then propose a
mechanism, in the spirit of Poisson, to explain the presence of the 11-year in
the magnetic field. We test this mechanism with observations and finally we
study closely the evolution of the g10 coefficient of the IGFR over time
Tracking the Late Jurassic apparent (or true) polar shift in U-Pb-dated kimberlites from cratonic North America (Superior Province of Canada)
Different versions of a composite apparent polar wander (APW) path of variably selected global poles assembled and averaged in North American coordinates using plate reconstructions show either a smooth progression or a large (âŒ30°) gap in mean paleopoles in the Late Jurassic, between about 160 and 145 Ma. In an effort to further examine this issue, we sampled accessible outcrops/subcrops of kimberlites associated with high-precision U-Pb perovskite ages in the Timiskaming area of Ontario, Canada. The 154.9â±â1.1 Ma Peddie kimberlite yields a stable normal polarity magnetization that is coaxial within less than 5° of the reverse polarity magnetization of the 157.5â±â1.2 Ma Triple B kimberlite. The combined âŒ156 Ma Triple B and Peddie pole (75.5°N, 189.5°E, A95â=â2.8°) lies about midway between igneous poles from North America nearest in age (169 Ma Moat volcanics and the 146 Ma Ithaca kimberlites), showing that the polar motion was at a relatively steady yet rapid (âŒ1.5°/Myr) pace. A similar large rapid polar swing has been recognized in the Middle to Late Jurassic APW path for Adria-Africa and Iran-Eurasia, suggesting a major mass redistribution. One possibility is that slab breakoff and subduction reversal along the western margin of the Americas triggered an episode of true polar wander
On the external forcing of global eruptive activity in the past 300 years
The decryption of the temporal sequence of volcanic eruptions is a key step
in better anticipating future events. Volcanic activity is the result of a
complex interaction between internal and external processes, with time scales
spanning multiple orders of magnitude. We review periodicities that have been
detected or correlated with volcanic eruptions/phenomena and interpreted as
resulting from external forces. Taking a global perspective and longer time
scales than a few years, we approach this interaction by analyzing three time
series using singular spectral analysis: the global number of volcanic
eruptions (NVE) between 1700 and 2022, the number of sunspots (ISSN), a proxy
for solar activity, the polar motion (PM) and length of day (lod), two proxies
for gravitational force. Several pseudo-periodicities are common to NVE and
ISSN, in addition to the 11-year Schwabe cycle that has been reported in
previous work, but NVE shares even more periodicities with PM. These
quasi-periodic components range from ~5 to ~130 years. We interpret our
analytical results in light of the Laplace's paradigm and propose that,
similarly to the movement of Earth's rotation axis, global eruptive activity is
modulated by commensurable orbital moments of the Jovian planets, whose
influence is also detected in solar activity
Cold atom Clocks and Applications
This paper describes advances in microwave frequency standards using
laser-cooled atoms at BNM-SYRTE. First, recent improvements of the Cs
and Rb atomic fountains are described. Thanks to the routine use of a
cryogenic sapphire oscillator as an ultra-stable local frequency reference, a
fountain frequency instability of where
is the measurement time in seconds is measured. The second advance is a
powerful method to control the frequency shift due to cold collisions. These
two advances lead to a frequency stability of at 7\times 10^{-16}^{87}^{133}$Cs fountains.
Finally we give an update on the cold atom space clock PHARAO developed in
collaboration with CNES. This clock is one of the main instruments of the
ACES/ESA mission which is scheduled to fly on board the International Space
Station in 2008, enabling a new generation of relativity tests.Comment: 30 pages, 11 figure
Determination of rapid Deccan eruptions across the Cretaceous-Tertiary boundary using paleomagnetic secular variation: 2. Constraints from analysis of eight new sections and synthesis for a 3500-m-thick composite section
International audienceThe present paper completes a restudy of the main lava pile in the Deccan flood basalt province (trap) of India. Chenet et al. (2008) reported results from the upper third, and this paper reports the lower two thirds of the 3500-m-thick composite section. The methods employed are the same, i.e., combined use of petrology, volcanology, chemostratigraphy, morphology, K-Ar absolute dating, study of sedimentary alteration horizons, and as the main correlation tool, analysis of detailed paleomagnetic remanence directions. The thickness and volume of the flood basalt province studied in this way are therefore tripled. A total of 169 sites from eight new sections are reported in this paper. Together with the results of Chenet et al. (2008), these data represent in total 70% of the 3500-m combined section of the main Deccan traps province. This lava pile was erupted in some 30 major eruptive periods or single eruptive events (SEE), each with volumes ranging from 1000 to 20,000 km 3 and 41 individual lava units with a typical volume of 1300 km 3. Paleomagnetic analysis shows that some SEEs with thicknesses attaining 200 m were emplaced over distances in excess of 100 km (both likely underestimates, due to outcrop conditions) and up to 800 km. The total time of emission of all combined SEEs could have been (much) less than 10 ka, with most of the time recorded in a very small number of intervening alteration levels marking periods of volcanic quiescence (so-called ''big red boles''). The number of boles, thickness of the pulses, and morphology of the traps suggest that eruptive fluxes and volumes were larger in the older formations and slowed down with more and longer quiescence periods in the end. On the basis of geochronologic results published by Chenet et al. (2007) and paleontological results from Keller et al. (2008), we propose that volcanism occurred in three rather short, discrete phases or megapulses, an early one at $67.5 ± 1 Ma near the C30r/C30n transition and the two largest around 65 ± 1 Ma, one entirely within C29r just before the K-T boundary, the other shortly afterward spanning the C29r/C29n reversal. We next estimate sulfur dioxide (likely a major agent of environmental stress) amounts and fluxes released by SEEs: they would have ranged from 5 to 100 Gt and 0.1 to 1 Gt/a, respectively, over durations possibly as short as 100 years for each SEE. The chemical input of the Chicxulub impact would have been on the same order as that of a very large single pulse. The impact, therefore, appears as important but incremental, neither the sole nor main cause of the Cretaceous-Tertiary mass extinctions
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