Jean-Pierre Valet
Institut de Physique du Globe de Paris
(IPGP)
1, rue Jussieu,
75238 Paris Cedex 05, France
tel : 33 - 1- 83-95-75-03
Research
interest
Geomagnetic secular variation
Geomagnetic reversals and excursions
Relative and absolute paleointensity
Magnetotactic bacteria
Magnetization acquisition processes
Rock magnetism
Paleoenvironment and diagenesis
Magnetostratigraphy
I have been actively studying the geomagnetic field variations recorded
by magnetization of lava flows and sediments with typical time constants from
hundreds to millions years. Measurements of the total field vector allow us to
address questions such as: what drives the fluctuations of the field refered as
secular variation, what causes field reversals and what happens during the
transition between the two polarity states, why does the field sometimes fails
to reverse, what happens during field excursions, whether the mantle has a
significant influence on some field instabilities etc.. Exploring the field
behaviour in the past requires to assess the nature and the geomagnetic origin
of the signals that are recorded by the magnetization of rocks. This can only
be achieved by detailed knowledge of the processes involved in the acquisition
of magnetization that are documented by rock magnetic experiments and analyzes.
I have also been interested in various other aspects linked to
magnetism, such as studies related to the signature of climatic changes in rock
magnetic parameters, in depositional processes, in mineralogical studies and in biomagnetism.
A
few results
My initial interest went to geomagnetic reversals. After several years
spent to decipher reversal records from sedimentary sequences, it became clear
that the response of magnetization to the field was not appropriate to record
the very rapid field changes prevailing during periods of very low field
intensity and that sediments do not provide the appropriate resolution.
An impressive piece of work has been conducted with Emilio
Herrero-Bervera after sampling huge piles of lava flows in the Waianeae
sequence of Oahu, Hawaii. For the first time, we could deal with a succession
of reversals from volcanic flows. We also obtained duplicate records of the
same reversals from different locations. The results indicate complex field
behaviour with precursors and rebounds, similarly to what would be observed
with the secular variation of the present field in presence of a very weak or
zero axial dipole field of the non dipole field. We have also shown that
clustering of directions frequently reported in records were caused by periods
of intense volcanic activity and not by a standstill in the reversal porcess.
These results have been integrated in a compilation of the most
detailed records from volcanics from sequences of overlying lava flows with
well defined reversed and normal polarities prior to the transition and several
distinct transitional directions. Very few records meet these basic criteria,
but they all show the same characteristics. There is no systematics in the
apparent reversal geometry, all transitions being characterized by complex pole
paths between the two polarities. It appears also that these detailed volcanic
records do not show any preference of the pole for a specific longitude when
switching between the two polarities.
However the most striking feature is a remarkable coherency when
plotting all records in terms of angular changes form the axial dipole. It
appears clearly that processes and time constants have remained unchanged for
all reversals. The reversal process can be divided in three phases, a precuror,
a rapid transit between the two polarities and/or a rebound. We can estimate
the duration of the transition by comparison with paleosecular variation. The
precursor and rebound correspond to secular variation of the non-axial dipole
in presence of a very weak axial dipole Thes phases do not last longer than
2500 yrs while the the transit between north and south if of the order of 1ka
only.
Problems encountered with sedimentary records have
generated interest to further constrain the mechanisms involved in the
magnetization of sediments using artifical redeposition in laboratory. The
results obtained with carbonated sediments point out the linearity of magnetization
with field strength which is a very good news for studies of relative
paleointensity in sediments. In contrast, we have seen no linearity and no
suitable orientation of magnetic grains observed for experiments conducted with
clayish sediments. This points out the importance of using carbonated sediments
for studies related to the geomagnetic field behaviour. Awe also reported that
post-depositional reorientation of the magnetic grauns is extremely limited. Other
diagenetic processes that remain to be documented would thus favour a
time-averaging of the paleomagnetic records which affects the resolution of the
signal in sediments.
Dynamics of
reversals and duration of the 3 successive phases.
Geomagnetic
Excursions
Geomagnetic excursions
represent short episodes that last a few thousand years at most during which
the field considerably exceeds its normal range of variability during a
polarity state. The Laschamp
event discovered in lava flows of Massif central (France) is the youngest and
the most studied field excursion. Its geomagnetic origin has been controversial
due to the existence of self-reversal processes. We have confirmed that reverse
polarity flows are affected by self-reversals, but we found that this is also
the case for normal flows. A direct consequence is that self-reversals cannot
be taken as responsible for the reverse directions. We sampled a large collection
of flows. New K-Ar dates by Gilles Gurin provided a coherent chronology of the
successive polarities. The 37 ka old reverse directions of the Olby flow are
consistent with the 41.9 kyr old normal polarity flows preceding the event. A
younger flow with intermediate directions dated at 33.3 ka may be related to
the Mono Lake event.
Surprisingly, no attention has
been given to the fact that the Laschamp and Mono Lake events occurred during
the period of Neandertal extinction (this is even clearer after recalibration
of the 14C ages) which lasted for several thousand years. During
this time interval the geomagnetic field strength was considerably reduced and shielding
efficiency of the magnetosphere lowered, leaving energetic particles reach
latitudes as low as 30Ħ. The enhanced flux of high-energy protons (linked to
solar activity) into the atmosphere yielded significant ozone depletion down to
latitudes of 40-45Ħ. A direct consequence was an increase of UV-B radiations at
the surface by at least 15-20% in Europe with significant impacts for human
populations. With Hlne Valladas we suggest that these conditions, maybe added
to other factors, have contributed to the demise of Neanderthal population.
We have compiled the most
detailed records of excursions that occurred during the Brunhes and Matuyama
chrons. The virtual geomagnetic poles (VGPs) of at least one record of each
event are able to reach the opposite polarity. It is impossible to reach the
ratio of reversed to intermediate VGPs present in the paleomagnetic records if
the excursions were not associated with a short period of reversed dipole
field. Therefore, excursions should be regarded as two successive reversals
bracketing an aborted polarity interval. We propose that the same underlying mechanisms
prevail in both situations (excursions or reversals) and that below a certain
strength, the field reaches an unstable position which preludes either the
achievement of a reversal or its return to former polarity.
Relative
and absolute paleointensity
Sediments provide continuous long-term records of the axial dipole
intensity. Our first studies focused on 5 distinct mediterranean cores. The
similarity of the results between all cores provided the first positive test
for relative paleointensity in sediments and the coherence with the existing
volcanic dataset further validated the concept. The demonstration has been
completed with Laure Meynadier after comparing the results from mediterranean
cores with 5 other cores from the Indian ocean for the past 140 ka. This very
first check revealed the coherency of geomagnetic changes recorded at very
distinct locations, and thus validated the approach. Subsequently, newly
acquired records yielded the construction of Sint-200 with Yohan Guyodo from 15
worldwide distributed records of relative paleointensity. The overall agreement
of Sint-200 with variations in 10Be production has been another
significant step. Following the same track we produced a composite curve
covering the entire Brunhes period (Sint-800).
The next challenge was to investigate the evolution of the field
across reversals. With Laure Meynadier we had the chance of finding a
carbonated sequence at site 851 during the ODP leg 138 with a very accurate
time reference completed by an excellent stable and homogeneous magnetization.
The records of relative paleointensity spanning the past 4 Ma revealed a slow
decrease of the field prior to the reversals likely caused by diffusion and which
is in sharp contrast with the intense and rapid recovery marking the end of the
transition and the advection processes associated with the restoration of the
dipole. Studies conducted on volcanic sequences in the Canary islands and
Hawaii (see above) further confirmed the pattern of field intensity across
recent reversals including the last one. Subsequently we produced a composite and
global record for the past 2Ma (Sint-2000) which serves also as a constraint to
the dynamo models.
Francois Ptrlis and Stphane Fauve have been working on the Van
Karman sodium experiment, the first experimental dynamo that produced a field
evolution with reversals. It is surprising that the field changes accompanying
the reversals are quite similar to Sint-2000, particularly the asymmetrical
pattern of field intensity and the overshoot occurring at the end of the
recovery phase. These features are also predicted by Petrelis et al reversal
model involving a coupling between the dipolar and the quadrupolar modes during
reversals.
Evolution
of the geomagnetic dipolar field intensity during the past 2Ma calibrated
with volcanic records
Success met for the recent million years incited us to scrutunize the
other end of the field history in archean rocks. The results obtained with
Mlina Macouin and Jean Besse on canadian dykes 1.1 to 2.4Ga old suggest a
reduced field intensity with possible implications on formation of the inner
core.
Paleosecular
variation
Using a simple dipole model calculated from the directional changes
recorded at 5 different sites during the past 2 ka, we have shown that field
changes derived from the archeomagnetic data at other multiple sites are of
equivalent quality to those derived from complex inversion models with a
spherical harmonic description of the field up to degree 5 and more. We thus
consider that these models are not fully relevant in the present state of the
databases and given the uneven geographical distribution of the sites.
The geocentric axial dipole (GAD) field describes most of the
structure in the time-averaged geomagnetic field. However the paleomagnetic
record displays small but persistent departures. Successive analyses of the new
databases have revealed that departures from the GAD can be fitted by an axial
quadrupole component amounting 3 to 5% of the axial dipole. Using the Sint-2000
composite curve and the most recent directional volcanic database for the past
4Ma we have shown that the deviations from the GAD are larger when the dipole
field is smaller, suggesting that
it keeps more or less the same strength.
The geodynamo shows evidence for long time scales that cannot be
connected with fluid dynamical processes in the core. Slowly evolving processes
like the modulation in reversal frequency are linked to the influence of the
mantle. Thermal interaction and /or chemical exchange between core and mantle
should be associated with heterogeneities which would modify the circulation in
the core and hence be reflected by magnetic anomalies. The existence of
long-term standing field anomalies is critical to test these concepts. In this
perspective, the debate regarding the persistence of the Pacific anomaly
reflected by a weak non dipole field
is critical. The lava flows that we have studied in Hawaii do not
indicate any significant anomaly except for the past 2ka, but this period may
be too short for a complete time-averaged of the field changes.
Going further south with Agns Elmaleh we have measured marine cores
from the Fiji basin. They are characterized by a large inclination anomaly in
agreement with previous measurements in the same area. However, we have not
detected any deviation in a large set of lava flows sampled in the Java island
and thus conclude to the absence of a long-term persistent non zonal anomaly of
the non-dipole field. Tthe large deviations in the Fiji area were likely caused
by local tectonics.
Rock
magnetism and magnetization signals
Because magnetization
acquisition depends on complex processes in sediment we developed experiments
aimed at describing the limits that they imposed on records of field
variations. Redeposition experiments using a gelatin allowed us to measure the
proportion of magnetic grains that are mechanically locked for different water
contents. The magnetic moment normalized by concentration was then compared to
the natural magnetization to define the concentration level at which the
magnetization has been locked-in. With Simo Spassov we are have investigated
different kinds of sediments (see above in the reversal section).
Much effort has been
carried out at improving techniques for relative and absolute paleointensity
using different approaches for normalization. Among those we have investigated
the influence of rock magnetic parameters that are sensitive to climatic changes.
We proposed to detect the residual climatic components using principal
component analysis.
Studies of absolute
paleointensity from lava flows are very tedious and poorly successful. Various
techniques have been proposed during the past 20 years. We are convinced that
the double heating technique of Thellier remains the unique approach. We
conducted multiple studies of contemporaneous flows. They all revealed that
samples with single domain grains of pure magnetite are the most suitable, if
not the only ones, capable of providing accurate determinations of the
paleofield intensity. Therefore, we consider that robust paleointensity
experiments can only be conducted using samples with high unblocking
temperatures of the natural magnetization.
We have carried out
mineralogical analyses on volcanic rocks, especially on several lava flows that
cooled down during the last reversal. Surprisingly, a few flows recorded the
two polarities, which given their thickness would imply that the transition was
as short as a few weeks. The most coherent
interpretation is that baking by the overlying flow was accompanied by
low-temperature oxidation of titanomagnetite to cation deficient magnetite and
produced partial remagnetization. Similarities with rapid field changes reported
in the Steens Mountain record suggest that these events should be interpreted
with caution.
Magnetotactic
bacteria are well known for their ability of mineralizing magnetite grains with
potential interest for industry and medicine. A freshwater
population of magnetotactic bacteria has been extracted from the Seine River
(France). Transmission electron microscopy by Aude Isambert and Nicolas Menguy
has revealed seventeen different morphotypes based on morphological criteria.
X-ray energy dispersive spectroscopy and electron diffraction analyses have
also shown cells containing Ba-rich and CaO inclusions. Two major modes of
magnetite crystals have been identified. The population does not meet some of
the criteria for biogenicity, which have been used so far for biomagnetites,
particularly those concerning the [111] elongation axis.
Selection of significant scientific contributions:
- First record of two successive reversals
(Nature 304, 1983; Nature 309, 1984)
- Sedimentary records of multiple reversals
with recurrent characteristics (Nature 311, 1984)
- Persistance of secular variation during a reversal (Nature 322,
1986).
- Records of the last transition suggest the a non-dipole field during
reversals (Earth Planet Sci. Lett. 87, 1988; Earth Planet Sci. Lett. 94,
1989).
- The poles lie 90Ħ away from the site longitude in sedimentary records
of reversals (Nature 356 , 1992).
- Records of relative paleointensity from Mediterranean and the Indian
ocean demonstrate the suitability of paleointensity records in sediments (Earth
Planet Sci. Lett. 114, 1992).
-
First record of field intensity for the past 4Ma -
asymmetrical evolution (Nature 366, 1993).
-
Effect of magnetic fabric on reversal records from
sediments (Nature 374, 1995).
-
First composite curve of field intensity changes
during the past 200 ka (Earth Planet. Sci. Lett., 143, 1996 )
-
First composite and reference curve of
paleointensity for the past 800 ka (Nature 399, 1999).
- Absolute paleointensity across reversals in volcanics confirms
asymmetry (J. Geophys. Res. 101, 1996).
- Time variations in geomagnetic intensity (Rev. Geophys, 41, 2003).
- Stable Mn siderite produces stable magnetite: implications for
carbonate spherules in meteorites (J. Geophys. Res., 108, 2003)
- Geomagnetic field strength
and reversal rate over the past 2Ma (Nature 435, 2005).
- Low Precambrian dipole recorded in Canadian dykes (G-cubed, 7, 2006).
- A selective procedure for
experiments of absolute paleointensity (Geophys. Res. Lett. 37, 2010)
Link between the Laschamp event and Neandertal demise (Quatern. Sci.
Rev. 29, 2010).
- Dynamics of reversals in
three phases - transitions are shorter than 1ka (Nature 49, 2012)
Publications
Analyse
bibliomtrique lĠadresse : http://scholar.google.fr/citations?user=QZlwr_cAAAAJ
1- Valet J.-P., Laj C., Paleomagnetic
record of two successive miocene geomagnetic reversals in western Crete, Earth Planet. Sci. Lett.,
54 , 53-53, 1981.
2- Valente J.-P., Laj C., Sorel D., Roy S. Valet J.-P., Paleomagnetic results from
mio-Pliocene sedimentary series in Crete,
Earth Planet. Sci. Lett., 57,
159-172, 1982.
3- Valet J.-P., Laj C., Langereis C.G.,Two
different geomagnetic reversals with identical VGP paths recorded at the same
site, Nature, 304, NĦ5924, 330-332,
1983.
4- Valet J.-P., Laj C., Reply to the
comment by K. Hoffman, Nature, 309, NĦ5963,"Matters Arising" NĦ 5963, 90-91, 1984.
5- Valet J.-P., Laj C., Invariant and
changing transitional field configurations in a sequence of geomagnetic
reversals, Nature, 311, NĦ5986, 552-555, 1984.
6- Valet J.-P., Laj C., Tucholka P.,
Volcanic record of reversal, Nature, 316, Ç News and
Views È, 217-218, 1985.
7- Valet J.-P., Laj C., Tucholka P., Reply to the comment by Gromme et
al, Nature, 318, 487, 1985.
8- Sen S., Valet J.-P., Ioakim C.,
Magnetostratigraphy of the Neogene of Kastellios Hill in Crete, Paleogeogr. Paleoclimat. Paleoecol., 53,
321-334, 1986.
9- Valet J.-P., Laj C., Tucholka P., High
resolution sedimentary record of a geomagnetic reversal, Nature, 322, NĦ6074, 27-32, 1986.
10- Valet J.-P. and the shipboard
scientific party of leg 108, Paleoclimate studied in the east Atlantic, Geotimes, October, 21-25, 1986.
11- Sen S., Valet J.-P., Magnetostratigraphy of
late Miocene continental deposits in Samos, Greece, Earth Planet. Sci. Lett., 80, 167-174, 1986.
12- Faugres J.-C,
Mannivit H., Valet J.-P.,
L'enregistrement des palocirculations et des paloclimats dans les dpots
nognes de l'Atlantique est subtropical et quatorial: apports du leg ODP
108, C.R. Acad. Sci. Paris, t 303, Srie II, NĦ17, 1986.
13- Valet J.P. with the shipboard
scientific party of ODP leg 108 Paleoclimatic linkage between high and low
latitudes, Nature, 322, Ç News
and Views È 211-212, 1986.
14- Bloemendal J.,
Tauxe L., Valet J.-P.,
High-resolution whole-core magnetic susceptibility logs from leg 108, Proc.Ocean Drilling Program PtA 108,1005-1013 , 1988.
15- Valet J.-P., Tauxe L., Clark D., The
Matuyama-Brunhes polarity transition recorded from lake Tecopa sediments
(California), Earth Planet. Sci. Lett. 87, 463-472, 1988.
16- Valet J.-P., Laj C. Langereis C.G.,
Sequential geomagnetic reversals recorded in upper tortonian marine clays in
western Crete (Greece), J. Geophys. Res.
Vol 93, NĦB2, 1131-1151, 1988.
17- Tauxe L., Valet J.-P., Relative paleointensity of the Earth's magnetic field from
marine sedimentary records: a global perspective, Phys. Earth Planet. Inter., 56, 59-68, 1989.
18-Valet J.-P., Tauxe L., Bloemendal J.,
The Matuyama-Brunhes geomagnetic reversal from two deep-sea cores of the
east-equatorial Atlantic, Proceedings of
the Ocean Drilling Program, Scientific Results Vol 108, 441-454 , 1989.
19- Tauxe L., Valet J.-P., Bloemendal J.,The
magnetostratigraphy of leg 108 sediments, Proceedings
of the Ocean Drilling Program, Scientific Results Vol 108, 429-440, 1989.
20- Valet J.-P., Tauxe L., Clement B.M.,
Equatorial and mid-latitudes records of the last geomagnetic reversal from the
Atlantic Ocean, Earth Planet. Sci. Lett.,
94,371-384,1989.
21- Bloemendal J.,
King J.W., Tauxe L., Valet J.-P., Rock magnetic stratigraphy of leg 108
sites 658, 659, 661 and 665 eastern equatorial Atlantic. Proceedings of the Ocean Drilling Program, Scientific Results Vol 108,
415-428, 1989.
22- Weaver P.P.E.,
Backman J., Baldauf J.-G., Bloemendal J., Mannivit H., Pokras E.M., Raymo M.E.,
Tauxe L., Valet J.-P., Chephow-Lasty
A. Glafsson G., Biostratigraphic synthesis ODP leg 108 Tropical Atlantic, Proceedings of the Ocean Drilling Program,
Scientific Results Vol 108, 455-462, 1989.
23- Tric E., Laj C.,
Jehanno C., Valet J.-P., Mazaud A.,
Kissel C., Iaccarino S.,A detailed record of the upper Olduvai polarity
transition from high sedimentation rate marine deposits of the Po valley, Phys. Earth Planet. Inter., 65,319-336, 1991.
24- Valet J.-P., Les transitions de
polarit du champ gomagntique, Rapport
quadriennal XXme assemble de l'Union Godsique et Gophysique Internationale
125-140, 1991.
25- Tric E., Laj C.,
Valet J.-P., Tucholka P., Guichard
F., The Blake geomagnetic event: transition geometry, dynamical characteristics
and geomagnetic significance, Earth
Planet. Sci. Lett., 102,1-13,1991.
26- Valet
J.-P., Leg 138 Participants, Ancient ocean -Climate link, Nature , Vol 353, Ç News and
Views È, 304-305,1992.
27- Valet J.-P., Courtillot V.,, Les
inversions du champ magntique terrestre, La
Recherche, 23, 1002-1013, 1992.
28- Valet
J.-P. Leg 138 Participants, High-resolution studies of the eastern
equatorial Pacific, Eos Transactions 73 , 257-262, 1992
29- Tric E., Valet J.-P., Tucholka P., Paterne M.,
Labeyrie L., Guichard F., Tauxe L. and Fontugne M, Paleointensity of the
geomagnetic field during the last eighty thousand years, J.
Geophys. Res., Vol.97, NoB6, 9337-9351, 1992
30- Valet J.P., Tucholka P., Courtillot V.,
Meynadier L., Paleomagnetic constraints on the geometry of the geomagnetic
field during reversals, Nature , 356,
400-407, 1992
31- Quidelleur X., Valet J.-P., Thouveny N.,
Multicomponents of magnetization in paleomagnetic records of reversals from
continental sediments in Bolivia, Earth
Planet. Sci. Lett., 111, 23-39, 1992
32- Courtillot V., Valet J.-P., Hulot G., Le Moul
J.-L.,The earth's magnetic field: a chaotic, randomly reversing geocentric
axial dipole with superimposed white noise at the core mantle boundary, Eos transactions, 73, 337-342, 1992
33- Meynadier L., Valet J.-P., Weeks R., Shackleton N.,
Hagee V.L., Relative
paleointensities of the geomagnetic field during the last fourteen hundred
thousand years, Earth Planet. Sci. Lett.,
114, 39-57, 1992
34- Meynadier L., Valet J.-P., Vincent E., Leg ODP 138:
un tournant dans les analyses haute rsolution des sdiments ocaniques, Gochronique NĦ41, 18, 1992
35- Shackleton N.J.,
Hall M.A., Meynadier L., Pate D., Valet
J.-P., High resolution stable isotope stratigraphy from bulk sediments, Paleoceanography, vol 8, No.2, 141-148, 1993
36- Nagy E., Valet J.-P., New advances for
paleomagnetic studies of sediment cores using U-channels, Geophys. Res. Lett., vol 20, No 8, 671-674, 1993
37-Valet J.-P., Meynadier L., Geomagnetic
field intensity and reversals during the last four million years. Nature, Vol 366, 234-238, 1993.
38- Valet J.-P., Meynadier L., Bassinot F.,
Garnier F., Relative paleointensity across the last geomagnetic reversal from
sediments of the Atlantic, Indian and Pacific oceans. Geophys. Res. Lett. , Vol 21, NĦ6, 485-488, 1994.
39- Gallet Y., Valet J.-P., Magnetic stratigraphy in
deep-sea sediments: from the superchron to the microchron, Oceanus Vol 36, NĦ4, 99-102, 1994.
40- Quidelleur X., Valet J.-P., Paleomagnetic records of
excursions and reversals: possible biases caused by magnetization artefacts? Phys. Earth Planet. Inter., 82, 27-48, 1994.
41- Tric E., Valet J.-P., Gillot P.Y., Le Meur I.,
Absolute paleointensities between 60 and 160 kyrs B.P. from Mount Etna
(Sicily), Phys. Earth Planet. Inter. 85,
113-119, 1994.
42- Quidelleur X., Valet J.-P., Courtillot V., Hulot G .,
Long-term geometry of the geomagnetic field derived from an updated base on
secular variation for the last 5 million years, Geophys. Res. Lett.,
Vol 21, NĦ15, 1639-1642, 1994.
43- Meynadier L., Valet, J.-P., Bassinot, F. Shackleton, N., Guyodo Y., Asymmetrical saw-tooth pattern of the
geomagnetic field intensity from equatorial sediments in the Pacific and the
Indian oceans, Earth Planet. Sci. Lett. ,
126, 109-127, 1994.
44- Meynadier L. Valet, J.-P., Grousset, F., Magnetic properties and origin of late
quaternary sediments in the Somali basin (Indian ocean), Paleoceanography, Vol. 10,
N0.3, 459-472, 1995.
45- Quidelleur X. ,
Holt, J., Valet J.-P., Confounding
influence of magnetic fabric on sedimentary records of a field reversal, Nature, Vol 374, 246-249, 1995.
46- Valet J.-P., Les inversions du champ
magntique: de la peau de Zbre aux dents de scie, Geochronique, 1995.
47- Meynadier L, Valet J.-P., Shackleton N.J., Relative
geomagnetic intensity during the last 4Ma from the equatorial Pacific, Proceedings of the Ocean Drilling Program,
Scientific Results Vol 138, 779-795, 1995.
48- Quidelleur X., Valet, J.-P., LeGoff M., Bouldoire, X., Field dependence on magnetization
of laboratory redeposited deep-sea sediments: first results, Earth Planet. Sci. Lett., 133 , 311-325, 1995.
49- Valet J.P, ODP Leg 154 scientific
party, Ceara Rise sediments document ancient climate change, Eos
Transactions 76, NĦ5, 41-42, 1995.
50- Courtillot V., Valet J.-P., Secular variation of the
earthĠs magnetic field : from jerks to reversals, C.R. Acad.Sci., 320, IIa, 903-922, 1995.
51- Quidelleur, X., Valet J.-P., Geomagnetic changes across
the last reversal recorded in lava flows from La Palma (Canaries islands), J. Geophys. Res., Vol 101, NĦB6 13755-13774,
1996.
52- Meynadier L. and
Valet J.-P., Post-depositional
realignment of magnetic grains and saw-toothed variations of relative
paleointensity, Earth Planet. Sci..
Lett., 140, 123-132 , 1996.
53- Guyodo Y., Valet J.-P., Relative variations in
geomagnetic intensity from sedimentary records: the past 200 thousand years, Earth Planet. Sci. Lett., 143, 23-36, 1996.
54- Valet J.-P., Brassart J. , Le Meur I.,
Soler V., Quidelleur X., Tric E., Absolute paleointensity and
magnetomineralogical changes, J. Geophys.
Res. , Vol101, NoB11, 25029-25044, 1996
55- Richter C., Valet J.-P., Solheid P. A., Rock
magnetic properties of sediments from Ceara Rise and implications for the
origin of their magnetic susceptibility, Proceedings
of the Ocean Drilling Program, Scientific Results 154, 169-180, 1997
56- Solheid P.,
Banerjee S.K., Richter C., Valet J.-P.,
High resolution rock magnetic study of Ceara Rise sediments (Site 925), Proceedings of the Ocean Drilling Program,
Scientific Results 154, 181-188,
1997.
57- Brassart J.,
Tric E., Valet J.-P.,
Herrero-Bervera E., Absolute paleointensity between 60 and 400 ka from the
Kohala Mountain (Hawai), Earth Planet.
Sci. Lett., Vol 148, 141-156, 1997.
58- Meynadier, L., Valet, J.-P., Guyodo, Y., Richter, C., Saw-toothed variations of relative
paleointensity and cumulative viscous remanence: testing the records and the
model? J. Geophys. Res., Vol. 103, B4,
7095-7105, 1998.
59- Valet, J.-P., Meynadier, L., A comparison of different techniques of
relative paleointensity, Geophys. Res.
Lett., Vol. 25, NĦ1, 89-92, 1998.
60- Valet,
J.-P., Kidane, T., Soler, V.,
Brassart, J., Courtillot ,V., Meynadier, L., Remagnetization in lava flows recording pretransitional
directions, J. Geophys. Res., Vol
103, NĦ B5, 9755-9775, 1998.
61- Rousseau,
D.-D., Zller, L., Valet, J.-P., Late pleistocene climatic
variations at Achenheim, France, Based on a magnetic susceptibility and TL
chronology of loess, Quaternary
Research 49, 255-263, 1998.
62- Valet, J.-P., Tric, E., Herrero-Bervera, E., Meynadier, L., Lockwood, J.
,P., Absolute paleointensity from hawaiian lavas younger than 35ka, Earth and Planet Sci. Lett., 161, 19-32,
1998.
63- Valet, J.-P., Gallet, Y., Paleomagnetism: ancient inclinations, Nature, ÇNews and ViewsÈ, Vol 396, 315-316, 1998.
64- Valet, J.-P. , Brassart, J., Quidelleur, X., Soler, V., Gillot, P.-Y., Hongre, L., Paleointensity variations across the
last geomagnetic reversal at La Palma (Canary islands, Spain, J. Geophys. Res. Vol. 104 , No. B4 , p.
7577, 1999.
65- Guyodo, Y., Valet, J.-P., Global changes in geomagnetic intensity during the past 800
thousand years, Nature, Vol 399, 249-252,
1999
66- Carlut, J., Valet, J.-P., Quidelleur, X., Courtillot, V., Kidane, T., Gallet, Y.,
Gillot, P.Y., Paleointensity across the Runion event in Ethiopia, Earth Planet. Sci. Lett., 170, 17-34,
1999.
67- Herrero-Bervera, E., Valet, J.-P., Paleosecular variation during
sequential geomagnetic reversals from Hawaii, Earth and Planet. Sci. Lett., 171, 139-148, 1999.
68- Valet, J.-P., Soler, V., Magnetic anomalies induced by lava flows and
possible consequences for paleomagnetic records, Phys. Earth Planet. Inter., 115 , 109-118, 1999.
69- Richter, C.,
Hayashida, A., Guyodo, Y., Valet, J.-P., Verosub, K., Magnetic intensity
loss and core diagenesis in long-core samples from the East Cortez Basin and
the San Nicolas Basin (California Borderland), Earth, Planets and Science 51, 329-336, 1999.
70- Guyodo, Y.,
Richter, C., Valet, J.-P., Paleointensity record from pleistocene sediments off the
California Margin, J. Geophys. Res.,Vol.
104, NĦB10, 22953-22965, 1999.
71- Guyodo Y., Valet, J.-P., Integration
of volcanic and sedimentary paleointensity data, Constraints imposed by
irregular eruption rates, Geophys. Res.
Lett. Vol. 26 , No. 24 , p. 3669,
1999.
72- Valet,
J.-P. and Herrero-Bervera, E.,
Paleointensity experiments using alternating field demagnetization, Earth and Planet. Sci. Lett. 177, 43-53,
2000.
73- Meynadier, L., Valet, J.-P., Reply to the comment on Ò Saw-toothed variations of relative
paleointensity and cumulative viscous remanence : Testing the records and
the modelÓ, J. Geophys. Res. Vol. 105 ,
No. B7 , p. 16,613, 2000.
74- Herrero-Bervera,
E., Vinuela, J.-M., Valet, J.-P., Paleomagnetic study of lavas of
the island of LanaĠi, Hawaii, J. Vol.
Geoth. Res., 104, 21-31, 2000.
75- Dormy, E., Valet, J.-P.,
Courtillot, V., Observational constraints and numerical dynamos., Geochem.
Geophys. Geosyst, Vol 1, October 18, 2000.
76- Valet, J.-P., Meynadier, L., Comment on Ç A relative paleointensity
stack from Ontong-Java plateau sediments for the Matuyama È by Yvo S. Kok
and L. Tauxe, J. Geophys. Res. Vol. 106
, No. B6 , p. 11,013, 2001.
77- Elmaleh, A., Valet J.-P., Herrero-Bervera, E., A map
of the Pacific anomaly during the Brunhes chron, Earth Planet. Sci. Lett., 193 NĦ3-4, p. 315-332, 2001.
78- Kravchinsky, V.A., Konstantinov, K.M., Courtillot,
V., Valet, J.-P., Savrasov, J.I., Cherniy, S.D., Mishenin, S.G., Parasotka,
B.S., Paleomagnetism of east Siberian traps and kimberlites: two new poles and
paleogeographic reconstructions at about 360 and 250 Ma, Geophys. J. Int., 148, 1-33, 2002.
79- Valet, J.-P., La Terre perd-elle son magntisme ? La Recherche, NĦ 351, 36-39, 2002.
80- Herrero-Bervera,
E. , Valet J.-P., Paleomagnetic
secular variation of the Honolulu volcanic series (33-700ka), OĠahu, Hawaii, Physics Earth Planet. Inter., 133,
83-97, 2002.
81- Valet,
J.-P., Time variations in
Geomagnetic Intensity, Reviews of
Geophysics, 41, 1/ 1004, 2003.
82- Isambert, A., Valet, J.-P., Gloter, A., Guyot F., Stable Mn-magnetite derived from
siderite by heating in air, J. Geophys.
Res. Vol. 108, No. B6, 2283, 2003.
83- Valet, J.-P., Herrero-Bervera E., Characteristics of geomagnetic reversals
inferred from detailed volcanic records, C.R.
Gosciences, 335, 79-90, 2003.
84- Quidelleur, X.,
Carlut,J., Soler, V., Valet, J.-P., Gillot P.Y., The age and
duration of the Matuyama-Brunhes transition from new K-Ar data from La Palma
(Canary islands) and revisited 40Ar/39 Ar ages, Earth Planet. Sci. Lett. 208, 149-163,
2003.
85- Herrero-Bervera,
E. , Valet J.-P., Persistent
anomalous inclinations recorded in the Koolau volcanic series of the island of
Oahu (Hawaii, USA) between 1.8 and 2.6 Ma, Earth
Planet. Sci. Lett 6633, 1-14, 2003.
86- Macouin, M., Valet, J.-P., Besse, J., Buchan, K. , Ersnt, R., LeGoff, M,
U.Schrer U., Low Paleointensities recorded in 1 to 2.4 Ga old Proterozoic dykes,
Superior province, Canada, Earth Planet.
Sci. Lett., 213, 79-95, 2003.
87- Glen, J., Valet, J.-P., Soler, V., Renne, P.R., Elmaleh, A., A Neogene geomagnetic polarity
transition record from lavas of the Canary islands, Spain: episodic volcanism
and/or metastable transitional fields ? Geophys. J. Int., Vol. 154,
Issue 2, Page 426-440, 2003.
89- Macouin, M., Valet J.-P., Besse J., Long-term evolution of the geomagnetic
dipole moment, Phys. Earth Planet. Inter.,
Vol 147, 2-3, 239-246, 2004.
90- Herrero-Bervera, E., Ubangoh, R., Aka, F.T., Valet, J.-P. Paleomagnetic and paleosecular variation study of the Mt
Cameroun volcanics (0.0-0.25 Ma), Cameroun, West Africa, Phys.
Earth Planet. Inter., Vol 147, 2-3, 171-182, 2004.
91- Herrero-Bervera, E., Valet, J.-P., Absolute
paleointensity from the Waianae volcanics (Oahu, Hawaii) between the
Gilbert-Gauss and the upper Mammoth reversals, Earth Planet. Sci. Lett., Vol 234, 279-296, 2005.
92- Bouquerel, H., Valet, J.-P., Le prote est-il quip pour le
magntotactisme? C. R. Goscience.,
337, 806-813, 2005.
93- Valet,
J.-P., Meynadier L., Guyodo, Y.,
Geomagnetic field strength and reversal rate over the past 2 Million years, Nature, Vol 435, 802-805, 2005.
94- Letard, I. , Sainctavit, P., Menguy, N., Valet, J.P., Isambert, A., Dekkers, M.,
Gloter, A., Mineralogy of greigite, Physica Scripta, Vol.
T115, 489-491, 2005.
95- Macouin, M., Valet, J.-P., Besse,
J., Ernst R.E., Absolute Paleointensity at 1.27 Ga from
the Mackenzie dyke swarm (Canada), G-cubed,
Vol. 7, Q01H21,
doi:10.1029/2005GC000960, 2006.
96- Isambert, A.,
T., De Resseguier, Gloter A., Reynard, B., Guyot, F. Valet, J.-P., Magnetite-like
nanocrystals formed by laser-driven shocks in siderite, Earth Planet. Sci. Lett. 243, 820-827, 2006.
97- Carter-Stiglitz, B., Valet, J.-P., LeGoff,
M., Constraints on the acquisition of remanent magnetization in fine grain
sediments imposed by redeposition experiments, Earth Planet. Sci. Lett. 245, 427-437, 2006.
98- Guyodo, Y., Valet,
J.-P., A comparison of relative
paleointensity records of the Matuyama chron for the period 0.75-1.25Ma, Phys. Earth Planet. Inter., 156, 3-4,
205-212, 2006.
99- Valet, J.-P., Paleointensity, absolute, techniques, Encyclopedia of Geomagnetism and Paleomagnetism, D. Gubbins & E. Herrero-Bervera edts., 753-757, 2007
100- Isambert, A.,
Menguy., N,. Larquet, E., Guyot, F., Valet,
J.-P., Morphological study of
magnetites in a freshwater population of magnetotactic bacteria, Am. Mineral., Vol 92, 621-630, 2007.
101- Valet,
J.-P., Herrero-Bervera, E.,
Geomagnetic reversals, archives, Encyclopedia
of Geomagnetism and Paleomagnetism, D. Gubbins & E. Herrero-Bervera edts., 339-346, 2007.
102- Herrero-Bervera, E., Browne, E.J., Valet J.-P., Singern B.S, Jichan B., Cryptochron C2r.2r-1 recorded 2.51
Ma in the Koolau Volcano at Halawa, Oahu, Hawaii, US : palomagnetic and 40Ar/39Ar
evidence, Earth Planet. Sci. Lett.
254, 3-4, 256-271, 2007.
103- Herrero-Bervera, E., Valet, J.-P., Holocene
paleosecular variation from dated lava flows in Maui, Phys. Earth Planet. Int., 161 (3), 267-280, 2007.
104- Plenier, G., Valet, J.-P., Gurin,
G., Lefvre, J.C., Carter-Stiglitz, B., Origin and age of the directions
recorded during the Laschamp event in the Chane des Puys (France), Earth Planet. Sci. Lett., 259, 414-431, 2007.
105- Narteau, C.,
LeMoul, J.-L., Valet, J.-P., The oscillatory nature of the
geomagnetic field during reversals, Earth
Planet. Sci. Lett., Vol 262, 1-2, 66-76, 2007.
106- Valet,
J.-P., Herrero-Bervera, E., LeMoul,
J.L., Plenier, G., Secular variation of the geomagnetic dipole during the past
2 thousand years, Geochem. Geophys.
Geosyst., 9, Q01008, doi:10.1029/2007GC001728, 2008.
107- Valet, J.-P., Plenier, G., Simulations of a
time-varying non dipole field during geomagnetic reversals and excursions, Phys. Earth. Planet. Inter.,169,
178-193, 2008.
108- Valet, J.-P., Plenier, G., Herrero-Bervera,
E., Geomagnetic excursions reflect an aborted polarity state, Earth and Planet. Sci. Lett. 274,
472-478, 2008.
109 – Ptrlis, F., Fauve, S., Dormy, E., Valet, J.P., Mechanism for magnetic
field reversals, Phys. Rev. Lett .,
102,14, 144503, 2009.
110 - Herrero-Bervera, E.,Valet, J.P., Testing
Determinations of Absolute Paleointensity from the 1955 and 1960
Hawaiian Flows, Earth Planet. Sci. Lett.,
http://dx.doi.org/10.1016/j.epsl.2009.08.035, 2009.
111 – Valet, J.P.,
Herrero-Bervera, E., Carlut, J., A selective procedure for absolute
paleointensity in lava flows, Geophys.
Res. Lett., 37, L16308, doi:10.1029/2010GL044100,
2010.
112 – Valet, J.P.,
Valladas, L., The Laschamp-Mono lake geomagnetic events and the extinction of
Neanderthal : a causal link or a coincidence?, Quatern. Sci. Rev 29, 3887-3893, 2010.
113 - Spassov, S., Valet, J.P., Kondopoulou, D., Zananiri,
I., Casas, L., Le Goff, M., Rock magnetic property and paleointensity
determination on historical Santorini lava flows. Geochemistry Geophysics Geosystems, 11, Q07006, 2010.
114 - Valet, J.P.,
Herrero-Bervera, E., A few characteristic features of the geomagnetic field
during reversals, The Earth's Magnetic Interior, IAGA Special Sopron Book Series,
Springer, Germany, 138-151, 2011.
115 – Valet, J.P.,
Herrero-Bervera, E, Meynadier, L.,
Time-averaged and mean axial dipole field, The
Earth's Magnetic Interior, IAGA Special
Sopron Book Series, Springer, Germany, 131-137, 2011.
116 - Valet, J.P., Moreno, E., Bassinot, F. et al, Isolating climatic and
paleomagnetic imbricated signals in two marine cores using
principal component analysis, Geochem.
Geophys. Geosyst., 12, 8, Q08012, doi:10.1029/2011GC003697, 2011.
117 –
Petrelis, F., Besse, J. , Valet, J.P.,
Plate Tectonics may control reversal frequency, Geophys. Res. Lett., 38, L19303, 2011.
118 – Blanco, D., Kravchinsky,
V.A., Valet, J.P., Ali, A., Potter, D.K., Does the Permo-Triassic
geomagnetic dipole low exist? Phys. Earth
Planet. Inter. 204, 11-21,
doi:10.1016/j.pepi.2012.06.005,
2012
119 – Moulin, M., Courtillot, V.,
Fluteau, F. , Valet, J.P., The van
Zijl Jurassic geomagnetic reversal revisited, Geochem. Geophys. Geosyst., 13, doi:10.1029/2011GC0039101, 2012.
120 -
Spassov, S., Valet, J.P., Detrital
magnetization from redeposition experiments of natural sediments, Earth Planet. Sci. Lett., 351, 147-157,:
doi 10.1016/j.epsl.2012.07.016, 2012
121 - Petrelis, F., Valet, J-P., Besse, J., When North heads South, Physics world, 25- 3, 51-55 , 2012
122
- Valet, J.P., Fournier, A.,
Courtillot, V., Herrero-Bervera, E., Dynamical similarity of geomagnetic field
reversals, Nature Vol.490, 7418, 89-+94, doi: 10.1038/, 2012.
123 - Bouilloux, A., Valet,
J.P., Bassinot, F., Joron, J.L.,
Blanc-Valleron, M.M., Moreno, E., Dewilde, F.,
Kars, M., Lagroix, F., Diagenetic modulation of the magnetic properties in sediments from the
Northern Indian Ocean, Geochem. Geophys. Geosyst., 14, 9,
3779-3800, doi: 10.1002/ggge.20234, 2013.
124 - Bouilloux, A., Valet,
J.P., Bassinot, F., Joron, J.L.,
Dewilde F., Blanc-Valleron,, M.M., Moreno, E., Influence
of seawater exchanges across the Bab-el-Mandab Strait on sedimentation in
the Southern Red Sea during the last 60 ka, Paleoceanography,
in press.