Direction and intensity of Earth's magnetic field at the Permo-Triassic boundary

The Earth's magnetic field is generated by the motion of liquid
iron-rich material in the outer core. One of the most drastic
manifestations of the dynamics in the outer core are polarity
reversals of the magnetic field. The processes controlling
geomagnetic reversals, however, are still poorly understood. The
mathematical formulation of the dynamics of the liquid outer core
show such a degree of complexity that a universal numerical model
still remains elusive. Given that the last reversal occurred about
780,000 years ago, direct observations of a reversal have never
been possible. Thus we are left with records of ancient reversals
recorded in sequences of sedimentary and igneous rocks. Documenting
any systematics in reversal processes will provide substantial
information about the outer core and core mantle boundary
conditions. However, despite the advances in deciphering the
behaviour of the field during polarity transitions, reversal
records yield controversial results and thus answers to several key
questions are still enigmatic. Detailed studies of
palaeodirectional and absolute palaeointensity patterns of
geomagnetic reversals are scarce and are restricted to the Cenozoic
so far. In order to verify or reject concepts developed on the
basis of this dataset, reversal records which occurred in the more
distant geological past of the Earth are needed. This work presents
the results obtained from the Siberian Trap Basalts (Russia) which
are coeval with the Permo-Triassic boundary (250 Ma). The sequence
yields the by far oldest hitherto studied detailed record of a
geomagnetic transition from reversed to normal polarity and
provides new insights in transitional field behaviour. Three
sections (Talnakh, Listvjanka and Abagalakh) comprising a total of
86 lava flows have been sampled in the Noril'sk region, located at
the northwestern rim of the Siberian Trap Basalt province. They
provide a complete coverage of the lava pile outcropping in the
area. The samples have been subjected to palaeomagnetic direction
analysis and to Thellier-type palaeointensity experiments.
Extensive rockmagnetic investigations and microscopical studies
have been carried out to asses the reliability of the
palaeomagnetic information recorded by the lava flows. Magnetite
and Ti-poor titanomagnetites were identified to be the carriers of
the characteristic remanent magnetisation. The reversibility of the
thermomagnetic curves and the observation of exsolution lamellae by
ore microscopy give clear evidence for a primary high-temperature
oxidation of the titanomagnetite. It can thus be inferred that the
measured palaeodirectional and intensity information obtained from
these flows was acquired shortly after extrusion of each flow. The
demagnetisation of the natural remanence reveals only one direction
of magnetisation for most samples. Thermal and alternating field
demagnetisation methods are equally effective in isolating the
characteristic remanent magnetisation. Occasional overprints have
maximum unblocking temperatures of 350°C or remanence coercivities
less than 20 mT. Reliable palaeointensity estimates were obtained
for approx. 50% of the samples. The relatively high success rate
can be attributed to the enhanced magnetic and thermal stability of
high-temperature oxidised titanomagnetites. In the lower part of
the sequence reversed polarity of the Earth's magnetic field is
identified. The associated palaeointensities yield values around 10
µT. The subsequent flows recorded transitional configurations. A
tight cluster of virtual geomagnetic poles (VGPs) in mid northerly
latitudes, comprising the results of 15 flows, is observed during
the transition. Within the cluster the record shows a pronounced
and well defined increase in intensity from around 6 to 13 µT. A
doubling of local field intensity infers that large scale dynamic
processes in the outer core are responsible for this feature,
making a strong case for a reasonable temporal stability (several
hundreds to a few thousand years) of the VGP cluster. Moreover, the
VGP clustering is identified in two parallel sections (Talnakh and
Listvjanka). This observation makes it unlikely that this feature
is an artifact of a localised burst in volcanic activity and
supports the concept of stabilised phases of the geomagnetic field
during reversals. The VGPs of the overlying flows move towards the
position expected for normal polarity. After rotating of the VGPs
into the Late Permian/Early Triassic geographic reference system it
is evident that most of the transitional VGPs are strongly confined
to a narrow longitudinal band which is perpendicular to near- or
far-sided VGP paths. Such near- or far-sided paths would be
indicative for the dominance of zonal, and thus axis-symmetric,
non-dipole fields. The VGP path of this transition suggests the
contribution of strong sectorial components of the Earth's magnetic
field. Following the transition itself, normal polarity is reached
for a brief time interval. Subsequently, the VGPs depart from this
position to form another well defined directional cluster recorded
by 14 successive flows. During this clustering, which is
interpreted as an excursion of the Earth's magnetic field, no
characteristic variation in palaeointensity is identified (mean
value 14 µT). Such post-transitional excursions are frequently
observed in younger reversal records and are explained by
instabilities of the geodynamo after the reversal. However, VGPs
associated with post-transitional excursions usually reach
positions similar to those occupied by VGPs during the transition.
In contrast to such "rebound" effects, the excursion-related VGPs
of this record are still confined to the latitudinal band defined
by the transition, but "overshoot" normal polarity. This
geometrical constraint suggests that non-dipole components similar
to those dominating the transitional VGP path are responsible for
this observation. Remarkably, the geomagnetic polarity transition
described here shares many similarities - such as directional
clustering, longitudinal confinement of the VGP path, the existence
of a post-transitional excursion and generally low
palaeointensities - with previously published reversal records of
mainly Tertiary age. It may, therefore, be inferred that the
underlying reversal processes are similar to those observed for the
Cenozoic. The results obtained of the superjacent 41 flows, which
were extruded immediately after the reversal-related excursion,
indicate that only at this stage of the record stable normal
polarity is reached allowing to determine several characteristic
parameters of the Early Triassic Earth's magnetic field. The mean
palaeointensity for this part of the sequence is 19 µT, which
corresponds to a virtual geomagnetic dipole moment (VDM) of 2.3 *
10^22 Am^2. These findings confirm that the Mesozoic dipole low
extends at least down to the Permo-Triassic boundary. Calculation
of the recorded secular variation yields values similar to those
averaged over the last 5 Ma, a period with distinctly higher mean
VDM (5.5 * 10^22 Am^2) compared to the data presented here. The
hypothesis of enhanced secular variation during phases of a low
mean VDM can, therefore, not be substantiated by this study.
Secular variation and the strength of the dipole moment seem to be
- at least in the Early Mesozoic - more complexly coupled than
previously assumed. Magnetostratigraphic results of borehole
samples obtained from basalts related to the Siberian Trap
volcanism including the West Siberian basin yield in total 6
polarity intervals. Comparison to the global magnetostratigraphic
scale indicates that the volcanic activity lasted no more than 3.2
Ma. However, the lava sequence in the Noril'sk area (more than 1700
m thick), representing the bulk of the erupted material, recorded
only one polarity transition. This finding has been supported by
data derived from boreholes in close vicinity to the surface
sections which makes the presence of further undetected polarity
transitions highly unlikely. It can be thus inferred that the
emplacement of the sequence occurred much faster than the
aforementioned 3.2 Ma. Radiometric ages suggest an upper limit for
the duration of the emplacement of approximately 1 Ma. Based on the
assumptions of similar rates of angular secular variation in the
Early Triassic and in the Holocene and an average duration of the
transition itself the time interval covered is estimated to be in
the order of 15000 years. This value has to be regarded as a
lowermost limit for the duration of the emplacement. Such a rapid
development of the volcanic province in the Noril'sk area would
imply an enormous eruption rate making a strong case for the
Siberian Trap basalts as cause for the Permo-Triassic crisis.