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Lecture IV Recap of Laschamp directions & relative
paleointensity
Magnetic anisotropy
P4.3 2020 Paleo- and Geomagnetism, S. Gilder
Stepwise alternating field (af) demagnetization from the A core isolates linear trajectories that decay toward the origin on “Zijdervelt” plots above ~30 mT.
We systematically fit line segments between the 30 and 60 mT steps and did not force the line to the origin.
“A”
P4.3 2020 Paleo- and Geomagnetism, S. Gilder
Stepwise alternating field (af) demagnetization from the D core isolates linear trajectories that often do not decay toward the origin. We systematically fit line segments between the 30 and 70 mT steps and did not force the line to the origin.
P4.3 2020 Paleo- and Geomagnetism, S. Gilder
Low field susceptibility (Xlf) reflects the contribution from all minerals (dia-, para- and ferri- magnetic). Because the susceptibility of magnetite is 1000x that
of other minerals, Xlf of magnetite often dominates the total contribution. SIRM reflects the concentration of ferrimagnetic minerals only.
Variations in magnetic parameters correlate well in the two cores. There might be a shift below 18.6 m between the two.
P4.3 2020 Paleo- and Geomagnetism, S. Gilder
ARM vs. SIRM correlate better with each other than with bulk susceptibility (X), so we will not use X as a normalizer for relative paleointensity.
P4.3 2020 Paleo- and Geomagnetism, S. Gilder
Proxies for high-coercivity magnetic minerals, such as hematite and
goethite, vs. soft magnetic minerals (magnetite) can be estimated by the S-
ratio and hard IRM (HIRM):
The S-ratio300mT (−IRM−300mT/SIRM) is scaled so that a value of 1 reflects
100% of magnetic remanence-bearing minerals with a coercivity lower than 300 mT, (magnetite or maghemite).
When the S‐ratio is close to zero or if it has negative values, contributions
from hematite and/or goethite will be significant.
The HIRM100mT is defined as 0.5 × (SIRM + IRM−100mT), where IRM−100mT represents the remanent magnetization obtained by first
saturating the sample in a high field (e.g., 1 or 1.5 T), and then applying a back‐field of −100 mT to reverse the
saturation isothermal remanent magnetization (SIRM) contributed by
magnetite or maghemite.
S-ratios and HIRM values are very coherent between the two cores. The consistently high S-
ratios indicate the cores contain mostly magnetite.
P4.3 2020 Paleo- and Geomagnetism, S. Gilder
We imparted partial anhysteretic magnetizations in the 30-60 mT and 30-70 mT windows, then stepwise AF demagnetized the ARM.
All samples show very similar demagnetization spectra, which means the destruction of the NRM magnetization by AF fields should be comparable (uniform) for all
samples.
P4.3 2020 Paleo- and Geomagnetism, S. Gilder
Here is a comparison of the normalized AF demagnetization between the NRM and the ARM data as a function of depth.
Note how consistent the ARM data are compared to the NRM. From this data we can calculate the relative paleointensity.
“A core”
P4.3 2020 Paleo- and Geomagnetism, S. Gilder
Here is a comparison of the normalized AF demagnetization between the NRM and the ARM data as a function of depth.
Note how consistent the ARM data are compared to the NRM. From this data we can calculate the relative paleointensity.
P4.3 2020 Paleo- and Geomagnetism, S. Gilder
Here are two ways to calculate the relative paleointensity plotted against inclination.
“A”
P4.3 2020 Paleo- and Geomagnetism, S. Gilder
Relative Paleointensity Four ways to estimate the relative paleofield strength are quite similar--they are all based on the magnetization intensity of the NRM after AF demagnetization at 30 mT. The one using 51 mT gives a comparable, yet
somewhat different picture.
P4.3 2020 Paleo- and Geomagnetism, S. Gilder
Relative paleointensity plotted against inclination.
P4.3 2020 Paleo- and Geomagnetism, S. Gilder
Here is the combined data set of relative paleointensity plotted against inclination.
P4.3 2020 Paleo- and Geomagnetism, S. Gilder
Here is the combined data set of relative paleointensity plotted against normalized declination and inclination. Note that pInt minima coincides where inclination = 0, but not declination, which never goes “antipodal”.
P4.3 2020 Paleo- and Geomagnetism, S. Gilder
MAGNETIC ANISOTROPY Because magnetic grains have shape anisotropy or crystalline anisotropy,
their magnetizations vary as a function of the direction of the measurement. This is also true for an assemblage of grains in a rock.
Magnetic fabrics
P4.3 2020 Paleo- and Geomagnetism, S. Gilder
The magnetization (M) of magnetically isotropic material is proportional to a weak external field (H):
M=kH where k is magnetic susceptibility.
In magnetically anisotropic material: M1 = k11H1, M2 = k22H2, M3 = k33H3
where k11 ≥ k22 ≥ k33 = k1 ≥ k2 ≥ k3 = kmax ≥ kint ≥ kmin
with k1/k3 = P (degree of anisotropy); k1/k2 = L (magnetic lineation) and k2/k3 = P (magnetic foliation)
Anisotropy of magnetic susceptibility measurements take 5-15 minutes/ sample.
P4.3 2020 Paleo- and Geomagnetism, S. Gilder
In magnetically anisotropic material:
M1 = k11H1, M2 = k22H2, M3 = k33H3 where k11 ≥ k22 ≥ k33 = k1 ≥ k2 ≥ k3 = kmax ≥ kint ≥ kmin
with k1/k3 = P (degree of anisotropy); k1/k2 = L (magnetic lineation) and k2/k3 = F (magnetic foliation)
Other definitions : T = shape parameter = (2η2 - η1 - η3)/( η1 - η3) = 2·ln·F/ln·P - 1
T = 0 = triaxial; T > 0 to 1 = oblate; T < 0 to -1 = prolateη1 = ln k1; η2 = ln k2; η3 = ln k3
P’ = corrected degree of magnetic anisotropy = exp (2[(η1 - ηm)2 + (η2 - ηm)2 + (η3 - ηm)2])1/2
Mean susceptibility = km = (k1 + k2 + k3)/3
P4.3 2020 Paleo- and Geomagnetism, S. Gilder
One can thus quantify the fabric
in rocks using magnetic methods.
P4.3 2020 Paleo- and Geomagnetism, S. Gilder
A plot of lineation vs. foliation = Flinn diagram
P4.3 2020 Paleo- and Geomagnetism, S. Gilder
AMS measurement schemes-scalar (AGICO) 15 position manual vs. 3 position “spinning” = (64
measurements / revolution)
P4.3 2020 Paleo- and Geomagnetism, S. Gilder
Anisotropy of anhysteretic remanent magnetization (AARM) 12 (6x2) measurement scheme (SushiBar)
P4.3 2020 Paleo- and Geomagnetism, S. Gilder
Magnetic susceptibility changes as a function of
composition and grain size.
P4.3 2020 Paleo- and Geomagnetism, S. Gilder
Single domain grains yield
“inverse” fabrics because the
susceptibility measured along the long axis of
the grain ≈ 0.
AARM does not have this problem.
P4.3 2020 Paleo- and Geomagnetism, S. Gilder
AMS of redeposition experiments under flowing conditions:
(1) k1 points in the direction of flow for low stream velocities
(2) k1 spreads out, with inclination to 0, as stream velocity increases.
One can thus deduce flow direction and velocity in sediments.
P4.3 2020 Paleo- and Geomagnetism, S. Gilder
Evolution of a sedimentary fabric getting increasingly overprinted
by a tectonic fabric as one approaches the Pyrenees
P4.3 2020 Paleo- and Geomagnetism, S. Gilder
A sedimentary fabric gets increasingly overprinted by a tectonic fabric approaching the Pyrenees
P4.3 2020 Paleo- and Geomagnetism, S. Gilder
Here is another example from Japan. Note the use of P’ and T--
these are quite useful.
P4.3 2020 Paleo- and Geomagnetism, S. Gilder
P
T
Superposition of two magnetic fabrics
Housen et al. (1993) P
T
k3
Two equal fabricsGrowth of a new fabric
P4.3 2020 Paleo- and Geomagnetism, S. Gilder
Central Asia Fast uplift rates, very good exposures
P4.3 2020 Paleo- and Geomagnetism, S. Gilder
Kuitun
Jinguohe Yaha
Jinguohe The Tianshan Sections
P4.3 2020 Paleo- and Geomagnetism, S. Gilder
Throughout the Tianshan domain, Cenozoic sections coarsen upward-- going from red-yellow, clay-rich mudstone to gray conglomerate.
Does this coarsening sequence reflect changes in climate and/or uplift?
Does the stratigraphy correlate in time?
Xiy
u Fo
rmat
ion
P4.3 2020 Paleo- and Geomagnetism, S. Gilder
Jinguohe : Northern flank of the Tianshan Mountains
P4.3 2020 Paleo- and Geomagnetism, S. Gilder
Magnetostratigraphy allows one to precisely date the ages of
sediments by comparing against the global geomagnetic polarity time
scale.
Our magnetostratigraphic study of the 4571-m-thick Jingou River section (Xinjiang Province, China) identified 67 polarity chrons, which correlated with the reference geomagnetic polarity time scale between ~1 Ma and ~23.6 Ma.
P4.3 2020 Paleo- and Geomagnetism, S. Gilder
The magnetostratigraphy allows us quantify sedimentation rates, which increase through time at Jinguohe.
P4.3 2020 Paleo- and Geomagnetism, S. Gilder
Do increases in sedimentation rate occur smoothly or punctually?
At Jinguohe, we found that sedimentation rate correlates with magnetic susceptibility,
which is a proxy for magnetite concentration, with important changes seen at 10-11 Ma and
at 15 Ma.
Charreau et al. (2009)
P4.3 2020 Paleo- and Geomagnetism, S. Gilder
Housen et al. (1993)
P
T k3
Superimposition of fabrics can explain k3 directions but not changes in P and T
P4.3 2020 Paleo- and Geomagnetism, S. Gilder
Magnetite concentration increases from the base to
the top of the section, which reflects a change in
sediment source No change in relative grain
size
P4.3 2020 Paleo- and Geomagnetism, S. Gilder
Anisotropy of anhysteretic remanent magnetization vs. anisotropy of magnetic susceptibility from the same section
P4.3 2020 Paleo- and Geomagnetism, S. Gilder
ODP 1063 magnetic fabrics are typical of sedimentary rocks with k3
perpendicular to bedding and k1+k2 in the sedimentation plane.
P4.3 2020 Paleo- and Geomagnetism, S. Gilder
There should be a relationship between magnetic field intensity and magnetic anisotropy. When the field is stronger, the torque to align the
magnetic minerals is greater. High fields should lead to increasingly prolate fabrics with k1 axes pointing toward the magnetic field direction.
There is a hint of a relationship in this in the ODP1063 cores.
P4.3 2020 Paleo- and Geomagnetism, S. Gilder
Here is a plot of the virtual geomagnetic poles of the Laschamp event.
P4.3 2020 Paleo- and Geomagnetism, S. Gilder
P4.3 2020 Paleo- and Geomagnetism, S. Gilder