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Paleomagnetism is the study of the record of the Earth's magnetic field in rocks. Rocks (e.g. volcanic, sedimentary rocks) can record direction and intensity of the field as it has changed over geologic time.
Vector of magnetization recorded into a rock sample
The magnetization of a rock is described by a vector (F) and defined by:
Declination (D): azimuth
Inclination (I): dip
Intensity of magnetization (H)
Down (positive) or Up (negative)
Magnetostratigraphy : magnetism into the stratigraphic sequence
GPTS: geomagnetic polarity time scale
Magnetostratigraphy uses records of changes in polarity of the geomagnetic field preserved in sedimentary sequences to correlate between wells/outcrops and to date the sediment.
Individual normal (positive) and reverse (negative) polarity intervals (called "Chrons") typically range from ~10 thousand to 10 million years in duration.
The magnetic field can be measured in each part of the world: it changes in space and in time
The Earth magnetic field is a vector quantity
(magnetic field vector, F)
Declination (D): azimuth
Inclination (I): dip
Intensity of magnetization (H)
Up or Down
Simple model
Complex model
Magnetic dipole field (dipolar)
Dinamo triggered by external fluid core motion
Brunhes and Matuyama were the first researchers to find reverse magnetic polarity (these reversals are recorded into the rocks)
Normal polarity
Reverse polarity
‘bar code’ of alternating normal (north directed) and reverse (south directed) polarity chrons with characteristic durations
GPTS 2000
Normal polarity
Reverse polarity
Development of the geomagnetic polarity time scale
(GPTS) over the last half century of study.
The initial assumption of periodic behaviour (Cox, 1963) was soon abandoned as new data became available.
The first modern GPTS based on marine magnetic anomaly patterns was established by Heirtzler et al. (1968).
Subsequent revisions by other authors show improved age control and increased resolution.
A major breakthrough came with the astronomical polarity time scale (APTS) in which every individual reversal is accurately dated (e. g. Hilgen et al., 1997).
Astrochronostratigraphic polarity time scale (APTS)
Langereis et al. 2010
100 yr – 10 kyr
secular variation
excursions
inversions
10 kyr – 1 Myr
(sub)chron
10-100 Myr
superchron
Short-term variation
Secular variation diagram for United Kingdom for years 1600 AD to the present (Butler, 1992) - Greenwich (UK)
Langereis et al. 2010
Formation of marine magnetic anomalies during seafloor spreading. The oceanic crust is formed at the ridge crest, and while spreading away from the ridge it is covered by an increasing thickness of oceanic sediments.
The black (white) blocks of oceanic crust represent the original normal (reversed) polarity of the magnetisation acquired
upon cooling at the ridge.
The black and white blocks in the drill holes represent normal and reversed polarity of the sediments.
The magnetization of a rock can be acquired through different processes, synchronous to or later than the formation of the rock, which produce a respectively primary or secondary magnetization.
Three types:
Rocks containing magnetic minerals are able to be magnetized
Magnetization that is acquired by igneous rocks while they cool
TRM of the lava
Earth's magnetic field
Sedimentary rocks, DRM reflects depositional history
How can we measure the fossil magnetism into the rocks?
1. Field survey to identify the suitable rocks for paleomagnetic analyses (e.g. lavas, clay, silt, etc.)
2. Sampling of oriented cores
3. Paleomagnetic analyses in laboratory
Managing cores
Drilling
Extraction
Re-sampling
Lab measurements
Stratigraphic and sedimentological obs
Example of integrated stratigraphy
LAqui-core (Porreca et al., 2016)
core portion
stratigraphic and sedimentological chart
Paleomagnetic laboratory
2G cryogenic magnetometer
Spinner magnetometer
Samples and paleomag data projections
Oven
Data projection
Spinner magnetometer
L. Sagnotti
Identification of primary magnetization:
Declination (D)
Inclination (I)
Intensity (H)
Global Boundary Stratotype Section and Point (GSSP)
https://timescalefoundation.org/gssp/
Cretaceous-Tertiary boundary (Umbria)
a key role in three major geological controversies:
- continental drift/paleomagnetism
- the Cretaceous-Tertiary (K-T) boundary layer, related to the meteorite impact and extinction of the dinosaurs
- more recently, paleoclimate studies of the Paleocene and Eocene.
G. Pialli
Comparison between the Upper Cretaceous polarity record from 150 m of pelagic limestone at Gubbio, and the polarity record from several hundred Kms of sea floor in three different oceans.
(Lowrie & Alvarez, 1977; Alvarez, 2009)
Neogene part of the GPTS
(Monte dei Corvi section, Italy)
Normal polarity
Reverse polarity
Magnetostratigraphy for the composite Monte dei Corvi beach section and calibration to the Neogene astronomical time scale.
The Tortonian GSSP is indicated by the red dashed line. The Monte dei Corvi section is suggested as the Tortonian reference section
Langereis et al. 2010
Global boundary Stratotype Section and Point (GSSP)
Paleomagnetic and cycle stratigraphic analyses of nearly 7000 m of section from continuous cores in the Newark
basin and an overlapping 2500 meter-thick composite outcrop provide
an astrochronostratigraphic polarity time-scale (APTS) for practically the entire Late Triassic (Carnian, Norian and Rhaetian) and the Hettangian (233 to 199 Ma)
Kent al. 2017
Newark-Hartford APTS
Kent al. 2017
Sedimentary record of climate cycles at Pizzo Mondello (outcrop section shown in photograph at top) correlated to portion of Newark-Hartford APTS.
Pizzo Mondello (Italy)
Marine sequence
Kent et al. 2017
pCO2 estimates from paleosols and the isotope 13C record from marine carbonates
Kent et al. 2017
Estimates of atmospheric pCO2 from paleosol carbonates in the Newark and Hartford basins
Reconstruction of Pangea is based on continental rotation parameters of Lottes and Rowley (1990) and mean global pole from Kent and Irving (2010).
Some key continental localities are indicated by filled circles (for positions at 220 Ma) connected to their relative positions at 200 Ma by open circles.
Relative positions of localities at 230 Ma and 210 Ma indicated by crosses.
Kent et al. 2017