Paleomagnetics and Marine Oxygen Isotope
stratigraphy
Time and the search
for the Golden Spike
Stratigraphic approaches
• Lithostratigraphy - Generally time transgressive
• Biostratigraphy - Provides information on relative time; complicated by biology and
environmental factors
• Geophysical logging- Varied methods can provide detailed stratigraphy; facies
reconstruction and relative time
How do we know seafloor age?
Image of sea floor age courtesy of NOAA
Earth’s Magnetic Field
Surprisingly, the answer involves the Earth’s Magnetic field
Rotation of the Earth and its metal core creates a global magnetic field
The polarity of the magnetic field switches through time
Magnetic Stripes on the Sea floor
The magnetic properties of seafloor rocks record changes in Earth’s magnetic field
Radiometric dating of seafloor rocks yield ages predicted by Plate Tectonics
Our first look at absolute time:
Radiometrically calibrated Paleomag
• Earth’s magnetic field varies in both intensity and direction (declination and
inclination) through time
• Events should thus be of global scale!
• Magnetic minerals record the paleo-intensity and direction during cooling (hard rock)
or within sediments.
• Magnetometers can remove the modern overprint to reveal “remnant” paleomagnetic
properties.
Magnetic recording
Hard Rocks
•
•
•
•
Signal acquired as the rock cools past the Curie point
These rocks can be radiometrically dated to obtain absolute ages!
Oceanic basalts can provide fairly continuous record
Continental lava flows record discrete events
Soft Rocks
•
•
•
Microscopic magnetic grains orient to the Earth’s field in sediments
Can provide very detailed records
Results sensitive to sediment disturbance, mineralogy (e.g. ferromagnetic, paramagnetic, diamagnetic
minerals)
ODP’s shipboard
Cryogenic Magnetometer
Downhole Paleomagnetics
•Geological High-Resolution Magnetic Tool (GHMT)
•Measures Magnetic susceptibility and total magnetic inductance
•These can be processed together to infer paleomagnetic remnant polarity
•Results are not as precise as shipboard data.
Development of the timescale
•
•
Rapidly refined during the 1960’s due to intensive scientific competition
Played a crucial role in the acceptance of the Theory of Plate Tectonics
Paleomag in marine sequences
• Results
match with biostratigraphic datums
Paleomagnetics over deep time
State-of-the-art
Magnetic stratigraphy
• Current globally accepted magnetic stratigraphy is based on field
polarity (Timing of transition between “normal” and “reversed”
fields).
• Current area of intensive study is the development of a
paleointensity timescale
– proper normalization for differences in magnetic mineralogy
– removal of diagenetic overprints
Global Paleo-intensity Signal
Recent example: Leg 162 Paleomag
Site 981
•
•
•
Continuous shipboard and discrete postcruise samples identify:
Gauss N Chron
Matuyama R Chron
•
•
•
•
•
Reunion N Event
Olduvai N Event
Cobb Mt. N Event
Jaramillo N Event
Brunhes N Chron
Depth: ~50 mbsf
Age: 0.78 Ma
Site 983
Regional Paleomagnetic correlation
• Provides ability to generate global TIME stratigraphic framework
• Which site has lowest accumulation rate?
• Economic implications?
Oxygen Isotope Stratigraphy
Principles of operation• Rayleigh distillation and isotope fractionation
• Eustacy - sea level change
• Mixing time of the ocean is short relative to geologic time (1-2 ka)
Milankovitch Forcing
Milankovitch Response
Oxygen isotope
stratigraphic curve
•
•
•
•
Marine Isotope Stages (MIS) are numbered
Odd numbers are warm interglacials
Even numbers are cold glacials
Provide estimates of relative time
Applying the
MIS stratigraphy
Deep sea benthic foraminfera are the taxa of choice for
stratigraphic correlation due to weaker temperature contribution
and thus stronger ice volume signal
Middle-Upper Miocene d18O
Integrating Magnetostratigraphy and Oxygen
Isotope stratigraphy
Cross-calibrating stratigraphic methods