II. BIOSTRATIGRAPHIC
-MAGNETOSTRATIGRAPHIC- 
CHEMOSTRATIGRAPHIC
CORRELATION

A. MAGNETIC FIELD OF THE EARTH AND MAGNETIC REVERSALS

• Currents in the liquid outer core produce the Earth's dipole magnetic field.
• Periodically the orientation of the Earth's magnetic field reverses.
• Today's magnetic field has south-seeking magnetic lines of force at the North magnetic pole.
• Intervals in the past with the same orientation as today are called normal polarity intervals; whereas, intervals the opposite of today are called reversed.

B. THE RECORD OF MAGNETIC REVERSALS and THE MAGNETIC POLARITY TIME SCALE

1. Magnetic Anomalies of the Ocean Crust

• new ocean crust is formed at ocean ridges (e.g. Mid-Atlantic Ridge)
• as new ocean crust cools, it acquires a signature of the magnetic field at the time it forms (normal or reversed).
• further crust forming at the ridge spreads the former crust away from the ridge (sea floor spreading)
• over time, parallel stips of normally or reversed magnetized crust are formed as the ocean widens
• these strips are recorded as magnetic anomalies
• postive anomalies are normally magnetized and negative anomalies are reversed magnetized.

2. The Magnetic Anomaly Time Scale

• a composite record of these anomalies has been constructed for the Cenozoic (0-66m.y.), Cretaceous and portions of the Jurassic. Interpolation of the ages of all anomalies was constructed by interpolation between the ages of a few well known anomalies to produce the Magnetic Anomaly Time Scale.

C. Magnetic Reversals in Volcanic Rocks and Sediments and the Paleomagnetic Time Scale.

1. Magnetic Reversals in Volcanic Rocks and Sediments

• cooling continental volcanic rocks are sediments accumulating in the ocean acquire a remnant magnetism of the magnetic field that can produce a reversal chronology
• combined with radiometric dating this chronolgy began to produce a paleomagnetic time scale, independent of the magnetic anomaly time sacle

2. Paleomagnetic Time Scale

• young portions of the paleomagentic time scale were generated from the study of volcanic rocks (Late Miocene-Recent)
• older portions were pieced together from the study of long continuously recovered sediments

3. INTEGRATION OF THE MAGNETIC ANOMALY AND PALEOMAGNETIC TIME SCALES

• both the magnetic anomaly time scale and paleomagnetic time scales were generated somewhat independently with different terminology for reversals
• recently these time scale have been merged to produce a geomagnetic time scale
• long duration reversals are called Chrons
• short duration reversals are called Subchrons
• Youngest Chrons are named: Brunhes, Matuyama, Gauss, and Gilbert
• Youngest Subchons are also named: Jaramillo, Olduvai, Kaena, Mammoth, etc.
• Older (>~5 m.y.) reversals are numbered
• Studies of fossils in paleomagnetically calibrated sections has produced a means of identifying individual reversals
• Reversal ages have been interpolated between radiometrically dated calibration points

4. LIMITATIONS OF THE PALEOMAGNETIC TIME SCALE

• There are only two types of reversals, normal and reversed.
• Disconformities may superimpose two separate normal or reversed polarity intervals, making them appear as a single reversal.
• Disconformites, if undetected, may result in an incorrect interpretation of the vertical sequence of reversals
• Identifying individual reversals requires preexisting correlation of biostratigraphic data to individual reversals, thus providing a biologic fingerprint to differentiate them from other reversals.

A. STRONTIUM ISOTOPE STRATIGRAPHY

• two isotopes of strontium (Sr-86 and Sr-87) generally occur with the same relative abundance throughout the oceans.
• the ratio of these isotopes has changed through time, partly with changes in rates rocks yield strontium to the ocean by exposure and erosion.
• changes of the ratios of these isotopes through time have been dated for the Cenozoic
• the abundance of Sr-87 has increased relative to Sr-86
• the rate of change is uneven with periods of little change and other intervals with more rapid change
• periods of rapid change offer a means of dating sediment by determing the Sr ratios within the shells of calcareous fossils
• dates are inaccurate if the fossil are diagentically altered.

B. OXYGEN ISOTOPE STRATIGRAPHY

• isotopes (species of an an element with different atomic mass) of oxygen-16 and oxygen-18 occur in seawater (H₂O).
• although water temperature and salinity influence the ratios of these isotopes, the primary control on the global ratio of these isotopes in seawater has been large scale fluctuations in Northern Hemispher ice volume.
• during the process of evaporation the lighter isotope (O-16) is preferentially taken up over O-18. Precipitation of this O-18 enriched water on continents, in the form of snow,  occurs in huge amounts to form ice sheets. This process changes the oxygen isotopic composition of the oceans. If these changes can be dated they can provide a means of high resolution stratigraphy.

• Large scale glaciation began in the Northern Hemisphere much later than in the Southern Hemisphere.
• Some small scale glaciation occurred in the Northern Hemisphere by about 5-6 Ma.
• Widespread ice sheets, though smaller than more recent ones, began waxing and waning by 3.5 Ma.
• During the past 1 million years, the largest of all ice sheets covered vast areas of North America.
• What caused the large scale glacial-interglacial variations in the Northern Hemisphere?
• Clues to this question come from the more complete deep-sea sediment

record.
• It is from this record that the most detailed record of glacial-interglacial

variations is revealed from oxygen isotopes of fossil planktonic foraminifera.
• These records indicate the most important cycles of climate change have

frequencies of 400,000, 100,000, 41,000, and 23,000 years.
• Additional insight became evident with the discovery that these climate

frequencies corresponded to changes in orbital parameters of the Earth as
noted by the Yugoslavian scientist Milankovitch.
• This discovery of matching frequencies of climate with the Milankovitch

orbital cycles, led to publication in the late 1960s of the theory "Orbital
Pacemaker to the Ice Ages". Yes, glacial-interglacial variations appear to be
related to changes in these parameters : MILANKOVITCH CYCLES
• 1. Axial Tilt (period = 41,000 yr.) 21.5-24.5^o range tilt shallow=greater polar snow because less summer melt
• 2. Eccentricity of Earth orbit (period= 400,000 & 100,000 yr.) 0.017-0.053 eccentricity, greater eccentricity= colder
• 3. Precession of the Equinoxes Wobble of the axis (period=23,000 yrs) causes the position of the equinoxes and solstices to shift in position around the Earth's elliptical orbit today closest approach of Earth to Sun is in Jan. so N. Hemisphere winter warmer 10,000 yr. ago in July so colder winter closest approach changes by 1 day per 60 yrs

• 116 major cycles (oxygen isotope stages) of O-18 enrichment or depletion have been documented in the sedimentary record of the last 2.8 M.Y.
• These stages represent glaciations and interglacial events.
• Since the frequency of these glacial-interglacial variations matches the Milankovitch factors, the ages of stage boundaries can be orbitally tuned to provide absolute ages.
• In high sedimentation depositional environments this allows a dtermination of sediment age to within hundreds of years.
• Today's interglacial is stage 1, all interglacials are odd numbered
• The last glacial is stage 2, all glacials are evene numbered
• there are 116 stages over the last 2.8 M.Y.
• Calibration of biostratigraphic data to oxygen isotopic stages allows them to be used in subsequent sections as high resolution age datums.