TIME

CONCEPTS ABOUT TIME

• Biological Time - Rhythms of activities of "internal clocks"
• Inherited cyclical behavior
• Astronomic Time - Relationship of Earth to other heavenly bodies
• Solar Time - 1 rotation based upon Atomic Clock; slows 2 msec each 100 years
• Historic Time - Sense of direction and significance of man above beast

HOW DO WE ACCOUNT FOR TIME?

• Earliest calendars based upon Lunar Cycle
• Assyrian calendar 364 days (full moon cycle is 29 ½ days)
• After 3 years, lunar calendar off by one month!!
• Best approximation was a 19 year cycle; 7 having 13 months (leap years) adopted by Israelites
• Roman calendar had 29 or 31 day months (except February had 28)
• Added extra month every 2nd year (Mercedonius)
• In 45BC Caesar reformed calendar and had one year with 445 days - Julian calendar

HOW DO WE ACCOUNT FOR TIME?

• Julian calendar 30 or 31 day months with leap year; date of vernal equinox drifted
• Pope Sixtus IV attempted reform, but astronomer Regiomontanus died of plague
• Council of Trent (1545) authorized Pope Gregory XIII to reform calendar
• Every 4th year is leap year except for century years not divisible by 400 - Gregorian Calendar
• Julian calendar used until either 1698 or 1752 in protestant countries; Russians changed in 1918



• GEOLOGICAL TIME CONSIDERATIONS

• Geochronology - the organization of exposures of rocks into a standard chronological sequence
• Before the advent of radiometric dating techniques, the Geologic Column was assembled based upon the basic Laws of Stratigraphy .

Divisions of Geologic Time

• Phanerozoic
• 543 Ma to present

• Proterozoic
• Time interval from 2500 to 543 Ma (geochemical change reflected in rock record).

• Archean
• Begins with oldest radiometrically dated rocks 3800 Ma

• Hadean
• Time before oldest known rocks on Earth (determined from radiometric dating of meteorites that have hit the earth's surface).
• Largest division is the EON

Divisions of Geologic Time

• Phanerozoic Eon is subdivided into three Eras
• Boundaries based on major global extinctions.
• Biotic extinctions principally in the zoological realm.

• Cenozoic - the last 66 MY interval.

• Mesozoic - 170 MY interval
• beginning at 245 MY and ending at 66 MY BP
• Paleozoic -397 MY interval
• beginning at 543 MY and ending at 245 MY BP

Radiometric Dating Techniques

• Isotopes differ in # neutrons; stable & unstable
• Spontaneous decay
• Parent-to-Daughter
• Loss of Alpha particle
• Helium atom (2p, 2n; At # -2; At Mass -4)
• Loss of Beta particle
• Electron loss turns neutron to proton (At # +1; At mass 0)
• Capture of Beta
• Turns proton to neutron (At # -1; At mass 0)

Useful Radiometric Isotopes

• Rubidium87 - Strontium 87 48.6 BY
• Traceamounts in Ign. & Meta Rx; >100 MY

• Thorium232 - Lead 208 14.0 BY
• Zircons widespread in Ign. & Meta Rx

• Potassium 40 - Argon 40 1.3 BY

• Uranium 238 - Lead 206 4.5 BY
• Zircons widespread in Ign. & Meta Rx

• Uranium 235 - Lead 207 0.7 BY
• Zircons widespread in Ign. & Meta Rx

• Carbon 14 - Nitrogen 14 5730 Yr
• ¹⁴Carbon

Non-Radiometric Dating Techniques

• Varved deposits
• Couplet comprised of summer (coarse) & winter (very fine from suspension) clastics deposited in response to yearly fluctuation in sedimentation. Capable of determining lake histories (e.g., Lake Baikal >200,000 varves).
• Dendrochronology
• Independent test for C¹⁴ dating.
• Oldest living plant (Pinus longaeva -bristlecone pine) > 5000 years old.
• Pinus longaeva specimen WPN-114 located on Mt. Wheeler, eastern Nevada was 5100 yrs old in 1964 (cut).
• Pinus longaeva living on California side of White Mts was 4600 yrs old (1974).
• Thermoluminescence
• Emission of light from a mineral by application of heat less than incandescence.
• Hydration of Obsidian
• The edge of volcanic glass may be effected by water
• Diffusion from the edge can be seen optically
• Time can be calculated using a diffusion equation.

• Amino Acid Racemization
• Thermal instability of stereoisomers.
• In polarized light there is a central asymmetry (l-) and (d-)
• When subjected to increased depth and age, there is an exponential rate of change from (l-) to (d-)
• Determination of (l-) and (d-) isomers and comparison to rate of change allows for age determination

GE142