Lecture 19:  April 15.

• Chapter 10 textbook
• Geological Time (USGS)
• Callomon, J. H., 2001, Fossils as geologcal clocks. Geol. Soc. London Spec. Publ. 190, 237-252.

• History of the Geological TimeScale
• How to do stratigraphy

Browse through:

• Modern stratigaphic databases
• The 2004 time scale
• The website of the International Commission on Stratigraphy (ICS)

Development of modern understanding of Time in Western Science

• Mid eighteenth through mid twentieth century
• 'Enlightenment': decreasing sense of importance of humans in universe
• From thousands to billions of years...

• Most familiar: Age of Earth ~ 6000 years, created 4004 BC: One is born, one dies (directional time: time's arrow).
• Ecclesiastes:'[One] generation passeth away, and [another] generation cometh: but the earth abideth for ever': Seasons of the year; astronomical cycles (cyclical time: time's cycle)

• Astronomical cycles, historical accounts, biblical chronology
• 'In the beginning, God created heaven and Earth, Gen. I.V.I. Which beginning of time, according to our chronology, fell upon the entrance of the night preceding the twenty third day of October in the year of the Julian Calendar 710', which translates into October 22, 4004 BC.
• William Lloyd, bishop of Winchester, put that date in the 1701 'Great Edition' of the King James bible

James Hutton: 'Theory of the Earth' (1788, 1795); explained by John Playfair in 'Illustrations of the Huttonian Theory of the Earth' (1802)

• "If the succession of worlds is established in the system of nature, it is vain to look for anything higher in the origin of the Earth, The result, therefore, of our investigations is, that we find no vestige of a beginning, - no prospect of an end"
• Realized that the world does not just erode away into the end of the world, but that the rocks show superposition of numerous cycles of eriosion and deposition and uplift.

Charles Lyell: 'Principles of Geology, Being an Attempt to Explain the Former Changes of the Earth's Surface by Reference to Causes Now in Operation' (1830-1833)

Lyell's cyclical time: life and climate: "Then might those genera of animals return, of which the memorials are preserved in the ancient rocks of our continents. The huge Iguanodon might reappear in the woods and the ichthyosaur in the the sea, while the pterodactyl might flit again through the umbrageous groves of tree ferns."

• Cyclical changes: count numbers (e.g., seasonal layering in ice cores, tree rings)
• Things that change unidirectionally: estimate rate from present-day processes, extrapolate back in time - must assume rates were constant, process started at formation of Earth

Must be sure these things are unidirectional : More salt in the sea, Earth started out hot, cooled down; Accumulation/ Erosion of sediments; evolution of organisms

• Bible : 169 AD, Theophilus of Antioch; Ussher: 1650 AD (Latin), 1658 AD (English)
• Decline of sea level: da Vinci (1452-1519); Benoit de Maillet (Telliamed); 1748 (fossils on land)
• Cooling of Earth/Sun: Isaac Newton, 1687; Comte de Buffon (1774); Lord Kelvin (1860-1897)
• Orbital Physics (tidal effects Earth-Moon: Moon flung out of Pacific Ocean): George Darwin, ~1879-1898
• Ocean chemistry (sulfate, chloride, sodium): Edmund Halley, 1715; John Joly 1899-1900
• Erosion-Sedimentation: John Joly, Archibald Geikie (1868-1908)
• Radioactive isotopes: 1920s- on; Arthur Holmes (1927-1947)

Theory that Earth was originally an ideal sphere covered with water, which water later retreated to its present level. Evidence: fossils looking like shells of ocean dwelling animlas found on mountain tops. Problems: sea level moves fairly rapidly, in response to melting or forming ice caps, no correlation to formation of Earth. Fossil are on mountain tops as result of mountain building processes and uplift.

• Physicist: second law of thermodynamics. There is a direction in time; can not be infinite.
• Laws of conservation and dissipation of energy; 661 papers on scientific subjects; 70 inventions.
• 400 - 10 million years
• Problem: did not know source of Sun's energy (nuclear energy); thought energy came from collision of many meteorites falling together. If that had been so, the Sun would have een rapidly losing energy since its formation and could not have existed for more than tens to hundreds of millions of years at most. Similarly, estimates of how rapidly the Earth would have lost heat from its original status a molten sphere resulted in an age range of 10-400 million years (Ma). In later estimates Kelvin refined the numbers to a few tens of millions.

Second son of Charles Darwin: Orbital Physics : Earth-Moon system looses energy by friction; Earth turns around slower, moon moves further away during Earth history 

• Mathematical astronomer; worked on age of Earth (Orbital Physics) through tidal effects Earth-Moon in ~1879-1898
• Most well known: theory that Moon was thrown out of hole which is now Pacific Ocean by rapid rotation
• Present theory on formation of Moon: Moon formed when large, Mars size asteroid hit Earth early on (about 4 billion years ago), its core was added to Earth, its mantle went on to form Moon. If you are interested:  read more on the web on the Origin of the Planets and the Origin of the Moon; or watch the video-modeling of moon-forming from 'moonlets'.

• Most common constituents of sea salt: sulfate, chloride, sodium
• Edmund Halley, (1656-1742): astronomer, geophysicist; 1715 - regretted there were no data
• John Joly (1857-1933): engineer, physicist, geologist. 1899: 'An Estimate of the Geological Age of the Earth'

Rivers are fresh, contain some salts. Flow into ocean: water evaporated from ocean. Salt remains behind. What we get from the calculation (divide 'how much salt is in ocean' by 'how much salt gets into the ocean every year by rivcers') is NOT age of Earth, but 'residence time'

• MANY geologists attempted this calculation
• Well known attempts: John Joly, Charles Walcott, Archibald Geikie (1868-1908)
• Estimate time needed for accumulation of all sediments on Earth
• First, estimate thickness of sediment accumulated during all geological periods
• Second, estimate accumulation rate (meters/year)
• Many methods, many uncertainties

• 1896: Henri Becquerel, Marie and Pierre Curie: natural radioactive decay
• 1905: Lord Ernest Rutherford: first suggestion to use radioactivity for measuring geologic time
• 1907, Bertram Boltwood published a list of geologic ages based on radioactivity

Radioactive elements decay: parent isotopes changes into daughter isotope, as parent decreases, daughther increases .

• Relative time (which is older than which): superposition, correlation using fossils
• 'Absolute time' or Numerical time: time in (millions or billions of) years, using radioactive minerals from, igneous rocks
• Igneous rocks (cooled lava flows) have no fossils

Radioactive elements: in igneous rocks (lavas, ashes), sometimes intercalated in rocks with fossils. That's how we derive linkage between geological and numerical time (age in years of geological periods).

Erosion and Sedimentation: Note that correlation by fossils, and the use of geological time periods long predated the concept of evolution (Darwin published Origin of Species 1859), and was used in an empirical way since William Smith's publication of the first geological map of England in 1801.

Hierarchy of names:

EON (Phanerozoic)

ERA (Cenozoic)

PERIOD (Neogene)

EPOCH (Miocene) 

Biostratigraphic zones: correlation of sediment layers (strata). Geological correlation of strata requires the determination of a sequence of unique points for non-recurrent events common to the sedimentary record as observed at different sites. The first and last occurrences of fossils [First appearance datum (FAD) and last appearance datum (LAD) ] are examples of unique events that can be used for correlation between stratigraphic sections.

Not so easy: sample spacing, rare fossils hard to find, many fossils rare close to beginning and end of range.

FADs and LADs may not be time-equivalent at different places (depth, climate bound): they may result from evolution (extinction), immigration (emigration). 

Various types of zones:

• Range zone (from FAD to LAD of one species)
• Interval zone (part of range of one species, where it does or does not co-occur with others)
• Concurrent range zone (overlapping parts of ramges of species)
• Assemblage Zone (overlapping ranges of many species)
• Oppel Zone (overlapping ranges of many species; not all species necessarily present)
• Acme zone (peak abundance)

Complications:

• There may be a hiatus or unconformity: time not represented in sediment
• Many first and last appearances at same place in sediment
• Difficult to distinguish from mass extinction (must look at many places)

Misleading effects:

• Signor-Lipps effect: abrupt extinction may appear to be gradual
• Lazarus species: appear to be extinct but re-appear (environmental effects)
• Elvis species: do not really re-appear: impersonator (parallel evolution)
• Zombie species: reworked older species

• Combine various statistical evaluations of data, graphical data evaluation
• First and last appearance data of different fossil groups (as many as possible)
• Paleomagnetic data
• Stable isotope/trace element data
• Lithologic data (e.g., volcanic ash)

The geological time scale remains a 'work in progress'! For instance, see the Chronos project web site (the one on geological time, not the interactive game site)