Bugs Isotopes Magnets and Orbits Earth s clock Cinzia

Bugs, Isotopes, Magnets, and Orbits: Earth’s clock Cinzia Cervato Chronos Iowa State University Dali: The persistence of memory, 1931

Time w Everything changes with time w Geologists “tell” time using various methods – – © Chronos “Bugs” = fossil organisms “Isotopes” = radioisotope geochronology “Magnets” = magnetostratigraphy “Orbits” = astronomical cyclostratigraphy American Geophysical Union Lecture, NSTA Convention April 1, 2005

Two ways to ‘tell’ time w In layered sedimentary rocks, the oldest rocks are at the bottom: relative time w Igneous rocks => numerical ages (in millions of years) w The trick is to give numerical ages to sedimentary rocks, the archive of Earth history © Chronos American Geophysical Union Lecture, NSTA Convention April 1, 2005

Earth’s archive Younger Older © Chronos American Geophysical Union Lecture, NSTA Convention April 1, 2005

Photo by Steven Pinker © Chronos American Geophysical Union Lecture, NSTA Convention April 1, 2005

History of the Grand Canyon Original artwork by Gary Hincks, W. W. Norton © Chronos American Geophysical Union Lecture, NSTA Convention April 1, 2005

4 billion years of life history in 2 minutes © Chronos American Geophysical Union Lecture, NSTA Convention April 1, 2005

Dinosaurs disappear 90% of life extinct First humans First land plants Primitive higher animals © Chronos American Geophysical Union Lecture, NSTA Convention First life April 1, 2005

Dinosaurs disappear 90% of life extinct First humans First land plants Primitive higher animals © Chronos American Geophysical Union Lecture, NSTA Convention April 1, 2005

Dinosaurs disappear First humans 90% of life extinct © Chronos American Geophysical Union Lecture, NSTA Convention April 1, 2005

“Bugs” w Biostratigraphy uses overlapping ranges of fossil organisms - animal or plant w Identifies first appearance and extinction events within a series of sedimentary rocks TAXA © Chronos American Geophysical Union Lecture, NSTA Convention April 1, 2005

© Chronos Cenozoic fossil mammals and turtles, American Geophysical Union Lecture, NSTA Convention Big

© Chronos American Geophysical Union Lecture, NSTA Convention April 1, 2005

Numerical ages w Various methods to give a numerical age (in years or millions of years) http: //web. utk. edu/~grissino/ http: //www. mccormick-violins. com/images/Hill. jpg © Chronos American Geophysical Union Lecture, NSTA Convention April 1, 2005

From dendrochronology to geochronology w Tree rings can be dated with 14 C to calibrate them w Radiocarbon can only be used to date organic material (plant or animal) younger than ~ 60, 000 yrs w For rocks and older material, we need other methods: e. g. uranium/lead http: //web. utk. edu/~grissino/ © Chronos American Geophysical Union Lecture, NSTA Convention April 1, 2005

Isotopes w Certain minerals (e. g. , zircons, zirconium silicates) contain radioactive elements like uranium w When the mineral is formed within an igneous rock, the radioactive element begins to ‘decay’ w The rate of decay is constant (like the ticking of a clock) and expressed as ‘half-life’ © Chronos http: //www. mala. bc. ca/~earles/zircon. jpg American Geophysical Union Lecture, NSTA Convention http: //www. geotimes. org/sept 03/NN_zircons. jpg April 1, 2005

Radioisotope geochronology w If rocks containing datable minerals (volcanic rocks) are interlayered with sedimentary rocks, we can give numerical ages to the rock sequence ash sedimentary rock http: //userpage. fu-berlin. de/~allggeo /Scheuber/Magmatismus/Abbildungen/Tephra-1. jpg © Chronos American Geophysical Union Lecture, NSTA Convention April 1, 2005

Eruption of Mt Etna, May 2000 © Chronos http: //boris. vulcanoetna. com/April. May 2000/GS_26042000_5. jpg American Geophysical Union Lecture, NSTA Convention April 1, 2005

ash sedimentary rock © Chronos American Geophysical Union Lecture, NSTA Convention April 1, 2005

Volcanic ash Uranium/lead ages of zircons extracted from ash http: //volcanoes. usgs. gov/Imgs/Jpg/Tephra/30410914 -076_large. jpg http: //www. geotimes. org/sept 03/NN_zircons. jpg © Chronos American Geophysical Union Lecture, NSTA Convention April 1, 2005

Half-life © Chronos American Geophysical Union Lecture, NSTA Convention April 1, 2005

Rate of decay is constant w How do we know this if the half-life is really long? w It is a very well tested assumption w Multiple minerals from single rock specimens dated by different, independent methods give consistent results © Chronos American Geophysical Union Lecture, NSTA Convention April 1, 2005

From geochronology to the time scale w Biostratigraphy - relative sequence of rocks and events w Uranium/lead geochronology - numerical ages w Calibrate geologic time scale w But what do we do where we do not have ash layers? © Chronos American Geophysical Union Lecture, NSTA Convention April 1, 2005

Applying geochronology w We use other methods to identify sequences and events w Magnetostratigraphy reconstructs sequences of rocks with different magnetization w Biostratigraphy and geochronology are required to “date” the sequence © Chronos Permo-Triassic boundary, China American Geophysical Union Lecture, NSTA Convention April 1, 2005

Magnets w The Earth’s magnetic field reverses orientation w Reversals have no periodicity Today 800, 000 yrs ago © Chronos American Geophysical Union Lecture, NSTA Convention W. W. Norton April 1, 2005

Preserving magnetization in rocks w When igneous rocks solidify, magnetic minerals ‘freeze’ the Earth’s magnetic field orientation at that time w Magnetic minerals in sedimentary rocks do the same Becomes solid W. W. Norton © Chronos American Geophysical Union Lecture, NSTA Convention April 1, 2005

Putting together the record of magnetic reversals W. W. Norton Geochronology dates lava flows/ash layers. Then, we place the magnetic intervals in a temporal sequence © Chronos American Geophysical Union Lecture, NSTA Convention April 1, 2005

Orbits and cyclicity Lake varves Sediments Ice http: //people. ucsc. edu/~mikeloso/images/varves. jpg © Chronos http: //www. aad. gov. au/asset/images/525_ul-core. jpg American Geophysical Union Lecture, NSTA Convention April 1, 2005

Periodicity (n): the quality of recurring at intervals w Orbital parameters vary with a known periodicity w Like the heartbeat! http: //www. ecglibrary. com/norm. html © Chronos American Geophysical Union Lecture, NSTA Convention April 1, 2005

Periodicity of orbital cycles 100, 000 yr cycles 41, 000 yr cycles 21, 000 yr cycles © Chronos American Geophysical Union Lecture, NSTA Convention http: //www. jhu. edu/~eps/staff/hinnov/ April 1, 2005 hinnovresearch/earthsorbitalparameters. htm

Cyclicity in the sedimentary record w Seasonal, annual to centennial cycles – Lake varves, tree rings, corals, cave deposits w Millennial and higher cycles – Lake and ocean sediments, loess w Orbital ‘tuning’ applied sequence of cyclical sediments => cyclostratigraphy w Cyclostratigraphy requires validation with radioisotope geochronology © Chronos American Geophysical Union Lecture, NSTA Convention April 1, 2005

Geologic Time Scale w 2004 Global Time Scale published by International Commission on Stratigraphy w Available digitally at www. chronos. org and www. stratigraphy. org © Chronos American Geophysical Union Lecture, NSTA Convention April 1, 2005

Teaching resources on geologic time and stratigraphy w Online resources from the www. chronos. org web site © Chronos American Geophysical Union Lecture, NSTA Convention April 1, 2005

www. paleoportal. org © Chronos American Geophysical Union Lecture, NSTA Convention April 1, 2005

Partner with scientists! w Help scientists develop educational resources on geologic time that you can use in your class - we need your help! w Questionnaire on geologic time and Earth history w For teachers and 6 -12 grade students w Go to www. chronos. org © Chronos American Geophysical Union Lecture, NSTA Convention April 1, 2005