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UMD GEOL 342 - Lecture 21: geochronology

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Astronomical DatingG342 Sedimentation and StratigraphyLecture 21: geochronology 5 May 2005Assoc. Prof. A. Jay Kaufman Everyone is required to fill out the on-line course evaluation. Please do this as soon aspossible. Undergraduate director John Merck will provide me with a list of respondents, whowill receive extra credit on their final examination. https://www.courses.umd.edu/online_evaluationGeochronologyAll we have learned about thus far is relative dating. Today we will discuss aspects ofnumerical (sometimes referred to as absolute) dating. Determining the numerical ages on ancientrocks became possible through the discovery of radioactive decay in the early 20th century,starting with Marie Curie’s discovery of radium. Although most radiometric studies areconducted on igneous and metamorphic rocks, the understanding of geochronology is essential tostratigraphy.Lacking a mass spectrometer, however, some geoscientists have gone to the effort of counting varves in glacial lakes and in ice to determine numerical ages. Deep drill cores in ice from Greenland and Antarctica have captured ice layers dating back over 180,000 years. Alternatively, dendrochronology is the study of the annual variability of tree ring widths,which has been extended back to 8000 years ago. The study of trees provides climateinformation regarding temperature, runoff, precipitation, and soil moisture.1Radiometric techniques rely on the decay of radioactive elements and counting the parentand daughter products. These different techniques rely on different half-lives for differentradioactive elements, so they fit different stratigraphic bandwidths. Similarly, they requireseparate materials, and thus are appropriate for different circumstances.Parent-Daughter Typical materials HalfLifeTypical Range OtherCarbon (14C) – Nitrogen (14N)Charcoal, plantmaterial5730 0-30,000(60,000 rarely)High precision; uncertaintyin <2000 yearsPotassium (39K) – Argon (40Ar)Feldspars, micas,ashes1.3billion 100,000 – 5 Ga(50,000 rarely)Usually 40Ar/39Ar todayUranium (238U) – Lead (206Pb)Zircon, Monazite,Badellyite4.5billion> 10 Ma Usually done in concertwith 235U/207PbUranium (234U) – Thorium (230Th)Carbonates 25,000 0 – 200,000 One of many in UraniumSeries datingRubidium (87Rb) – Strontium (87Sr)Many mineralsand rock types47billion> 10 Ma Old technique – rarelyusedOther (less quantitative) radiometric age techniques However, there is a big gap between radiocarbon dating and 40Ar/39Ar, where manygeomorphological and stratigraphic questions sit. As such, there are many other types of datingschemes with lower precisions, but with strong capabilitiesthermoluminescence dating (TL) – The elements uranium and thorium in minerals, like zirconand quartz, decay to produce alpha particles. These can get trapped in the crystal lattice ultimatelyleading to saturation. Since this is a background process the accumulation of alpha particles canbe used to constrain the age of ambient minerals. Heat and light will release the trapped particles,producing luminescence, which can be quantified. If the minerals are pristine, one can exposethem to light and heat and count their scintillation to get an age. The errors are large, but thetechnique is good for direct dating of river and beach sediments. Range: 5,000 – 300,000 yrs.2fission track – Alpha particle decay also makes tracks (holes) in crystals that can be seen andmeasured. Again, the rate of this process is understood, so one can count the tracks and get anage. The tracks close at relatively low temperatures (100 – 200oC for different minerals), and assuch, this technique is useful only for young, surface sediments. It DOES provide an upliftclosure age of minerals (chiefly apatite). Range: 100,000 – 20 Macosmogenic nuclide dating – The earth is constantly bombarded by cosmic rays, which createradioactive element during nuclear collisions (e.g., 14C). They do this with many elements,creating nuclides like 10Be, 26Al, and 39Cl. These nuclides appear in MANY different mineraltypes (e.g., olivine, quartz, calcite). As long as these minerals are exposed at the surface, they willcollect cosmogenic elements until reaching a balance between the acquisition and decay rates.Estimates are more precise if multiple elements are cross-compared. This technique is good forcalculating uplift and erosion rates. Range: 10,000 – 300,000.amino acid racimization – Biological amino acids all have the same “handedness” – they spiral tothe left. This spiraling, or chirality, changes through molecular kinetics after the organism dies.The process in sensitive to temperature and time, but if these can be constrained, and age can bedetermined by calculating a ration between left- and right-handed amino acids. This technique isgood for dating shells and uplift terraces, although the uncertainties are often large. Range:10,000 – 100,000 yrs.tephrachronology – Some volcanic eruptions have enormous ranges, and cover big chunks of thecontinent with ash. The ash may not contain any crystals, but will have distinct chemicalsignatures (e.g., trace element ratios). If these signatures are known, the ash can be analyzed andcompared to a database of well calibrated ages. This requires clean samples and a good catalog of3dated events, but is widely applied where these conditions are filled. Examples include theBishop Tuff (0.78 Ma) and the Mazama ash (6000 yrs). Range: 0 – 2 Ma.Astronomical DatingThe Earth changes its orbital patterns in repeated and predictable ways. This wasrecognized in the early 1940’s by Serbian mathematician, Milankovich. He calculated the typeand rate of these changes, since called Milankovich cycles. These patterns affect the amount anddistribution of sunlight coming to the eath’s surface, and thus change the climate. Thesesignatures can be seen in the ice records, as well as in sedimentary records throughout much ofearth history.Milankovitch CylcesThe three orbital parameters of this cyclicity are listed here:Name Motion Cycle duration OtherPrecessionRolling like a top orgyroscope19 ky & 23 ky Determines insolation in northernhemisphereObliquityChanging tilt


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