Lecture  3  Radiometric Dating and Sedimentary Processes

Focus Questions:  How can we measure the numerical age of rock layers?  and How are past environments recorded in sedimentary rocks?

 

I.                     A word about atoms…

A.       The nucleus of an atom contains the protons (+ charge) and neutrons (0 charge).  The electrons (- charge)  are outside the nucleus, and circle in  clouds or “shells”.  Chemical reactions involve the electrons of an atom, not the nucleus. 

 

B.      The number of protons in the nucleus of an atom is the Atomic Number, and identifies the element. 

Examples:  An atom with 8 protons is an atom of Oxygen.  An atom with 7 protons is an atom of Nitrogen.

C.      The number of protons plus the number of neutrons is the Atomic Mass.  Atoms that have the same number of protons (same atomic number) but different numbers of neutrons are the ISOTOPES of an element.  Example

There are 3 isotopes of carbon:  C-12 (6 protons plus 6 neutrons)

                                                                        C-13 (6 protons plus 7 neutrons)

                                                                                        C-14 (6 protons plus 8 neutrons)

They are all carbon, but differ in their number of neutrons.  They are Isotopes.

II.  Isotopes can be STABLE  or UNSTABLE.  If they are UNSTABLE, we say they are RADIOACTIVE.  Radioactive isotopes spontaneously decay to atoms of other elements.

A.       Radioactivity was discovered by Henri Becquerel in 1896, and early in the 20th century, scientists including Marie Curie discovered elements that had radioactive isotopes.

 

B.       The Half-Life-the amount of time it takes for half of the radioactive atoms in a sample to decay to other elements-the terms for these are Parent atoms decaying to Daughter atoms.

 

C.       Different Parent/Daughter pairs have different half lives.  Two groups of Parent/Daughter Isotopes are used in radiometric dating:

 

1.        Primordial isotopes– these radioactive isotopes have very long half-lives, and are part of the original material present when the solar system formed.

Parent/Daughter pair                     Half Life                                Material Used

Rubidium87-Strontium 87            48.8 billion years               igneous/metamorphic rocks

Potassium40-Argon40                      1.3 billion years               igneous/metamorphic rocks

Uranium 238-Lead 206                   4.6 billion years                igneous/metamorphic rocks

Uranium 235-Lead 207                   0.7 billion years                igneous/metamorphic rocks

 

2.       Cosmogenic isotopes-these radioactive isotopes have very short half-lives, and are made continuously in the upper atmosphere.

        Parent/Daughter pair                     Half Life                Material Used

        Carbon-14/Nitrogen-14                 5,730 years         organics

III.  Sedimentary environments

Focus Question:  How are past environments recorded in sedimentary rocks?

1)                  Much of the rock record we will be looking at is made of sedimentary rocks.   There are lots of kinds of sedimentary rocks, but for our purposes, we will look at two big groups-

those composed of silicate minerals, for the most part, such as

Conglomerates (very large grain sizes), sandstones, shales, claystones (very tiny grain sizes)

       And those composed of calcium carbonate that has either precipitated from water, or supplied from shell fragments, or both-these are limestones.

(Other common precipitated mineral deposits that form rock units  include gypsum and salt).

2)                  Energy of the environment of deposition is visible in the sizes of the grains in sedimentary rocks.

It takes more energy to move big grains, less energy to move small ones.  So…

Boulders, cobbles, gravel---higher energy

Sand-medium energy

Silt and clay-low energy

 

3)                  Sedimentary rocks start as sediments that have been eroded and transported to basins where they collect.  Typically, the source of energy doing this job is water or wind.  Water accounts for a lot of this.  Think of the rivers carrying sediment down to the sea, for example.

4)                  Sediments are formed in various kinds of environments.  Generally, we can see the “trail”.  Examples of this are

 delta sediments-where rivers meet the sea, or a lake, you can see big piles of sediment.

                Mississippi delta “birdsfoot deltas”  river controls the sediment distribution

                Nile delta-sea waves control the sediment distribution

5)                  Sea floor sediments-most of the earth’s surface is covered by ocean, with sediments collecting in the ocean basins.  How does sediment get in there?

 

Terrigenous sediment-from the continents-large grains (sand and gravel) near shore, smaller grains further out-the deep ocean basins have very fine grained sediments

        Rivers bring sediment down to the sea

        Wind blows fine grains out across the oceans, these settle to the bottom

 

Biogenic sediment-tiny shells actually made by plankton-microscopic plants and animals in the surface ocean-these collect as oozes on the ocean floor

        Calcareous oozes-shells are made of calcium carbonate

       Siliceous oozes-shells are made of silica

 

6)                   Walther’s Law of Facies-depositional environments that are next to each other at one point in time will stack vertically through time (if there are no unconformities…)

Sea level rise -coastal sediment package of layers will show facies (the look of the rocks) with a pattern of coarse grains at the bottom, fining upward with fine grains at the top

Sea level drop- (coastal sediment package of layers will show facies with a pattern of fine grains at the bottom, coarsening upward with coarsest grains at the top)

7)                   Carbonate rocks – limestones, are very common, and can be made from shells and/or precipitated calcium carbonate.

Florida-a big carbonate platform-modern reef environments of the National Marine Sanctuary (in the keys)

Reefs have a characteristic structure –forereef with talus slope, the reef core, the backreef and lagoon.  These can be recognized in the rock record, same formation, full of reef fossil organisms. 

Next Lecture:  Sea floor spreading and mountain building processes