Forensic science-geological time scale.

Geological time scale

The table below was adapted from www.pubs.usgs.gov

EON	ERA	PERIOD	EPOCH
Phanerozoic	Cenozoic	Quaternary 1.6 million years ago	Holocene Pleistocene
	Cenozoic	Tertiary 1- 66 million years ago	Pliocene Miocene Oligocene Eocene Paleocene
	Mesozoic	Cretaceous 66-138 million years ago	Late Early
		Jurassic 138-205 million years ago	Late Middle Early
		Triassic 205-240 million years ago	Late Early
	Paleozoic	Permian 240-290 million years ago	Late Early
		Pennsylvania 290-330 million years ago	Late Middle Early
		Mississippian 330-360 million years ago	Late Early
		Devonian 360-410 million years ago	Late Middle Early
		Silurian 410-435 million years ago	Late Middle Early
		Ordovician 435-500 million years ago	Late Middle Early
		Cambrian 500-570 million years ago	Late Middle Early
Proterozoic	Late Proterozoic Middle Proterozoic Early Proterozoic	2.5 billion years ago
Archean	Late Archean Middle Archean Early Archean	3.8 billion years ago

Long before carbon or uranium dating geologists expressed Earth's geological time scale as a sequence of distinct stages characterized by the development of plant and animal life or geological activity.
This time scale was developed over many years as new fossil remains were uncovered and studied. The length of each distinct stage did not become apparent until the advent of radioactive dating.

So, this geological time scale is separated into eons, which are subdivided into eras, which are subdivided into periods which in turn are subdivided into epochs. The names of these subdivisions follow a pattern that may not appear obvious at first.
For example the word:

- zoic means animal life;
- phanero means obvious or visible;
- paleo means ancient;
- meso means middle;
- ceno means recent;
- protero means first or early.
Knowing the meanings of these words we can decipher the meaning of each subdivision.
For example:

- Proterozoic contains protero and zoic. This means early life and as such this period was characterized by the advent of microscopic life forms such algae, bacteria and simple worm like animals.
- Phanerozoic contains phanero and zoic. This means obvious life and so during this period of time complex organisms such as dinosaurs, mammals and trees evolved.

1) The phanerozoic eon is subdivided into three eras, cenozoic, mesozoic and paleozoic.
Was life present during these three eras?
2) In which era would we expect to find the most advanced, complex creatures?
3) In which era woud the life form be expected to be in its simplest form?

4) The single celled paramecium shown on the right appeared on Earth during which one of the two eons, phanerozoic or proterozoic? Explain.

5) The tracks shown on the left were most likely to have been formed during which eon?

6) The uranium bearing rock sample containing the foot prints on the left was analyzed for lead and uranium isotopes. It was found that uranium-238 and lead-206 isotopes were present. If the half-life of uranium-238 is 4.5 billion years what is the likely ratio of uranium-238 to lead-206 isotopes?
a) uranium-238:lead-206 = 1:9
b) uranium-238:lead-206 = 1:1
c) uranium-238:lead-206 = 19:1
d) uranium-238:lead-206 = 4:1

Explain your answer.

7) The red arrow on the left shows a thin layer of clay thought to have been deposited after a large impact that ended the rein of the dinosaurs late in the cretaceous period.
The rock just below the layer was analyzed for uranium-238 and lead-206. What should the ratio of uranium-238 to lead-206 be?
a) uranium-238:lead-206 = 1:9
b) uranium-238:lead-206 = 1:1
c) uranium-238:lead-206 = 19:1
d) uranium-238:lead-206 = 145:2
Explain your answer.