Week 2 - sample lab exam question

WEEK 2 - SAMPLE LAB EXAM QUESTION

QUESTION:

Figure A shows a banded iron formation. These structures were formed from 2.5 billion years to 1.8 billion years ago.

Figure B shows a stromatolite. Stromatolites were first formed 3.5 billion years ago. They became abundant approximately 2.5 billion years, and remained abundant until the first appearance of animals in the Cambrian.

Both structures give us important information about the nature and cause of atmospheric changes that have taken place on our planet since the appearance of life.

Explain:
1. the connection between structures A and B.
2. what information about atmospheric changes on the earth is indicated by these structures.
ANSWER:
1. Stromatolites (B) are structures made by cyanobacteria on the early earth. Cyanobacteria were the first organisms to generate oxygen by photosynthesis. The oxygen initially oxidized reduced minerals such as iron. The layers of oxidized iron in the banded iron formation (A) were oxidized by oxygen produced by cyanobacteria as a byproduct of photosynthesis.
2. Stromatolites preserved from the early earth show that cyanobacteria - very similar to modern forms - were abundant on earth. This demonstrates the source of the increasing atmospheric oxygen.
  
  The banded iron formations show that, during the period of their formation, levels of atmospheric oxygen varied, producing alternating layers of reduced and oxidized iron.

Back to Biology 102 Home Page.