Most of the world's iron ore, including that found in northern Minnesota, formed during the middle Precambrian period, ranging in age from 1.8 to 2.5 billion years old. They are composed of alternating layers of iron-rich material, most often magnetite or
hematite, and silica. These iron oxide minerals formed on the ocean floor, creating the banded iron deposits you see here. What
was the source of the oxygen that helped to produce these deposits?
A)
The oxygen that was released into the atmosphere as a by-product of
photosynthesis caused iron deposits to oxidize creating the iron oxide
minerals.
B) Photosynthetic organisms making oxygen as a by-product of
photosynthesis and the oxygen reacted with the iron dissolved in seawater
to form iron oxide minerals.
C) The decomposition of carbon-based life forms in the oceans released
oxygen and carbon dioxide. These gases reacted with the iron deposits
forming the minerals mentioned.
D) In Earth's early biosphere, there were not organisms that underwent
aerobic cellular respiration and an excess of oxygen built up in the
atmosphere. This oxygen reacted with iron deposits to form iron oxide
minerals.