Today, banded iron formations, a type of sedimentary rock found in many location
ID: 295009 • Letter: T
Question
Today, banded iron formations, a type of sedimentary rock found in many locations, provides evidence for the dramatic increase in oxygen released by cyanobacteria. Oxygen reacted with iron dissolved in the ocean. This created black iron oxide minerals (like hematite and magnetite). These minerals, together with reddish-colored chert (a very fine-grained quartz that contains small amounts of iron), precipitated out of solution, collected at the sea floor, and eventually turned into banded iron formations.
The majority of banded iron formations were formed about 2.2 billion years ago during the Precambrian Eon. During this time, widespread iron oxidation, in large part due to oxygen production by cyanobacteria, provided the minerals that make up banded iron formations.
Take a look at the image showing banded iron-rich rocks deposited during the Precambrian eon. The statements below describe conceptual relationships between iron-rich deposits and Earth’s early atmosphere. Determine if these statements are true or false.
Drag the appropriate items into their respective bins. Note that not all bins need to receive a label.
Reset Help- These rocks are evidence that oxygen and iron became iron oxide during the Precambrian.
- These rocks are evidence that oxygen must have been present during the Precambrian. The oxygen was released as a by-product of photosynthesis.
- These rocks are evidence that significant amounts of iron were released on the ocean floor billions of years ago.
- These rocks are evidence that there was very little and isolated amounts of iron deposited with chert during the Precambrian.
- These rocks are evidence that oxygen was not yet present during the Precambrian.
- These rocks are evidence that Earth's early atmosphere remained the same during the Precambrian.
- These rocks provide evidence that photosynthetic bacteria existed 3.5 billion years ago.
Explanation / Answer
The first statement that says ' These rocks are evidence that oxygen and iron became iron oxide during the precambrian'. This statement is true as the free oxygen present and the dissolved ferrous ions in ocean reacted to form the iron oxide.
The second statement is also true. Oxygen must have been present during the proterozoic period .Green plant photosynthesis is the accepted source of this free oxygen.
The third statement is also true. The Banded Iron Formations from around the world are all of age between 3 billion years to 1.3 billion years. The formation of BIF is believed to take take place in the ocean where huge reserves of iron ions were present.
The fourth statement is false- These rocks or the banded iron formation are evidence that there were huge amounts of iron and silica deposited during the precambrian.the silica was deposited in the form of jasper and chert.
The fifth statement is also false- If oxygen was not present than it was not possible to form iron ore deposits during the precambrian. The iron ore deposits consist mainly of two iron oxide minerals, 1. Magnetite and 2. Hematite
The sixth statement is false. Oxygen was absent in the primordial atmosphere of the earth and during late archean it was also present in negligible amounts. From the rock evidence we can infer that the amount of oxygen began increasing gradually about 2.5 billion years ago. This was contributed by the photosynthesis of the green algae and bacteria.
The seventh statement is true. Some biogenic organism activity have been seen present in the rocks of precambrian like the lake superior formation rocks. Primitive bacteria existed about 3.5 billion years ago and they slowly began to evolve and produce green algae which produced oxygen by photosynthesis.