Banded Iron Formation

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Terra Dinâmica: Um Curso para Educadores

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American Museum of Natural History

Terra Dinâmica: Um Curso para Educadores

98 classificações

The AMNH course The Dynamic Earth: A Course for Educators provides students with an overview of the origin and evolution of the Earth. Informed by the recently released Next Generation Science Standards, this course examines geological time scales, radiometric dating, and how scientists “read the rocks.” We will explore dramatic changes in the Earth over the last 4 billion years, including how the evolution of life on Earth has affected its atmosphere. In addition to looking at geology on a global scale, participants will take to their own backyards to explore and share their local geologic history. Course participants will bring their understanding of the dynamic Earth - along with content resources, discussion questions, and assignments - into their own teaching.

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Evolution of the Atmosphere

You will learn how the evolution of photosynthetic life changed the concentration of oxygen in the oceans and atmosphere, and how this is reflected in the rock record. You will also become familiar with how the Next Generation Science Standards connect to this week’s content. Finally, you will complete a written assignment: an analysis of a local geologic feature.

Evolution of Early Earth's Atmosphere11:06

Banded Iron Formation3:19

Stromatolites2:42

Conheça os instrutores

Edmond Mathez, Ph.D.
Curator and Professor
Department of Earth and Planetary Sciences
Ro Kinzler, Ph.D.
Senior Director
Science Education

We are standing in the Hall of Planet

Earth and I'm here before a banded iron formation.

And the reason this sample is on display here

is that it is the primary evidence that the early

atmosphere contained no oxygen, and so that's what I

would like to explain to you, the logic behind that.

But first let me describe to you what this is.

obviously banding refers to the fact that this banded iron formations

are layered and the layers consist of this red stuff.

which is basically jasper, a form of

silica, and the individual, very small grains

in jasper are coated with iron oxide, and that's what gives them the red color.

And these dark layers are made of the iron oxide mineral magnetite.

the formula of which FE304. The question is

why is it that this sample provides

evidence for no oxygen in the early atmosphere?

It turns out that first of all, these samples only

exist in the geologic record before about 1.8 billion years ago, or so ago.

We don't see them in the relatively, young record.

And there's a good reason for that.

Present day seawater contains basically no iron.

The concentrations

are so low, they're almost impossible to measure.

and the reason it contains no iron is because

as soon as one puts a little bit of iron

in ocean water, it immediately reacts with the oxygen

in the ocean water to precipitate as, iron oxide minerals.

So that's probably what happened in the early earth.

This graphic illustrates the process by

which we think banded iron formations formed.

In the early earth there was hot spring

deposits driven by volcanic activity, causing circulation

of sea water through the ocean crust.

And as that sea water moved through the ocean

crust it dissolved some of the minerals in the

crust and became enriched in metals, especially iron, and

so introduced large amounts of iron into the ocean.

So that ocean at that time contained no oxygen.

So because it contained no oxygen, it was able

to dissolve significant quantities of iron.

Eventually, stromatolites appeared on the scene.

Organisms that engaged in photosynthesis and

thus introduced oxygen into the system.

The earliest known stromatolites appear in the

geologic record about 3.4 billion years ago.

So the stromatolites were producing oxygen, but that

oxygen was immediately reacting with the iron in

ocean water.

To precipitate the iron oxide minerals that form the band and iron formations.

The ocean basically ran out of iron

and other things that were removing oxygen from

the system, that the oxygen content of the

ocean and the atmosphere were able to rise.

And we know pretty much when that was, because we have means

of dating that time and it started, that rise started about 2.45 billion

years ago, and it ended about, 2 billion years ago.

In that interval, the oxygen content of the atmosphere increased from essentially

nothing to perhaps a percent or so, and it didn't reach its

present of about 20% until the end of the Proterozoic, era.

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