Banded Iron Formation

Banded Iron formations occur in Proterozoic rocks, ranging in age from 1.8 to 2.5 billion years old. They are composed of alternating layers of iron-rich material (commonly magnetite) and silica (chert). Each layer is relatively thin, varying in thickness from a millimeter or so up to several centimeters. Here is one theory as to how they might have formed:

It is theorized that the Earth's primitive atmosphere had little or no free oxygen. In addition, Proterozoic rocks exposed at the surface had a high level of iron, which was released at the surface upon weathering. Since there wasn't any oxygen to combine with it at the surface (like happens now in our oxygen-rich atmosphere), the iron entered the ocean as iron ions. At the same time, primitive photosynthetic blue/green algae was beginning to proliferate in the near surface waters. As the algae would produce O2 as a waste product of photosynthesis, the free oxygen would combine with the iron ions to form magnetite (Fe3O4), an iron oxide. This cleansed the algae's environment. As the biomass expanded beyond the capacity for the available iron to neutralize the waste O2 the oxygen content of the sea water rose to toxic levels. This eventually resulted in large-scale extinction of the algae population, and led to the accumulation of an iron poor layer of silica on the sea floor. As time passed and algae populations re-established themselves, a new iron-rich layer began to accumulate. Unfortunately, the algae were of relatively low intelligence and were unable to learn from their past excesses (this was also before the EPA), so they would again proliferate beyond the capacity of the iron ions to clean up their waste products, and the cycle would repeat. This went on for approximately 800,000,000 years!

 

 

Following are note from a somewhat more extensive GeoFantasy I have concerning the reality of their formation

Oh, the waste of it all...

Supposed to be on Banded Iron Formations

We'll get there eventually

Start with a couple questions: tough one's so give this some thought

What would happen if I put a plastic bag over my head for 10 minutes

What do the Giant Panda and Northern Spotted Owl have in common

What we have here is a classic story with something for everyone

Wanton excess

Total disregard for public health and safety

Environmental disrespect in its most blatant form

And, best of all, lots and lots of irresponsible mating

This broad of a topic obviously requires some serious arm waving

I love geology: one of the things I love the best is

No real laws or rules

We borrow from the real sciences and use as we see fit

But none that are really ours

Actually there are several laws, one of which is helpful in this case

Strickler's 1st Law of GeoFantasy

"All good regional theories break down at the local level"

This covers a lot of territory

No time to explore all possible ramifications, but

Part of what this means is that we are free to...

Look at the "big picture" and

Wave our arms as much as we want

"GeoFantasize" at will w/o fear of serious contradiction

Only requirement is that we recognize the limits of our interpretations

And accept them for the "progress reports" that they are

One final thing:

I assume we all have sufficient background for this discussion

EXAMPLE: If I say "subduction" most of you will nod your heads wisely and look smug, as opposed to a vacant stare

So with all this in mind let's engage in some serious "geofantasy"

The earth's been here for 4.5 BY (give or take a week)

What's it been doing for this time?

In simplest terms Differentiation: define

Not a perfect process

Yes, there are high density materials in the crust

And probably some lower density stuff at depth

But over the course of geologic time it will sort itself out

This is a "law" of physics: not much we can do to restrict or control the process

In any event, surely not completed yet

Definitely an on-going process

One result of differentiation is the iron/nickel core

Another result: a low density crust

Composed of 2 fundamentally different rock types

Basalt: relatively heavy and covers 70% of surface

Granite: lighter than basalt, and covers 30%

If we observe the way the 2 interact, several reasonable interpretations can be made

Subduction where they meet

Feeds on itself

Granite formation is associated with subduction zones

As granite is formed it further stabilizes the low density raft

Which we call a craton

Causes more subduction

Which leads to the formation of more granite

Granite represents the final "purified" product of the differentiation process

The scum of the earth, if you will

At this time, the earth has purified enough to cover 30% of the surface

Possible to imagine a time with less granite, more basalt

In general, a more mafic crust

Also seems certain that the hydrosphere and atmosphere have evolved thru time

And life: from simple to more complex organisms

Put all together

Easy to imagine a time when land, sea, air, and life was a little bit to a lotta bit different from what it is today

Evidence is in the rocks at current erosional level

Very incomplete record

What we don't know almost surely exceeds what we do know

By several orders of magnitude

Also open to varying interpretations

Remember: no real laws or rules, just some common sense guidelines

Therefore, we are free to geofantasize about the past all we want

Remembering always that it's only one dog's opinion

Anyway, we have here several rocks

Called Banded Iron Formation (no surprise - its what the talk's about)

Represents the world's greatest reserve of iron ore

Geologists are trained to observe & record - so let's be geologists for a bit

Describe: get audience input

Alternating layers of silica (chert) and iron

Commonly magnetite or some other oxide

Layers vary in thickness; up to a centimeter or so

Silica throughout

Probably looking at times of excess iron, not excess silica

Certainly appear to represent marine deposition of some sort

Banded Iron formation comes in thick, widely scattered sequences

100's of meters thick in places

Repetitive layers: chert / magnetite / chert / magnetite (or another oxide of iron)

Most are very old: 1.8 to 2.6 billion years

Although some limited exposures in late pre-Cambrian

How could these have formed?

GeoFantasy is always the fun part of geology

Usually aided by a liberal dose of liquid refreshment

So, let's set the "Wayback Machine" for 2.61 billion years ago

Physical setting at this time:

Crust: more mafic

Not as much granitic scum

Atmosphere: Quite a bit different

No O2 but lots of CO2

Hydrosphere: Water's water, so probably not much overall difference

But what's in the water is important, too

Similar ions, but different concentrations

Probably more silica

No silica-secreting planktonic organisms

Probably more iron

More mafic crust, and

No free O2 in atmosphere to combine with iron exposed at surface

So what was living at this time?

Not much: life still a relatively new thing

Certainly less complex and diversified than today

Dominant lifeform was the humble Blue/Green Algae

Blue/Green bacteria, or Cyanobacteria to be politically correct

Kings (and Queens?) of the world

Single cells, living in colonies

Although they were prokaryotic photosynthesizers, they really weren't all that different from us

Same basic requirements:

Obtain energy

Eliminate waste products

Cruise around looking for a way to reproduce

We're talking sex here

Cleverly disguised as "the basic drive to propagate the species"

And like us, it was the second two that gave the algae the biggest problems

So here we are: 2.61 BY ago

Algae's in the water

Population has been expanding for quite some time now

Some of the algae colonies are getting a bit crowded

But no problem the sea's got a whole bunch of surface area

And it's a good life - the ultimate utopia

No real predators, or other pressure on the population

All they have to do all day is:

Obtain energy (Eat)

Deal with their waste products

Cruise around with their honey making baby algaes

Obtain energy: that's easy

CO2 and sunlight did the trick

Didn't even have to worry about running out of charcoal for the Hibachi

Mating even easier

No senses, so easy to choose a sweetheart

And when it comes right down to it

Don't really even need a mate

Able to do it all by themselves

Expelling waste the easiest of all: after all, it was just O2

No sweat - just dump it into the ocean

Magically just goes away

Like I said the ultimate utopia

Anyway, let's get back to the procreation part

We're talking lots of B/G Algae here

And all of them over the age of 3 hours procreating like crazy

"In order to propagate the species"

They really weren't oversexed

It's just that they were real small and the ocean real big

Room for a whole herd of them

And just like the bunnies, the population increased

Local colonies merged into algae cities

Which developed into major algae metropolitan areas

And the population continued to increase

And over time there was noticed a developing social consciousness and public awareness

And the population continued to increase

Rumbles of discontent begin to be heard throughout the land

(Or the sea, as the case may be)

Whispers begin to be heard

"The population is expanding too fast"

"How long can we continue to expand the population without damage to the environment"

Some radicals even suggest that some method of population control should be considered

Birth control, sexual monogamy, even selective sterilization in cases of wanton promiscuity

Many began fleeing the urban areas, searching for a better life

Anyway, none of this did much good

They kept talking but

The increase in population was out of control

Doubling every 3 hours or so

And at that rate, and over the course of geologic time...

The population continued to increase

After a while some began to notice a foul odor in the air (or the sea, as the case may be)

A smell like raw oxygen waste products

Which weren't going away like they were supposed to

At first it was no a big deal, only an irritant

But the population continued to increase

And the reek got worse, growing into a foul stench which could not be ignored

Citizen's groups formed

Brought together by fears of environmental pollution by waste O2

Concerned for their families, loved ones, and the basic algae way of life

Oxygen disposal became the "cause of the decade"

The sides polarized along expected lines

Heated, emotional dialog was exchanged

Algae environmental activists pleaded for responsible disposal of waste O2

Algae industrialists claim overreaction by the militant left

Stating for public record: "All data point to the obvious conclusion that the sea is infinite in it's capacity to absorb waste O2. The continued hysterical response by a few well-meaning citizens can only serve to undermine public morale, and spread fear and dissension throughout the population."

Militant environmental groups began to spring up throughout the land

(Or sea as the case may be)

Demanding an end to degradation of the environment

March (float?) around with signs, chanting "What are we gonna do with the free O2! What are we gonna do with the free O2!"

Get audience chanting

You guys are great: give you a sign, teach you the backstroke, and toss you into the water

All this hubbub forces the government to act

Orders a full investigation of the O2 problem

A "scientific study," with recommendations for mitigation

Extensive research is undertaken

Focus was on finding out what happened to the O2 when the process was working

It was an exhaustive program

In the time it took, the population doubled 73,537 more times

What they found was startling

Free O2 in the water was definitely toxic if in high enough concentrations

The free O2 in the water was combining with iron to form magnetite

Which was heavy and sunk to the bottom

Effectively cleansing the environment of waste O2

Several brave divers actually made it to the seafloor, where they discovered nearly a centimeter of the stuff piled up

So what went wrong? They theorized that:

There must be rocks exposed somewhere at the surface which were full of iron

There couldn't be any oxygen in the atmosphere or it would combine with the iron before the iron ever got a chance to wash into the sea

But the erosion and transport of iron to the sea worked at a finite rate

While the concentration of O2 in the water was rapidly increasing

Conclusion: the algae population was producing far too much waste O2 for the amount of iron available in the water to neutralize it

A serious health hazard was imminent, and...

The population continued to expand

Well, shit, they said (no pun intended)

What to do

Very few options left

The time to fix it was long past

The more realistic begin anticipating mass poisoning of the population

Emotions run high

New groups cruising around with new signs stating:

"The End is Near," and "Prepare to Meet thy Doom," and so on

Responsible citizens vow to learn from past excesses "if only some of us can survive"

But unlike us, no EPA to write it into law for all future generations

And racial memory was poorly developed

In any event, as I said before, it was way too late for a fix

Huge numbers of the population begin to die from O2 poisoning

Entire population centers (urban areas if you will) are destroyed

Only the relatively sparsely populated fringes have any survivors

But some do survive

Life isn't easy for the few who survive

Total disruption of all they've ever known

But hey, wait, they're only algae

With simple need, wants, and desires

They suck it up and forge on

Pretty soon, the livin's easy

And it's a good life

No real predators, or other pressure on the population

All they have to do all day is:

Eat, expel waste products, and cruise around with their honey making baby algaes

Seems like we may have the beginning of a pattern here

Sure enough, we have evidence for repeated cycles

Iron rich layers - expansion of the biomass

Iron-poor layers - mass extinction

How many times did large portions of the earth's biomass die off

Only to re-build to the point where pollution again caused mass extinction

Let's do a rough estimate

300 meter sequence = 30,000 cm

2 cm/cycle (1cm each for Fe+ and Fe-)

=15,000 cycles

Geologic record indicates that this was going on for 800,000,000 years

800,000,000 years / 15,000 cycles = 53,000 years/cycle

Anything here we can relate to our own existence?

What sort of waste products are we spreading around?

The Blue/Green Algae kept it simple

All they threw away was oxygen

It was even usable to sustain other forms of life

Once they finally evolved

And why did they evolve?

Availability of oxygen for aerobic respiration has to play some part

How about "higher" forms of life

"Does the bear shit in the woods?"

You bet! But the waste decomposes

And like the algae's O2, is in the service of life

How about us?

Some of our waste products go beyond natural

And biodegradable

Think of the stuff we've made!

And who's gonna clean it all up?

Do you really believe

That 2 weeks before we go extinct a bell will sound

We'll put it all back and tidy up a bit

Or does it even matter?

53,000 years of algal supremacy translates into 1 cm of earth's history

How much of a layer will we leave

How distinctive will be our marker horizon?