Ice Cores: Part 3 Dust and Ashes

Continued from Part 1 and Part 2

Layered Snow Melting

I was going to move right on to the dusty stuff in the ice, but ran into some very interesting University study pages I have to point out.  Columbia opens its mathematical calculation class with an introduction we’re going to look at:

“These annual layers provide a record of the earth’s climate that reaches back as much as 200,000 years.”

What?  I thought they went back 1.4 million years!  That is one giant discrepancy.

Columbia recommended their students check out this U. of Michigan (go Wolverines!) page.  The coolest thing is a cross-section drawing showing a lake (as in unfrozen water) “the size of Lake Ontario” underneath the 4km [13,000ft+] elevation Vostok Ice Station in Antarctica.  That is awesome.  Anyone for a swim?  Oh, yes, the U. of Mich drawing states the ice goes back 420,000 years.  All right, what’s going on?

Have a look at what they say about the squashed bottom ice:

“The… age of the ice… is obtained by counting layers of ice and, when layers are no longer clearly visible, modeling the flow of merged ice layers.”

Wait a minute.  In really squashed ice, “layers are no longer clearly visible?”  I don’t remember the Natural History Museum tell us that.  What is this “modeling,” and is this going to end up being as full of assumptions as radiometric dating is?

I searched nearly an hour for a page that would explain how you figure out a model to calculate “the flow of merged ice layers.”  If any of you know what they’re talking about please leave a comment!

Snow layers

The other, better known way scientists study really deep, squashed ice from an ice field is by looking at the layers of “dust.”  Today we’re going to find out what this dust is and where it came from before settling onto Antarctica or Greenland.

The best page I found on this is a technical paper abstract.  Those are the high-falooting, jargon filled short versions of what the scientists were studying so other scientists can keep up with what they need to know in a few minutes.

Major features in Antarctica: (1)South Pole, (...

(9) Ice core drill sites (EPICA), (16) Lake Vostok (Photo credit: Wikipedia)

They were studying an ice core from Antarctica using a core called EPICA Dome C.  First, they say they’ve been using X-rays to study the dusty ice.  Cool.  Then they go on to their ideas about the climate at different depths of the ice.  I’m with them for one Ice Age, but that would be near the bottom, not where they put it.

Turns out, the stuff blown onto the ice has changed a lot from time to time. A lot of the dust at certain levels has the same ingredients as dirt at the bottom of South America.

At other levels there is dust from lots of different sources.  The study authors are pretty sure some of that mixed dust came from Australia.  I also found a similar study from Greenland saying the dust up there is either local or blown in from East Asian deserts.

But this isn’t the whole story.  When a large volcano erupts with a lot of ash, the tiny particles are carried by the wind to the ice sheets.  Even in recent times, volcanic ash has been suspended in the air around the globe for many months by eruptions like Krakatoa’s in 1883. Another helpful thing is that every volcano spews out different things.  I don’t usually use PBS:NOVA because it is so biased, but they were the clearest on what’s going on:

Snow-like ashfall caused by heavy rain mixing ...

Snow-like ashfall caused by heavy rain mixing with ash columns

“Ash from a particular volcano has its own unique characteristics, much like a person’s fingerprints. These characteristics include chemical composition, and the size and shape of crystals and glass shards. They can be used to determine not only the particular volcano that produced the ash, but the particular eruption from that volcano as well.”

So a really major eruption will leave traces in the ice on all the ice sheets.  Scientists use these layers to check if they are counting at the same rate as other researchers.

BTW, this doesn’t always work.  A moving ice sheet (glacier) in Alaska didn’t have ash from an 803 AD volcanic eruption as expected, leaving scientists to try and figure out why.

What they don’t tell you is that the other records they are comparing with don’t
have dates on them either.  Once we move beyond written history, it is
all guesswork to calculate how old any of these layers are, whether they are ice sheets, ocean floor layers, or anything else.

The LORD is slow to anger, and great in power, and will not at all acquit the wicked: the LORD hath his way in the whirlwind and in the storm, and the clouds are the dust of his feet. Nahum 1:3

Ice Cores: Part 2 Annual Layers?

Continued from yesterday’s post

IT University of Copenhagen

IT University of Copenhagen (Photo credit: João Ramos)

OK, it took some searching to find people willing to explain just what is being used to calculate the dates they give to ice cores, but the University of Copenhagen was very helpful in describing what’s going on.

Near the surface, where the layers haven’t been squashed by the weight of the snow/ice too badly, it is easy to see differences in the ice.

Each depositional event (e.g. a snow storm) is clearly seen as a distinct layer. Summer and winter snow can often be distinguished by a hard surface, or even sometimes a melt layer, at the top of each summer layer. This is very handy, as the layers in this way can be used for dating purposes, counting from the top how many summer and winter layers there are above a given depth.

Snow layers

Snow layers (Photo credit: brewbooks)

They admit that a single snow storm forms a “distinct layer,” but don’t worry about that too much because Summer “can often be distinguished by a hard surface” each year.  I wonder if they have kept track of freak Summer storms to see if they look like an annual (yearly) layer after a while?

For deep, squashed ice they have to use other ways to determine age.  This gets a bit thick, so I hope I can make sense of this.  Sometimes it seems like they want to make these things so complicated we just have to believe them!

The U. of Copenhagen page sends you on to Ice core dating using stable isotope data to find out what’s going on.  Are your eyes crossed yet?

Alright, here’s how it works.  We know that there are different isotopes (see yesterday’s post for the definition) present in the ice that forms at different temperatures.  So Summer ice has a different amount of these little guys than Winter ice.  From their graph, it looks like there are more isotopes in Summer than Winter.  Since everyone knows there is a Summer once a year and a Winter once a year it is easy to tell one year from another, right?

Oh, wow, look at this:

Thin section of an ice core from Antarctic sea...

Thin section of an ice core from Antarctic sea ice; microscope view under polarized light

As the ice layers get older, the isotopes slowly move around and gradually weaken the annual signal. This process is called diffusion and sets the limit for far back in time annual layers can be identified using δ18O data.

(The delta sign thingy is just the type of isotope they are looking at)  Do you see what they are admitting?  After a certain depth, these atoms get all mixed together and you can’t use them to determine heat/cooling cycles any more.

That’s the end of the page, so now what?  Let’s follow the link Dating using impurity measurements to see how else they figure the age of the ice.

When dating an ice core by counting annual layers, one can use data of any kind that has an annual cycle.

I knew it, I knew it, they are assuming each warming/cooling trend must equal one whole year!

The dust content and the concentration of many chemical impurities in the ice also show seasonal variations and can therefore be used for annual layer counting. The advantage is that the impurities are unaffected by diffusion and can be used to identify annual layers in ice of any age, and that high-resolution measurements of ice impurities produce several parallel data series that can be used for dating, thereby making the annual layer identification process more robust.

They use the dusty layers to determine the age of the ice.  According to this website, there is a distinct layer laid down once a year, no more, no less.

Where does this dust come from?  Well, you’ll have to wait ’til tomorrow!

Then Noah built an altar to honor the Lord. Noah took some of all the clean birds and some of all the clean animals and burned them on the altar as a gift to God.

The Lord smelled these sacrifices, and it pleased him. The Lord said to himself, “I will never again curse the earth as a way to punish people. People are evil from the time they are young, but I will never again destroy every living thing on the earth as I did this time. 

As long as the earth continues, there will always be a time for planting and a time for harvest. There will always be cold and hot, summer and winter, day and night on earth.”  Genesis 8:20-22  Easy-to-read Version