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

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