Ammonites: Finding and Destroying Fossils for Science

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Some of you may have seen my ammonite donation. If not and you are curious it is linked here:

It is an important player in this trip. Anyway I thought I might describe my first trip to this locality and why you might realize it is a little more rare than the normal finds there. Some of this is part of a poster for the now cancelled Rocky Mountain Section of the Geological Society of America so if I ever do get to put my poster online I will post a link here so you guys can look at it too. For now here is a trip report.

 

 

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Part 1: The Field

First our stratigraphic column from http://utahgeology.com/utah-stratigraphic-columns/?var=strat_66

 

We will be working in the Mancos Shale at about 90 million years old. This was a couple million years after the Ocean Anoxic Event 2 or OAE2 when wide spread parts of the ocean’s waters experienced anoxic (lacks oxygen) and euxinic (lacks oxygen but full of hydrogen sulfide or the rotten-egg smell chemical some of you may be familiar with). This event killed off most of the ichthyosaurs and almost all of the pliosaurs we know of. One of our many questions is how the ammonites faired during this time.

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Picture of the field site

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This location has a couple modes of preservation some ammonites are in concretions…

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…some are crushed in the shale layers...

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The ones we are specifically after are the crushed ones in the shale for our project. They are rarely preserved as whole partially because they didn’t have much sediment infill so they would get crushed in early diagenesis. Since they don’t have infill they are quite thin and usually end up weathering out of the rock like this in broken pieces.

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Besides the ammonites there were lots of other creatures in very select horizons; in this case, some small Inoceramids.

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Every now and then you find a large portion of the ammonite intact. Here I am with my unkept field beard and the largest ammonite find of the day, one of the few selected for our study.

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I’ll admit they aren’t the prettiest looking fossils but that’ll probably make you feel less bad about what we had to do to them.

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But first let's look at some eye candy that will stay unharmed in my collection from an undisclosed Jurassic location nearby.

 

Before

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After

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Can you see the spiral snail shell here?

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Every now and then you even get a chunk of petrified wood in the concretions here. 

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Up close and personal

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Part 2: The Lab

Before I show you the pictures of the torture rack, here is the reasoning why their sacrifice is important. Inside the shells of the ammonites is the chemical calcium carbonate (CaCO3) in the form of the biominerals calcite and aragonite. The elements had been pulled out of the water either from the food they were eating or from them taking it out of the water themselves. Carbon and Oxygen in particular are controlled by how much is in the atmosphere. The ocean surface and the atmosphere are usually at equilibrium or trying to reach equilibrium. If carbon levels in the atmosphere go up, it goes up in the oceans generally, given the temperature remains the same. But if the temperature changes, it can change how the different isotopes of oxygen behave. We can use this property to figure out the temperature of the water they lived in if the shells preserve those original conditions. Carbon, on the other hand, is controlled by temperature to a lesser effect but the bigger influences at the time were the volcanic eruptions that spewed out a lot of carbon dioxide and shifted the carbon isotope composition in the atmosphere. We can use this fact to pinpoint when eruptions occurred and when the ammonites lived. But for us to observe these elemental ratios we have to extract some of them.

Okay now onto the destruction.

LOOK AWAY IF YOU ARE SQUEAMISH TO FOSSIL GORE, YOU HAVE BEEN WARNED!

 

 

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LAST CHANCE TO TURN BACK

We used a Micromill which is basically a fancy drill press essentially to take powdered samples of ammonite shell material. The scratches are from the drill press. If you would like to see values, you’ll just have to wait for my poster.

Before

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After

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Even more after

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Now we will return to some tranquil scenery

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And now to end my trip here is a herd of wild Pronghorn Antelope.

Thank you for reading!

[Al Tahan]

Very cool! Love the science. 

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15 minutes ago, Al Tahan said:

Very cool! Love the science. 

Thanks, I quite like it too!