Quick Science lesson sought regarding fossils in JB . . .

[aw]

I'm no science buff. So help me out. The sea biscuit fossils (or whatever they are) that you see throughout JB, are they sea biscuits? Are the flat ones the same? What might their scientific names be? They are primarily saltwater creatures right (when they were alive, of course)? So, wouldn't this be evidence of when the earth was covered in seawater?

Sorry for the dumb questions. I was talking to a non-diver about what I have seen and wanted to make sure I had the right info.

[Webmaster]

It would be evidence that the sediments, fossils and all the other assorted junk that got consolidated into that rock was lain down when that particular patch was underwater.

I'm not especially familiar with the geology of FL, but for instance there are patches of limestone on top of some Mtns. in the Rockies. That doesn't mean the water level was at the level of the top of the mountain. It was laid down, lithified into rock and uplifted when the Rockies were pushed up.

Geology is not static, it's just usually under very slow processes. And when it's not insurance companies start to sweat.

[Jerry]

The sea-biscuits are from the phylum Echinodermata (echinoids); this group includes: crinoids, brittle stars, starfish, and sea urchins. What you are seeing are several varieties of sea urchin. The flat and the more rounded are both echinoids but there my knowledge breaks down---dead bugs were not my specialty.

Jerry

[Limestone Cowboy]

Sand dollars too! Here's a good reference:

http://www.flmnh.ufl.edu/invertpaleo...lery=echinoids

FGS used to have a poster on them too, but I couldn't find a link to it...

Echinoids are a pretty good marker species for the different limestones in Florida, along with other things like forams. I'm not as familar with the Marianna Limestone, but Rhyncholampas gouldii is a very common species in the Suwannee Limestone further south. The little needle-like pieces of rock on the floor of JB are the spines from the echinoids.

One interesting thing about Echinoids is they have a slightly higher Magnesium content in their shells (called tests) so they preserve a little better than other things like clams or snails.

Drew

PS Hi Jerry, still a little embarrassed over our argument long ago in Mike T's apartment on Suwannee vs Ocala limestone. You were right of course...

[Kelly Jessop]

One interesting thing about Echinoids is they have a slightly higher Magnesium content in their shells (called tests) so they preserve a little better than other things like clams or snails.

...

That is interesting. I've always been told the reason Jackson Blue has such a blue hue evidenced by the first glimmer of light is blue,is due to the magnesium content. Correct?

[DeWayne]

One interesting thing about Echinoids is they have a slightly higher Magnesium content in their shells (called tests) so they preserve a little better than other things like clams or snails.

...

That is interesting. I've always been told the reason Jackson Blue has such a blue hue evidenced by the first glimmer of light is blue,is due to the magnesium content. Correct?

That is what I have always heard as well. If you can deco as the sun goes down it is the most beautiful color of blue you might ever see. I wonder if the mag from the abundant fossils has permeated the rock.

[Gibby]

That is what I have always heard as well. If you can deco as the sun goes down it is the most beautiful color of blue you might ever see. I wonder if the mag from the abundant fossils has permeated the rock.

Remembering back to geology class.
A fossil is a replacement of the biological material with minerals that are abundant in the current environment. This does take a very long time to happen so soft tissues are able to decay to nothing before the replacement can happen. Hard tissues like bone or shell decay so slowly that it gives the minerals time to move in to the spaces left behind by the decaying tissue. This is why we find dinosaur bones but no clue on the soft tissue structure.

As for the biscuits having more magnesium in them I believe this would only happen if the environment they were sealed in had abundant amounts of magnesium as well. Also the biscuits having a more porous "shell" structure would lend itself to easier mineral replacement than a denser snail shell or clam shell. So to guess at DeWayne's question, the magnesium in the rocks should have permeated the fossil. Not the other way around. The fossils should show a higher concentration of magnesium because the magnesium has seeped from the rock to replace shell. In the rock the magnesium should be fairly evenly dispersed.

And with the blue water is primarily caused my magnesium but other minerals contribute to this as well. It’s just the magnesium that has the highest "bluing" effect. Something like this can be seen at blue grotto and devils den. Although the two systems are very close and most likely connected in some minor way, the magnesium content at the grotto is much higher than the den.

All of this of course is an educated guess from what I remember from class many years ago. I could be way off.


And my favorite deco was under a full moon.

[Ken Roop]

It would help to know which strata the fossil came from and it would still
need to be examined. There are several genus of sea biscuts with several
species within each ( if my memory from many geology classes serves me correctly).

The light reflection and absorbtion question is Physics. The blues (colors that the H2O appears) is the last color to be absorbed due to its higher energy. Reds and yellows are first colors to be absorbed and are not reflected as the blue and violets are.

From Chemistry there are several elements that will turn the water blue,
but not the way deep clear water of a spring appears blue.

Hope this helps.

[Ken Roop]

The dolomite process occurs when salt water inundates limestone and a replacement reaction begins on the CaCO3. Magnesium is the third most abundant element in seawater. It begins to replace some of the Calcium
in the limestone. When the next iceage occurs and sea levels go down the process stops.

But this process occurs many times in geologic history.

Hope this helps.

[aw]

I knew you guys had answers. Now I can just e-mail the link to this thread to my non-diving friend and he can read the explanations himself. Saves me a lot of confusion about phylums on the forum. Thanks.