Reef Chemistry Question of the Day #69 Lysocline

Randy Holmes-Farley

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Some types of plankton deposit calcium carbonate internally. Coccolithophores, for example, deposit internal skeletal like structures:

Emiliania huxleyi - Wikipedia, the free encyclopedia

220px-Emiliania_huxleyi_coccolithophore_%28PLoS%29.png


When these organisms die, their bodies may drift down toward the bottom in the deep ocean.

When they reach a certain depth, the calcium carbonate begins to dissolve. This depth is called the lysocline.

Which of the following is not a reason that calcium carbonate becomes more soluble in the deep ocean than at the surface?

A. The magnesium concentration is higher
B. The pressure is higher
C. The temperature is lower
D. The pH is lower

Good luck!
















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treemanone2003

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The higher pressure allows The breakdown to happen better, so B is not the answer. I'm gonna have to say temp is inconclusive because that drops as depth increases and I've already determined higher pressure helps the breakdown process. My gut tells me that pH will also have some effect in the process and the most reasonable answer is "A".
 

Cory

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With temperature the hotter it gets the more the atoms move, more speed more dissolution. Pressure with enough of it would increase temperature and this dissolution. Lower ph would speed it up. So I'm guessing magnesium doesn't effect the rate of dissolution. I agree A. But I've been wrong before!
 

leptang

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D because deep ocean doesn't have light to bring up the pH and oxygen maybe lower and makes the water more acidic. Im just guessing
 
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Randy Holmes-Farley

Randy Holmes-Farley

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D because deep ocean doesn't have light to bring up the pH and oxygen maybe lower and makes the water more acidic. Im just guessing

Wouldn't that make it a reason for better dissolution, rather than "not a reason" that the question asked?
 
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Randy Holmes-Farley

Randy Holmes-Farley

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And the answer is...A. The magnesium concentration is higher.

Magnesium, like all the major ions, does not change with location in the ocean or with depth, except as salinity changes locally. That property is called being "conservative". So it is not any higher in the deep ocean.

The main factor in driving the increased solubility of calcium carbonate with depth is the pressure. It turns out that dissolved calcium and carbonate ions (and the tightly bound water molecules that are attached to them) take up less volume than the equivalent amount of solid calcium carbonate and water molecules. So the tremendous pressure pushes the equilibrium between solid and dissolved calcium carbonate toward more dissolution.

Temperature generally decreases with depth in the ocean, and calcium carbonate is more soluble at lower temperatures in seawater.

The pH is also typically lower as you move away from the surface. Photosynthesis uses up carbon dioxide and raises pH near the surface, but in deeper water, that happens less or not at all, reducing pH. As you get to very deep water, the pH begins to rise again due to the dissolution of calcium carbonate that is the topic of this question, but it does not get as high as the surface water. pH hits a minimum around 1000 meters deep.




 

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