Reef Chemistry Question of the Day 294: The CCD

Randy Holmes-Farley · Jul 25, 2023

Reef Chemistry Question of the Day 294

The CCD is a chemically-related aspect of the ocean that has strong implications for organisms that form calcium carbonate skeletons or shells.

What is the CCD?

1. Calcium Coupling Density
2. Calcite Concretion Demineralization
3. Carbonate Compensation Depth
4. Calcium Constricted Dissolution

Good luck!

Previous Reef Chemistry Question of the Day:

Reef Chemistry Question of the Day 293: Let's Count Ions!

Reef Chemistry Question of the Day 293 The number of ions in a reef tank is truly mind boggling. In 100 gallons of seawater, there are about 255,000,000,000,000,000,000,000,000 individual ions. Let's count them one by one! Someone had to do it, right? How else would we know? OK, we'll...

www.reef2reef.com

BigMonkeyBrain · Jul 25, 2023

Deep reef tank !

Something else I did not know.

Probably happens !

Randy Holmes-Farley · Jul 26, 2023

Anyone else what to try before I give the answer?

Troylee · Jul 26, 2023

Randy Holmes-Farley said:
Anyone else what to try before I give the answer?

Nah! That’s why we got you!

Reefer Matt · Jul 26, 2023

3. Carbonate Compensation Depth

taricha · Jul 26, 2023

Carbonate compensation depth sounds like a real thing I read somewhere about pressure making carbonates more soluble...

X-37B · Jul 26, 2023

taricha said:
Carbonate compensation depth sounds like a real thing I read somewhere about pressure making carbonates more soluble...

Yep ccd

paragrouper · Jul 26, 2023

Three

Randy Holmes-Farley · Jul 26, 2023

I was spurred to write this question of the day based on a very interesting new study that folks might want to read. I'll summarize it when I give the answer a bit later, but here it is for those who want it now:

‘Dividing line’ for undersea wildlife discovered in the deep Pacific Ocean

The pattern of diversity in the deep sea is more complex than first thought.

www.nhm.ac.uk

Randy Holmes-Farley · Jul 28, 2023

And the answer is:

What is the CCD?

1. Calcium Coupling Density
2. Calcite Concretion Demineralization
3. Carbonate Compensation Depth
4. Calcium Constricted Dissolution

The CCD is an extremely important phenomenon for the ocean, and its existence at all does not follow from simplistic chemistry ideas.

In the equilibrium that exists between calcium carbonate and its dissolved counterparts, calcium and carbonate ions, there's an important change that is not obvious from looking at the chemical equation.

Ca++ + CO3-- <----> CaCO3

Specifically, when all aspects of this reaction are taken into account, such as the way water molecule interact with the ions, the solid CaCO3 takes up more physical space than the two ions.

The unexpected implication of that effect is at at truly huge pressures, the equilibrium is shifted toward Ca++ and Co3-- and away from solid CaCO3. In essence, the pressure causes calcium carbonate to dissolve.
Thus, at some depth in the ocean, calcium carbonate is no longer supersaturated and fairly stable as it is near the surface, but rather dissolves.

This depth can vary a bit by location depending on the local pH, but is typically between 4,000 and 5,000 meters. Vey deep!

One implication of the CCD is that any calcium carbonate minerals that are formed at the surface and fall to great depth will dissolve as they fall, leaving the bottom covered in mud that is called a silaceous ooze (from diatoms, mostly) as opposed to the deposits at shallower depths that is often called a calcerous ooze (from calcifying organisms bodies, shells, etc.)

A second implication of the CCD is even more interesting, and new demonstrated. Above the CCD, organisms depositing calcium carbonate can thrive. But below it they have a very hard time fighting against the dissolution, and at great depth, organisms without calcium carboante prevail. This is what was recently shown in the article I linked above:

‘Dividing line’ for undersea wildlife discovered in the deep Pacific Ocean

The pattern of diversity in the deep sea is more complex than first thought.

www.nhm.ac.uk

from it:
The upper reaches of the abyssal zone, from around 4,000 to 4,800 metres, are described as the ‘shallow abyss’. These ecosystems are initially dominated by soft corals and molluscs, but as depth increases the effects of the boundary layer begin to appear.

Known more formally as the carbonate compensation depth (CCD), this layer marks the point where the combination of pressure and temperature increases how likely calcium carbonate is to dissolve, and so makes it harder for animals to use it to make shells and skeletons.

This means that moving through this gradual boundary, molluscs which depend on their calcium carbonate shells disappear first, and soft corals begin to decline. While these corals may not have hard exoskeletons like their shallow relatives, they tend to have a hard core or other calcium carbonate components to help maintain their structure.

Beyond 4,300 metres, soft coral ecosystems are replaced by those dominated by brittle stars, close relatives of starfish which are less dependent on calcium carbonate.

However, towards the bottom of the CCD at around 4,800 metres even these cannot create enough calcium carbonate, and anemones and sea cucumbers take over to become the dominant groups of the ‘deep abyss’.