Alkalinity in the ocean?

Wow, I question that I know the answer to.

It replaces it in 2 ways.

1)  HCO3 + H+ <---> H2CO3 <--> H20 + CO2

In words this means that carbon dioxide in the atmosphere slowly reacts with ocean water to produce alkalinity.  It must be a slow process and probably needs a lot of water surface area or it would be more effective at maintinaing alkalinity in our aquariums.

2)  At the very bottom of the ocean, below about -20000 feet is an area called the CCD.  I think it stands for carbonate compensation depth... can't remember for sure.  All the carbonate materials (calicte, aragonite, dolomite, etc) that have fallen down there slowly dissolve because of the intense pressure and cold water.  Carbonate minerals tend to precipitate (crystalize) when the pressure on them is reduced, like on the edge of your bathtub faucet as the pressurized water escapes, and they redissolve under extreme pressure, like at the bottom of the ocean.  The Ca and CO3 are then brought back to the surface of the ocean by slow moving deep ocean currents that rise upward near land masses.  This is also called upwelling.  Upwelling also brings nutrients, especially phosphate, to the surface for phytoplankton to use.

Steve

Edited by faviasteve

Hi Steve,

1)  HCO3 + H+ <---> H2CO3 <--> H20 + CO2

In words this means that carbon dioxide in the atmosphere slowly reacts with ocean water to produce alkalinity.  It must be a slow process and probably needs a lot of water surface area or it would be more effective at maintinaing alkalinity in our aquariums.

Adding CO2 to seawater does not change it's alklanity at all!  All it does is shift more of the carbonate to bicarbonate.  RHF explains it better then I can (I'm too lazy to type my answer ).  From http://www.advancedaquarist.com/issues/feb2002/chemistry.htm

Alkalinity Facts

There are several facts about total alkalinity that follow directly from the definition. Unfortunately, some of these have been misunderstood by some hobby authors.

One of these facts is termed The Principle of Conservation of Alkalinity by Pankow ("Aquatic Chemistry Concepts", 1991). He shows mathematically that the total alkalinity of a sample CANNOT be changed by adding or subtracting CO₂. Unfortunately, there is an article available on line, which claims otherwise, and encourages people to "lower alkalinity" by adding CO₂ in the form of seltzer water. This is simply incorrect.

Forgetting the math for the moment, it is easy to see how this must be the case. If carbonic acid is added to any aqueous sample with a measurable alkalinity, what can happen?

Well, the carbonic acid can release protons by reversing equations 1 and 2:

(5) H₂CO₃ ==> H⁺ + HCO₃^-

(6) HCO₃^- ==> H⁺ + CO₃^--

These protons can go on to reduce alkalinity by combining with something that is in the sample that provides alkalinity (carbonate, bicarbonate, borate, phosphate, etc). However, for every proton that leaves the carbonic acid and reduces alkalinity, a new bicarbonate or carbonate ion is formed that adds to alkalinity, and the net change in total alkalinity is exactly zero. The pH will change, and the speciation of the things contributing to alkalinity will change, but not the total alkalinity.

 

I like your answer #2 though.  BTW, the average lysocline depth in the pacific is 2,500 ft for calcite and 1,600 ft for aragonite.  In the atlantic it's 14,100 ft for calcite and 4,600 ft for aragonite (from Chemical Oceanography, Millero).

Okay, so please let me know if I understood it right. 

The two ways we figure that Alk is supplied to reefs are:

1. Alkalinity is made available to reefs through upwelling water that has become saturated with carbonate/bicarbonate, but in a round-about way. "This deeper water circulates from the equator to the poles and rises to the surface and eventually flows back to the equator and repeats this cycle.  It's this cycle that supplies the vast majority of alkalinity for ocean surface water."

2. "The rest enters the ocean in river water." I heard the other day that the Amazon dumps about a billion gallons of water a minute into the ocean. (Would this perhaps be one of the benefits of Kold-Steril purified water?)

My next question is:

"What is the percentage contribution of each?"

I totally forgot about river water.  River water is getting its carbonate alkalinity from carbonate rocks (mostly limestone) dissolving on land becasue of low PH rainwater.  The Ca and CO3 for most limestone (according to what I learned in geology class) originated in the ocean as coral skeletons or calcareous mud.  Most of the carbonate materials that aren't redissolved into the ocean eventually get pressed into a rock.  Becasue this has gone on for billions of years most of the Earth's Ca and CO3 (which includes most of the Earth's carbon) is in carbonate rocks and not dissolved in the ocean.

Geologists think that most of the Earth's early carbon was carbon dioxide from outgassing (eruptions) of volcanoes.  Somehow most of that atmospheric carbon dioxide got converted into carbonate rocks.  If carbonate rocks originate from oceanic calcite and aragonite and oceanic critters got thier carbonate from the water, how did all the atmospheric carbon get converted to bicaronate if my #1 reason isn't correct?  I'm not trying to argue.  That's just the way I've always understood it.

So what is the best way to lower dKH?
Mine is 15dKH and I haven't found a good way to get it down.

Any other way's ?
I just changed out ~20gal of rio water and instant ocean a couple of days ago.
dKH read correct on change water.
The tank is 55 gal but I have a ~120 gal refug/sump.
The other problem is PH and CA read low all of the time.

PH=7.8 day/night (rev light on refug)
CA=300

I am out of ideas.

Initially it had been running stable at ~9.5dKH.
I had run out of the test kit for a month and when I tested about a month later, it was high.

I have also had a problem keeping my CA up, it has now dropped down to 240.
It is like all of my CA has turned into ALK. (And not the hicc kind .)

To solve it I had tried turning off my CA reactor and did a large RIO change.
This has dropped it down to 13.5dKH.
However, my CA is still going down.

I was told by someone to start adding TurboCA or Kalk and that would help.

Now they are:

    CA=240
    PH=8.0
    dKH=13.5
    Mg=?? Can't remember exactly but it was just a tad low.

Sounds good I'll give it a whirl.

I don't remember adding a buffer but then again I have been accused of selective amnesia in the past.

Thanks for your help

Adam, Thanks.

Here is the link to the article:

http://www.nature.com/nature/journal/v437/n7059/pdf/nature04 095.pdf

If I understand this correctly, the study is telling us that Alkalinity is significantly more dependent on Atmospheric CO2 than was previously assumed. The effect of this is going to be the continuing loss of coral over the coming decades. And that's about 10 times faster than previously predicted.

(  We could have told them what would happen to those 'pods exposed to low alkalinity.)

Now I'm even more confused. They were talking about carbonate concentration in the water. I thought you told us that this was Alkalinity? How can saturation of aragonite in the water not be something akin to Alk.

I believe what you said. They really don't yet understand the true chemistry of the ocean. But one thing is for sure, the chemistry is changing. In the Cook Islands, the reef near Raratonga is 99% dead. According to locals this has happened just within the last two years.