4.4 Video The chemistry of the global ocean conveyor belt

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From the course by Universitat de Barcelona

Oceanography: a key to better understand our world

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Universitat de Barcelona

Oceanography: a key to better understand our world

24 ratings

The course for a round the world regatta leads the boats through the Mediterranean Sea and the Atlantic, Southern Indian and South Pacific Oceans. On their voyage, the skippers come across a large variety of oceanographic situations: strong currents, floating ice, wave formations of every kind and biological diversity. In this course the student will learn about the foundations of the science of oceanography. You'll learn about the classification and formation of the ocean floor, how current sea satellite analysis systems work, the chemistry of the oceans and the processes that led to its formation. To follow this course does not require previous knowledge of oceanography.

From the lesson

4. Ocean chemistry: a chemical industry with wall-less pipes

In this section you will learn ocean chemistry what is it? how does it work? What is the vertical axis of the biosphere and the vertical segregació of the elements and how it works? What is carbon carbonate system and what is its relationship with the ocean conveyor belt?. The answers to all these questions will help us understand how the oceans work, the importance for navigation and its relationship to the planet's climate.

4.4 Video The chemistry of the global ocean conveyor belt17:44

Meet the Instructors

Jordi Serra
Dr.
Grupo de Geociencias Marinas

In this final lesson we will look at how the processes we have looked at so far combine

with the global distribution of the chemical elements

we are interested in and dealt with in this module.

We have not named all of those which are important in marine chemistry

but we've looked at those linked to the fundamental part

of the bio-geochemical functioning

and we will look at how these combine with the

distribution of these elements in the world's oceans.

The title of the lesson has been chosen because anybody

who has an interest in the oceans will have heard about

or will have seen this diagram a thousand times in different versions.

In this drawing we simply see that there are certain places

where deep water is formed which circulates towards the bottom of the Pacific

and then returns on the surface towards the place of origin.

So, the idea is that there is a certain amount of continuity of water

and if water sinks in one place it must be replaced by surface water,

but if that water which sinks circulates up to thousands of km far away

then water must come in,

a certain proportion of water must return from these remote locations.

The diagram for this general circulation exist in many different versions

and this would be a version where there has been

an attempt to give details about the circulation

and in blue we have deep water circulation and in red surface circulation,

but we're not going to get into that now.

in fact we are going to go back in time somewhat and

we're going to look at where this idea came from.

The idea comes from a paper by Broecker which he published with Peng in 1972,

in which they conceived a model on which we have the Atlantic,

the Indian and the Pacific,

indicating where there is deep water production.

In the North Atlantic and along the Antarctic

and there are sources of deep water which circulates

preferably along the eastern coast of the American continent

and then must circulate towards the Indian and towards the Pacific underneath.

Avoiding certain details about deep water circulation and its return,

and this is a current which results from certain gyros, etc.

But basically this is the idea they had and then there is a return

which happens at points.

Along this journey what is fundamental for chemistry

is that we have deep transportation of water

and a return on the surface, but along this path anywhere,

in some places more than others,

there is a production of particle which sediment and which

dissolve and are recycled and remineralised at a depth.

it's obvious that if we have a train or a lorry which circulates in this direction

and is loaded up throughout its journey, it will be empty here,

but when it reaches the Pacific it will be full;

so there is an enrichment of deep water,

an enrichment with chemical elements

which form part of the composition of the material which

sediments and which drops down towards the sea bed.

In some places water rises up,

and water reaches the sea floor zone because there is a certain amount of supply,

because otherwise this production at the surface wouldn't be possible,

but in general the surface concentration of nutrients is 0,

so very low, except in certain special places.

However in the majority of the oceans, the

concentration of nutrients at the surface is very low.

What is the result of this?

Firstly that we see that if we trace a profile;

this would be a series of stations in the North Pacific and

this would be a series of stations in the North Atlantic.

We can see, for example, that at a depth, this reaches down to 5.000 m,

we have a concentration of nitrate which on the surface is almost nil

and which increases up to the area just below the central waters,

and then more or less a steady concentration is maintained.

The same outline occurs for the Pacific,

with some small differences, but what is significant is that the

Atlantic and the Pacific have very different concentrations;

20 micro moles per kilo in the Atlantic and almost 40 micro moles in the Pacific.

The concentration of deep water has doubled. This is enrichment.

This occurs also with phosphate.

You'll see that this color,

this color scale corresponds to total carbon, so the total organic carbon.

The red is for high concentrations and the blue low concentrations.

You can see that on the surface the total carbon would be relatively low concentrations,

but as we move down, the concentration increases.

Above all what is important is that it is very high

in the Pacific and relatively low in the Atlantic;

there is an enrichment with inorganic carbon.

Then there is also another important question; this is silicate.

Silicate is one of the nutrients needed by the diatoms and other organisms.

It is more resistant to dissolution;

when a particle drops down, the first to dissolve quickly in a relatively basic medium,

which is sea water, are phosphates, they are easily lost and they turn into water.

So there are very few phosphates and demand is high.

Then there is "fast carbon", which we'd call the

carbon which is easily respired, which is digested.

Then there is nitrogen and then, what takes most effort,

what gets furthest because it is harder for it to dissolve is

silicon, forms of silicon and silicon dioxide, for example.

So the maximum concentrations at a depth of silicon always correspond to greater depths

than we would find for the maximum phosphate,

which means a difference of almost 1000 m. in terms of

the maximum concentration of one nutrient and another.

So, first some nutrients are released and then others

and that's why deep waters in proportion are richer in silicon

and the result is that the concentration of silicates in the Pacific is not double,

but three times greater than the concentration of silicate in the Atlantic.

Whilst for example we saw that nitrogen, nitrate was double.

So, this is a fundamental difference.

Why all of this?

Because the bacteria, microorganisms are doing their jobs, they are recycling,

they are remineralising organic matter and that

organic matter is a source of nutrients for bacteria,

but is also has different processes according to the amount of oxygen there is, etc.

But basically, the most important activity, whilst there is oxygen,

is to burn organic matter with oxygen and as it burns CO2 is produced.

That is why there is an increase in total inorganic carbon in deep waters.

Nitrate is produced and phosphate is produced or

released, so there is a consumption of oxygen,

and what can be perfectly seen is that the

concentration of oxygen is maximum in surface waters

and it is even over-saturated at times

and falls at speed and the minimums for oxygen are given just below the main thermocline.

So, there is an area of central waters and then

the main thermocline with intermediate waters

and that's where the minimums for oxygen are generally found.

What is also seen is that these older Pacific waters have

a much lower concentration of oxygen than the Atlantic.

The reason is that in the North Atlantic,

North Atlantic Deep Water is produced which circulates at a depth loaded with oxygen.

This water of mixes of this water, and there is water

from the Antarctic etc., reach the North Pacific.

The minimums of oxygen in the oceans, and there are

very significant minimums in the North Pacific,

not here, but in this area of the minimums of oxygen.

What is true is that these deep waters are a lot poorer in oxygen;

the oxygen has been exhausted and nutrients have been produced

and so there is an increased amount of nutrients and less oxygen.

There is an important concept in bio-geochemistry and

it's what we call the apparent utilization of oxygen.

This consist of looking at what the concentration of oxygen was in equilibrium

with the atmosphere of the water when it was formed.

You will recall that all of the water which are at

the bottom were at some point formed on the surface

and they have acquired density and have sunk in some way or another

but when they were at the surface they had a certain

temperature, a certain concentration of salts,

a certain density and therefore had the capacity to have a certain amount of oxygen.

That amount of oxygen can be estimated, if it is not known exactly,

and then it is possible to study how the water that

sinks saturated with oxygen utilizes that oxygen

and that oxygen is consumed.

When we reach a certain point and we measure the oxygen,

the difference between the original oxygen and the

oxygen measured is the apparent oxygen utilization.

There has to be a relationship,

a chemical relationship between the composition

of the matter which has been used in respiration

and the apparent production of nitrate phosphate,

for example, of this water and its alkalinity,

logically, because the carbonate is also dissolved.

So the chemical composition of the matter which sediments,

which drops down and dissolves can be estimated

based on studies of calculations of this type.

So really, this gives rise to relationships of this type,

so phosphate and nitrate with data for the whole

world which would fill a diagram of this type.

You can see that the oxygen which gives color to the dots would be relatively high here,

but basically it's a reduction in oxygen as the nitrates and phosphates increase.

if we take the data for the Atlantic,

we see that this relationship between the concentration

of oxygen and the appearance of phosphate.

The nitrate and phosphate are correlated logically.

if we look at examples fro the Pacific we can see the same type of relationship

but reaching much higher nutrient concentrations

and lower concentrations of oxygen.

if we look at the concentrations of oxygen at

3.000 m. depths, the colours here are very light,

with minimum concentrations at 3.000 m. which are not in the minimum oxygen zone,

but at 3.000 m. we have minimums in the Pacific and maximums

in the Atlantic in the water which has just descended,

to put it one way and intermediate values for the Indian Ocean.

In parallel we have the phosphates and the same, less and more phosphates,

less and more nitrates and in general we can draw

a diagram which appears in many publications

and the idea is that we can make a scale of phosphates

and nitrates according to a relationship which we call the Redfield relationship.

That would be approximately that for each phosphorous

we'd have approximately 15-16 of nitrogen;

we'd put the silicates here or the silicons, if you prefer,

and logically along a line like we saw before we'd have cold surface waters,

deep water from the Atlantic, Indian deep water and Pacific deep water.

Respiration would act in this direction and if there was

photosynthesis in the water which rose to the photic zone,

the quantity of nutrients would decrease because

they would be consumed by organisms.

In parallel there would be an increase in inorganic

carbon and an increase in alkalinity too,

so alkalinity which would mean the proportion of cations like calcium,

magnesium which are balanced out by carbonates and bicarbonates

and we'll see that the alkalinity is greater in the Pacific than in the Atlantic,

but the alkalinity is greater because the calcium carbonate has dissolved,

but organic matter has been respired which does not add alkalinity;

respiration of organic matters does not add calcium but it does add CO2.

If we add carbonate and CO2 we saw before that basically what is produced is bicarbonate.

The result is that in the end in the Pacific on one hand as CO2 is added the PH drops

and on the other hand as CO2 is added we dissolve carbonate,

because we're adding carbonate and doing that we're adding alkalinity

and the result is that in the northern Pacific

deep waters we have greater levels of alkalinity,

greater amounts of total inorganic carbon, a lower pH and a lower amount of carbonate.

What does a lower amount of carbonate mean?

Well in the Pacific the particles of carbonate which

drop down dissolve more easily than in the Atlantic

and this is something that will affect sediment.

It's a question we'll look at in another module.

Finally we have the pH diagram.

You can see that the pH difference, the pH in the upper area, in surface waters,

is in the order of 8.2, as we said before,

and it reduces logically in waters towards places where there is a minimum of oxygen.

Ir recovers later on, but we have higher pHs in the Atlantic than in the Pacific.

This is the link with the previous slide.

Therefore we have less inorganic carbon,

higher levels of inorganic carbon in the Pacific and in this

series of diagrams for alkalinity, the same thing happens.

Lower alkalinity, higher alkalinity and for the

pH we have higher pH values, lower pH values.

This distribution you can see is the consequence of

the activity of chemical processes which are local.

That means to say chemical reactions which are

produced, organisms which eat each other, etc.

but in a medium where there is gravity,

which acts only on particles

and where there is a horizontal circulation at different

levels which works to distribute all of this.

So, in reality this idea of huge industry with

wall-less pipes is somewhat true of this ocean.

So there's this big industry with gravity,

circulation, where there are places which heat up,

where there is solar energy in some places, there are places where cooling takes place;

there are mechanisms, a series of processes of interaction with the atmosphere, etc.

and all of this works in a very coherent way,

and is something which gives rise to a perfectly organized

structure which really is one of the wonders of the planet.

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