[speaker001:] or the Open University
[Chris:] Yeah but the Open University ones... cover all of the material and they're what I actually wrote the lectures from... erm but there's six of them which is why the o the only advantage of the other one is that it's one not six and so... but I mean the material is all covered text of which there are three copies of most of them in the library one on short loan and one on sort of a general loan so you
[speaker001:] Yeah. What's the other one like? The
[Chris:] Tha that's quite good for, for the biological stuff, it doesn't really contain much... sort of useful for the last three or four lectures, it doesn't contain much on sort of ocean circulation and... the physics, you know but it covers the biology interactions quite well. And also it's quite useful for the... Right then we, we'll leave some of the physics and stuff that we've been doing behind now and just spend one lecture looking at some chemistry which I know will be equally popular.
[speaker001:] Oh good.
[Chris:] Most of the ninety two naturally occurring elements, that's leaving aside the, the elements that have been created artificially in particle accelerators and things, have been found in sea water and it's quite likely that those that haven't yet been recorded from sea water will be recorded as our analeti er as our analytical techniques get better. So basically you can consider sea water as being a solution containing salts of all of the naturally occurring elements.... Can you just copy that down. Would I, would I do that to you?
[speaker001:] Mhm...
[Chris:] You will probably be relieved to hear that you're not expected to memorize that table and to regurgitate it in the exam.... What you should be aware of though is which elements are the most common in sea water... and you'll not be too surprised to learn that sodium and chlorine, as in sodium chloride, as in common salt... are in fact the two most elements in sea water... followed by magnesium, sulphur and calcium.... Now there is inherently a problem in carrying out any chemical analysis of sea water and that is the fact that sea water contains lots of lumpy bits.... These particles, this particular matter is generally given the sort of generic term seston... some of those particles are mineral particles... some of them are colloidal aggregates... as the iron tends, iron, iron, ions tend to come together to form colloids... in solution... and some of them are particles of biological origin. So there are a whole range of different origins to these particles but they're all floating around in the sea water. So if you come to try and analyze sea water you first of all have to separate the er aqueous base and those things in solution from this particular material.... For no real reason other than tradition... samples are normally passed through a forty five mu sieve... or membrane filter.
[speaker001:] Is that seston or sestron
[Chris:] Seston T O N... So sea water analysis then is carried out on the water which passes through the forty five mu filter.... Typically in the open ocean situation... sea water contains thirty fives parts per thousand of salt, usually expressed as that symbol, sometimes you'll see it written as parts per thousand P P T... and what that means is that there are thirty five grams of salt per... kilogram of water... and that's usually given the symbol S and is referred to as the salinity.... We can divide the components of sea water into three erm categories... firstly those things which we can consider to be major constituents... these are substances which are present... are concentrations greater than one part per million.... So if you look down your table you've got your concentrations here in parts per million and you can see that everything above fluorine would be a major constituent and everything below it would be a minor constituent. Major constituents account for ninety nine point nine percent of the material dissolved.... Minor or the trace constituents then are present in concentrations less than one part per million... they obviously count for less than point one percent of the total salts... They are useful as chemical tracers of particular waters. If we know a water has a particular site of, of origin then we can often use minor trace constituents as a marker of that water to follow its fate and its path through the ocean, okay? So minor constituents are often useful as chemical tracers.... Our third category then is substances which are, are nutrients, and this is nutrients in the widest sense. These cut across the major and minor boundaries in that some of them are in concentrations greater and some lesser... one part per million.... These are substances which have biological significance.... The principal ones are nitrogen, which is present as nitrate, nitrite... and various oxides of nitrogen... phosphorous which is present as phosphate... silica present as silicate... which requires silica?
[speaker001:] ...
[Chris:] And carbon present as carbonate which is used in skeletal material... or as carbonate or C O two or H C O three minus... which are all used as substrates for photosynthesis.... Because these are biologically active compounds, they don't follow the simple... chemical processes of the other elements in the water, their concentration at any given time or place is a function of the biological activity and history of the water. So for example in a region of high productivity you may find very low concentrations of carbonate because it's been stripped out and converted to skeletal material being used in photosynthesis.... Substances whose concentrations are not affected by biological activity you'll often see described as bio-unlimited... compounds... so you might come across that in your reading. Right? So that's substances whose abundance is not influenced by biological activity, okay? So that's the opposite if you like of those nutrients.... In addition to salts, ocean water contains dissolved gases.... All of the gases present in the atmosphere are also present in surface waters.... That occurs simply by physical processes, primarily gas being trapped as bubbles under the action of breaking waves.... Carbon dioxide is the most soluble... of the gases... because as it dissolves it doesn't just go through a physical solution it goes through a chemical conversion... such that carbon dioxide dissolving in water forms... carbonic acid... which, in water... will dissociate into a hydrogen iron and a hydrogen bicarbonate iron... which can further dissociate... this is why I've got a nice wide blackboard and you've only got a piece of A four paper... so you end up with a carbonate iron two hydrogen ions... This system is a dynamic equilibrium. If you put more C O two into the system the concentrations of all of these go up, if you take carbonate out, for example for conversion to skelet skeletal material, it will pull material in this direction through the, through the s through those equations. The actual point of the equilibrium, where the balance occurs on each of these, is a product of temperature and pressure.... Topically this system is extremely important. If you measure the atmospheric rise in carbon dioxide due to fossil fuel burning, you'll see that it's only about one half of that predicted if all of the coal and oil that had been burnt since the industrial revolution had gone into the atmosphere, we would probably have er an increase in C O two double what we can actually measure. The rest has entered this system and has been absorbed by the oceans. Okay?... So we know that so far about fifty percent of our anthropogenic C O two has been locked away in this system in the ocean. And at the moment there is considerable er research effort being directed to try and work out just how much more carbon dioxide the ocean will continue to absorb.
[speaker001:] What does anthropogenic mean?
[Chris:] From human sources.... For example can we continue burning fossil fuel and will half of it continue to be absorbed by the ocean or is the ocean reaching saturation... such that these equilibrium terms are being reached and future increases in C O two will be reflected in atmospheric build up, and it's only atmospheric C O two that contributes to the greenhouse effect. So the exact er nature of this balance and how waters between contact with the atmosphere are being mixed into the deep sea, cos remember this equilibrium can only be occurring in surface waters cos only they are in contact with the gas phase... how that turnover of deep water's occurring affects just how much capacity we've got for absorbing C O two in the oceans and therefore mitigating the greenhouse effect.
[speaker001:] How can we work that out?
[Chris:] What's that?
[speaker001:] How can you work that out?
[Chris:] Well that's what they're trying to do at the moment by measuring rates of turnover of deep to surface waters
[speaker001:] Mhm
[Chris:] which is actually poorly understood, we'll talk a bit more about that in the next lecture. Okay? But you can see if this lot gets converted to carbonate and then that water then gets mixed down to the deep water, it will be replaced at the surface with water which has a low carbonate concentration which will suck more C O two out of the atmosphere. So it's quite critical as to how that rate is turning
[speaker001:] [cough]...
[Chris:] It's then oxygen... surface waters are saturated, in fact they're often super-saturated with oxygen. In part this is the result of photosynthesis which is pumping oxygen into solution but again primarily it's down to the physical saturation due to breaking waves and air bubbles being mixed in to the system.... Below the photic zone oxygen is consumed by biological activity... so immediat from immediately below the photic zone you tend to see a decrease in oxygen with depth... reaching a minima somewhere between five hundred and a thousand metres depth
[speaker001:] Re reaching a what?
[Chris:] Reaching a minimum.... Below about a thousand metres oxygen concentrations are fairly static... down to the a minimum between five hundred and a thousand metres depth... so they're fairly static.... That's in the open ocean... in enclosed basins, for example the Black Sea, many fjords and sea lochs, the deep waters are not renewed by water masses moving in from other areas in the way that they are in the open ocean... and there anoxia can occur in the deep waters, that is the oxygen can be completely removed by biological activity... particularly in degradation processes of organic matter, bacterial respiration... so anoxic conditions can occur in isolated deep basins... but low oxygen concentrations are actually very rare in the open ocean....
[speaker001:] Why is that? Why is it... erm well yeah... er le less of a level of oxygen at erm medium depth
[Chris:] Yeah?
[speaker001:] now surely it's far more difficult to get oxygen down deep?
[Chris:] Yeah. It's because... that's the sort of trace you get with an oxygen minimum around eight hundred metres. The photic zone is, this is percent saturation of oxygen... fully saturated... and what we're seeing here is oxygen being utilized by respiration. In general biological activity decr decreases with, with increase in depth so you see er respiration using up this material. But when you start getting down here you've got low biological activity
[speaker001:] Yeah but sh sh
[Chris:] but you've also got water masses which are being moved in... which are rich in oxygen. The reason they're rich in oxygen is because they were formed at the polar regions, alright? Where, because the water is very cold, it will absorb an awful lot of oxygen. We'll look, we'll actually look at the processes of deep water formation tomorrow, so you'll actually see how the waters are formed, but the reason why this is high is basically the act of A, low biological activity removing it and B, the fact that the source waters have not come from below, they've come in horizontally from an area where they were formed which was very rich in oxygen.... Now if you start analyzing the composition of sea salt, or the salt in, in sea water, what you find is that there is a remarkable constancy in the ratio of one element to the other. No matter where you go in the world ocean you find that although the total salinity may vary, the actual ratio of say silicon... no that's, let's say potassium to aluminium is the same er so there is a constancy of composition... there is a constant ratio between the elements.... That constancy of composition relationship breaks down in enclosed seas and bays for example where er addition processes, I E er salts which have been eroded from river water, may alter the composition. So may, you may have a bay for example that's in an area where all the rivers draining into it are rich in copper because they're running over rocks which are copper. So in enclosed areas like that this breaks down but in the open ocean system you've got this very strong constant ratio. Other areas where it may break down are areas of high biological activity... for example tropical reef flats... very high biological activity there, very high demand for calcium carbonate to build all those coral skeletons, so carbonate will be stripped out.... And waters passing through the earth's crust at hydrothermal vents etcetera will also have undergone chemical changes so again pool waters being emitted will vary in their composition.... So how do we actually go about measuring this thing called salinity?... Well if the definition of salinity is the amount of salt dissolved in a particular volume of water, perhaps the logical way to do it is simply to take a volume of water and evaporate it and weigh the amount of salt that's left... so that might be the simplest approach. There are however problems with that approach... of how much do you dry the salts, for example. Depending the amount of drying concerned actually alters the composition of some salts. For example magnesium chloride holds water within its crystal lattice and if you dry it that water comes out, but as it comes out it also strips out... the chlorine as hydrogen chloride gas for example. So that would be one salt whose actual nature and therefore weight and therefore your measurement of salinity will vary depending on the degree of drying.... Carbonates will combust at relatively low temperatures so if you dry your water in an oven you may find you're actually burning off some of the carbonates.... So that would be another source of error.... Well... our constancy of composition gives us a way round some of these problems. If we can establish through very careful analysis what the ratio are between certain elements and each other then that ratio also holds between any given element and total salinity, yes?...
[speaker001:] Yes.
[Chris:] Yes. So by measuring just one substance we can work out the overall total salinity. The substance that's most frequently measured is actual the chl is actually the chlorine content... and the salinity is one point eight zero six five five times the chlorine concentration.
[speaker001:] [cough]...
[Chris:] Chlorine is relatively easy to measure... I'm sure you'll all of done it at A level chemistry... titration, silver nitrate? Yeah? To establish the concentration of chlorine in a solution. And that was the technique that was really used up until the sixties for the determination of salinity. Whole research cruises where water samples were being taken at many depths, every sample that came back was titrated on board ship to get the silver nitrate to establish the chlorinity and hence the salinity. These days we can do it much easier and we do it electrically, so we can use a relationship between conductivity... which has to be er compensated for temperature and pressure at which you're doing your readings, and that gives us a measure of our chlorinity and then that gives us a measure back to our salinity.... And modern salinometers will automatically compensate for the temperature and pressure and do this conversion so you can get a direct read out from an electrical instrument of the salinity.... But one must still be wary... the fact that although we've got this nice electric gadget, that we drop the probe into a bucket of water and it gives us er a salinity... it is still entirely dependent on this ratio... and this ratio is based on the constancy of composition which is very good for open ocean waters but breaks down in coastal waters where erosional processes, where fresh water additions and the sediment loads of the rivers... may actually alter this ratio, okay? So although we've got a device we must use it with care when working in coastal waters.... Another way... of measuring salinity is optically.... This is an this is er another... this is a good one for use in the field, it can be quite accurate... it's, it's certainly a very easy one to use in the field cos all you need is a thing that looks a bit like a telescope about four inches long... and what tha the principle it's working on here is the fact that ref the refracted index... of water changes as you dissolve salts in it. So what it really is is a fr refractometer... okay? It's measuring a refractive index of the solution but, and giving you a read out in terms of salinity, okay. So a useful field technique....
[speaker001:] Do you have to sieve it first?
[Chris:] Sorry?
[speaker001:] Presumably you sieve it first?
[Chris:] Well again all of these things should be done on water that's been passed through a filter to separate the water from the sestron yeah.... However again, in the field that's frequently not... and you do literally drop your probe in a bucket....
[speaker001:] As the actress said to the bishop.
[Chris:] Exactly, I was just thinking the same thing.
[speaker001:] [LAUGHTER]...
[Chris:] Well that's the sort of composition... which immediately begs the question where do these salts come from... and where do they go to? So sources and sinks of salts.... What do you think the most obvious source is? Yeah. Weathering of rocks... by rain water, by frost, by chemical action... which is then leaked into rivers and carried from rivers into
[speaker001:] [cough]...
[Chris:] However some salts are very abundant in river water but are very rare in crustal rocks. In particular chlorine... the most dominant element in sea water... it's quite common in river water but is extremely rare in the rocks that form the earth's crust. So the simple weathering model will not explain our distribution of chlorine.... Chlorine is an example of an element that is being recycled continually. The chlorine we detect in the rivers has actually come from the ocean. The chlorine is picked up as aerosols... droplets containing salt for example from breaking waves... taken up by the atmosphere, carried over the land, rained down again, gets into the rivers and ends up back in the sea. So chlorine is continually going through this cycle. Ocean to aerosol... into the atmosphere, carried over, deposited as rain, leaks back through the river system into the sea.... Looking in ballpark figures, river water is about three hundred times more dilute than sea water....
[speaker001:] How do you in the first place?
[Chris:] I'll come to that in a second.
[speaker001:] Ah right....
[Chris:] Sea water though contains proportionately less... hydrogen carbonate... silicate... and calcium than river water. So river water... is greater than sea water, proportionately, for those elements. Okay?... Which implies that these must be precipitated in some way out of the marine environment. They're obviously being continually carried in from the river water but are being taken out of the system somewhere in the marine environment. Looking at that list would anybody like to suggest what sort of processes those substances are all involved in?
[speaker001:] Biolo biological. skeletal.
[Chris:] Yeah, biological.
[speaker001:] [LAUGHTER]
[Chris:] Diatoms... calcium hydrogen carbonate... skeletal material... these things are probably ending up predominantly as deep sea sediments... pelagic sediments, yeah, remember? Diatomic oozes... regularian oozes and so on?... So that's a sink for some material. Let's come back to our chlorine then and think about... well if it's not being weathered out of the rocks, what's its ultimate source?... And the simple answer is it would appear that it's volcanic in origin. Volcanoes emit very large concentr very large amounts of hydrochloric acid as a gas... H C L gas... and earlier in the earth's history volcanic activity was much, much more widespread than it is now and during this period vast amounts of H C L were emitted. This is highly soluble, so it is immediately washed from the atmosphere by rain into solution. And since then this chlorine has been continually recycled through the hydrosphere.... Sorry... One of the other tables you've got... shows you a comparison between the elements which are present in crustal rock and in the sea and from that you can see which ones are a good reflection, I E whi which elements have been transferred to the sea by direct weathering, and which of them are undergoing other processes, either other sources or other sinks.... So for example you can see chlorine... the percentage of chlorine in solution expressed as a, a per centimetre solution is something like twenty four thousand but its percentage in crustal rock is nought point nought one three so there's this big excess of chlorine to explain.... Now arguments such as that in terms of composition of the sea relative to the composition of crustal rock
[speaker001:] [cough]
[Chris:] imply a steady state system... or over a long term and if over the long term... the rate of addition of material to the sea is equalled by the rate of removal.... If there's a continual turnover of material we can actually calculate its residence time, that is the average length of time a particular molecule of substance X spends in the ocean.... And that would be calculated as the total amount dissolved in the ocean divided by its rate... either of addition... or removal... so if we're assuming steady state the two will be the same, whichever is easiest to ma measure... and that would normally be expressed in years...
[speaker001:] [cough] Sorry, what does R T stand for again?
[Chris:] Residence time....
[speaker001:] So w w what rate of addition or removal addition or removal?
[Chris:] Yeah, right. Ei either the rate of addition or the rate of removal cos if we're assuming a steady state the two will be the same and it's just a question of for some substances it might be easier to measure th the rate of addition, for others it might be easier to measure the rate of removal. You should get the same answer if you did them both.
[speaker001:] Mhm....
[Chris:] There are some residences, residence times, we'll note that the units are actually millions of years.... Chlorine has a residence time of infinity, that's because it's being recycled round all the time so it's in the system forever,. Very low residence time for hydrogen carbonate... biologically active, and an intermediate time for things like sodium and potassium which are common in crustal rock and are also commonly being deposited through sedimentation and so on.... In general there's a good correlation between a substance's residence time and its concentration. The more important it is in sea water, the longer its residence time.... The principal removal mech mechanisms for salts are inorganic precipitation... a chemical reaction between the dissolved substance and a particle, such that it then becomes part of the particle... and obviously for biologically mediated substances biological processes such as skeleton formation... or conversion to biological tissue.... just want to look at the carbonate system in a bit more detail.... Calcium carbonate... is... teetering on the brink of solubility in sea water.... The surface oceans are actually super-saturated with calcium carbonate... so there's a lot in solution, there's more in solution than in theory is possible, in super-saturated condition and in addition there's a lot floating around in the solid form in biological material.... Spontaneous precipitation from surface waters is actually fairly rare because most of the carbonate a ions are weakly bound up with magnesium ions... in surface waters. So it's the presence of magnesium ions in surface sea waters that helps keep the calcium carbonate from precipitating.
[speaker001:] Sorry, what's bound up with...?
[Chris:] The carbonate is bound up with magnesium ions, weakly. Very weakly... but it is sufficient to pr prevent them pairing with calcium ions to form... precipitate....
[speaker001:] What's that?
[Chris:] Er two plus and two minus.... Now as th this weak bou bonding's overcome by biological processes to actually produce solid calcium carbonate in skeletons, now as that skeletal material sinks down through the water column, it moves out of the region of super-saturation... and begins to dissolve. The region where it begins to dissolve... is known as the lysochine... okay? So that's the region where calcium carbonate is no longer super-saturated and so that skeletal material, as it floated down through the water column, would begin to dissolve.... If you go deeper through the water column you'll come to a depth where all of the calcium carbonate has dissolved... and that's known as the carbonate compensation depth, or C C D... the carbonate compensation depth.... These are, are not single depths, they're, they're more regions... and the actual depth where the two regions occur varies around the ocean. The depth of the lysochine is basically controlled by water chemistry.. the P H and the concentration of carbonate in the water.... The C C D is controlled both by the water chemistry, the P H and carbonate concentration, and by the rate of supply. So below an area with a very high biological activity, where there's a very large amount of calcium carbonate raining down, the C C D will be deeper because it will take that much longer for this material that's rained down to actually dissolve. Yeah?... So, okay? So the C C D will be deeper under biologi biologically productive regions....
[speaker001:] ?
[Chris:] Yeah. So the C C D will be deeper under regions with high biological productivity.... And if you remember your distributions of deep sea sediments you will recall that calcium carbonate based deposits are rare in deep waters... because it's all dissolved... but are more common under regions where there is a high productivity because the C C D will have been depressed. So you can get calcium carbonate deposits for example, on the top of isolated sea mounts or mid-ocean ridges.... The carbonate system is also important in the control of sea water P H.... Typically sea water is alkaline around about eight plus or minus nought point two P H units... and P H is defined as the negative log... of the concentration of hydrogen ions.... Variations in PH away from this are typically controlled by the equilibrium we've already seen... between the hydrogen bicarbonate ion and carbonate and H plus. So this equilibrium... is the principal mechanism for maintaining sea water P H. If it shifts that way you get a greater liberation of, of ion, of H plus ions which moves the P H down, making it more acid.... How much chemistry can you remember? Equilibrium constants? If K is the equilibrium constant for that reaction... then we can actually redefine our terms such that the concentration of hydrogen ions will be K multiplied by the concentration of... hydrogen bicarbonate ions over the concentration of... bicarbonate ions.... In practice it's very difficult to measure the P H of sea water. Because there are so many substances in there an and the relative concentrations of many of these ion species are dependent on equilibrium reactions such as this, it makes it very difficult to measure using the traditional approaches. One way round that is to actually measure... the alkalinity of the water.... Alkalinity is not a measure of how alkaline a solution is, it is therefore a stupid name
[speaker001:] [LAUGHTER]
[Chris:] but it's the one the chemists give it. Alkalinity is defined as the amount of hydrogen ions required to neutralize... the negative charge... on the anions, I'll go through it again Roly
[speaker001:] [LAUGHTER]
[Chris:] in the solution. Okay? So that is the amount of hydrogen ions required to neutralize the char the negative charge on the anions. And therefore it can be found very easily, very directly, simply by titration.... And having measured the alkalinity... you can simply convert back... to work out the concentrations of the carbonate and bicarbonate ions and from that you can calculate the P H using that relationship. Okay? So in practice it's easier to measure the alkalinity and then back-calculate to get that and hence the P H... than to actually try and measure the P H directly from sea water.... Are you sure you wouldn't physics?... Right just to finish off I just wanna look at actual vertical distributions of... elements in the ocean.... We can actually characterize... three types of distribution.... Firstly those substances which increase relatively rapidly with depth and then become constant... and this group are usually referred to as the bio-limiting... element or a bi having a bio-limiting distribution.... Substances which are required... nutrients, micro-nutrients, copper and zinc are required as micro-nutrients by plant cells for example, would show distributions like this... de decreased levels in surface waters where er in the photic zone biological activity is high they'll be continually stripped out of the water column... there in the concentrations then increase as biological activity decreases and then becomes constant with depth.... We then have a group of substances which are bio-unlimiting... this is concentration and that's depth... barium, bromine, chlorine... iron... magnesium, potassium,, sodium, sulphate... will all be examples of bio-unlimiting substances.... And of course because this is biological oceanography it's not as simple as that there is a third category which is bio-intermediate.... These show some decrease in surface waters but not as severe as the bio- limiting substances... calcium, barium... carbon... would be examples of bio-intermediate substances....
[speaker001:] Why doesn't it tail off at the bottom then if it equals out at depth?... I thought you said with the bio- limiting it, it evened out at depth as a concentration
[Chris:] Well it is, this is the concentration across here, this is depth. So that's a constant concentration.... Yeah?... Everybody happy? Follow all of that?... Good.... Do deep sea circulation tomorrow.
[speaker001:] Radium... radium That's a washing powder.
[Chris:] That's right, no that's Radion
[speaker001:] Ra radium microbes sea water. Keeps all the fishes clean. [LAUGHTER] Keeps them sparkling white [LAUGHTER]
[Chris:] You've, you've got... you've got a video which is scheduled for one thirty because you've got something else at two o'clock, but you can have it now if you prefer to have it now rather than one thirty