Arguments
Understanding the long-term carbon-cycle: weathering of rocks - a vitally important carbon-sink
Posted on 2 July 2013 by John Mason
above: the processes of the long-term carbon-cycle that this post explores. Graphic: jg.
This post delves into the long-term carbon cycle that involves the interactions of the atmosphere with rocks and oceans over many millions of years. Because of its length, I've broken it up into bookmarked sections for easy reference: to come back here click on 'back to contents' in each instance.
Contents
Introduction: what is weathering?
Carbon dioxide and rock weathering: the chemistry
Limitations to the precipitation of calcium carbonate: the Carbonate Compensation Depth
The significance of weathering as a carbon-sink
Deep weathering of rocks: an illustrated example from Mid-Wales, UK
How breaking up minerals affects their weathering-rate: mountain-building as an accelerant
Picking up signals of major weathering episodes in the geological record
Introduction: what is weathering?
Weathering is a familiar process to us all. It involves the chemical reactions between chemical compounds in the atmosphere and chemical compounds on the planet's surface. When your car's exhaust pipe falls apart noisily, it is because the steel from which it was constructed has, over several years, reacted with oxygen and rainwater to form rust. It has weathered. But that's a relatively fast example involving a relatively unstable compound. The compounds making up the vast majority of Earth's land surface - the minerals that make up rocks - are, by and large, very slow to react. As a consequence, large-scale weathering is a process that takes place on a timescale of millions of years, over which periods it constitutes a critically important carbon-sink.
Why a carbon-sink? Because, via weathering of rocks and reprecipitation of weathering products as carbonate sediments (e.g. limestones), huge quantities of atmospheric carbon dioxide end up locked away for a very, very long time. The process begins when CO2 dissolves in droplets of water, up there in the clouds. The resulting solution, which reaches the surface as rainwater, is weakly acidic:
CO2 + H2O = H2CO3 (or carbonic acid - the old name for carbon dioxide was carbonic acid gas)
Rainwater containing carbonic acid is able to react with most minerals at varying rates according to their chemical stability. Now, some naturally-occurring minerals are extremely stable. Think about gold, eroded mechanically from ore deposits and then recovered by prospectors, maybe hundreds of thousands of years later, from river-gravels by panning. Or quartz (silicon dioxide), found as hard, white pebbles on beaches. They're both pretty bomb-proof.
At the other end of the scale are the very unstable minerals, such as sulphides, compounds of various metals with sulphur. Sulphides readily weather in the surface or near-surface environment, so that most ore-deposits have near-surface zones in which the products of primary sulphide weathering - the secondary minerals - are to be found. The copper carbonates, azurite and malachite, are examples of secondary minerals that will be familiar to most readers. Such minerals can occur in large quantities where the weathering has been prolonged - over a few million years - but the relative rapidity of the process is demonstrated by the fact that thin films of often colourful secondary minerals are a common sight along the walls of mine-tunnels only a century or two old.
above: primary (left) and secondary (right) copper minerals. The common primary ore, chalcopyrite, is a sulphide of copper and iron that reacts readily with air and moisture at the surface. Prolonged weathering forms colourful secondary minerals in large quantities, like the blue and green copper carbonates, azurite and malachite (the illustrated specimen is about 6 inches long). These copper carbonates constitute a localised, and therefore relatively minor, long-term carbon-sink, compared to the very widespread occurrence of limestones. Photos: author
In between these extremes, from the bomb-proof to the downright unstable, there exists a whole spectrum of mineral stabilities, but an important point is that most of the numerous mineral species that make up rocks lie towards the stable end of that spectrum: whilst they do react with carbonic acid, they do so at a very slow rate, on geological rather than human timescales. So let's now take a look at what happens.
Carbon dioxide and rock weathering: the chemistry
When carbon dioxide dissolves in droplets of water to form carbonic acid, it dissociates (splits into reactive charged particles or ions):
H2CO3 ? HCO3− + H+
The two ions that occur as a result of this process are the bicarbonate ion, HCO3− and the hydrogen ion, H+. Put simply, hydrogen ion activity within a solution is a measure of its acidity: the more active hydrogen ions in a solution, the more acidic it is and the better at dissolving other compounds. The 'H' in pH - the scale by which acidity is measured - is hydrogen. The pH value of 7 (pure water) is neutral, values of 7 up to 14 are increasingly alkaline and values of 7 down to 0 are increasingly acidic.
Carbonic acid is a weak acid: ordinary rain thus has a pH of around 5 to 5.5, but there is lots of it available in the environment, so that over prolonged timescales it does a lot of weathering. Other acids may also be present: for example, sulphur dioxide, outgassed during volcanic eruptions or released by industry, likewise dissolves in droplets of water in the atmosphere to produce sulphuric acid, hence the term 'acid rain', with a pH of around 4 to 4.5. However, over geological timescales things like major eruptions and human industry are mere punctuation marks in a long sentence.
Rocks that contain carbonates - limestone and dolomite being common examples - react rather more quickly too because the minerals they are largely made from, such as calcite (calcium carbonate) are more reactive than silicates. Carbonate dissolution leads to the formation of karstic landscapes complete with cave-systems lined with deposits of reprecipitated calcium carbonate forming stalactites and other features. The dissolution reaction goes as follows:
H2CO3 + CaCO3 = Ca(HCO3)2
carbonic acid + calcium carbonate = calcium bicarbonate (in solution)
Silicate minerals occur widely in nature. Some globally-abundant rocks - such as basalts - are mostly composed of silicates. There exists a great variety of different silicate minerals - for example the feldspars, micas, olivines, pyroxenes and amphiboles to name but a few groups - that combine silicon and oxygen with potassium, sodium, calcium, magnesium, aluminium, iron and many other elements. Silicates weather via rather more complex reactions, but let's simplify things with a generalised equation for the process using the calcium silicate CaSiO3, which occurs naturally as the mineral wollastonite:
2CO2 + 3H2O + CaSiO3 = Ca2++ 2HCO3– + H4SiO4
carbon dioxide + water + calcium silicate = calcium ions + bicarbonate ions + silicic acid (in solution)
The dissolved calcium and bicarbonate ions travel in groundwater to the rivers and thereby find their way to the sea, where they are reprecipitated as calcium carbonate. The reprecipitation is mainly biogenic - it involves various creatures making their shells or skeletons from calcium carbonate:
Ca2++ 2HCO3– = CaCO3 + CO2 + H2O
calcium ions + bicarbonate ions = calcium carbonate + carbon dioxide + water
above: a hand-sized cut slab of limestone, dating from early Carboniferous times (~340 million years ago), with fossil exoskeletons of ancient corals, consisting of calcium carbonate obtained by the organisms from the seawater back at that time. A classic example of an ancient and long-term carbon-sink. Photo: author.
The silica from the silicic acid is used similarly by organisms such as the abundant planktonic diatoms:
H4SiO4 = SiO2 + 2H2O
silicic acid = silicon dioxide + water
Significantly, via the calcium carbonate-generating part of the chemical process, there is a net loss of readily-mobile carbon: we start with twice as much as we end up with. That's why weathering of silicates is so important. The missing carbon is locked up in the calcium carbonate in the following way: as shelled creatures die and their remains accumulate, we get a carbonate-rich sediment. As the sediments continue to accumulate, our carbonate-rich layer will be progressively buried under new layers of sediment and in time it will turn into solid rock - limestone. Globally, limestones are very common and often occur in great thicknesses (think of the White Cliffs of Dover, for example). That carbon is thereby locked away and immobilised for a very long time. The overall process may be expressed as follows:
2CO2 + 2H2O + CaSiO3 = CaCO3 + CO2 + 2H2O + SiO2
carbon dioxide + water + calcium silicate = calcium carbonate + carbon dioxide + silica + water
The basis of the equation written above has been understood since the middle of the 19th Century. A more complex version could be written involving magnesium too: that element can be coprecipitated with calcium to form dolomite, calcium magnesium carbonate with the formula CaMg(CO3)2. Because of their natural abundance and their chemical properties, calcium and magnesium are the two vital elements in this long-term carbon sink. It is important here to point out that weathering of silicates containing e.g. only potassium or sodium does not trap carbon in the same way, because the carbonates of these elements are water-soluble so they do not precipitate to form sediments. Calcium and magnesium are the key.
above: limestones of upper Cretaceous age forming the famous White Cliffs of Dover on the south coast of the UK. Think just how much carbon is locked up in these cliffs! Photo: Remi Jouan.
Limitations to the precipitation of calcium carbonate: the Carbonate Compensation Depth
An important factor regarding the precipitation (or otherwise) of calcium carbonate in the oceans is the Carbonate Compensation Depth (CCD). The reaction:
Ca2++ 2HCO3– = CaCO3 + CO2 + H2O
is reversible: if the physical conditions favour precipitation over dissolution then you will get carbonate precipitates accumulating. But if calcium carbonate dissolution is happening at a faster rate than calcium carbonate precipitation, there is no net gain in solid calcium carbonate: instead calcium carbonate that is already present in solid form starts to dissolve.
What are favourable physical conditions for precipitation or dissolution? Calcium carbonate is more soluble in water at a) lower temperatures, b) at higher pressures and c) in the presence of increased dissolved CO2. These conditions are all satisfied in the world's deeper oceans, at depths below ca. 5000m, which in broad terms is the Carbonate Compensation Depth. Sediments on ocean floors below this depth contain no calcium carbonate. As the image below shows, such depths are not uncommon.
above: ocean depths worldwide. Many of the major ocean basins are deeper than the Carbonate Compensation Depth at ~5000m. Graphic - NOAA.
There is a slight complication to this because, like many substances, calcium carbonate occurs in nature as two minerals - polymorphs - that have different crystal structures. These are calcite (trigonal) and aragonite (orthorhombic). Aragonite is also precipitated in the oceans - molluscs use it to build their shells and corals their endoskeletons - but it has a much shallower compensation depth which varies from less than 1000m in some low latitudes to over 3000m in the North Atlantic.
One effect of increasing dissolved CO2 in oceanic water (ocean acidification) is to lessen the compensation depths of both calcite and aragonite. Whilst such a shift places a further limit to marine carbon sequestration, it also impacts severely on the marine ecosystem. Organisms that need to precipitate aragonite in order to build their skeletons or shells will clearly find themselves in dire straits if the aragonite compensation depth shifts upwards, so that their habitat now lies beneath it. Such a point applies particularly to fixed or benthic faunas that dwell on the sea-bed because such communities cannot simply up sticks and move: hence they are particularly vulnerable to rapid changes in the chemistry of our oceanic waters.
The significance of weathering as a carbon-sink
The weathering of rocks is estimated to involve the drawdown of about a gigaton of atmospheric carbon dioxide a year. That sounds a bit hopeless when compared to the ~30 gigatons emitted by humans burning fossil fuels every year. Over geological timespans, however, the amount of carbon dioxide removed from the atmosphere via weathering is huge, at around a million gigatons per million years. Globally, limestones and other carbonate-based sedimentary rocks are a phenomenally important carbon sink that is relatively stable in nature: they are estimated to hold over 60 million gigatons of carbon - compared e.g. to the estimated total of 720 gigatons carbon dioxide that is present in the atmosphere and the 38,400 gigatons present in all of the oceans.
Limestones do not give up their carbon again readily. The key processes by which they may yield it up include metamorphism (recrystallisation under heat and pressure during mountain-building episodes), subduction (being forced down into the Mantle at a tectonic plate boundary, where the carbon is liberated during melting, leading to CO2 making its way towards the surface in magmas and being outgassed by volcanoes) or industry (people heating limestone to high temperatures in order to make cement). However, compared to human industry, the processes of metamorphism, subduction and volcanism operate over geological timescales, so whilst there is a flux of carbon dioxide going into weathering and thence to limestones, there is also an outward flux going back into the atmosphere. Just as well: it is thought that with no geological resupply, and even with ocean outgassing, the weathering process would remove all atmospheric carbon dioxide in less than half a million years. Just as a rapidly-forming excess of the gas is very bad news, losing it all like that would be disastrous. The long-term carbon cycle makes sure that this isn't going to happen!
Deep weathering of rocks: an illustrated example from Mid-Wales, UK
On a year-to-year basis, the process of rock-weathering is a slow one. How slow? Well here in Mid-Wales, where I am based, we have a useful example of weathered and unweathered rocks outcropping at surface that demonstrate the time factor as well as anything.
A short crash-course into the geology of Mid-Wales is needed to get this story into context. The rocks of the district consist of slate-grey mudstones, siltstones and sandstones, which were originally deposited as muds, silts and sands on an ancient seabed in late Ordovician and early Silurian times, some 445-435 million years ago. They were then uplifted and folded during a phase of mountain-building and the area has probably remained land ever since. There - done and dusted.
above: slate-grey, relatively unweathered late Ordovician mudstone, typical of the rocks of Mid-Wales. Photo: author
Now we come to the weathering. We know that these mudstones, siltstones and sandstones underwent massive-scale weathering. Why? Because, at scattered localities here and there in the Mid-Wales hills, we find remnants of the weathered rocks. Instead of the normal, slate-grey colour, rocks at such localities are instead various pale pinkish or buff shades. It's not just a surface thing either: if you whack a piece open with a hammer, you will see that it pervades right through.
above: a broken boulder of pervasively weathered Silurian mudstone from Mid-Wales. The slate-grey colour has been changed to pale pinkish and buff shades. Photo: author
Why are there only remnants of such rocks left behind? The answer is almost certainly glaciation. Glacial ice is a remarkably efficient agent of mechanical erosion. The series of glaciations that took place between 2.5 million and 12,000 years ago fashioned the modern Welsh landscape. Ice-caps over a kilometre thick and valley-glaciers carving their way down to the lowlands are estimated to have eroded away tens to hundreds of metres of strata. As a consequence, in most places in Mid-Wales, the rocks outcropping at surface are the normal, unweathered slate-grey colour, the glaciers having eroded away the deep-weathered stuff. In the 12,000 years since the end of the last glaciation, these rocks have remained visibly unweathered - there hasn't been enough time for the slow chemical reactions to have had anything more than superficial effects.
How breaking up minerals affects their weathering-rate: mountain-building as an accelerant
In the same area of Wales, the rocks contain numerous mineral-lodes that in the 17th-19th Centuries supported a respectably-sized metal mining industry, with lead, silver, copper and zinc being produced. As with the rocks of the area, the deep-weathered zones of these ore-deposits have (barring a few notable exceptions) been eroded away during glaciation and metal sulphides can often be found quite close to the surface. The old mines neatly demonstrate another critical principle concerning weathering which is that the greater the surface area available to weathering agents, the greater the amount of weathering that can take place in a given time.
Let's visualise this first with a simple example: imagine a cube of something nice and reactive - iron, for example, 10 centimetres on edge. To calculate its surface area, we first obtain the area of one face: 10cm x 10cm = 100cm2. As a cube has six identical sides, we multiply the area of one face by six, thus getting 600cm2 of iron waiting to react with moist air and go rusty.
Now let's saw the cube up into 1cm cubes (OK, before anyone says, there'll be some wastage from the saw-cuts but let's forget about that). The cube's volume is 10 x 10 x 10 = 1000cm3, so once the saw has done its work, we have a thousand 1cm cubes. Each will have a surface area of 1 x 1 x 6 = 6cm2. The total surface area of all the cubes will be 6 x 1000 = 6000cm2. By dividing the original cube up into all these little ones, we have increased the surface area available to react by a whole order of magnitude. Cut those centimetre cubes into millimetre cubes and we now have 60,000cm2 of surface available to react, and so on.
Now let's see the process demonstrated in those mines. When they were being worked, mostly between 100 and 200 years ago, tunnels were driven into the ore deposit and the ore was blasted into small fragments for removal. Invariably, quantities of debris, rich in freshly broken sulphides, were left behind when the mines were abandoned. Both air and moisture then had access to a much larger surface area of reactive sulphides to get to work on. The result was that in just a century or two, the debris became coated or even cemented together by secondary minerals. It's a good example of a major uptick in the weathering-rate caused by surface area changes. The wide range of secondary minerals present in such coatings and cements (mostly complex sulphates) is indicative of an unstable and immature chemical system that is constantly evolving.
above: small amounts of recently-formed secondary minerals, the result of a few centuries of weathering of freshly-broken sulphides in the metal mines of Mid-Wales. (L): sky-blue rosettes (~2mm across) of the copper zinc carbonate-sulphate schulenbergite with deep blue crystals of the lead copper sulphate linarite. (R): bottle-green crystals of the copper zinc sulphate, ramsbeckite. Photos: D.I. Green
Now, let's apply the same principles to rocks in general. What processes in nature break up rocks to create a massively increased surface area? Many, but let's think about the really obvious ones: volcanic eruptions and mountain-building episodes. Both build elevated piles of rock that are prone to collapse by the actions of gravity, earthquakes, rainstorms, explosions in the case of volcanoes and frost action in the case of mountain ranges. In each case, there is a big increase in the surface area of rock available to weathering agents.
Weathering may not, however, take place to any great extent in a high mountain-range like the Himalayas. Temperature is a critical factor: when a landmass is uplifted into a mountain range it will enter a colder climatic zone due to the increase in altitude. Although frost can force rock faces apart and cause rockfalls making a greater surface area of rock available to weather, if it is so cold that there is little or no liquid water available, weathering is inhibited.
The most intense chemical weathering of silicate-bearing rocks instead occurs where temperatures are well above freezing for prolonged periods and rainfall is high, conditions that are especially satisfied in the Tropics - where deeply weathered profiles can extend down from the ground surface to well over a hundred metres depth. Higher temperatures speed up the rates of the chemical reactions and higher rainfall delivers more weathering agents per annum.
Combine such parameters and the answer to the riddle of why mountain-building may lead to enhanced carbon dioxide drawdown becomes clear. Mountain-building episodes facilitate the creation of a vast supply of fresh-broken rock. That rock is constantly being transported away from their cold heights, firstly via gravity (rockfalls), then by glaciers and finally by rivers to a warmer, lowland climate, where the weathering takes place. The Himalayan foothills and the Ganges basin are a good example of such areas where enhanced weathering of rock debris, transported from the mountains and deposited as fine-grained sediment, occurs.
Picking up signals of major weathering episodes in the geological record
Mountain-building episodes occur due to continental collisions that are driven by the slow process of plate tectonics and they typically take place on timescales of millions or tens of millions of years - far more slowly than, for example, the Milankovitch cycles that swing Earth in and out of warm or cool periods on scales of tens to hundreds of thousands of years. For this reason, perturbations to the rate of weathering - and thereby the flux of carbon dioxide being consumed by weathering - typically occur slowly, although they may be quite profound, to the extent that significant past changes to climate may have, at least in part, been due to them.
Basaltic weathering is a particularly strong candidate in this respect - basalts are rich in calcium-bearing silicates like plagioclase feldspar and magnesium-bearing silicates like olivine. A major period of basaltic eruption (with much attendant carbon dioxide outgassing) followed by prolonged intense weathering could potentially create a corresponding peak and dip in atmospheric carbon dioxide levels as outgassing falls and weathering-related carbon dioxide drawdown rises. Can such weathering episodes be detected in the geological past? The answer is quite possibly yes, by studying the ratios of isotopes of the alkaline earth metal, strontium, part of the same group of elements as calcium and able to substitute for calcium in minerals such as calcite (in limestones) and feldspars (in basalts, for example).
Strontium has four stable, naturally occurring isotopes as follows (abundances in brackets): 84Sr (0.56%), 86Sr (9.86%), 87Sr (7.0%) and 88Sr (82.58%). Of these, 87Sr is radiogenic, which means that it has been formed by radioactive decay of an unstable isotope of another element, in this case rubidium 87. The ratio of 87Sr and 86Sr is the one used in the context of weathering: in igneous rocks like basalt it is typically around 0.704 but in other rock-types it is a little higher. Therefore, a period of intense weathering of basalts ought to show up in the isotope record: weathering feldspars release Sr2+ ions in solution which, like the calcium ions, are reprecipitated in limestones, which should then show lower-than-usual 87Sr/86Sr ratios.
This is work-in-progress, but results are promising, taking into account parameters including signals of major erosion, which manifest themselves as upticks in the rates of sediment deposition, and the different weathering-rates of different rock-types. The modelling of fluctuations in the carbon and sulphur cycles for the whole of the Phanerozoic, taking weathering episodes into account, gives plots like the following:
above: ratio of mass of atmospheric CO2 (RCO2) at a past time to that at the pre-human present, calculated via the GEOCARBSULF model, with (volc) and without (no volc) volcanic rock weathering. From Berner (2006).
Several things stand out in this plot, of which the most drastic is the precipitous fall in CO2 levels beginning around 375 million years ago during the Devonian, this marking the wide colonisation of the land by photosynthetic plants. It marks an important divide in atmospheric chemistry between a land-plant poor (or plantless, before the Ordovician) world and the richly vegetated world that has existed ever since. The decline in carbon dioxide levels from the Hothouse of the late Mesozoic and early Cenozoic to the cooler world of the past 15 million years is apparent at the RHS of the plot. Older glaciations - the Karoo at 360-260 million years ago and the Andean-Saharan at 460-430 million years ago, are both marked by sharp declines in CO2.
That atmospheric carbon dioxide acts as a key driver with respect to global temperatures is without doubt: it is the detailed reconstruction of atmospheric chemistry that becomes more difficult the further back in geological time one goes. By getting a sound grasp on major, unusually intense episodes of past silicate weathering, another piece of the jigsaw will go into place and those reconstructions will continue to improve.
"The climate has always changed" has been a favourite arm-wave of the contrarian brigade for many years. What they don't tell you is why, by how much and what were the consequences. In some cases, the consequences were dire, including some of the greatest mass-extinctions this planet has experienced. Causation and magnitude of these events is a major field of research, but perturbations to the long-term carbon cycle, including major outgassing and weathering episodes, are an integral part of the overall picture and may be, in some instances, a prime cause. That we are causing one such major perturbation right now is an indication of the footprint we ourselves are going to leave in the geological record - and given our current track-record, it is not going to be something to be proud of. Indeed, I wonder what the rocks might tell, say 200 million years from now? Perhaps readers might make that the focus of the discussion!
We'll return to some of these disastrous climatic changes of the past in future posts.
Further reading:
For a good in-depth discussion of weathering, the following book is well worth finding:
Berner, R. A., 2004, The Phanerozoic Carbon Cycle: CO2 and O2: Oxford, Oxford University Press, 150 p.
The figure above was taken from:
Berner, R.A. 2006: Inclusion of the Weathering of Volcanic Rocks in the GEOCARBSULF Model. American Journal of Science, May 2006 vol. 306 no. 5 295-302.
Acknowledgement:
My thanks to Bob Berner of Yale for helpful discussions and additional reading material.
This is a minor point, but I believe there is a typo in the silicate weathering equation above the graphic. The first symbol on the right-hand side of the equation should be Ca (superscript)(2+), not (subscript 2)(superscript +).
Great post. Very informative. There are a couple of minor errors in the first graphic. The calcium ion should have the "2" after "Ca" as a superscript, not a subscript. Also there is no charge on CaCO3.
Oops, the same typo is in the equation below the first.
I see we have some great proofreaders amongst the readership! Many thanks - we'll get this fixed ASAP!
Done :)
Interesting reading John, looking forward to the next in series.
It's worth adding (I'm not sure if & how well you're going to explain it later) this rock weathering process is also considered to be primary planetary thermostat. That is the balance of the sides of Urey reaction you've shown, is self regulating: e.g. increased rate of rock weathering in response of increased solar forcing, draws down CO2 therfore cooling the planet. The opposite would slow down the weathering, causing CO2 increase from volcanic outgassing, therefore warming the planet.
Such self-regulating properties of these processes are used to explain the "young sun paradox" - the puzzle why Earth maintained relatively stable surface temp during her 4.6Ga evolution despite sun becoming 30% brighter during that time. The answer: weathering thermostat lowered CO2 levels accordingly. The gradual decline of RCO2 on your last figure in Ga timescale can also be thought as said "thermostat at work", because the sun's intensity has increased signifficantly during that time. You can see for example, that Andean-Saharan glaciation at 460-430Ma happened at CO2 levels relatively high, perhaps higher than during early Cenozoic that enjoyed hothouse conditions. That's because sun's output was weaker at that time so glaciation threshold in CO2 was higher.
Yes - I'll be writing a piece about that glaciation before too long. It's one of the most interesting episodes of drastic climate change in the geological record, it coincided with the second biggest mass extinction in the fossil record and the rocks around here record its passing. Indeed the relevant stage of the Ordovician - the Hirnantian - is named after a place not that far from where I live!
chriskoz
Yeah. This post touches on that, what happened during the Ordovician/Silurian mass extinction.
The Earth has a basic thermostat. But under the right circumstances it can go haywire. A mass extinction was the result.
Another minor typo:
CaSiO3- (ie showing an overall -1 charge) in equation in top illustration should be simply CaSiO3 (ie with no charge).
LOL I missed that one when I corrected the others. It was around 0600 this morning when I did the other ones and I hadn't had enough coffee at the time! Sorted now :)
Silicate rocks are a complex mix of (mostly) silicate minerals. These minerals do not have a fixed composition: in feldspars, for example, sodium, potassium and calcium (and other) atoms can substitute for one another, and the exact composition of the feldspar crystal depends upon both the composition of the melt from which it crystallised and the temperature and pressure at which it did so. Thus, the composition of silicate rocks is more easily and accurately recorded as a list of oxides of various amounts, rather than of minerals per se. For example, a typical basalt lava has the following composition:
SiO2 49.06%
TiO2 1.36%
Al2O3 15.70%
Fe2O3 5.38%
FeO 6.37%
MnO 0.31%
MgO 6.17%
CaO 8.95%
Na2O 3.11%
K2O 1.52%
H2O 1.62%
P2O5 0.45% (from Turner and Verhoogan, "Igneous and Metamorphic Petrology").
Giving the composition of silicate rocks in this way enables one to immediately appreciate their potential for carbon sequestration via weathering, in this case, from the 8.95% of CaO and 6.17% MgO. Another (less common) extrusive igneous rock called rhyolite has only 1.22% CaO and 0.38% MgO and has thus much less potential for carbon sequestration.
@Slioch - absolutely - the key minerals are the plagioclase feldspars (the calcic ones), mg-rich members of the olivine group and Ca/Mg rich pyroxenes and amphiboles. Granite - the coarse-grained, intrusive equivalent of rhyolite - likewise tends to have Ca and Mg contents in a similar range, present in plagioclase feldspar. However, it could be argued that the weathering of granite is still significant simply because granite is such an abundant rock-type, both in its primary state and as detritus making up sedimentary rocks. Incidentally, the propensity of rok-forming minerals to weathering is roughly as follows, starting at most susceptible and ending with least susceptible (note that there is a continuous compositional series between the sodi and calcic end-members of the plagioclase feldspar group):
Olivine, Plagioclase (calcic)
Pyroxene
Amphibole
Biotite (black mica), Plagioclase (sodic)
Orthoclase
Muscovite (white mica)
Quartz
If you visit the famous china clay-pits of Cornwall, which exploit altered and weathered granite, you will find that it is the feldspar that has decomposed: the waste-heaps left after the kaolinite (one of the weathering-products) has been extracted are the residual, relatively unscathed quartz and mica.
Absolutely a fine article, John. A great deal of the information will aid me in updating the Ph.D. I received in 1968, although I did use Turner and Verhoogan as a textbook in "Igneous and Metamorphic Petrology." I look forward to your next post. BTW, in the next version of the excellent illustration beginningthe article could you show a more obviously continuous and recently uplifted mountain range and label it as such? This would possibly have more significance to incipient geologists and emphasize the importance of exposing newly elevated rock to the weathing (re: rock) cycle.
Our decendants a few thousand years from now will be grateful for the geological carbon sink but we need to encourage and preserve carbon sinks that act a tad quicker if we are to live to leave such decendants. Fabulous article.
http://mtkass.blogspot.co.nz/2012/02/carbon-sinks.html
For the denialists, you'll need to explain
how nearly all geologic records were created by one global flood
and
how all of this occurred within 6124 years.
Bear with me.
The mindless universe started with inflation.
Soon after cooling enough atoms of H, and He and a bit of Li formed.
Some of these atoms some time later gravitationally collapsed into stars.
These early stars were responsible for nucleosynthesis of heavier nuclei.
It was only supernovae that actually produced nuclei above Iron 56.
All this newly formed matter that was strewn everywhere then finally at some cool places formed new stars and planets.
The third stone from the Sun was geologically active with ordinary chemistry to make a near habitable planet. The above is very good article.
Early mono cellular life transformed the oceans and atmosphere and forever changed the simple chemistry that was in effect up till then and it took four billion years!
Finally multi cellular life evolved. Until we came along. We are all trashing our only home because of greed.
Worth a look.
http://www.youtube.com/watch?v=9nnwvoH-4XI
Bert
As a geologist, this thread finally feels like I've found my people....:-)
Thanks to the fellow geologists who've helped me refresh my phase reaction diagrams!
(haven't commented here in a while; wow, this looks different)...
K, Na - don't ordinarily form carbonates - so what happens to them to remove them from the ocean? When more CO2 is making the ocean more acidic, would more Na and K ions act the same way on pH as dissolving CaCO3?
How much more more CaCO3 would be in solution if precipitation were all abiogenic?
Patrick 027:
Ever wondered where table salt comes from? Perhaps mined from sodium chloride precipitated from the sea? How about potash (potassium salts). which are mined extensively for use as fertilizer?
As they say, if you aren't part of the solution, you're part of the precipitate...
Bob Loblaw - thank you - (I sure hope I'm soluble enough to help) - but I assume those types of deposits form under rather restricted/special conditions - a body of water has to be isolated from the rest of the ocean to some extent (maybe not completely - epeiric seas) so that it can dry up enough to form NaCl deposits, for example (I don't know as much about potash). I didn't think the oceans continually become more saline over time in between such evaporite deposits forming. I've heard that ocean chemistry is maintained over geologic type via cycling through hydrothermal vents - are they a sink for these elements? Is biogenic sediment a significant sink?
Patrick:
Yes, specialized conditions come into play. That's why salt mines and potash mines are are found in some places, but not most. Shallow seas, in hot locations, where evaporation is high. Most of the potash that I know of is from ancient seas, currently far from any salt water body. Ditto for many salt deposits.
I'm not a geologist, so I don't know offhand where such deposits may be currently forming, but I know where I'd start to look.
Greetings. A wonderfully informative site.
Currently, I am proposiing that the weathering products of Granite, assembled conveniently in depths of over 8,000 feet thickness in "dry lake playas" located in the "Great Basin" of Nevada, be utilized to sequester CO2 in gigatonne quantities, through the importation of Pacific Ocean water; and through the simple expedient of spraying the water above the playas, whose supernatant water ranges from pH approx. 8.5 to approx. ph 10 .
in fact, i have "patent pending" staus on a patent entitled "Carbon Dioxide Direct Air Capture and Sequestration utilizing Endorheic Basin Alkaline Deposits to effect Mineral Carbonation"
excerpt:
Presuming then a 150 foot radius spray fan, and an average wind speed of 10 MPH, the volume of CO2 which will pass through the plane of the fan half-circle, per year, is ~ 400,000 metric tonnes. This is for one "spray rig", and, ultimately, thousands are envisaged. (See note __)
==
Tests are being designed to see how much of that 400,000 tonnes of CO2 can be captured.
i GUESS that between 8% and 15% can be 'caught": but this is a guess.
Obviously there is more to it than this, but I would invite consideration, questions, rebuttle, arguments, or any input whatsoever.
We are all in this together.
Thank you.
DavidNewell
David,
How much energy will it take to pump that much water from the Pacific up to the Great Basin? Where will that energy come from? Is that really the best use of that much energy? What will you do with the left over salt?
Michael:
Thank you for your question.
PHYSICAL PLANT PROPOSAL
Scale of the proposal, by comparison to an existing pumping structure
1/3 of the electricity used in California, year to year, is employed to power the Edmonston Pumping Station, which lifts fresh water across the Tehachapies, from Northern to Southern California
Edmonston Pumping Station profile:
Normal static head: 1,970 ft
Total flow at design head: 315 ft³/s (9 m³/s) (per unit)
Motor rating: 80,000 hp (60MW)
Flow at design head: 315 ft³/s (9 m³/s) (per uniit)
Total flow at design head: 4410 ft³/s (450,000 m³/h) (Combined)
Total Motor rating: 1,120,000 hp (835 MW)[2]
========================
Hypothesizing broadly, a 3/4 to 1 gigawatt power plant
could lift 1/3 the water, or ~1470 cu. feet of water second (150,000cu.M/ Hr.)
3 times as high, or ~ 6,000 feet.
This would enable a 2,000 foot "head" of pressure to effect delivery of the ocean water to wherever you may want to run the pipes: in Nevada, whose playas are at approximately 4,000 feet above sea level.
This comparison is made so that the project can be seen as imminently "doable" if the will to do so is established.
The envisioned structure would consist of pumps to convey Pacific Ocean water from (probably) the vicinity of Eureka, to lined retention basins located on the Eastern slope of the Sierra Nevada, and thence released and conveyed by piping as circumstances dictate through spray nozzles arranged on the upwind side of the playas to be so utilized. (Sketches are found in the attachment # 1)
In my sample, I used the Black Rock Desert Playa as the "example.": it is both relatively close, and better characterized than any of the many other playas which could be employed. (And is a "saline" playa, and thus not changed in this protocol..)
The spraying of ocean water through adjustable nozzles in fans of approximately 150 foot radius will generate "customizable " droplet sizes to effect varying degrees of CO2 absorption and water evaporation.
Presuming then a 150 foot radius spray fan, and an average wind speed of 10 MPH, the volume of CO2 which will pass through the plane of the fan half-circle, per year, is ~ 400,000 metric tonnes. This is for one "spray rig", and, ultimately, thousands are envisaged. (See note __)
============
It would be desirable to isolate portions of the Playa downwind of the sprayers to retain the water in shallow local ponds. The pH of the impounded waters would be monitored, and deeper levels of the playa soils would be accessed by the use of water jets to"drill down" through the consolidated clays, when necessary. As precipitated carbonates and salts build up in the impoundment, the location of the spray nozzle may be moved to a new area.
It remains the results of field tests to determine which method of spraying water will be the most satisfactory:
—The direct spraying of ocean water above the playa surface
—The vacuum induction of supernatant playa water into the nozzle stream, similar to an "aspirator" mechanism
or
— The regeneration of electricity at the spray head location so that supernatant playa water can be pumped through the nozzles.
In addition, imparting an electrostatic charge to the droplets through electrification of the spray nozzles may be contemplated, to both keep the droplets discrete from one another, and/or modify the residence time of the flight path of the droplets, as described in exhibit 2.
==============
The 3% NaCl content of the water would be of no significant concern on the already saline playa under discussion.(Of which many others of similar saline character can be found) The volume of soils in which it would be mixed, over decades, would be be of no concern. On the other hand, if it is seen as a desireable commodity, the locus of the spray heads and their localized retention podns can be changed frequently, and the prior spray surface allowed to dry, at which time the NACl can be scraped up and utilized Other places. One way, or the other, it's not a problem.
===============
Cost estimates, AKA Wild Guesses:
Estimated cost of the proposal implementation:
Edmonston Pumping Station was referred to earlier as an example of scale. It cost ~$6 billion dollars to build. It flows more water than the proposal, and therefore has larger pipes, but the distance of pumping is shorter than the proposal requires. These facts tend to offset one another.
Obviously, there are many variables beyond my best guesswork which will modify the actual cost.
Should the State of California redirect the energy for the Tehachapi pumps to this project, there would be no need for the construction of another power plant.
Since this seems unlikely, in would not be unrealistic to anticipate another $10 billion in the construction of another power plant.
It may be noted that if the power plant was co-located with the pumping plant, near the ocean, that the cooling water for the power plant could be part of the water pumped, thus alleviating the environmental problems related to hot-water discharge.
==============
The scale of the investment needed is (again a guess) about what farmers have put into central pivot irrigation systems over the past 40 years or so.
"my" proposal dwells with conditions extant in my "local endorheic basin", whose relatively recent collection of huge quantities of cations from the local fresh granite surface seem, to my eyes, to be a relatively rare occurrence:
however, there may be many other places in the world where it would be easier to provide ocean water to, and spray over alkaline clay-like soils: but I'm not familiar with them. It IS unlikely that the huge QUANTITIES of material are to be found elsewhere: (as is found in the Great Basin...) but if the source water is cheaper to obtain, then it's worth considering.
==================
If it works, it is of course worth the energy, despite the APA's summation of 18 months ago, which is that "Direct Air Capture" is a non-runner.
The differences in this are:
the huge area of sprayed liquid constitutes "the collector", and this offsets the fact that th pH is "moderate", not "highly" alkaline.
The "10 mph average breeze" consitutes the "air mover"
The "reactant" doesn't have to be "desorbed" from the collected CO2, at 600 degrees plus, and recycled.
There is no concern about taking "desorbed and liquified" CO2 to some other site and injecting it...
There is a certitude that the mineralization of CO2 will in fact provide "geologic interval" sequestration.
(gratuitous commentary alert..)
Also, on a personal level, I find this to be an exemplification of the (to me) apparent fact that "what we are for" on this planet, is to "help out" the planet "work", not drive towards individualized "wealth". ALL this is, is an elaboration and an "assist" to "how the planet works".
(end gratuitous comment.)
Please throw me another objection
I appreciate the opportunity to dialogue with people about this premise, who have an understanding of the perilous times we are in.
Sincerely,
David
Oh yeah, and obviously the technique is imminently scaleable.
David,
It strikes me that it is unrealistic to imagine that 100% of the CO2 that passes the waer jet would be absorbed. Find some data to support your claim, or withdraw it. The volume of water you want to move seems to me to be much too small to absorb a significant amount of CO2. You realize that
The volume transport of the overturning circulation at 24 N has been estimated from hydrographic section data ([4]) as 17 Sv (1 Sv = 106 m3/s), source
The currents off of Chile also move a tremendous volume of water. How can you compare the amount of CO2 your scheme would remove to the CO2 those currents absorb?
Where will all the salt go?
According to the state of California, the total electricity generation in California is about 74,000 MW. Since you claim at post 24 that 835 MW is 1/3 of the power used in California I am inclined to think you need to check your math much more carefully. It is unreasonable to imagine that 1/3 of the power in California is used to pump that single water station. In general, the amount of water that would need to be pumped is far beyond any reasonable amount that could be pumped. Where would the gigantic amount of energy come from?
Michael, not that it make a lot of difference but 74GW is capacity not generation. From the actual generation, it seems average generation is 22000MW.
I have sometimnes wondered about an alternative approach to capturing and sequestering CO2, piggy-backing of the fact that we may well need to look at geo-engineering answers down the track.
The most technically viable geoengineering answer seems to be injecting large volumes of aerosols into the atmosphere to reflect sunlight and cause some cooling. Certainly it is a very far from ideal answer since you still get a lot of other climate impacts, even if aggregate warming were reversed, and it does nothing about ocean acidifcation. But we may need to go down that road anyway.
So, since we are possibly going to put 'something' up there any way, why not look at whether we can get that 'something' to do double duty and help with removing CO2 as well as reflecting sunlight. If we lofting large quantities of microscopic particles, the surface area of all that material is enormous - perfect for enhancing rates of chemical reactions.
What if we simply crushed the very terrestrial rocks that are involved in weathering into fine dust and lofted that into the atmosphere as an aerosol, massively accelerating the natural rate of weathering. Instead of Carbonic acid falling from the sky and weathering happening on the ground, we might instead have see things like calcium bicarbonate falling in the rain.
Would that sequester carbon? Or would it just mess up the chemistry of the oceans even further?
Failing that, could some smart chemist come up with another reaction that we could use that would achieve a similar result. Up there wwe have some wonderful resources; CO2, H2O, O2, N2 and energy in the form of sunlight. All the buidling blocks for most of the known chemicals found in Organic Chemistry. Surely someone can think of a chemical pathway that will work.
The surface area of all those aerosols up there is huge. A sphere with a surface area of 1 m2 has a diameter a bit over 1/2 meter. Split that into micro spheres with a diameter of a micron and the total surface area increases by a factor of over 1/2 a million!
A precious, precious resource.
And if we can get the chemistry right, a far far more efficient method of trying to draw down CO2 than any of the more mechanical approaches being considered down on the surface.
I would suggest that discussions of geoengineering methods (as per davidnewell above) be taken to an appropriate thread on geoengineering.
KR: My apologies, I am new to this site, and have not explored all it's nooks and crannies.
this seeme to be "on subject", dealing essentially with "weathering of rock" , but I would be glad to start another thread, or do whatever I can to cooperate.
meanwhile, the following is in response to Michael's observations
In response to:
michael sweet
"David,
It strikes me that it is unrealistic to imagine that 100% of the CO2 that passes the waer jet would be absorbed. Find some data to support your claim, or withdraw it."
Michael:
I cannot withdraw an assertion I did not make.
No assertion that 100% would be captured was projected. If you COULD capture 100%, you most definitively would not want to, as downwind vegetation would be significantly affected.
My WAG as stated above, is here replicated for your reading convenience:
=
quote:
"i GUESS that between 8% and 15% can be 'caught": but this is a guess."
Unquote:
Further, the following tests are indicated:
from the paper:
"Attachment three" details these tests, (as well as a proposed field trial), but following is a summary of them.
1. Drift droplets of pure water, ocean water, and ocean water from over playa soils, through various sized nozzles, through an enclosed cylinder of air, while monitoring the reduction in the amount of CO2 in the air over a time interval.
This experiment will show the effectiveness of the proposed water source options, and the effects of changing the droplet size.
2. Perform the same tests through an extended archway with a specified rate of air flow through it, measuring the humidity and depletion % of CO2 in the air at the exit.
This experiment will generate a better profile of anticipated effectiveness of air droplets rising and falling in the proposed construct.
Proposed field trial:
3. An approximately 30 HP water pump should be provided to pump basic pH ~9.1) and saline Carson Sink water into the air in a breeze, utilizing the "tuneable spray nozzle" of "attachment one" (or similar), with upwind and downwind measurements of CO2.
This test will demonstrate the more expensive option, (which is likely to be the most effective), which relates to dumping water on the playa after it has been used to generate electricity, and using the power produced to drive a pump which sucks up the water again and produces the spray.
A "Request for Proposal" (RFP) is being drawn up in regard to these tests, and will generate actual quotes
==================
You (Michael) said:
"The volume transport of the overturning circulation at 24 N has been estimated from hydrographic section data ([4]) as 17 Sv (1 Sv = 106 m3/s),
The currents off of Chile also move a tremendous volume of water. How can you compare the amount of CO2 your scheme would remove to the CO2 those currents absorb?
Mr. Sweet, your source, http://www.pik-potsdam.de/\u126 ~stefan/thc_fact_sheet.html, doesn’t open.
Perhaps I’m missing something, but it appears you are speaking of the mass transport of the current, whereas the key point of the above proposal is a function of the surface area .
Would you like to see a calculation of surface area on the flow of water subdivided into various size droplets sprayed through the air?
(As stated by Glenn in post # 28:
The surface area of all those aerosols up there is huge. A sphere with a surface area of 1 m2 has a diameter a bit over 1/2 meter. Split that into micro spheres with a diameter of a micron and the total surface area increases by a factor of over 1/2 a million!)
michael,
Do you think that the principle mode of ocean CO2 absorption occurs directly across the surface of the ocean, or do you think it may significantly derive from rainfall and ocean spray?
What do you think happens to mists of neutral water collected in a tray after falling through 8 feet of fall?
The pH goes through interesting curves,, but winds up around pH 5.6 if I recall.
You asked (again, and I will answer again)
Where will all the salt go?
The 3% NaCl content of the water would be of no significant concern on the already saline playa under discussion.(Of which many others of similar saline character can be found) The volume of soils in which it would be mixed, over decades, would be be of no concern. On the other hand, if it is seen as a desireable commodity, the locus of the spray heads and their localized retention podns can be changed frequently, and the prior spray surface allowed to dry, at which time the NACl can be scraped up and utilized Other places. One way, or the other, it's not a problem.
Uou observed:
According to
the state of California, the total electricity generation in California is about 74,000 MW. Since you claim at post 24 that 835 MW is 1/3 of the power used in California I am inclined to think you need to check your math much more carefully. It is unreasonable to imagine that 1/3 of the power in California is used to pump that single water station. In general, the amount of water that would need to be pumped is far beyond any reasonable amount that could be pumped. Where would the gigantic amount of energy come from?
Thank you, Michael, I can’t find my source, and it appears to be in error anyway, I will retract that.
The salient point intended to be communicated, was that the "power source" for the enterprise can be compared to existing plant used for the same purpose: moving water.
Regards,
David
KR:
I've read the "comments policy" , KR, and also have read the "geoengineering" thread.
I manage another large bulletin board, and encounter your "this belongs better elsewhere" notation fairly frequently. However, once there have been responses and evolution of the thread, generally moving it causes more problems of meaning.
Yes, the proposal is "geoengineering", but more specifically, it utilizes the "weathering of rock" propereties to effect it's end. (As stated, "effectiveness" needs to be quantified by a third party independent lab..) (will be done within 6 months.)
I am of the opinion that we MUST effect direct air capture, or reap a whirlwind that will be unthinkably destructive. I hope for all our sakes and subsequent generation's sakes, that this approach works, because it is one of very few that appear to be scaleable.
thank you.
David
davidnewell - Sites vary, but on SkS most visitors follow the Recent Comments page, leading directly to conversations in progress.
I would opine (personal opinion) that weathering and the long term natural carbon cycle are quite distinct from geoengineering, whether that involves granite chips or artificial aerosols or orbital mirrors or iron seeding of oceans, etc. And that discussions of natural checks and balances are a very different topic from modifying nature for our goals.
KR, whatever Skeptical Science decides is appropriate is fine with me.
In point of fact, I am loathe to look at "geoengineering" wiith anything but a seriously "jaundiced eye", as in general they (the techniques) smack of just more wild-eyed demonstration of the human intellect's capacity to dick around with things we really have a poor understanding of: trying to use a hammer to adjust a wristwatch.
That being said, "this" technique takes a natural process (mineralization of CO2) and enhances it, in a natural way. (Spraying water in the air, duh!)
"This" technique, if shown to be harmful in any way whatsoever, can be discontinued, modulated,
: and if shown to be valuable, enhanced, expanded, etc..
============
It is an unfortunate fact that the "intellect / ego" has ignorantly gotten us into this mess,
but unlike what the flower children might want, which is to return to teepees and hunter-gathering, (which by the way might be OK in the long run, I've no opinion on the matter..)
what WE have to do is employ the SAME imagination and intellect, but in a different way, to reduce the ramifications of the die we've already cast.
"What do we do to make life prosper????" on this planet, whose wholeness is sometimes called "Gaia" by some: (although it does expose onto some abuse in these parts. )..THAT's the question we each should be continuously tryiung to answer and act on. .
So how do we decrease circulating CO2?
This methodology, utilizing the weathering products of granite, to both sequester CO2 and increase water vapor in the air, appears to be the best, most innocuous, route suggested, to the best of my knowledge of specific geoengineering suggestions, such as the others in your response. .
All speculative, but if there's a better speculation out there, let's hear it!
Thank you.