The Carbon Cycle: a geologist’s view

Executive Summary

  1. IPCC AR5 carbon cycle model sees 8.9±1.4GtC emitted in a non-specified year. 2.6±1.2GtC was sequestered in land biomass, 2.3±0.7GtC sequestered in the oceans mainly by plankton and 4GtC remained in the atmosphere. These numbers are estimates but seem to be a reasonable rendering of current understanding.
  2. Since the beginning of the industrial revolution a total of 240GtC from human emissions have accumulated in the atmosphere while a similar amount has been sequestered by the non-permanent reservoirs of the deep ocean and terrestrial biomass, soils and biodetritus. What will be the fate of the emissions C in these non-permanent reservoirs and of that which remains in the atmosphere?
  3. The IPCC favour the Bern model for removal of C from atmosphere to sinks. This is founded upon the concept of reservoirs that have different “speeds of response” using 4 Taus (half life) of 1.2, 18.5 and 173 years and 1 Tau with infinity. The latter two represent 48% of emissions which basically says that 48% of emitted CO2 is going to hang around for a very long time. I am not aware of any physical basis for this approach which I argue is deeply flawed.
  4. Emissions can be matched to the evolution of the atmosphere using a single exponential decay model with decline rate of 2.8% representing a half life of about 24 years. If emissions can be modelled using a single exponential, why use 4?
  5. The concept of “reservoir speeds” is flawed. The appropriate way to approach this problem is to look at the amount of C sequestered by individual reservoirs on an annual basis. This approach shows that annual primary production of marine life of 50GtC that exports 13 GtC to the ocean depths is far more significant in the short term than rock weathering that perhaps deposits 0.2GtC into the shallow ocean.
  6. The concept that rock weathering by carbonic acid represents an important sink for CO2 emissions is a difficult one for a geologist to understand. All that this process does is to convert a relatively minuscule quantity of CO2 to HCO3 and deposits this in the ocean via river water. The CO2 simply takes a different route compared with direct absorption of CO2 into the oceans from the atmosphere where it is converted to HCO3 automatically by the pH equilibrium.
  7. The deep oceans have far higher C content and lower pH than surface waters. This makes it virtually impossible for C to be absorbed into the oceans to be sequestered by the solubility pump that does not appear to exist at the present day apart from the initial solution of CO2 into surface waters as part of the annual atmosphere – ocean flux.
  8. It appears that virtually all of the manmade emissions that are sequestered are removed by photosynthesis, trees on land and phytoplankton in the oceans. Land based sequestration is in live and dead biomass, biodetritus and soils. In the oceans, dead plankton sink quickly by gravity taking organic material and carbonate into the deep ocean where it is stored. It is relevant to ask for how long these sinks can go on absorbing ever larger quantities of CO2.
  9. If emissions were to be switched off, the biological pumps would continue working and would pump down atmospheric CO2 to pre-industrial levels in around 120 years (5* the 24 year half life). The fate of the emissions stored in these temporary reservoirs is another issue. Rates of geological (permanent) removal appear to be very slow, roughly 0.4 GtC per year. And so it would take  about 3250 years to remove a 1300 GtC slug of C. This “slow” process is removing C from non-permanent reservoirs, not from the atmosphere that will return to pre-industrial levels of CO2 quickly.

This post has grown out of a lengthy conversation with climate concerned commenter Dennis Coyne following my last post on this topic. We are trying to find the answer to two basic questions 1) where has the 56% of manmade emissions that have been sequestered gone? and 2) how long will the 44% of emissions that have accumulated in the atmosphere linger if emissions were to be switched off?

The latest IPCC carbon cycle model, which looks like a credible piece of work, is a good place to begin (Figure 1). In summary (all figures GtC per year for a non-specified year in the recent past):

Fossil fuel emissions and cement production 7.8 ± 0.6
Net land use change 1.1 ± 0.8
Total manmade emissions 8.9 ± 1.4

Net land biomass uptake 2.6 ± 1.2
Net ocean uptake (mainly biomass) 2.3 ± 0.7
Atmospheric increase 4.0
Total atmosphere + sequestered 8.9 ± 1.9

In this post I question some popular beliefs surrounding fast and slow sinks, the solubility pump, the rock weathering sink and CO2 sequestration by ocean currents. I also dwell on the ocean carbon cycle that is dominated by biological activity where 11 Gt of organic carbon derived from plankton dies and sinks to the ocean depths every year! This is a lengthy and complex post where I lay my neck on the line on a number of issues.

Figure 1 The carbon cycle from IPCC AR5.

When I began writing this post I was working off the Grid Arendal carbon cycle that I guess should now be viewed as an outdated cartoon. That model had so much wrong with it my blood pressure was up. I’m pleased to say that most of the concerns I had are addressed by the AR5 carbon cycle (Figure 1). And so part of the objective of this post is to provide a summary of the more up to date data and to try and lay to rest many misconceptions circulating in the minds of bloggers, journalists, politicians and I dare say some climate scientists. It is a vast and complex topic. It is inevitable that some of what I have to say will be wrong. Some of you may think it is all wrong 😉

Where have the emissions gone?

A good starting point is to understand that about 56% of manmade emissions have been sequestered and that even as emissions have grown, about 50% are sequestered every year. I am using the emissions and atmosphere model developed by Roger Andrews, considered superior to official data since it combines emissions from fossil fuels, cement and deforestation. Roger’s model runs from 1910 to 2010. Because of the complexity of carbon geochemistry (next section) it is also convenient to express all numbers as billions of tonnes of carbon (GtC) as opposed to CO2. All numbers are approximate.

Atmosphere 1910: 640 GtC
Atmosphere 2010: 828 GtC
Increase 1910 – 2010: 188 GtC
Emissions 1910 – 2010: 425 GtC
Sequestered emissions: 425-188 = 237 GtC = 56% of total emissions for the period

And what are the active sinks and their sizes:

Terrestrial vegetation (trees): 450 to 650 GtC
Soils and bio-detritus: 1500 to 2400 GtC
Marine organisms: 3GtC*
Surface ocean water: 900 GtC
Intermediate and deep oceans: 37,800 GtC
Atmosphere: 829 GtC
Total: 41,482 to 42,582 GtC
Total less atmosphere: 40,653 to 41753

* 3 GtC marine organisms alive at any one time masks a gigantic 50GtC annual productivity.

The total emissions 1910 to 2010 represent 1.0% of all sinks and sequestered emissions of 237 GtC represent 0.6% of all sinks excluding atmosphere. Emissions are relatively tiny compared with the size of the sinks. Whether or not the deep oceans are a potential sink for removal of man made emissions is clearly an important part of the story.

The carbon dioxide – bicarbonate continuum

Carbon dioxide – carbonate geochemistry is complex and to be honest I don’t fully understand this myself. I’m not sure many people do. Four main chemical species are involved:

carbon dioxide: CO2
carbonic acid: H2CO3
bicarbonate: HCO3-
carbonate: CO3–

The reaction goes:

CO2 + H2O   =   H2CO3   =   HCO3- + H+

The reaction can go either way, from left to right or from right to left. The equilibrium is dependent upon temperature, pressure and pH (acidity). Note the production of an H+ ion that is the cause of concern in ocean acidification. In the surface ocean environment it is pH that exerts main control and Figure 2 shows that ocean surface pH of around 8 strongly favours the bicarbonate species often referred to as dissolved inorganic carbon or DIC.

Figure 2 The dependence of carbon molecule / ion species on pH. The surface ocean has pH of about 8 and hence strongly favours bicarbonate ions. The deeper oceans have pH approaching 7 where a little CO2 will co-exist with bicarbonate.


The Misconception of Fast and Slow Sinks

If we look at the oceanic part of the Grid Arendal Carbon Cycle (Figure 3) we see three different kinds of data 1) the size of Carbon reservoirs in GtC , 2) the flux between the reservoirs in GtC and 3) the rate of process denoted by colour (See Figure 2). The rate of process is a link between the size of the flux and the size of the reservoir – fair enough. The flux figures are all per annum and so in fact the amount of CO2 exchanged each year between the atmosphere and ocean surface is about the same as between surface and deep water. These two processes are operating at the same rate, its just that the deep ocean cycling is slow compared with the vast size of the reservoirs involved. This kind of representation and others like it have perpetuated the misconception of sinks operating at different speeds.

As far as I can tell from looking at the AR5 carbon cycle virtually all CO2 is removed from the atmosphere by photosynthesis (trees and phytoplankton) and sequestered emissions are being stored in the active non-permanent sinks. The rate of permanent removal from those non-permamnent sinks is so slow (geological) it can all but be ignored for human time scales. The question boils down to for how long can the non-permanent sinks go on absorbing manmade emissions?

Figure 3 The ocean carbon cycle according to Grid Arendal.

The Myth of The Solubility Pump

I have always understood there to be two main marine processes for the removal of CO2 from surface to greater ocean depths. The first being the biological pump and the second being the solubility pump. Houghton in fact devotes little space to describing these processes on pages 32 to 36 of his book [1]. For the solubility pump I envisaged either diffusion leading to surface C being pumped away or mechanical mixing processes burying C laden surface waters in the depths.

I was consequently fairly surprised to learn that the deeper ocean layers have considerably higher C contents than the surface. This is particularly so for the Pacific Ocean that is significantly different to the Atlantic Ocean (Figure 4). Not far below the surface of the Pacific Ocean we in fact encounter water with pH below 7.3 – it is almost acid! (see also Figure 5). The reason for this is rotting marine organic matter at depth the dissolution of carbonate rain from the surface (see below).

Diffusion normally works from high to low concentration, hence it is impossible for diffusion to remove C from the surface since deeper waters have the higher content.

Furthermore, mixing shallow with deeper water will inevitably result in an increase not a decrease of the C in the surface layer. I may stand to be corrected but I cannot see how C in solution in surface waters can be sequestered into deeper waters by a solubility pump.

Figure 4 Comparison of pH and C content of the Pacific and Atlantic Oceans [2]. The deep Pacific has much lower pH and higher carbon content than the Atlantic.

Figure 5 Pacific Ocean pH with depth along a transect from Alaska (right) to Hawaii (left). Note different depth scales on upper and lower panels. The upwelling of low pH deep water is I believe “the far end” of the oceanic thermohaline circulation (Figure 6) [3]. Deep water upwelling such as this brings vital nutrients for plankton from the deep ocean into surface layers. Note that the source of this image is a teaching pdf that contains many great slides.

The Myth of The Rock Weathering Sink

It seems to have become engrained in the folk lore of climate science that weathering rocks somehow creates a sink for CO2. The first time I read about this some years ago in a paper by J. Hansen I didn’t understand it and I do not understand it today. The storyline goes CO2 combines with water to make carbonic acid that weathers rocks and is subsequently fixed into river water as calcium bicarbonate ions and shortly thereafter gets dumped in the sea. The flux is small, around 0.4 GtC (gross) according to IPCC AR5.  This gets blown into a huge amount by integrating the effect over geological time. I contest that this is not valid. The correct approach is to compare the size of the annual flux with other processes. Rock weathering merely moves a tiny quantity of CO2 from atmosphere to ocean via a different route. Furthermore, since speciation is pH dependent, and waters associated with silicate bedrocks are often acid, it is not clear to me that the bicarbonate ion would be favoured.

The AR5 carbon cycle does in fact show 1GtC per annum being released back to atmosphere from the riverine flux (freshwater outgassing). The riverine system works as follows:

Rock weathering: 0.4 GtC
Export from soils to rivers: 1.7 GtC
Total input to rivers: 2.1 GtC

Freshwater outgassing: 1 GtC
Burial in estuaries and deltas: 0.2 GtC
Export from rivers to oceans: 0.9 GtC
Total export from rivers: 2.1 GtC

Half of the rock weathering component seems to go straight back into the atmosphere and the approximate 0.2 GtC that makes it to the sea is insignificant compared with the ±80 GtC ocean-atmosphere and ±120 GtC biosphere-atmosphere exchanges. I find it hard to conceive how rock weathering, a process that accounts for 0.1% of the annual carbon cycle has been elevated by climate scientists to be one of the most important controls on Earth’s CO2 budget and temperature history.

What rock weathering does do is to liberate cations into solution that may eventually become the salt in sea water or the Ca in limestone. But there are a host of weathering processes that can do this, for example the hydrolysis of plagioclase feldspar to the clay mineral kaolinite. Much of rock weathering in fact takes place as a result of soil formation where organic acids released by tress play a crucial role.

Totally absent from the rock weathering debate is the potential liberation of CO2 from the vast C reservoir that rocks contain. Grid Arendal sees between 66,000,000 and 100,000,000 GtC in those sinks, the biggest number by far on the chart. Bacterial processes for example consume C and produce CO2.

The Myth of Ocean Currents Sequestering CO2

Time to bust a myth that I myself flirted with recently (though I never stated this explicitly) and that is that oceanic thermohaline and meridional overturning may remove C laden waters from the surface and bury them at depth. You just need to look at Figure 4 to see how impossible this concept is. For a start, surface waters are depleted in C relative to deep waters. The oceanic carbon cycle (Figure 1) shows that overturning of deep to shallow waters introduces 11 GtC  from deep to shallow layers each year instead of removing it. Oceanic circulation in general may sequester surface water depleted in C in the seas between Norway and Greenland but these return to surface as deep waters enriched in carbon and other nutrients in the Indian and Pacific Oceans (Figure 6). Upwelling of deep low pH, nutrient enriched waters is a key part of the biological pump component of the ocean carbon cycle (see below).

Figure 6 Oceanic thermohaline circulation. Source is UK Met Office.


The Ocean Biological Pump

The representation of the ocean biological pump [4] shown in Figure 7 bears important information. It shows 3 Gt organic carbon in the surface ocean and this tallies with the number in the AR5 carbon cycle (Figure 1). But it also shows net primary production of 50 Gt per year and net export of 10 Gt C per year. This is shown in Figure 1 as the 50Gt per annum transfer from surface water to marine biota. The Tau (half life) for surface ocean C is only 0.06 years – 22 days! This is not the Amazon forest, growing slowly over centuries but an extremely rapid turn over of microscopic plankton.

Figure 7 The ocean  biological pump from Sigman and Haug [4]. According to this scheme the surface oceans never contain more than 3 Gt organic carbon and yet they export 10 GtC per year brought about by the extremely fast cycle of growth, death and removal of dead organisms mainly by gravity. 

There are two main types of plankton in the oceans that form the base of the food chain. Phytoplankton (Figure 8) contain chlorophyll and fix CO2 from the upper ocean layer via photosynthesis and they are considered to be plants. Zoo plankton eat the phytoplankton. Ocean algal blooms are one manifestation of phytoplankton growth (Figure 9). The deeper ocean is subject to a continuous rain of this dead organic and carbonate material that locks atmospheric C into microscopic creatures that die and sink evidently transferring about 10 GtC from upper to deeper ocean layers each year. That is a large amount!

Figure 8 Phytoplankton 100 times magnified. “Phytoplankton are single-celled, free-floating, non-swimming plants. Zooplankton, which consist of small animals and the larval forms of invertebrates and fish, together with phytoplankton make up the group called plankton. The predominant forms of phytoplankton are diatoms, golden brown algae, green algae, blue green algae, and dinoflagellates. Over 20,000 species of diatoms alone exist in the world. They have an exoskeleton composed of silica and have no means of locomotion.” Source.

Figure 9 Given right conditions, algae (phytoplankton) sometimes grow out of control to produce blooms that can be harmful. But they do remove large amounts of CO2 from surface waters. The image shows an algal bloom off the SW coast of England.

There is in fact major uncertainty over the size of this flux. The UK National Oceanographic Centre (NOC) saying [5]:

The biological carbon pump is a major term in the global carbon cycle, transferring approximately 5-15 GT C yr-1 from the surface ocean to the oceans interior (Henson et al., 2011). It is of comparable magnitude to the annual increase in CO2 in the atmosphere driven by anthropogenic remobilisation of fossil fuel reserves and without it we believe that atmospheric CO2 would be order 200ppm higher (Parekh et al., 2006). Small changes in its functioning and or strength could radically affect ocean atmosphere partitioning of CO2.

AR5 sees 13 GtC, Sigman and Haug see 10 GtC per year and NOC sees a range of 5 to 15 GtC transported from surface to deep ocean each year. Compared with annual emissions of 9Gt, the uncertainty here is quite significant. So much for settled science.

The whole picture is made more complicated by annual returns from the depths to surface that cancel much of the biological removal of C. But the main conclusion is that this process is fast, driven by photosynthesis and is storing some emissions (2.3±0.7 GtC per year) in the vast non-permanent reservoir of deep ocean water.

One aspect of the Sigman and Haug scheme shown in Figure 6 that I don’t fully understand is the bottom panel showing 15,000,000 Gt organic carbon in sedimentary rocks. That’s fine. And they also show a return to atmosphere of some of that C which tallies with what I say above must happen. It is the rate of the burial process that I don’t understand. They show 0.05 Gt organic carbon per year being sequestered by ocean sedimentation which seems far too low. The AR5 carbon cycle has a figure 0.2GtC sequestered to the oceans and another 0.2 GtC sequestered in deltas. These numbers seem very low and so I did a cross check on geological sedimentation rates to find that these figures for geological burial are in fact quite high. One concern must be that removal of emissions from non-permanent reservoirs into the geological record occurs on geological time scales that are too long to be relevant to Mankind.

Models for the sequestration of CO2

The Bern model, favoured by the IPCC, is based on sinks acting at different speeds removing CO2 from the atmosphere which I have already argued is an invalid approach. From the preceding it is possible to see that the 1.2 and 18.5 year Taus are perhaps representations of the “fast” oceanic biosphere and terrestrial biosphere pumps (see below). But as far as I can tell these fast photosynthetic processes are responsible for the removal of virtually all CO2 from the atmosphere into non-permanent reservoirs – plants, soils, bio-detritus and deep ocean water.

Time constant (Tau)   % of annual pulse removed at that rate
1.2 y                                18%
18.5 y                             34%
173 y                              26%
∞                                    22%

It seems to have become engrained in the folklore of climate science that manmade CO2 emissions may linger for many millennia in the atmosphere [6]. I do not know what evidence exists to support this notion. The scientific logic hurdle that needs to be overcome is how one views the 44% of emissions to date that have not been sequestered. The Bern view seems to be that they are hanging around waiting for “slow processes” to grab them. My view is that photosynthesis is rate / capacity limited and has not yet had time to grab them. Stop emissions today and photosynthesis will draw down the 44% of emissions that linger in the atmosphere long before glacially slow rock weathering has the chance to do anything meaningful.

The pressing question today is for how long can these non-permanent reservoirs go on absorbing ever larger quantities of carbon since permanent sequestration of organic matter into mudrocks and bicarbonate into limestones appears to take place so slowly so as to not be significant on human, multi-century time scales?

If we consider a pulse of Carbon about 1,300Gt is size (comment by Dennis Coyne) being human emissions and just over half of this will be absorbed by these non-permanent sinks over the time span that the pulse is emitted and the remainder will be absorbed in the decades following the cessation of emissions. Can the deep ocean absorb 610Gt of carbon? With a reservoir that is already 41,000 Gt it would seem to me that adding an additional 1.5% should be no major challenge.

The terrestrial biomass side of the story is perhaps not so straightforward where the reservoirs are perhaps 2500 GtC. Absorbing an additional 690Gt would represent an uplift of 28% in that C budget. Is it possible for trees to simply double in size under the influence of CO2 fertilisation? And the terrestrial bio-mass side of the story is already in difficulty owing to deforestation. IPCC AR5 actually sees a net reduction of 30Gt in that non-permanent store since pre-industrial times even although 2.6Gt are being sequestered via that route every year for the time being (Figure 1).

It should be abundantly clear that every effort needs to be made to halt or even reverse deforestation at all levels on Earth. Carbon stored in a living tree is carbon stored. Carbon stored in a fallen tree is carbon stored some of which will enter the longer lived store of bio-detritus. The carbon stored in our forests is little different to that stored in fossil fuel – it has simply been removed from the atmosphere more recently. The BIG difference between burning wood and burning coal is that the tree had the possibility to grow bigger and to remove even more carbon from the atmosphere. The practice of clearing hardwood forests to generate electricity that is labelled ‘Green’ has to be one of the most crazy policies ever devised by the Green movement. Second only to clearing forests to grow biofuel in Brazil and Indonesia.

How long will manmade emissions linger?

This series of posts on the carbon cycle began with Roger Andrews pointing out that emissions can be matched to what has actually happened in the atmosphere (Mauna Loa) using a single exponential decline function [7]. If emissions appear to fit a single exponential decline why seek a more complex explanation? Phil Chapman introduced an interesting concept based on equilibrium distribution of increased carbon between the various active sinks. At that time, the very large deep ocean sink was not in our minds active whilst in fact it appears to be one of the main destinations for emissions. Adding the deep ocean to the equilibrium distribution model makes any residual manmade CO2 in the system trivial.

Figure 10 shows what would have happened to emissions had we switched off combusting fossil fuel in 1995 according to my (and Roger’s) model. The biological pumps, energised by elevated CO2 levels, would keep pumping and would quickly draw down atmospheric CO2. But their effect declines exponentially as the PCO2 in the atmosphere is drawn down by their action.

Figure 10 The chart shows annual additions of emissions since 1965, based on Roger Andrews’ emissions model that has a component for deforestation, and how they decline. The wedge at the bottom is the pre-1965 emissions stack. A 2.8% per annum exponential provides an excellent fit between atmosphere and non-sequetered emissions. The black line shows how the atmosphere actually evolved. The red arrow shows how the atmosphere would have evolved had emissions been switched off in 1995. According to the IPCC Bern model if emissions were switched off then the atmosphere would move side ways following a horizontal trajectory. It is not at all clear to me why the processes that were sequestering the pre-1995 emissions should suddenly stop doing so if emissions were switched off.

The “best fit” single exponential for my model has a half life of 24 years and following the rule of thumb that after 5 half lifes most of the perturbation is consumed I will assert that the atmosphere will have returned close to pre-industrial levels in 120 years post switching emissions off.

At this point it is possible to build a bridge towards Bern and IPCC since in this model virtually all emissions are stored in non-permanent sinks. The AR5 carbon cycle sees only 0.4GtC per annum going into long-term geological storage. Thus, a 1300 GtC “pulse” may take 3250 years to eventually be removed from the non-permanent active sinks “forever”. That is quite a long time in which mishaps may occur. Much would depend upon the stability of equilibrium. It is possible to envisage forests grown fat on the CO2 binge, having problems when the nutrient supply gets switched off. A forest dying may return emissions CO2 to the atmosphere before it becomes geologically sequestered. But this would be a self regulating process since the increase in atmospheric CO2 that this may cause would prevent other forests dying elsewhere. In the oceans I suspect that rotting organic matter, given time, will become part of deep marine mudstones.

Up to the point when emissions are switched off 56% of all prior emissions are already sequestered into the non-permanent reservoirs. I can think of no reason why the biological processes active before 1995 should somehow stop working. The pre 1995 emissions that remain in the atmosphere would continue to be sequestered at the same rate as before, i.e. 2.8% per annum, drawing down the atmosphere to pre-industrial levels after about 120 years.


[1] Houghton Global Warming, The Complete Briefing
[2] Fun with Krill
[3] Teaching slide deck – well worth a look!
[4] The Biological Pump in the Past D. M. Sigman and C. H. Haug
[5] UK National Oceanographic Centre
[6] David Archer: Fate of fossil fuel CO2 in geologic time
[7] Roger Andrews The residence time of CO2 in the atmosphere is …. 33 years?

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24 Responses to The Carbon Cycle: a geologist’s view

  1. Peter Shaw says:

    Your Fig 1 has a small but important omission.
    I’ll simplify the chemistry slightly for the general reader.

    Most of the Earth’s solid surface is basalt rock (the seabed).
    A major mineral in this is forsterite (a magnesium silicate). Alteration of forsterite by water produces several minerals, including brucite (magnesium hydroxide).
    In natural waters, this strips dissolved CO2, with end-product some form of limestone.
    (Water from submarine mud volcanoes associated with ocean trenches can have a remarkably high pH.)

    If the AR5 diagram includes terrestrial volcanic emissions, it should also include this geochemical sequestration.

    Geology teaches of episodes of flood-basalts, probably deposited where rapid weathering can occur. As this chemistry is well capable of stripping all CO2 from the air, it shouldn’t be discounted.

    • Euan Mearns says:

      Hi Peter, I remain skeptical that weathering of sub-areal basalts would actually sequester any CO2 but rather dump HCO3- into runoff where it becomes indistinguishable from the HCO3- already in the ocean. And as you know much of the basalt on the ocean floor is covered by ocean floor sediments, probably too far removed from seawater to have any impact.

      But basalts when they are extruded at mid ocean ridges are a completely different matter where we have large scale hydrothermal systems pumping seawater through the newly formed ocean crust. Do you know if carbonate mineralisation is common in that environment? I did a quick search and did find some references to carbonate mineralisation in ophiolites.

      If you had any mineral / chemical reactions to share I’d be interested to see them. Carbonate mineralisation of mid ocean ridge basalts, if it takes place, would be a real sink, removing CO2 / HCO3 – from seawater and locking it away. Carbonate mineralisation within ocean sediments (diagenesis) is another possible process.

      • Peter Shaw says:

        Euan –
        The usual alteration sequence is olivine -> serpentine -> talc, where MgO:SiO2 ratio is (resp) 2, 1.5 & 0.75. So MgO (as hydroxide) is produced at each step.
        The hydroxide (brucite) is found in sea-bed cores around ridges low in the alteration zone, where you would expect it (ref. mislaid, sorry).
        Magnesium carbonate is (relatively) soluble, but as seawater also contains calcium (as temporary hardness) the end-product is eg dolomite, which isn’t.
        This carbon cycle has a period of ~1E7 yrs, so might be the Bern “infinity”.
        Although generally slow, this sequestration can locally be fast (as you imply for hydrothermal regions).
        Consider the numbers: 25000 km of MOR times 500m fracture depth times 5cm/yr spreading is a lot of potential sequestration.
        I see debate about how much CO2 is introduced via undersea volcanoes, but none on the removal which must accompany it.
        I think your Fig 1 (ex AR5) is incomplete.

  2. Rui N Rosa says:

    Thanks for this comprehensive study. Very relevant issues to me.

    I refer to te Executive Summary:

    Point 6:
    Comment: CO2 dissolves carbonate rocks, for instance openning up caves and carving karstic landscapes – – insoluble carbonates becoming soluble bicarbonates such that every atmospheric CO2 molecule carries an extra molecule of CO2 by underground or susface water to sea. But in silicate rock (mostly eruptive either plutonic or volcanic) silicates are converted from insoluble silicates to insoluble carbonates – and “free” CO2 is replaced by the mostly abundant SiO2 (sand, sandstones). Thre is no runoff. This natural weathering processes is thought to be a promising path to accelerate the geological sequestration of CO2 into basalts and ultramafic rocks. The extent of the ongoing natural process is difficult to quantify (missing sink?!).

    Point 7:
    Comment: It is necessary that the deep ocean is saturated in CO2 to maintain the continuous sedimentation of organic and inorganic carbon by combination of biological pumping and gravity – to form many of the sedimentary rocks we find all around and are permanently being produced.

    Point 9:
    Comment: CO2 is continually sent into the atmosphere from the crust and the mantle, through techtonic activity either volcanic or diffuse. Most of it derives from the breakup of carbonate sedimentary rocks, which are converted into silicate rocks in subduction zones, closing the geological branch of the carbon cycle. Much of that CO2 is emmitted in the oceanic ridges and volcanic arcs.

    Thanks and Regards

    • Euan Mearns says:


      On point 6 – I still don’t get it. To become a sink it has to make carbonate. There is zero permanent carbonate mineralisation going on in eroding mountain terrains. And I believe most of the chemical weathering will in fact be going on in soils. If weathering removes some CO2 then it is a tiny effect and what we would be interested in is the change in that effect with time – it is going to be minuscule.

      On point 9 – I think it is a gross simplification to see crustal carbonate going down a subduction zone to re-emerge in a volcano as the whole story. Of course this goes on but it is only a small part. Where did all the “crustal carbon” come from? Well it came from the mantle as part of the on-going differentiation of asthenosphere – lithosphere – and crust. Kimberlite pipes contain a much loved form of C we call diamond. I’m sure there is literature on this but I suspect that is primitive mantle derived C. And much of volcanism on Earth goes on along the mid ocean ridges where grabbing crustal CO2 is less straight forward than above a subduction zone.

      Much of the leaking CO2 from solid Earth to atmosphere I believe is linked to natural seeps from oil and gas deposits. But I reckon it is likely a slow process since if it were rapid there would be no oil and gas deposits left for us to exploit.

      Not sure I like your converting silicates to carbonates and vice versa.

      • Rui N Rosa says:


        CO2(g) + H2O(l) + CO3Ca(s) = (CO3H)2Ca (aq)
        so that weathering of carbonates removes a CO2 molecule from the atmosphere and one from the rock, both flowing towards the sea (where eventually are incorporated in sediment)

        Regarding weathering silicate rock,CO2 is incorporated in the altered rock to be much later returned by techtonic activity (volcanoes and methamorphism):
        CO2(g) + SiO3Ca = CO3Ca + SiO2

        Most crustal CO2 came from the primitive atmosphere, mostly captured by photosynthesis, little is left still evolving from the mantle.

  3. Roger Andrews says:

    In my geological career I’ve looked at lots of the “leached caps” that commonly cover mineral deposits. These consist of thick masses of oxidized rock – I’ve seen oxidation in drill core at depths of over 500m – in which reactive species have been oxidized to carbonates, sulfates, silicates etc. by percolating rainwater, which is mildly acidic because of its CO2 content, although the process is later enhanced by the oxidation of sulfides. These are cases in which “weathering” unquestionably acts as a carbon sink.

    • Euan Mearns says:

      Roger, I have no issue with weathering sequestering CO2 where there is evidence for carbonate mineralisation associated with it. But what we are looking for here is a process that responds quickly to produce increased uptake in proportion to increased PCO2. We know we see that in land plants. I still need to find out if this is observed in phytoplankton. The quantities of C involved in weathering in the AR5 carbon cycle are so small that any small change associated with increased CO2 will be wholly trivial unless the amplification is by factors and not fractions.

  4. Roger Andrews says:

    On a more mundane note. Euan, you say

    In summary (all figures GtC per year for a non-specified year in the recent past):
    Fossil fuel emissions and cement production 7.8 ± 0.6
    Net land use change 1.1 ± 0.8
    Total manmade emissions 8.9 ± 1.4

    I think I can specify the year – it was 2004, ten years ago

    Approximate figures for 2013 are:
    Fossil fuel emissions and cement production 9.7
    Net land use change 1.5
    Total manmade emissions 11.2

    The ±s would probably be similar.

    • The IPCC’s carbon cycle also isn’t quite as well balanced as the IPCC claims (pics from a 2013 powerpoint presentation from Columbia which contains some other interesting graphics.)

      • Euan Mearns says:

        Roger, I find it quite astonishing that they are banging on about a missing sink that just happens to be the exact same size as the net bio-mass uptake in the AR5 (2004) carbon cycle.

        The last slide, the one with the genetically modified shrunken Hobbit in it, is interesting. Are you able to work out where we are on that curve. Is 400 µmol/mol intercelular = 400 ppm CO2 in atmosphere. If it is and the effect goes assymptotic at 600 to 800 ppm then there has to be an evolutionary reason for that.

      • Dennis Coyne says:

        It is pretty clear that land would be the missing sink. Note that the error on the land use change is quite large. Have you read the 2005 and 2009 Archer papers? I would think that a geophysicist would see that see that the carbon sequestered as dissolved inorganic carbon will cycle back to the surface and be released. This and the CaCO3 cycle should not be ignored.

        • This has nothing to do with geophysics. The question is whether there are enough good observational data to allow the IPCC to present a carbon cycle cartoon that gives the impression that everything is known. I’m still working on this, trying to separate fact from fiction, and may (or may not) have something to present later. But at the moment the answer to the question is no, there aren’t.

          • dennis coyne says:

            Hi Roger,

            The cartoon is presented with uncertainties, you should read chapter 6, there is more information in the text. Your reading of the IPCC work (assuming you have done so), is very different from my own. There is a great deal of uncertainty, and generally in science we never reach the point that everything is known, that is a pretty high bar.

            Euan’s post has everything to do with geophysics, but probably more to do with geochemistry.



          • Hi Dennis

            Your link contains a couple of gigatonnes of chemical and mathematical formulae but not an ounce of hard data.

            Here’s what I mean by hard data, or as hard as we are going to get in the context of the carbon cycle:



            You might try looking at some.

          • Dennis Coyne says:

            Hi Roger,

            I have seen plenty of data. Data without a model is not useful. How well does a single exponential fit atmospheric CO proxy data from 10000 bce to 1850? The 250 ppm CO2 10000 years ago rose to about 280 ppm over 9000 years most likely due to the development of agriculture and land use change as human population grew.

            One might ask why the decline of atmospheric carbon dioxide was different in the earlier period. In other words why didn’t it return to 250 ppm?

            I assert that ignoring the carbon cycle beyond simply the biological pump is a flawed model. We need to look at how quickly DIC cycles from deep ocean back to the atmosphere and the CaCO3 cycle and ocean buffering capacity to understand the Carbon cycle.

          • Euan Mearns says:

            Ah Dennis,

            Data without a model is not useful.

            But a model built without data is useless.

            The 250 ppm CO2 10000 years ago rose to about 280 ppm over 9000 years most likely due to the development of agriculture and land use change as human population grew.

            Or maybe it just continued to rise as the oceans warmed to the interglacial. I’m certainly not arguing that the larger scale carbon cycle should be ignored, simply observing that it functions so slowly as to be totally irrelevant in trying to understand observations from the last 200 years. One of the main points of my post is to wonder about where the emissions CO2 stored in temporary sinks might end up. Those wedded to the deeply flawed Bern model are doomed to never understand what is actually going on.

    • Euan Mearns says:

      Its rather disappointing to find that AR5 is not using updated numbers. The year should actually be stamped on the diagram. If their numbers for fluxes and sequestration etc have not been updated either for 10 years then we may conclude that the IPCC have learned nothing new in those 10 years and that they take the view they were right then and they are right now.

      • One does get the impression of a “let’s not rock the boat” attitude.

        And a great deal of work on the carbon cycle has indeed been done in the last ten years.

        Another interesting exercise is to see if you can find out where the numbers in the IPCC’s carbon cycle cartoon came from – like the 123 GtC/yr into land sinks and the 118.7 GtC/yr out. I checked back through the IPCC’s references a year or two ago to see if I could discover the sources of these numbers, but without success.

  5. Euan,

    I found this article very informative, well written and argued. I was a pleasure for me to read it and i was able to follow your arguments despite not being climatologist.

    Thank you !

  6. I’ve tried to find chemical analyses of rainwater and so far have not found one that lists carbonic acid. The CSIRO published some really detailed chemical analyses of rainwater, and that data also lacks any reporting of HCO3 etc. They reported almost every element under the sun but carbon.

    So on what basis is the belief that rainwater has weak carbonic aside in it based on? pH only quantifies H+.

  7. Peter F Gill says:

    Most of the above discussion relies on the tacit assumption that if it were not for mankind the atmospheric carbon dioxide content would remain close to constant. However if atmospheric carbon content has been increasing from other causes – a consequence of past warming for example – then the focus on such things as missing sinks disappears. Partial pressure considerations suggest that in such circumstances burning fossil fuels would merely suppress carbon dioxide that would have been emitted through other causes. By the way free protons are somewhat of a rarity down here on Earth. Perhaps we should remember to talk about hydronium ions rather than H+ and while we are at it remember that this says nothing about what is providing the negative ions.

  8. dennis coyne says:


    I agree you need both data and a model. A “model” which ignores the cycle which brings carbon from the deep ocean to the surface and the CaCO3 cycle is much more deeply flawed than the modiified Bern model with Tau 13000 (21%), Tau 300 (27%) and Tau (1) (52%) which I presented earlier. The present levels of carbon sequestered are driven by high emissions, drop the emissions and the rate that carbon is upwelling from the deep ocean will exceed carbon moving to the deep ocean through the biological pump.

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