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What they are discussing is simply an acceleration of a natural process: the carbonate-silicate cycle[1]. Silicate rocks are transformed into carbonate rocks th

by dmm 8y ago

What they are discussing is simply an acceleration of a natural process: the carbonate-silicate cycle[1]. Silicate rocks are transformed into carbonate rocks through weathering. This captures CO2 from the atmosphere. When carbonate rocks are transformed back into silicates through metamorphosis or magmatism the CO2 is released back into the atmosphere. This definitely has the potentially for the long-term capture of CO2. For example in about 600 million years increased solar output will disrupt the carbonate-silicate cycle, resulting in the increased weathering of rocks and the capture of enough CO2 that C3 plants, which make up 99% of existing plant species, will no longer be able to live on earth. [2] [1] https://en.wikipedia.org/wiki/Carbonate%E2%80%93silicate_cycle https://en.wikipedia.org/wiki/Carbonate%E2%80%93silicate_cyc... [2] https://en.wikipedia.org/wiki/Timeline_of_the_far_future https://en.wikipedia.org/wiki/Timeline_of_the_far_future

DoctorOetker 8y ago
The Carbonate Silicate cycle you linked also states: >These dissolved minerals are eventually carried by water to the ocean, where they are used by living organisms such as foraminifera, radiolarians, coccolithopores, and diatoms to create shells of CaCO3 (calcite) or SiO2 (opal) through the reactions Ca2+ (aq) + 2HCO3− (aq) → CaCO3(s) + CO2(g) + H2O(l) (for calcite precipitation) and SiO2(aq) → SiO2(s) (for opal precipitation). Observe how half of the carbon is transformed back into CO2, and of the remaining half most will simply return carbonate after death and it's just a small portion of the latter half that falls to the sea floor to end up buried...
- dmm 8y ago
  That makes sense, thanks
- ajnin 8y ago
  Why would the CaCO3 return to carbonate after death ? In the past skeletons of diatoms dropped down to the ocean floors in huge layers which are now limestone rock.
  - DoctorOetker 8y ago
    You are certainly correct that there is limestone rock and that it derives from skeletons of diatoms etc.. Consider this (caricaturized) argument: suppose most artificial mummies are mummified humans, that does not mean most humans end up mummified! please check out the sibling comment by philipkglass, the referenced text has a section on calcite compensation, and the previous section detailing the saturation horizon. At high enough pressure calcite prefers being dissolved again. Consider a calcite diatom cap, close to the beach, since it is above the saturation horizon, it won't dissolve soon, now consider a diatom dropping dead over the mariana trench, so the calcite starts sinking, until it passes the saturation horizon, now the pressure is clearly high enough to prefer being dissolved...
- philipkglass 8y ago
  As btilly has mentioned a few times, ocean acidification is a major problem because atmospheric CO2 has risen so quickly. It takes a few thousand years for the atmosphere, the upper portion of the ocean, and the deep ocean to reach equilibrium. There is an especially elevated CO2 concentration and corresponding lowered pH in the upper layers because there hasn't been time to reach equilibrium with the larger volumes of water in the deep ocean. These upper layers of the ocean, sadly, are also where the vast majority of ocean life and photosynthetic activity exist. Adding alkalinity to the oceans via accelerated silicate weathering can protect sea life against acidification. More alkalinity in the ocean also increases its absorption capacity for CO2, which reduces warming feedback effects caused by radiative forcing from CO2 in the atmosphere. The ocean contains ~60x as much inorganic carbon as the pre-industrial atmosphere; to draw down the content of the present atmosphere to pre-industrial levels, it would require increasing the dissolved inorganic carbon content of the oceans by less than 1%. Converting some of it to solid carbonate minerals is a nice-to-have but not especially critical, because average residence time of dissolved inorganic carbon is about 200,000 years, which will take us well beyond the age of fossil fuels. https://www.soest.hawaii.edu/oceanography/faculty/zeebe_files/Publications/ZeebeWolfEnclp07.pdf https://www.soest.hawaii.edu/oceanography/faculty/zeebe_file...
  - DoctorOetker 8y ago
    I am really grateful for the text you mentioned, but it did not change my opinion on this technology as a carbon removal technology, it only confirmed what I understood. However, it is very well written and I will probably reference others to it whenever I can. The only minor mistake I could detect was a "boron" when clearly carbon was intended. If you have a similar quality text detailing the relevant parameters of adding carbonated rocks to counter the acidification I would be interested. I was discussing the claimed carbon removal aspect, so I now disregard the acidification aspect... In my analogy I stand corrected and should have stated that the bottle of water represented the surface seawater, since there is no free fast oceanic mixer. The surface water is equilibrating much faster with atmosphere than the whole water column of the ocean. You defend the technology by mentioning that the average residence time is about 200.000 years, but that is for all inorganic carbon across the whole depth of the ocean, not just the surface layer! As you say there is not enough mixing, so dissolved inorganic carbon in surface seawater has a much shorter residence time. The long residence time is dominated by the slow movement of deep sea water... So unless the proposal is augmented with either a huge oceanic mixer, or with dumping the carbonate over the mariana trench (if it dissolves there, it will take a long time before it reaches atmosphere again), lacking these augmentations we are dissolving carbonates in surface seawater, and the carbonate ions can equilibrate back to CO2 so it really is just emissions foisted of as capture... If it stays in the solid say calcite CaCO3 state as opposed to being dissolved, then it does not affect the alkalinity of the surface water...

by dmm 8y ago

DoctorOetker 8y ago

The Carbonate Silicate cycle you linked also states: >These dissolved minerals are eventually carried by water to the ocean, where they are used by living organisms such as foraminifera, radiolarians, coccolithopores, and diatoms to create shells of CaCO3 (calcite) or SiO2 (opal) through the reactions Ca2+ (aq) + 2HCO3− (aq) → CaCO3(s) + CO2(g) + H2O(l) (for calcite precipitation) and SiO2(aq) → SiO2(s) (for opal precipitation). Observe how half of the carbon is transformed back into CO2, and of the remaining half most will simply return carbonate after death and it's just a small portion of the latter half that falls to the sea floor to end up buried...

dmm 8y ago
That makes sense, thanks
ajnin 8y ago
Why would the CaCO3 return to carbonate after death ? In the past skeletons of diatoms dropped down to the ocean floors in huge layers which are now limestone rock.
- DoctorOetker 8y ago
  You are certainly correct that there is limestone rock and that it derives from skeletons of diatoms etc.. Consider this (caricaturized) argument: suppose most artificial mummies are mummified humans, that does not mean most humans end up mummified! please check out the sibling comment by philipkglass, the referenced text has a section on calcite compensation, and the previous section detailing the saturation horizon. At high enough pressure calcite prefers being dissolved again. Consider a calcite diatom cap, close to the beach, since it is above the saturation horizon, it won't dissolve soon, now consider a diatom dropping dead over the mariana trench, so the calcite starts sinking, until it passes the saturation horizon, now the pressure is clearly high enough to prefer being dissolved...
philipkglass 8y ago
As btilly has mentioned a few times, ocean acidification is a major problem because atmospheric CO2 has risen so quickly. It takes a few thousand years for the atmosphere, the upper portion of the ocean, and the deep ocean to reach equilibrium. There is an especially elevated CO2 concentration and corresponding lowered pH in the upper layers because there hasn't been time to reach equilibrium with the larger volumes of water in the deep ocean. These upper layers of the ocean, sadly, are also where the vast majority of ocean life and photosynthetic activity exist. Adding alkalinity to the oceans via accelerated silicate weathering can protect sea life against acidification. More alkalinity in the ocean also increases its absorption capacity for CO2, which reduces warming feedback effects caused by radiative forcing from CO2 in the atmosphere. The ocean contains ~60x as much inorganic carbon as the pre-industrial atmosphere; to draw down the content of the present atmosphere to pre-industrial levels, it would require increasing the dissolved inorganic carbon content of the oceans by less than 1%. Converting some of it to solid carbonate minerals is a nice-to-have but not especially critical, because average residence time of dissolved inorganic carbon is about 200,000 years, which will take us well beyond the age of fossil fuels. https://www.soest.hawaii.edu/oceanography/faculty/zeebe_files/Publications/ZeebeWolfEnclp07.pdf https://www.soest.hawaii.edu/oceanography/faculty/zeebe_file...
- DoctorOetker 8y ago
  I am really grateful for the text you mentioned, but it did not change my opinion on this technology as a carbon removal technology, it only confirmed what I understood. However, it is very well written and I will probably reference others to it whenever I can. The only minor mistake I could detect was a "boron" when clearly carbon was intended. If you have a similar quality text detailing the relevant parameters of adding carbonated rocks to counter the acidification I would be interested. I was discussing the claimed carbon removal aspect, so I now disregard the acidification aspect... In my analogy I stand corrected and should have stated that the bottle of water represented the surface seawater, since there is no free fast oceanic mixer. The surface water is equilibrating much faster with atmosphere than the whole water column of the ocean. You defend the technology by mentioning that the average residence time is about 200.000 years, but that is for all inorganic carbon across the whole depth of the ocean, not just the surface layer! As you say there is not enough mixing, so dissolved inorganic carbon in surface seawater has a much shorter residence time. The long residence time is dominated by the slow movement of deep sea water... So unless the proposal is augmented with either a huge oceanic mixer, or with dumping the carbonate over the mariana trench (if it dissolves there, it will take a long time before it reaches atmosphere again), lacking these augmentations we are dissolving carbonates in surface seawater, and the carbonate ions can equilibrate back to CO2 so it really is just emissions foisted of as capture... If it stays in the solid say calcite CaCO3 state as opposed to being dissolved, then it does not affect the alkalinity of the surface water...