Ocean Carbonate From Rocks

When looking at the oceans, and where all that CO2 comes from, we saw a lot of it bubbling up from the ocean floor as raw CO2 (even as blobs of liquid CO2 at depth). But much of the CO2 in the ocean is in the form of dissolved carbonates.

There is a lot of angst on the part of the AGW crowd about added carbonate in the ocean from higher atmospheric levels of CO2 gas. That this would cause, somehow, various critters to have problems. (Even though CO2 levels have been much higher in the past and those same critters lived just find then). The basic premise is that we burn carbon, that makes CO2, that dissolves into the water as carbonate ions, that is evil.

Well, they better stop all the rivers from flowing then…

http://ak.water.usgs.gov/yukon/hypothesis/Eberl.php

Denny Eberl

Are carbonates (calcite + dolomite) dissolving in the Yukon River?

The Yukon River is one of the few large rivers in the world that carries large quantities of carbonate minerals (calcite + dolomite). Quantitative X-ray diffraction analyses of bottom and suspended sediments shows that these minerals originate as glacial flour derived from the St. Elias Range, where they then travel down the Donjek and White Rivers to enter the Yukon River in the Yukon Territory north of Whitehorse. In the graph below it can be seen that Yukon River bottom sediments (collected by kayakers) near Whitehorse contain about 2 weight percent carbonates. When the White River tributary enters the Yukon river, this value jumps to about 14%. (Is the White River made white by the carbonates it carries?) Thereafter the carbonates decrease to about 2 % near the end of the river at Pilot Station. Is this decreasing trend related to the progressive dissolution downstream of carbonates in the Yukon River, or is the decrease related to dilution of carbonates by tributary sediments? In order to understand the movement of carbon in the river, which we want to do to know in order to determine whether or not permafrost is melting in the basin, it is necessary to account for the possible addition of inorganic carbon to the system by carbonate dissolution.

An approximate mass balance calculation, using average percentages for the minerals in suspended sediments at the fixed stations (data for three years were used in the simple averages) and sediment flow data indicates that approximately 1.2 x 10^6 metric tons of suspended carbonate, roughly 1/4 of that carried by the river, may dissolve between Eagle and Pilot Station each year, a river distance of about 2500 km. This total includes about 30% of the suspended calcite and 15% of the suspended dolomite. Similarly, roughly 3 x 10^6 metric tons of feldspar dissolve. This includes approximately 1% of the alkali feldspar carried by the river between Eagle and Pilot Station, and 25% of the plagioclase. Roughly 6 x 10^6 metric tons of quartz dissolves over the same reach, or 3% of that carried. It is estimated that 10% of the yearly calcium ion load for the river comes from the dissolution of carbonates and plagioclase between Eagle and Pilot Station.

But what’s a few million tons of carbonate per year, every year, for millions of years… (Note that the first million tons is what dissolves just between two measuring points. The rest is still in the river being dissolved and / or carried to the ocean.)

OK, but they start with saying that it is one of the few rivers that carry a lot of carbonate minerals. Doesn’t that mean it is unusual?

It is only unusual in the concentration of suspended SOLIDS (those minerals), not in the presence of feldspar, calcite, or dolomite in the drainage basin. For those who don’t know much about rocks, the names can be daunting. I loved my geology classes, but after a couple decided I needed a different major just due to the mineralogy zoo of names driving me batty. Seems like there are a few million ways the chemistry of rocks can manifest and each little variation gets a new name (often named for some person who identified it and with little logical or predictive structure – it doesn’t ‘compress’ well). At any rate, I can relate to folks who don’t know a plagioclase from an orthoclase… But we will spend just a moment on that kind of thing. Don’t worry, it won’t be painful ;-)

Feldspar is a very common mineral. It has a metal ion or two, and then some silicon and oxygen in it. You can think of it as being like quartz (silicon dioxide) with some added sodium or potassium and then aluminum oxide mixed in.

The orthoclase family has a lot of Potassium, Aluminum and Silicon Dioxide in it. It’s also called “alkali feldspar” or potassium feldspar for that reason. So that “alkali feldspar” they talked about was this kind of mineral.

The plagioclase family is basically the same stuff, but with the sodium and calcium ions in it instead. As the percentage of any given metal ion changes, the rock physical properties change somewhat, and you get another specific name to memorize… (It was at about this point, while staring through a rock microscope and trying to identify particular grains in a rock in lab, that I made the mistake of ‘looking ahead’ in the book and seeing just where this was headed… At that point I decided I was not all that keen on spending the rest of my days memorizing all the things other folks had found. There’s a zillion different rocks…)

So we are not all that interested in the feldspar part of that story, other than to note that it means a lot of sodium, potassium, and calcium are dissolving into that water too. Every year the ocean gets a giant load of alkali metals dumped into it. So much for “acidification”…

So those feldspars matter, but to the ‘acidification’ question. It is the carbonate rocks that have the CO2 in them. While fledspars are some of the most common minerals on the earth, what about that calcite and dolomite?

The wiki says: “Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable” The first thing to realize is that there are even more kinds of carbonate rocks around the world than just the two mentioned in the article, and even the calcite comes is a few forms. The second thing to notice is that aragonite is the form used by shell forming critters to make their shells.

This raises the interesting question of just how all those fresh water clams can ever make their shells when clearly millions of tons of carbonate can be dissolved into a single river and the poor dears ought to just be leached away /sarcoff>. But that aside, it’s basically a calcium ion along with some carbonate.

Dolomite has a mixture of magnesium and calcium as carbonates. (You can also find some deposits of things like lithium carbonate or sodium carbonate too. These are more rare on land as they are very soluble in water and usually wash away. They are typically found in the desert side of mountain drainage basins) The wiki describes it as:

Dolomite (play /ˈdɒləmaɪt/) is a carbonate mineral composed of calcium magnesium carbonate CaMg(CO3)2. The term is also used to describe the sedimentary carbonate rock dolostone.

Dolostone (dolomite rock) is composed predominantly of the mineral dolomite with a stoichiometric ratio of 50% or greater content of magnesium replacing calcium, often as a result of diagenesis. Limestone that is partially replaced by dolomite is referred to as dolomitic limestone, or in old U.S. geologic literature as magnesian limestone. Dolomite was first described in 1791 as a rock by the French naturalist and geologist, Déodat Gratet de Dolomieu (1750–1801) from exposures in what are now known as the Dolomite Alps of northern Italy.

Which brings us back to that calcite. While there are pure crystal forms of calcite, most folks know it as limestone. Yes, all that limestone all over the world is calcite or dolomitic limestone. How much is there? Well, “only” about 10% of all sedimentary rocks…

Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate (CaCO3). Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera.

Limestone makes up about 10% of the total volume of all sedimentary rocks. The solubility of limestone in water and weak acid solutions leads to karst landscapes, in which water erodes the limestone over thousands to millions of years. Most cave systems are through limestone bedrock.

That Karst formation is what you find from up about Kentucky all the way down to the end of Florida. I shudder to think how many mega-tons of carbonate erode from there into the ocean. It is named for where it was first found in Serbia, but with the German name Karst rather than the original Kras of Serbia. It extends on into Italy where it is called Carso. The point? There is one heck of a lot of the stuff and it is spread pretty much all over the world. Gigantic quantities of the stuff dissolve into the ocean each and every year. And that’s a very good thing!

http://jfr.geoscienceworld.org/content/27/4/271.abstract

Our first order production estimate shows that reef foraminifera contribute approximately 43 million tons of CaCO 3 per year, 34 million tons of which accumulate in reef sediments. Eighty percent of the global foraminiferal reef carbonate is estimated to be produced by larger symbiont-bearing species alone. The annual production of 43 million tons represents roughly 0.76 percent of the present-day CaCO 3 production in the world’s oceans and approximately 4.8 percent of the global carbonate reef budget.

See, these reef formers need to to make all those reefs everyone is so worried about. 43 Million Tons of it, per year, for reef framininfera alone. Now notice that next bit: 0.76% of global calcium carbonate production in the ocean… Hmmm…. I could probably find the original number if I put the time into it, but a quick shortcut would be to just use that 43 million and back figure it. I get 5,658 Million Tons per year. Each and every year. That’s a heck of a lot of carbonate.

Guess these guys are ‘good guys’ taking all that nasty carbonate out of the ocean (and indirectly CO2 out of the air), right?

The biochemical production of calcium carbonate in the ocean leads to the release of the greenhouse gas CO 2 which can be discharged into the atmosphere. Using our first order approximation, the annual production of 43 million tons of foraminiferal reef carbonate results in the release of 11.4 million tons of CO 2 . This represents only 0.05 percent of anthropogenic CO 2 emission in 1991.

Oh Dear! Living things creating EVIL CO2 in the oceans… (Perhaps we could tell that author that CO2 is a fundamental part of the life cycle of the planet and that plants need it, so this is just the coral making sure that the plants don’t die…) One has to wonder, though, with them making their own CO2, just how they manage to put up with it. I mean, if a few parts per million in the air is supposed to make them dissolve, how can they release 11.4 million tons of it right at their surfaces and not have a problem?

We have another ‘back figure’ opportunity here, too. Presuming the other animals making ocean carbonate have similar respirations, we can figure about 1,500 Million Tons of CO2 from them. We can also back figure the ‘anthropogenic CO2 number they used via 11.4 / 0.0005 = 22,800 million tons. That’s over 22 Billion tons. As CO2 is only 12/44 carbon, that’s 6.2 Billion Tons of carbon. As that roughly matches world coal production in 2006 and all the other fossil fuels are used in lesser amounts and with lower carbon content, it ‘sanity checks’ at about right.

Some large part of that gets turned into carbonates on the land, or consumed by plants. But still, it looks at first blush like we make more CO2 than the carbonate formers can remove from the ocean. And then there is all that washing in from the rocks, too.

http://adsabs.harvard.edu/abs/1993GBioC…7..927M

Production and accumulation of calcium carbonate in the ocean: Budget of a nonsteady state

Milliman, John D.
Global Biogeochemical Cycles, Volume 7, Issue 4, p. 927-957

Present-day production of CaCO3 in the world ocean is calculated to be about 5 billion tons (bt) per year, of which about 3 bt accumulate in sediments; the other 40% is dissolved. Nearly half of the carbonate sediment accumulates on reefs, banks, and tropical shelves, and consists largely of metastable aragonite and magnesian calcite. Deep-sea carbonates, predominantly calcitic coccoliths and planktonic foraminifera, have orders of magnitude lower productivity and accumulation rates than shallow-water carbonates, but they cover orders of magnitude larger basin area. Twice as much calcium is removed from the oceans by present-day carbonate accumulation as is estimated to be brought in by rivers and hydrothermal activity (1.6 bt), suggesting that outputs have been overestimated or inputs underestimated, that one or more other inputs have not been identified, and/or that the oceans are not presently in steady state. One “missing” calcium source might be groundwater, although its present-day input is probably much smaller than that of rivers. If, as seems likely, CaCO3 accumulation presently exceeds terrestial and hydrothermal input, this imbalance presumably is offset by decreased accumulation and increased input during lowered sea level: shallow-water accumulation decreases by an order of magnitude with a 100 m drop in sea level, while groundwater influx increases because of heightened piezometric head and the diagenesis of metastable aragonite and magnesian calcite from subaerially exposed shallow-water carbonates.

Here we have a number of ‘about 5 billion tons’, a bit lower than the earlier number. They then say that 40% dissolves again. If it’s dissolving, why don’t the animals shells dissolve? (Hint: Maybe they have evolved to deal with it…) But the interesting bit is that they have a carbonate deficit. They figure 1.6 Billion Tons of carbonate in, per year, but ‘about 3’ billion tons sequestered. And that doesn’t even allow for the recently discovered ‘fish gut rocks’ sequestration. (Fish were found to bind calcium to carbonate to get rid of the ‘excess calcium’ they take in with sea water, then poop out the rocks, that sink to the ocean floor. This was recently discovered, but the above paper is from 1993.)

Now I find three things particularly interesting here. First off, there is a factor of 2 mis-match in the in / out numbers. Second, there is added ‘out’ that they are also missing. Then there is that final line about “an order of magnitude” variation in production from sea level changes. So during an ice age even less will be sequestered.

That the fish have evolved a method to deal with ‘excess’ calcium implies the ocean levels have risen from what they were when the basic cellular structure first evolved. Over millions of years, the ocean is getting saltier and, it would seem, with higher levels of calcium too. At the same time, we have a calcium carbonate influx deficit rate right now in the ocean (per that paper). If you put those two together, I get that we are just getting the ocean back a little bit toward the original calcium level. (And maybe it would benefit from a bit more carbonate flux to help that along).

BTW, the implications for total ocean productivity from that ‘order of magnitude’ drop in shallow water during ice ages is more than just a bit disturbing. We gain more land above water for farming, but have less of the highly productive continental shelf areas for fish…

Back At Carbon

But it is also clear that carbonate rocks are not going to be pulling all the CO2 out of the air as we make it. While there is a deficit of influx, it’s only about 1.5 Billion Tons, so we’ve got to look elsewhere.

This has some folks all a-twitter about where that excess can go. Some of them make truly nutty statements:

http://harvardmagazine.com/2002/11/the-ocean-carbon-cycle.html

Besides the slow pace of ocean turnover, two more factors determine the rate at which the seas take up carbon dioxide. One is the availability of carbonate, which comes from huge deposits of calcite (shells) in the upper levels of the ocean. These shells must dissolve in ocean water in order to be available to aid in the uptake of CO2, but the rate at which they dissolve is controlled by the ocean’s acidity. The ocean’s acidity does rise with increased CO2, but the slow pace of ocean circulation prevents this process from developing useful momentum. It takes a long time for the increased acidity to reach the vulnerable calcite deposits, to dissolve them, and then to bring the carbonate cations to the surface where they can combine with CO2 in the surface waters of the ocean. There is no hope, says McElroy, that this process will take place fast enough to help control the build-up of CO2.

This is just SO backwards and SO unphysical. Carbonate washes into the oceans from land. It EXITS the ocean as carbonate deposits. He wants them to dissolve, so they can absorb CO2 with the “carbonate cations”. Cations are the positively charged metal parts. That’s the calcium and magnesium. So he wants to dissolve Calcium Carbonate, that will put carbonate into the ocean, so that the calcium can then bind to OTHER carbonate from the CO2 dissolving into the ocean? Then, one presumes, to precipitate again back where it was (and thus do nothing at all, net.) And he figures that rising ‘acidity’ (by which is actually meant lowering of the truly alkaline ocean pH rather than actual acidity) will be helpful? Because it is going to dissolve the carbonates that will put MORE CO2 into the ocean as carbonate ions? AND he thinks that making a solution more acidic will increase the tendency for a weak acid to suck in more CO2? Someone needs to tell him that CO2 leaves acid solutions and dissolves more into alkaline solutions…

At any rate, it’s from Harvard, and given the kind of folks they give degrees to (Michael Moore got one), clearly it doesn’t take much thinking skill. So it looks like I’ll have to resort to the wiki rather than Harvard for useful information on detritus deposition…

Carbon Cycle

Original and larger sized image

Here we can see that the ocean surface layers alone have over 1,000 GIGA tons of carbon in them. 1,020 Billion tons. Somehow I think our 6 Billion tons could be accommodated into it without much of a notice… But soils hold another 1,580 Billion tons, and much of the atmospheric CO2 is going to be put into plant roots, soil bacteria, black charcoal from fires, etc. Vegetation is 610 Billion tons. So if we just let 10% more plants grow we suck it all up. ( I think we put that many phone books and newspapers in landfills when I was a kid… maybe we just need to stop recycling ;-)

BTW, on the graph you will see ‘fossil fuels and cement production’ with 4,000 next to it. That is in black, so is a ‘storage’, not a ‘flux’. It’s also a bit misleading as the production of cement turns a carbonate rock into lime, that is made into cement, that then consumes CO2 slowly over it’s lifespan as it eventually turns back into carbonate. Cement production does not permanently release CO2, it just lets us move the rocks around easier… That the coal and oil of the world ‘store’ carbon is pretty much a given… but it’s nice to know we have so much of it. 4,000 Billion Tons. Heck, allow a billion for the cement. Call it 3,000 Billion Tons. That gives about 500 years at 6 Billion Tons a year…

Also note that all the carbon in carbonate rock is being ignored on this graph. Wonder how may giga tons is 10% of all sedimentary rocks? I don’t see any indication of carbonate rock erosion being noted. The whole carbon cycle is being shown as between living things, the air, the oceans, and fossil fuels (other than the 150 GT of ocean storage as ‘sediments’). It doesn’t even show the 3 GT of sequestration from carbonate rock formers each year, just 200 Mt of what I presume are organic sediments.

So even the Wiki is not very useful for judging the flows of carbon and carbonate. I’m shocked, shocked I tell you! ;-)

So where is all this going

There are several points scattered through here, but the big lumps are these.

There are massive quantities of carbon as carbonate floating around all over the world. Giga-tons of the stuff erode into the ocean each year in rivers and ground water. Our estimates of it are way off, and our understanding of the ocean carbonate budget is dodgy at best. Even the wiki recognizes this:

http://en.wikipedia.org/wiki/Carbon_cycle

Much remains to be learned about the cycling of carbon in the deep ocean. For example, a recent discovery is that larvacean mucus houses (commonly known as “sinkers”) are created in such large numbers that they can deliver as much carbon to the deep ocean as has been previously detected by sediment traps. Because of their size and composition, these houses are rarely collected in such traps, so most biogeochemical analyses have erroneously ignored them.

and from the larvacean link:

Appendicularians have greatly improved the efficiency of food intake by producing a test (skeleton) known as a “house” of protein and cellulose that, in most cases, surrounds the animal like a bubble and which contains a complicated arrangement of filters that allow food in the surrounding water to be brought in and concentrated prior to feeding. Even in those species in which the house does not completely surround the body, such as Fritillaria, it is always present and attached to at least one surface. The high efficiency of this method allows larvaceans to feed on much smaller nannoplankton than most other filter feeders.

These houses are discarded and replaced regularly as the animal grows in size and the filters become clogged; in Oikopleura, a house is kept no more than four hours before being replaced. No other Tunicate is able to abandon its test in this fashion. Discarded larvacean houses account for a significant fraction of organic material descending to the ocean deeps.(Robinson, Reisenbichler & Sherlock 2005)

The presence of 10% of surface rocks as carbonates tell us that the ocean has had a long history of sequestering carbonate, that it will continue to sequester it, and that the quantities are truly gigantic. It also testifies to the fact that carbonate is a fundamental of life. CO2 is just a small part of the global carbon budget, there is vastly more in carbonates in the ocean and on land, and the movement of that carbonate between rocks, the oceans, the air, and into life; that is what keeps life going.

That we have a carbonate flux deficit into the oceans right now is not a good thing. We need more carbonate, not less. At some point there will be a new ice age and that flux will increase, but until that time, more carbonate is being sequestered than is being eroded back in (per the numbers – that I’m not so sure I’d trust…)

There is a very large flux of “alkaline feldspar” (and one presume also other rocks) into the oceans. The paranoia about the gigatons of CO2 making carbonic acid ignores the gigatons of potassium, sodium, and calcium from those rocks that will react with it and make alkaline carbonates. We live on a rocky planet, and those rocks determine our chemistry balance. Not the air. Not even the water. It is the rocks that take things out of solution, and it is the rocks that put things back into solution. Any attempt to get too far from the present balance will be met with a wall of stone…

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