R.D.Schuiling, Faculty of Geoscience, Utrecht
Many people claim that the rate of weathering of olivine is slow, but they base this on experimental data in sterile laboratories, under exclusion of biotic and other environmental factors. These laboratory conditions are irrelevant for the real world, and weathering models based on these data, therefore, produce nonsense (I am not providing references to these modelling papers, to save the authors further embarrassment. One would expect that reviewers should have spotted their blunder).
In this note I provide some data on the weathering of olivine in nature, which make clear that the weathering of olivine is fast enough to play an important role in the cycle of CO2 capture and its safe and sustainable storage as carbonate rocks. The CO2 released by volcanism has always been captured by the weathering of rocks since the origin of the Earth. Without this mechanism the Earth would be a lifeless planet with a CO2 atmosphere in the order of 100 bar, as our neighbor planet Venus demonstrates.
Keywords: Climatic change, Olivine, rate of weathering, biotic effects
The Eifel volcanism provided a first clue for the author that weathering in nature proceeds considerably faster than in the laboratory. As soon as the Eifel volcanism began, contemporaneous Rhine sediments at the Dutch border suddenly started to contain plenty of volcanic minerals like augite and basaltic hornblende. Strangely enough, although olivine is one of the commonest minerals in the Eifel volcanics, these sediments contained no olivine. Apparently, all the olivine grains had completely disappeared by weathering on the short trip along the Rhine from Bonn to Lobith, at the Dutch border. Although evidently much faster than weathering rates obtained in laboratory studies, unfortunately it is difficult to derive hard data for the rate of weathering of olivine from this evidence.
From studies of weathering of large dunite massifs in tropical countries it is possible to derive realistic weathering rates of olivine over long geological times. A first example comes from the Kaloum peninsula near Conakry, Guinea, West Africa. The backbone of this 50 km long peninsula is formed by a vertical plate of dunite, with a cross-section of approximately 5 km (fig.1). The rock is completely covered by a thick lateritic weathering crust, which makes it stand out on a satellite image. Although I have criss-crossed the area a number of times, I have not seen a single outcrop of dunite. The only sample in my possession of this dunite is from a drillcore, provided by the Geological Survey of Guinea.
Only some basic information on the dunite body is needed to calculate its average (minimum) weathering rate. The required data are its radiometric age, the average thickness of the weathering crust, and the composition of the laterite compared to the composition of the fresh rock (1), from which it was formed. During tropical weathering, essentially all the magnesium and silica were removed, leaving a lateritic crust mainly composed of iron (hydr-)oxides. As olivine is essentially a magnesium-silicate, this means that around 90% of the rock has vanished, so 1 meter of laterite is equivalent to 10 meters of fresh dunite. The thickness of the weathering crust is in the order of 100 meter, so during its existence at least 1 km of dunite has weathered, and even more if some of the laterite had been eroded in the meantime. As the radiometric age (the time it was emplaced) of the rock is 195 million years (2), its average weathering rate is 1000 meters/ 195 million years = 5.1 micron/year. A similar number results from a dunite massif at Jacupiranga/Brazil (3). Due to the fact that there the thickness of the weathering crust is not exactly known, because the drill core stopped at 40 meter depth, still in lateritic weathering crust, the results are even less well constrained.
5.1 micron/year is already more than ten times faster than the weathering rates obtained in sterile laboratories, but is this the real rate of weathering of fresh olivine rock in nature, when exposed to tropical conditions? In fact, there are 3 factors that cause the real weathering rate of exposed rock to be even considerably larger. The first is the fact that the dunite body is an intrusion, that means that after its emplacement it took some time before it was exposed to the atmosphere, probably several tens of millions of years later. First its overburden had to be removed by erosion, before it could start to weather. This shortens the time available for weathering. The second consideration is the fact that the rate of weathering will go down as the thickness of the overlying lateritic weathering crust increases, because it will make the contact of the rock with the atmosphere increasingly difficult. It is likely that the real weathering rate would be around 20 to 30 micron/year when the rock is exposed. This is the outcome for solid rock. The third consideration refers to the difference between the rate of weathering of solid rock and the rate of weathering of individual loose grains. If we spread olivine grains, they are attacked on all sides by weathering. This would make the rate of weathering of a grain 4 times more than its rate of weathering for a flat circle with the same radius. The outcome of these three factors is that under favorable conditions of wet tropics, weathering rates of olivine grains will be 50 micron/year or more.
From the above considerations, it should be clear that it is impossible and meaningless to give an exact number for the rate of weathering of olivine in the real world, but it is also clear that it is several orders of magnitude larger than what is observed in the laboratory under sterile conditions, using distilled water. As people have used the low number for olivine weathering observed in the laboratory to model its behavior in nature, many people were led to believe that olivine weathering is not fast enough to play a role in halting climate change.
Reasons for the discrepancy
Why is the rate of weathering of olivine in nature so vastly higher than rates determined in the laboratory? An obvious reason is that the laboratory experiments were carried out with distilled water. In natural waters the reaction goes faster, particularly if the waters are saline like sea water, and even much faster in brines, as we have observed in our weathering experiments (author’s unpublished data). The main difference, however, are the biotic factors. Let me briefly note some of these, but it is likely that there are other, yet unknown factors, like the role of soil bacteria and earth worms.
Another, non-biotic but very important reason for rapid weathering is motion. In rivers, or on beaches in the surf, or in shallow waters with strong bottom currents, olivine grains are worn down by collision and abrasion. Slivers with sizes around a micron are knocked off. We found in our very modest imitation of a surf, by moving olivine grains slowly along the bottom of an erlenmeyer, that these slivers weathered extremely fast, in the order of days (10). A second reason of fast weathering caused by motion is that olivine grains in a fixed position often develop a silica-rich rim which slows down further weathering. When the grains are moving, this rim is rapidly scratched off.
Implications for climate change
It is a logical choice to look at the process of enhanced weathering to counteract climate change, because weathering is the only process that has captured huge volumes of CO2 during the whole history of the Earth. Weathering produces bicarbonate solutions that are carried by rivers to the sea, where corals, shellfish and plankton turn them into carbonate rocks (limestones and dolomites). These rocks contain approximately a million times more CO2 than the oceans, the atmosphere and the biosphere together. They are the safe and sustainable storage rooms for CO2. This process has removed almost completely all the CO2 that was ever produced by volcanoes, and thus provided the conditions for a livable planet. In the present situation we need to enhance this weathering process to make up for the vastly higher emission of CO2 caused by our burning of fossil fuels. The choice of olivine, or its hydrated relative serpentine, to enhance weathering is logical, because both minerals are very abundant, and both weather fast. To make the spreading of these minerals even more effective to capture CO2, one should select environments that increase the weathering rates. Among these a lush vegetation and a high-energy fluviatile or marine environment are the most effective. Mining olivine rocks, milling them, and spreading the grains over these environments is probably the most effective, safe and low-cost way to counter climate change and the accompanying ocean acidification. We make use of processes that have shown to be effective over the whole history of the Earth, and it is highly unlikely that these processes will all of a sudden produce negative effects, when we apply them in the geotherapy of the Earth.
There is no such thing as THE rate of weathering of olivine. What nature tells us is that olivine weathers several orders of magnitude faster than shown by weathering experiments in sterile laboratories. Olivine weathering can become the most effective and safest way to counter climate change in a cost-effective way (11).
Acknowledgment: I want to thank prof.dr.M.A.Elburg/Johannesburg for her critical remarks.