Olivine – (Mg,Fe)2SiO4 – is touted as a means of geoengineering as it weathers to metal ions, bicarbonate ions and silicic acid, absorbing carbon dioxide in the process. Dissolving 1 g of the mineral in the ocean would lead to the sequestration of 0.28 g of carbon, equivalent to roughly 1 g of carbon dioxide. The scientists calculate that adding 3 Pg of olivine to the ocean each year could adsorb nearly 10% of annual man-made carbon-dioxide emissions.

However, the team estimates that, with present-day technology and energy infrastructure, about 30% of the carbon dioxide absorbed by the ocean by olivine dissolution would be re-emitted by grinding the olivine into small particles.

This is the first time the effects of enhanced silicate weathering on marine biology have been considered and modelled in detail, the researchers believe. "Olivine dissolution in such large quantities as are of interest in the geoengineering context...does increase the carbon-dioxide uptake capacity of the ocean by about 10%, based on results from our marine ecosystem model," Peter Köhler told environmentalresearchweb. "Silicic-acid input leads in our model to changes in the phytoplankton species distribution and to changes in the amount of carbon that is transported to the sea floor via sinking of dead organic matter – the so-called biological pump."

In some areas silicate is a limiting nutrient for the growth of diatoms, a group of algae that have silica shells. In such cases, the addition of olivine boosts the productivity of diatom species, the researchers found, whilst slightly shrinking the growth of non-diatom species. The team used the biogeochemical model REcoM-2 coupled to the Massachusetts Institute of Technology general circulation model (MITgcm), modelling the input of up to 10 Pg of olivine into the ocean per year.

"The dominant effect (∼90%) for marine carbon-dioxide uptake is caused by the alkalinity input [introduction of bicarbonate ions], which changes the chemistry of the marine carbonate system and raises the uptake capacity for carbon dioxide," said Köhler. The remainder is due to the increased strength of the biological pump.

The researchers reckon that 100 dedicated ships could distribute 1 Pg of olivine over the ocean each year. In the Southern Ocean south of 60° and most regions of the southern hemisphere, concentrations of silicate are naturally high so the addition of olivine would create little biological response, according to the team. The largest increase in diatom growth would occur in the North Atlantic and equatorial regions.

Köhler said that using olivine in a large-scale application would alter marine biology to a large extent because of the associated input of nutrients, not just silicate but also iron and other trace metals. "This should be taken into consideration if assessing the pros and cons of the approach," he said. "These nutrient issues are also relevant for land-based approaches, because dissolved matter eventually reaches the ocean via riverine input."

The addition of olivine could also reduce the amount of light reaching plankton.

"This approach should be considered as an ocean fertilization experiment, with all the related policy implications and discussions," said Köhler. "It probably also has consequences for oxygen conditions in the deep ocean because altered export of dead marine organic matter changes the remineralizeation rates there, which consumes oxygen."

For olivine to dissolve in the ocean's upper layers it must be in the form of small particles, just one micron or so across. "Large particles will sink un-dissolved, and then immediate carbon-dioxide uptake by the ocean does not occur," said Köhler. "This might also pose a problem for land-based enhanced weathering, because wind might disperse particles distributed on land, which then might be deposited in the ocean, where they might sink un-dissolved if too large."

Now the team, who reported the study in Environmental Research Letters (ERL), plans to investigate the effect of iron input on marine biology and oxygen conditions.

"Taking all our conclusions together – mainly the energy costs of the processing line and the projected potential impact on marine biology – we assess this [geoengineering] approach as rather inefficient," said Köhler.