Olivine Dissolution in Seawater: Implications for CO₂ Sequestration through Enhanced Weathering in Coastal Environments

Abstract

Enhanced weathering of (ultra)basic silicate rocks such as olivine-rich dunite has been proposed as a large-scale climate engineering approach. When implemented in coastal environments, olivine weathering is expected to increase seawater alkalinity, thus resulting in additional CO2 uptake from the atmosphere. However, the mechanisms of marine olivine weathering and its effect on seawater–carbonate chemistry remain poorly understood. Here, we present results from batch reaction experiments, in which forsteritic olivine was subjected to rotational agitation in different seawater media for periods of days to months. Olivine dissolution caused a significant increase in alkalinity of the seawater with a consequent DIC increase due to CO2 invasion, thus confirming viability of the basic concept of enhanced silicate weathering. However, our experiments also identified several important challenges with respect to the detailed quantification of the CO2 sequestration efficiency under field conditions, which include nonstoichiometric dissolution, potential pore water saturation in the seabed, and the potential occurrence of secondary reactions. Before enhanced weathering of olivine in coastal environments can be considered an option for realizing negative CO2 emissions for climate mitigation purposes, these aspects need further experimental assessment.

Model‐Based Assessment of the CO2 Sequestration Potential of Coastal Ocean Alkalinization

“…Olivine dissolution in aqueous environments is complex and an analytical function to describe how olivine dissolves in coastal seawater is not available. When olivine is added to coastal waters, potential pore water saturation in the seabed and secondary reactions will likely occur (Montserrat et al, ). In addition, the alternating layers (Mg‐poor layers, SiO 4 ‐coatings) generated on the olivine mineral surface can lead to nonstoichiometric dissolution (the number of dissolved atoms cannot be expressed as a ratio presented in the chemical formula) and temporal variations of the area‐normalized olivine dissolution rates (Maher et al, ).…”

“…We focus only on the effects of temperature and pH on dissolution because other potentially important factors such as reduced pH in sediments due to the degradation of organic matter, benthic macrofauna sediment bioturbation and ingestion, and wave action (Montserrat et al, ) are not resolved in our model.…”

“…Given these likely limitations on implementing AOA in the open ocean and on coastal beaches, we consider the climate mitigation potential and environmental side effects of implementing AOA in “coastal seas” (Meysman & Montserrat, ; Montserrat et al, ), which includes oceanic continental shelves shallower than approximately 200 m; referred to hereafter as coastal ocean alkalinization (COA). For conceptual simplicity, we consider these coastal waters well mixed from the surface to seabed on at least an annual basis, driven, for example, by tidally and wind‐induced turbulence and seasonally cycling surface buoyancy fluxes.…”

Assessing carbon dioxide removal through global and regional ocean alkalinization under high and low emission pathways

“…We do not consider the biogeochemical response to other minerals and elements that can be associated with the sourcing of alkalinity from the application of finely ground ultra-mafic rocks such as olivine and forsterite, nor dissolution processes required to increase alkalinity (e.g. Montserrat et al, 2017).…”

Olivine Weathering against Climate Change

“…Experiments have shown that pieces of basalt weathered fast during digestion, and were returned as small heaps of clay [8] [9]. It is likely that this will also hold for olivine, when olivine grains are spread over tidal flats [10]. Another, non-biotic but very important reason for rapid weathering is motion.…”