The efficacy of enhancing carbonate weathering for carbon dioxide sequestration

Abstract

Enhanced weathering is a geoengineering strategy aiming to increase continental weathering rates, thereby increasing the delivery of atmospheric carbon (as HCO3-) to the oceans. Most enhanced weathering studies focus on the capacity of silicate rocks (e.g., basalt) and minerals (e.g., olivine, Mg2SiO4, or wollastonite CaSiO3) to remove atmospheric CO2. However, carbonate minerals (e.g., calcite, CaCO3) could provide an additional, rapid way to increase HCO3- export to the oceans. Recent studies suggest that 0.84 Gt C yr−1 could be removed from the atmosphere through the enhanced dissolution of calcite in soils, provided carbonic acid is the main dissolution agent. What is not clear is whether atmospheric CO2 dissolved in soils can be transported by rivers, which typically have lower [pCO2], to the oceans. This difference in calcite solubility between soils (where weathering occurs) and rivers (where HCO3- is transported) may lead to large amounts of secondary carbonate formation during transport, releasing the CO2 consumed through dissolution. Here, we present a modeling study comparing the estimated soil dissolution capacity (SDC) in 149 of Earth's largest river basins, to the potential transport capacity of carbon (PTCC) in corresponding rivers. We find the SDC can only be exported to the oceans, without secondary carbonate precipitation, if rivers are in disequilibrium with respect to calcite (i.e., SIc = 1). In this instance, 0.92 Gt C yr−1 may be sequestered above background weathering rates, which is ~20% of annual increases in atmospheric carbon. If rivers are at equilibrium with calcite (i.e., SIc = 0), approximately two-thirds of the carbon dissolved in soil waters are lost due to calcite precipitation in rivers, and just 0.26 Gt of additional atmospheric C yr−1 can be transported to the oceans. Overall, the efficacy of enhanced carbonate weathering is a function of the capacity rivers have for transporting the products from carbonate weathering to the oceans, rather than the dissolution capacity of soils. These findings have implications for the efficiency of enhancing silicate weathering for ocean alkalinity enhancement, as secondary carbonate precipitation during transport is not always considered.

Life Cycle Assessment of Coastal Enhanced Weathering for Carbon Dioxide Removal from Air

“…This limitation is less constraining in CEW; since the oceans are affected by acidification, seawater’s average pH is much higher, i.e., 8.1, and marine carbonate chemistry is less sensitive to alkalinity addition at a global scale . Therefore, in CEW, the added alkalinity can rapidly dilute, and this could also help prevent the formation of secondary minerals, such as clays, via reverse weathering . For these reasons, research has also focused on CEW, particularly when using olivine, which combines widespread abundance with fast dissolution .…”

“… 18 Therefore, in CEW, the added alkalinity can rapidly dilute, 19 and this could also help prevent the formation of secondary minerals, such as clays, via reverse weathering. 20 For these reasons, research has also focused on CEW, particularly when using olivine, which combines widespread abundance with fast dissolution. 21 However, unlike EW 22 and other ocean-based CDR approaches such as ocean liming, 23 CEW’s environmental sustainability remains largely unknown.…”

Quantifying CO2 Removal at Enhanced Weathering Sites: a Multiproxy Approach

“…This is because river chemistry integrates the suite of water− rock interactions occurring in a given catchment, encompassing mineral dissolution, but also secondary processes (e.g., calcite precipitation, clay formation), which can impact carbon removal efforts. 5 Studying dissolved inorganic carbon (DIC), major elements, and isotopes in rivers allows quantification of key parameters for calculating carbon fluxes such as (i) the proportion of carbon derived from the modern atmosphere (i.e., active sequestration of modern atmospheric reactions that are consequential for carbon sequestration; and (iii) the amount of DIC exported to the ocean. Indeed, monitoring, reporting, and verifying (MRV) enhanced weathering has been the focus of discussion in recent years, 13,14 particularly because of the potential importance of enhanced weathering in emerging voluntary carbon markets.…”

“…Enhanced weathering is a carbon removal strategy aimed at removing carbon dioxide (CO 2 ) from the atmosphere. Typically, this involves the spreading of silicate − and carbonate − rock powders on a variety of land types (e.g., arable, forested). The powders may be sourced from naturally occurring minerals (e.g., wollastonite, calcite) or derived from industrial activities, e.g., mine tailings or blast furnace slags (hereafter referred to as slag deposits) .…”

“…Analyzing river (or groundwater) chemistry from catchments undergoing enhanced weathering trials provides an ideal method to quantify the rate of atmospheric carbon removal. This is because river chemistry integrates the suite of water–rock interactions occurring in a given catchment, encompassing mineral dissolution, but also secondary processes (e.g., calcite precipitation, clay formation), which can impact carbon removal efforts …”