Consider the following activities: 1. Spreading finely ground basalt rock on farmlands extensively 2. Increasing the alkalinity of oceans by adding lime 3. Capturing carbon dioxide released by various industries and pumping it into abandoned subterranean mines in the form of carbonated waters How many of the above activities are often considered and discussed for carbon capture and sequestration?

Let's evaluate each activity: 1. Spreading finely ground basalt rock on farmlands extensively: This is a climate change mitigation technique known as enhanced rock weathering. Crushed silicate rocks like basalt react with CO2 from the atmosphere and soil to form stable carbonate minerals, effectively sequestering carbon. Spreading it on agricultural land also potentially improves soil health. This is considered for carbon capture and sequestration. 2. Increasing the alkalinity of oceans by adding lime: This is a form of Ocean Alkalinity Enhancement (OAE), a proposed geoengineering technique for carbon removal. Increasing ocean alkalinity enhances its capacity to absorb atmospheric CO2. This is discussed for carbon capture and sequestration. 3. Capturing carbon dioxide released by various industries and pumping it into abandoned subterranean mines in the form of carbonated waters: This is a description of Carbon Capture and Storage (CCS). CO2 emissions from industrial sources are captured and stored underground in geological formations such as depleted oil/gas fields, saline aquifers, or, as mentioned here, abandoned mines (though less common than saline aquifers). Injecting it as carbonated water is one variation, aiming for mineral carbonation. This is a well-established concept for carbon capture and sequestration. All three activities are actively discussed and researched as methods for carbon capture and sequestration.