Close weathering and carbonate burial across a geological carbon boundary

1. Define the evidence and process

Weathering links exposed rocks, water, atmosphere and sediment burial. Dissolved products can travel far beyond the original outcrop, so interpreting a carbon effect requires a boundary that follows those products. An initial uptake during dissolution is not necessarily equal to persistent net removal after downstream reactions.

2. Connect mechanism, geometry and boundary

An ideal calcium-silicate reaction is CaSiO₃ + 2 CO₂ + H₂O → Ca²⁺ + 2 HCO₃⁻ + SiO₂. The carbonate-rock counterpart is CaCO₃ + CO₂ + H₂O → Ca²⁺ + 2 HCO₃⁻. In the latter, one carbon atom came from the original rock and one from the external CO₂ reservoir; labeling both as newly removed atmospheric carbon would double count.

If each transported calcium later precipitates carbonate, Ca²⁺ + 2 HCO₃⁻ → CaCO₃ + CO₂ + H₂O. For silicate extent a and carbonate extent b, weathering consumes 2a+b CO₂, precipitation returns a+b and net uptake is a. Carbonate dissolution and reprecipitation cancel in this ideal complete boundary, although they relocate rock carbon.

This is an atom-conserving boundary comparison, not a measured global flux or a carbon-removal project claim. Sulfuric-acid weathering, organic carbon, incomplete precipitation, transport and metamorphic return can change the accounting. Uplift and climate affect exposure and reaction conditions, but stoichiometry alone gives neither a rate nor a timely offset for modern emissions.

3. Reconstruct the worked packet

At a=2 mol silicate and b=3 mol carbonate, weathering consumes 7 mol CO₂ and transports 5 mol calcium. Precipitation returns 5 mol CO₂, leaving net uptake 2 mol. The 5 mol carbonate carbon deposited includes 3 mol carbon originally supplied by carbonate rock.

4. Predict and investigate

Predict first: Why is gross CO₂ consumption during dissolution an incomplete measure of net removal?

Compare pure silicate, pure carbonate and mixed cases. Retain carbon origins and the downstream return, then identify the consequence of moving the boundary to before precipitation. State which extra observations would be required to estimate a rate.

5. Test changed evidence

Apply the evidence to a changed case

For a=0 and b=4, compare gross consumption, return and net removal.

Retain the evidence frame, calculations and assumptions before opening public feedback.

Compare a worked response

Gross consumption is 4 mol CO₂ and precipitation returns 4 mol. Net removal is zero within this completed pathway; 4 mol rock carbon are relocated into new carbonate. This does not mean carbonate weathering has no local chemical or geomorphic effects.

6. Retain a bounded decision

Preserve the first plan, source identity, original observations, units and competing interpretations. All numerical packets and diagrams in this category are invented teaching cases, not measured site conditions. Retain earlier actual USGS archives separately; do not merge unrelated sites into an invented actual history. Human review, unfamiliar transfer and any chosen supervised practical pathway remain separate. No mining, treatment, slope access or environmental clearance is authorized by these exercises.

Mechanism reference, checked 30 October 2026. Original prose, calculations, questions and diagrams.