During the first billion years of Earth's existence, the atmosphere was rich in carbon dioxide. Over geological time, the concentration of atmospheric carbon dioxide decreased significantly as it was locked up in sedimentary rocks.
Which of the following describes a correct chemical pathway by which carbon dioxide was removed from the early atmosphere to form these sedimentary rocks?
Carbon dioxide dissolved in the oceans, forming carbonate ions (CO32−\text{CO}_3^{2-}CO32−), which reacted with dissolved calcium ions (Ca2+\text{Ca}^{2+}Ca2+) to form insoluble calcium carbonate (CaCO3\text{CaCO}_3CaCO3) precipitates that deposited on the seabed.
Dissolved carbon dioxide was directly reduced by volcanic sulfur dioxide (SO2\text{SO}_2SO2), producing solid calcium sulfate (CaSO4\text{CaSO}_4CaSO4) and carbon deposits that eventually compressed into limestone.
Early photosynthetic marine organisms absorbed dissolved carbon dioxide and converted it into calcium carbonate (CaCO3\text{CaCO}_3CaCO3) shells via anaerobic respiration.
Gaseous carbon dioxide reacted directly with silicon dioxide (SiO2\text{SiO}_2SiO2) minerals in surface rocks during dry weathering to produce solid calcium carbonate (CaCO3\text{CaCO}_3CaCO3) and quartz.
Practise Edexcel GCSE Chemistry Earth and atmospheric science with exam-style questions for Foundation and Higher tier. 62 questions, matched to the Edexcel GCSE Chemistry (1CH0) specification and written in Paper 1 and Paper 2 style. Every question includes a full worked solution and mark scheme, so you can see where marks are awarded rather than just whether you got the answer right.
