Which of the following is a form of carbon in the long-term carbon cycle?

In the long-term carbon cycle, carbonate deposits are indeed a primary form of carbon. These deposits, which mainly consist of minerals like limestone and dolomite, are formed when carbon dioxide from the atmosphere or oceans reacts with calcium, leading to the precipitation of calcium carbonate. This process often occurs in marine environments where organisms such as coral and some types of shellfish extract calcium carbonate from seawater to build their shells and skeletons. Over geological timescales, these carbonate-rich sediments can become lithified into rock formations, effectively sequestering carbon from the atmosphere for millions of years.

While carbon dioxide gas is a significant component of the carbon cycle, it primarily participates in short-term processes like photosynthesis and respiration rather than serving as a stable long-term carbon storage mechanism. Glucose is an organic molecule that is part of the biological carbon cycle but does not represent a long-term storage form. Ethanol, produced through fermentation, is also part of a short-term cycle and does not contribute to the long-term sequestration of carbon. Therefore, carbonate deposits are the correct answer, highlighting their critical role in storing carbon over extended periods in the geological record.