Final answer:
Deep marine deposits with abundant calcite-shelled organisms are most commonly found in the open ocean, where conditions such as water depth, low sedimentation rates, and stable water conditions allow for the accumulation and preservation of the calcite from these organisms.
Step-by-step explanation:
In which environment would you find deep marine deposits with abundant calcite-shelled organisms? One would expect to find such deposits in environments where conditions are favorable for the life and preservation of such organisms. Calcite-shelled organisms are typically found in the open ocean, where they inhabit the top layers of the water column and upon death, their shells can sink to the ocean floor, contributing to sedimentary deposits over geological time scales. Examples of these organisms include planktonic foraminifera and coccolithophores, which are abundant in deep marine settings far from land-based sediments, such as those found in a river delta or around volcanic islands.
When considering sedimentary environments, places like coral reefs, which are found in shallow ocean waters, can also be rich in calcite from the skeletons of corals and other marine life. However, these are not deep marine deposits.
During periods of low sea level (lowstand), continental shelves and carbonate platforms may become exposed, limiting, the abundance of deep marine deposits at these times. Subsequently, during a rise in sea level or in depositional environments where sedimentation rates are low and water conditions are stable, the accumulation of calcitic shells in sediments is best preserved.
C. Open Ocean, in about 20 words: Deep marine deposits with calcite-shelled organisms are most common in the open ocean.