Question

Where is most of the carbon in the ocean?

a) On the ocean's surface within phytoplankton
b) Freely circulating in the ocean currents
c) In the form of dead organisms on the ocean floor
d) In living animals and plants

Answer 1

The ocean floor holds the largest carbon reservoir in the form of calcium carbonate in sediments, which are formed from dead marine organisms over geologic time, contributing to long-term carbon storage.

Step-by-step explanation:

The largest reservoir of carbon in the ocean is found in the form of carbonate sediments on the ocean floor, primarily as calcium carbonate (CaCO3). The process begins with atmospheric carbon dioxide (CO2) that dissolves in the ocean water. Phytoplankton, which are photosynthetic microscopic organisms in the surface waters, incorporate this dissolved CO2 into organic compounds. Over time, marine organisms with calcium carbonate shells die, and their remains contribute to sedimentary deposits on the ocean floor. These deposits, over geologic time, become limestone, which holds a vast amount of Earth's carbon.

Thus, while phytoplankton and living organisms play important roles in the ocean's carbon cycle by fixing carbon and moving it through the food web, the most significant carbon storage is in dead organisms that form sediments on the ocean floor, becoming part of the long-term geologic carbon cycle.