Which of the following ions and dissolved gases maintains a constant concentration for sediment depths of 10 cm or more?

• A. Sulfide (S2-)
• B. Carbon dioxide (CO2)
• C. Ferric iron (Fe3+)
• D. Oxygen (O2)

Answer: (D)

The correct answer focuses on the behavior of oxygen in sediment environments. In sediment depths of 10 cm or more, the concentration of oxygen tends to remain relatively constant compared to the other options. This is primarily due to the continuous diffusion of oxygen down through the sediment layer, as well as its production through photosynthesis in the upper layers where light penetrates. This dynamic allows for a somewhat stable concentration of dissolved oxygen throughout the sediment profile.

In contrast, the other ions and gases behave differently. Sulfide, for instance, is often derived from the decomposition of organic matter and can accumulate in deeper sediments, leading to variable concentrations depending on the biological activity and the presence of sulfate-reducing bacteria. Similarly, carbon dioxide is produced during decomposition processes and its concentration can increase significantly in lower sediment layers where organic matter is broken down, causing fluctuations rather than stability. Ferric iron exists in a state that can change depending on the redox conditions in the sediment, which can lead to varying concentrations as it is reduced to ferrous iron under anaerobic conditions.

Therefore, it is the unique nature of oxygen dynamics in the sediment that allows for a more stable concentration over greater depths, making it the best choice among the provided options.