Which of the following ions is least affected by sediment depth based on the information provided?

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

Answer: (D)

The least affected ion by sediment depth is sulfide (S2-). In aquatic environments, sulfide is often produced in anaerobic conditions where bacteria reduce sulfate in deeper sediments. As sediment depth increases, there tends to be less oxygen available, which can lead to higher concentrations of sulfide. This is because, in deeper sediments, organic matter decomposition consumes available oxygen, creating conditions that favor sulfate reduction and the accumulation of sulfide.

In contrast, ferric iron (Fe3+), oxygen (O2), and carbon dioxide (CO2) are more sensitive to sediment depth changes. Iron generally precipitates and is less soluble in oxygenated environments, which makes its concentration variable depending on aerobic versus anaerobic conditions at different depths. Oxygen levels tend to decrease significantly with increased sediment depth, particularly in areas where organic material is decomposed, which affects its availability and behavior markedly. Carbon dioxide can accumulate in sediments, but its concentration can also fluctuate with biological activity and gas exchange, making it impacted by sediment depth as well.

Thus, sulfide's behavior in deeper, anaerobic sediments makes it the ion that is least affected by sediment depth compared to the others listed.