A. Sediments from the anoxic basin of the Black Sea are laminated, i.e., the new
ID: 107314 • Letter: A
Question
A. Sediments from the anoxic basin of the Black Sea are laminated, i.e., the newest layer of sediment is always on top followed by the next oldest layer and so on. Would you expect to find such a nicely organized layering of sediment below an oxic water column? Explain.
B. would you expect the exchange rates of solutes between the sediment and the water column to be higher, similar, or lower in oxic compared to anoxic basins? Explain.
The answers require mention of bioturbation. Please give me the correct answer that is very detailed.
Explanation / Answer
There are three fundamental principles of stratigraphy, now known as Steno's Laws:
1) Law of Original Horizontality– Beds of sediment deposited in water form as horizontal (or nearly horizontal) layers due to gravitational settling.
2) Law of Superposition– In undisturbed strata, the oldest layer lies at the bottom and the youngest layer lies at the top.
3) Law of Lateral Continuity– Horizontal strata extend laterally until they thin to zero thickness (pinch out) at the edge of their basin of deposition.
Thus, according to the second principle, the lamination will be followed in an undisturbed strata. The living organisms are one of the most common driver of disturbance. Their activities such as, burrowing, ingestion and defecation of sediment grains etc. displace sediment grains and mix the sediment matrix. This is known as bioturbation.
A. Now, anoxic environments are the environments with little or no oxygen. All life on the earth are oxygen dependent for their physiological needs. Thus, the absence of oxygen implies the absence of life. Therefore, the abcsence of life in an anoxic basin such as the Black Sea means no bioturbation, and hence the laminated sedimentary deposits. However, in an oxic water, these organisms will be present, and disturb the sediment columns. Thus, we don't expect the lamination of sediments in such oxic basins.
B. Bioturbation inducing organisms randomly mix sediment by free burrowing through the sediments, particularly the small-bodied surface dwelling species, and some of them can form more permanently occupied burrows that can penetrate deeper into the sediments and convey sediment upwards or downwards. This increase sediment permeability, hence reductions of the rates of these bioturbation components would result in increased consolidation of sediment, decreasing the area of the sediment-water interface and limiting exchange between the sediment and water column. Thus, the exchange rates in the oxic basins would be higher than that of anoxic basins.