Question

Although clay is widely dispersed in the oceans, in some areas, deep-sea sediments are dominated by clay, while in others they are dominated by carbonate or silica ooze. Why do these differences exist?

Short answer

In conclusion, the differences in deep-sea sediment distribution, specifically clay, carbonate, and silica ooze, can be attributed to various factors such as sediment sources, biological productivity, depth of the ocean (CCD and lysocline), and water acidity. Terrigenous sediments, like clay, originate from the land, while biogenous sediments, like carbonate and silica ooze, are formed from the remains of marine organisms. The presence of carbonate or silica ooze depends on the productivity of surface waters and their subsequent rate of deposition. Furthermore, the carbonate compensation depth (CCD) determines the dissolution of carbonate, and water acidity affects sediment composition. By considering all these factors, we can understand the distribution of these sediments in different regions of the ocean floor.

Step-by-step solution

Understanding the Sources of Deep-Sea Sediments

Deep-sea sediments mainly come from three sources: terrigenous, biogenous, and cosmogenous. Terrigenous sediments originate from land, mainly in the form of minerals like clay. Biogenous sediments are formed from the remains of marine organisms, resulting in carbonate and silica deposits. Cosmogenous sediments are dust particles from space, but they comprise a minor portion of deep-sea sediments.

Evaluating Sedimentation Rates

The rate at which sediments accumulate on the ocean floor depends on their source. For example, terrigenous sediments such as clay are carried into the ocean via rivers and wind, and their rate of deposition depends on weather patterns, erosion, and other geographical factors.

Role of Biotic and Abiotic Factors

Biogenous sediments, such as carbonate and silica ooze, are derived from the skeletal remains of marine organisms. The presence of such ooze depends on the productivity of the surface waters above, which can be influenced by nutrient availability, light penetration, water temperature, and other factors. Hence, regions with high productivity will have more carbonate or silica ooze deposition.

Carbonate Compensation Depth (CCD) and the 'Lysocline'

The depth of the ocean also plays a significant role in sediment composition. Below a certain depth, known as the carbonate compensation depth (CCD), carbonate starts dissolving. This depth varies but is usually around 4500 meters. Additionally, there is a transitional depth range, known as the 'lysocline', where the rate of carbonate dissolution increases sharply.

Acidification and Sediment Composition

Water chemistry, particularly acidity, is an essential factor in sediment distribution. More acidic waters can dissolve carbonate shells, limiting their presence in the sediments. In contrast, slightly alkaline waters allow for the accumulation of carbonate sediments.

Compilation and Conclusion

In conclusion, the existence of these differences in deep-sea sediment distribution stems from several factors, including sediment sources, biological productivity, the depth of the ocean (CCD and lysocline), and the acidity of the waters. By considering all these components, we can understand why particular regions of the ocean floor are dominated by clay, while others by carbonate or silica ooze.

Key concepts

Terrigenous Sediments

Terrigenous sediments are particles that originate on land and make their way into the ocean. These sediments are primarily composed of clay minerals and sediments like sands and silts. When considering their source, these sediments are typically transported to the ocean through rivers, wind, glaciers, and even volcanic eruptions.
Understanding how they get transported is crucial in predicting their deposition pattern. For instance, the proximity of a landmass will often result in more terrigenous material found on the ocean floor nearby. Weathering and erosion are continuous processes that help break down rocks and minerals into smaller particles.
This material then gets carried by rivers and streams into the ocean.

• Clay is a very fine-grained sediment that can be carried long distances from its source due to its lightweight.
• The wind can also transport clay across large oceanic expanses before settling on the ocean floor.
• Glaciers can carry large boulders and finer sediment to the ocean, which then gets deposited once the ice melts.

This varied transport mechanism leads to wide dispersion of terrigenous sediments across the oceans, impacting the sedimentary layout of the sea floor.

Biogenous Sediments

Biogenous sediments are primarily derived from the remains of living organisms in the ocean, including both plants and animals. The main components of biogenous sediments are the shells and skeletons of marine organisms, which are predominantly composed of carbonate (CaCO₃) and silica (SiO₂).
These sediments are often referred to as "oozes" based on whether carbonate or silica predominates:

• **Calcareous ooze** is dominated by calcium carbonate from organisms like foraminifera, coccolithophores, and pteropods.
• **Siliceous ooze** is rich in silica from organisms such as diatoms and radiolarians.

Biogenous sediments usually accumulate in areas of high productivity, where nutrient levels are sufficient to support large populations of such organisms.
Regions with high productivity typically lead to large deposition of these biological materials on the ocean floor.
The availability of nutrients, sunlight, and optimal temperatures are some of the factors that significantly influence biogenous sedimentation.

Carbonate Compensation Depth (CCD)

One fascinating aspect of ocean sediment distribution relates to the carbonate compensation depth (CCD). The CCD is the depth in the ocean below which calcium carbonate starts to dissolve significantly.
This process affects the presence and distribution of carbonate sediments. Below this depth, you will not find much carbonate material because it tends to dissolve under the increased pressure and acidity.
The typical CCD in the ocean is around 4500 meters, but it can vary depending on water temperature, pressure, and carbon dioxide concentration.

• Above the CCD, carbonate production from organisms is often sufficient to accumulate significant sediments.
• At or below the CCD, carbonate inputs decrease as dissolution processes dominate, leading to diminished carbonate sedimentation.
• The lysocline is a more gradual transition zone near the CCD where dissolution rates sharply increase.

This depth dependency explains why some oceanic regions might be devoid of carbonate sediments, particularly those areas deeper than the CCD. Knowing the CCD helps oceanographers predict the types of sediments found at various depths of the sea bed.