Deep Sea Exploration - Schmidt Ocean Institute

 

Schmidt Ocean Institue: Nazca Ridge High Seas

 

The Composition and Distribution of Ocean Sediments (III)

The Dominance of Biogenous Sediments in the Oceanic Zone

While lithogenous sediments dominate the continental margins, biogenous sediments make up the majority of the seafloor surface in the deep ocean basins. This is due to the abundance of plankton in the surface waters and the lack of strong currents to transport larger sediment grains to these depths.

The two main types of biogenous sediments found in the oceanic zone are:

Siliceous oozes, which are dominated by the shells of diatoms and radiolaria.

Calcareous oozes, which are dominated by the shells of coccolithophores and foraminifera.

The distribution of these biogenous sediments is largely determined by the availability of nutrients in the surface waters and the depth of the seafloor.

Siliceous Oozes

Siliceous oozes tend to dominate in areas where nutrient-rich, cold waters upwell to the surface, such as along the eastern equatorial regions and in polar areas. These nutrient-rich waters provide the necessary resources for silica-shelled plankton to thrive.

Calcareous Oozes

Calcareous oozes, on the other hand, are more common in warmer, less nutrient-rich surface waters. However, the distribution of calcareous oozes is also limited by the depth of the seafloor. Below the calcium carbonate compensation depth (CCD), which is around 4,500 meters on average, the acidity of the seawater is high enough to dissolve the calcium carbonate shells before they can accumulate on the seafloor. As a result, calcareous oozes are typically found in shallower areas, such as along the mid-ocean ridges.

Abyssal Clays

In the deep ocean basins, where neither siliceous nor calcareous oozes can accumulate, the dominant sediment type is abyssal clays. These fine-grained, lithogenous sediments are derived from the settling of airborne dust and ash, as well as the weathering of rocks on land.

The Importance of Ocean Sediments

The study of ocean sediments is crucial for understanding the Earth's history, climate, and geological processes. The composition and distribution of these sediments can provide valuable information about past environmental conditions, changes in sea level, and the movement of tectonic plates.

For example, the presence of certain types of biogenous sediments, such as calcareous or siliceous oozes, can indicate changes in ocean productivity and nutrient availability over time. Similarly, the accumulation of lithogenous sediments can reveal information about past patterns of erosion and weathering on land.

By studying the complex and diverse world of ocean sediments, scientists can gain a deeper understanding of the Earth's dynamic systems and how they have evolved over geological timescales. This knowledge is essential for addressing pressing environmental challenges, such as climate change and ocean acidification, and for informing sustainable management of marine resources.

The Composition and Distribution of Ocean Sediments (II)

Biogenous Sediments

Biogenous sediments are composed of the shells and debris of organisms that live in the surface waters of the ocean and settle to the seafloor when those organisms die. The most important contributors to biogenous sediment are the single-celled autotrophic and heterotrophic plankton, such as diatoms, coccolithophores, radiolaria, and foraminifera. These organisms produce shells made of either silica (siliceous oozes) or calcium carbonate (calcareous oozes).

Hydrogenous Sediments

Hydrogenous sediments are minerals that are precipitated directly from the ions dissolved in seawater. This can occur in shallow, evaporative environments, such as inland seas, where minerals like halite (table salt), gypsum, and calcite can precipitate. Hydrogenous sediments can also form in deep ocean environments, where dissolved phosphates and manganese become supersaturated and precipitate as nodules.

Cosmogenous Sediments

Cosmogenous sediments are the rarest type of ocean sediment, consisting of meteorites and tektites (glass nodules formed from the melting of Earth's crust after a meteorite impact). These extraterrestrial materials are found in greater abundance in the ocean than on land due to the slow sedimentation rates in the deep ocean, which allow them to accumulate more easily.

The Dominance of Lithogenous Sediments

While biogenous sediments cover the majority of the seafloor surface, lithogenous sediments make up the largest percentage by volume of marine sediments. This is because lithogenous sediments, particularly the fine-grained muds, are able to be transported long distances by rivers and winds and accumulate in the deep ocean basins.

However, the thickest accumulations of sediment are found in the continental margin environments, where lithogenous sediments can pile up to depths of up to 9 kilometers. This is due to the much higher sedimentation rates in these areas, which can range from 0.5 to 800 meters per 1,000 years, compared to the slow sedimentation rates of 1 centimeter per 1,000 years in the deep ocean basins.

The Composition and Distribution of Ocean Sediments (I)

The ocean floor is a vast and complex environment, with a diverse array of sediments that have accumulated over millions of years. These sediments play a crucial role in our understanding of the Earth's history, climate, and geological processes. In this comprehensive blog post, we will delve into the different types of ocean sediments, their sources, and their distribution across the seafloor.

Understanding the Vocabulary of Sediment Size

To begin, let's familiarize ourselves with the terminology used to describe the size of sediment grains. While there are many terms used to describe sediments of all sizes, we can simplify them into three main categories: muds, sands, and gravels. Gravels are the largest sediment grains, with a diameter greater than 2 millimeters. Sands are the intermediate-sized grains, ranging from 1/16 millimeter to 2 millimeters in diameter. Muds are the smallest grains, less than 1/16 millimeter in diameter.

The distribution of these different grain sizes in the ocean is closely tied to the energy levels of the water environments in which they are found.

Sediment Distribution in the Ocean

The ocean can be divided into two major geographic zones: the neritic zone, which covers the continental shelf, and the oceanic zone, which encompasses the continental slope, rise, abyssal plains, mid-ocean ridges, and trenches.

Neritic Zone

The neritic zone is characterized by faster-moving currents that interact with the shallow seafloor. In this environment, larger sediment grains, such as gravels and coarse sands, can be picked up and transported. Rivers that enter the neritic zone tend to retain their mud-sized sediments in suspension, allowing them to be carried a considerable distance offshore. Fine sands may collect in medium-energy coves and be carried out to the edge of the continental shelf, while muds typically settle out only in closed-off, low-energy lagoons.

Oceanic Zone

In contrast, the oceanic zone is characterized by deep, slow-moving waters with little interaction with the seafloor. In this environment, the only sediments that can be transported are the fine-grained muds, either through suspension in the water column or by floating icebergs. Gravels and sands are unable to be transported to the deep ocean by normal currents and instead remain closer to the continental margins.

The Sources of Ocean Sediments

There are four major sources of sediment that contribute to the ocean floor:

Lithogenous or Terrigenous Sediments

Lithogenous or terrigenous sediments are those that originate from the weathering and erosion of rocks on land. These sediments can be transported to the ocean by rivers, glaciers, or coastal erosion. Lithogenous sediments are typically composed of bits of rocks and minerals and can be classified as either "immature" or "mature" based on the degree of physical and chemical weathering they have undergone.

Upcoming events: 38th International Meeting of Sedimentology in Huelva, Spain

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Marine Carbon Biochemistry

 

Upcoming events: 7th World Conference on Marine Biodiversity

The 7th World Conference on Marine Biodiversity will be organized in Bruges between November 17th and 20th 2026.

The World Conference on Marine Biodiversity series (WCMB) is conducted tri-annually. The WCMB is a high-level international meeting that focuses on the conservation and sustainable use of marine biodiversity. The conference provides a platform for policymakers, scientists, conservationists, industry representatives, and other stakeholders to discuss current and emerging marine biodiversity issues and identify ways to protect and sustainably manage marine ecosystems and their resources.

Previous editions were held in:

• Valencia, Spain (2008) - Outcome: Valencia Declaration - A Plea for the Protection of Marine Biodiversity
• Aberdeen, Scotland (2011) - "Our Oceans, Our Future"
• Qingdao, China (2014) - "Life in the Changing Ocean"
• Montreal, Canada (2018) - "Connecting with the Living Ocean"
• Auckland, New Zealand (2020) - "Understanding the current scale and importance of biodiversity in the marine environment"
• Penang, Malaysia (2023) - "Marine Biodiversity Challenges in the Anthropocene"

 Further information