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.