Marine Sediments and Stratigraphy

From windblown dust to the shells of plankton, material settling onto the seafloor builds layered deposits whose composition and sequence record the history of the ocean, climate, and life.

Definition

Marine sediments are the unconsolidated particles that accumulate on the seafloor from biological, terrestrial, chemical, and extraterrestrial sources; stratigraphy is the study of their layering and the chronological record it provides.

Scope

This topic covers the principal types of marine sediment — terrigenous, biogenous, hydrogenous, and cosmogenous — their sources and transport, the controls on their distribution across the seafloor, the processes of accumulation and diagenesis, and the stratigraphic interpretation of layered deep-sea deposits.

Core questions

• What are the main types and sources of marine sediment?
• What controls where different sediment types accumulate on the seafloor?
• How do biogenic oozes form, and why are they restricted by depth and chemistry?
• How is the layered sediment record dated and interpreted stratigraphically?

Key theories

Sediment provenance and distribution

Sediment type reflects the balance of inputs from land, biological productivity, and seawater chemistry, so coarse terrigenous material dominates margins while fine clays and biogenic oozes blanket the deep sea.

Carbonate compensation depth

Below a depth where dissolution exceeds supply, calcium carbonate shells dissolve before burial, so calcareous oozes are confined to shallower seafloor while siliceous and clay deposits dominate deeper basins.

Mechanisms

Particles reach the seafloor from rivers and wind (terrigenous), the sinking remains of plankton (biogenous), precipitation from seawater (hydrogenous), and space (cosmogenous); their distribution depends on supply, productivity, and dissolution. Calcareous shells dissolve below the carbonate compensation depth, and accumulating layers are progressively compacted and chemically altered during burial.

Clinical relevance

Marine sediments host hydrocarbon and mineral resources, record submarine hazards such as turbidity flows, and preserve the microfossil and geochemical archives that underpin reconstructions of past climate and ocean conditions.

History

The classification of deep-sea sediments dates to John Murray's analyses of Challenger expedition samples in the late nineteenth century; twentieth-century coring and scientific ocean drilling refined the understanding of sediment distribution, the carbonate compensation depth, and deep-sea stratigraphy.

Key figures

• John Murray
• Wolfgang Berger
• Eugen Seibold

Seminal works

• seiboldBerger2017
• kennett1982

Frequently asked questions

What is a deep-sea ooze?

An ooze is a fine deep-sea sediment made up largely of the microscopic shells of plankton, classified as calcareous or siliceous depending on whether those shells are calcium carbonate or silica.

Why is there little calcium carbonate in the deepest ocean?

Below the carbonate compensation depth, seawater dissolves calcium carbonate faster than it arrives, so carbonate shells dissolve before they can accumulate, leaving clays and siliceous remains instead.