Marine sediments are layers of material that accumulate on the ocean floor, forming a geological record that covers most of the seabed. These deposits range in size from fine clay particles and mud to coarse sand and gravel, creating a blanket over the Earth’s oceanic crust. The material originates from various sources, including eroded continental rock, the skeletal remains of marine life, and chemical reactions within seawater.
Sediment thickness varies dramatically, from a few millimeters on younger crust near mid-ocean ridges to several kilometers on continental margins. The slow, continuous accumulation of these particles, often referred to as a “stupendous snowfall,” creates an archive that preserves information about Earth’s history, ocean chemistry, and past climate.
Where Marine Sediments Originate
The source of marine sediments dictates their chemical and physical composition. The largest volume comes from land, categorized as terrigenous material, derived from the weathering and erosion of continental rocks.
Most terrigenous sediment is transported by major river systems, which discharge enormous quantities of fine-grained material like silt and clay into the coastal zone. Wind also sweeps dust and volcanic ash into the atmosphere, carrying it over the ocean where it settles. Glaciers and icebergs further contribute by carrying large, unsorted fragments of rock, known as ice-rafted debris, which drop to the seafloor as the ice melts. Once in the ocean, powerful turbidity currents—dense, sediment-laden underwater flows—transport material down continental slopes onto the deep abyssal plains.
Another major source is the ocean itself, where marine organisms create biogenous material from their skeletal remains. The shells and tests of microscopic organisms like plankton and foraminifera continuously rain down through the water column, accumulating as organic-rich sediment. Minor inputs include materials from submarine volcanoes and cosmogenous dust from extraterrestrial sources.
Categorizing Sediment Types
Marine sediments are classified into four main types based on their ultimate source, which determines their composition once deposited.
- Terrigenous sediments: Also known as lithogenous sediments, these are the most abundant by volume, making up about 55% of the ocean floor. They consist of rock fragments, quartz, feldspar, and clay minerals derived from the physical and chemical breakdown of land-based rocks.
- Biogenous sediments: Covering about 45% of the ocean floor, these are composed of the accumulated hard parts of marine organisms. When these deposits contain at least 30% biogenic material, they are classified as oozes. Calcareous oozes consist of calcium carbonate shells, while siliceous oozes are made of silica tests.
- Hydrogenous sediments: These are formed through chemical reactions within the seawater, causing dissolved minerals to precipitate out and solidify. Examples include manganese nodules and metal-rich precipitates found near hydrothermal vents. These sediments cover less than 1% of the seafloor.
- Cosmogenous sediments: This is the least abundant category, originating from extraterrestrial sources. This material is primarily composed of interplanetary dust and debris from meteorites.
Sediments as Climate Archives
The layered structure of marine sediments creates a chronological record of Earth’s history, making them invaluable archives for paleoceanography and climate research. As particles settle, they trap physical, chemical, and biological indicators, or proxies, that reflect ocean and atmospheric conditions at the time of deposition. Scientists access this record by conducting deep-sea coring, extracting long, cylindrical columns of sediment from the seafloor.
Analysis of microfossils preserved in these cores is a primary method for reconstructing past climates. For example, the ratio of oxygen isotopes within the calcium carbonate shells of organisms like foraminifera reflects the ocean’s temperature and ice volume when the organism was alive. The distribution of different species can also indicate past water mass characteristics, such as salinity and nutrient content.
Another chemical proxy involves organic molecules, such as alkenones synthesized by algae, whose structure changes depending on water temperature. The ratio of these compounds acts as a “paleothermometer,” allowing researchers to reconstruct sea surface temperatures over long timescales. Analyzing these layers allows researchers to create high-resolution records of climate variability, including glacial cycles and extreme weather events, extending back hundreds of thousands of years.
Modern Scientific Exploration
Modern scientific exploration of marine sediments employs a combination of advanced technology and laboratory analysis to unlock the information contained within these archives.
Methods and Analysis
Geophysical methods, such as sub-bottom profile surveys and seismic reflection, are used to map the subsurface layers of the seafloor without physical disturbance. These techniques use acoustic pulses to penetrate the seabed, generating images that reveal the thickness and structure of buried sediment layers and underlying rock. Physical sampling remains a fundamental step, involving the use of remotely operated vehicles (ROVs) and specialized drilling vessels to collect cores from the deep ocean.
Once retrieved, samples undergo sophisticated laboratory analysis. Microanalytical approaches measure chemical proxies at high resolution, sometimes allowing for seasonal or annual climate reconstructions. Researchers also use radiocarbon dating on microfossil shells to determine the precise age of the layers and assess how mixing by burrowing organisms, called bioturbation, affects the archive’s time resolution.
Economic and Ecological Roles
The economic significance of marine sediments drives exploration for deep-sea mineral resources. Deposits like polymetallic nodules, which are hydrogenous sediments rich in manganese, nickel, copper, and cobalt, represent potential sources of metals needed for modern technology. Research is also focused on the ecological role of these sediments, which form complex habitats for unique deep-sea organisms, including the microbenthos that play a role in carbon cycling.