Introduction
Every day, a silent rain of particles falls through the ocean water column and settles on the seafloor. Dead phytoplankton, the shells of foraminifera and coccolithophores, the remains of diatoms and radiolarians, fine clay carried by wind from continents, volcanic ash from eruptions, and even microscopic cosmic dust from space — all accumulate layer by layer on the ocean floor. In the deep ocean far from any continent, sedimentation proceeds at extraordinary geological slowness: 2–20 millimetres (0.08–0.79 in) per 1,000 years. A layer the thickness of a penny might represent 10,000 years of Earth history.
These sediments, however mundane they sound, are the planet's most complete and continuous archive of its recent geological history. Unlike rocks on land, which are constantly being eroded, weathered, buried, and metamorphosed, the deep-sea sediment record is largely undisturbed — laid down in cool, dark, quiet water and left to accumulate. A sediment core pulled from the deep North Atlantic might contain a continuous record of ocean temperatures, ice volume, atmospheric CO₂, ocean circulation patterns, and plankton evolution extending back 50 million years. No other geological archive matches this: ice cores from Antarctica reach back 800,000 years, tree rings a few thousand, coral records a few centuries. The deep-sea sediment record is in a class of its own.
The international effort to drill and recover these sediment cores — the Deep Sea Drilling Project (DSDP, 1968–1983), succeeded by the Ocean Drilling Program (ODP, 1985–2003) and the Integrated Ocean Drilling Program / International Ocean Discovery Program (IODP, 2003–present) — has produced one of the most productive archives in the history of science. Cores from around the world are stored in refrigerated repositories in the United States, Germany, and Japan, available to any researcher who applies for access.
Key Terms
Sediment that settles slowly through the water column from the surface ocean, far from any continental source. Includes calcareous ooze (from foraminifera, coccolithophores, pteropods), siliceous ooze (from diatoms and radiolarians), and red clay (fine mineral particles remaining after all biogenic material dissolves). Accumulates at 1–20 mm (0.04–0.79 in) per 1,000 years.
A pelagic sediment composed predominantly of calcium carbonate (CaCO₃) shells of planktonic foraminifera and coccolithophores. Covers about 48% of the deep ocean floor. Does not form below the carbonate compensation depth (CCD), where the rate of dissolution exceeds the rate of supply and CaCO₃ dissolves before reaching the seafloor.
The depth (typically 4,000–5,000 m (13,124–16,405 ft), variable by ocean basin) below which seawater is so corrosive to calcium carbonate that CaCO₃ dissolves faster than it can accumulate. Below the CCD, the seafloor is covered by red clay rather than calcareous ooze. Foraminifera shells above the CCD sink slowly; as they pass through the CCD they dissolve.
A sedimentary deposit formed by a turbidity current — a dense, sediment-laden flow that rushes down the continental slope and fans out across the abyssal plain. Turbidites are recognisable as graded beds (coarser at the base, finer at the top) interbedded with fine pelagic sediment. They can transport continental sediment thousands of kilometres into the deep ocean.
The ratio of the heavy oxygen isotope (¹⁸O) to the lighter (¹⁶O) in a sample, expressed relative to a standard. In foraminifera shells, δ¹⁸O reflects both ocean temperature and the volume of ice on land (which preferentially sequesters ¹⁶O). Changes in δ¹⁸O down a sediment core record glacial-interglacial cycles over millions of years.