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Ocean sediments accumulate slowly over millions of years and preserve a detailed archive of past ocean conditions. Pelagic sediments include biogenic oozes (made of calcareous or siliceous microfossils such as foraminifera and radiolarians), terrigenous clays blown or washed from land, and hydrogenous deposits (e.g., manganese nodules). Deep-sea sediment cores provide records of past ocean temperatures (via oxygen isotope ratios in foraminiferal shells), ice volume, ocean circulation, and productivity. The carbonate compensation depth (CCD) — the depth below which CaCO₃ dissolves — controls the global distribution of calcareous sediments.
Interpret a simplified oxygen isotope record from a foraminifera core, identifying glacial-interglacial cycles. Map the distribution of sediment types across an ocean basin and explain the pattern using water depth relative to CCD and proximity to land.
From your study of sedimentary rocks, you know that sediments accumulate in layers and that each layer records conditions at the time of deposition. From seafloor spreading, you know that oceanic crust forms at mid-ocean ridges and moves outward, aging as it goes. Ocean sediments sit on top of this crust, and the fundamental insight is that they provide the most continuous and detailed record of Earth's climate history available anywhere — far more complete than most terrestrial records, which are frequently interrupted by erosion.
Ocean sediments come in three main varieties. Biogenic sediments (or oozes) are made from the skeletal remains of microscopic organisms that lived in the surface waters and sank to the bottom when they died. Calcareous ooze comes from organisms like foraminifera and coccolithophores that build shells of calcium carbonate; siliceous ooze comes from diatoms and radiolarians that build shells of silica. Terrigenous sediments are particles weathered from continents and delivered to the ocean by rivers, wind, or ice — they dominate near coastlines and downwind of major deserts. Hydrogenous sediments precipitate directly from seawater through chemical reactions, forming features like manganese nodules that grow at rates of millimeters per million years.
The distribution of these sediment types across the ocean floor is not random — it follows predictable rules. The most important is the carbonate compensation depth (CCD), typically at 4,000–5,000 meters. Above this depth, calcareous shells accumulate on the seafloor; below it, the water is so cold and under such high pressure that it becomes corrosive to calcium carbonate, dissolving shells faster than they accumulate. This means the deep abyssal plains are covered in terrigenous clay (the only material that survives at any depth), while shallower mid-ocean ridges and plateaus are blanketed in calcareous ooze. The CCD itself has shifted up and down through geological time in response to changes in ocean chemistry, and tracking these shifts in sediment cores reveals past changes in ocean circulation and carbon cycling.
The real power of ocean sediments lies in what the microfossils record chemically. When a foraminifer builds its calcite shell, it incorporates oxygen atoms from seawater. Oxygen comes in two stable isotopes — lighter ¹⁶O and heavier ¹⁸O — and the ratio between them in the shell (written as δ¹⁸O) depends on two things: the temperature of the water the organism lived in, and the isotopic composition of the seawater itself (which changes as ice sheets grow and shrink, preferentially locking up light ¹⁶O on land). By measuring δ¹⁸O down a sediment core, paleoceanographers reconstruct a timeline of glacial and interglacial periods stretching back tens of millions of years. Each centimeter of core may represent thousands of years of history, and the global network of deep-sea cores has allowed scientists to correlate these records across ocean basins, revealing the synchronized rhythm of ice ages driven by orbital variations in Earth's path around the Sun.