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Place 04 · The Ridge and the Trench · Chapter 7 of 7
What the Mud Remembers
The deep seafloor is buried under a slow snow of dust, shells and metal, and every layer is a page of Earth’s history that nobody has finished reading.
About 15 minutes · 1 video · 1 3D model · 2 quick activities
Start here
A history book written in mud
Every year a slow rain of dust, shells and bits of rock settles onto the seafloor, each layer landing on top of the last. It comes in four kinds: from the land, from living things, from seawater itself and, in tiny amounts, from space. Pull up a sediment core and you can read that history in order, oldest at the bottom, and the shells inside even record how cold the ocean was, back through the ice ages and millions of years before them.
Watch this
See it move
Media & Methods Lab UTL ETH Zurich — Marine sediments as climate archives. Four minutes from ETH Zurich on why geologists get excited about mud.
While you watch, look for
- What the layers of seafloor sediment are made of
- What the layers can tell scientists about past climate
- Something you did not expect. Hold onto it.
The short version
Three things going on here
From the land
Lithogenous: the continents, crumbled
Rivers, wind and glaciers wear down the continents and carry the bits to sea as sand, silt and clay; about 90 percent of it is thought to arrive by river. Most of the coarse material settles near the coast. Only the finest clay drifts all the way to the middle of the ocean, where it builds up at about 1 mm every thousand years as abyssal clay.
From the living
Biogenous: a snow of shells
Tiny plankton build shells of calcium carbonate (foraminifera and coccolithophores) or of glassy silica (diatoms and radiolarians), and when they die the shells sink. Where they make up at least 30 percent of the sediment it is called ooze. Calcareous ooze covers about 48 percent of the seafloor, but mostly where the water is less than about 4 km deep, because deep, cold water dissolves carbonate; the White Cliffs of Dover are ancient coccolith ooze turned to chalk.
From the water
Hydrogenous: grown in place
Some sediment crystallizes straight out of seawater. Manganese nodules, lumps of manganese and iron oxides with copper, nickel and cobalt mixed in, grow around a bit of shell or bone by just a few millimeters every million years, so they survive only where almost nothing else falls on top. That is why the quietest abyssal plains of the Pacific are strewn with them, and why mining companies are so interested.
Photos: Martin Škeřík, Valentin Onu, Jake Ganse / Pexels
Play with it
Hold a grain of ooze
This is a single foraminifer shell, Globigerinoides ruber, about half a millimeter across, scanned with X-rays and blown up so you can spin it. Piled up by the billions, shells like this make calcareous ooze, and the chemistry of foram shells is one of the main ways scientists work out how warm the ocean was in the past.
3D model: “Foraminifera – Globigerinoides ruber” by Secteur Sciences et Technologies – AMU on Sketchfab (X-ray scan by Thibault de Garidel-Thoron and Vladimir Vidal, CEREGE; CC BY-NC-SA).
Two questions to answer while you are in there
- Count the chambers. Did this shell grow all at once, or one chamber at a time, each bigger than the last?
- This shell is calcium carbonate. What would happen to it if it sank to 5,000 m in the deep Pacific?
Try it yourself
Check yourself
Try it
Land, Life or Water?
Eight quick calls: decide where each seafloor sediment came from.
Try it · interactive
Core Driller
Drill sediment cores near a river mouth, under the rich equatorial Pacific, out on a quiet abyssal plain and beside a mid-ocean ridge, then read the layers: lithogenous mud, calcareous and siliceous ooze, abyssal red clay and manganese nodules, and why each one ends up where it does.
Open the Core Driller in its own window (opens in a new tab).
Words you will hear
Four terms, that is all
Lithogenous sediment
Sediment made of rock bits from land
Biogenous ooze
Sediment at least 30% plankton shells
Sediment core
Tube of layered seafloor mud, oldest at bottom
Manganese nodule
Slow-growing metal lump on the deep seafloor
Check your understanding
Practice the whole chapter
Everything in this chapter, in eight quick questions. Retry as often as you like. Read the feedback on every answer, right or wrong, because that is where the learning is.
Practice
Practice: What the Mud Remembers
Eight questions covering the video, the three cards, the four terms and the interactive above.
If you want more
Go deeper
- Webb 12.1 — Classifying Sediments — LibreTexts · 8 minutes
- Webb 12.4 — Hydrogenous Sediments — LibreTexts · 8 minutes
- NASA Earth Observatory — Paleoclimatology: A Record from the Deep — NASA · 8 minutes
Photo credits (Pexels): Magda Ehlers, Rodion Kutsaiev, Martin Škeřík, Valentin Onu, Jake Ganse.