The Density Conveyor
Water that sinks off Greenland surfaces in the Pacific roughly a thousand years later, carrying heat and carbon with it.
Phase 0 · The phenomenon
A thousand-year journey
A schematic loop: a blue deep-water path starts at a sinking point near Greenland in the North Atlantic, runs south along the Atlantic floor, eastward around Antarctica, and north into the Indian and Pacific Oceans, where it rises; a red surface path returns westward and north to the Atlantic. Labels mark sinking near Greenland and upwelling in the North Pacific roughly a thousand years later.
Every winter, in the seas between Greenland, Iceland and Norway, surface water gets so cold and salty that it sinks two to three kilometres to the sea floor. It does not come back up nearby.
Chemists who measure radiocarbon in seawater find that the deep water of the North Pacific last touched the air roughly a thousand years ago. Along the way the deep water carries the carbon dioxide it dissolved at the surface, out of contact with the atmosphere.
What makes surface water sink, what could stop it, and how do we know how old deep water is? This lab builds the conveyor one station at a time.
What makes ocean water sink, what could stop it from sinking, and how can water that sinks off Greenland take about a thousand years to reach the surface of the Pacific?
What this lab works toward
HS-ESS2-4Use a model to explain how changes in the energy flowing into and out of Earth's systems change climate.HS-ESS2-6Build a quantitative model of how carbon cycles among ocean, air, rock and living things.
Plain-English summaries. The course maps these performance expectations to this lab; the official wording is published at nextgenscience.org.
The stations
- Density BenchHow do temperature and salinity together set seawater's density?
- Sea Ice FactoryHow does making sea ice turn surface water into deep water?
- Meltwater LidHow much fresh meltwater does it take to stop surface water from sinking?
- Conveyor ClockHow does radiocarbon date deep water, and how much carbon does cold sinking water carry down?
Each station runs the same loop: predict before you see any data, investigate the model and record runs, reveal what the measurements show, then explain the gap between your prediction and the result. A claim-evidence-reasoning checkpoint and an exit ticket close the lab.
- Density uses a straight-line approximation that matches real seawater well for open-ocean temperatures and salinities. Real seawater density is curved: cold water is less sensitive to temperature than warm water.
- The conveyor is drawn as one loop. Real overturning has several branches, eddies and mixing, and is driven by winds and deep mixing as well as by sinking.
- In the sea-ice and meltwater stations, all the salt or fresh water mixes evenly through the surface layer at once.
- The radiocarbon age measures the time since the water last exchanged carbon with the air, corrected for the surface ocean's own small radiocarbon offset. Mixing of waters of different ages blurs it.