(Also see Ocean Currents for a related activity)
The freezing temperature of water depends on the amount of dissolved salts (salinity). Normal ocean water, with a salinity of about 3.4%, begins to freeze when temperatures reach about -1.9°C. At its maximum extent in the winter, sea ice covers about 19 million square kilometres of the Southern Ocean near Antarctica. Approximately 80% of this ice melts each summer, contributing to the global mixing of ocean water.
Vertical mixing of ocean water, known as ‘overturning circulation’ or ‘thermohaline circulation’, is an important aspect of the global current system that is driven primarily by rising and sinking of water masses at high latitudes in both the northern and southern hemispheres.
The seasonal formation and melt of sea ice is the dominant factor controlling the salinity and density of surface ocean waters in the polar regions. Salt is expelled as the ocean water freezes to form sea ice. This creates dense brine that sinks and flows down the continental shelf of Antarctica to form Antarctic Bottom Water – the densest water in the open ocean. This water flows outward from the Southern Ocean and through other ocean basins as part of the global ocean circulation ‘conveyor belt’ that distributes heat, nutrients and gases around the world. There are only a few areas in the world where this dense water is formed.
Sea ice extent in the Arctic has been declining rapidly over the past few decades. Although there are currently no clear trends in the extent of Antarctic sea ice, with the exception of the area around the Antarctic Peninsula, numerous climate models agree that Antarctic sea ice will decline in the future. A decrease in the amount of sea ice may slow the overturning circulation, decreasing the ocean’s ability to absorb atmospheric carbon dioxide and affecting the distribution of heat around the globe.
MAKING SEA ICE
*see below for directions to make your own hydrometer
NOTE: Neither the hydrometer nor the multimeter measures salinity directly.
A hydrometer measures the density of a liquid. Density is a measurement of the quantity of matter contained in a given volume and is usually expressed as grams per cubic centimetre. When salt is dissolved in water, the volume of the water does not change, but the density increases. Buoyancy is a function of density – it is the ability of an object to float on a liquid because of the greater density of the liquid. The higher the salinity, the denser the liquid and the higher the hydrometer will float. You can purchase hydrometers in stores that sell aquarium equipment.
A multimeter, set on the ohm scale, measures electrical resistance, which is a measurement of properties that limit the ability of a substance to conduct electricity. Salt in water decreases the resistance, which results in a lower reading on the scale. Therefore, the more saline the water, the lower the reading on the meter. Keep the two probes an equal distance apart when making comparative measurements and hold the probes still until the numbers on the scale stop moving. Inexpensive multimeters are available at electronics supply shops.
Attach a small piece of modelling clay or plasticine to one end of a drinking straw. Make sure the clay completely seals the straw so that no water can get in. Adjust the amount of clay so that the hydrometer floats upright with about 5 cm out of the water. If it sinks to the bottom, remove some clay. If it tilts to one side, add more clay.
To calibrate your hydrometer, place it in a deep clear container (such as the bottom of a 2-litre soft drink bottle) of fresh water. Grab the straw at the water line and pull it out of the water Draw a line on the straw at this point with a permanent marker. This represents the density of fresh water. Use the same volume of liquid, at the same temperature, when you test your other samples.