Science: Salinity Explained

The following is based on a webinar featuring Dr. Stephen Riser (Univ. of Washington) [View the video].

What do we mean by salinity? Anybody who has ever taken an ocean swim knows it is salty. That salty taste is mostly sodium and chloride ions. In fact, every one of the 92 naturally occurring elements are present in seawater to some extent, most of them in vanishingly small extents.

At right are the top ten or so ions that are present in seawater. Sodium and chloride together are about 85% by weight of all the dissolved stuff in sea water. But there are other ions, as well.

When we talk about salinity, it’s not just sodium and chloride, we’re talking about everything. The beauty of this whole idea is the concept of salinity involves all of these ions, and the salinity varies from place to place. The amazing thing is that these ions are in almost constant proportions in most of the world ocean. No matter the actual concentration, if you just measure one of these ions, you can infer what all of the other ones are from that.

Substance Symbol % of Seawater % of Total Weight of Salt
Chloride Cl- 19.980 55.04
Sodium Na+ 10.556 30.61
Sulfate SO4-2 2.649 7.68
Magnesium Mg+2 1.272 3.69
Calcium Ca+2 0.400 1.16
Potassium K+ 0.380 1.10
Bicarbonate HCO3- 0.140 0.41
Bromide Br- 0.065 0.19
Boric Acid H3BO3 0.026 0.07
Strontium Sr+2 0.013 0.04
Fluoride F- 0.001 0.00
Total 34.482% 99.99%

The table shows that in a kilogram of water, there are 34.482 grams of dissolved salt that we call “salinity.” The other 965 or so grams is freshwater. So when we’re talking about salinity, we’re talking about the number at the bottom of the table: 34.482.

The dissolved matter is mostly sodium chloride. However, other ions are also derived from the internal chemistry of the ocean, the dissolving of biological things, the sediments from the sea bottom and, of course, runoff from the land. That’s how this dissolved matter gets into the ocean.

Figure 1 shows information similar to that in the table above. A kilogram of seawater is depicted at left. Light green represents water in that kilogram. The orange-colored slice – 34.4 grams – represents the ions, mostly sodium and chloride. But there is other matter, as well. Together, these are the major constituents of seawater.

Major constituents of seawater
Figure 1. Major constituents of seawater.

Looking at observations of the ocean going back a century, it’s amazing how many temperature measurements exist. In fact, even Benjamin Franklin collected temperature data in the Gulf Stream during the 18th century. Even in his time, you could measure ocean temperature reliably with a thermometer.

But even though salinity is a basic property of the ocean, it is much harder to measure. You have to actually measure what’s in seawater. Traditionally, that meant titrating samples of seawater to figure out the percentage of “dissolved stuff.” Imagine doing this on a rolling ship in bad weather! This helps to explain why there are much fewer historical measurements of salinity than temperature.

In recent years, salinity has been determined by measuring the electrical conductivity of seawater. After this technique became popular, the number of reliable salinity measurements increased significantly.

And as soon as we started putting temperature and salinity instruments on thousands of Argo floats, the amount of data increased by an order of magnitude. With NASA satellites and Argo floats, we’re finally in a position here to say something quantitative about salinity, and why it matters, and what it can tell us about the ocean circulation.

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