Introduction
Some people have noted that planet Earth should really be called planet Water, or perhaps planet Ocean, because the dominant geographical feature is the huge, interconnected single ocean. What is the structure of the ocean? In this lesson, we will look at the geography and chemistry of the seas of planet Ocean. The next lesson will follow up with the ocean's circulation.
Geography of the Oceans
Where did the ocean's water come from? No one knows exactly. Because comets in space contain water (they are, in essence, great balls of ice around a rocky core), it is possible that comets contributed to Earth's water soon after the formation of the solar system. At that time, huge numbers of comets would still have been flying around the inner solar system, and as they impacted the earth, they released their water to Earth's surface.
This theory seems to present a problem, because our neighboring planets of Venus and Mars are relatively dry. But Venus probably lost its water through breakdown of the water in its hot temperatures. And the rovers that recently surveyed the chemistry and geology of Mars have provided evidence that Mars once had liquid water on its surface, probably even a shallow ocean. How much water Mars lost to space and how much is locked up frozen as ice beneath the Martian surface is still unknown.
By whatever means Earth got its water, there is plenty of it. All the water on the planet—including rivers, lakes, ice caps, groundwater, and of course the oceans—is collectively called the hydrosphere.The greatest amount of water in the hydrosphere, more than 97%, resides in the oceans and is salty.
Geographically, the oceans cover about 71% of Earth's surface. But is there one ocean or several? We are all familiar with three oceans: the Pacific, the Atlantic, and the Indian oceans. These are fairly well distinguished by the shapes of the continents. Oceanographers cite two more areas worthy of being called oceans. One is the Arctic Ocean, the area bounded to the south by Russia and Canada, and in the center of which is the North Pole. The Arctic Ocean is mostly covered with ice in the winter but large portions of it open up in the summer. Because it is blocked by land around a large part of its perimeter, and it exchanges water with the Atlantic around Greenland and with the Pacific through the narrow Bering Strait, the Arctic Ocean is the most isolated of all the oceans and differs most from the others in its chemistry.
The other region worthy of being called an ocean is hardly bounded at all on one side. It is the Antarctic Circumpolar Ocean, also referred to as the Southern Ocean. This is the belt approximately 10° of latitude in width, which rings Antarctica. To its south, it is obviously bounded by the circular continent of Antarctica, which sits over the South Pole and extends to about 70° south latitude. But to its north side, the Antarctic Circumpolar Ocean connects with the southern ends of the Atlantic, Pacific, and Indian oceans.
The reason that the Antarctic Circumpolar Ocean is a well-defined region is because it is also called the Antarctic Circumpolar Current. It sweeps around Antarctica, around and around, from west to east, like a giant whirlpool. Its motion stirs in water from the other oceans and gives water back to them as well.
But the question remains: Are there many oceans or one? As we have seen, the oceans are interconnected with each other. The Atlantic, Pacific, and Indian oceans are connected by the Antarctic Circumpolar Ocean, and the Arctic Ocean has its own connections to the Atlantic and Pacific oceans (but not the Indian). In a sense, a single world ocean exists, with different zones that are only partially but not completely isolated from each other. For convenience, though, we will continue to talk about the different regions as oceans. Just keep in mind that they are all connected.
Let us now follow a path starting at a beach, say along the Atlantic coast, perhaps at the famous Jersey shore of New Jersey, and follow the ocean's bottom as it changes in depth. You very quickly get into water over your head and go to a depth of about 100 meters. But then the ocean stays at that depth for perhaps a hundred miles. You are on the continental shelf.
The continental shelf is geologically still part of the continent—in this case North America. The shelf rock is continental crust, not ocean crust. Worldwide, the continental shelf is a significant amount of area, about 10% in area of the world's oceans. In places, the shelf is very narrow, for example, off Peru. In other places, the shelf is extensive, for instance, in Indonesia. The Gulf of Alaska, the famous fishing ground for wild salmon, exists over the continental shelf. The continental shelf, in fact, contains most of the world's most productive fishing regions, or fisheries, as well as much of the world's petroleum producing rock formations.
At some point as you continue heading out toward the center of the ocean, the continental shelf starts to bend downward. You have reached the shelf break. This is the upper edge of the continental land mass, and as you continue outward, you start to head down in depth quite rapidly. You are on the continental slope. The slope is still part of the continent, but it does head downward to the ocean floor itself.
When you leave these underwater parts of the continents, you reach a depth of about 4 kilometers, a part of the ocean called the abyss, benthos, or simply the deep ocean. For most of the world's oceans, the deep ocean changes little in depth, varying from about 3 to 5 kilometers in depth, with the average of 4 kilometers. Here you are on ocean crust, all of which emerged at some point in the past from the volcanic ridges, as described in Lesson 5.
Deviations from the average depth of the ocean are, in fact, caused by plate tectonics. As we saw in Lesson 5, the Mariana Trench, in the western Pacific Ocean near the island of Guam, is more than 11 kilometers deep. Here the ocean's crust is subducted into the depths of the lithosphere. The fact that much of the ocean's floor is always moving into subduction zones is what makes the ocean's floor very "young" by geological standards, only one or at most two hundred million years old. The continents, on the other hand, contain rocks that are billions of years old.
The other deviation from the average depth of the ocean occurs at the oceanic ridges, or mid-ocean ridges (so-called, because they are rarely at the continental boundaries, though they are not always really in the middle, either). As you walk across the bottom of the Atlantic Ocean, 4 kilometers down beneath the ocean's surface, where it's pitch dark because no light reaches those depths, you eventually start walking uphill, as if climbing a gently sloping mountain range. You have started up the Mid-Atlantic ridge, where the phenomenon of seafloor spreading was discovered.
As you climb the Mid-Atlantic ridge, you rise for a kilometer or 2, until at the top, you reach regions where volcanic gases emerge from vents in the ocean's crust. This is where the ocean's crust is born. So at the ridges, the ocean is less deep, typically 2 kilometers, or about half the world ocean average depth of 3 kilometers.
The ridges are important in affecting the average level of the ocean itself, over long, geological time periods. Geologists know that sometimes Earth's tectonic activity increases, and sometimes it decreases, throughout periods that last tens of millions of years and longer. If the tectonic activity increases, the volume of the world's ocean ridges increases because ridges that spread faster create more buoyant shallow oceanic lithosphere. The ridges are higher, which pushes the water higher up onto to the continents. It's like getting into a bathtub—the water level rises. Alternatively, if the overall activity of plate tectonics decreases, the volume of the ridges drops, and the sea level goes down. Changes such as these are the reasons why the ocean was over large parts of what is now Colorado, Wyoming, Texas, and Oklahoma many millions of years ago.
One other deviation from the average depth of the world's ocean occurs at hotspots in the deep ocean. Hawaii is the most famous example. The Hawaiian Islands are volcanic islands that have arisen all the way from the deep ocean bottom to break the ocean's surface and present us with a lovely tropical paradise in the middle of the Pacific Ocean. Most oceanic hotspots (and oceanic islands) are in the Pacific Ocean basin.
Figure 13.1 shows the features that we have described. Look at it, then move on to the practice questions.
Chemistry In the Oceans
The ocean is not just water, as any mouthful that you get while swimming at the beach can tell you. There are chemicals in seawater. When you evaporate the water from a cup of seawater, the salt remains. It is typically about 3% by weight of the seawater, quite a significant amount.
Here is the breakdown of the contents of the ocean's salt:
Chloride (chlorine ions) 55%
So just the top five ions equal 98% of the total salt. The top two—chloride and sodium—make 85% of the salt, which is why we typically call salt sodium chloride. But if you actually use sea salt, manufactured by evaporating salt water, then the other ions are present as well. Indeed, dozens of other ions are found in the 2% of "all others" in the list above. Some of these lower-ranking ions are crucial to life, which we will see in a moment. But first, you'll need some more information about the ocean's salt.
The ocean does vary a bit from place to place in its salinity. Rather than speaking of percent salt, oceanographers use a related term called parts per thousand. After all, percent is actually parts per hundred. So if the ocean's salinity is 3%, that is the same as 30 parts per thousand. The symbol for parts per thousand adds another zero to the denominator of the percent sign. Therefore, 30 parts per thousand is written as 30‰. Much of the ocean's salinity is around 34, but it varies from about 33‰ to as high as 36‰. What causes this variation?
Some local variation is caused by the fresh water of rivers that flow into the ocean—for example, the Amazon. But much of the variation in salinity of the open ocean, away from the coast, is caused by different balances of the two processes that add or subtract water from the ocean's surface. These two processes, respectively, are precipitation and evaporation.
While precipitation (rain and snow) and evaporation occur everywhere across the ocean, they are not always in balance. Where precipitation exceeds evaporation, that area of the ocean will become a little fresher, or less salty. Where evaporation exceeds precipitation, the region will become more salty, or have a higher salinity. No area of the ocean is isolated, as we have seen, so no zone gets ever saltier or fresher through time. Zones where evaporation exceeds precipitation will just have a higher salinity than the ocean average.
The Mediterranean Sea is especially salty. Rainfall is low, and the sea doesn't have that many freshwater rivers flowing into it. But the Mediterranean Sea has plenty of evaporation. It has some of the saltiest water in the world, and it flows out into the Atlantic Ocean through the Straits of Gibraltar.
The Arctic Ocean is relatively fresh. It is cold and thus evaporation is low. It does receive snow and rain, but another major factor is that it receives large amounts of fresh water from the rivers in Russia that flow north and empty into the Arctic Ocean. In essence, the Arctic Ocean gets the precipitation that falls on Russia.
The ocean's equatorial belts have a relatively high salinity. Sure, it rains a lot over the tropical oceans because of the Hadley cells. But that rain came from the ocean, from evaporation and then condensation into clouds. The important point is that some of the evaporated water is transported by winds to the continents, feeding rain to the tropical rainforests. Thus, more water is evaporated from the tropical ocean than falls back to the ocean as rain. Both evaporation and precipitation is high in these regions, but evaporation exceeds precipitation, making the tropical ocean belts saltier than the world ocean average.
Now we will look at two of the minor ions of the ocean's chemistry: phosphate and nitrate (with the chemical formulas: PO43–and NO3–). These are present in tiny amounts in the ocean overall and even less so in the surface ocean, where most of the ocean's life exists. Photosynthesizing algae and bacteria, which together form the phytoplankton, are a major component of the life in the surface ocean.
Phosphate and nitrate are taken up from the ocean's water by the phytoplankton during photosynthesis, the process that uses sunlight to convert chemicals from the environment into living bodies. Two of the substances absolutely essential for all living things are phosphorus (P) and nitrogen (N). These are present in the ocean ions phosphate and nitrate. Thus, the plankton, when living in the sun-drenched upper portion of the ocean, actively take phosphate and nitrate into their bodies.
During this uptake, phosphate and nitrate are removed from the ocean water. The living things actually change the chemistry of the water. In fact, the phytoplankton can be so highly active that the amounts of phosphate and nitrate are reduced to nearly zero across the world ocean, on average. You can see this effect in Figure 13.2, where the amounts of P and N near the ocean's surface are close to zero.
You can also see in Figure 13.2 that the amounts of P and N increase with depth, reaching levels of nearly 80 for P and more than 500 for N at a depth of 1 km, then decreasing a bit but remaining quite abundant all the way to the bottom of the ocean at the average 4 kilometer depth.
The difference in the concentration of phosphorus and nitrate at different depths is caused, as noted, by the consumption of these two nutrient elements by the phytoplankton at the ocean's surface. But another biological factor is at work as well. As the phytoplankton die, their bodies fall downward in the ocean (while alive, the phytoplankton have ways to remain buoyant). Also, other creatures, such as tiny swimming crustaceans, feed on the phytoplankton and excrete wastes. These wastes also float downward into the deep ocean.
The dead cells and various kinds of wastes (from fish, too) carry bacteria on them. The bacteria feed on the falling wastes, and, as they feed, convert the wastes back into phosphate and nitrate, both of which are returned (regenerated) to the seawater by the bacteria. This is the reason that phosphate and nitrate are so abundant in the deep water; they are regenerated back into the water by the bacteria who feed on the falling waste products of the surface life. We see in this example how life in the ocean influences the actual chemistry of our huge expanse of salty water.
Practice problems of this concept can be found at: What's the Ocean Practice Questions
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