5.2 Origin of the Oceans

Portions modified from Karla Panchuk in "Physical Geology" by Steven Earle*

So how did the oceans form in the first place? Remember from section 3.1 that the early Earth was formed through the accretion of various materials, and that a period of melting and intense volcanic activity followed. The materials that accreted on the early Earth contained the components that would eventually become our oceans and atmosphere. There are a few hypotheses concerning the origin of the oceans. One suggests that under the high pressures found in the Earth’s interior, gases remain dissolved in magma. As these magmas rise to the surface through volcanic activity, the pressure is reduced and the gases are released through a process called outgassing. Volcanic activity releases many different gases, including water vapor, carbon dioxide (CO2), sulfur dioxide (SO2), carbon monoxide (CO), hydrogen sulfide (H2S), hydrogen gas, nitrogen, and methane (CH4). Lighter gases such as hydrogen and helium dissipated into space, but the heavier gases remained and formed Earth’s early atmosphere, and potentially surface water. Another idea is that during this early bombardment, water was brought to Earth through comets, which are mostly dust and ice, and/or meteorites that may have contained traces of water that could have accumulated on the Earth’s surface. These hypotheses are not mutually exclusive, and it’s possible that all of them contributed to the formation of the oceans.


The rise of atmospheric oxygen

It’s worth noting that the early atmosphere lacked free oxygen (O2), the form of oxygen that we breathe. We know this in part because prior to 2 billion years ago, there were no sedimentary beds stained red from oxidized iron minerals. Iron minerals were present, but not in oxidized form. At that time, O2 was produced in the atmosphere when the Sun’s ultraviolet rays split water molecules apart; however, chemical reactions removed the oxygen as quickly as it was produced. It wasn’t until the appearance of life that Earth’s atmosphere began to become oxygenated. Photosynthetic organisms used the abundant CO2 in the atmosphere to manufacture their food, and released O2 as a by-product. At first all of the oxygen was consumed by chemical reactions, but eventually the organisms released so much O2 that it overwhelmed the chemical reactions and oxygen began to accumulate in the atmosphere, although present levels of 21% oxygen didn’t occur until about 350 Ma. Today the part of our atmosphere that isn’t oxygen consists largely of nitrogen (78%). The oxygen-rich atmosphere on our planet is life’s signature. If geologic process were the only processes controlling our atmosphere, it would consist mostly of carbon dioxide, like the atmosphere of Venus.


As the early Earth cooled, the water vapor in the atmosphere condensed and fell as rain. By about 4 billion years ago, the first permanent accumulations of water were present on Earth, forming the oceans and other bodies of water. Water moves between these different reservoirs through the hydrological cycle. Water is evaporated from the oceans, lakes, streams, the surface of the land, and plants (transpiration) by solar energy (Figure 5.2.1). It is moved through the atmosphere by winds and condenses to form clouds of water droplets or ice crystals. It comes back down as rain or snow and then flows through streams and rivers, into lakes, and eventually back to the oceans. Water on the surface and in streams and lakes infiltrates the ground to become groundwater. Groundwater slowly moves through the rock and surface materials; some returns to other streams and lakes, and some goes directly back to the oceans.


Figure 5.2.1 Earth’s hydrological cycle (Steven Earle, “Physical Geology”).

Water is stored in various reservoirs as it moves through this cycle. The largest, by far, is the oceans, accounting for 97% of the volume (Figure 5.2.2). Of course, that water is salty. The remaining 3% is fresh water. Two-thirds of our fresh water is stored in ice and one-third is stored as groundwater. The remaining fresh water — about 0.03% of the total — is stored in lakes, streams, vegetation, and the atmosphere.


Figure 5.2.2 Proportions of Earth’s water found in the various reservoirs (Steven Earle, “Physical Geology”).

To put that in perspective, let’s think about putting all of Earth’s water into a 1 L jug. We start by almost filling the jug with 970 ml of water and 34 g of salt. Then we add one regular-sized (~20 mL) ice cube (representing glacial ice) and two teaspoons (~10 mL) of groundwater. All of the water that we see around us in lakes and streams and up in the sky can be represented by adding three more drops from an eyedropper.

Although the proportion of Earth’s water that is in the atmosphere is tiny, the actual volume is huge. At any given time, there is the equivalent of approximately 13,000 km3 of water in the air in the form of water vapor and water droplets in clouds. Water is evaporated from the oceans, vegetation, and lakes at a rate of 1,580 km3 per day, and just about exactly the same volume falls as rain and snow every day, over both the oceans and land. The precipitation that falls on land goes back to the ocean in the form of stream flow (117 km3/day) and groundwater flow (6 km3/day).

How did the oceans get salty?

Outgassing was responsible for ocean formation, but how did the ocean water get salty? Most of the salts and dissolved elements in the ocean were probably outgassed along with the water vapor, so the ocean has probably always been about as salty as it is now. But we know that rainfall and other processes weather rocks on the Earth’s surface, and runoff carries dissolved substances into the ocean, contributing to its salinity. Yet despite this constant input, the ocean’s salt composition remains essentially the same. Therefore, the rate of input of new material must be balanced by the rate of removal; in other words, the oceans are in a steady state in regards to salinity.

There are multiple pathways through which dissolved ions enter the ocean; runoff from streams and rivers, volcanic activity, hydrothermal vents (see section 4.11), dissolution or decay of substances in the ocean, and groundwater input. Ions are removed from seawater as they are incorporated by living organisms (for example in shell production) or sediments, sea spray, percolation of water into the crust, or when sea water gets isolated from the ocean and evaporates.

The relationship between the input and removal of an ion can be examined through the concept of residence time, which is the average length of time a single atom of an element remains in the ocean before being removed. Residence time is calculated as:

\text { Residence time} =\frac{\text{amount of the substance in the ocean}}{\text {the rate at which the substance is added or removed}}

There is great variation in residence times for different substances (Table 5.2.1). Generally speaking, substances that are readily used in biological processes have short residence times, as they are used up as they become available. Substances with longer residence times are less reactive, and may be a part of long-scale geological cycles.

Table 5.2.1 Residence times for some constituents of sea water

Constituent Residence time (years)
Chloride (Cl-) 100,000,000
Sodium (Na+) 68,000,000
Calcium (Ca2+) 1,000,000
Water 4100
Iron (Fe) 200

So what about lakes? They are subjected to runoff and river input, so why aren’t they salty like the oceans? One reason is that compared to the oceans, lakes and ponds are relatively temporary phenomena, so they do not last long enough to accumulate the same levels of ions as the oceans. Furthermore, lakes often have rivers flowing both into and out of them, so many ions are removed through the outflow, eventually finding their way to the oceans. The oceans only receive river input; there are no rivers flowing out of the ocean to remove these materials, so they are found in greater abundance in sea water. It should be noted that there are some lakes that contain water whose salt content may rival or exceed that of the ocean; these lakes usually lack river outflow. The Great Salt Lake in the western United States is an example.

*”Physical Geology” by Steven Earle used under a CC-BY 4.0 international license. Download this book for free at http://open.bccampus.ca



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