Human earth shapers/ETHS101/Earth Systems Exchange/Ocean Chemistry
Ocean Change: A Chemistry Focus
The chemistry of the ocean has changed significantly over time. Some of these changes are recorded in the geochemistry of ancient ocean sediments, and some of these changes we can only infer. Let’s look at a few key aspects of ocean chemistry that are particularly relevant to life in the oceans: oxygen and acidity. Later in the unit we'll return to both of these changes, and examine how human activity is currently changing ocean oxygen and acidity.
Note: We can observe the changes in Oxygen percentages using the National Centers for Environmental Information website. If you follow use this resource
- Click on the "show Figure" option.
- Use the red arrows at the top of the page to move "down one depth level"' and observe the oxygen change.
Oxygen
The oxygen content of the ocean probably largely followed the evolution of oxygen in the atmosphere. So the early ocean was devoid of oxygen, and this would have profoundly affected the chemistry of other elements in the ocean. For example, the cycles of iron, sulphur, and most trace metals are strongly affected by oxygen concentrations.
On top of the changes in oxygen in the atmosphere that we saw in the previous lecture, the ocean also experienced a series of dramatic decreases in oxygen concentrations, called Ocean Anoxic Events (OAEs). Anoxia refers to the absence (‘an’) of oxygen (‘oxia’). The events were most common during warm, CO2-rich periods of earth’s history, especially the Cretaceous and Jurassic periods. They typically lasted less than a million years. During these geologically brief episodes, it’s believed oxygen levels in the ocean plunged to almost zero. Not surprisingly then, given how life is dependent on oxygen, the OAEs are associated with mass marine extinctions.
What caused the OAEs? There are different theories to explain their occurrence. Dissolved oxygen enters the ocean from the atmosphere, and through the production of oxygen at the surface of the ocean by marine plants. Oxygen is moved around the oceans by ocean circulation. Oxygen is consumed in the ocean by animals and microbes during the decomposition of plant material. A warm climate is going to be conducive to oxygen loss simply because oxygen, like all gases, is less soluble in warm water- so less oxygen will tend to be dissolved in the sea when the temperatures are higher. Most of the theories for OAEs involve a massive increase in plant production, driving increased consumption of oxygen when bacteria decompose the plant material. This is analogous to what happens in some coastal areas today impacted by agricultural runoff- you can get fish kills as a massive algae bloom stimulated by the fertiliser is then consumed by bacteria, which uses up all of the oxygen, killing the fish. Because some OAEs are associated with periods of increased volcanism, one line of thinking is that the volcanism released CO2, which caused the climate to warm, which accelerated weathering and delivery of nutrients to the ocean, which increased plankton production. Others think the volcanism itself may have supplied essential micronutrients such as iron.
Acidity
pH refers to the hydrogen ion concentration in a solution, specifically the negative logarithm of the H+ concentration. A pH of 7 is neutral, a pH < 7 means a solution is acidic, while pH >7 is a basic solution. Ocean pH today is on average 8.1. The pH of the ocean is actually a result of many aspects of ocean chemistry, including the levels of carbon dioxide, and concentrations of major dissolved cations and anions in seawater. When CO2 dissolves in seawater it forms carbonic acid, which decreases seawater pH. Today, the pH of the ocean is decreasing as human-emitted CO2 enters the ocean. This is known as ocean acidification. Changes in ocean pH have also occurred in the geological past, due to natural variations in the carbon cycle, which resulted in more CO2 entering the oceans.
Now we’ll look at one example of how the ocean changed dramatically in the past, during an event known as the Paleocene Eocene Thermal Maximum, or the PETM. The PETM took place 56 million years ago, at the boundary between the Paleocene and the Eocene. This geological event has been proposed as an analogue to our current climate, where anthropogenic CO2 is causing the earth to warm. The PETM was indeed a time of exceptional warmth- a thermal maximum. The sediment record tells us temperatures rose by 6 degrees in 20,000 years, and it appears this was caused by a sudden influx of carbon dioxide to the atmosphere, perhaps due to a pulse of methane from the seafloor caused by climate warming. The ocean response to this climate event included widespread loss of oxygen (anoxia) and decrease in pH (acidification). The pH of surface waters decreased by 0.3 units in a few thousand years. This is the same as decrease in pH that’s projected to occur by the end of the century due to human activity, but the current rate of acidification is 10 times faster than during the PETM. The PETM was associated with widespread extinctions in the ocean, particularly among a group called the benthic foraminifera- single-celled animals with shells made of calcium carbonate (chalk), who live on the seafloor.
