Day 2 Part 5
As with the terrestrial biosphere, the oceans (the largest component of the hydrosphere) can function as either a carbon source or a carbon sink. Look at the image below as a reminder of the different processes by which the ocean can act as a carbon sink and a carbon source.
Typically, the oceans take up approximately two petagrams of carbon each year. More specifically, the oceans have taken up half of all the CO2 emitted by human activities since pre-industrial times. With the increased production of CO2 by humans over the past two centuries, more and more carbon has been added to the oceans. When CO2 enters the ocean, a complex series of reactions occur in which the carbon dioxide is transformed into carbonic acid (by reacting with the water), the carbonic acid breaks apart into hydrogen ions and bicarbonate ions (which are found in baking soda), and some percentage of the hydrogen ions combine with carbonate ions (which are found in washing soda) to form additional bicarbonate ions. Why it is not important to remember all the parts of that complex series of reactions, it is valuable to understand one of its effects: the removal of carbonate ions that it makes it more difficult for marine organisms (known as calcifiers) to grow shells.
Q23: What other effects do you think anthropogenic CO2 emissions are having on marine organisms besides the impact on calcifiers described above?
As a result of data collected in Hawaii, we are better able to understand the effects of the increasing amounts of CO2 that the oceans are absorbing. Scientists working on the Hawaii Ocean Time-series (HOT) program have been making repeated observations of the hydrography, chemistry, and biology of the water column at a station north of Oahu, Hawaii U.S.A. since October 1988 (http://hahana.soest.hawaii.edu/hot/). As you have seen earlier in this lab, measurements of atmospheric CO2 concentrations at Mauna Loa, Hawaii have been made since 1958. Click HOT_MaunaLoa to open the file in Google™ Earth and see the location of the HOT measurements relative to the Mauna Loa station. The image below shows changes in atmospheric CO2 concentrations and the amount of dissolved CO2 in the middle of the Pacific Ocean (i.e. Hawaii).
Q24: What is the general relationship between atmospheric CO2 concentrations and the amount of CO2 dissolved in seawater?
As noted above, changes in the amount of CO2 in seawater also will affect the pH of the water as as well as the availability of carbonate ions. The image below shows changes in seawater pH and the concentration of carbonate ions near Hawaii.
Q25: What effect did the increased absorption of CO2 by the oceans have on the pH? Did the pH level ever get to the point where the ocean water was acidic?
There is one last issue to consider before leaving the topic of the hydrosphere as sink in particular and the carbon cycle in general. Some people have suggested boosting the oceans capacity to act as a sink by (1) seeding the ocean with iron (http://www.sciencedaily.com/releases/2012/12/121219101252.htm) and (2) adding lime to seawater (http://www.sciencedaily.com/releases/2008/07/080721001742.htm). It remains to be seen whether these are effective forms of carbon capture. Even if they are effective, the question remains …
Q26: Do you think that we should be exploring means like this to increase the ocean’s capacity to absorb CO2? Why or why not?
