Lab 11.2

Lab 11.2 -Sky to Sea: Unraveling the Carbon Connection

Fundamental concept: The ocean exchanges CO₂ with the atmosphere and can act as a sink or source for this greenhouse gas.
Estimated time to complete: 15 minutes
Data skills preparation:
Materials needed: None

As discussed at the end of Lab 11.1, the ocean and the atmosphere are intimately linked and are constantly exchanging gases like O₂ and CO₂. In this activity, we are going to dive a bit deeper into these interactions and discuss when the ocean is a sink versus a source of CO₂. The ocean is said to be a sink for CO₂ when the net movement of CO₂ is from the atmosphere to the ocean (when the ocean serves as a net absorber of atmospheric CO₂). The ocean is a source of CO₂ when the net movement of CO₂ is from the ocean to the atmosphere (when the ocean is releasing CO₂ to the atmosphere).

The figure above helps to summarize these interactions within the ocean carbon uptake system. We will learn about the role of biological processes and upwelling in later activities (Lab 11.3 and 11.4, respectively). For now, let’s focus on gas exchange between the atmosphere and ocean.

The following figure shows three different time series relating to CO₂.

The top panel shows CO₂ concentrations in the atmosphere (in gold) and CO₂ concentrations in seawater (blue). The principle of diffusion states that substances move from areas of higher concentration to areas of lower concentration. If CO₂ concentrations in seawater are higher than CO₂ concentrations in the atmosphere, there will be a net movement of CO₂ out of the ocean and into the atmosphere; under those conditions the ocean is a source of CO₂.

The bottom panel shows how much CO₂ is being transferred from the ocean to the atmosphere – also called the “flux” of CO₂ from the ocean to the atmosphere. Positive flux values indicate that CO₂ is moving from the ocean to the atmosphere and that the ocean is a source of CO₂. Negative flux values indicate the opposite: that CO₂ is moving from the atmosphere into the ocean and that the ocean is a sink for CO₂. Negative flux occurs when CO₂ concentrations in seawater are lower than CO₂ concentrations in the atmosphere. During the time period shown, atmospheric CO₂ was approximately 405 ppm. First review your understanding of flux and the movement of CO₂, then analyze the graphs below and answer the questions that follow.

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An Aside on Units: MAKE A TOGGLE Show more, show less

• ppm – Refers to “parts per million” (i.e., as opposed to “parts per hundred”, which would be percent!) and is a way that the amount of CO2 in seawater or atmosphere is measured. It is a measure of the concentration of a substance. The amount of CO2 may also be reported in another measure of concentration, µatm, or microatmospheres.
• mol m-2 s-1 – Can be read as “the amount of substance (measured in moles) that is transferred or produced over an area of one square meter per second”. Unlike ppm, which is a measure of concentration, this is a measure of flux or movement – of how much gas is being exchanged between the ocean and atmosphere, in this case.

Quick Check Questions

Orientation Questions:

1. What time period is displayed in the graph?
2. What is the range of values seen in the air pCO₂ values on the Oregon Shelf? 
3. What is the range of values seen in the ocean pCO₂ values on the Oregon Shelf? Are these values more or less variable than those seen for the air?

Interpretation Question:

4. At the Oregon shelf surface mooring, give an example of a month when the ocean generally a source for CO₂ during the time period shown? Give an example of a month when it is a sink? 
5. Now switch to look at the Pioneer NES Inshore Surface mooring, which is located off the east coast of the U.S. in the Atlantic Ocean, by clicking in the corresponding circle at the bottom of the graphs. Is the seasonal pattern here similar to the Oregon pattern?

Reflection Question:

6. When averaged throughout the year, is the ocean typically absorbing more CO₂ from the atmosphere or releasing CO₂ into the atmosphere? 
7. Remember that anthropogenic atmospheric CO₂ is a greenhouse gas and traps heat near the earth’s surface.  Explain how your answer to question 6 could impact global temperature.