Oceans in the carbon cycle

Source: NASA

Hey fellows!

To wrap up past weeks' discussions here is a video. It overviews components of the carbon cycle in relation to the oceans. Here are the major points that relate to this blog's issues.

According to NASA, oceans would contain 50 times the amount of atmospheric carbon, making their bed the largest carbon reservoir.
They play a key role in the carbon cycle, to the extent that they have been slowing down global warming. By absorbing carbon, they are mitigating the effects of fossil fuel combustion.

I used to ask myself why the intensive use of fossil fuel led to what seemed to me a gradual, slow change. The consequence seemed relatively small in comparison to the proportion of the underlying cause. I guess one reason to that would be the ocean's carbon uptake.

How then can one measure the effects of climate change on the oceans? To what extent have they alleviated rising CO2 levels? Monitoring the oceans is extremely difficult. Satellites help in this regard (NASA). However they are currently unable to monitor most deep oceans' properties. Currently, remote sensing is far more efficient at gathering terrestrial data than oceanic data.

The video also introduces what I will talk about next: the consequences of increasing levels of atmospheric carbon for ocean acidification and marine life.
 

Feeding back loops

We suspect that the biological pump is not working at its full capacity (De La Rocha 2007). Research is being conducted to find ways to improve its current capacity. These include carbon capture and storage methods whereby carbon is taken from point sources and transported to a storage 'facility'.

Numerical model simulations proved that CO2 could be directly injected to the deep ocean through the moving ships method (Se-min, Sato, and Baixin 2008). Although this may reduce atmospheric CO2 levels in the short term, sequestrated CO2 has negative impacts on marine biology (Orr et al. 2005).

We ought to take these CO2 sequestration forcings with caution. We can rarely get rid of something without consequences being felt.

We suspect increasing temperatures will negatively affect the carbon pump. Carbon storage will

decrease, ultimately inflating atmospheric CO2 levels (Henson et al. 2011). The oceanic carbon pump

is thereby linked to another positive feedback. Increased atmospheric CO2 is likely to raise

temperatures in turn decreasing ocean particles’ ability to pump carbon. And the loop goes on.

That is because oceans and atmosphere are two interwoven systems. A change in air temperature within the ocean/atmosphere system affects mechanisms at work in oceans. In turn oceanic responses affect atmospheric levels of CO2.

I guess that needs to be stressed. Positive feedbacks will increase climate change.

It seems that global warming is not dramatically alarming in itself. To us human beings, 2°C do not represent a huge difference to our body temperature.

Perhaps that’s a reason why a good number of people are not realising how much global warming will affect the environment.

But does anything function in isolation on earth?

We have come to think of systems as separate wholes. Conceptualisation makes our brain represent systems as being divided into independent parts. I can picture an ocean floating alone in emptiness.

Concepts are extremely useful to understand mechanisms in complicated, intricate systems. But let’s bear in mind that the environment does not function like that. There is no change that does not potentially bear consequences on its surroundings.

How much is marine biology pumping?

Last week I described how carbon gets pumped into the ocean by diatoms. It is of great interest then, to quantify the extent of remineralised carbon.

The extent of primary production varies geographically. In upwelling regions, primary production is greater. Satellite imagery shows that near the coast and in the Northern Hemisphere, primary production is larger (figure 1).

Figure 1 Oceans net primary production. Source: NASA

Large seasonal variation exist in oceanic primary production.

Figure 2 Seasonal variation in net primary production (index). Source: Lutz et al. 2007

This makes quantifying the biological pump difficult. How can we infer primary production from one measurement to another spatial and temporal condition if primary production is spatially and temporally distributed?
We can look into which methods are used and how reliable they may be.

One way of measuring the biological pump is called the f-ratio. Primary production can be divided into recycled production (RP) and new production (NP). The extent of NP roughly represents how much carbon gets pumped into the ocean each year, if we look at long time scales (Henson et al. 2011). The f-ratio is given by NP/ NP+RP and estimates the uptake to 12 GtC yr−1 (Laws et al., 2000).

However this result should be taken with caution. NP is fed by nitrate coming from deep water through vertical mixing. Results were derived from this assumption. But it appeared in more recent studies that nitrate can be fuelled from surface waters as well; representing up to half of NP’s supply (Yool et al. 2007). To tackle uncertainties, Henson offers an alternative method using thorium-234 (²³⁴Th). It traces particles exporting organic carbon. The ThE ratio then gives how much carbon is pumped with: ²³⁴Th- derived export/ Primary Production

Using the f and the ThE ratios, results can be generalized globally depending on sea surface temperature. A correlation exists between temperature and production. Indirectly, cold waters induce more production than in warmer, tropical waters. The thorium approach brings a result of 5 GtC yr−1. (Henson et al. 2011)

Henson cites other methods which range from ∼5.7 GtC yr−1 to ∼20 GtC yr−1 (2011). We can acknowledge that the difference is great. The largest result date back to the late 1970s, containing higher uncertainty due to reduced scientific knowledge. Nonetheless many contemporary papers point out to about 10 GtC yr^-1(Dunne et al. 2007; Laws et al. 2000). How can we interpret this wide range of results? In such a context, can we trust these values?

First, what leads to such discrepancy should be looked at. Researchers deal with high uncertainty. Little is known about oceanic data, and the ways they are measured are imperfect. What is more, data is gathered at specific time and location. It overlooks temporal and regional variability because conclusions are derived on the assumption that oceans are homogeneous (Henson et al. 2011). Then, it would be useful to use a consistent approach used at different places and time of year. We could choose the thorium method which appears to be more efficient.

The extent of sediment trapped carbon is hard to quantify. It is crucial however to measure the extent of this reservoir since it is this sequestrated carbon that is responsible for lowering CO2 levels. It is done by collecting particles in sediment traps. Controversy surrounds this method. Although there is some disagreement, we expect the sediment reservoir is about 150 GtC (see figure, IPCC 4th report 2007).


We suspect that the biological pump is not working at its full capacity (De La Rocha 2007).

How is the biological pump reacting to increasing atmospheric CO2 as a result of anthropogenic changes? Are there other current environmental changes likely to affect oceanic primary production?
 

To conclude we observe high uncertainty attached to measuring the biological pump. In the end, the largest carbon sink remains widely unknown (Henson et al. 2011). More research on methodology ought to be conducted as it is crucial to gain knowledge on oceanic carbon fluxes. For the simple reason that current and future anthropogenic changes may alter such fluxes. And we need to know what their past and present state is to see in which direction we are heading.

Lastly, we can see that the extent of the biological pump may have been inflated. This is alarming for future atmospheric carbon levels. In predicting these, we may have measured the carbon pump to absorb more carbon than in reality.

Pumping Biologically?

Today I will talk about the other mechanism through which atmospheric carbon dioxide enters oceans, and is sequestrated within them. It is called the biological pump and can be decomposed into 3 steps:

Figure 1 Schematic showing the biological pump mechanism. Source: De La Rocha 2007

- Inorganic carbon is fixed into organic particles through photosynthesis. Light in the surface water, or euphotic zone enables phytoplankton to absorb CO2 which gets converted into organic matter.

CO2+H2O + light ----> CH2O + O2

- Most of this primary production (the process through which inorganic carbon is converted into organic carbon) decomposes in the upper layer. Some of it, though, reaches the deep sea where it faces the same fate: it is recycled and becomes CO₂ once again. This happens through decomposition by bacteria (De La Rocha 2007).

 

- Fixed carbon gets remineralized as part of the nutrient cycle. It will be used in primary production again. However, a portion of sinking primary production escapes decomposition and reaches the ocean floor. Particles aggregate forming marine snow, thereby increasing the sinking rate. Upon reaching the bottom, primary production is fixed as sediments and it can be sequestrated for thousands of years. Out of the 50–60 Pg C fixed into organic matter per year in the upper layer, only 0.3% (0.16 Pg C) becomes sequestrated in sediments (De La Rocha, 2007).

It is that sequestrated carbon that is ultimately responsible for lowering atmospheric CO₂ (De La Rocha 2007). Supposedly, levels of pre-industrial atmospheric carbon would have increased by two thirds without the biological pump (Broecker, 1982).