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.