June 18, 2009
As I have stated before, I am participating in cruise TN250, the 6th and final cruise of the Bering Sea Ecosystem Study, otherwise known as BEST. The scientists that I am working with are Roger (Pat) Kelly, a marine research technician, and Matt Baumann, a PhD student. Both of them work under Dr. S. Bradley Moran at the University of Rhode Island's Graduate School of Oceanography. Dr. Moran and Pat are interested in the impact that changes in sea ice cover will have on the primary productivity (phytoplankton) and particle export in the Eastern Bering Sea. The eastern Bering Sea is one of the most productive fishing grounds in the world for a reason. The cold, deep, nutrient-rich waters of the Pacific upwell onto the shelf of the eastern Bering Sea and have a profound influence on the food web. As the ice retreats and more sunlight enters the upper 50-100 meters of the water column, the phytoplankton begin to flourish (or bloom) and set in motion a chain of events that results in a mass migration of marine animals to the north for late spring and summer feeding.
After the phytoplankton bloom occurs, one of two things may happen. If the phytoplankton are consumed before they sink, then the carbon that they took in (in the form of dissolved CO2) stays in the upper part of the water column known as the pelagic region. If on the other hand they sink before they are consumed, then their carbon will be transported to the bottom of the ocean where it will become part of the benthic (bottom-dwellers) food cycle. It is this organic carbon export (as opposed to the inorganic carbon in the form of oxides and carbonates) from the surface to the bottom of the ocean that interests Dr. Moran, Pat, and Matt.
Pat and Matt are making use of an indirect method involving thorium-234 as a naturally occurring radioactive tracer to estimate the carbon export. But why calculate the activity of thorium in the water column if organic carbon is what they are really after? The answer is somewhat complex and I will do my best to explain. Bear with me on this one as I am still trying to grasp the concept myself. It appears that measuring the falling organic carbon is not as easy as one might think.
Particulate organic carbon (POC) can exist in the water column in one of three basic groups. It can exist as either suspended particles, as falling particles, or as particles stirred up from the sediment below. If a water sample is captured, there might not be a way to distinguish between the three types of POC without extensive organic analysis and therein lies the problem. If the sample is taken in deep water, then the organic carbon originating the sediment may be ruled out. But how do they distinguish between the suspended and falling particles? And what about samples taken in shallow water on the continental shelf where the currents regularly stir up the sediment on the bottom? As a result of these problems a model needs to be constructed that can incorporate data from multiple sample sites (deep and shallow water) and predict the carbon flux. Enter thorium.
Thorium adheres to the surface of marine particles. Whether they be mineral grains or small pieces of organic matter thorium tends to stick to them. When these particles sink in the ocean the water column gets stripped of its thorium. Thus the thorium export rate can be related to the carbon export if the ratio of thorium to sinking organic carbon is known. But of course that begs the question, how do you measure the thorium flux in the water column and how is the POC/thorium ratio determined?
The nucleus of a thorium-234 atom is unstable. All unstable nuclei will eventually undergo radioactive decay and turn into another element. What makes one radioactive isotope different from another is the manner by which it turns into another element (decay mode) and how long it takes to undergo this process (measured in half-lives). Thorium-234 undergoes ßeta decay, which means when it radioactively decays it ejects, or emits, a β particle (a high energy electron) from its nucleus.
A thorium sample can be prepared by complexing the radionuclide to manganese oxide and precipitating it out of the seawater. The first measurement of β-emission must wait until 72 hours after sampling due to the presence of other short-lived isotopes that emit β-particles as well. After the initial wait period is up the sample is then monitored for β emissions over a time period of 5 half-lives. The final measurement will be taken long after all of the Th-234 has decayed to obtain the background levels of radiation, mostly due to potassium-40. If the rate at which the β particles are produced is recorded, using the decay constant for Th-234 the original activity of the Th-234 in the sample can be calculated by constructing what is known as a decay curve.
However, this information only tells them how much thorium is being exported and not how much was there originally. The thorium-234 in seawater comes from another naturally occurring radioactive isotope, uranium-238, which has undergone a decay. Thankfully, the U-238 is conservative in seawater, meaning that its concentration is proportional to the water's salinity and can be calculated easily. In the case of particle free waters the activity of Th-234 is equivalent to the activity of the U-238. The amount of thorium missing from the water (the thorium deficit) is calculated relative to the amount of U-238 and the deficit of thorium is determined.
Sediment traps (open ended tubes into which particles settle) are the ideal method to capture falling particles but are hard to work with and cannot be deployed in shallow water. First, the sediment traps that they are working with span 100 meters over the water column length (to track the falling particles over the entire part of the water column that receives light from above). Much of the area along the Bering shelf is no more than 90 meters in depth so the trap would drag along the bottom. Second, as mentioned before, currents on the continental shelf the current regularly stir up the sediment so it would be hard to discern whether the particles captured in the traps are from above or below.
For these shallow water sites, the water samples taken at various depths are analyzed for POC and thorium content. Add in the info obtained from deep water sites (sediment traps and regular water samples) and an assumption that the characteristics of the falling carbon will remain roughly the same regardless of the overall depth of the water column and an equation can be developed which will calculate the organic carbon flux wherever the measurements are made.
It's that easy.