D341: Porcupine Abyssal Plain cruise 2009


Cruise blog

Tuesday 11 August 2009

Using radioactive tracers to estimate carbon fluxes

The biological pump is a major component of the global carbon cycle which transports approximately 10 gigatonnes carbon per year from the atmosphere to the oceans interior. For it, to effectively sequester carbon in the deep ocean, it must export organic carbon to depths below that of winter mixing. This occurs via the sinking of organic carbon aggregated into particles, large enough to attain high sinking velocities. The aim of our work in the D341 is to estimate the POC fluxes from the upper layers to the twilight zone in the PAP site using two different and complementary techniques based on the disequilibria of the radioactive pairs 234Th-238U and 210Po-210Pb.

Some of the elements that comprise the earth are radioactive. Those natural radionuclides are ideal tracers of a great variety of environmental processes for several reasons. First of all they are ubiquitous in the environment; secondly, their radioactive decay allow us to “easily” measure them (or we can try to measure  1mBq/L of 210Po or 6·10-9 ng/L using other techniques!). Finally, equilibriums between the activities of the parents and the daughters of the radioactive chains are established in close environmental compartments. As the compartments are rarely close  in the environment, these equilibriums are broken and the measurement of the correspondent desequilibrium allows us to establish different properties of the compartment of interest.

It is this last property the one that we use to estimate carbon fluxes from 234Th and 238U and 210Po and 210Pb disequilibria.

234Th (T1/2=24,1d), daughter of 238U (T1/2=4,47.109y), can be used to estimate how much POC is exported into the deep ocean. 238U is conservative in the seawater. But unlike 238U, 234Th is particle reactive in the water column. As particles sink through the water column, 234Th is scavenged with them and the secular equilibrium between 234Th and 238U is broken, the subsequent disequilibrium can thus be used to quantify carbon export fluxes.

First, 234Th export fluxes are calculated. Moreover, if we know the ratio POC/234Th, PIC/234Th or BSi/234Th (that can be measured in the sinking particles using in situ pumps, SAPS), it is possible to estimate the rate of carbon and biominerals export from the surface ocean.

In a similar way, a disequilibrium through the water column it is found between 210Pb (T1/2=22y) and its daughter 210Po (T1/2=138d). However, this disequilibrium has different characteristics than that of the pair 234Th-238U. 234Th is attached to the surface of the particles, however, 210Po it is assimilated by the organic matter and incorporated to the cell as a substituent of sulphur. This is an important difference because it is expected that 210Po-210Pb disequilibrium allow us to better estimate POC fluxes whereas 234Th will be used to estimate particle scavenging.

Furthermore, the different half lives of 234Th (days) and 210Po (half a year) would allow us to study different timescales, fast changes in fluxes (234Th) and also seasonal variations (210Pb).

During the cruise our aim is to measure 210Po, 210Pb and 234Th in the station in as much depths as possible (high resolution profiles if we want to integrate the disequilibrium properly!). For this reason, there where always so many carboys spread around our lab, and some of them flying all around during the storms!

After collecting the water, radionuclides are precipitated following different radiochemical procedures. Afterwards, their radioactive decay will be measured and the concentrations of the radionuclides are obtained.

Maria Villa & Fred Le Moigne

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