Automated analysis of 234Th in surface waters
In order to facilitate under-way measurements of 234Th in surface waters we have developed an automated system for filtration of surface water and subsequent precipitation of 234Th on MnO2. The instrument was built in cooperation between AWI and ISITEC.
The instrument is meant to obtain a geographical distribution of the depletion of 234Th, and thus of the export production of organic carbon in the preceeding approx. 2 months.
Automated 234Th analyzer showing one 5-L tube to contain the sample to be filtered (by air pressure) over first filtration unit, and two tubes for alternating intermediate storage of filtered seawater in which MnO2 particles are allowed to grow before they are filtered (by membrane pump) over the second filtration unit. The filtered volume is read from pressure sensors. The front view from the lower part of the instrument shows the pneumatically driven membrane pumps for water and air pressure and the tank for intermediate storage of samples after first filtration.
Example: approaching the Laptev Shelf from the Arctic Ocean
Blue squares show the 234Th/238U ratio in surface water on a transect from the central Arctic Ocean towards the shelf of the Laptev Sea. Data of Pinghe Cai, Ingrid Vöge, Michiel Rutgers van der Loeff, Kate Lepore, Brad Moran
On our Polarstern Expedition ARK XXII/2 to the Arctic, Summer 2007, the downward flux of particles from the surface layers was extremely low throughout the central Arctic Ocean. Only in the shallower regions on the Siberian shelves could we measure clearly reduced 234Th, caused probably by settling remains of plankton production.
Example on transect Bremerhaven-Cape Town
Distribution of total 234Th/238U in surface waters on a transect Bremerhaven-Cape Town (Polarstern Expedition ANT XXIII/1, Oct/Nov 2005) as measured with the manual 20-L technique (triangles) and with the automated 5-L procedure (dots).
First results show the general validity of the automated procedure compared to the conventional 20-L filtration method. The latitudinal distribution of the 234Th/238U ratio distinguishes oligotrophic areas (ratio around 1 implying no removal, like the regions around 30°N and around 5°N) from areas where particles have been exported to depth as a result of plankton production. Some removal is found in the zone of 10-20 °N (influenced by dust inputs and upwelling off the African coast) and in the Biscay area. The strongest removal is found south of the equator in the South Atlantic Gyre implying appreciable particle export in the preceeding weeks.
References
- Rutgers van der Loeff M. M., Vöge I., and Lilienthal H. (2004) Automatisierung einer Fällungs- und Filtrationsprozedur für Wasserproben. German Patent.
- Rutgers van der Loeff M., Sarin M. M., Baskaran M., Benitez-Nelson C., Buesseler K. O., Charette M., Dai M., Gustafsson O., Masque P., and Morris P. J. (2006) A review of present techniques and methodological advances in analyzing 234Th in aquatic systems. Marine Chemistry 100(3-4), 190-212.