Optimization of In Situ Pumps
Matter fluxes within the ocean are carried by two main vectors 1) oceanic currents are transporting the matter both in the dissolved and particulate phases and 2) particles are themselves mobile within the water column, mainly sinking.
Both dissolved and particulate phases significantly interact with each other. Those interactions have been shown to play a crucial role in the cycle of several elements. One example is the reversible scavenging model for Th and Pa [Bacon et Anderson, 1982].
Whereas, for several elements, the particulate fraction may be significantly larger than the dissolved fraction, the former has been poorly documented for most elements, compared to the latter. This is due to the difficulty of particle sampling. This lack of documentation, and therefore of knowledge and understanding, of the particle cycles prevents improvements of our understanding of oceanic biogeochemical cycles in general.
In situ pumping is one of the two ways of collecting particles (the other is water collection followed by filtration onboard), and the only way to collect large volume particulate samples. However, in situ pump deployment is extremely time consuming, typically 8 hours for a deep cast, with typically 6 to 8 pumps and therefore, 6 to 8 samples.
Typically, on GEOTRACES like cruises, in situ pumps are used to collect
- Suspended particles for carbon analyses
- Suspended particles for trace element and isotopes analyses
- Suspended particles for their direct observation (e.g. Scanning Electron Microscopy, XANES etc.)
- Dissolved radionuclides, via adsorption
Each of the above requires a specific type of medium (membranes or cartridges), incompatible with the others. Therefore, with the standard in situ pump configuration, one pump was required for each analysis, so that 4 pumps would be required at each targeted depth. In the context of the GEOTRACES program, planning numerous trans-oceanic sections, including the documentation of suspended particle chemical composition, optimizing such sampling was necessary. Such optimization had already been carried out by our US colleagues, from WHOI.
Organization, Results and schedule
The French seagoing instrumentation facility, DT INSU, holds 5 McLane, and 6 Challenger Oceanics in situ pumps. Those pumps are available for the entire French research community. This project was therefore a project for the French GEOTRACES community as a whole. Several discussions were therefore organized between various investigators using in situ pumps in GEOTRACES like cruises and DT INSU. These discussions lead to set the following desired configuration for the pumps:
- One 142mm filter holder for collection of trace elements and isotopes (SUPOR membranes)
- One 142mm filter holder for collection of particulate carbon and 234Th (QMA membranes)
- One 47mm holder equipped with a flat membrane (Nuclepore) allowing particle observations (e.g. MEB)
- 2 cartridges holding Mn impregnated media for collection (adsorption) of dissolved radionuclides (notably Ra).
Participants: F. Lacan, M. Souhaut, H. Planquette, O. Desprez de Gésincourt, L, Scouarnec, L. Fichen, A. Royer, P. Lam.
LEGOS : Laboratoire d'Etudes en Géophysique et Océanographie Spatiales (Toulouse)
DT INSU : French seagoing instrumentation facility (Brest)
LEMAR : Laboratoire des sciences de l'Environnement MARin (Brest)