Hungry for Particles
By Ole Valk |
The in-situ-pumps (ISP) are a very important scientific instrument on this expedition for the GEOTRACES-Team. These pumps are deployed on a rope and then they are going to the deep, distributed over several depths. There they pump for 2.5 hours sea water, which is going through a filter. Thus we can filter relatively large volumes of water (ca. 400-800 liters) directly in the water column. We need these big volumes, due to the low concentration of the tracers we want to measure and the even lower portion of the particulate fraction of these elements (in case of Thorium and Protactinium: 10%). Another reason is that the Arctic Ocean is starving of particles, due to the ice coverage. We use the filters to measure the concentrations of certain trace elements and their isotopes over the whole water column, back at our home labs. The largest part of the filters will be used for analyzing 230Thorium, 231Protactinium and Neodymium isotopes. Data about these tracers in the Arctic Ocean is still very limited. Additional subsamples are cut out of the filters in order to measure Silicon concentrations and Silicon isotopes, as well as other trace elements. We measure 234Thorium directly on board by detecting the beta decay. In combination to these methods we collect samples to analyze Radium isotopes, using special cartridges.
This approach is very important for our scientific work, because we also sample the dissolved fraction of these tracers using the CTD-rosette to collect water samples. So we can get information about the particle composition within the water column and the processes responsible for exchange between the particulate and dissolved fraction. Another interesting part is to determine which particles are preferred by which isotope to react and scavenge from the water column. Especially in the field of radionuclides (230Th and 231Pa) there is still a lack of knowledge. Combined with the water samples we hope to get information about the exchange and interaction between the shelves and the deep sea basins and to detect changes in deep water circulation.
The ISP stations take a lot of time, as mentioned before, because it is not only the 2,5 hours pumping time, but also the fixing of our 13 pumps to the cable, as well as the time to go down to the deep. Usually an ISP station takes 5-6 hours. Therefore we are limited in the amount of stations where we can sample, and for sure we would like to do much more than we actually can! After a lot of ISP-stations it is still always very exciting to get the pumps back on deck and to see if and how much they pumped. Because some of them seem to have their own will and need a lot of care and love in between the different stations. And of course the deployment and recovering of the pumps is always a real pleasure, independent of the time and state of tiredness, as it is happening in a beautiful environment with a breath taking background.
