Thorsten Dittmar

The Polar Oceans

My past research in the polar oceans was focused on the Arctic Ocean, where the Siberian rivers transport huge amounts of organic compounds from taiga and tundra into the ocean. By using amino acids as molecular tracers I could show that the riverine input is distributed over the Arctic Ocean and a considerable fraction reaches the abyssal ocean (Figure 1). There it is trapped for many hundreds to thousands of years. This process efficiently removes carbon from active cycles. For this research I closely collaborated with colleagues from the Alfred Wegener Institute, the leading facility for Polar and Marine Research in Germany.

Graph Fig. 1: Dissolved organic nitrogen (DON) in the eastern Arctic Ocean. Percentage of terrigenous DON to total DON, estimated from principal component analyses of molecular amino acid data (Dittmar 2004, Limnology and Oceanography 49, 148-156).

 

Anarctica Photo I will add a new perspective to my research by moving to the Southern Ocean, that surrounds the Antarctic continent. There, surface water which is rich in algal-derived dissolved organic carbon (DOC) is very efficiently transported into the deep abyssal ocean. This deep-water formation is directly linked to sea-ice formation, when salt is rejected and dense brine-enriched waters penetrate the deep ocean. Sea ice is one of the most productive marine environments, and DOC concentrations in the brine are among the highest measured in marine waters. The biogeochemistry of sea ice is widely unknown and it is not clear whether sea-ice DOM is persistent enough to survive downward convection.

The objective of my research is to investigate the formation of persistent DOM in sea ice and to trace its fate into the deep sea. I seek to answer the question: Do ice-covered oceans act as a "DOM pump" to the abyssal ocean and so sequester carbon from active cycles? This research should yield first quantitative estimates on the amount of carbon annually sequestered by this process in ice-covered oceans worldwide.

Graph Fig. 2: Conceptual model of water and organic carbon circulation in the Southern Ocean. Labile ice-algal derived DOC is mineralized in ventilated surface waters. Semi-labile and refractory compounds can be exported via deep- and bottom water formation into the abyssal ocean. Mineralization products and refractory compounds are trapped there for hundreds to thousands of years, respectively. Figure adapted, after Speer et al. (2000).

 

For this study I am closely collaborating with colleagues from Germany and UK, within the framework of ISPOL (www.ispol.de). ISPOL is a multi-national, interdisciplinary field experiment in the Weddell Sea (Antarctica) - the prime site for bottom-water formation. In collaboration with colleagues from the National High Magnetic Field Laboratory, the ultimate facility for analytical chemistry, I will apply molecular tracer techniques to trace DOM from ice-algal into the deep ocean. The results from this molecular fingerprinting will be integrated into physical models. For this, I collaborate with colleagues from Physical Oceanography including Dr. Kevin Speer.