Interactions of dissolved and particulate organic matter in the ocean

Dissolved organic matter (DOM) is one of the largest organic carbon reservoirs on our planet. In the deep sea, DOM has an average age of about 6.000 years, which indicates that part of it is resistant to microbial degradation. At the sea surface, photosynthetic primary producers fix a part of carbon dioxide into particulate organic matter (POM). Sinking particles transport POM into the ocean’s interior. This biological pump is a significant process that removes carbon from the sea surface. Phytoplankton also release dissolved organic matter (DOM) while they live and upon death, forming the nutritional basis for heterotrophic microorganisms that again release DOM. Adsorption of DOM onto sinking particles could be the most important removal process for DOM in the deep ocean. Such removal process could be highly relevant in the context of global biogeochemical cycles because it might control the pool size of organic carbon in the oceans. Yet, the interaction of DOM and POM in the water column is still not fully understood.

In our project we study POM and DOM interactions in laboratory experiments and at long-term ocean-monitoring stations such as the upwelling region off Cap Blanc, Mauritania. At the Cap Blanc site, intermediate and bottom nepheloid layers regularly occur due the lateral transport of shelf erosion. Sedimentary POM and DOM in these layers may affect carbon cycling on a large spatial scale. Preliminary data suggest that aggregation and adsorption onto freshly formed sinking particles may be significant removal process for aged DOM at Cap Blanc. We investigate interactions of DOM with settling particles at the molecular level and through radiocarbon dating. Molecular analyses are being done at the Institute for Chemistry and Biology of the Marine Environment (ICBM, University of Oldenburg), using ultra-high-resolution mass spectrometry (FT-ICR-MS), NMR and quantitative HPLC methods. For radiocarbon dating we collaborate with Hendrik Grotheer and Gesine Mollenhauer at AWI.