Production and sequestration of dissolved organic carbon in the Weddel Sea: Tracing deep-water formation with molecular methods
Final Report Abstract
Dissolved organic matter (DOM) is an ultimate chemical product of all life on earth. It integrates energy, carbon dioxide and nutrients into a vast compositional and structural variety of molecules – further modified by biological, chemical, and physical processes. In the ocean, organic matter production depends mainly on the photosynthetic activity of autotrophs and most of it is immediately consumed and respired by heterotrophs. Some of this fresh organic matter, however, escapes immediate turnover and accumulates in dissolved form in the entire water column. During transport and seasonal convective overturn, microbial, photochemical, and physical processes remove most of the fresh DOM. The remaining organic matter is an old, chemically poorly characterized heterogeneous mixture of small, partially oxidized and unsaturated molecules: refractory DOM. Chemical fingerprints for marine dissolved organic matter (DOM) were unraveled using ultrahigh resolution mass spectrometry (FT-ICR-MS). These fingerprints were suitable to depict the age of DOM in the Weddell Sea – an important prerequisite to estimate residence times and sequestration potentials for organic compounds in the ocean. As part of the analytical approach, a method was established that enabled for the first time separation and quantification of organic phosphorus and sulfur in marine DOM. Quantitative (DOC) and qualitative results (FTMS) showed that the DOM in the Weddell Sea is very homogeneous. However, compared to the lower latitude surface and deep water masses as observed in the East Atlantic, DOM in the Southern Ocean is older and more degraded. This indicates that although the sea ice, in combination with strong seasonal phytoplankton bloom is a highly productive habitat, fresh organic matter is quantitatively reworked and remineralized by heterotrophic processes. The observed increased DOC concentrations in bottom water sample from the Weddell Sea could not be related to fresh organic matter as hypothesized in our application to this project. The final evidence for the source of the higher DOC concentrations in the bottom water still needs additional studies. However, our data indicate that the dominant pool of DOM is derived from very old and degraded material, likely resulting from the large contribution of old North Atlantic Deep Water entering the Weddell Sea. These findings are in agreement with previous results but contrast with the standard paradigm of radiocarbon age and DOC concentration relationship in the deep ocean and contemporary concepts of marine DOC cycling. Ultra-refractory compounds are potentially indicative of an enhanced capability of the Southern Ocean to sequester organic carbon as refractory DOM and thus long-term carbon storage. The underlying source and formation mechanism remain to be revealed and are likely related to the specific oceanographic conditions in the Weddell Sea. Studying the details of this surplus of old DOC will contribute substantially to an improved understanding of the marine DOC cycle.
Publications
- (2011). Inorganics in organics: Quantification of organic phosphorus and sulfur and trace element speciation in natural organic matter using HPLC-ICPMS. Anal Chem 83: 8968-8974
Lechtenfeld, O.J., B.P. Koch, W. Geibert, K.-U. Ludwichowski, and G. Kattner
- (2012). A molecular perspective on the ageing of marine dissolved organic matter. Biogeosciences 9: 1935-1955
Flerus, R., O.J. Lechtenfeld, B.P. Koch, S.L. McCallister, P. Schmitt-Kopplin, R. Benner, K. Kaiser, and G. Kattner
(See online at https://doi.org/10.5194/bg-9-1935-2012)
