Quantification and molecular characterisation of dissolved organic sulphur at the benthopelagic interface in the Weddell Sea

jana.geuer [ at ] awi.de


The Weddell Sea is an important region for Antarctic bottom water formation and thereby affecting global ocean currents and biogeochemical cycles. Few previous studies revealed that elevated dissolved organic carbon (DOC) concentrations could be found in the bottom water of the Weddell Sea. Such elevated concentrations could either originate from downwelling of water masses or a sedimentary DOC efflux. Furthermore, little is known about the molecular identity of dissolved organic sulphur (DOS) in the ocean. This study aims at a DOC and DOS source identification in the Southern Weddell Sea using bulk and molecular analytical approaches. In addition, the reproducibility of solid phase extraction (SPE) for dissolved organic matter concentration and desalting using different sample volumes were assessed. The water column was sampled at 40 stations in the outflow region of the Filchner Depression in the Weddell Sea. At 14 stations sediment pore water was sampled in addition. SPE was performed using different sample volumes for water column and sediment pore water. Quantification of DOC and SPE-extractable DOC (SPE-DOC) was performed via high-temperature catalytic oxidation. Quantification of SPE-DOS was performed via inductively coupled plasma mass spectrometry, molecular characterisation via Fourier transform ion cyclotron resonance mass spectrometry. Elevated DOC and partly also elevated SPE-DOS concentrations (average water column DOC: 52 ± 11 µmol L-1, SPE-DOS: 0.26 ± 0.22 µmol L-1) were found in surface water (DOC up to 69 ± 13 µmol L-1, SPE-DOS up to 1.5 ± 0.5 µmol L-1) and also the lower pelagic zone within water layers of high potential density (DOC up to 61 ± 13 µmol L-1, SPE-DOS up to 1.2 ± 0.4 µmol L-1). In sediment pore water both DOC and SPE-DOS concentrations were higher (DOC: 205 ± 202 µmol L-1, SPE-DOS: 4.2 ± 6.2 µmol L-1) and showed more molecular differences than water column samples. Pore water and water column samples were clearly distinguishable on a molecular level. The water layer above the sediment showed a high molecular similarity to the pore waters but lower concentrations of DOC and SPE-DOS than the pore water. The increase of the extraction volume led to a decrease in SPE-DOC extraction efficiency and a relative decrease of SPE-DOS. Elevated DOC concentrations in the lower pelagic zone were attributed to downwelling of water masses rather than sedimentary efflux. Still, the sediment most likely contributes DOC and DOS to the lower water column, which could be observed on a molecular level. Differences in the extraction volume of pore water and water column samples led to significant changes in SPE-DOC/SPE-DOS ratios (larger volume: 76 ± 39, smaller volume: 24 ± 12). The extraction volume was furthermore likely to affect the molecular composition of the samples.

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Geuer, J. (2015): Quantification and molecular characterisation of dissolved organic sulphur at the benthopelagic interface in the Weddell Sea , Master thesis, University of Applied Sciences Bremerhaven.

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