Dissolved organic carbon in sediments of the central Arctic Ocean collected during POLARSTERN cruise ARK-XXVII/3 from August-September 2012

Marine organic matter (OM) sinks from surface waters to the seafloor via the biological pump. Benthic communities, which use this sedimented OM as energy and carbon source, produce dissolved organic matter (DOM) in the process of remineralization, enriching the sediment porewater with fresh DOM compounds. We hypothesized that in the oligotrophic deep Arctic basin the molecular signal of freshly deposited primary produced OM is restricted to the surface sediment pore waters which should differ from bottom water and deeper sediment pore water in DOM composition. This study focused on: 1) the molecular composition of the DOM in sediment pore waters of the deep Eurasian Arctic basins, 2) whether the signal of marine vs. terrigenous DOM is represented by different compounds preserved in the sediment pore waters and 3) whether there is any relation between Arctic Ocean ice cover and DOM composition. Molecular data, obtained via 15 Tesla Fourier transform ion cyclotron resonance mass spectrometer, were correlated with environmental parameters by partial least square analysis. The fresher marine detrital OM signal from surface waters was limited to pore waters from < 5 cm sediment depth. The productive ice margin stations showed higher abundances of peptides, unsaturated aliphatics and saturated fatty acids formulae, indicative of fresh OM/pigments deposition, compared to northernmost stations which had stronger aromatic signals. This study contributes to the understanding of the coupling between the Arctic Ocean productivity and its depositional regime, and how it will be altered in response to sea ice retreat and increasing river runoff.

Epibenthic foraminiferal d13C in the Recent deep Arctic Ocean

Low planktic and benthic d18O and d13C values in sediments from the Nordic seas of cold stadials of the last glaciation have been attributed to brines, formed similar to modern ones in the Arctic Ocean. To expand on the carbon isotopes of this hypothesis I investigated benthic d13C from the modern Arctic Ocean. I show that mean d13C values of live epibenthic foraminifera from the deep Arctic basins are higher than mean d13C values of upper slope epibenthic foraminifera. This agrees with mean high d13C values of dissolved inorganic carbon (DIC) in Arctic Bottom Water (ABW), which are higher than mean d13CDIC values from shallower water masses of mainly Atlantic origin. However, adjustments for oceanic 13C-Suess depletion raise subsurface and intermediate water d13CDIC values over ABW d13CDIC ones. Accordingly, during preindustrial Holocene times, the d13CDIC of ABW was as high or higher than today, but lower than the d13CDIC of younger subsurface and intermediate water. If brine-enriched water significantly ventilated ABW, brines should have had high d13CDIC values. Analogously, high-d13CDIC brines may have been formed in the Nordic seas during warm interstadials. During cold stadials, when most of the Arctic Ocean was perennially sea-ice covered, a cessation of high-d13CDIC brine rejection may have lowered d13CDIC values of ABW, and ultimately the d13CDIC in Nordic seas intermediate and deep water. So, in contrast to the idea of enhanced brine formation during cold stadials, the results of this investigation imply that a cessation of brine rejection would be more likely.

(Table 1) Sample locations and clay mineral data of sediment surface samples from the Barents Sea region

A new surface sediment sample set gained in the western Barents Sea by the MAREANO program has been analysed for basic clay mineral assemblages. Distribution maps including additional samples from earlier German research cruises to and off Svalbard are compiled. Some trends in the clay mineral assemblages are related to the sub-Barents Sea geology because the Quaternary sediment cover is rather thin. Additionally, land masses like Svalbard and northern Scandinavia dominate the clay mineral signal with their erosional products. Dense bottom water, very often of brine origin, that flows within deep troughs, such as the Storfjorden or Bear Island Trough, transport the clay mineral signal from their origin to the Norwegian-Greenland Sea.

(Table 1) Sulfur contents and isotope composition in rocks and minerals from DSDP/ODP Hole 504B

DSDP Hole 504B is the only hole in oceanic crust to penetrate through the volcanic section and into hydrothermally altered sheeted dikes. We have carried out petrologic and sulfur isotopic analyses of sulfide and sulfate minerals and whole rocks from the core in order to place constraints on the geochemistry of sulfur during hydrothermal alteration of ocean crust. The nearly 600 m-thick pillow section has lost sulfur to seawater and has net d34S = -1.8 per mil due to degassing of SO2 during crystallization and subsequent low temperature interaction with seawater. Hydrothermally altered rocks in the 200 m-thick transition zone are enriched in S and 34S (4300 ppm and +3.0 +/-1.2 per mil, respectively), whereas the more than 500 m of sheeted dikes contain 720 ppm S with d34S = +0.6 +/-1.4 per mil. These data are consistent with the presence of predominantly basaltic sulfur in hydrothermal fluids deep in the crust: following precipitation of anhydrite during seawater recharge, small amounts of seawater sulfate were reduced at temperatures >250°C through conversion of igneous pyrrhotite to secondary pyrite and minor oxidation of ferrous iron in the crust. The S- and 34S-enrichments of the transition zone are the results of seawater sulfate reduction and sulfide deposition during subsurface mixing between upwelling hot (up to 350°C) hydrothermal fluids and seawater. Seawater sulfate was probably reduced through oxidation of ferrous iron in hydrothermal fluids and in the transition zone rocks. Alteration of the upper crust resulted in loss of basaltic sulfur to seawater, fixation of minor seawater sulfur in the crust and redistribution of magmatic sulfur within the crust. This caused net increases in sulfur content and d34S of the upper 1.8 km of the oceanic crust.