Penetration velocity and deceleration of penetrometer stations N1 to N97 during cruise Senckenberg_11_2008

In-situ geotechnical measurements of surface sediments were carried out along large subaqueous dunes in the Knudedyb tidal inlet channel in the Danish Wadden Sea using a small free-falling penetrometer. Vertical profiles showed a typical stratification pattern with a resolution of ~1 cm depicting a thin surface layer of low sediment strength and a stiffer substratum below (quasi-static bearing capacity equivalent: 1-3 kPa in the top layer, 20-140 kPa in the underlying sediment; thickness of the top layer ca. 5-8 cm). Observed variations in the thickness and strength of the surface layer during a tidal cycle were compared to mean current velocities (measured using an acoustic Doppler current profiler, ADCP), high-resolution bathymetry (based on multibeam echo sounding, MBES) and qualitative estimates of suspended sediment distributions in the water column (estimated from ADCP backscatter intensity). The results revealed an ebb dominance in sediment remobilization, and a general accretion of the bed towards low water. A loose top layer occurred throughout the tidal cycle, likely influenced by bedload transport and small events of suspended sediment resettlement (thickness: 6 +-2 cm). Furthermore, this layer showed a significant increase in thickness (e.g. from 8 cm to 16 cm) related to periods of overall deposition. These findings imply that dynamic penetrometers can conveniently serve to (1) quantify potentially mobile sediments by determining the thickness of a loose sediment surface layer, (2) unravel sediment strength development in potentially mobile sediments and (3) identify sediment accumulation. Such data are an important complement and add a new geotechnical perspective during investigations of sediment remobilization processes in highly dynamic coastal environments.

Iron and aluminium and hydrogen peroxide measured on water bottle samples during Marion Dufresne cruise ANTARES-II

Total dissolvable iron (TDFe), particulate iron (PFe) and hydrogen peroxide (H2O2 measurements were performed along a N-S transect in the upper 250 m in the Southern Ocean (62°00E/66°42S - 49°00S, ANTARES II cruise, February 1994). TDFe was organically extracted (APDC/DDDC-chloroform) and analysed by Graphite Furnace Atomic Absorption Spectrometry (GFAAS), PFe was analysed by GFAAS following a strong mixed-acid leach, and H2O2 was analysed on board by fluorometry. The respective detection limits are equal to 0.13 nmol/kg, 0.02 nmol/kg, and 3.0 nmol/kg. TDFe concentrations vary from 0.4 to 6.2 nmol/kg and profiles are not completely depleted in the surface. PFe concentrations vary from 0.02 to 0.2 nmol/kg. Iron/carbon (Fe/C) uptake ratios for phytoplankton were calculated either from seawater or particle measurements. They are variable along the transect but are consistent when they could be compared. All the observed ratios are within the range of values proposed for the Fe/C uptake ratios by phytoplankton. Using our uptake ratio calculated in the Permanent Open Ocean Zone (4 x 10**?6 mol/mol), we estimate that the primary production which can be supported by the iron input flux into the surface waters is two times higher than the measured primary production in the same area. In the surface waters, H2O2 concentrations vary from 5.0 to 19.7 nmol/kg. Such low concentrations are due to strong vertical mixing, low dissolved organic matter concentrations and the latitude of the site.