Tobias Roeske

Shelf‐basin exchange times of Arctic surface waters estimated from 228Th/228Ra disequilibrium

The transpolar drift is strongly enriched in 228Ra accumulated on the wide Arctic shelves with subsequent rapid offshore transport. We present new data of Polarstern expeditions to the central Arctic and to the Kara and Laptev seas. Because 226Ra activities in Pacific waters are 30% higher than in Atlantic waters, we correct 226Ra for the Pacific admixture when normalizing 228Ra with 226Ra. The use of 228Ra decay as age marker critically depends on the constancy in space and time of the source activity, a condition that has not yet adequately been tested. While 228Ra decays during transit over the central basin, ingrowth of 228Th could provide an alternative age marker. The high 228Th/228Ra activity ratio (AR = 0.8–1.0) in the central basins is incompatible with a mixing model based on horizontal eddy diffusion. An advective model predicts that 228Th grows to an equilibrium AR, the value of which depends on the scavenging regime. The low AR over the Lomonosov Ridge (AR = 0.5) can be due to either rapid transport (minimum age without scavenging 1.1 year) or enhanced scavenging. Suspended particulate matter load (derived from beam transmission and particulate 234Th) and total 234Th depletion data show that scavenging, although extremely low in the central Arctic, is enhanced over the Lomonosov Ridge, making an age of 3 years more likely. The combined data of 228Ra decay and 228Th ingrowth confirm the existence of a recirculating gyre in the surface water of the eastern Eurasian Basin with a river water residence time of at least 3 years.

Rare Earth Elements in the surface Ocean under the Saharan dust belt

The supply of trace metals to the surface ocean via dust deposition is important for primary productivity and the global biogeochemical cycle of many elements. Here we utilise the systematic variation of the chemical properties of yttrium and the rare earth elements (YREE) to investigate trace metal release from dust in the equatorial Atlantic Ocean. We present YREE data for the dissolved (<0.45 μm) and suspended particulate matter (SPM) collected from the mixed layer during Polarstern cruise ANT-XXIII/1 in Oct-Nov 2005. Saharan dust can be traced with the Al content of the SPM revealing a broad maximum extending from 15° to 3°N. The PAAS normalised YREE patterns of the dust dominated SPM are relatively flat with a broad peak centred around Eu and Gd. This dust dominated SPM is also characterised by lower Y/Ho and Er/Nd ratios than the particulate material from outside the high Al zone. The dissolved YREE distributions show normal seawater patterns with the relative enrichment of heavy REE over light REE. The samples with dust dominated SPM are enriched in the light and middle REE by a factor of approximately 2 compared to the other samples and a Sargasso Sea surface water. The dissolved Y/Ho and Er/Nd ratios obtained from the dust dominated SPM zone are also low compared to the samples outside the zone but display a fractionation between the SPM and the dissolved phase. This comparison indicates a consistent incongruent dissolution of the dust associated YREE which are probably mainly hosted by oxide coatings on the particles.In order to analyse differences in concentration, speciation and total mobility of arsenic two different locations were studied near the Helgoland Mud Area, North Sea. The first location is characterised by natural sedimentation, the second by deposited sediments dredged from the port of Hamburg. Porewater as well as sediment profiles were analysed with respect to arsenic compounds (As (III) and total As) and major redox species as total and reactive manganese and iron. The sediment samples were handled under inert atmosphere before and during extraction by water, phosphate, hydrochloric acid and aqua regia. Total element contents in porewater and leachable extracts of sediment fractions were analysed. The results show a strong redox coupling of arsenic with manganese and iron. Oxidized arsenic seems to adsorb to manganese- and iron-oxyhydroxides in surface sediments. In contrast to the solid samples, the pore water data shows a release of As (III) into porewater when manganese- and ironoxyhydroxides are reduced in the upper part of the cores. Also a remobilisation of As (V) occurs. Downward diffusing arsenic can be fixed by carbonate below the zone of manganese and iron reduction. In the anoxic parts of the sediments As (III) and As (V) are released and could be fixed at authigenic iron sulphide or arsenic sulphides formation. A sulfidic precipitation of arsenic in iron-dominated systems is limited by the occurrence of HS-. Total solid-phase contents in leachable extracts of sediment fractions of the natural area show significant higher arsenic concentrations than the core of the anthropogenic dumping area. This is due to the higher fines content of the Helgoland mud area. Higher total porewater contents of iron and arsenic in the core of the anthropogenic dumping area thus due to higher turnover rates of organic matter by iron reduction. Higher concentrations of arsenic may be due to a higher availability of iron in the dumped sediments.