(Table 2) Uranium and thorium isotopic composition, activity ratios and ages of bivalvia shells in sediment core GIK14350, northern Germany

A new chemical procedure for cleaning marine carbonates was applied to remove detritus as well as metaloxide contaminations of marine shells from Eemian deposits and adjoining succession of a sediment core from Dagebüll, Schleswig- Holstein. Hence, one can significantly reduce the contamination with detrital uranium and thorium. Thermal ionisation mass spectrometry was applied to determine the uranium and thorium activities used for 230Th/U dating of these shells. At least ten samples of marine bivalves of five different core sections were analysed. Samples located below a five meter thick clay layer at 19-24 m yielded an average age of 132±18 ka. Shells located above the clays at 15-20 m depth were strongly influenced by percolating groundwaters of an open system. Therefore, a reliable dating of these samples was not possible.

Net community production, Thorium-234 activity and particle flux during the Southern Ocean iron fertilization experiment LOHAFEX

A closed eddy core in the Subantarctic Atlantic Ocean was fertilized twice with two tons of iron (as FeSO4), and the 300 km**2 fertilized patch was studied for 39 days to test whether fertilization enhances downward particle flux into the deep ocean. Chlorophyll a and primary productivity doubled after fertilization, and photosynthetic quantum yield (FV/FM) increased from 0.33 to >0.40. Silicic acid (<2 µmol/L) limited diatoms, which contributed <10% of phytoplankton biomass. Copepods exerted high grazing pressure. This is the first study of particle flux out of an artificially fertilized bloom with very low diatom biomass. Net community production (NCP) inside the patch, estimated from O2:Ar ratios, averaged 21 mmol POC/m**2/d, probably ±20%. 234Th profiles implied constant export of ~6.3 mmol POC/m**2/d in the patch, similar to unfertilized waters. The difference between NCP and 234Th-derived export partly accumulated in the mixed layer and was partly remineralized between the mixed layer and 100 m. Neutrally buoyant sediment traps at 200 and 450 m inside and outside the patch caught mostly <1.1 mmol POC/m**2/d, predominantly of fecal origin; flux did not increase upon fertilization. Our data thus indicate intense flux attenuation between 100 and 200 m, and probably between the mixed layer and 100 m. We attribute the lack of fertilization-induced export to silicon limitation of diatoms and reprocessing of sinking particles by detritus feeders. Our data are consistent with the view that nitrate-rich but silicate-deficient waters are not poised for enhanced particle export upon iron addition.