Anja Reitz

Biogenic barium and the detrital Ba/Al ratio: a comparison of their direct and indirect determination

Biogenic barium concentrations obtained from direct determination by a three-step sequential extraction procedure are compared to those obtained from the widely used indirect normative calculation based on total digestion. A comparison of the biogenic barium from the direct/sequential extraction and the indirect/normative calculation clearly shows that the detrital Ba/Al ratio is the critical factor in the normative approach, and that erroneous assumptions based on this ratio may introduce significant errors to the calculated biogenic barium. Overall, the crustal average Ba/Al ratio of 0.005–0.01 is much higher than our directly determined global average of 0.0037. This would result in an underestimation of the biogenic barium and thus of the primary productivity calculated using the Ba-flux obtained from the normatively calculated Ba record. Using our (Ba/Al)det ratio of ∼0.0037 leads to normatively calculated biogenic barium results that are in reasonable agreement with the biogenic barium from sequential extraction for samples of the Atlantic, Pacific and Indian Ocean. Our directly determined ‘regional’ (Ba/Al)det ratios deviate from those calculated or assumed from hinterland ratios and our global average (Ba/Al)det is lower than the one commonly reported. Therefore, in sediments with a significant terrigenous fraction, the sequential extraction technique is always required.

Discovery of a natural CO2 seep in the German North Sea: Implications for shallow dissolved gas and seep detection

A natural carbon dioxide (CO2) seep was discovered during an expedition to the southern German North Sea (October 2008). Elevated CO2 levels of ∼10–20 times above background were detected in seawater above a natural salt dome ∼30 km north of the East-Frisian Island Juist. A single elevated value 53 times higher than background was measured, indicating a possible CO2 point source from the seafloor. Measured pH values of around 6.8 support modeled pH values for the observed high CO2 concentration. These results are presented in the context of CO2 seepage detection, in light of proposed subsurface CO2 sequestering and growing concern of ocean acidification. We explore the boundary conditions of CO2 bubble and plume seepage and potential flux paths to the atmosphere. Shallow bubble release experiments conducted in a lake combined with discrete-bubble modeling suggest that shallow CO2 outgassing will be difficult to detect as bubbles dissolve very rapidly (within meters). Bubble-plume modeling further shows that a CO2 plume will lose buoyancy quickly because of rapid bubble dissolution while the newly CO2-enriched water tends to sink toward the seabed. Results suggest that released CO2 will tend to stay near the bottom in shallow systems (<200 m) and will vent to the atmosphere only during deep water convection (water column turnover). While isotope signatures point to a biogenic source, the exact origin is inconclusive because of dilution. This site could serve as a natural laboratory to further study the effects of carbon sequestration below the seafloor.