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A natural carbon dioxide (CO 2) seep was discovered during an expedition to the southern German North Sea (October 2008). Elevated CO 2 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 CO 2 point source from the seafloor. Measured pH values of around 6.8 support modeled pH values for the observed high CO 2 concentration. These results are presented in the context of CO 2 seepage detection, in light of proposed subsurface CO 2 sequestering and growing concern of ocean acidification. We explore the boundary conditions of CO 2 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 CO 2 outgassing will be difficult to detect as bubbles dissolve very rapidly (within meters). Bubble‐plume modeling further shows that a CO 2 plume will lose buoyancy quickly because of rapid bubble dissolution while the newly CO 2 ‐enriched water tends to sink toward the seabed. Results suggest that released CO 2 will tend to stay near the bottom in shallow systems (

[1] The residual dark unit of the most recent eastern Mediterranean sapropel (S1) is usually overlain by sediments with enhanced concentrations of MnOx in two separated layers. The variability and magnitude of the Mn enrichment at different locations and water depths indicate that Mn must have been added preferentially to sediments at intermediate (1–2 km) water depths. We propose a two-stage mechanism for the Mn enrichment that involves decreasing oxygenation with increasing water depth. This mechanism involves the loss of reduced Mn2+ from the deepest sediments (>2 km water depth) into overlying anoxic waters and a variable gain of MnOx in sediments in contact with oxygenated waters at shallower depth. In the S1 unit that receives the extra MnOx input, an upper higher Mn-enriched zone (>3 wt %) is maintained continuously at the top of the accumulating S1 unit because the pore waters are anoxic at shallow sediment depth while bottom waters are oxic to some degree. In a reactive-transport model, the Mn enrichment in the upper zone could not be supported by normal sediment diagenesis. Thus the MnOx in the upper Mn horizon must have formed mainly in the water column. The MnOx in the upper Mn-enriched zone adsorbed Mo and Li from seawater in a similar manner as other Mn-enriched oxic sediments, nodules, and crusts, with a Mn:Mo ratio of ∼600:1, a Mn:Li ratio of ∼750:1, and a δ98/95MoMOMO of −2.5 ‰.