Ocean urea fertilization for carbon credits poses high ecological risks

The proposed plan for enrichment of the Sulu Sea, Philippines, a region of rich marine biodiversity, with thousands of tonnes of urea in order to stimulate algal blooms and sequester carbon is flawed for multiple reasons. Urea is preferentially used as a nitrogen source by some cyanobacteria and dinoflagellates, many of which are neutrally or positively buoyant. Biological pumps to the deep sea are classically leaky, and the inefficient burial of new biomass makes the estimation of a net loss of carbon from the atmosphere questionable at best. The potential for growth of toxic dinoflagellates is also high, as many grow well on urea and some even increase their toxicity when grown on urea. Many toxic dinoflagellates form cysts which can settle to the sediment and germinate in subsequent years, forming new blooms even without further fertilization. If large-scale blooms do occur, it is likely that they will contribute to hypoxia in the bottom waters upon decomposition. Lastly, urea production requires fossil fuel usage, further limiting the potential for net carbon sequestration. The environmental and economic impacts are potentially great and need to be rigorously assessed. (C) 2008 Elsevier Ltd. All rights reserved.

Post-cyclic Strength Degradation of Undisturbed and Remolded Marine Silty Clay

A series of static and cyclic-static tri-axial compression tests under consolidated-undrained conditions are carried out to study the characteristics of post-cyclic strength of the undisturbed and the remolded samples of marine silty clay. It is found that the post-cyclic monotonic strength decreases if the cyclic strain or pore pressure is over a certain value. The maximum degradation is 10% for undisturbed samples while 70% for remolded ones. The relationship between normalized undrained shear strength and apparent overconsolidation ratio, which is determined by the excess pore pressure induced by cyclic loading, is also established. Static consolidated-undrained tests on overconsolidated remolded samples are also performed. It is proposed that the static consolidated-undrained tests may be substituted for the cyclic-static consolidated-undrained tests if the post-cyclic strength degradation of remolded silty clay is needed to be evaluated simply.