An experimental approach to microbial carbonate precipitation in improving the engineering properties of sandy soils

Purpose Stabilization of weak soil can be achieved through different methods, some of which include jet column, cement stabilization and fly ash stabilization. Unfortunately, the use of the aforementioned methods of soil improvement affects the environment negatively thereby leading to environmental degradation. With the aforesaid impediment in mind, the need for devising methods of weak soil improvement becomes pertinent. Methods Bacillus sp. — a non-pathogenic organism found abundantly in soil — was investigated in this study as a potential agent of soil improvement. The usability of Bacillus sp. in soil improvement was investigated with direct shear tests and permeability tests under optimum conditions in this study. Result Time-dependent study on the effect of the ureolytic bacteria Bacillus sp.-induced calcium carbonate precipitation shows reduction in permeability and increase in the strength of the soil under study. On exhaustion of the available nutrients in the soil, however, the strength of the soil is not negatively impacted. Conclusion Microbially induced calcium precipitation by Bacillus sp. is effective in soil improvement as such it may serve as substitute for conventional soil stabilization techniques. The ability of the bacteria to precipitate calcium carbonate in the soil leads to reduction in the permeability and increase in the shear strength of the soil.

The uranium isotopic composition of modern stromatolites forming in Shark Bay, Western Australia

<p>Stromatolites represent some of the earliest evidence for life and are valuable geochemical archives for understanding the rise of oxygen on early Earth. Metal redox proxies in carbonates, such as stable uranium isotopes (<sup>238</sup>U/<sup>235</sup>U), are useful for assessing the oxidation state of ancient waterbodies, but may also be sensitive to local water chemistry and early sedimentary diagenesis. This requires the validation of such proxies in modern environments before applying them to ancient carbonates. Here we measure the U isotopic composition of modern stromatolites forming in Hamelin Pool in Shark Bay, Western Australia – a large hypersaline marine embayment and the largest modern example of stromatolite development globally. Actively-growing stromatolite tops from Shark Bay exhibited a narrow range of δ<sup>238</sup>U from -0.30 to -0.33‰, corresponding to an offset of ca. +0.1‰ from seawater. Such an offset has not been found in other biotic marine carbonates, which exhibit seawater-like δ<sup>238</sup>U (ca. -0.4‰), but is consistent with findings from carbonate co-precipitation experiments. One hypothesis for our measured +0.1‰ offset is the elevated Ca concentration of the hypersaline Shark Bay seawater relative to open seawater. This results in a greater proportion of dissolved U present as Ca<sub>2</sub>UO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub>, which is expected to be isotopically lighter than other U species and not incorporated during carbonate mineral formation. Higher δ<sup>238</sup>U up to +0.11‰ were measured in the deeper stromatolite laminae, consistent with the expected U isotope signatures for U reduction. Stromatolite radiocarbon ages show that the diagenetic modification of U occurs within ~1 ka and may be considered syndepositional on geological timescales. These results from the deeper stromatolite laminae support the application of a ca. -0.4‰ correction factor to the δ<sup>238</sup>U of stromatolites formed in oxic waterbodies, similar to other biotic carbonates. It is unclear whether the additional +0.1‰ offset found in stromatolite tops is particular to seawater chemistry of Shark Bay or a general feature of microbial carbonate precipitation. This warrants investigation of the δ<sup>238</sup>U proxy in other modern environments where stromatolites proliferate.</p>