Early Diagenetic Silica Deposition in Algal Cysts and Spores: A Source of Sand in Black Shales?
ABSTRACT Devonian black shales deposited on the North American craton contain abundant Tasmanites cysts. Although these are typically flattened because of compaction, a small proportion are filled with diagenetic silica. The latter are spherical to ellipsoidal (0.1-0.5 mm), and filled with chalcedony, microquartz and megaquartz, and single quartz crystals. Chalcedonic cyst fillings are preserved best in cheat and phosphate nodules, whereas megaquartz and single quartz crystals are most common in shale matrix. Together with colloform textures, this suggests that the various silica types originated from recrystallization of early diagenetic silica deposits. Thin sandstone beds in the Chattanooga Shale (e.g., Bransford Sandstone) contain abundant quartz sand that is much coarser than the detrital quartz component of the underlying black shales. Because of this, their quartz component is thought to have been transported over considerable distances from the basin margin. However, because certain shale horizons contain as much as 10% silicified cysts that upon reworking could have yielded quartz grains of fine to coarse sand size, the quartz component of these sandstone beds may actually have formed in situ. Indeed, petrographic examination of the sandstone beds shows them to contain quartz grains with morphological and textural features of "cyst" quartz (e.g., rounding, sphericity, chalcedony, pyrite inclusions, lobate grain margins). Thus, silica deposition in algal cysts may provide a significant component of intrabasinal quartz sand in shale sequences. Distinction of this type of quartz from extrabasinal detrital quartz is important to the reconstruction of the depositional history of shale sequences.
Sulfate-reducing bacteria and silica solubility: a possible mechanism for evaporite diagenesis and silica precipitation in banded iron formations
Selective replacement of sulfate-evaporite minerals by silica and the precipitation of silica in association with sulfide mineral phases in banded iron formations may be mediated by the metabolic activities of sulfate-reducing bacteria. Hydrogen sulfide is known to be a product of this metabolism and is often called upon as a source of sulfur for metallic sulfides in sedimentary rocks. We report here on the influence that chemical changes induced by bacterial sulfate reduction have on silica solubility.Controlled in vitro growth experiments with Desulfovibrio desulfuricans and silica show (1) this organism can grow in silica concentrations as great as 400 ppm with no inhibition and (2) growth in the presence of silica yields a decrease in dissolved silica.Growth experiments with 80 ppm silica produced a lowering in dissolved silica from 80 ppm to 60 ppm, a 25% decrease, in just 30 h. Control experiments in the absence of cells resulted in no effective decrease in dissolved silica. The ability of sulfate-r...
Exploring the Textual Alchemy of Legal Gender: Experimental Statutes and the Message in the Medium
This article draws on original empirical research to explore the politics of experimental feminist statutes. It has two main aims. First, it traces how conventional legislative drafting techniques have participated in the wider social creation and continuance of sex/gender norms. It shows how dominant statutory expressions of sex and gender that might otherwise appear timeless have shifted with social change and legal innovation. The article contributes to debates in feminist legal studies, legal anthropology, and legislative drafting by making visible, and analysing the particular power of legislative text, its ‘alchemy’, in expressing and re-creating sex/gender as a social, cultural and political artefact. Second, drawing on this research, the article explores what the Future of Legal Gender project might consider and do when drafting an experimental statute to decertify legal gender. Addressing questions of positionality, believability, legal form and the use of potentially innovative or contested drafting techniques (the singular “they”, the second person), the article explores tensions between legislative drafting and feminist legal method, as well as the benefits for bringing feminist analysis and perspectives to this important aspect of legal practice. Given that legislative drafting does not merely inscribe pre-agreed policy ideas into legal text but helps to shape emerging ontologies of gender, then drafting an experimental statute invites feminists to pay attention to interlinked questions of substance and form in the exploration of prefigurative legal futures.