Silica botryoids from chemically oscillating reactions and as Precambrian environmental proxies

In this petrographic and geochemical study, we differentiated diverse quartz botryoids, including circular-concentric, twinned, columnar, wavy, and stromatolite-like structures versus synchronous biotic patterns of similar geometry and size dimensions (filamentous traits and stromatolites) in Precambrian cherts of Barberton, South Africa, and Gunflint, Canada. The botryoidal habits explored retained self-similar patterns of radially aligned acicular quartz with concentric laminae, which are not documented in biologically built stromatolites. These ancient fractals and their composition imitate those in chemically oscillating reactions, implying that the precipitation of botryoids was fueled by abiotic diagenetic degradation of organic matter and subsequently metamorphosed into chert

Biogenic silica diagenesis under early burial in hemipelagic marine sediments

Four aspects of biogenic silica diagenesis are researched under this project: signatures of ongoing silica diagenesis in pore water, drivers leading to anomalous compaction (abrupt petrophysical changes), bottom-water temperature controls on silica dissolution and recycling, and temperature-time relationships in silica later diagenesis. Results of this project are discussed in Chapters 4â7. Chapter 4 demonstrates, based on pore chemistry, mineralogy, and thermodynamic analyses, that solubility equilibrium is reached for diagenetic silica in pore water at opal-A to opal-CT transition zone captured by Ocean Drilling Program Sites 794 and 795 in Japan Sea; thus opal-CT is precipitating across the transition. It is also argued that ion transport mechanisms have failed to erase signatures of ongoing diagenesis. Chapter 5 utilises texture and mineralogical analyses of hemipelagic sediments from Sites 794 and 795 to place anomalous compaction across the transition zone in a diagenetic context. A sharp decrease in opal-A content in the transition under dissolution causes a significant decrease in sediment stability which results in a porosity drop and other petrophysical variations. Subsequent precipitation of opal- CT has, however, had far less effects on porosity. Chapter 6 models variations in silica dissolution and recycling with bottom-water temperatures using data from 22 representative Ocean Drilling Program sites. Though model expression of early diagenesis confirms a relationship between temperature and opal-CT formation in young sediments (&LT; 4 Ma) near the seabed in the Antarctic, the established inverse temperature-time correlation in silica diagenesis contradicts main controls from low bottom temperature over a short geological time. Chapter 7 investigates temperature-time relationships in silica diagenesis by examining 67 scientific drill sites where the opal-A to opal-CT transition zone lies in Cenozoic sediments. Based on these constraints, the reference opal-CT stability field is modified. The chapter also applies a model that formulates time-temperature dependence of silica diagenesis in representative Sites 794 and 795.</p

Biogenic silica diagenesis under early burial in hemipelagic marine sediments

Four aspects of biogenic silica diagenesis are researched under this project: signatures of ongoing silica diagenesis in pore water, drivers leading to anomalous compaction (abrupt petrophysical changes), bottom-water temperature controls on silica dissolution and recycling, and temperature-time relationships in silica later diagenesis. Results of this project are discussed in Chapters 4–7. Chapter 4 demonstrates, based on pore chemistry, mineralogy, and thermodynamic analyses, that solubility equilibrium is reached for diagenetic silica in pore water at opal-A to opal-CT transition zone captured by Ocean Drilling Program Sites 794 and 795 in Japan Sea; thus opal-CT is precipitating across the transition. It is also argued that ion transport mechanisms have failed to erase signatures of ongoing diagenesis. Chapter 5 utilises texture and mineralogical analyses of hemipelagic sediments from Sites 794 and 795 to place anomalous compaction across the transition zone in a diagenetic context. A sharp decrease in opal-A content in the transition under dissolution causes a significant decrease in sediment stability which results in a porosity drop and other petrophysical variations. Subsequent precipitation of opal- CT has, however, had far less effects on porosity. Chapter 6 models variations in silica dissolution and recycling with bottom-water temperatures using data from 22 representative Ocean Drilling Program sites. Though model expression of early diagenesis confirms a relationship between temperature and opal-CT formation in young sediments (&amp;LT; 4 Ma) near the seabed in the Antarctic, the established inverse temperature-time correlation in silica diagenesis contradicts main controls from low bottom temperature over a short geological time. Chapter 7 investigates temperature-time relationships in silica diagenesis by examining 67 scientific drill sites where the opal-A to opal-CT transition zone lies in Cenozoic sediments. Based on these constraints, the reference opal-CT stability field is modified. The chapter also applies a model that formulates time-temperature dependence of silica diagenesis in representative Sites 794 and 795.</p

Supplemental Material: Silica botryoids from chemically oscillating reactions and as Precambrian environmental proxies

Materials and methods, Figures S1–S4, and Table S1.  </p

Botryoidal quartz as an abiotic signature in Palaeoarchean cherts of the Pilbara Supergroup, Western Australia

Chemically oscillating reactions (COR) are abiotic processes that generate self-repeating circularly concentric morphologies during decarboxylation of organic acids. The geometry and millimetre to centimetre size dimensions of laminated quartz botryoids recorded in the Palaeoarchean Apex and Strelley Pool chert horizons in Western Australia simulate the self-similar fractal patterns arising in vitro from COR using classical and modified reactants of the Belousov-Zhabotinsky (B-Z) reaction. The botryoidal patterns akin to those developed by the COR include circular concentric laminae, open-book features, microbialite- and stromatolite-like laminations, wavy and domal structures, and rosettes. The mineral composition and organic matter (OM) distribution of these objects indicates an origin from the early diagenetic decarboxylation of carboxylic acids. Pore-water alkalinity decrease is likely due to the decomposition of OM, the generation of carbonic acid, and the cessation of chemical wave diffusion. The chemical waves developed from primary circular oxidation spots, under solubility equilibrium with respect to silica could trigger diagenetic precipitation of quartz. The presence of Fe2+-bearing hematite in various botryoidal geometries in the Apex chert is an analogue of ferroin-derived Fe in B-Z solutions, whereas the presence of Fe and Zn sulphides in the Strelley Pool quartz botryoids is akin to sulphur redox intermediates in B-Z experiments. Correlated microscopy and Raman spectral analysis corroborate that the metamorphosed OM associated with these Palaeoarchean botryoids is indigenous and syngenetic with the host chert. OM in botryoidal quartz displays circular concentric laminations as well as gradients of its density. Accordingly, an abiotic model of COR for the diagenetic growth of the studied Apex and Strelley Pool quartz botryoids is proposed. Comparisons with the chemical compositions, geometric morphology and range of size dimensions of self-similar patterns have been the criteria for development of this model expression. The explored ancient botryoids can thus represent abiotic sedimentological signatures of carbon cycling

Botryoidal quartz as an abiotic signature in Palaeoarchean cherts of the Pilbara Supergroup, Western Australia

Chemically oscillating reactions (COR) are abiotic processes that generate self-repeating circularly concentric morphologies during decarboxylation of organic acids. The geometry and millimetre to centimetre size dimensions of laminated quartz botryoids recorded in the Palaeoarchean Apex and Strelley Pool chert horizons in Western Australia simulate the self-similar fractal patterns arising in vitro from COR using classical and modified reactants of the Belousov-Zhabotinsky (B-Z) reaction. The botryoidal patterns akin to those developed by the COR include circular concentric laminae, open-book features, microbialite- and stromatolite-like laminations, wavy and domal structures, and rosettes. The mineral composition and organic matter (OM) distribution of these objects indicates an origin from the early diagenetic decarboxylation of carboxylic acids. Pore-water alkalinity decrease is likely due to the decomposition of OM, the generation of carbonic acid, and the cessation of chemical wave diffusion. The chemical waves developed from primary circular oxidation spots, under solubility equilibrium with respect to silica could trigger diagenetic precipitation of quartz. The presence of Fe2+-bearing hematite in various botryoidal geometries in the Apex chert is an analogue of ferroin-derived Fe in B-Z solutions, whereas the presence of Fe and Zn sulphides in the Strelley Pool quartz botryoids is akin to sulphur redox intermediates in B-Z experiments. Correlated microscopy and Raman spectral analysis corroborate that the metamorphosed OM associated with these Palaeoarchean botryoids is indigenous and syngenetic with the host chert. OM in botryoidal quartz displays circular concentric laminations as well as gradients of its density. Accordingly, an abiotic model of COR for the diagenetic growth of the studied Apex and Strelley Pool quartz botryoids is proposed. Comparisons with the chemical compositions, geometric morphology and range of size dimensions of self-similar patterns have been the criteria for development of this model expression. The explored ancient botryoids can thus represent abiotic sedimentological signatures of carbon cycling