Hidekazu Yoshida1, Ryusei Kuma2, Hitoshi Hasegawa3, Nagayoshi Katsuta4, Sin-Iti Sirono2, Masayo Minami5, Shoji Nishimoto6, Natsuko Takagi7, Seiji Kadowaki8, Richard Metcalfe9. 1. University Museum, Nagoya University, Chikusa, Nagoya, Japan. dora@num.nagoya-u.ac.jp. 2. Graduate School of Environmental Studies, Nagoya University, Chikusa, Nagoya, Japan. 3. Faculty of Science and Technology, Kochi University, Kochi, Japan. 4. Faculty of Education, Gifu University, Yanagido, Gifu, Japan. 5. Institute of Space-Earth Environmental Research, Nagoya University, Nagoya, Japan. 6. Nagoya City Science Museum, Nagoya, Japan. 7. Faculty of Science, Nagoya University, Nagoya, Japan. 8. University Museum, Nagoya University, Chikusa, Nagoya, Japan. 9. Quintessa, Videcom House, Newtown Road, Henley-on-Thames, Oxfordshire, UK.
Abstract
Isolated silica concretions in calcareous sediments have unique shapes and distinct sharp boundaries and are considered to form by diagenesis of biogenic siliceous grains. However, the details and rates of syngenetic formation of these spherical concretions are still not fully clear. Here we present a model for concretion growth by diffusion, with chemical buffering involving decomposition of organic matter leading to a pH change in the pore-water and preservation of residual bitumen cores in the concretions. The model is compatible with some pervasive silica precipitation. Based on the observed elemental distributions, C, N, S, bulk carbon isotope and carbon preference index (CPI) measurements of the silica-enriched concretions, bitumen cores and surrounding calcareous rocks, the rate of diffusive concretion growth during early diagenesis is shown using a diffusion-growth diagram. This approach reveals that ellipsoidal SiO2 concretions with a diameter of a few cm formed rapidly and the precipitated silica preserved the bitumen cores. Our work provides a generalized chemical buffering model involving organic matter that can explain the rapid syngenetic growth of other types of silica accumulation in calcareous sediments.
Isolated silica concretions in calcareous sediments have unique shapes and disn class="Chemical">tinct sharp boundaries and are considered to form by diagenesis of biogenic siliceous grains. However, the details and rates of syngenetic formation of these spherical concretions are still not fully clear. Here we present a model for concretion growth by diffusion, with chemical buffering involving decomposition of organic matter leading to a pH change in the pore-water and preservation of residual bitumen cores in the concretions. The model is compatible with some pervasive silica precipitation. Based on the observed elemental distributions, C, N, S, bulk carbon isotope and carbon preference index (CPI) measurements of thesilica-enriched concretions, bitumen cores and surrounding calcareous rocks, the rate of diffusive concretion growth during early diagenesis is shown using a diffusion-growth diagram. This approach reveals that ellipsoidal SiO2 concretions with a diameter of a few cm formed rapidly and the precipitated silica preserved the bitumen cores. Our work provides a generalized chemical buffering model involving organic matter that can explain the rapid syngenetic growth of other types of silica accumulation in calcareous sediments.
Authors: Chloé Plet; Kliti Grice; Anais Pagès; Michael Verrall; Marco J L Coolen; Wolfgang Ruebsam; William D A Rickard; Lorenz Schwark Journal: Sci Rep Date: 2017-10-23 Impact factor: 4.379