Seawater silica cycling and chert formation at the Neoproterozoic–Cambrian transition: Insights from δ.sup.30./sup.Si and Ge/Si systematics of hydrothermal cherts from the Bohemian Massif

0573253 - GLÚ 2024 RIV NL eng J - Journal Article
Ackerman, Lukáš - Poitrasson, F. - Magna, T. - Polák, Ladislav - Ďurišová, Jana
Seawater silica cycling and chert formation at the Neoproterozoic–Cambrian transition: Insights from δ³⁰Si and Ge/Si systematics of hydrothermal cherts from the Bohemian Massif.
Chemical Geology. Roč. 634, September (2023), č. článku 121598. ISSN 0009-2541. E-ISSN 1872-6836
R&D Projects: GA ČR(CZ) GA20-13644S
Institutional support: RVO:67985831
Keywords : Silicon isotopes * Germanium * Chert * Cycling * Ediacaran * Cambrian
OECD category: Geology
Impact factor: 3.9, year: 2022
Method of publishing: Limited access
https://www.sciencedirect.com/science/article/pii/S000925412300298X

The genesis of cherts, rocks with >90 wt% SiO₂, is suggested to be linked with dissolved seawater silica and its elevated precipitation in the Phanerozoic due to massive biological activity in the Earth’s oceans compared to the Precambrian where hydrothermal fluids played a more dominant role. Because cherts may record a history of global seawater Si cycle, their Si isotope systematics may serve as a proxy for their origin and depositional environment. In this study, Si isotope compositions, combined with Ge/Si ratios, were determined for a suite of shallow-water, deep-water and stromatolitic cherts from the Blovice accretionary complex (Bohemian Massif, Czech Republic), representative of deep marine and subduction settings, to establish the nature of cherts and processes of their formation, identify the sources of Si and Ge, and discuss the possible effect of the Neoproterozoic–Cambrian transition. The predominantly high δ³⁰Si values > +0.23‰, associated with low Ge/Si, in deep-water cherts suggest their precipitation from seawater-derived low-temperature fluids at the seawater–sediment interface with subordinate admixture of terrigenous material. Moreover, the tight relationship between δ³⁰Si and organic matter contents in these deep-water cherts is indicative of either equilibrium precipitation of isotopically lighter Si as the organic content of the medium increases or global decrease of δ30Si values of seawater during the Meso–Neoproterozoic related to coupled Si and C cycles. On the other hand, a δ³⁰Si variation of ~2.5‰ (–0.94‰ to +1.60‰) in shallow-water cherts argues for precipitation from seawater interacting with hydrothermal fluids of variable temperature. Organic matter, Fe-(oxy)-hydroxides and clays represent important sinks for Ge (and Si in case of organic matter) and thus play a determining role in global marine Si–Ge cycling. This, together with the presence of abundant cherts at the Neoproterozoic–Cambrian transition, appears to be associated with redox-controlled Fe cycle which itself is related to microbial reduction and the increasing presence of organic matter towards the Phanerozoic.
Permanent Link: https://hdl.handle.net/11104/0344927