What is it about?

Silica-rich sedimentary rocks (chert) record past life on Earth because they host microfossils and organic molecules (OMs). A long-standing knowledge gap was how cherts preserve these OMs. This means that the spatial location of these OMs was an issue related to their preservation. Exploiting X-ray scattering interactions with rocks, our study shed light on this issue showing that differently-aged cherts are composed of nanometric quartz crystals (at least 100 nm long). Most impressively, nanometric spaces (pores) occur between those tiny crystals. We contribute by showing that these nanometric pores are filled by OMs. To this aim, the X-ray scattering properties of samples before and after their combustion permitted us to quantify the volume and size of pores holding OMs. Spatial distribution and abundance of some chemical elements and measurement of OMs helped us to delineate a full sample characterization.

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Why is it important?

We contributed to understanding how the size and shape of pores evolve over time in chert and how this correlated to organic matter preservation. This issue was scarcely addressed in previous literature about this type of rock, largely suggested as a record of life in past geological time.


Chert formation largely relies on biosilica formation, which controls the abundance of chemical elements in the Hydrosphere through interactions between microorganisms and dissolved cations. Biosilica forms in oxygen-bearing media at atmospheric P-T conditions, leading to sub-micrometric aggregates. Under compaction, these aggregates become cherts, preserving organic matter (OM). In aqueous media, remineralisation promotes bacterial uptake of heavy metals (HMs). The solubility and no biological use of divalent HMs allow their bioaccumulation. HMs are 10000X enriched in marine chert compared to modern seawater, supporting their bioaccumulation. Concomitantly, some experiments also showed aggregation of SiO2 nanoparticles in the presence of E. coli. These evidences suggest that HMs are possibly also an additional record of bacterial activity linked to biosilica and chert formation. Thus, in future research, we will focus on the initial stages of biosilica formation to explore how silica aggregation could act as a mechanism of HM encapsulation.

Patricio Montecinos Munoz
University of Sao Paulo

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This page is a summary of: (U)SAXS characterization of porous microstructure of chert: insights into organic matter preservation, Journal of Applied Crystallography, November 2023, International Union of Crystallography,
DOI: 10.1107/s1600576723008889.
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