Stable silicon isotopes in chert and silicified sediments

Chert mostly forms in low temperature environments close to the surface of the Earth and thus records processes that have occurred during its formation in the marine realm but possibly also records changes that have occurred on land. We use silicon isotopes to assess the information contained in chert about the geological past and to decipher the processes of silicification, i.e. the processes that turn silica into a solid rock of quartz chert.

As chert does not precipitate directly as solid quartz but as (a)biogenic opal or opal-CT, it remains unclear to what extent the large range of silicon isotope compositions found in chert (normalized 30Si/28Si ratio *1000= δ30Si= -3 ‰ to +5 ‰) hold information on environmental conditions or on diagenetic processes that converted silica precursor phases into quartz chert.

To discern diagenetic ‘fingerprints’ from information inherited from the environment, we study the youngest products of the silicification process, i.e. porcelainites embedded in Pliocene and Pleistocene siliceous sediments of the Southern Ocean. Using state-of-the-art femtosecond laser ablation coupled to a MC-ICP-MS (multi-collector inductively , we determine their micro-scale isotopic composition to understand silicification and to evaluate the actual meaning of silicon isotopes in chert.

Knowledge obtained from these studies, we apply to chert deposited at the Precambrian-Cambrian boundary; that is the time when metazoans develop drastically to form almost all of the modern phyla within only a short time span of the geological past.

Ediacaran Chert-layers in Hunan, China

The widespread occurrence of chert and phosphorites in Ediacaran (Late Precambrian) rocks suggest that the marine ecosystem was subjected to changes relevant to the development of higher life. As part of the FU Berlin Forschergruppe 736 (Research Group: The Precambrian-Cambrian Ecosphere (R)evolution: Insights from Chinese microcontinents: we evaluate the silicon isotope record of ancient cherts from China to contribute to the understanding of conditions in the former oceanic realm and on the land surface, both of which might have been groundbreaking for the development of most modern life forms.

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Dr. Michael Tatzel
Geochemistry of the Earth's surface