Towards a new understanding of the genesis of chalk: Diagenetic origin of micarbs confirmed by clumped isotope analysis

Chalk is usually thought to be a homogeneous sediment with a relatively simple early diagenetic history. Here, clumped isotope analyses of samples from a core of Campanian Maastrichtian chalk are presented, indicating that material smaller than 5 mu m has a different origin than the coccolith-dominated coarser fraction. The smallest size fraction (1 to 5 mu m) of chalk is dominated by calcite particles without a distinct morphology (micarbs). Clumped isotope data of the micarbs reveals formation temperatures of 14 to 18 degrees C which is 8 to 10 degrees C colder than those derived from coeval coccoliths. The micarbs are interpreted as the product of calcite neoformation, precipitated in the uppermost part of the sediment column (100 metres below sea floor) and linked to early dissolution of aragonitic fossils. These findings prove that early cements can be an abundant component in chalk, and thus challenge the common notion that chalk is always largely composed of calcareous nannofossils, and differs only in terms of minor constituents and degree of lithification.

Automated summary

Clumped isotope analyses of chalk samples revealed different origins of materials in the chalk, with the smallest size fraction showing significantly lower formation temperatures compared to coccoliths from the same period. The micarbs, interpreted as products of calcite neoformation, challenge the common notion that chalk is mainly composed of calcareous nannofossils.