Towards a new understanding of the genesis of chalk: Diagenetic origin of micarbs confirmed by clumped isotope analysis MATTIA TAGLIAVENTO* , CE ´ DRIC M. JOHN † , KRESTEN ANDERSKOUV* and LARS STEMMERIK ‡ *Department of Geoscience and Natural Resource Management, University of Copenhagen, Øster Voldgade 10, Copenhagen K 1350, Denmark (E-mail: tagliavento@geo.uni-frankfurt.de; mattiatagliavento@outlook.com) †Department of Earth Science & Engineering, Imperial College London, London SW72AZ, UK ‡Geological Survey of Denmark and Greenland (GEUS), Øster Voldgade 10, Copenhagen K 1350, Denmark Associate Editor – John Reijmer ABSTRACT Chalk is usually thought to be a homogeneous sediment with a relatively sim- ple early diagenetic history. Here, clumped isotope analyses of samples from a core of Campanian Maastrichtian chalk are presented, indicating that mate- rial smaller than 5 μm has a different origin than the coccolith-dominated coarser fraction. The smallest size fraction (1 to 5 μm) 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°C which is 8 to 10°C colder than those derived from coeval coccoliths. The micarbs are inter- preted 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. Keywords Chalk, clumped isotopes, early diagenesis, micarbs. INTRODUCTION The Cretaceous chalk of north-west Europe is a very pure, fine-grained carbonate rock composed almost entirely of low-Mg calcite. It is consid- ered to be formed of coccoliths, with minor con- stituents being calcareous microfossils and a highly specialized benthic fauna of brachiopods, bryozoans and other low-Mg calcite shells (H  akansson et al., 1974; Surlyk, 1997; Lauridsen & Surlyk, 2008). Evidence of an aragonitic fauna is limited to impressions of cephalopods, moulds after micromorphic bivalves and gas- tropods in hardgrounds, and gastropod borings in larger shells, like echinoderms (Carter, 1972; Heinberg, 1999; Hansen, 2010). The absence of an aragonitic fauna has been interpreted as a product of very early dissolution, either on the seafloor or in the shallow subsurface. Chalk thus shares features characteristic of early aragonite dissolution with many other carbonate records (Cherns & Wright, 2000; Munnecke et al., 2001; Wright & Cherns, 2004; James et al., 2005; Cherns et al., 2008) such as: (i) monomineral composition of low-Mg calcite; and (ii) lack of preservation of an aragonitic fauna. Aragonite shells are often a major component of carbonate sediments, and even in cool-water carbonates like on the Australian Shelf (James et al., 2005) they can account for up to 50 to 80% of the total sediment. Their disappearance thus produces a strong taphonomic bias, where the original 1 © 2020 The Authors. Sedimentology © 2020 International Association of Sedimentologists Sedimentology (2020) doi: 10.1111/sed.12802