Changes in the pelagic fine fraction carbonate sedimentation during the Cretaceous–Paleocene transition: contribution of the separation technique to the study of Bidart section

Abstract

The Bidart section (southwestern France) displays an almost complete Late Maastrichtian–Early Danian transition. As the Cretaceous–Paleocene (K/P) crisis mainly affects carbonate producers, the aim of this study is to precisely evaluate the evolution of pelagic sediments composition across the boundary. A new protocol for separation of fine carbonates into discrete size fractions was used (1) to precisely evaluate (in weight of carbonate and not only number of particles per gram of rock) the contribution of each carbonate component type (foraminifera, calcareous nannofossils and “nonbiogenic” carbonates, calcite microparticles, and calcareous macrocrystals), and (2) to measure their isotopic signature and thus constrain the origin of these carbonate particles. The Maastrichtian pelagic carbonates are mostly biogenic (70% of the bulk carbonate) but also contain “nonbiogenic” carbonates: calcite microparticles (<4 μm) and calcareous macrocrystals (>4 μm). Due to their isotopic signature, calcite microparticles cannot result from fragmentation and partial dissolution of biogenic remains, and the most likely origin appears to be early diagenetic processes and/or export from shallow-water platforms. The calcareous macrocrystals derive from an early diagenetic process. The lowermost Danian sediments are dominated by “nonbiogenic” carbonates (between 50% and 80% of the bulk carbonate). Nevertheless, two kinds of biogenic components are present: Thoracosphaera operculata and Maastrichtian-like calcareous nannofossils. Thoracosphaera operculata is an opportunistic calcareous dinoflagellate, which frequently occurs at high abundances in basal Danian sediments and is certainly in situ, this is confirmed by it showing a Danian isotopic signature. The Maastrichtian-like calcareous nannofossils are nannofossils of taxa that indisputably occur in the Maastrichtian, but which do not occur consistently in later Danian strata, they show a Maastrichtian-type isotopic signature, which strongly supports the inference that they are mainly reworked from the Maastrichtian. This study shows that “nonbiogenic” carbonates are important components of the pelagic carbonate even during periods of normal sedimentation. In the “crisis interval”, the biogenic carbonate production drops sharply, while calcite microparticles and carbonate macrocrystals remain almost unaffected and represent the major part of the carbonate in the earliest Danian sediments. Finally, the sediments return to a composition similar to that of the Maastrichtian in M. pseudobulloides foraminifera zone.