How Do Vertebrate Droppings Fossilise? Biogenic Versus Diagenetic Signals in Coprolites Evidenced by Elemental Mappings

Abstract

Thousands of coprolites were unearthed from the Lower Cretaceous swamp/lacustrine sediments at the locality of Angeac-Charente, Southwest France, alongside tens of thousands of bones and teeth from around 40 vertebrate taxa. This work focuses on the chemical composition of 50 coprolite specimens to determine their elemental variability in relation to their taphonomy and morphotypology. By using μXRF, 31 chemical elements were selected to compare eight morphotypes. Combined with FTIR-ATR and LIBS, our analyses highlight homogeneous carbonate-fluorapatite (CFAp) composition for all samples. Elements of clear detrital origin such as Si, Al and K are enriched in the margin, which points to early surficial contamination of the faeces by siliciclastic particles. Also, higher in the margin, Ce, Y and Sr concentrations result from inwards migration of mobile elements initially adsorbed onto the host clays and their substitution for calcium within the CFAp lattice. Coprolite central regions were more affected by different elements, such as Fe, S, As and Zn, reflecting reducing conditions under which pyrite minerals precipitated. Rapid hardening of the coprolite margin by permineralisation enhanced reducing conditions in the centre of the coprolite, favouring precipitation of sulphide minerals. Coprolite composition results from the reduction/oxidation conditions during early diagenesis, rather than from the recent oxidative front, overprinting possible biological signal between morphotypes. Only the two smallest morphotypes are geochemically distinct from the others, with higher concentrations of Ca, compatible with a piscivorous diet. This indicates that the chemical composition of the coprolites reflects the burial environment rather than the specific composition of the digested food.