Soluble humic substances from in situ precipitated microcrystalline calcium carbonate, internal sediment, and spar cement in a Cretaceous carbonate mud-mound

Albian carbonate mud-mound limestones exposed near Iraneta, northern Spain, show a fabric- and particle-specific fluorescence. Intense fluorescence is restricted to in situ precipitated microcrystalline (automicritic) fabrics , calcified demosponges, and coralline sponges. Intermediate intensity deri ves from micritized bioclasts, pellets, and a rim of marine bladed cement. Most invertebrate skeletons, late-diagenetic equant cement, and crosscuttin g zones of dolomitization are weakly to nonfluorescent. Internal microcryst alline sediment (allomicrite) and red algae debris have variable fluorescen ce. Correlation between rock fluorescence and soluble humic substances was evaluated from 3 g of automicrite, allomicrite, and cement. Time-resolved l aser-induced fluorescence spectroscopy (TRLFS) with ultra-short pulses on t wo extracrystalline fractions (NaOH-soluble) and two intracrystalline fract ions (HCl-soluble and NaOH-soluble) showed that most of the soluble humic s ubstances of automicrite are within the crystals; but conversely, are signi ficantly enriched on outer surfaces of allomicrite. Spar cement is close to detection limits. Fluorescence lifetimes are in the range of 0.5-2 ns and 3.5-6 ns. We conclude that precipitation of automicrite took place during o xidative organic matter diagenesis, i.e., during condensation reactions of degradation products of marine biopolymers. By contrast, allomicrite formed by skeletal breakdown followed by ingestion, organic coating, and reingest ion during deposit feeding. A humic-substance-based model of marine polymer gels represents a new approach for the understanding of ancient polygeneti c carbonate muds, so typical of Phanerozoic mud-mounds in deeper water sett ings.