Petrology of Lower Cretaceous carbonate mud mounds (Albian, N-Spain): insights into organomineralic deposits of the geological record

The process of organomineralization is increasingly well understood with re spect to modern carbonate sediments accumulating adjacent to tropical reef atolls and reef caves. Mineralization related to non-living organic substra tes results in autochthonous micrite production ('automicrites'). 'Automicr ites' are the main constructive element of Lower Cretaceous (Albian) carbon ate mud mounds in northern Spain. These slope mud mounds occur within trans gressive and early highstand system tracts encompassing several macrobenthi c ecological zones. They are clearly separated from the biocalcifying carbo nate factory (Urgonian carbonate platforms), in both space and time. Within these build-ups, most 'automicrites' were initially indurated and accreted to form a medium-relief growth framework. 'Automicrites' have a uniform, p resumably high-Mg-calcite precursor mineralogy. They show an inorganic stab le-isotope signature (partial derivative(13)C around +3.3 parts per thousan d,) within the range of early marine cements, and skeletal compounds lackin g major vital effects. Epifluorescence microscopy shows that they have faci es-specific fluorescence, which is similar to skeletal compounds of Acantho chaetetes, but clearly different from allomicritic sediment and cements, wh ich are mostly non-fluorescent. The EDTA-soluble intracrystalline organic f raction (SIOF) of Albian automicrites shows an amino acid spectrum that is similar to shallow subsurface samples from their modern counterparts. Gel e lectrophoresis of the SIOF demonstrates an exclusively acidic character, an d a mean molecular size range between 20 and 30 kDa. Experiments in vitro ( inhibition tests) indicate that the SIOF has a significant Ca2+-binding cap acity. Fluorescence and chemical characteristics of SIOF point to a main su bstance class, such as humic and fulvic acids, compounds that form from pri stine organic matter during early diagenesis. Biomarker analyses provide ev idence for the crucial role of biodegradation by heterotrophic microorganis ms, but no biomarker for cyanobacteria has been found. Primary sources of o rganic material should have been manifold, including major contributions by metazoans such as sponges. It is concluded that many carbonate mud mounds are essentially organomineralic in origin and that the resulting fabric of polygenetic muds ('polymuds') may represent ancestral metazoan reef ecosyst ems, which possibly originated during the Neoproterozoic.