Questioning the microbial origin of automicrite in Ordovician calathid–demosponge carbonate mounds

Abstract

Calathid-demosponge carbonate mounds are a feature of Early to Middle Ordovician shallow-marine carbonate depositional environments of tropical to subtropical palaeolatitudes. These mounds contain an important amount of autochthonous non-skeletal microcrystalline calcium-carbonate (automicrite) conventionally considered microbial in origin. Here, the automicrite of calathid-demosponge carbonate mounds (Tarim Basin, north-west China) is broken down into five distinct fabrics: an in situ peloidal-spiculiferous fabric (AM-1), an in situ peloidal fabric (AM-2), an aphanitic-microtubular fabric (AM-3), a minipeloidal fabric (AM-4) and a laminoid-cerebroid fabric (AM-5). Type AM-1 occurs with AM-2 being succeeded by an assemblage of AM-3 and AM-4. Types AM-4 and AM-5 are separated by an erosional disconformity. A good correlation of fluorescence and cathodoluminescence of automicrites indicates that induced and supported organomineralization produced automicrite, probably via the permineralization of non-living organic substrates adsorbing dissolved metal-humate complexes. Using a spreadsheet with six parameters and 17 characters, AM-1 to AM-4 turn out to be non-microbial in origin. Instead, these automicrites represent relics of calcified metazoan tissues, such as siliceous sponges, non-spiculate sponges or the basal attachment structures of stalked invertebrates. Fabric AM-5 is a microbial carbonate but is post-mound in origin forming a drape within a reefal framework established by AM-4. The five automicritic fabrics, individually or as an assemblage, are a common element of Ordovician calathiddemosponge carbonate mounds in general. The reassessment of the origins of these automicritic fabrics holds consequences for understanding of the Great Ordovician Biodiversification Event in terms of community structure, reef ecology and reef evolution. Episodically, these fabrics are also present in other carbonate build-ups stretching from the Neoproterozoic over the entire Phanerozoic Eon. The massive calcification of metazoan soft tissue (AM-1 to AM-4) characterizes episodes and conditions of enhanced marine calcification and might be of value to refine secular trends of pCO 2 , Ca concentration and Mg/Ca ratio at the scale of individual sedimentary basins.

Possible poriferan body fossils in early Neoproterozoic microbial reefs

“…2a-c) and vermiform microstructure either demonstrated or interpreted to be sponge-derived in diverse Phanerozoic microbial, reefal and non-reefal carbonate rocks [6][7][8]10,24,25,27,37 . The compositional and textural homogeneity of the microspar groundmass supports an origin through permineralization of a pre-existing biological substance 9 , rather than incremental accumulation of detrital sediment or microbial carbonate that passively incorporated complexly anastomosing tubular microfossils. Variable preservation and association with geopetal peloid accumulations are familiar aspects of Phanerozoic sponge taphonomy 9,21,22,38 .…”

“…The compositional and textural homogeneity of the microspar groundmass supports an origin through permineralization of a pre-existing biological substance 9 , rather than incremental accumulation of detrital sediment or microbial carbonate that passively incorporated complexly anastomosing tubular microfossils. Variable preservation and association with geopetal peloid accumulations are familiar aspects of Phanerozoic sponge taphonomy 9,21,22,38 . In previous work, detailed comparison of the three-dimensional characteristics of vermiform microstructure with branching cylindrical organism types yielded no convincing alternative to the sponge interpretation 6 .…”

“…Sponges are the most basic known animal type 2,3 ; it is possible that body fossils of hitherto-undiscovered Proterozoic metazoans might resemble aspect(s) of Phanerozoic fossil sponges. Vermiform microstructure 4,5 , a complex petrographic feature in Phanerozoic reefal and microbial carbonates, is now known to be the body fossil of nonspicular keratosan demosponges [6][7][8][9][10] . This Article presents petrographically identical vermiform microstructure from approximately 890-million-year-old reefs.…”

“…The calcification of decaying sponge soft tissue has been documented in modern sponges 21,22 , and produces sponge 'mummies' 22 as well as a range of subtle carbonate sedimentary textures (such as peloid clusters) in living and Phanerozoic fossil sponges 7,9,21,22 . Taphonomic sediment textures (polymuds) that may be poriferan-related have previously been identified in the reefs that are the subject of this Article 23 .…”

Environmental controls on the development of Mississippian microbial carbonate mounds and platform limestones in southern Montagne Noire (France)

“…The Coteau de Bergue, Roque Redonde and Roc de Murviel formations are composed mostly of micropeloidal fabrics ( sensu lato ), commonly recorded in Mississippian bioconstructions and of widely accepted microbial origin (Chafetz, ; Monty, ; Neuweiler & Reitner, ; Pratt, ; Pickard, ; Neuweiler et al ., ; Riding, ); but see Shen & Neuweiler () for a different opinion. Similar microbial textures are currently recorded in stressed and hypersaline environments, peritidal settings, lakes, caliches and patch reefs (e.g.…”