Authigenic anatase within 1 billion-year-old cells

The siliciclastic similar to 1 Ga-old strata of the Torridon Group, Scotland, contain some of the most exquisitely preserved three-dimensional organic-walled microfossils (OWMs) of the Precambrian. A very diverse microfossil assemblage is hosted in a dominantly phosphatic and clay mineral matrix, within the Diabaig and the Cailleach Head (CH) Formations. In this study, we report on several microfossil taxa within the CH Formation (Leiosphaeridia minutissima,Leiosphaeridia crassa,Synsphaeridiumspp. andMyxococcoidesspp.) that include populations of cells containing an optically transparent and highly refringent mineral, here identified using electron microscopy as anatase (TiO2). Most anatase crystals occur entirely within individual cells, surrounded by unbroken carbonaceous walls. Rarely, an anatase crystal may protrude outside a cell, interpreted to correspond to zones where the cell wall had broken down prior to anatase precipitation. Where an anatase crystal entombs an organic intracellular inclusion (ICI), the ICI is large and well preserved. These combined observations indicate that the intracellular anatase is an authigenic sedimentary phase, making this the first report of in situ precipitated anatase intimately associated with microfossils. The ability of anatase to preserve relatively large volumes of intracellular and cell wall organic material in these cells suggests that the crystallisation of anatase entombed cellular contents particularly quickly, soon after the death of the cell. This is consistent with the strong affinity of Ti for organic material, the low solubility of TiO2, and reports of Ti occurring in living organisms. With the data currently available, we propose a mineralisation pathway for anatase involving Ti complexation with organic ligands within specific cells, leading to localisedpost-mortemanatase nucleation inside these cells as the complexes broke down. Further overgrowth of the anatase crystals was likely fuelled by very early diagenetic mobilisation of Ti that had been bound to more labile organic material nearby in the sediments.

Automated summary

This study reports microfossil assemblages within the CH Formation of the Torridon Group in Scotland, containing anatase crystals entirely enclosed within individual cells, marking the first report of in situ precipitated anatase intimately associated with microfossils. The ability of anatase crystals to preserve large volumes of intracellular organic material suggests rapid entombment of cellular contents post-mortem, likely driven by the strong affinity of Ti for organic material and the low solubility of TiO2. A proposed mineralisation pathway for anatase involves Ti complexation with organic ligands within specific cells, leading to localized post-mortem anatase nucleation and subsequent overgrowth fueled by early diagenetic mobilisation of Ti.