Paleoenvironmental context and paleoecological significance of unique agglutinated polychaete worm tube-ferruginous microstromatolite assemblages from the Middle Jurassic of the Southern Carpathians (Romania)

Tubular structures showing well-preserved agglutinated walls are recorded from Middle Jurassic (Bathonian-Lower Callovian) hardgrounds and condensed beds cropping out in the Southern Carpathians, Romania. The agglutinated tube-wall is formed of 2-3 concentric layers of ooids, bioclasts, and silicate grains, and tube length is highly variable (maximum 60 mm) with the external diameter ranging from 0.64 to 8.85 mm. The agglutinated tubes are associated with ferruginous microstromatolites, forming small buildups of 10-25 mm thickness within hardground crusts and coatings of macro-oncoids. This is the first record of agglutinated tube-worms associated with ferruginous microstromatolites. Scanning electron microscope investigations revealed that the ferruginous microstromatolites that encrust the tubes were formed by the activity of putative bacterial and fungal filaments. Integrated analyses reveal the paleoecology of these unique fossil assemblages and allow a paleoenvironmental assessment of their genesis in a relatively deep-water, open-marine shelf environment, below fair-weather wave base or near to storm wave-base. This environment was characterized by low rates of sedimentation and calm waters alternating with periodic episodes of moderate to high water agitation. Different types of ferruginous microstromatolite were formed in poorly oxygenated microenvironments, where iron bacteria and fungi could have precipitated ferric oxyhydroxides during intervals with calm to moderate water energy. These became protected hard substrates, allowing the settlement of benthic organisms. Increase of water energy led to the availability of food resources and of specific-sized grains, as subsequent essential paleoenvironmental parameters necessary for the settlement and development of polychaete agglutinated worm tubes.