Phosphatic microspherules and their geobiological implications from the Frasnian-Famennian transition at the Yangdi section, Guilin, South China

Abstract

Abundant perfectly-preserved phosphatic microspherules have been discovered across the Frasnian-Famennian (F-F) transition from the Yangdi section in Guilin, Guangxi, South China. They are mostly spherical or elliptical in shape and about 150 μm in diameter with smooth exterior surfaces. Each microspherule consistently possesses a small dimple on the surface. The internal texture of microspherules consists of concentric light-colored apatite and dark-colored organic matter bands alternating around a central core. Conodonts have also been found preserved together with phosphatic microspherules in the same horizon, and the abundance of the former is obviously higher than that of the latter. Laser Raman spectral studies show close similarities in spectral patterns between the outer shells of phosphatic microspherules and the blade of the conodont genus Palmatolepis sp., as well as between the microspherule nucleus and the platform of the same conodont genus. Furthermore, the statistical results and geochemical data demonstrate that the elevated abundance of phosphatic microspherules roughly coincides with the blooms of bacteria and algae, but is later than the sharp increase of oceanic nutrients. The phosphatic microspherules are interpreted here to be the ‘otoliths’ secreted by conodont animals based on the compositional similarities between phosphatic microspherules and conodonts and their interrelated abundances. In addition, an analogous study reveals morphological and textural similarities between fish otoliths and phosphatic microspherules. The formation of phosphatic microspherules is probably related to seawater eutrophication. We speculate that the explosive growth of bacteria and algae is probably caused by the enrichment of nutrients that is most likely associated with the increase of terrestrial inputs, submarine hydrothermal activities or the upwelling anoxic bottom waters in the late Devonian, which would stimulate the conodont animals to secrete phosphatic microspherules—the ‘otoliths’ of conodont animals. This study reveals the coupling relationship between organisms and environments from the perspective of phosphatic microspherules and provides new evidence for the cause of faunal crisis during the Late Devonian F-F transition.