The calcareous nannofossil Prinsiosphaera achieved rock-forming abundances in the latest Triassic of western Tethys: consequences for the δ13C of bulk carbonate

[摘要] The onset of pelagic biomineralization was a milestone in the history of thelong-term inorganic carbon cycle: as soon as calcareous nannofossils becamemajor limestone producers, the pH and supersaturation state of the globalocean were stabilized (the so-called mid-Mesozoic revolution). But althoughit is known that calcareous nannofossils were abundant already by the end ofthe Triassic, no estimates exist on their contribution to hemipelagiccarbonate sedimentation. With this work, we estimate the volume proportion ofPrinsiosphaera, the dominant late Triassic calcareous nannofossil,in hemipelagic and pelagic carbonates of western Tethys. The investigatedUpper Triassic lime mudstones are composed essentially of microspar and testsof calcareous nannofossils, plus minor bioclasts. Prinsiosphaera hadbecome a significant component of lime mudstones since the late Norian, andwas contributing up to ca. 60% of the carbonate by the late Rhaetian inperiplatform environments with hemipelagic sedimentation. The increasingproportion of Prinsiosphaera in upper Rhaetian hemipelagic limemudstones is paralleled by an increase of the δ¹³C of bulkcarbonate. We interpreted this isotopic trend as related to the diagenesis ofmicrospar, which incorporated respired organic carbon with a lowδ¹³C when it formed during shallow burial. As the proportion ofnannofossil tests increased, the contribution of microspar with lowδ¹³C diminished, determining the isotopic trend. We suggest that asimilar diagenetic effect may be observed in many Mesozoic limestones with asignificant, but not yet dominant, proportion of calcareous plankton.

[学科分类] 地球化学与岩石