Abstract
Abstract. The onset of pelagic biomineralization was a milestone in the history of the long-term inorganic carbon cycle: as soon as calcareous nannofossils became major limestone producers, the pH and supersaturation state of the global ocean were stabilized (the so-called mid-Mesozoic revolution). But although it is known that calcareous nannofossils were abundant already by the end of the Triassic, no estimates exist on their contribution to hemipelagic carbonate sedimentation. With this work, we estimate the volume proportion of Prinsiosphaera, the dominant late Triassic calcareous nannofossil, in hemipelagic and pelagic carbonates of western Tethys. The investigated Upper Triassic lime mudstones are composed essentially of microspar and tests of calcareous nannofossils, plus minor bioclasts. Prinsiosphaera had become a significant component of lime mudstones since the late Norian, and was contributing up to ca. 60% of the carbonate by the late Rhaetian in periplatform environments with hemipelagic sedimentation. The increasing proportion of Prinsiosphaera in upper Rhaetian hemipelagic lime mudstones is paralleled by an increase of the δ13C of bulk carbonate. We interpreted this isotopic trend as related to the diagenesis of microspar, which incorporated respired organic carbon with a low δ13C when it formed during shallow burial. As the proportion of nannofossil tests increased, the contribution of microspar with low δ13C diminished, determining the isotopic trend. We suggest that a similar diagenetic effect may be observed in many Mesozoic limestones with a significant, but not yet dominant, proportion of calcareous plankton.
Highlights
Calcareous nannofossils have contributed significantly to pelagic sedimentation starting in the late Triassic (Bellanca et al, 1995; Bralower et al, 1991; Bown, 1998; Bown et al, 2004; Gardin et al, 2012; Preto et al, 2013)
We focused on the late Norian, Rhaetian and early Jurassic portions of the succession (Fig. 2), which are well constrained by conodont biostratigraphy (Mazza et al, 2010, 2012), calcareous nannofossils and radiolarians (Preto et al, 2012)
The ultrafacies and calcareous nannofossil abundance are referred to the reworked lime mudstone lithoclasts that make up the coarse portion of the calciturbidite
Summary
Calcareous nannofossils have contributed significantly to pelagic sedimentation starting in the late Triassic (Bellanca et al, 1995; Bralower et al, 1991; Bown, 1998; Bown et al, 2004; Gardin et al, 2012; Preto et al, 2013). Before pelagic biomineralization by calcareous nannofossils, carbon was sequestrated, in the form of calcium carbonate, only in platform areas, i.e., in areally limited portions of the global ocean This implied a poor stabilization of carbonate species concentrations in seawater and, a variable seawater pH before the Triassic. From the Jurassic on, precipitation of carbonates occurred both on platforms and in open oceans in the form of calcitic tests of planktonic organisms (nannoliths, coccoliths, calcareous dinocysts, and, later on, planktonic foraminifers). An ocean in this state buffers the carbonic acid and the products of its dissociation more efficiently, resulting in a more stable seawater pH through geological times. The switch from the “Neritan” to the “Cretan” mode was a milestone in the evolution of ocean chemistry, and was baptized the “mid-Mesozoic revolution” (Ridgwell, 2005)
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