Where and when the earliest coccolithophores?

Abstract

Gardin, S., Krystyn, L., Richoz, S., Bartolini, A. & Galbrun, B. 2012: Where and when the earliest coccolithophores? Lethaia, Vol. 45, pp. 507–523. New calcareous nannofossil analyses from the Northern Calcareous Alps of Austria are herein used to update and improve the state of knowledge about the oldest occurrence of coccolithophores reported in the literature. Previously reported Norian occurrences of coccoliths were based on an obsolete Triassic chronostratigraphy, in which the Rhaetian stage was subsumed into the Norian (‘Sevatian 2’). The oldest stratigraphical record of coccoliths spp. lies just below the Norian‐Rhaetian boundary and the first coccolith species, Crucirhabdus minutus, is recorded from the base of Rhaetian stage. The latter bio‐event is located just above the First Occurrence of the conodont Misikella posthersteni and the first occurrence of the ammonoid Paracochloceras suessi in the Steinbergkogel section (Austria), Global Stratotype Section and Point (GSSP) candidate for the Norian–Rhaetian boundary. The appearance of the coccolith Crucirhabdus minutus is seen as a robust biochronological datum that will provide useful constraints for Triassic biostratigraphy, palaeoclimatic modelling and phylogenetic reconstructions. The new calcareous nannofossil biochronology of Steinbergkogel that we present herein completes the existing biostratigraphic characterization of the Norian‐Rhaetian transition based on conodonts and ammonoids and strengthens the position of Steinbergkogel as the best GSSP proposed section for the base of the Rhaetian. The record of coccolithophores across the Norian–Rhaetian boundary at Steinbergkogel takes place along with a discernible increase in abundance of Prinsiosphaera triassica, as well as the appearance of Euconusphaera zlambachensis, which are the two most important Rhaetian pelagic carbonate producers. This succession of bio‐events is interpreted as the initiation of the pelagic carbonate production driven by the successful spreading of calcareous nannofossils in the Western Tethys during the Rhaetian. □Austria, Calcareous Alps, coccolithophores, Triassic.

The calcareous nannofossil <i>Prinsiosphaera</i> achieved rock-forming abundances in the latest Triassic of western Tethys: consequences for the <i>δ</i><sup>13</sup>C of bulk carbonate

“…Our aim is to provide a quantification of nannofossils' contribution to pelagic and hemipelagic carbonates. This study should thus be considered complementary to that of Gardin et al (2012), both in terms of methods and because of the different paleogeographical position of the studied successions.…”

“…These taxa apparently become common only from the latest Sevatian (upper Norian), i.e., much later than Bralower et al (1991) suggested for ODP sites 759-761 and 764 on the Wombat Plateau offshore of northwest Australia. The study of Gardin et al (2012) is based on condensed successions of the Northern Calcareous Alps in Austria, and for the first time provides semi-quantitative estimation of calcareous nannofossil abundances in the Triassic, tightly constrained by conodont biostratigraphy. The quantification approach followed by Gardin et al (2012), however, can only identify trends in nannofossil abundances, and gives little information on the volume contribution of nannofossils to carbonate sedimentation.…”

“…The study of Gardin et al (2012) is based on condensed successions of the Northern Calcareous Alps in Austria, and for the first time provides semi-quantitative estimation of calcareous nannofossil abundances in the Triassic, tightly constrained by conodont biostratigraphy. The quantification approach followed by Gardin et al (2012), however, can only identify trends in nannofossil abundances, and gives little information on the volume contribution of nannofossils to carbonate sedimentation. Preto et al (2013) showed that "calcispheres" of uncertain taxonomic affinity become significant components of limestone from the base of the upper Carnian, but their contribution to deep-water carbonate sedimentation is hard to quantify due to strong epitaxial calcite overgrowth.…”

“…A Lower Jurassic (Pliensbachian) portion of the section instead yielded a rich coccolith flora and abundant calcispheres. Gardin et al (2012) attempted to pin down precisely the first (common) occurrence of key late Triassic nannofossil taxa as Prinsiosphaera and "coccolithophores". These taxa apparently become common only from the latest Sevatian (upper Norian), i.e., much later than Bralower et al (1991) suggested for ODP sites 759-761 and 764 on the Wombat Plateau offshore of northwest Australia.…”

“…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). The spreading of calcareous nannofossils had a major consequence on seawater chemistry, as well as on the long-term carbon cycle.…”