Deciphering processes controlling mid-Jurassic coccolith turnover
Introduction
During the Mesozoic Era, the Middle Jurassic was a key period for plankton evolution in marine environments, probably related to the opening of large oceanic domains, such as the Alpine Tethys and the central Atlantic Ocean. Nannofossil plankton carbonate production increased at that time and contributed significantly to the pelagic carbonate sedimentation in open oceans (Suchéras-Marx et al., 2012, Suchéras-Marx et al., 2014). The most remarkable pattern in nannofossil assemblages identified during this time is the transition between the coccolith genera Lotharingius, dominant in the Toarcian (Cobianchi, 1992, Cobianchi et al., 1992, Mattioli and Erba, 1999) and Watznaueria, which was dominant from the Middle Jurassic to the end of the Early Cretaceous, and ubiquitous in Upper Cretaceous assemblages (Erba, 1990, Lees et al., 2005, Linnert and Mutterlose, 2009, Tiraboschi and Erba, 2010). The transition between these two genera is well known and well defined in terms of phyletic relationships (Bown, 1987, Cobianchi et al., 1992, Mattioli, 1996). However, abundance changes precisely documenting this transition across the Early–Middle Jurassic transition are generally limited to semi-quantitative data, published as distribution charts for the western Tethys (Erba, 1990, Cobianchi et al., 1992, Reale et al., 1992, Stoico and Baldanza, 1995, Fauconnier et al., 1996, Chiari et al., 2007, Sandoval et al., 2008, Tiraboschi and Erba, 2010), and there is no up-to-date assessment of these data, nor any report on the quantitative abundance changes through time. We present in this study the abundance data for Lotharingius and Watznaueria through the Early–Middle Jurassic transition using new and published data. Correlation of these data with external proxies for paleoenvironmental change has allowed us to conjecture on which processes, biotic or abiotic, triggered this significant shift.
Sea-surface temperatures and surface-water nutrient levels are the two major parameters controlling nannoplankton distribution and proliferation (McIntyre et al., 1970, Okada and Honjo, 1973, Balch, 2004). In the geological record, these two parameters can be reconstructed, to some extent, using stable oxygen and carbon isotope data. For the Jurassic, several data, coming from both Tethyan and boreal regions, are available. These carbon and oxygen isotope data were measured on different fossil remains, such as bivalve shells (Paris Basin: France, Brigaud et al., 2009); and belemnite rostra (Lusitanian Basin: Portugal, Jenkyns et al., 2002; Basque–Cantabrian Basin: northeastern Spain, Gomez et al., 2009). Carbon and oxygen isotope ratios have also been measured on bulk carbonates (Umbria–Marche Basin: central Italy, Bartolini et al., 1996 and Bartolini and Cecca, 1999; Betic Cordillera: southern Spain, O'Dogherty et al., 2006 and Sandoval et al., 2008; Lusitanian Basin: Portugal, Suchéras-Marx et al., 2012; Subalpine Basin: southeast France, Suchéras-Marx et al., 2013) while oxygen isotope ratios have been measured in fish-tooth apatite (Paris Basin: northern France, Lécuyer et al., 2003; Luxemburg, Dera et al., 2009). Fossil-derived δ18O data are, however, sparse for the Middle Jurassic of the western Tethys. We present new carbon and oxygen isotope data measured in well-preserved and biostratigraphically well-dated brachiopods selected from France (western Tethys).
For the first time, we made a compilation of published and new nannofossil data, which evidence the Lotharingius–Watznaueria turnover in the nannoplankton community through the Early–Middle Jurassic transition. We compared these data to newly-acquired and literature-derived carbon and oxygen isotope data. Correlating both datasets has allowed us to meaningfully discuss the paleoceanographic changes in terms of possible mechanisms (abiotic versus biotic) for triggering the Lotharingius–Watznaueria turnover.
Section snippets
Compilation of nannofossil data
We compiled 860 semi-quantitative and quantitative abundances of Lotharingius and Watznaueria species from 28 sections, located in six different western Tethyan areas (central Italy; northern Italy and Switzerland; southeast France; southern Spain; western central Portugal; and southern Morocco; Fig. 1A). Among the 860 data, 274 are our own unpublished results from central Italy, Switzerland and Morocco (Fig. 1A). All the data have been calibrated with ammonite and/or ascribed to nannofossil
Changes through time of Lotharingius and Watznaueria abundances
Fig. 3 shows the variation through time of the abundances of the three morphogroups of Watznaueria and of Lotharingius. Lotharingius species were dominant from the Late Toarcian through the Aalenian, whereas low abundances are recorded in the Late Bajocian–Bathonian interval. This group shows a long-term decreasing trend during the studied interval with a small rebound located around the Aalenian/Bajocian boundary. Watznaueria with a cross shows a large abundance peak at the Aalenian/Bajocian
Discussion
Diagenetic re-equilibration of the chemical composition of a marine calcium carbonate may occur through a mechanism of dissolution–precipitation involving waters of meteoric origin. In this framework, thermodynamics predict that recrystallized shells must be Sr-depleted and Mn-enriched, relative to their pristine chemical compositions (Brand and Veizer, 1980, Veizer, 1983). Therefore, any record of the progress of the water–carbonate chemical reaction must be illustrated by a positive
Conclusions
A compilation of abundance changes of Lotharingius and Watznaueria, which were the successive dominant taxa in coccolith assemblages of the Early–Middle Jurassic, has been made for the first time. A comparison of this significant overturn in the coccolith community with the δ13C record and δ18O-derived temperatures has led us to identify likely forcing through major paleoceanographic and climatic events that took place during a short interval around the Aalenian–Bajocian transition.
In the
Acknowledgments
We are grateful to the Editor Frans Jorissen, Jackie Lees and an anonymous reviewer for both English corrections and comments which greatly improved the quality of an earlier version of the manuscript. This study was supported by the projects BQR 2006 of the Université Lyon 1 to FG, BQR 2010 and INSU 2011 Syster/Interrvie to EM. The Laboratories UMR 5275 at the University Grenoble 1 and UMR 5276 at the University Lyon 1 are also acknowledged. This study was supported by a post-doctoral grant of
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