Research paperMiddle Jurassic coccolith fluxes: A novel approach by automated quantification
Introduction
Coccoliths are microscopic calcite platelets produced by planktonic algae called coccolithophores. They constitute about one half of the pelagic carbonate production in modern oceans and significantly contribute to the carbon cycle (Westbroek et al., 1993). Coccolithophores first appeared in the fossil record at the end of the Triassic (~ 210 Ma ago; Prins, 1967), and gradually colonized the open ocean during the Jurassic and Cretaceous (Roth, 1989, Hay, 2004, Erba, 2006). The impact of nannoplankton-derived carbonates on the carbonate budget of the Mesozoic ocean is highly debated. Some authors have proposed a significant increase in pelagic carbonate production during the Late Jurassic (Roth, 1989, Hay, 2004, Erba, 2006). However, mass accumulation of nannofossil-derived carbonate in Jurassic and Lower Cretaceous successions remain generally lower than the carbonate produced in shallow-water environments and exported basinwards (Mattioli and Pittet, 2002, Suan et al., 2008, Gréselle et al., 2011, Suchéras-Marx et al., 2012).
During the Early Bajocian (~− 170 Ma, Middle Jurassic), the Watznaueria genus diversified and started dominating coccolith assemblages until the end of the Early Cretaceous (Cobianchi et al., 1992, Mattioli and Erba, 1999, Lees et al., 2005, Erba, 2006). This great change in the composition of coccolith assemblages is well documented (Cobianchi et al., 1992, Mattioli and Erba, 1999, Bown, 2005), but no studies have yet quantified the coccolith absolute abundance variation in relation to this diversification. Unfortunately, such quantifications are extremely time consuming and difficult to perform.
Dollfus and Beaufort (1999) and Beaufort and Dollfus (2004) developed an automated system that greatly reduces the time spent collecting data. They coupled a system of automated microscopy with a system of automatic coccolith identification called SYRACO (SYstème de Reconnaissance Automatique des COccolithes). This system has been successfully used for the quantification of coccoliths from living and recent coccolithophores (Beaufort et al., 2008, Beaufort et al., 2011). In this study, we have tested and applied the SYRACO automatic counting for coccolith flux quantification for the first time in Mesozoic deposits. This study was conducted on the latest Aalenian–Early Bajocian interval for two sites: Cabo Mondego, Portugal and Chaudon-Norante, France. The results of coccolith automated optical quantification were compared with estimates from identifications made by classical optical methods in order to test the application of automated quantification to Mesozoic deposits. Coccolith absolute abundances and fluxes are discussed in regard to other paleoenvironmental proxies, such as carbon stable isotopes.
Section snippets
Cabo Mondego
The Cabo Mondego section is located on the western Atlantic coast of Portugal near Figueira da Foz (Fig. 1). This section is in the Lusitanian Basin, and is bounded eastward by the Iberica Meseta. The sedimentary succession is represented by marine deposits of Late Toarcian to Kimmeridgian age (Ruget-Perrot, 1961). Cabo Mondego is the Global Stratotype Section and Point (GSSP) for the Aalenian/Bajocian boundary (Pavia and Enay, 1997) as well as the Auxiliary Stratotype Section and Point (ASSP)
Slide preparation
Samples were prepared following the random settling method for absolute abundance quantification described by Beaufort (1991) and modified by Geisen et al. (1999). This method was used to produce homogeneous slides, and absolute abundance was estimated from the number of coccoliths found per field of view. Thus, the estimations require counting a reasonable number of fields of view rather than a fixed number of coccoliths. Two different sample preparations were made. The preparation for classic
Comparison between classic and automatic counting
Although we successfully applied the automated system to Mesozoic nannofossils, a part of the photographs collected automatically was not of sufficient quality to support effective species recognition. Nevertheless, the photographs were good enough to distinguish coccoliths from “invaders”. The results presented here were obtained after a visual checking of SYRACO results in order to avoid possible misinterpretation induced by SYRACO and eliminate the effects of residual “invaders”. The
Impact of preservation on absolute abundance
The correlation of absolute abundances calculated using classic and automatic counting might be dependent on coccolith preservation, which can render identification by SYRACO difficult. We tested the impact of preservation state on absolute abundance using the Bonferroni/Dunn test with 95% confidence interval (Fig. 7). This test was performed on both classic counting of absolute abundances (Fig. 7A and B) and automatic counting of absolute abundances (Fig. 7C and D). Classic counting of
Conclusions
In this study, we quantified coccolith fluxes during the latest Aalenian–Early Bajocian at Cabo Mondego, Portugal and Chaudon-Norante, France by applying for the first time an automatic counting methodology to Mesozoic samples. The main results of this study are as follows:
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Automatic counting using SYRACO (Beaufort and Dollfus, 2004) is a reliable method for absolute abundance and coccolith flux quantification for Mesozoic deposits with strong linear correlations (r > 0.8) between classic and
Acknowledgments
Remarks by Jorijntje Henderiks and Paul Bown greatly improved an earlier version of the manuscript. We are also grateful to Matthew Makou for English corrections and two anonymous reviewers and Richard Jordan for comments on an earlier version of the manuscript. This paper is a part of B.S-M.'s PhD project funded by the French Ministry of Research and supported by BQR UCBL Lyon1 2006 (F.G.), BQR UCBL Lyon1 2010 and INSU 2011-12 Syster/Interrvie (E.M.). This paper is a contribution of the team
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2018, Palaeogeography, Palaeoclimatology, PalaeoecologyCalcareous nannofossil assemblage turnover in response to the Early Bajocian (Middle Jurassic) palaeoenvironmental changes in the Subbetic Basin
2017, Palaeogeography, Palaeoclimatology, PalaeoecologyCitation Excerpt :n/m2/yr.), which is rather similar to that calculated for the Cabo Mondego (Lusitanian Basin) section (6.66 × 1010 n/m2/yr.; Suchéras-Marx et al., 2012) and slightly higher than for the Chaudon-Norante (French Subalpine Basin) section (1.11 × 1010 n/m2/yr; Suchéras-Marx et al., 2014) for the same interval. Absolute abundances and fluxes (Fig. 4) display peaks in the Ovalis Subzone (lower part of the Laeviuscula Zone) and in the Patella Subzone (lower part of the Propinquans Zone).
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2016, Marine MicropaleontologyCitation Excerpt :The peak of Watznaueria with a cross also coincides with the onset of a gradual decrease in seawater 87Sr/86Sr (Jenkyns et al., 2002; Wierzbowski et al., 2012), reflecting a major input of mantle-derived Sr by hydrothermal fluids linked to increasing mid-ocean ridge activity. In the Early Bajocian (Hyperlioceras discites and Witchellia laeviuscula ammonite zones), the abundance increases in both Watznaueria without a central-area structure and Watznaueria with a bar correspond to a major diversification of the Watznaueria genus lasting for about 1.5 Myr (Suchéras-Marx et al., 2013, 2014, 2015), and was associated with diversification or radiations of other marine planktic (radiolarians; Bartolini et al., 1999), nektonic (ammonoids; O'Dogherty et al., 2006) and benthic (foraminifera, brachiopods; Andrade, 2004; Vörös, 2005; Canales and Henriques, 2013) groups. At that time, typical Middle Jurassic forms in the different fossil groups first appeared and replaced Lower Jurassic assemblages.