Middle Jurassic coccolith fluxes: A novel approach by automated quantification

doi:10.1016/j.marmicro.2014.06.002

Marine Micropaleontology

Volume 111, September 2014, Pages 15-25

https://doi.org/10.1016/j.marmicro.2014.06.002 Get rights and content

Highlights

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First time adaptation of automated microscopy to Mesozoic samples.
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Automated microscopy and SYRACO are good estimators of coccolith fluxes.
•
Increase of coccolith fluxes during the Early Bajocian.
•
Possible link between climate change and enhanced oceanic nutrient supply.

Abstract

Coccolithophores, major pelagic carbonate producers, underwent important changes during the latest Aalenian–Early Bajocian (ca. − 170 Ma, Middle Jurassic). Specifically, a new genus called Watznaueria diversified and started dominating the coccolith assemblages. While this change in the coccolith assemblages is already known, the impact on coccolith absolute abundances and fluxes is still misunderstood. Here we tested for the first time on Mesozoic deposits an automated counting method using the SYRACO system, and then compared these results with a classic counting method. Two neural networks were created based on the shape, birefringence and size of coccoliths, the first of which contained only one class and the second of which was composed of six classes. Based on the morphological criteria used for the neural networks, coccoliths were not identified at the species level. We quantified coccolith absolute abundances and estimated fluxes for the latest Aalenian–Early Bajocian in two well-dated and correlated sections, Cabo Mondego in Portugal and Chaudon-Norante in France. There is a strong linear correlation between classic and automatic counting (r > 0.8) supporting the latter as a reliable method for coccolith absolute abundance and flux quantification. In spite of under- or over-estimation by automatic counting with respect to classic counting, the absolute abundance stratigraphic trends are remarkably similar. Under- or over-estimation may be reduced by further technical improvements such as automatic focus. At Cabo Mondego, coccolith fluxes increased from 0.2 × 10⁹coccoliths/m²/yr in the latest Aalenian to 700 × 10⁹ coccoliths/m²/yr in the middle of the Laeviuscula ammonite Zone (Early Bajocian), remaining high for the rest of the Early Bajocian. At Chaudon-Norante, a similar trend was observed. This coccolith flux increase is therefore considered to be a supra-regional event matching an important δ¹³C_{Bulk carbonate} positive excursion. The carbon isotopic positive excursion, along with the onset of radiolarian sedimentation and coccolith fluxes is related to a gradual increase in the fertility of the oceans during the Early Bajocian and is likely associated with warmer and more humid climates and associated enhanced continental weathering.

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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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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Research paperMiddle Jurassic coccolith fluxes: A novel approach by automated quantification

Highlights

Abstract

Introduction

Section snippets

Cabo Mondego

Slide preparation

Comparison between classic and automatic counting

Impact of preservation on absolute abundance

Conclusions

Acknowledgments

C. R. Acad. Sci.

Palaeogeogr. Palaeoclimatol. Palaeoecol.

Mar. Micropaleontol.

Palaeogeogr. Palaeoclimatol. Palaeoecol.

Sediment. Geol.

Cretac. Res.

Neural Netw.

Palaeogeogr. Palaeoclimatol. Palaeoecol.

Palaeogeogr. Palaeoclimatol. Palaeoecol.

Cretac. Res.

Palaeogeogr. Palaeoclimatol. Palaeoecol.

Mar. Micropaleontol.

Palaeogeogr. Palaeoclimatol. Palaeoecol.

Mar. Micropaleontol.

Palaeogeogr. Palaeoclimatol. Palaeoecol.

Palaeogeogr. Palaeoclimatol. Palaeoecol.

Cretac. Res.

Palaeogeogr. Palaeoclimatol. Palaeoecol.

Glob. Planet. Chang.

Glob. Planet. Chang.

Pacific microplate and the Pangea supercontinent in the Early to Middle Jurassic

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Middle and Late Jurassic carbon stable-isotope stratigraphy and radiolarite sedimentation of the Umbria-Marche Basin (Central Italy)

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Calcareous nannofossil and δ13C records from the early Cretaceous of the western Atlantic ocean: evidence for enhanced fertilization across the Berriasian–Valanginian transition

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Causes and consequences of calcareous nannoplankton evolution in the Late Jurassic: implications for biogeochronology, biocalcification and ocean chemistry

Evolutionary trends of calcareous nannofossil genera Lotharingius and Watznaueria during the Early and Middle Jurassic

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Research paper
Middle Jurassic coccolith fluxes: A novel approach by automated quantification

Calcareous nannofossil and δ¹³C records from the early Cretaceous of the western Atlantic ocean: evidence for enhanced fertilization across the Berriasian–Valanginian transition