Perturbation of the carbon cycle during the late Pliensbachian – early Toarcian: New insight from high-resolution carbon isotope records in Morocco

https://doi.org/10.1016/j.jafrearsci.2015.12.018Get rights and content

Highlights

  • High-resolution carbon isotope records from three sections in Morocco.

  • Decoupling between δ13Ccarb and δ13Corg at the Pliensbachian–Toarcian boundary.

  • Negative shift of δ13Ccarb linked to massive neritic carbonate demise event.

Abstract

Preceding the early Toarcian Oceanic Anoxic Event by ∼1 Myr, the Pliensbachian–Toarcian boundary event is in many aspects as severe and disturbing for the environment as its better-studied successor. Both events are associated with rapid and pronounced global warming, major faunal and floral turnover, increased hydrological cycling and dramatic collapses of carbonate production. To better characterize the Pliensbachian–Toarcian boundary event, a high-resolution, paired carbonate and organic matter carbon isotope survey of three sections from the Central High Atlas Basin of Morocco has been undertaken. A pronounced negative shift in the carbonate carbon-isotope record, not paralleled by a similar excursion in the organic carbon, can be linked to the collapse of the neritic carbonate factory in the earliest Toarcian. These results show that, contrary to the Toarcian Oceanic Anoxic Event, a rapid and massive injection of 13C-depleted carbon into the atmosphere is not responsible for the environmental perturbations observed during the Pliensbachian–Toarcian boundary event. However, input of isotopically non-depleted carbon such as mantle source CO2 into the atmosphere as a potential cause for the Pliensbachian–Toarcian boundary event cannot be excluded. This would most probably be sourced from an early pulse of the Karoo–Ferrar Large Igneous Province.

Introduction

The Toarcian Oceanic Anoxic Event (T-OAE, ca. 181.5 Ma) was a geologically brief interval characterized by global climatic change, faunal and floral turnover, and profound changes in exogenic processes (e.g. Van de Schootbrugge et al., 2013). It represents the most prominent example of environmental change during the late Early Jurassic (Pliensbachian–Toarcian) and was likely initiated by the activity of the Karoo–Ferrar Large Igneous Province (LIP; e.g. Caruthers et al., 2013, Sell et al., 2014, Burgess et al., 2015). The T-OAE is moreover characterized by a rapid and ample (2.5–7‰) negative carbon isotope excursion (CIE) (Hesselbo et al., 2000, Hesselbo et al., 2007, French et al., 2014, Ullmann et al., 2014).

Environmental precursor and aftermath events are numerous during the late Early Jurassic. These events bracket the T-OAE within a frame of global secular changes (Suan et al., 2010, Dera et al., 2010, Korte and Hesselbo, 2011, Krencker et al., 2014). From the oldest to the youngest, these are: the Margaritatus Zone event (middle Late Pliensbachian), the Pliensbachian–Toarcian (P-To) boundary event, the Variabilis Zone event (late Middle Toarcian) and the Dispansum Zone event (middle Late Toarcian). Because of the pulsed nature of the Karoo-Ferrar LIP activity during the considered time interval (Jourdan et al., 2008), it is thought that this long-lasting volcanic episode is ultimately responsible for pacing these environmental perturbations (Dera et al., 2010, Caruthers et al., 2013). However, causal mechanisms have still to be firmly established.

An outstanding issue is whether the T-OAE represents the most extreme example of a series of recurring environmental perturbations, or if its drastic nature is due to the involvement of an extraordinary trigger. In this respect, comparison between the T-OAE and its most recent precursor, the P-To event may provide additional clues. Both events, separated by ca. 1 Myr, are associated with rapid and pronounced warming as well as major marine faunal and floral turnover (Suan et al., 2010, Dera et al., 2010, Dera et al., 2011, García Joral et al., 2011, Caruthers et al., 2013). Recent investigations have moreover suggested that the P-To event is associated with a negative CIE in the bulk carbonate record, whose magnitude rivals the CIE observed for the T-OAE (Hesselbo et al., 2007, Bodin et al., 2010, Littler et al., 2010, Trecalli et al., 2012). Sharing therefore manifold similarities, it could be postulated that a similar trigger drives both events. As such, any scenario proposed for the T-OAE must take into account the constraints brought in by the findings from the P-To event. Nonetheless, since the record of the P-To event is often highly condensed or absent (Morard et al., 2003), only limited studies are available so far to assess more precisely its extend and potential causes.

Here we present and discuss high-resolution paired organic-inorganic carbon isotope records spanning both P-To event and T-OAE at three different localities (Boumardoul n’Imazighn, Amellago and Foum Tillicht) from the Central High Atlas Mountains in Morocco. These sections are representative for three different depositional environments with the basin, respectively inner ramp, outer ramp and deep basinal settings. In conjunction with previously published data, these records are used to constrain the cause(s) and consequence(s) of the carbon cycle perturbation for both events. Different scenarios for their initiation and termination are discussed (see Fig. 1).

Section snippets

Geological settings

The High Atlas of Morocco (Fig. 1) was formed during the Alpine Orogeny (Frizon de Lamotte et al., 2008, Kaislaniemi and van Hunen, 2014). It is composed of a thick Mesozoic sedimentary succession deposited within the eastern part of an Atlantic aulacogen rift system. The High Atlas rift opened during the early phase of the dislocation of Gondwana as the result of left-lateral transtensive movements (Piqué et al., 2000). The rift system was composed of several highly-subsiding sub-basins

Definition of T-OAE and P-To event

No strict definitions for the T-OAE and P-To event have yet been proposed and officially ratified. The recognition that organic-rich marine rocks characterize the early Toarcian Falciferum Chronozone (eq. Levisoni Chronozone, cf. Caruthers et al., 2013) provided the basis for the initial definition of the T-OAE (Jenkyns, 1988). Similar to other Mesozoic oceanic anoxic events, recent high-resolution and multi-proxy studies have however casted doubt on the global distribution of organic-rich

Lithological descriptions

The three studied sections (Boumardoul n’Imazighn, Foum Tillicht and Amellago) are located within the southern part of the Central High Atlas Basin (Fig. 2). The first section is situated in the Dades Valley, ca. 30 km north of Boumalne along the R704 road, next to the village Boumardoul n’Imazighn. The second one is situated ca. 10 km northwest of the city of Rich, on the eastern side of the N13 main road. The third section is located ca. 60 km from Rich, west of the Amellago village, in the

Global perturbation of the C-cycle at the onset of the T-OAE

One unresolved question with regard to the T-OAE is the exact mechanism leading to the abrupt negative CIE at its onset. Although the global occurrence of the negative CIE was initially challenged by belemnite data (Van de Schootbrugge et al., 2005, Gómez et al., 2008), its widespread documentation in bulk carbonate and marine OM (Al-Suwaidi et al., 2010, Bodin et al., 2010, Caruthers et al., 2011, Gröcke et al., 2011, Kemp and Izumi, 2014), in fossil wood (Hesselbo et al., 2007, Littler

Conclusions

In Morocco, both T-OAE and P-To event are associated with a distinct negative shift in the δ13Cmicrite trend. A similar shift, with an amplitude of 3–4‰ is also recorded in the bulk δ13Corg record at the onset of the T-OAE. This is however not the case for the δ13Corg trend associated with the P-To event, where a similar negative shift is lacking. If a perturbation of the global carbon isotope signal can thus be invoked for the T-OAE, a similar scenario is discarded for the P-To event, whose δ13

Acknowledgment

This research was financed by the Deutsche Forschungsgemeinschaft (DFG, project n° BO 3655/1–2). We would like to thank Martin Hönig for its assistance in the field. Guillaume Suan and Felix Schlagintweit are gratefully acknowledged for their help in the field, and incertae sedis identification, respectively. Analytical work in the isotope laboratories at Bochum and Hannover was supported by Andrea Niedermayr and Christiane Wenske, respectively. We acknowledge Karl Föllmi and Matias Reolid for

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