Abstract
Lower Jurassic sedimentary successions in the Atlantic margin basins include several organic-rich intervals, some with source rock potential; time-equivalent units are also identified in on- and offshore areas worldwide. Despite decades of research, it is still unclear which mechanisms lead to the deposition of organic-rich sediments during the Early Jurassic. The objectives of this study are to construct a detailed temporal and geographical framework of Sinemurian–Toarcian organic matter preservation intervals (OMPIs; subdivided into local, regional, and superregional) and roughly constrain the relationship of OMPIs with the Lower Jurassic δ 13 C record. This survey combines an in-depth analysis of literature on the distribution of organic-rich facies in the Sinemurian–Toarcian with new geochemical studies [total organic carbon (TOC) and organic matter pyrolysis] from selected outcrop sections from Portugal, Spain, and Morocco. Strong local control on OMPIs during most of the Sinemurian is suggested. Regionally widespread organic-rich facies are associated with the most negative δ 13 C values of the broad Sinemurian–Pliensbachian negative carbon isotopic trend recorded in organic matter (including the Sinemurian–Pliensbachian Boundary Event). Pliensbachian OMPIs are expressive in the areas bordering the proto-Atlantic Ocean and are often linked with positive δ 13 C excursions and short-lived warm intervals, but OMPIs are also defined for the Late Pliensbachian cool interval. Early Toarcian superregional OMPIs are associated with some of the most pronounced δ 13 C excursions of the Mesozoic. Toarcian maximum TOC content occurs with the positive δ 13 C (recovery) trend following the δ 13 C negative shift typically linked with the Early Toarcian Oceanic Anoxic Event (T-OAE), supporting the notion that peak carbon sequestration/ocean anoxia post-dated the main phase of carbon input into the atmosphere, as also suggested by recent modelling efforts. However, additional superregional OMPIs predate and postdate the T-OAE, indicating that conditions favouring preservation of organic matter (increased productivity and/or enhanced preservation) during the Early Toarcian were not restricted to the T-OAE interval. The compilation of Sinemurian–Toarcian OMPIs presented in this paper demonstrates that organic-rich intervals of regional and superregional expression in the Lower Jurassic sedimentary record are ubiquitous and may even be more numerous than in the Cretaceous. Considering the association of some of the Sinemurian, Pliensbachian, and Toarcian (not taking into account the T-OAE related OMPIs) regional and superregional OMPIs with well-defined carbon isotopic excursions, it is here suggested that these hold the same relevance as the secondary OAEs of the Cretaceous, such as the Valanginian OAE (Weissert Event), Hauterivian OAE (Faraoni Event), and Late Aptian–Early Albian OAE (OAE 1b cluster).
Highlights
The compilation of Sinemurian–Toarcian organic matter preservation intervals (OMPIs) presented in this paper demonstrates that regional and superregional sedimentary sequestration of organic matter (OM) during the Early Jurassic was more common than previously thought (Fig. 5)
The Early Toarcian superregional OMPIs T1, T5, T6, and the regional OMPI T7 occur in association with the Lower Toarcian pCIE, and their spatial spread supports some degree of equivalency to the secondary Cretaceous OAEs. These OMPIs demonstrate that the establishment of environmental and depositional conditions leading to widespread preservation of OM in the Late Pliensbachian–Early Toarcian was not exclusively associated with the Toarcian Oceanic Anoxic Event (T-OAE)
The recognition of several Early Jurassic organic matter preservation intervals (OMPIs) of regional and superregional extent gives new insight into the role that OM sequestration played in Earth System process and provides support to current theoretical models for carbon cycle perturbations in the Early Jurassic
Summary
The deposition and preservation of organic matter-rich sediments during the Early Jurassic has led to the formation of prolific hydrocarbon source-rocks in many locations worldwide (e.g. Powell, 1978; Hallam and Bradshaw, 1979; Fleet et al, 1987; Jenkyns, 1988; Kodina et al, 1988; Baudin et al, 1990; Baudin, 1995; Bessereau et al, 1995; Scotchman, 2001; van de Schootbrugge et al, 2005b; Duarte et al, 2010; Silva et al, 2011, 2017; Suan et al, 2011; Silva and Duarte, 2015; Ruhl et al, 2016; Gómez et al, 2016a; Xu et al, 2017a; Campana et al, 2017). The first debate concerns the role of primary productivity and associated carbon flux from surface waters to the sediment-water interface (Pedersen and Calvert, 1990) vs marine anoxia and associated enhanced OM preservation (Demaison and Moore, 1980; Tyson, 1995) in controlling the stratigraphic and geographic occurrence of organic-rich facies; the second relates to the mechanisms leading to the preservation and transformation of OM at geological time scales: degradation/recondensation (Huc and Durand, 1974; Tissot and Welte, 1978, 1984); selective preservation (Philip and Calvin, 1976; Tegelaar et al, 1989; Largeau and Derenne, 1993; Silva et al, 2012); sorptive mineral protection (Bishop et al, 1992; Mayer, 1994; Hedges and Keil, 1995; Kennedy et al, 2002; Burdige, 2007; Kennedy and Wagner, 2011), OM sulfurization (Sinninghe Damsté et al, 1988; Adam et al, 1998), and OM-mineral interaction (van Buchem et al, 1995; Kennedy et al, 2002; Hemingway et al, 2019). Organic-rich sedimentary rocks characterized by lipid-rich OM with elevated Hydrogen Index (HI) seems to require low availability of O2 in bottom waters (e.g. Espitalié et al, 1977, 1985; Demaison and Moore, 1980; Tissot and Welte, 1984; Kohnen et al, 1990, 1992; Goodarzi et al, 1993; Schouten et al, 1994; Sinninghe Damsté et al, 1995; van Buchem et al, 1995; Rodger Harvey et al, 1995; Sun et al, 2002; Farrimond et al, 2003; Lee et al, 2004; Bowden et al, 2006; Zonneveld et al, 2010; Silva et al, 2012)
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