Astronomical calibration of the Toarcian Stage: Implications for sequence stratigraphy and duration of the early Toarcian OAE

Abstract

The Toarcian Oceanic Anoxic Event (T-OAE) of the early Jurassic period involves one of the largest perturbations of the carbon cycle in the past 250 Ma, recorded by a pronounced negative carbon-isotope excursion (CIE). Numerous studies have focused on potential causes of the T-OAE and CIE, but are hampered by an uncertain timescale. Here we present high-resolution (!2 kyr) magnetic susceptibility (MS) measurements from the marine marls of the Sancerre-Couy drill-core, southern Paris Basin, spanning the entire Toarcian Stage. The MS variations document a rich series of sub-Milankovitch to Milankovitch frequencies (precession, obliquity and eccentricity) with the periodic g 2 -g 5 (405 kyr) and quasi-periodic g 4 -g 3 (!2.4 Myr Cenozoic mean periodicity) eccentricity terms being the most prominent. The MS-related g 4 -g 3 variation reflects third-order eustatic sequences, and constrains the sequence stratigraphic framework of the Toarcian Stage. In addition, MS variations reveal a modulation of g 2 -g 5 by g 4 -g 3 eccentricity related cycles, suggesting that sea-level change was the main control on the deposition of the Toarcian Sancerre marls, in tune with the astro-climatic frequencies. The stable 405 kyr cyclicity constrains a minimum duration of the Toarcian Stage to !8.3 Myr, and the well documented CIE, associated with the T-OAE, to !300 to 500 kyr. The 405 kyr MS timescale calibrates the periodicity of the prominent high-frequency ! 13 C cycles that occur in the decreasing part of the CIE to 30 to 34 kyr, consistent with the Toarcian obliquity period predicted for an Earth experiencing sustained tidal dissipation.

Carbon sequestration in an expanded lake system during the Toarcian oceanic anoxic event

“…In addition to plotting the data separately, we consider the values as a whole to determine a more statistically meaningful Re-Os date, given: 1) the small amount of geological time between the sampled intervals (both intervals are within the stratigraphic interval of the negative CIE and the duration of the negative CIE has been estimated to be 300-900 kyrs [14][15][16] , which is well below the uncertainty of the ReOs date; 2) the similar initial 187 Os/ 188 Os compositions of each sampled interval (Fig. S1 a, b); and 3) the better spread of the range of the Re-Os isotope compositions.…”

“…The observed early Toarcian perturbation to the exogenic 38 carbon cycle has been linked to volcanism of the Karoo-Ferrar large igneous province 39 (LIP) and associated release of volcanogenic CO 2 , thermogenic methane (CH 4 ) from 40 sill intrusion into Gondwanan coals, and biogenic methane from dissociation of sub-41 seafloor clathrates 3,6,[11][12][13] . Early Toarcian elevated atmospheric pCO 2 likely induced 42 climatic and environmental change 5,12,[14][15][16] by accelerating the global hydrological 43 cycle and increasing silicate weathering, thereby increasing delivery of riverine 44 nutrients to the oceans and potentially also to large inland lakes 17 . In the marine 45 realm, the consequential increase in primary productivity and carbon flux to the sea 46 floor is credited with enhancing the burial of planktonic material in relatively deep 47 continental-margin sites, whereas in shallower water semi-restricted marine basins, 48 chemical and physical water-column stratification likely aided the burial of organic 49 matter 17 .…”

Stratigraphic continuity and fragmentary sedimentation: the success of cyclostratigraphy as part of integrated stratigraphy

“…The exception is the basal Jurassic, which is highly precisely radioisotope dated and intercalibrated with cyclostratigraphy (Blackburn et al 2013). Multi-million year long cyclic marine sequences from the KimmeridgianTithonian (Weedon et al 2004;Boulila et al 2008;Huang et al 2010b), Oxfordian (Boulila et al 2010), Toarcian (Boulila et al 2014), and Sinemurian -Hettangian (Ruhl et al 2010;Hüsing et al 2014) with high-resolution records of total organic carbon, carbon isotopes, carbonate content and magnetic susceptibility show evidence for distinct 405 kyr orbital eccentricity cycles. Most of these sequences have been tuned to interpreted 405 kyr cycles, resulting in a sharpening of higherfrequency power preferentially in the obliquity and precession bands.…”

Revision of Saurorhynchus (Actinopterygii: Saurichthyidae) from the Early Jurassic of England and Germany

“…It was deposited over MAXWELL E.E. & STUMPF S., Revision of Saurorhynchus 2.7-2.88 million years (Boulila et al 2014). In the latest tenuicostatum Zone (Dactylioceras semicelatum Subzone), continuous deposition of organic-rich laminated black shales began, and continued through most of the serpentinum Zone (Röhl et al 2001).…”

“…In southwestern Germany, specimens of Saurorhynchus are known from the latest tenuicostatum Zone to the end of the serpentinum Zone (see Hauff 1938 for approximate stratigraphic distribution and raw abundance data), a range of approximately 1.5-1.62 million years (Boulila et al 2014). Most of the material consists of isolated skulls and skull fragments, strongly compressed and exposed in lateral view; however, rare articulated specimens have been recovered (Hauff 1938;Urlichs et al 1994).…”