Duration of the Early Toarcian carbon isotope excursion deduced from spectral analysis: Consequence for its possible causes

Abstract

The marked 3-8‰ negative carbon isotope excursion associated with the Early Toarcian oceanic anoxic event (OAE;~183 myr ago) in the Early Jurassic period is thought to represent one of the most important perturbations of the C-cycle in the last 200 myr. However, the origin of this excursion remains strongly debated, primarily due to uncertainties in the estimation of its duration, which ranges from~200 kyr to 1 myr. Here we present a new orbital calibration of the Early Toarcian carbon isotope excursion, based on spectral analyses of two independent datasets generated from the sedimentary record of two hemipelagic sections from Portugal (Peniche) and SW Germany (Dotternhausen), in order to better constrain the timescale and hence the origin of this excursion. These analyses reveal that orbital cycles exert a strong influence on both the calcium carbonate content in Portugal and on the greyscale of black shales in Germany, which allow us to propose a duration of ≥ 1.9 myr for the Early Toarcian and of~900 kyr for the entire carbon isotope excursion. The shift towards lower carbon isotope values lasted~150 kyr, and carbon isotope values remained low for~450 kyr; the subsequent increase of carbon isotope values lasted~300 kyr. This calibration suggests that the sustained input of isotopically light carbon at the origin of the excursion occurred over~600 kyr and thus dismisses causal mechanisms implying relatively small source reservoirs such as the massive dissociation of methane hydrates. In the light of our new cyclostratigraphic timescale, the massive input of isotopically light carbon associated with the emplacement of the Karoo-Ferrar basaltic province appears as the most likely cause of the Toarcian global carbon isotope excursion. We also show that the C-isotope perturbation coincided with a transition from precession-eccentricity-dominated cycles to obliquityeccentricity-dominated cycles, suggesting that the OAE was marked by a fundamental change in the response of the climate system, which allowed the obliquity signal, normally better recorded at high latitudes, to be a dominant forcing factor of short-term sedimentary cycles at tropical latitudes.

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

“…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 .…”

“…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.…”

“…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 . Particularly in northern Europe, the evidence points to regional to global 50 development of anoxic/euxinic (sulphide-rich) bottom waters that strongly affected 51 palaeoceanographic conditions and marine ecosystems 15,17,18 . Globally significant 52 burial of 13 C-depleted photosynthetically derived organic matter commonly produced 53 an overarching positive carbon-isotope excursion (CIE) interrupted by the 54 Previous study on a section from Bornholm, Denmark suggested a sharp 155 increase of atmospheric pCO 2 reconstructed from terrestrial leaf stomatal density at 156 the onset of the negative CIE 12 .…”

“…Gondwanan coals or the dissociation of sub-sea-floor gas hydrates, also resulting in 205 enhanced early Toarcian atmospheric pCO 2 levels 3,11,12,15 . The typical early Toarcian 206 δ 13 C pattern, with a stepped negative shift interrupting an overarching positive 207 excursion, has been observed in marine and terrestrial organic matter and shallow-208 water platform and deep-water pelagic carbonates and manifestly affected the entire 209 ocean-atmosphere system 3,5,6,8 .…”

“…This 220 chemostratigraphic pattern is very similar to that developed in marine sections from 221 northern Europe, where sedimentary TOC-levels can locally reach ~20%. Box-model 222 studies for the early Toarcian carbon cycle suggest that the release of ~9000 Gt 223 carbon from methane clathrates (with δ 13 C of ~ -60‰) or ~25,000 Gt carbon as 224 thermogenic methane (with δ 13 C of ~ -35‰), is required to generate a negative δ 13 C 225 excursion compatible with the mean change in bulk carbonate of 4-6‰, and which 226 would have caused an increase in atmospheric pCO 2 of ~1000 ppm 7,8,15,41 . Excess 227 atmospheric CO 2 is assumed to have been sequestered both by enhanced weathering 228 of Ca-Mg silicates due to greenhouse warming, and by massive burial of organic 229 carbon in marine dysoxic, anoxic and euxinic depositional environments 17 .…”