Orbital pacing of the Early Jurassic carbon cycle, black‐shale formation and seabed methane seepage

Abstract

The Early Jurassic (ca 201 to 174 Ma) was marked by a series of rapid perturbations in climate, the environment and global geochemical cycles, which have been linked to volcanic outgassing and the release of biogenic or thermogenic methane into the ocean-atmosphere system. The state of the global carbon cycle and prevailing climatic and environmental conditions that existed at this time are, however, poorly constrained. Here, mudrocks of the Lower Sinemurian Arietites bucklandi ammonite Biozone at coastal exposures at Kilve, Somerset, UK, have been studied. This succession includes laminated organic-rich black shales, which are present throughout the Bristol Channel Basin, and coincides with a 2 to 3& negative carbon-isotope excursion, distinct changes in inferred land vegetation and abundant marine prasinophytes (green algae). The event itself does not represent a single perturbation of the regional environment, but follows in a sequence of eccentricity-modulated, precession-paced perturbations that occur throughout the preceding Hettangian stage, with the periodic formation of organic-rich laminated black shales in the Bristol Channel Basin. However, the Early Sinemurian event studied herein is more extreme in nature, with sedimentary total organic carbon values of 5 to 11% persisting over about 2 m, representing ca 100 kyr, possibly in phase with short (ca 100 kyr) and long (ca 405 kyr) eccentricity forcing. The formation of methane-seep carbonatecemented mounds took place relatively soon after the deposition of laminated black shales. Biogenic methane probably formed in response to microbial methanogenesis in the organic-rich black shale, which was subsequently channelled to the sediment-water interface approximately 5 m above the source bed, and ca 200 kyr after cessation of formation of the black shale.

Orbital pacing and secular evolution of the Early Jurassic carbon cycle

“…15), and the Hettangian to lower Sinemurian record from St Audries Bay/East Quantoxhead/Kilve (Bristol Channel Basin, England, United Kingdom; refs. 4 and [17][18][19], which together represent the hitherto longest high-resolution δ 13 C TOC record of the Lower Jurassic, demonstrates that numerous medium-amplitude δ 13 C TOC shifts recorded in Mochras have previously been merged into what appears as stratigraphically more expanded shifts, or were missed entirely (Fig. 3).…”

“…3. δ 13 C TOC record of the Hettangian to Pliensbachian of the Mochras core, upper Hettangian to Pliensbachian of the Sancerre-Couy core, Paris Basin (15) and the composite Hettangian to lower Sinemurian record from the Bristol Channel Basin (4,(17)(18)(19). Key for identified ammonite subzones in Mochras given in Fig.…”

“…Controversial astronomical duration estimates are currently debated for the Hettangian Stage, ranging between 1.7/1.9 My and 4.1 My (17-19, 67, 68). The shorter estimates are derived from multiple astrochronological studies (17)(18)(19), with duration estimates from the Hartford Basin, United States, and the biostratigraphically well-constrained St Audries Bay succession, Somerset, United Kingdom, being effectively indistinguishable and supported by correlation to the geomagnetic polarity time scale, as well as radioisotopic ages for the Triassic-Jurassic boundary and earliest Sinemurian from the Pucará Basin, Peru (18,67,(69)(70)(71). The longer estimate of 4.1 My is based on cyclostratigraphic interpretation of spliced magnetic-susceptibility records from multiple UK locations, the individual sections of which suggest durations of 2.9 to 3.2 My (68).…”

“…Stratigraphically expanded shifts were recorded at the Sinemurian-Pliensbachian boundary (8)(9)(10)(11)(12)(13)(14) and the upper Pliensbachian margaritatus and spinatum zones (10,15,16). Furthermore, multiple stratigraphically less extended short-term δ 13 C shifts of ∼0.5 to 2‰ magnitude have been recognized throughout the Hettangian (17)(18)(19), in the Sinemurian (17,(20)(21)(22)(23)(24), and Pliensbachian (10,11,16,22,(25)(26)(27)(28), where they are recorded as individual shifts or series of shifts within stratigraphically limited sections. Some of these short-term δ 13 C excursions have been shown to represent changes in the supraregional to global carbon cycle, marked by synchronous changes in δ 13 C in marine and terrestrial organic and inorganic substrates and recorded on a wide geographic extent (e.g., refs.…”

“…However, due to the previous lack of a continuous dataset capturing and contextualizing all isotopic shifts in a single record, there is no holistic understanding of the global nature, causal mechanisms, and the chronology and pacing of these CIEs. Therefore, these δ 13 C shifts have largely been interpreted as standalone events, linked to a release of 12 C from as-yet-undefined sources, reduced organic productivity (leaving more 12 C in the ocean-atmosphere system) and/or 13 C-depleted carbon sequestration and orbitally forced environmental change affecting the carbon cycle on the scale of Milankovitch cyclicity (17,20,24,25). Evidence for the latter is so far limited to the Hettangian to early Sinemurian and the early Toarcian, where high-resolution isotope records provide the basis for cyclostratigraphic analysis (17)(18)(19)(29)(30)(31).…”