Mechanisms and Effects of the Toarcian Oceanwide Anoxic Event
Final Report Abstract
The Earth’s cryosphere represents a huge climate-sensitive carbon reservoir, capable of releasing carbon dioxide (CO2) and methane (CH4) upon rising global temperatures that will accelerate global warming. The role of the cryosphere carbon reservoir during ancient periods of global warming is however poorly known. In contrast to previous views of a constantly warm Early Jurassic (190-180 Ma BP) period a current reassessment identified icehouse conditions preceding the Toarcian greenhouse climate. During the Early Toarcian (~183 Ma) global warming led to a perturbation of the global carbon cycle, manifested in recurring negative carbon isotope excursions (CIEs). We propose that an initially volcanicdriven minor warming triggered a subsequent melt-down of the Earth’s mid-latitudinal and thus labile cryosphere. The rapid release of greenhouse gases, mainly as 13C-depleted CH4, is recorded in the carbon isotope ratios of sedimentary organic matter displaying a series of orbitally-forced negative CIEs. These reveal a frequency shift from eccentricity (mid-latitude sensitive processes) to obliquity (high-latitude sensitive processes) cycles comparable to Quaternary climate rhythms. This pattern is explained by a self-sustaining destabilization of labile mid-latitudinal cryosphere carbon reservoirs, which then rhythmically progressed poleward. The hitherto underestimated presence of a temperature-sensitive cryosphere constituted an essential precondition of the Early Toarcian climate change as reflected in the C isotope excursions and may serve as a natural fossil analogue against which current anthropogenically impacted climate change scenarios can be compared. The cryosphere melt-down pulses to a large extent controlled the degree of surface water freshening and stratification on the Toarcian NW-Tethyan shelf, which is recorded best in the Lorraine Basin, where northern-derived freshwater mixed with southern saline Tethys water. The freshwater inflow from terrestrial run-off and via the Viking corridor constantly resupplied nutrients into the NW-Tethyan shelf that promoted high bioproductivity. Due to the physical water column stratification, chemical water stratification led to permanently anoxic to euxinic conditions in the water column, often with extension of euxinia into the photic zone as evidenced by Chlorobiaceae biomarker. Orbitally induced changes in water properties (salinity, nutrients temperature) affected the planktonic communities thriving in the Lorraine Basin, which is recorded in the steroid and hopanoid biomarker composition, indicative of predominantly algae and cyanophytes. Freshwater stratification was less intensive in the Eastern part of the NW Tethyan shelf due to restricted inflow via the Viking corridor but sufficient to establish short lived periods of photic zone euxinia also in this region.
Publications
- (2018) Environmental response to the early Toarcian carbon cycle and climate perturbations in the northeastern part of the West Tethys shelf. Gondwana Research 59 144–158
Ruebsam, Wolfgang; Müller, Tamás; Kovács, János; Pálfy, József; Schwark, Lorenz
(See online at https://doi.org/10.1016/j.gr.2018.03.013) - (2019) Cryosphere carbon dynamics control early Toarcian global warming and sea level evolution. Global and Planetary Change 172 440–453
Ruebsam, Wolfgang; Mayer, Bernhard; Schwark, Lorenz
(See online at https://doi.org/10.1016/j.gloplacha.2018.11.003) - (2014). Chronology of the Early Toarcian environmental crisis in the Lorraine Sub-Basin (NE Paris Basin). Earth and Planetary Science Letters 404, 273–282
Rübsam, W., Münzberger, P. Schwark, L.
(See online at https://doi.org/10.1016/j.epsl.2014.08.005) - (2015) Reply to the comment by Boulila and Hinnov towards “Chronology of the Early Toarcian environmental crisis in the Lorraine Sub- Basin (NE Paris Basin)”. Earth and Planetary Science Letters 416, 147–150
Rübsam, W., Münzberger, P. Schwark, L.
(See online at https://dx.doi.org/10.1016/j.epsl.2015.02.022) - (2016) Microbially-mediated fossil-bearing carbonate concretions and their significance for palaeoenvironmental reconstructions: A multi-proxy organic and inorganic geochemical appraisal. Chemical Geology 426, 95-108
Plet, C., Grice, K., Pagès, A., Ruebsam, W., Coolen, M.J.L., Schwark L.
(See online at https://doi.org/10.1016/j.chemgeo.2016.01.026) - (2017) Multiproxy reconstruction of oceanographic conditions in the southern epeiric Kupferschiefer Sea (Late Permian) based on redox-sensitive trace elements, molybdenum isotopes and biomarkers. Gondwana Research 44, 205-218
Ruebsam, W., Dickson, A.J., Hoyer, E.M., Schwark, L.
(See online at https://doi.org/10.1016/j.gr.2016.10.017) - (2017) Palaeobiology of red and white blood cell-like structures, collagen and cholesterol in an ichthyosaur bone. Scientific Reports Nature 7: 13776
Plet, C., Grice, K., Pagès, A., Verrall, M., Coolen, M.J.L., Ruebsam, W., Rickard, W.D.A., Schwark, L.
(See online at https://doi.org/10.1038/s41598-017-13873-4)
