The response of marine carbon-nitrogen-zinc geochemical cycling to the ~183 Myrs Toarcian global warming

Abstract

An extreme perturbation of carbon cycle to the global climate arising from the injection of massive amounts of CO2/CH4 into the atmosphere-ocean system occurred during the Early Jurassic (Toarcian: ~183 Myrs). This carbon-cycle perturbation was dictated by negative carbon isotope excursions (T-CIEs) of ~3-7 ‰ in δ13C, which were globally recorded by the coeval marine carbonate and organic matter reservoirs. The severe Early Toarcian carbon cycle was accompanied by expanded marine anoxia which was dubbed Toarcian Oceanic Anoxic Event (T-OAE), and considerable accumulation of organic-rich sediments especially in the northern European epicontinental shelf-sea region. The T-CIEs on carbonate or organic matter from the Early Toarcian marine sedimentary sections have been widely identified, however, it is not well exemplified how and to what extent local-scale carbon cycles are able to disturb the T-CIE records. Furthermore, the traditionally deemed T-OAE was increasingly challenged by the fact that most of the Early Toarcian sedimentary sections investigated for the argument of global deep ocean anoxia were deposited in hydrographically restricted anoxic basins, which were appreciably isolated from the open ocean. Interestingly, the widespread organic-rich sediments deposited on the epicontinental shelf across the T-CIE interval generally contain high amounts of zinc (Zn) which is a sulfide-forming and bio-essential element. The response of Zn and its isotopic composition to changes in the bioproductivity during the T-OAE, however, has so far remained unexplored.

In this dissertation local-scale carbon cycles were investigated using the combined δ13Ccarb and δ13Corg for the Dotternhausen T-CIE sedimentary section, SW Germany (see section 3.2.1). A two-step δ13Ccarb-δ13Corg decoupling – superimposing the general T CIE trends – was identified, which corresponded to an increase in green sulfur bacteria (GSB) prosperity. This δ13Ccarb-δ13Corg decoupling was thus related to GSB disturbance, with the larger magnitude second-step δ13Ccarb-δ13Corg decoupling being further exaggerated by early diagenesis. Local-scale carbon cycles can thus considerably upset the general expression of the T-CIE. Furthermore, this dissertation observed contrasting decoupled δ13Ccarb-δ13Corg patterns from paleogeographically distinct localities of the Tethys region, demonstrating that the local carbon-cycle perturbations have pervasively and independently impacted the Early Toarcian global carbon cycle.

To address the controversy of the Early Toarcian open ocean redox structure, a combination of δ15N of bulk samples and extracted kerogens (expressed by δ15Nbulk and δ15Nker) from the Dotternhausen T-CIE sedimentary section was used in this dissertation (see section 3.2.2). Both δ15Nbulk and δ15Nker values imply enhanced N2 fixation by cyanobacteria using the molybdenum (Mo)-based nitrogenase enzyme. N isotope compositions of +0.3 to +2.5 ‰ produced by this process are in stark contrast to the typical sedimentary δ15N values (> 3 ‰) induced by partial water-column denitrification and/or anammox in oxygen minimum zones of the modern ocean, thus indicating the existence of restricted local oxygen-depleted settings on the northern European epicontinental shelf. Such restricted environments with severe oxygen depletion on the northern European epicontinental shelf are unlikely representative of the open-ocean redox landscape.

The Zn biogeochemical cycling in response to the Early Toarcian anoxia on the northern European epicontinental shelf-sea region, was explored by measuring δ66Znbulk values for the Yorkshire (northeastern England) and DotternhausenT-CIE sedimentary sections, respectively (see section 3.2.3). The authigenic δ66Znauth values of these two sedimentary sections are comparable to that of the modern global deep ocean, indicating that the Zn biogeochemical cycle in the Early Toarcian ocean has already reached present-day conditions. A short-lived ocean oxygenation was likely recorded by a negative excursion of δ66Znbulk observed in both Yorkshire and Dotternhausen sections, implying changing ocean redox states during the T-CIE. Near-quantitative removal of dissolved Zn from the seawater to early diagenetic minerals in the sediment with minimal isotopic effects is observed under excess hydrogen sulfide conditions, similar to the behaviour of the redox-sensitive element molybdenum (Mo). Compared to molybdenum, however, zinc as a non-redox sensitive element is less prone to local redox effects in restricted basins and thus δ66Znauth values of euxinic marine sediments bear the potential to accurately record global seawater redox variations through geological time.