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(2017) 'Carbon sequestration in an expanded lake system during the Toarcian oceanic anoxic event.', Nature geoscience., 16 (2). pp. 129-134. Further information on publisher's website:https://doi.org/10.1038/ngeo2871Publisher's copyright statement:Additional information: Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-pro t purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. The early Toarcian Oceanic Anoxic Event (T-OAE at ~183 Ma) is recognized as one 32 of the most intense and geographically extensive events of oceanic redox change and 33 accompanying organic-carbon burial in the Mesozoic Era 1,2 . The T-OAE is marked by 34 major changes in global geochemical cycles, with an apparently rapid negative shift 35 of as much as ~7‰ in bulk marine and terrestrial organic-carbon isotope records and 36 a typically smaller (3-6‰) negative excursion in carbonate archives and specific 37 organic compounds 3-10 . 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 . 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 3 characteristic abrupt negative shift that invariably characterizes the T-OAE (early 55Toarcian tenuicostatum−falciferum ammonite biozones) 1,2,18 . 56

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Molybdenum (Mo)‐isotope chemostratigraphy of organic‐rich mudrocks has been a valuable tool for testing the hypothesis that the Toarcian Oceanic Anoxic Event (T‐OAE, Early Jurassic, ~183 Ma) was characterized by the spread of marine euxinia (and organic matter burial) at a global scale. However, the interpretation of existing Mo‐isotope data for the T‐OAE (from Yorkshire, Cleveland Basin, U.K.) is equivocal. In this study, three new Mo‐isotope profiles are presented: from Dotternhausen Quarry (South German Basin, Germany), the Rijswijk core (West Netherlands Basin, Netherlands), and the Dogna core (Belluno Basin, northern Italy). Precise biostratigraphic and chemostratigraphic correlation between the three sites allows a direct comparison of the data, enabling some key conclusions to be reached: (i) The Mo‐isotope composition of seawater during the peak of the T‐OAE was probably close to ~1.45‰, implicating a greater removal flux of sulphides from seawater, and a larger extent of global seafloor euxinia compared to the present day; (ii) Mo‐isotope cycles previously identified in the Yorkshire sedimentary succession are attributed to changes in the degree of local Mo drawdown from overlying Cleveland Basin seawater; (iii) The consistency of the new multisite Mo‐isotope data set indicates a secular reduction in the burial of Mo globally in the late stages of the T‐OAE, implying a contraction in the extent of global marine euxinia; (iv) Subtle differences in the Mo‐isotope composition of deposits formed in different euxinic subbasins of the European epicontinental shelf were probably governed by local variations in basin hydrography and rates of water renewal.

Despite its assumed global nature, there are very few detailed stratigraphic records of the late Cenomanian to the early Turonian Oceanic Anoxic Event 2 from the Southern Hemisphere. A highly resolved record of environmental changes across the Cenomanian–Turonian boundary interval is presented from Ocean Drilling Program Site 1138 on the central Kerguelen Plateau (southern Indian Ocean). The new data lead to three key observations. Firstly, detailed biostratigraphy and chemostratigraphy indicate that the record of Oceanic Anoxic Event 2 is not complete, with a hiatus spanning the onset of the event. A decrease in glauconite and highly weathered clays after the onset of Oceanic Anoxic Event 2 marks the end of the hiatus interval, which can be explained by a relative sea‐level rise that increased sediment accommodation space on the Kerguelen Plateau margin. This change in depositional environment controlled the timing of the delayed peak in organic‐matter burial during Oceanic Anoxic Event 2 at Site 1138 compared with other Oceanic Anoxic Event 2 locations worldwide. A second key observation is the presence of cyclic fluctuations in the quantity and composition of organic matter being buried on the central Kerguelen Plateau throughout the latter stages of Oceanic Anoxic Event 2 and the early Turonian. A close correspondence between organic matter, sedimentary elemental compositions and sediments recording sea‐floor oxygenation suggests that the cycles were mainly productivity‐driven phenomena. Available age‐control points constrain the periodicity of the coupled changes in sedimentary parameters to ca 20 to 70 ka, suggesting a link between carbon burial and astronomically forced climatic variations (precession or obliquity) in the Southern Hemisphere mid‐latitudes both during, and after, Oceanic Anoxic Event 2: fluctuations that were superimposed on the impact of global‐scale processes. Finally, trace‐metal data from the black‐shale unit at Site 1138 provide the first evidence from outside of the proto‐North Atlantic region for a global drawdown of seawater trace‐metal (Mo) inventories during Oceanic Anoxic Event 2.

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