Eine Phasen-spezifische geochemische Studie von mariner Produktivität und Nährstoffzyklen während des Perm/Trias-Massenaussterbens (PHoSPhoR)
Mineralogie, Petrologie und Geochemie
Zusammenfassung der Projektergebnisse
Mechanisms for a globally pervasive marine redox change, compatible with the time-span of the end-Permian mass extinction, are changes in ocean circulation decreased O2 solubility under globally rising temperatures and increased respiratory O2 demand under large-scale eutrophication. Recent studies have postulated that changes in the marine inventory of phosphorus (P); the ultimate limiting nutrient for marine productivity on geological timescales can be held accountable for eutrophication-induced oxygen depletion and hydrogen sulfide (H2S) build-up. This notion rests on the observation that robust evidence exist to support elevated continental fluxes of nutrients at the Permian-Triassic (P-Tr) boundary; a potential feedback of globally elevated temperatures and linked increased chemical weathering as well as destruction of soil-stabilizing vegetation. This mechanism would result in a vertical profile of oxygen availability with maximum O2 depletion at intermediate depths (or the oxygen minimum zone: OMZ), where aerobic respiration outcompetes O2 production through photosynthesis. Up to now, there has been little focus on reliably judging productivity levels and nutrient availability/cycling across the P-Tr boundary. Approaches estimating the vertical gradient in dissolved inorganic carbon (DIC)-δ13C are unreliable due to a lack of carbonate biogens. Instead, previous research has focused on micritic bulk rock from depth transects (multiple sections) have been used to establish vertical δ13Cdic of the latest Permian and Early Triassic ocean. Carbonate from allochthonous sources and varying dominant-mineralogy; aragonite or calcite, differences in the amount of organic matter and remineralization by microbial guilds are all potential causes that can leave a distinct δ13C imprint on the deep-water carbonate signal. My numerical investigation of δ13C variability warrants against over-interpetation of local δ13C deviations superimposed on the long-term first-order trend towards 13C-depleted values across the P-Tr boundary. Hence, depth-gradients in δ13C, based on bulk-rock are suspect of alteration, and inferences on the biological pump should be supported by independent proxy records. The sedimentary records of reactive P (Preac) and total organic carbon (TOC) are an alternative approach to gauge ancient nutrient levels and productivity. Reactive phosphorus represents P phases that have the potential to be, or have been, mobilized in the environment, including iron-bound, organic and authigenic P. However, previous P-Tr studies applying these elements as productivity and nutrient availability estimates might be hampered, as they are mere approximations P reactive, based on bulk-rock elemental measurements. Such bulk-rock measurements potentially fail to (fully) correct for incorporation of P other than the targeted compounds; such as, the detrital sediment fraction. Conversely (post)-depositional mobilization of P reactive is known to occur, especially under euxinic conditions, and might therefore be of great importance when investigating this time-interval with supposed widespread occurrence of sulfidic water masses. To more reliably investigate the role of marine P cycling in the mass extinction, I undertook a study to establish the sedimentary P records in the context of local redox conditions, by a combined P and Fe speciation approach, complemented with redox-sensitive trace elements and pyrite sulfur isotope analysis. The outcome of this study suggests that anoxia developed first in the proximal parts of the shelf; a sudden redox-controlled shift in nutrient availability turned the deeper shelf environment anoxic; where after the whole shelf remains in a more sulfidic state through sustained high levels of P regeneration. In this scenario the proximal cause might still be sought in magmatic activity (evident as the Siberian Traps Basalts) starting before the main extinction pulse, setting in motion a complex web of interacting and destabilizing feedbacks.
Projektbezogene Publikationen (Auswahl)
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(2017) Disparate Permian-Triassic carbonate-carbon isotope trends explained by a diagenetic model forced with spatially heterogeneous organic matter fluxes, IGCP 630: Permian-Triassic climatic and environmental extremes and biotic response, Tohoku University, Sendai, Japan
Schobben, M., van de Velde, S. , Suchocka, J., Leda, L., Korn, D., Struck, U., Ullmann, C.V., Hairapetian, V., Ghaderi, A., Korte, C., Newton, R.J., Poulton, S.W., Wignall, P.B.
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(2017) Latest Permian carbonate-carbon isotope variability traces heterogeneous organic carbon accumulation and authigenic carbonate formation. Climate of the Past 13:1635-1659
Schobben M, Velde S van de, Gliwa J, Leda L, Korn D, Struck U, Ullmann CV, Hairapetian V, Ghaderi A, Korte C, Newton RJ, Poulton SW, Wignall PB
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(2017) Limitations and opportunities for Permian-Triassic carbonate-carbon isotope stratigraphy posed by microbial-controlled diagenetic mineral additions, EGU General Assembly 2017, Vienna, Austria
Schobben, M., van de Velde, S., Suchocka, J., Leda, L., Korn, D., Struck, U., Ullmann, C.V., Hairapetian, V., Ghaderi, A., Korte, C., Newton, R.J., Poulton, S.W., Wignall, P.B.
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(2018) Towards a mechanistic understanding of marine anoxia development during the end-Permian mass extinction, EGU General Assembly 2018, Vienna, Austria
Schobben M, van Soelen EE, Sleveland AR, Kürschner WM, Svensen H, Planke S, Bond DPG, Newton RJ, Wignall PB, Poulton SW
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(2018): Chemostratigraphy Across the Permian‐Triassic Boundary. In: Alcides N. Sial, Claudio Gaucher, Muthuvairavasamy Ramkumar und Valderez Pinto Ferreira (Hg.): Chemostratigraphy Across Major Chronological Boundaries: Wiley (Geophysical Monograph Series
Schobben, M., Heuer, F., Tietje, M., Ghaderi, A., Korn, D., Korte, C., Wignall, P.B.
