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Title: Carbon-isotope stratigraphy, geochemistry and geochronology of the Lower Jurassic of the Cardigan Bay Basin (Llanbedr (Mochras Farm) borehole), UK, and the Neuquén Basin, Argentina
Author: Storm, Marisa
ISNI:       0000 0004 7232 9331
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2018
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The lack of a continuous high-resolution carbon-isotope (δ13C) record based on a single material, and thus a single carbon reservoir, has so far inhibited investigation on the secular evolution of the Early Jurassic (~201-175 Myr) carbon cycle. The existing δ13C records for the Lower Jurassic Series tend to cover short stratigraphic intervals, originate from a variety of different sedimentary basins and different carbon reservoirs, or are of relatively low stratigraphic resolution. New δ13CTOC data from the continuous strata of the stratigraphically expanded Llanbedr (Mochras Farm) drill-core provides the most expanded Lower Jurassic δ13CTOC record to date. The record aggregates several δ13CTOC excursions of different stratigraphical extent and magnitude. Based on stratigraphic correlations, several of these excursions appear approximately coeval in several NW European sedimentary basins, in multiple carbon reservoirs. Geochemical characterization reveals an organic-lean character of the Lower Jurassic sediments, dominated by terrestrial organic matter. Changes in the relative admixture of marine vs terrestrial organic matter appear as a likely subordinate mechanism driving changes in δ13C. Consecutively appearing small-amplitude fluctuations (~0.5-2 ‰) of comparable magnitude and stratigraphic thickness, including those recorded elsewhere, and hitherto unrevealed excursions, appear to be largely orbitally paced, corresponding to long (405-kyr) eccentricity cycles. Orbital tuning of the continuous Hettangian to Pliensbachian record allows an astronomical timescale to be established, which closely agrees with previous timescales for the Hettangian and Pliensbachian stages, and provides the basis for a Sinemurian chronology. Furthermore, a stratigraphical framework is developed on Lower Jurassic sections in the Neuquén Basin, Argentina, providing the grounds for future work on the radioisotopic dating of the onset of the Toarcian oceanic anoxic event (T-OAE). A chemostratigraphic dataset allows the correlation to same-age records. Coeval appearance of siderite in the lower Toarcian of the Mochras core and the Neuquén Basin suggests a disturbance of the global sulphur cycle (possible low sulphate ocean) during the T-OAE.
Supervisor: Jenkyns, Hugh ; Hesselbo, Stephen ; Ruhl, Micha Sponsor: Shell International Exploration & Production B.V.
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available