Search

Hesselbo et al. (2006; hereinafter HJDO06) present stable-isotopic profiles through lower Toarcian sediments at Peniche, Portugal, and interpret them as evidence of a massive injection to the atmosphere of isotopically-light CO2 derived from methane oxidation. The atmospheric inputs supposedly gave rise to negative isotopic excursions in all carbon reservoirs. The paper follows others proposing that environmental change in early Toarcian time was driven by the release to the atmosphere of methane, either from marine clathrates (Hesselbo et al., 2000; Kemp et al., 2005), or from organic-rich shales that were baked by Karoo–Ferrar intrusions (McElwain et al., 2005). The methane hypothesis was tested by van de Schootbrugge et al. (2005), who highlighted the existence of sections that lacked a negative excursion in aC in one or more sample media: amongst them is the section on the coast of Yorkshire, UK, used to erect the methane hypothesis by Hesselbo et al. (2000), which lacks a negative excursion in the δC of belemnite calcite (McArthur et al., 2000; van de Schootbrugge et al., 2005). As the excursion is not seen in these sections, it cannot represent a global event and so could not have resulted from methane release, which would have influenced all reservoirs of carbon. New data are now provided by HJDO06 for a section (Peniche

Secular records of the elemental and isotopic composition of belemnite calcite were studied in Pliensbachian and Toarcian sections from the Yorkshire coast, UK, and Southern Germany, to investigate oceanographic change during an interval prior to and including the Toarcian Oceanic Anoxic Event (OAE). Records from Southern Germany are correlated to the UK stratigraphy using strontium isotope stratigraphy. The geochemical trends measured from belemnite calcite are consistent between the two sections, and are interpreted in terms of temperature and salinity of the northwest European epi-continental sea. The data suggest that a dramatic environmental change coincided with the Toarcian OAE. Belemnite Mg/Ca, Sr/Ca, and Na/Ca ratios increase by a factor of between 1.7 and 2 coincident with a 3‰ negative shift in δ18O from the mid-tenuicostatum zone until the lower falciferum zone of the UK ammonite biostratigraphy (a period of ∼0.6–0.7 Myr). Taken at face value, the Mg/Ca and δ18O data argue for an abrupt warming of 6–7°C and substantial freshening during this interval. Global warming accompanied by an accelerated hydrological cycle and increased runoff is proposed to explain these changes. Prior to these events, data from lower in the Yorkshire section suggest a possible cooling accompanied by a shift to more saline waters during the period from the upper Pliensbachian margaritatus zone to the Toarcian lower tenuicostatum zone. This earlier event may also have been important in causing density stratification in the northwest European epi-continental sea.

We profile 87Sr/86Sr, δ13C, δ18O, Sr/Ca, Mg/Ca, and Na/Ca in belemnites through Pliensbachian and Toarcian strata on the Yorkshire coast, UK, which include the early Jurassic oceanic anoxic event. The 87Sr/86Sr profile shows that the relative duration of ammonite subzones differ by a factor of up to 30: the Lower Jurassic exaratum subzone is 30 times longer than the clevelandicum subzone because the exaratum subzone in Yorkshire, which contains the anoxic event, is condensed by a factor of between 6.5 and 12.2 times, relative to adjacent strata. Using our 87Sr/86Sr profile, the resolution in correlation and dating attainable in the interval is between ±1.5 m and ±15 m of section, and better than 0.25 Myr. In parts of the sequence, this stratigraphic resolution equals that attainable with ammonites. A new age model is provided for late Pliensbachian and early Toarcian time that is based on the 87Sr/86Sr profile. Through the sequence, the Sr/Ca, Mg/Ca, Na/Ca and δ18O of belemnite carbonate covary, suggesting that elemental ratios may be useful for palaeotemperature measurement. Our δ13Cbelemnite data splits into three the previously reported positive isotope excursion (to +6.5‰) in the early Toarcian. We speculate that the excursion(s) resulted from addition to surface waters of isotopically heavy CO2 via ebullition of methanogenic CO2 from the sediment during early burial of organic rich (>10% TOC) sediments

Strontium isotope profiles, derived from minimally altered samples, across marine K/T boundary sequences exposed at Kjolby Gaard and Nye Klov, Denmark, and on Seymour Island, Antarctica, show neither the boundary excursions, nor the boundary spikes, in 87Sr/86Sr that have been reported for K/T boundary sequences from elsewhere. Nor do our data conform to modelled predictions of the Sr isotopic response of the oceans to a positive spike in 87Sr/86Sr at the terminal Cretaceous. Boundary values for 87Sr/86Sr (with 95% confidence intervals) are 0.707828±3 in Denmark and 0.707832±7 in Antarctica. Our data suggest that 87Sr/86Sr stopped increasing and started decreasing at least 90 ka before the K/T boundary.

Stable isotopic data are presented for 112 samples of francolite from 18 separate phosphate deposits. Values ofδ13C andδ34S in most offshore deposits suggest formation within oxic or suboxic environments either by carbonate replacement or direct precipitation of francolite from water of normal marine compositions. The exceptions are concretionary francolite from Namibia, which has an isotopic composition in keeping with its formation within organic-rich sediments, and that from offshore Morocco, which has an isotopic signature of the anoxic/suboxic interface. Onshore deposits from Jordan, Mexico, South Africa and, possibly, the Permian Phosphoria Formation in the western U.S.A., are substantially depleted in18O: they appear to be too altered for deductions to be made about their environments of formation. In other onshore deposits which are unaltered, or minimally altered, the isotopic composition suggests that some formed within sulphate-reducing sediments (Sedhura, Morocco) whilst francolite from the Georgina Basin of Australia formed at the oxic/anoxic boundary, where oxidation of biogenic H2S decreases theδ34S of pore water. In general, pelletal samples show non-oxic isotopic signatures, whilst non-pelletal samples show oxic isotopic signatures, but samples from Namibia, Peru (Ica Plateau) and the Californian and Moroccan margins are exceptions to this rule. Morphology may therefore be a misleading indicator of francolite genesis as no definitive relation exists between phosphorite type and isotopic signature.