Strontium isotope variations in Oxfordian seawater

• Autorzy: Wierzbowski H. , Zieliński G.
• Konferencja: International Congress on the Jurassic System (7 ; 06-18.09.2006 ; Kraków, Poland]
• Języki publikacji: EN

Abstrakty

EN

The Jurassic strontium isotope curve is characterized by a broad and deep minimum at the Callovian/Oxfordian boundary. However, there are some discrepancies among its datings and the 87Sr/86Sr values represented in various papers (cf. Jones et al. 1994; Podlaha et al. 1999). We analysed well-stratigraphically dated and well-preserved (Fe <150 ppm, Mn <50 ppm) belemnite rostra derived from the Submediterranean and Boreal Provinces (Polish Jura Chain, Swabian Alb, Isle of Skye). Rostra were carefully separated from surrounding sediment with microdrill, ground and dissolved in 2M acetic acid. After removal of insoluble residue, the leachates were evaporated with an admixture of concentrated nitric acid. Strontium separation was performed at two steps: with 100% nitric acid and cation exchange resin (Bio-Rad 50W-X8 with 2.5 M HCl as eluent). Total blank of chemical procedures has been determined to be less than 1 ng of total Sr. The blank value was not enough to affect the Sr isotope ratio at the average sample weight (50 mg). Analyses of strontium isotopic ratio (87Sr/86Sr value) were performed by a VG Sector 54 mass spectrometer (equipped with 7 faraday collectors) at the Institute of Geological Sciences PAS in Warsaw. All isotope ratios were instrumentally corrected for fractionation effect by using the 87Sr/86Sr value of 0.1194 and normalized with NIST 987 isotopic standard (87Sr/86Sr ratio = 0.710248). The scarcity of the data and the spread of results do not allow the exact recognition of the 87Sr/86Sr values in the lowermost Oxfordian (Fig. 1). However, the strontium isotope curve is characterized by a discernible minimum (87Sr/86Sr ratios between 0.70679 and 0.70681) in the uppermost Lower and the Middle Oxfordian. The 87Sr/86Sr ratio rapidly increases starting from the Middle/Upper Oxfordian boundary and reaches about 0.70685 in the uppermost Oxfordian (Bimmamatum and Planula zones). The 87Sr/86Sr ratio of Callovian-Oxfordian seawater, which was the lowest in the Mesozoic, might have been linked to enhanced hydrothermal activity of the seafloor. Nevertheless, the long time-span of this minimum is unusual. The Callovian-Oxfordian 87Sr/86Sr ratio minimum may also be correlated with a crisis in carbonate sedimentation. In addition, the increase of seawater 87Sr/86Sr ratio coincides with a new period of acceleration of carbonate sedimentation in the oceans. The present data may be used to obtain a best fit of the strontium isotope curve in the Oxfordian.

Słowa kluczowe

EN

strontium isotope stratigraphy; Oxfordian; belemnites

• Wydawca: Państwowy Instytut Geologiczny - Państwowy Instytut Badawczy
• Czasopismo: Volumina Jurassica
• Rocznik: 2006
• Tom: Vol. 4, no. 1
• Strony: 222--223
• Opis fizyczny: Bibliogr. 4 poz.

Twórcy

autor

Wierzbowski H.

autor

Zieliński G.

• Institute of Geological Sciences, Polish Academy of Sciences, ul. Twarda 51/55, PL-00818 Warszawa, Poland,

Bibliografia

• Głowniak E. 2002. The ammonites of the family Perisphinctidae from the Plicatilis Zone (lower Middle Oxfordian) of the Polish Jura Chain (Central Poland): their taxonomy, phylogeny and biostratigraphy. Acta Geologica Polonica, 52, 3: 307-364.
• Głowniak E. 2005. The Wartae Subzone – a proposal for the lower boundary of the unified Upper Oxfordian Substage in the Submediterranean Province. ISJS Newsletter, 32: 34-37.
• Jones C. E., Jenkyns H. C., Coe A. L. and Hesselbo S. P. 1994. Strontium isotopic variations in Jurassic and Cretaceous seawater. Geochimica et Cosmochimica Acta, 58, 14: 3061-3074
• Podlaha O. G., Chalimourda A. and Albeverio S. 1999. Nonlinear system analysis of a 87Sr/86Sr time series for Phanerozoic seawater. Chemical Geology, 161, 1-3: 241-252.