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Research Articles: Paleoceanography |
1 Departamento de Estratigrafía y Paleontología, Facultad de Ciencias, Universidad del País Vasco, Apartado 644, 48080 Bilbao, Spain; gqbrofri{at}lg.ehu.es
2 Departamento de Estratigrafía y Paleontología, Facultad de Ciencias, Universidad del País Vasco, Apartado 644, 48080 Bilbao, Spain
3 Repsol-YPF, Paseo de la Castellana 280, 28046 Madrid, Spain
Well preserved calcitic belemnite samples from the Basque-Cantabrian basin of northern Spain have been analyzed to improve our current understanding of the chemistry of seawater as an index of paleoceanographic changes during the Early Jurassic, a period punctuated by times of oceanic anoxia and global mass extinctions. Because the oxygen-isotope composition of calcite depends not only on temperature but also on salinity, we present here a combination of Sr/Ca, Mg/Ca, and 
18O analyses that helped us to evaluate both effects separately. Comparison between the temporal records and cross plots of these geochemical parameters shows a good correlation of Mg/Ca with 18O and a weak correlation of Sr/Ca with 18O.
Comparison of our belemnite 18O record with coeval O-isotope profiles from other paleogeographic domains suggests the existence, despite differences in the absolute isotopic values, of a major negative shift during the serpentinus Zone that is regionally reproducible. This event is linked to the Early Toarcian ocean anoxic event (OAE), and suggests the existence of global changes in paleoclimatic and paleoceanographic conditions that are reflected largely in the geochemical composition of seawater. Compared to our results, however, other Early Toarcian 18O data reported from northern European basins are generally depleted 
1.3-1.6
In contrast, Mg/Ca ratios display similar values. These differences in 18O data suggest a salinity component for the northern European data and provide evidence for a north-to-south salinity gradient within the north European interior (Boreal) seaway during the Early Jurassic. This study suggests that the secular variation of Mg/Ca is a useful proxy for discriminating between temperature and salinity effects on the oxygen isotope composition of fossil shells, and highlights the prospect of using the Mg/Ca ratios of belemnite calcite to calculate paleotemperatures independently of salinity. The findings of our study may help to calibrate 18O curves in other basins by removing the salinity component of their 18O records, which may help to calculate water density gradients between the Jurassic seas of northern and southern Europe.
This article has been cited by other articles:
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  K. LITTLER, S. P. HESSELBO, and H. C. JENKYNS A carbon-isotope perturbation at the Pliensbachian-Toarcian boundary: evidence from the Lias Group, NE England Geological Magazine, March 1, 2010; 147(2): 181 - 192. [Abstract] [Full Text] [PDF]
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 M. MALKOC and J. MUTTERLOSE THE EARLY BARREMIAN WARM PULSE AND THE LATE BARREMIAN COOLING: A HIGH-RESOLUTION GEOCHEMICAL RECORD OF THE BOREAL REALM Palaios, January 1, 2010; 25(1): 14 - 23. [Abstract] [Full Text] [PDF]
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H. WIERZBOWSKI and M. M. JOACHIMSKI STABLE ISOTOPES, ELEMENTAL DISTRIBUTION, AND GROWTH RINGS OF BELEMNOPSID BELEMNITE ROSTRA: PROXIES FOR BELEMNITE LIFE HABITAT Palaios, June 1, 2009; 24(6): 377 - 386. [Abstract] [Full Text] [PDF]
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 L. Emmanuel, M. Renard, R. Cubaynes, M. de Rafelis, M. Hermoso, L. Lecallonnec, A. le Solleuz, and J. Rey The "Schistes carton" of Quercy (Tarn, France): a lithological signature of a methane hydrate dissociation event in the Early Toarcian. Implications for correlations between Boreal and Tethyan realms Bulletin de la Societe Geologique de France, September 1, 2006; 177(5): 237 - 247. [Abstract] [Full Text] [PDF]
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