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- Title
Primary and diagenetic isotopic signals in fossils and hemipelagic carbonates: the Lower Jurassic of northern Spain.
- Authors
Rosales, Idoia; Quesada, Santiago; Robles, Sergio
- Abstract
Stable isotope and trace element analyses of 230 Jurassic (Pliensbachian–Toarcian) samples from northern Spain have been performed to test the use of geochemical variations in fossils (belemnites and brachiopods) and whole-rock hemipelagic carbonates as palaeoceanographic indicators. Although the succession analysed (Reinosa area, westernmost Basque–Cantabrian Basin) has been subject to severe thermal alteration during burial diagenesis, the samples appear to be well preserved. The degree of diagenetic alteration of the samples has been assessed through the application of integrated petrographic, chemical and cathodoluminescence analyses. It is demonstrated that brachiopods and whole-rock carbonates, although widely used for palaeoceanic studies, do not retain their primary marine geochemical composition after burial diagenesis. In contrast, there is strong evidence that belemnite rostra preserve original isotopic values despite pervasive diagenesis of the host rock. Well-preserved belemnite shells (non-luminescent to slightly luminescent) typically show stable isotope values of +4·3‰ to –0·7‰ δ13C, +0·7‰ to –3·2‰ δ18O, and trace element contents of <32 μg g–1 Mn, <250 μg g–1 Fe, >950 μg g–1 Sr and Sr/Mn ratios >80. This study suggests that the degree to which diagenesis has affected the preservation of an original isotopic composition may differ for different low-Mg calcite fossil shells and hemipelagic bulk carbonates, behaviour that should be considered when marine isotopic signatures from other ancient carbonate rocks are investigated. Multiple non-luminescent contemporaneous belemnite samples passed the petrographic and geochemical tests to be considered as palaeoceanic recorders, yet their δ13C and δ18O values exhibited moderate scatter. Such...
- Subjects
SPAIN; JURASSIC stratigraphic geology; TRACE element analysis; FOSSILS
- Publication
Sedimentology, 2001, Vol 48, Issue 5, p1149
- ISSN
0037-0746
- Publication type
Article
- DOI
10.1046/j.1365-3091.2001.00412.x