| dc.description.abstract | Here, I have investigated the stable isotope (δ13C, δ18O) and elemental composition (Mg/Ca, Sr/Ca, Fe/Ca, Mn/Ca) of Jurassic belemnites from Cabo
Mondego in Portugal (Bajocian-Bathonian) and Albstadt-Pfeffingen in southern Germany (Bathonian-Callovian). Both localities are stratigraphically significant, with Cabo Mondego representing the Auxiliary Stratotype Section and Point (ASSP) for the Bathonian Stage and Albstadt-Pfeffingen being a potential Global Stratotype Section and Point (GSSP) for the Callovian Stage.
Variations in the stable carbon-isotope ratios of macrofossil calcite can reflect changes in fluxes between reservoirs in the palaeo carbon cycle. Cabo
Mondego is an important site to link changes in stable isotope ratios directly to ammonite zones and subzones, and thereby allow chronostratigraphic
calibration. An abrupt positive δ13C isotope shift is observed in the belemnite stable isotope record from the bomfordi Subzone below the Bajocian-Bathonian
boundary. The δ13C shift appears not to have been caused by temperature changes as δ18O ratios remain stable throughout the studied section, nor is there a change in the 𝛿 13C values of coeval brachiopods. One possible cause of this shift in carbon-isotope ratios is a faunal turnover. Faunal turnovers of ammonites and belemnites are observed elsewhere in Europe within the bomfordi Subzone.
The potential of the Jurassic section at Albstadt-Pfeffingen (BadenWürttemberg, Germany) to become a GSSP, an internationally agreed reference point which defines the lower boundary of a stage on the geologic timescale, for the base of the Middle Jurassic Callovian Stage, makes detailed investigations, especially regarding the completeness of the strata, important. The ammonite
assemblages of this section have already been studied in detail, but few chemostratigraphic data from the Bathonian and Lower Callovian fossils of the
site have been published. Belemnite rostra are relatively abundant in the section and have undergone little diagenetic alteration according to chemical and optical screening techniques, making them a suitable target for isotopic analysis.
Chemostratigraphic signals in the rostra across the Bathonian-Callovian boundary show comparatively little change in δ13C values, but a gradual decrease
in δ18O may indicate a warming trend into the Callovian. Notably, a distinct stepchange in 87Sr/86Sr ratios across the stage boundary suggests an unconformity, or at least strongly condensed part of the section, confirming strong doubts on
the site's suitability as a GSSP. | en_GB |
