Original paper

Timing of the Early Aptian 13C excursion in the Boreal Realm

Malkoč, Matthias; Mutterlose, Jörg; Pauly, Sebastian

Abstract

Trace elements (Mg, Sr, Fe, Mn) and stable isotope data (δ13C, δ18O) have been obtained from 136 belemnite rostra of Barremian - early Late Aptian age from three sections of the Boreal Realm (northwest Germany). The δ13C signal is characterized by values of 0-2‰ for the Barremian, a shift towards more positive data of 4-5‰ in the upper part of the Neohibolites ewaldi belemnite Zone of the Early Aptian and a subsequent decrease to 2‰ in the Neohibolites clava belemnite Zone of early Late Aptian age. The maximum of the positive δ13C excursion is biostratigraphically well constrained by calcareous nannofossils, ammonites and belemnites. The peak of the carbon isotope excursion is located above a distinctive laminated black shale layer, the Fischschiefer, which corresponds to the Oceanic Anoxic Event 1a (= OAE 1a). The Fisch schiefer is sandwiched by the first occurrences of the calcareous nannofossil taxa Rhagodiscus angustus, Braarudosphaera hockwoldensis and Eprolithus apertior/Eprolithus floralis. Macropalaeontologically the Fischschiefer can be assigned to the Deshayesites tenuicostatus ammonite Zone and to the Neohibolites ewaldi Zone. The δ13C findings allow correlation with Tethyan successions, where a similar δ13C excursion has been described from the Selli Level attributed to the OAE 1a. The Boreal data document that the perturbation of the carbon cycle, reflected by the δ13C excursion and the OC-rich Fischschiefer, also affected the Boreal Realm. The good biostratigraphic dating of the studied sections allows the direct calibration of the δ13C excursion in the Boreal Realm. In particular the ammonite age of the OAE 1a and of the associated carbon isotope anomaly is documented. This offers the opportunity to correlate Boreal and Tethyan ammonite zones using the δ13C curve as a chemostratigraphic reference. It is noteworthy that the Boreal δ13C excursion occurs both in the basin centre and the basin margin, the latter represented by an iron-ore facies deposited in extremely shallow coastal waters.

Keywords

barremian/aptian intervalbelemnitesboreal realmnorthwest germanyδ13c excursionstratigraphyoae 1a