The Lutetian global stratotype section and point at Gorrondatxe revisited: Biomagnetostratigraphic refinements and astronomical tuning

Abstract

Biomagnetostratigraphic studies carried out at the Ypresian/Lutetian (Eocene) Gorrondatxe section solved major chronostratigraphic calibration discrepancies that had existed for several decades. The new calibration resulted in the definition of the Global Stratotype Section and Point (GSSP) for the Lutetian Stage at the Gorrondatxe layer containing the lowest occurrence (LO) of the calcareous nannofossil Blackites inflatus (base of Subzone CP12b). However, subsequent studies cast some doubt on the reliability of the Gorrondatxe biomagnetostratigraphic data and, consequently, the appropriateness of the criterion used for the definition of the Lutetian GSSP. In order to address the issues raised, the biomagnetostratigraphy of the Gorrondatxe section was revisited and an additional cyclostratigraphic analysis was undertaken using magnetic susceptibility data series. The present study shows that the Gorrondatxe biomagnetostratigraphy is reliable. The spectral analyses showed the dominance of precession (~20 ky) and short (~100 ky) eccentricity forcing on sedimentation, but the influence of obliquity (~38 and ~55 ky) cycles was also detected and modulation by long (405 ky) eccentricity cycles was deduced. The succession was tuned to the 49.122-48.084 Ma interval and the astronomical ages obtained for a significant number of biomagnetostratigraphic events compared well with those derived from other studies. One of the exceptions is the LO of B. inflatus, which seems to be older in Gorrondatxe than in the Mediterranean and Western Atlantic areas. This diachrony most likely also delayed the LO of B. inflatus in historical reference sections of the North Sea area, rendering previous claims that the Lutetian GSSP caused some historical Lutetian sections to be Ypresian in age invalid. In fact, the definition of the base of the Lutetian Stage by the LO of B. inflatus at Gorrondatxe, now astronomically age dated at 48.455 Ma, best guarantees that the original concepts of the Ypresian and Lutetian historical stratotypes are maintained. Combined with secondary biomagnetostratigraphic marker events and an astronomically tuned cyclostratigraphy, the Ypresian/Lutetian Gorrondatxe section provides the globally significant chronostratigraphic reference model expected of any GSSP.