Untangling the Palaeocene climatic rhythm: an astronomically calibrated Early Palaeocene magnetostratigraphy and biostratigraphy at Zumaia (Basque basin, northern Spain)

Abstract

The magnetostratigraphy of a 54-m-long section above the Cretaceous–Tertiary boundary at the sea-cliff section of Zumaia in the Basque basin (northern Spain) has been established. The section encompasses the entire Danian and the lower part of the Selandian stages as indicated by calcareous plankton biostratigraphy. The studied interval consists of (hemi)pelagic limestone–marl alternations in the form of couplets and bundles, which range from centimetre/decimetre to metre scale respectively and a few thin-bedded calcareous turbidites. The magnetostratigraphy, based on samples from about 200 stratigraphic levels, allows the identification of six reversal boundaries from chron C29r to C26r at a bed level. The spatial (or temporal) evolution of periodicities from a lithologically coded series is studied with the continuous wavelet transform technique. A preliminary age model based on the standard CK95 GPTS indicates that the basic lithologic carbonate–marl couplet corresponds to the 19–23-kyr precession cycle (21–31-cm cycle in the depth domain) and that a bundle cycle (usually groups of four to six basic couplets) with global periodicity centred at 1.22 m corresponds to the ∼110-kyr eccentricity cycle. We have tuned the bundle cycles to the Va03_R7 eccentricity orbital solution [Astrophys. J. 592 (2003) 620–630] following an initial match of a node of the ∼2.4-Ma eccentricity modulatory cycle in the target time series to particularly carbonate-rich bundles from the upper part of the Zumaia section that displays significant power of a 4.4-m-period cycle corresponding to the ∼404-kyr eccentricity cycle. Consistency between lithologic patterns and characteristics in the eccentricity target is reasonably met although the ∼404-kyr eccentricity cycle is not persistent throughout. The tuning, however, appears robust as it brings the age of the K/T boundary at ∼65.8 Ma. It is argued that a sea-level signal (tectonically driven?) is superimposed on the climatic forcing at the Milankovitch band masking the full expression of the low-frequency astronomical periods. We provide a cycle-tuned duration for all intervening Early Palaeocene polarity chrons and estimate relative ages for bioevents. The cycle-tuned chronology indicates that the CK95 GPTS overestimates the duration of chrons C28 and C27 by 20 and 26% respectively. Our data may prove useful to better constrain Early Palaeocene biostratigraphy of calcareous plankton and in the redefinition of the boundary between the Danian and Selandian stages.

Introduction

The tuning of primarily marine cyclic sedimentary successions to orbital solutions for the recent past has proved to be a powerful geochronometer (see [1] for a recent review). Milankovitch climate cyclicity tied to magnetic polarity stratigraphy for 0–5.23 Ma has resulted in an astronomical polarity time scale (APTS) accepted as the definitive chronology for the Pliocene and Pleistocene, superseding other chronostratigraphic frameworks (i.e. high-precision radiometric dating is actually calibrated against the APTS). Extensions of the astronomical calibration to the late Miocene (5.23 to ∼10 Ma) and older periods are increasingly being published [2], [3]. Developing orbitally tuned time scales in strata older than the limit of precise astronomical solutions for the Earth’s orbital variations (estimated to be 35–50 Ma in [4]) has so far been risky. Nevertheless, refinements in long-term integrations of planetary orbits make it possible to extend the accuracy of astronomical time series to increasingly older ages [5]. In any case, it is possible to build up an orbital chronology, albeit floating, if the more stable eccentricity modulation of precession can be identified in the geological record [1], [4], [6].

The aim of this paper is to present a detailed magnetostratigraphy and biostratigraphy along the cyclic hemipelagic lower Palaeocene succession in the Zumaia section from the Basque basin, northern Spain, and to assess the Milankovitch climate forcing on the lithologic stacking pattern to derive an Early Palaeocene cyclochronology.