High-resolution cyclochronology of the lowermost Ypresian Arnakatxa section (Basque-Cantabrian basin, western Pyrenees)

Abstract

The early Eocene paleoclimate record provides one of the best analogues for today’s 9 global warming. In order to reconstruct the evolution of the early Eocene paleoclimate, 10 and understand how environmental feedback mechanisms acted on it, an accurate time 11 framework is necessary. In this regard, the astronomically calibrated time scale (ATS) 12 provides the highest possible resolution, but models beyond 40-50 Ma are not fully 13 resolved and actual geological data are incomplete. With the aim of filling this gap, the 14 expanded lower Ypresian Arnakatxa section studied herein offered a potentially 15 valuable orbitally paced geological record. This outcrop displays a well-defined 16 arrangement of strata in couplets and bundles. The spectral analyses of colour data 17 series showed the dominance of two main periodicities, which were related to orbital 18 forcing on sedimentation by precession (20 kyr) and short (100 kyr) eccentricity cycles. 19 Despite not being represented in the spectrograms, the influence of long (405 kyr) 20 eccentricity on sedimentation was also deduced. Moreover, the disruption of the orbital 21 signal in the upper half of the Arnakatxa section correlates with a very long (2.4 Myr) 22 eccentricity node centred at ~54.6 Ma, which could also have caused the amplification 23 of the orbital signal related to obliquity (41 kyr). Taking everything into account, the 24 cyclostratigraphic analyses carried out in Arnakatxa resulted in a precessional scale orbital chronology for the time interval between 55.805 and 54.435 Ma (duration of 26 1.37 Myr). Thus, the Arnakatxa succession could be reliably correlated with Atlantic 27 ODP records, which are the main reference for Ypresian astrochronology, at 28 precessional scale. Furthermore, the results from Arnakatxa also help to identify the 29 astronomical solutions that better match actual geological data, contributing to the 30 construction of a definitive Ypresian ATS. In this regard, the Arnakatxa results are not a 31 good fit for solutions La10a, La10d, La11 and ZB18, but match well with the previously 32 thought to be less reliable solutions La10b and La10c.