Looking for orbital signals in stable isotopes of carbon and oxygen of belemnites from the Early Jurassic in the Basque Cantabrian basin through the smoothed Lomb-Scargle periodogram

Abstract

The use of time series of stable isotopes obtained from fossilized samples of macrofossils, such as belemnites from the early Jurassic, until now, have not been used to explore whether these paleoindicators may have recorded some kind of periodic events related with the orbital cycles of the Earth. This, despite the fact that they are one of the few existing geological records for the early Jurassic. For this reason, and taking into account several of the limitations of these types of data sets, in this work, it is used a powerful spectral analysis technique known as the smoothed Lomb-Scargle periodogram, which can be applied directly to non-equispaced time series contaminated by noise. Furthermore, this work takes into account the dating errors of the belemnite samples in the estimation of the spectrum. The objective of this paper is to identify the presence of orbital cycles that have been recorded in time series of d18O and d13C from samples of belemnites collected in the Basque-Cantabrian basin. Special emphasis is placed on the search for the one of the eccentricity cycles, such as the long-period eccentricity cycle located ca. 400 Ka or the short-period cycle located ca. 100 Ka, since the 400 Ka cycle is known to have remained unchanged from the Mesozoic to the present and since both cycles have been identified in stratigraphic records of the Mesozoic collected in different geographic areas. However, despite the rigorousness of our analyzes, the long-period signal was not found, although we have located one ca. 109 Ka, which might be related with one the of cycles of short-period of the eccentricity of the Earth. © 2021 CSIC.