New chronological constraints for the Plio-Pleistocene section of Dhar Iroumyane (Morocco) and the associated fossil locality of Guefaït-4

Abstract

We provide new numerical age constraints to the Plio-Pleistocene section of Dhar Iroumyane (Morocco) and the associated fossil locality of Guefaït-4, which has yielded an exceptionally rich and diverse faunal assemblage, through a combination of Electron Spin Resonance (ESR) and Terrestrial Cosmogenic Nuclides (TCN) dating of quartz. While challenging in many aspects, the application of these dating methods to the same deposits located about 5 m below the fossiliferous horizon returns consistent numerical dating results within respective uncertainties. More specifically, the age overlap enables to propose a combined ESR-TCN age of 2.87 ± 0.11 Ma (1σ), which may be regarded a maximum age constraint for Guefaït-4 from a chronostratigraphic point of view. These results allow us to narrow down the options of chronological interpretation proposed in a previous study by Parés et al. (2023), by supporting a correlation of the magnetic reversal initially identified a few meters above the site to the Gauss-Matuyama transition (2.61 Ma). A final age estimate of ∼2.7 Ma may be inferred for the fossil horizon through a series of sensitivity tests based on sedimentation rates. With this approach, the normal polarity interval identified in the upper part of the sequence is most likely correlated to the Olduvai Subchron (1.93–1.78 Ma), and the two short reversals in the lowermost deposits with dominantly normal polarity may be reasonably associated to the intra-Gauss Mammoth (3.33–3.21 Ma) and Kaena (3.12–3.03 Ma) Subchrons. Consequently, the present work help position Guefaït-4 as a key Late Pliocene fossil locality in N Africa for the establishment of a robust regional Plio-Pleistocene biochronology, although further work is required in the future to increase the chronological resolution along the sedimentary sequence. While we do acknowledge the intrinsic uncertainty associated with the ESR and TCN dating results (including the limited number of samples processed in the present study), these encouraging results nevertheless illustrate the interest of using a multi-technique approach to constrain Plio-Pleistocene deposits, and the importance of combining numerical dating with magnetostratigraphy and biostratigraphy/biochronology in non-volcanic sedimentary context. Such approach also contributes to a better understanding of the potential and limitations of each dating method, and helps providing critical insights into the avenues worth exploring in the future from a methodological and sampling point of view in order to ensure future successful ESR and TCN dating applications beyond 2 Ma.