Millennial to Centennial Cyclicity Within the Exoreic Saline System of Boujmel, Southern Tunisia

Introduction

Cyclostratigraphy is a branch of stratigraphy dealing with the interpretation of cyclic variations in the stratigraphic record. The most important, and also the first to be found out, of these cycles are the Earth's orbital cycles of precession, obliquity, and eccentricity (Milankovitch cycles), which result from perturbations of the Earth's orbit and its rotational axis. These cycles translate (via orbital-induced changes in insolation) into climatic, oceanographic, sedimentary, and biological changes that are potentially recorded in the sedimentary archives through geologic time. Nonetheless, the Milancovitch Theory explaining the ancient cyclostratigraphy does not file a case for the Holocene cyclostratigraphy. For instance, the decadal to millennial scales cycles of the Late Holocene sediment in lakes and sebkhas recorded based on geophysical and geochemical parameters were possibly not explained by this Theory. Recent studies (e.g., Baumgarten and Wonik 2015; Timothy and Swindles, 2014; Martinez-Ruiz et al., 2015) have highlighted other astronomical (such as the solar activity) and oceanographic (such as the thermohaline circulation) causes (e.g., Pérez-Asensio et al., 2020) coupled with atmospheric dynamics (Shimizu et al., 2020) to explain short climatic cycles.

As a matter of fact, the Sun activity and its effect on the climatic cyclicity have gained univocal popularity. The short term cyclicity of Sun activity is inferred through sunspots amountand activity. The Schawbe cycle (9 to 14 yr with an average of 11 yr) (Deng et al., 2015; Miyake et al., 2015; Ahluwalia et al., 2015) is governed by the Sun internal dynamics whereas the 22 yr Hale cycle is linked to solar convection (Sun et al., 2015). The Gleissberg cycle (87 yr) is due to the variability in Sun diameter (Kim et al., 2015). Other cycles such as that of Vriès (240yr) are still insufficiently explained (Herrera et al., 2015). These cycles are directly seen through telescopic sunspots and historical naked-eye sunspots observations of solar activity. Other proxies including the cosmogenic isotope ¹⁰Be, cosmogenic isotope ¹⁴C and the climate record allows inferring the longer cyclicity of the Sun activity (Herrera et al., 2015 and references therein). Nonetheless, the climatic cyclicities are not always related to the Sun activity. Instead, other oceanographic and atmospheric mechanisms and processes created hybrid cycles (Martinez-Ruiz et al., 2015). For instance, in the westernmost Mediterranean Sea, major periodicities at 1300 yr and 1515 yr, linked tothe North Atlantic climate variability and the African monsoon system were found out along the past 20,000 yr. Other major periodicities of 2000 yr and 5000 yr are related to the solar activity. Minor secondary millennial to centennial cycles stretching along 650 yr, 1087 yr, and 3000 yr are still debatable (Rodrigo-Gámiz et al., 2014). Cyclostratigraphy proves vital to give precision to ages found by absolute and relative dating techniques through the astrochronological calibration. Astrochronology is the dating of sedimentary units by calibration with astronomically tuned timescales, such as Milankovitch cycles (Krijgsman et al., 2001; Meyers, 2008) and sunspot cycles (Fairbridge, 1996; Liritzis & Fairbridge, 2003). When used in concern with radiometric dating, it allows the resolution of timescales to a high degree of accuracy. In the case of Milankovitch cyclostratigraphy, the dating error can be lower than 21000 years. As for the application of astrochronology for Holocene sediments, the use of astronomic cycles seems promising to give precision to radiocarbon dating and setting limit of some recent global modification on the planet such as the Anthropocene and the Great Acceleration.