Stable isotopes and palaeo-hydrological implications of Tunisian laminar calcretes during the late Pliocene - early Pleistocene

During the late Pliocene-early Pleistocene Tunisian calcretes were formed in a soil environment. Fabrics at the macro and micro scale show that these deposits are similar to rhizogenic modern analogue systems forming in Spain. We infer that a similar climate prevailed in Tunisia in the past, specifically winter-season rainfall and a dry summer with annual rainfall close to 430 mm/year in the center of Tunisia. Here, we provide further constraints on the climate under which the Tunisian calcrete formed by looking at vegetation structure, water advection and amount of seasonal rainfall.

To investigate (1) the palaeo-vegetation type(s) if they are C3 or C4 plants and (2) the source(s) of meteoric water of Tunisian calcretes during that era, stable isotopes of C and O were analysed in 25 samples taken from 5 horizontal laminar calcretes cores, from 3 sites: North (N36º.43.713; E10º.06.681’), Center (N35º.07.077’; E10º.14.545’), South (N33º.28.898’; E10º.23.602’). Results are expressed relative to the VPDB reference.
In the North, the δ¹⁸O samples show values varying from – 4.78 ‰ to -6.91 ‰. Likewise, central site cores are characterised by δ¹⁸O values ranging from -5.32 ‰ to -6.97 ‰. In contrast, the δ¹⁸O values from the South are more depleted (-8.82 ‰ to -7.20 ‰). Concerning the carbon isotope results, both central and southern sites show similarly enriched δ¹³C values with an average of -6 ‰, while the North site shows more depleted values (-11.3 ‰ to -9.6 ‰). The δ¹⁸O values are similar to those determined in the last deglaciation/early Holocene speleothem carbonates from caves in the Tunisian Atlas Mountains, indicating a North Atlantic source. The north-to-south difference in both isotope systems reveals a decoupling between precipitation δ¹⁸O and vegetation effects. The southern site shows more depleted water isotopes (a consequence of internal water recycling effects) and more enriched δ¹³C, consistent with C4 vegetation and/or lower soil respiration. The central site shows low water recycling, but southern-like dryland vegetation. The northern site shows low water recycling and C3 vegetation and/ or higher soil respiration. Consequently, although there is enhanced humidity in all three sites, the sites do not record the same amount of rainfall and the same response of the landscape to form calcretes.