Isotopic tracers of the origin of Ca in a carbonate crust from the Middle Atlas, Morocco

Carbonate crusts developed on basalt were studied at Tazouta in the Moroccan Middle Atlas. The chosen representative weathering profile is 2.8 m thick and is composed from base to top of a fresh basalt overlain by a coherent saprolite, a friable saprolite and a surface soil. Carbonate crust genesis occurs by replacement of primary and secondary clay minerals by low Mg calcite. The 14C dates of the calcite range from 15 to 14 ka in the coherent saprolite, and from 12 to 8.5 ka in the friable saprolite. tic weathering and replacement of Si-bearing minerals by carbonate are coupled processes leading to a strong calcium net gain (300 kg/m3) within the friable saprolite. The low weathering rate of basalt compared to the calcrete genesis rate, and the high mobility of calcium in supergene weathering mantles, limit the basaltic Ca contribution to calcrete genesis. The contribution of the Triassic sediment is also limited by the mineralogical unavailability of Ca. The 87Sr/86Sr ratios confirm the low contribution of basalt and indicate an external origin of Ca. The Sr isotope ratios of calcrete are different from those of the adjacent Liassic carbonates and are close to rainfall values. rbon isotope compositions of carbonates (−8‰ to −9‰ vs. PDB) imply a significant contribution of C3-dominated organic carbon during carbonate precipitation. Oxygen isotopic compositions of carbonates (average −5.24‰ vs. PDB), show that carbonates are 18O-enriched compared to meteoric waters (−4‰ to −3.5‰ vs. SMOW).