What Caused the Greening of Kuwait: An Isotopic Investigation of the Source of Moisture During Quaternary Pluvial Periods in Kuwait

Unlike the modern hyper-arid conditions across most of the Arabian Peninsula, the region experienced pronounced humid phases during the Quaternary, supporting a dense vegetation cover. Evidence for these humid periods and the associated greening is documented in regional geological and paleoenvironmental records, including speleothems, lake-level reconstructions, lacustrine sediment sequences, and the presence of soil carbonate. However, the timing, extent, and moisture sources of these humid phases remain poorly constrained in the northern Arabian Peninsula, particularly in the area now occupied by Kuwait. There is evidence that the area once experienced wetter intervals, but it is unclear whether they were driven by incursions of a Tropical Ocean monsoon (summer) or by enhanced Mediterranean (winter) westerlies.

This study investigates the nature of Quaternary humid conditions in Kuwait using petrographic, mineralogical, and oxygen- and carbon-isotope analyses (δ¹⁸O and δ¹³C) of relict pedogenic and paleosol carbonates. A total of 84 soil samples were collected across 21 sites in Kuwait, targeting calcic and petrocalcic horizons. Petrographic thin sections show progressive stages of carbonate development from stages I to III. Stage III carbonates are older and have δ¹⁸O values that cluster between −12 ‰ and +3 ‰ (VPDB) and δ¹³C values between −9 ‰ and 0 ‰. These ranges reflect the coevolution of soil moisture sources and vegetation types. During monsoon-influenced intervals, long-distance moisture transport and the amount effect produce isotopically light rainfall, resulting in carbonates with more depleted δ¹⁸O values. In contrast, carbonates have more isotopically enriched δ¹⁸O values during periods influenced by Mediterranean winter westerlies. The δ¹⁸O values of stage III soil carbonate suggest moisture sourced from both the tropical monsoon and the Mediterranean. Lower δ¹³C values reflect the contribution of soil-respired CO₂ from C₃ plants, whereas higher δ¹³C values reflect a greater contribution of C₄ plants. The δ¹³C values of stage III soil carbonate in Kuwait clearly reflect humid phases sourced from the tropical monsoon and supporting C₄ vegetation, as well as winter rainfall from the Mediterranean and supporting C₃ vegetation. The determination that Kuwait has experienced wetter conditions in the past from both tropical and Mediterranean sources is important for determining potential future precipitation amounts.