Deciphering paleoclimatic signals: Organic carbon isotope signatures in terrestrial higher plants and loess from the Iranian Loess Plateau

The stable carbon isotopic composition of bulk soil organic matter (δ13Corg) in paleosols is a key proxy for reconstructing paleovegetation and paleoclimate. While extensive δ13Corg-based reconstructions exist for regions like west-central Europe and monsoonal Asia, detailed studies are lacking for the western arid Central Asia (ACA) loess belt—a critical transitional zone between European loess, the Chinese Loess Plateau (CLP), and eastern Xinjiang loess. This gap limits the proxy’s robustness in this area. The Northeastern Iranian Loess Plateau (ILP), located within the westerlies domain at ACA’s western margin, features a pronounced precipitation gradient (~100 km) with minimal temperature variation, providing an ideal setting for such research. To address previous limitations (narrow precipitation range, lack of vegetation δ13C data), we collected 47 surface soil samples across an extended precipitation gradient and 27 dominant vegetation samples from corresponding sites. We systematically analyzed δ13Corg in vegetation, plant litter, and bulk soils to understand its variability and paleoclimatic significance, establishing a robust modern analog. Results show: (1) Vegetation δ13C values (-31.07‰ to -19.26‰, mean -24.60‰) confirm pure/dominant C3 vegetation; (2) New surface soil δ13Corg values (-28.46‰ to -22.78‰, mean -25.99‰) primarily reflect C3 plants; (3) Combining new and published soil data (n = 91) reveals a significant negative correlation between δ13Corg and mean annual precipitation (MAP) (δ13Corg = -0.0048 * MAP - 23.5905; r = -0.6803, p