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

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 &lt; 0.001), but no significant correlation with temperature, consistent with vegetation response. Global comparisons (6938 soils, 4488 C3 plants, 416 C4 plants) place ILP values within the global range while highlighting distinct regional precipitation-δ13Corg relationships. Applying this δ13Corg-MAP correlation to the well-dated Yellibadragh (YE) loess profile (ILP), we quantitatively reconstructed Holocene MAP. It indicates relatively arid early Holocene conditions, followed by gradually increasing precipitation from ~7.9 ka. Comparison with ACA and monsoonal CLP records supports the “Westerlies-dominated Climatic Regime” (WDCR) hypothesis and extends its western boundary based on terrestrial evidence. Preliminary analysis of δ13Corg changes along the vegetation-litter-soil organic matter (SOM) continuum revealed complex variations without statistically significant patterns, likely influenced by factors like lignin content, C4 decomposition rates, and sample size. Key unresolved questions regarding SOM decomposition, isotope fractionation mechanisms, and structure complicate precise regional fractionation value determination. Furthermore, Anthropocene alterations—rising atmospheric CO2 (favoring C3 growth), global warming (potentially favoring C4 physiology), and intensified human activities (e.g. deforestation)—challenge direct application of modern δ13Corg-climate relationships to paleoreconstructions. Rigorous cross-validation using multiple indicators, sites, proxies, and models therefore remains essential.