High-resolution carbonate isotope records of Holocene soil water balance and ecosystem dynamics in northern Iran loess

Abstract Stable carbon isotopes in Holocene soils provide key insights into past climate and ecosystems. This study presents high-resolution isotope analyses of pedogenic carbonates and organic carbon from modern loess-derived soils in northern Iran across a strong precipitation gradient (150–850 mm mean annual precipitation [MAP]). Eight soil profiles span five soil orders: Alfisols, Mollisols, Inceptisols, Aridisols, and Entisols. The mean δ 13 Cpc values show strong linear relationships with MAP (δ 13 Cpc = −0.0093 × MAP + 1.8878; R 2 = 0.98) and with the ratio of precipitation to potential evapotranspiration, P/PET (δ 13 Cpc = −8.6842 × P/PET + 1.608; R 2 = 0.99), indicating that δ 13 Cpc reliably reflects moisture availability during carbonate formation. Values range from −6.2‰ in wetter sites to −0.1‰ in dry areas, reflecting changes in soil respiration and CO₂ flux. δ 13 Coc values (−25.6‰ to −23.3‰) indicate dominant C₃ vegetation and exhibit a bimodal response to precipitation, increasing from arid to semiarid zones and decreasing in wetter forests. Oxygen isotopes in carbonate (δ 1 ⁸Opc = −7.9‰ to −6.6‰) show limited climate sensitivity, reflecting precipitation mainly from the Caspian Sea with minimal evaporative enrichment. Overall, δ 13 Cpc, δ 13 Coc, and δ 1 ⁸Opc provide robust proxies for soil moisture, vegetation structure, and water sources, supporting paleoenvironmental reconstruction in loess systems.