4-13 Pedogenic Carbonate as a Reliable Proxy for Soil Moisture Balance and Ecosystem Dynamics: High-Resolution Isotope Analysis of Holocene Loess Soils in Northern Iran

Session: Recent Advances in Soil and Paleosol Science

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Stable carbon isotopes in Holocene soils offer essential insights into past climate and ecosystem changes. This research provides high-resolution stable isotope data from pedogenic carbonate and related organic carbon in modern loess-derived soils of northern Iran, located in the southeastern Caspian lowlands. The main objective was to identify the key environmental factors influencing δ¹³C variation in pedogenic carbonate (δ¹³Cpc) and organic carbon (δ¹³Coc). Eight representative soil profiles were thoroughly sampled across a steep north-south precipitation gradient (150–850 mm, Mean Annual Precipitation, MAP) over approximately 70 km. The soils encompass multiple orders in Soil Taxonomy—Alfisols, Mollisols, Inceptisols, Aridisols, and Entisols—reflecting substantial climatic and geomorphic variation along the transect.

The δ¹³Cpc values display a strong linear relationship with MAP (δ¹³Cpc = -0.0093 × MAP + 1.8878; R² = 0.98) and precipitation to evapotranspiration ratio, P/PET (δ¹³Cpc = -8.6842 × P/PET + 1.608; R² = 0.99), confirming its usefulness as a proxy for soil moisture during carbonate formation. Values range from -6.2‰ in wetter southern profiles to -0.1‰ in drier northern soils, indicating shifts in soil respiration and CO₂ flux across the gradient.

In contrast, δ¹³Coc values (-25.6‰ to -23.3‰) indicate consistent C₃ vegetation dominance but show a non-linear pattern with precipitation—initially rising from the driest sites to around 500 mm MAP, then decreasing in the wetter forested zone. This pattern matches regional vegetation data, suggesting that δ¹³Coc is a sensitive indicator for canopy structure, plant physiology, and ecosystem composition.

δ¹⁸Opc values range narrowly from -7.9‰ to -6.6‰ and have a weak link to climate factors, suggesting they form from stable soil water primarily originating from Caspian Sea precipitation with little evaporative enrichment.

These findings highlight δ¹³Cpc as a dependable proxy for hydrology, δ¹³Coc as a marker of vegetation changes, and δ¹⁸Opc as an indicator of moisture origins in the region. Collectively, they provide a detailed framework to reconstruct Holocene rainfall and ecological shifts in loess areas, with broader significance for soil carbon dynamics and land use resilience in semi-arid zones.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

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