Abstract
The stable carbon isotopic composition of bulk organic matter (δ13Corg) in paleosols has been widely used as a proxy indicator for reconstructing past vegetation and climate. Previous studies generally show a negative correlation between the δ13Corg values of modern C3 plants and surface soils under C3 plant-dominated ecosystems and mean annual precipitation (MAP). However, the relationship between δ13Corg and MAP varies among different climatic regimes, resulting in uncertainties in paleo-precipitation reconstructions. In this study, we analyzed the δ13Corg values of surface soil samples collected along a north-south climatic gradient in northern Iran, in the western part of arid central Asia (ACA). Our aims were to explore the relationship between surface soil δ13Corg values and climatic factors; to use it to develop a transfer function for ACA; and then to apply the transfer function to a well-dated loess-paleosol sequence (AB1) to reconstruct early Pleistocene (2.4–1.8 Ma) MAP in northern Iran. The results show that: (1) the δ13Corg values of 44 surface soil samples range from −27.56‰ to −23.61‰, with an average of −25.77‰, indicating that the modern natural ecosystem in northern Iran is dominated by C3 vegetation; (2) The δ13Corg values of the surface soil samples are strongly negatively correlated with MAP (y = −0.0079x − 22.8418, R2 = 0.4419, p < 0.001), with a coefficient of −0.79‰/100 mm. We infer that the effects of mean annual temperature (MAT) and elevation on the relationship are minimal; (3) Reconstructed early Pleistocene MAP in northern Iran was ∼649 mm, much wetter than the present day (∼331 mm); (4) The loess δ13Corg values from ACA exhibit similar trends to records from monsoonal Asia, suggesting a similar history of Quaternary climate change between ACA and monsoonal Asia, which is likely related to the growth and decay of Northern Hemisphere ice sheets.
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