Middle to late Miocene Middle Eastern climate from stable oxygen and carbon isotope data, southern Alborz mountains, N Iran

Abstract

The Alborz mountains of northern Iran intercept and divert the northern hemisphere westerlies carrying moisture from the Mediterranean and Black Sea, and form an orographic barrier to moisture sourced to the north in the Caspian Sea. This implies that terrestrial deposits along the southern Alborz mountains (leeward side of northerly winds and windward side of westerlies with the respect to Tibet and other mountainous terrain in the Himalayan–Karakoram realm) potentially track changes in past moisture and erosional regimes and mirror rainfall patterns. Here, we present results of a stable isotope analysis and clay mineral study of the Miocene (ca. 17.5–7.6 Ma) Upper Red Formation in the foreland of the southern Alborz mountains. The changes recorded by stable oxygen and carbon isotope data from pedogenic and lacustrine/palustrine carbonate in the southern Alborz mountains suggest: 1) an increase in aridity possibly related to the evolution of the Alborz orographic rain shadow, which became more efficient between 17.5 and 13.2 Ma; 2) a steady increase in precipitation between 13.2 and 10.3 Ma with a significant increase in rainout along the southern slope of the Alborz mountains between 11 and 10.3 Ma, possibly related to perturbations in atmospheric circulation pattern in the northern hemisphere as suggested by coeval wetter phases in southern Europe; and 3) a decrease in aridity from ca. 9.6 to 7.6 Ma, possibly reflecting an increase in the seasonality of precipitation. Based on environmental and climatic changes observed across southern Asia and India starting from ca. 10 Ma, we speculate that the topographic evolution of the Himalayan–Tibetan system might have affected the late Miocene climate in Middle East.