Regional Patterns in Miocene-Pliocene Aridity Across the Chinese Loess Plateau Revealed by High Resolution Records of Paleosol Carbonate and Occluded Organic Matter

Paleosols preserved in the Red Clay depositional sequence of the Chinese Loess Plateau record information about vegetation and regional hydrology responses to global temperature variation throughout the late Miocene and Pliocene. Reconstructing spatial and temporal patterns of environmental change across the Loess Plateau from carbon isotopes of pedogenic carbonate (delta C-13(carb)) is complicated because multiple factors affect delta C-13(carb) values and higher resolution records do not exist along the northern margin of the Loess Plateau. To address these needs, we present paired carbon isotope records of pedogenic carbonate and occluded organic matter (delta C-13(org)) from 697 discrete nodules sampled from 119 different depths at the Jiaxian section, North Central China. Between 7.6 and 2.4 Ma, delta C-13(carb) values increase by nearly 5 parts per thousand, while delta C-13(org) values increase by 2.5 parts per thousand. These increases are explained by a progressive decline in moisture availability through time, and there is no definitive evidence from these delta C-13 data for C-4 vegetation at the Jiaxian site until after 3.6 Ma. Comparison of the Jiaxian record to other Loess Plateau sections reveals a consistent spatial gradient with delta C-13(carb) values becoming higher and more variable to the N-NW. Additionally, an independent index of monsoonal precipitation from a southern site corresponds to fluctuations in delta C-13(carb) values at Jiaxian, while southern delta C-13(carb) records remain more stable. These spatial patterns are explained by a progressive decline in moisture availability across the Loess Plateau through the Late Miocene and Pliocene, with delta C-13(carb) values being more sensitive to moisture availability under consistently more arid conditions to the NW.