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 (δ13Ccarb) is complicated because multiple factors affect δ13Ccarb 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 (δ13Corg) from 697 discrete nodules sampled from 119 different depths at the Jiaxian section, North Central China. Between 7.6 and 2.4 Ma, δ13Ccarb values increase by nearly 5‰, while δ13Corg values increase by 2.5‰. These increases are explained by a progressive decline in moisture availability through time, and there is no definitive evidence from these δ13C data for C4 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 δ13Ccarb 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 δ13Ccarb values at Jiaxian, while southern δ13Ccarb 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 δ13Ccarb values being more sensitive to moisture availability under consistently more arid conditions to the NW.Plain Language SummaryThe Loess Plateau region in North Central China currently receives most of its rainfall from the East Asian Summer Monsoon System. However, uncertainty persists about how the monsoon system and regional climate will respond to future anthropogenic-driven climate change. Reconstructions of environmental conditions from the geologic past can inform predictions about future regional environmental change. This study generates a record of past environmental change using carbon isotope data from organic matter and carbonate preserved in fossil soils deposited between 8 and 2 million years ago at the Jiaxian site on the northern edge of the Loess Plateau. These data indicate that, over this time period, plants became more water stressed and less CO2 was respired by roots and soil organisms, which is consistent with the local climate becoming more arid. The first conclusive evidence for warm-season grasses in this record occurred at 3.6 million years ago. We evaluated regional environmental changes by comparing the carbonate isotope data from Jiaxian with data from four other locations across the Loess Plateau. Spatial differences between sites are best explained by soil carbonate records at drier sites being more responsive to any changes in precipitation as compared to the relatively wetter southern sitesKey Points Pedogenic carbonate and organic δ13C values at the Jiaxian site (North Central China) increase throughout Miocene-Pliocene δ13C increase primarily forced by increased aridity over time and definitive evidence of C4 vegetation absent until ∼3.6 Ma Northern Loess Plateau carbon isotope records are more positive and variable than southerly sites throughout the late Miocene and Pliocene