SOIL CARBONATE RECORD OF LATE MIOCENE AND PLIOCENE ENVIRONMENTAL CHANGE FROM THE JIAXIAN RED CLAY SECTION, NORTHERN LOESS PLATEAU, CHINA

The Chinese Loess Plateau (CLP) hosts some of the most continuous and high resolution terrestrial records of Neogene climate change. At the Jiaxian Red Clay section we collected paleosol carbonate nodules nearly continuously at 25 cm resolution through 60 m of section deposited during the late Miocene and Pliocene in order to investigate changes in vegetation, ecosystem productivity and atmospheric CO₂. We measured the stable carbon isotope compositions of both calcite and organic matter occluded within the nodules. The δ¹³C values of the calcite and the occluded organic matter generally increase from 7.5 to 2.5 Ma (δ¹³C calcite from -8 to -2 ‰ and δ¹³C organic matter from -25 to -22 ‰). The difference between δ¹³C values of calcite and occluded organic matter (∆¹³C) also increases across this interval from 17 to 20 ‰. The δ¹³C values of occluded organic matter are consistent with nearly pure C₃ vegetation at this location during the late Miocene and Pliocene, and the observed shift to less negative values upsection may indicate an increase in aridity throughout this time interval. The increase in ∆¹³C values is also consistent with a drying-induced decrease in soil respiration or an increase in atmospheric CO₂ concentrations. In order to constrain changes in soil respiration rates, we also measured boron concentrations in the nodular calcite. B concentrations of the nodular calcite [B] measured thus far vary smoothly with maxima at 7 and 3.5 Ma and a minimum at 6 Ma. The increase in [B] from 6 to 3.5 Ma is consistent with a drying-induced decrease of soil respiration rates. We conclude that substantial coverage by C₄ vegetation in northern CLP did not occur until the Pleistocene, despite the increase in aridity observed during the late Neogene.