Astronomically forced climate evolution in a saline lake record of the middle Eocene to Oligocene, Jianghan Basin, China

摘要

Following the early Eocene climatic optimum some 50 million years ago, the marine paleoclimate record indicates that a long interval of global cooling took place, punctuated by a series of short-term warming reversals and culminating with the glaciation of Antarctica at the Eocene-Oligocene transition. We investigate a saline lake record from the Qianjiang and Jinghezhen formations, Jianghan Basin, China for the corresponding continental climate response. A 20.25 Myr long astronomical time scale is constructed based on 405-kyr orbital eccentricity cycle tuning of gamma ray (GR) series measured from the Qianjiang and Jinghezhen formations. The halite-rich interval between 41-40.4 Ma correlates well with the Middle Eocene Climate Optimum recorded in the deep-sea delta O-18 record, confirming predominantly halite deposition during the warming. Silt-mudstone-rich intervals were deposited during similar to 2.4 Myr orbital eccentricity minima. The evidence indicates a history of alternating fresh water (humid/cool) and saline water (dry/hot) lake cycles paced by similar to 100 kyr orbital eccentricity cycles. Analysis of GR series from the deepest part of the lake indicates strong astronomical forcing of halite deposition throughout the Middle-Late Eocene, and a shift to siliciclastic deposition and cooler climates at similar to 33.9 Ma. Early Oligocene wet/cool climate in the Jianghan Basin signals East Asian summer monsoon intensification, and a reduction from northwest winter monsoon in response to Tibetan Plateau uplift. Northern Hemisphere summer insolation minima in the Middle-Late Eocene with temperatures that are 4.6 degrees C higher than present may provide an analogy for near-future climate change in Jianghan Basin. (C) 2019 Elsevier B.V. All rights reserved.