Terrestrial and oceanic responses in the northern hemisphere during the Eocene-Oligocene transition

Abstract

As one of the most significant climate events during the Cenozoic era, the Eocene to Oligocene transition (EOT) marks the Earth entering into an icehouse word. However, the climatic responses of Northern and Southern Hemispheres to the EOT are suggested to be asymmetric and the thermal asymmetry was related to the heat transport change, which was likely driven by the strengthening of the Atlantic meridional overturning circulation (AMOC). Here we reconstruct three high-resolution records of the EOT, with two terrestrial records at low and middle latitudes in the Northern Hemisphere respectively, and an ocean record in the North Atlantic to investigate whether the AMOC imprint could be identified in the Northern Hemisphere. The terrestrial records from the Maoming Basin, southeast China, show that the proxies based on n-alkane and bulk 13C shifted abruptly across the EOT. The increasing Paq and decreasing CPI indicates that the environment shifted from lacustrine to delta conditions and water level reduced abruptly at low latitude Asia during the transition, which appears to correspond to Antarctic glaciations. Further, the terrestrial records from the Qaidam Basin, northwest China, show that the carbonate oxygen isotopes and the content of calcium carbonate increase during the EOT, suggesting that the aridification occurred at middle latitude Asia. The reducing moisture at both low and middle latitudes in Asia corresponded to the rapid cooling in the Southern Hemisphere, but was different from the gradual and delayed cooling trend in the North Atlantic. We suggest that the AMOC imprint across the EOT had not propagated into Asia. Comparing with the above terrestrial records from the two Asia localities, ocean records from IODP Site U1411 in the North Atlantic are not entirely consistent with the rapid cooling signal during the EOT. Our GDGT proxies document a gradual cooling trend and enhanced ocean mixing from the late Eocene to the early Oligocene, which are consistent with the surface ocean temperature (SST) cooling trend from another Site U1404 in the North Atlantic. The similar trends of these two sites suggest that the ocean circulation change provided more heat to the North Atlantic and further demonstrate the enhancement of AMOC during the EOT. Despite its enhancement during the EOT, the AMOC imprint appears to be constrained in the North Atlantic and have not propagated to Asian continent at the time.