Cooling-driven climate change at 12-11 Ma: Multiproxy records from a long fluviolacustrine sequence at Guyuan, Ningxia, China

To explore the evolution of climate through the Miocene, especially during the Middle Miocene climate transition, multiproxy continental records were analyzed by susceptibility (SUS), lightness (L*), total inorganic carbon (TIC) and total organic carbon (TOC) content from the 2900-m-thick fluviolacustrine sediment sequence at Guyuan, Ningxia, China and compared with the redness (a*) and the pollen humidity indices from the same section. The published Miocene records of sedimentation rate, sediment grain size, biota turnover, and palaeoproductivity from other regions were also compiled and compared. Results show that multiproxies of the Sikouzi section have changed substantially since 12-11 Ma ago. SUS, L* and TIC have increased while a*, TOC and the pollen humidity index decreased, all implying that the palaeoclimate in the study area has got cooler and drier since that time. This climate change also left imprints in many other regions, probably resulting from global cooling and the development of the East Antarctic Ice Sheet since about 14 Ma. Several positive feedback mechanisms are proposed to have effectively modulated and magnified the Mid-Miocene global cooling, including vegetation change, greenhouse gas (atmospheric CO2 and water vapor) fluctuations. They led to the strengthing of ocean and atmospheric circulation, significant lowering of sea level and increasing aridification over the globe, especially in middle-high latitudes. Since 12- 11 Ma ago, a continuously cooler and drier climate may have caused the substantial intensification of physical weathering and exhumation on the earth's surface, the increased sediment accumulation rates on the land and in the sea, and the inception of productivity increase in the oceans. Nevertheless, when and where these mechanisms operated and how they were tied to each other remain ambiguous, deserving further investigation.