温盐环流反转及其对新生代气候的影响

TRANSITION OF THERMOHALINE CIRCULATION MODES AND ITS IMPACT ON CENOZOIC CLIMATE

温盐环流是由海水温度、盐度差异驱动的全球洋流循环系统。在气候系统中,它对全球热量输送起到了十分重要的作用。在亚轨道尺度(千年时间尺度)上,温盐环流的改变导致了一系列快速的气候变化,因此备受关注。在构造时间尺度(百万年时间尺度)上,古海洋记录和数值模拟揭示出,温盐环流的反转对新生代气候也产生了非常显著的影响。在新生代,温盐环流由"南大洋深层水主控型"向"北大西洋深层水主控型"反转。这一反转改变了全球的热量输送,使得南半球强烈变冷,并有可能导致南极东部永久冰盖的形成。在这一反转事件中,热带海道的作用更加重要。

The thermohaline circulation（THC）is a large-scale ocean circulation driven by global density gradients created by surface heat and freshwater fluxes. It plays an important role in the globule heat transport. Changes of the circulation have large impacts on the Earth climate. They cause a series of abrupt climate variations on the sub-orbital time scale, i. e., the millennial scale. Thus, the thermohaline circulation receives much concern. Here, based on paleoceanographic evidence and numeral modeling,we show that the thermohaline circulation also affect the climate markedly on the tectonic time scale,i, e., the million-year scale. In the Cenozoic, the thermohaline circulation reversed from the southern ocean deep water （SODW）dominated mode to the North Atlantic deep water（NADW） dominated mode,though the time of this reversal is still under debates. Some evidence reveals that the NADW formation was active during a period of the late Early Miocene and dominated ocean circulation after about 15Ma,while the earliest evidence of the NADW formation has been found in deep sea cores of the Early Oligocene age. The recent modeling study simulated the transition of thermohaline circulation from the SODW to the NADW dominated mode. The simulation indicates that the transition to the NADW-dominated thermohaline circulation cause a strong cooling in most of the southern oceans and over East Antarctica. The narrowing/closing of the Tethys Seaway appears to be a key in the transition of ocean circulation from a SODW-dominated mode to the modern ocean circulation dominated by North Atlantic deep water （NADW）. The key role of the Tethys Seaway is also supported by the timing of key geological events,including the evolution of the Tethys Seaway,the variations of climate and the changes of NADW in the Neogene. The narrowing/closing of the Tethys Seaway associated with the transition of ocean circulation modes is potentially important for understanding the mechanism of the Cenozoic cooling, though other processes, for example, the large drop of atmosphere CO2 level,also contribute to the cooling. The linkage between the Tethys seaway and the Cenozoic cooling still needs to be studied further both from the geological and the modeling side.