从南海看第四纪大洋碳储库的长周期循环

LONG-TERM CYCLES IN CARBON RESERVOIR OF THE QUATERNARY OCEAN：A PERSPECTIVE FROM THE SOUTH CHINA SEA

近年来,长尺度、高分辨率的深海和冰芯记录为第四纪研究提供了新机遇.古气候研究不再局限于单个的冰期旋回,而是延伸到跨越多个冰期的长时间过程（≥10^5年）.1999年大洋钻探184航次在南海所得保存良好的沉积层中,发现更新世稳定碳同位素（δ13C）有3个105年尺度的长周期,属于全球大洋的普遍现象.随后的研究还证实水循环（冰盖变化）和碳循环（生物地球化学变化）中都有这种为期105年的长期过程,进而提出了这两种过程是否相关的问题.研究表明,δ13C和其他生物地球化学地质记录中普遍存在长偏心率周期.第四纪之前海洋δ13C序列中的40万年旋回,可以用＂溶解有机碳假说＂来解释：季风控制的营养物质输送,改变着海水中颗粒有机碳和溶解有机碳的比例（POC/DOC）,进而引起δ13C的40万年周期变化.然而,1.6Ma前的大洋改组将南大洋深水层隔离开来,使得海洋δ13C的40万年长偏心率信号变得模糊.近百万年来,地球气候系统经历了两次大改组,一次是大约90万年前的＂中更新世革命＂（MPT）,一次是约40万年前的＂中布容事件＂（MBE）.而在MPT和MBE事件之前都发生过碳同位素的重值事件（δ13Cmax）,分别为约100万年前的δ13Cmax-Ⅲ和50万年前的δ13Cmax-Ⅱ.如果将相应冰期旋回上类似的水文气候现象联系起来考虑,可以看出无论水文还是生物地球化学的事件,在很大程度上都是由南大洋的海洋变化所驱动的.因此我们认为,起源于南大洋的生物地球化学长周期过程,在第四纪冰盖的消长变化中起着关键作用.

In the recent years, high-resolution long records from the deep-sea and ice-cores have offered new research opportunities for Quaternary science. Paleoclimate studies are no more restricted only to individual glacial cycles but extended to cover long-term （≥ 10^5 yrs） processes across glacial cycles. Ocean Drilling Program （ODP） Leg 184 to the South China Sea in 1999 uncovered well-preserved sediment sections from which three long-term cycles in the Pleistocene carbon isotope （δ13C） sequences have been found and proved to be common to the global ocean. The subsequent discoveries confirm the existence of long-term processes of 10^5 yrs in both the hydrological cycle （ ice- sheet changes） and the carbon cycle （biogeochemical changes）, and pose a question whether the two long-term processes are related. The present review demonstrates that the long eccentricity cycles prevail throughout the δ13C and other biogeochemical records in the geological history, and the 400-ka cycles in the oceanic δ13C sequence before the Quaternary can be hypothetically explained by changes in the ratio between particulate and dissolved organic carbon （POC/DOC） in the ocean depending on the monsoon-controlled nutrient supply, a ＂DOC hypothesis＂. However, the ocean restructuring at 1.6Ma marked by the isolation of a sluggish abyss under the Southern Ocean has obscured the long eccentricity 400-ka signal in the oceanic δ13C. The last million years period has experienced two major changes in the climate regime： the Mid-Pleistocene Transition （MPT） centered at 0.9Ma and the Mid-Brunhes Event （MBE） around 0.4Ma. The MPT and MBE were preluded by δ13C max-Ⅲ about 1.0Ma and δ13C max-II about 0.5Ma, respectively. Together with other similar hydroclimatic phenomena over corresponding glacial cycles, the two groups of hydrological and biogeochemical events appear to have been largely driven by oceanographic changes in the Southern Ocean. Therefore, we interpret that the long-term biogeochemical processes originating from the Southern Ocean must have played a crucial role in the Quaternary ice-sheet waxing and waning.