白垩纪大洋缺氧事件:研究进展与未来展望

Cretaceous Oceanic Anoxic Events:Research Progress and Forthcoming Prospects

白垩纪古海洋中分别在Aptian-Albian、Cenomanian-Turonian和Coniacian-Santonian期间发生多次大洋缺氧事件(Oceanic Anoxic Events,OAEs),分别被称为OAE1、OAE2和OAE3,而OAE1又可以划分为OAE1a,1b,1c和1d四次,其中OAE1a和OAE2达到全球规模。白垩纪大洋缺氧事件一般对应海相碳酸盐岩的碳同位素正偏、海洋生物的快速更替和富有机质黑色页岩的大规模分布,指示了大范围的古环境、古气候变化。虽然历次大洋缺氧事件在成因上具有一定的差异,但是一般认为其与白垩纪时期海底大规模火成岩省的活动有关。大规模火成岩省活动将带来大气-海洋系统中CO2的上升,地表风化和水循环的加速,从而造成海洋中营养元素的过剩输入,引发海洋生产力的升高,诱发海洋中缺氧条件的发生。大洋缺氧条件下,由于碳-氧-硫-磷等元素之间的耦合循环关系,在大洋缺氧和海洋生产力之间形成快速的正反馈关系,有可能是白垩纪大洋缺氧事件得以达到全球规模的促进因素(如OAE1a和OAE2)。不同时期的大洋缺氧事件对白垩纪大气-海洋系统的发展具有不同的意义:OAE1a标志着白垩纪中期极端温室气候的开始,OAE2前后是温室气候的高潮时期,而OAE3的发生则伴随白垩纪温室气候的减弱。由于历次大洋缺氧事件产生的大规模有机碳的埋藏,对大气-海洋系统产生了深刻的影响,其主要作用是导致大气海洋系统中CO2下降、O2上升,以及伴随而来的全球变冷和海洋氧化能力的增强。虽已历经30多年的研究,白垩纪大洋缺氧事件的前因和后果还未完全阐明,后续研究中将继续对重点的缺氧事件如OAE1(a,b,c,d)、OAE2和OAE3,开展多学科的研究,获取缺氧事件发生期间的古大气CO2浓度、海水温度、营养状况、生物种类及其海平面变化等方面的信息。大洋缺氧事件的陆相响应方面的研究也已经陆续开展,我国科学家可望在这方面有所作为。

Oceanic Anoxic Events （OAEs） occurred during Aptian Albian, Cenomanian Turonian boundary and Coniacian Santonian, namely the OAE1, 2 and 3, with the OAE1 being further divided as OAEla, lb, lc and ld. But only the OAEla and OAE2 reached to a global scale with others being limited in the Atlantic and Tethyal realm. Cretaceous OAEs usually corresponded to the positive excursion of carbon stable isotope in the marine carbonates, rapid turnover of marine biota and large scale distribution of organic carbon enriched black shales, which indicated wide ranges of changes in paleoenvironment and paleoclimate. Though the genesis of each OAE was somehow different, it is usually thought that Cretaceous OAEs may have been related to the igneous activities from the Large Igneous Provinces （LIPs）. The CO2 content would have increased; chemical weathering and hydrological cycle would have been accelerated with the higher input of nutrients to ocean as the results of igneous activities of LIPs occurred at certain times of Cretaceous. The ocean would have got productive and prone to become anoxic in the deep water. The rapid positive feedbacks between the ocean primary productivity and ocean anoxia, due to the coupling of carbon, phosphorus, oxygen and sulfur cycle, would have contribute to the expansion of ocean anoxia to a global scale （for example the OAEla and 2. The different OAEs might have different implications for the evolution of Cretaceous ocean-atmospheric system, the OAEla marked the beginning, OAE2 signified the peak, and OAE3 indicated the decline of the greenhouse climate during Cretaceous. Due to enormous burial of organic carbon, the OAEs would have much effect on the atmospheric/oceanic system such as the decrease of CO2 and increase of O2, accompanied by the global cooling and enhancement of oxidation potential of the deep ocean. The causes, processes, and consequences of OAEs still need to be elucidated though more than 30 years has passed since it was put forward for the first time. Multidisciplinary studies are to be done for these OAE1 （a, b, c, d）, 2 and 3, to get the important information on the CO2 contents, seawater temperature, nutrient status, the change of sea level and biota when they occurred. The terrestrial response to the OAEs has also been studied in succession. Chinese scientists are expected to contribute much to this issue.