古海洋活性铁循环研究进展及对白垩纪缺氧-富氧沉积转变的启示

Progress of the study of reactive iron cycling in the paleo-ocean and its applications to the genesis of Cretaceous anoxic-oxic sedimentary transition

铁作为地壳中丰度最高的元素之一,广泛参与到一系列地球化学循环中。现代海洋中的铁主要来源于河流、冰川和风的铁氧化物颗粒和溶解铁的输入。陆源输入的铁氧化物在有机质埋藏、降解的早期成岩作用过程中,发生一系列转化过程而埋藏下来,该过程被称作活性铁循环。氧化-强氧化条件利于沉积物中氧化铁的持续产生或者至少保持不被溶解的状态,从而形成棕色—红色沉积物;还原条件利于沉积物中铁氧化物的溶解,形成菱铁矿、黄铁矿(铁硫化物)等形式的埋藏,并可能造成溶解铁在海洋内的迁移。Raiswell、Can-field、Poulton等通过对现代典型海洋环境活性铁循环研究,提出了一系列用于判别古海洋氧化-还原条件的活性铁指标体系,并成功地将太古宙以来的古海洋划分成为含铁的大洋、硫化的大洋和氧化的大洋等3个演化阶段。由于活性铁的不同形态对磷具有不同的生物地球化学效应,将造成"氧化条件下磷的优先埋藏、缺氧条件下优先释放的现象"。磷是海洋生产力的限制性元素,铁和磷循环的上述耦合关系将造成"缺氧的大洋生产力越高,富氧的大洋生产力越低"现象的出现。目前已在白垩纪古海洋缺氧-富氧沉积中初步证实了上述反馈关系的存在,但是对活性铁埋藏形式对该特殊沉积的贡献还需要进一步的工作。

Iron,as one of the most abundant elements in the Earth crust,cycles within a series of geochemical processes.The iron in the modern ocean mostly comes from riverine,glacial and aeolian by the input of iron oxide particles and dissolved iron ions.The iron oxides will change their form and finally be buried during the early diagenetic process characterized by the burial and degradation of organic matter,which is called the reactive iron cycling.Iron oxides would continue to be formed or at least not to be dissolved under oxic conditions,and the sediments in brown to red color will then be dominant.While in a reducing environment,iron oxides will be dissolved,with the possible migration of iron ion in the ocean,and then buried as siderite,pyrite,or other iron-bearing minerals.A series of iron proxies have been suggested to distinguish the redox condition of the paleo-ocean,by studying iron cycling in the modern ocean.On the basis of this methodology,scientists have reconstructed the redox history of paleo-ocean since the Archaean as a sequential appearance of the ferrous,sulfidic and oxic ocean.Owing to the adsorption of phosphorus onto the iron oxides,and the inability of reducing iron-bearing minerals to combine with the phosphate ion,more phosphorus will be buried in the sediments under oxidized conditions;while a reducing environment will enhance the release of phosphorus from the sediments.Because the phosphorus is the constraining element for the marine production,the ocean will become more productive under reducing conditions;but less organic matter will be produced under more-oxic environment.Such feedback mechanism has been preliminarily proved in the deposition of Cretaceous anoxic and more-oxic sediments.But more works are still needed to elucidate the contribution of different forms of iron burial to the occurrence of such special sediments.