新元古代氧化事件驱动海洋Ba循环变化

Changes in Oceanic Ba Cycle Driven by the Neoproterozoic Oxygenation Event

新元古代晚期出现大气和海洋氧气含量显著升高的现象,即新元古代氧化事件(NOE)。该事件可能促使了后生动物的出现与辐射以及复杂生态系统的建立。海洋中的氧化过程可改变海水的化学组成,如Fe、C、S的价态和种型,而海水中Ba的生物地球化学循环则主要受S的种型及浓度的控制,特别是硫酸根浓度。文章介绍了NOE如何改变海洋Ba循环:(1)NOE之前,海水硫酸根浓度极低,Ba循环处于保守状态;(2)NOE期间,海水硫酸根浓度升高,处于重晶石过饱和状态,导致沉积物中过剩Ba的富集以及层状重晶石的形成;(3)NOE之后,海洋重晶石过饱和状态一直持续到古生代末期,而中生代以后海洋硫酸根充足,Ba循环主要受控于生产力,呈现非保守性。此外,文章还指出,Ba同位素在重建新元古代晚期海洋Ba浓度,继而反演海水硫酸根浓度(即氧化程度)研究中均具有独特的优势。

The late Neoproterozoic witnessed an increase in the atmospheric and oceanic oxygen levels,namely the Neoproterozoic Oxygenation Event(NOE),likely resulting in the naissance and radiation of metazoans and the establishment of complex ecosystem.Oceanic oxygenation could change oceanic chemistry,such as species and valence states of Fe,C,and S,and the biogeochemical cycle of Ba in the ocean is strongly controlled by the S species and sulfate concentration.This review introduces how the NOE changed the oceanic Ba cycle:(1)Before the NOE,the oceanic sulfate concentration was low and the oceanic Ba cycle was conservative;(2)during the NOE,the oceanic sulfate increase led to excess Ba enrichments in sediments and formation of massive barite deposits;and(3)after the NOE,the ocean kept over-saturated relative to barite until the terminal Paleozoic and the Ba cycle was controlled by biological productivity afterwards.In addition,this review suggests to use Ba isotope system to reconstruct the oceanic Ba concentration,and indirectly to estimate the oceanic sulfate concentration(oxygenation extent)during the late Neoproterozoic.