Oxygen, climate and the chemical evolution of a 1400 million year old tropical marine setting
Oxygen, climate and the chemical evolution of a 1400 million year old tropical marine setting
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14 亿年前热带海洋环境的氧气、气候和化学演化
DOI：
10.2475/08.2017.01
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发表时间：
2017-10
影响因子：
2.9
通讯作者：
Canfield Donald E
中科院分区：
文献类型：
--
作者：
Wang Xiaomei;Zhang Shuichang;Wang Huajian;Bjerrum Christian J;Hammarlund Emma U;Haxen Emma R;Su Jin;Wang Yu;Canfield Donald E
下马岭组是一套保存极为完好的沉积序列，形成于14亿年前的海洋被动边缘。我们使用了多代理的方法，包括铁的形态，微量金属动力学和有机地球化学，探索海洋化学的演变，通过大部分的下马岭组。这种演化是在下马岭沉积体系古地理和沉积演化的背景下进行的。根据沉积学和地球化学标准，将下马岭组非正式地划分为6个单元。在这六个单元中，我们充分探索了其中的四个。第4单元是我们研究的最低单元，由深水红色泥浆组成，周期性地被绿色淤泥和桑迪浊积岩打断。铁的提取结果表明，红泥富含高活性铁，表明铁的水柱来源。然而，低有机碳含量，低氢指数（HI）值，和氧化的活性铁池的性质表明在氧化的底部沃茨沉积。我们解释单元4代表一个低生产力的含铁最小含氧区（OMZ）的环境，下伏含氧的底部沃茨。向单元3的过渡反映了初级生产力的增加，以及支持缺氧光合细菌的更具生物活性的OMZ的发展。尽管如此，在这个单元中，底部的沃茨仍然是含氧的。上覆单元2代表沉积物表面向深水脱氧和缺氧沃茨的过渡。这些沃茨在单元底部为含铁水体，在单元顶部为硫化物水体（富氧水体）。在最上面的单元1中，缺氧条件继续，被朝向单元上部的更频繁的水柱氧化打断。我们把这些化学动力学的演变的气候和气候变化的背景下，特别是，下马岭组的热带辐合带（ITCZ）和由此产生的Hadley细胞动力学的位置。此外，虽然我们的研究结果表明，缺氧水柱条件高的水柱在夏马岭组沉积过程中的持久性，他们也证明了近海底的底层水氧化过程中的四个单位的沉积，并在一个时间间隔延长到10的数百万年。
The Xiamaling Formation is an exceptionally well-preserved sedimentary succession deposited on a marine passive margin about 1400 million years ago. We used a multi-proxy approach, including iron speciation, trace metal dynamics, and organic geochemistry, to explore the evolution of ocean chemistry through most of the Xiamaling Formation. This evolution is put in the context of the paleogeography and the sedimentological evolution of the Xiamaling depositional system. Overall, the Xiamaling Formation is informally divided into six units based on both sedimentological and geochemical criteria. Of the six units, we fully explored four of them. Unit 4, the lowest unit we studied, is comprised of deep-water red muds, periodically interrupted by green-colored silt and sandy turbidites. Iron extraction results show that the red muds are enriched in highly reactive iron, indicating a water-column source for the iron. However, the low organic carbon contents, low hydrogen index (HI) values, and the oxidized nature of the reactive iron pool indicate deposition in oxygenated bottom waters. We interpret unit 4 to represent a low-productivity ferruginous oxygen-minimum zone (OMZ) environment, underlain by oxygenated bottom waters. The transition to unit 3 reflects an increase in primary productivity, and the development of a more biologically active OMZ, that supported anoxygenic phototrophic bacteria. Still, in this unit, the bottom waters remained oxygenated. The overlying unit 2 represents the transition to deep-water deoxygenation and anoxic waters at the sediment surface. These waters were ferruginous in the bottom part of the unit and sulfidic (euxinic) towards the top. In the uppermost unit 1, euxinic conditions continued, punctuated by more frequent water-column oxygenation towards the upper part of the unit. We place the evolution of these chemical dynamics in the context of climate and climate change, and in particular, the placement of the Xiamaling Formation in relation to the Intertropical Convergence Zone (ITCZ) and the resulting Hadley Cell dynamics. Also, while our results demonstrate the persistence of anoxic water-column conditions high in the water column during the deposition of the Xiamaling Formation, they also demonstrate bottom water oxygenation near the seafloor during the deposition of three of the four units, and over a time interval extending to 10's of millions of years.
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