Phytoplankton contributions to the trace-element composition of Precambrian banded iron formations
Phytoplankton contributions to the trace-element composition of Precambrian banded iron formations
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DOI：
10.1130/b31648.1
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发表时间：
2018-05-01
影响因子：
4.9
通讯作者：
Lalonde, Stefan V.
中科院分区：
文献类型：
--
作者：
Konhauser, Kurt O.;Robbins, Leslie J.;Lalonde, Stefan V.
带状铁矿层是地球前寒武纪岩石记录中具有重要经济意义的沉积矿床，由交替的富铁层（赤铁矿、磁铁矿和菱铁矿）和硅酸盐/碳酸盐层（石英、粘土矿物、白云石和铁白云石）组成。根据对西澳大利亚哈默斯利群 2.48 Ga Dales 峡谷成员带状铁地层单元的化学分析，之前有人提出，带状铁地层中的大多数（如果不是全部）铁可能被缺氧光养细菌（光铁营养菌）氧化，其细胞密度远低于现代富铁水环境中发现的细胞密度。然而，产氧浮游植物也可能能够提供必要的氧化能力。在这里，我们重新审视缺氧和含氧浮游植物种群的问题，以解释带状铁形成沉积，并量化可能与其生物量相关的选定微量元素（P、Mn、Co、Ni、Cu、Zn、Mo、Cd）的量。以戴尔斯峡谷部分的扩展地球化学数据集为例，我们发现，周转时间与现代生态系统中的周转时间相当，形成带状铁层所需的相同浮游植物种群可以提供在这个富铁矿床中发现的全部微量元素。此外，由于带状铁形成和缺氧浮游植物微量元素化学计量之间的相似性，我们认为带状铁形成中保存的大部分微量元素库存在某个时刻在水柱中被生物同化，从海底和沉积物堆中降解的光铁营养生物量中释放出来，并最终通过水铁矿的近定量吸附以大约化学计量比例固定在富铁沉积物中。我们的观察表明，与今天一样，浮游植物及其生物量的循环利用控制了古代海水和沉积物的微量元素组成。
Banded iron formations are economically important sedimentary deposits in Earth's Precambrian rock record, consisting of alternating iron-rich (hematite, magnetite, and siderite) and silicate/carbonate (quartz, claylike minerals, dolomite, and ankerite) layers. Based on chemical analyses from banded iron formation units of the 2.48 Ga Dales Gorge Member of the Hamersley Group in Western Australia, it has been previously suggested that most, if not all, of the iron in banded iron formations could have been oxidized by anoxygenic phototrophic bacteria (photoferrotrophs) at cell densities considerably less than those found in modern iron-rich aqueous environments. However, oxygen-producing phytoplankton may have also been capable of supplying the necessary oxidizing power. Here, we revisit the question of the anoxygenic and oxygenic phytoplankton populations necessary to account for banded iron formation deposition and quantify the amount of selected trace elements (P, Mn, Co, Ni, Cu, Zn, Mo, Cd) that could have been associated with their biomass. Using an expanded geochemical data set for the Dales Gorge Member as an example, we find that with turnover times comparable to those seen in modern ecosystems, the same phytoplankton populations required to form banded iron formations could have supplied the entirety of trace elements found in this iron-rich deposit. Further, spurred by the similarities between banded iron formation and anoxygenic phytoplankton trace-element stoichiometries, we suggest that much of the trace-element inventory preserved in the banded iron formation was at some point biologically assimilated in the water column, released from degrading photoferrotrophic biomass at the seafloor and in the sediment pile, and ultimately fixed in the iron-rich sediment in approximately stoichiometric proportions by near-quantitative adsorption to ferrihydrite. Our observations suggest that, as today, phytoplankton and the recycling of their biomass exerted control over the trace-element composition of ancient seawater and sediment.