Sedimentation velocities of various chemical sediments are typically calculated using Stokes's law. However, applying it to chemical sediments that form in situ in the water column is not ideal because the particle properties do not fulfill many of the assumptions underpinning the applicability of Stokes' law. As a consequence, it has been difficult to predict the sedimentation rate of ancient chemical sediments, such as Precambrian banded iron formations (BIF), because their primary sediments likely comprised aggregates of ferric hydroxides, such as ferrihydrite [Fe(OH)3], and marine bacterial biomass, including cyanobacteria. In this work we use a new experimental method to address the mechanisms by which primary BIF sediment, formed by the oxidation of dissolved Fe(II) by O2 and simultaneously incubated with cyanobacterium Synechococcus sp., were deposited to the Archean ocean. Specifically, we formed the aggregates in situ over a wide range of initial pH and Fe(II) concentrations, continuously recorded the entire settling processes of aggregates under each condition, and then processed the data in MATLAB according to different settling mechanisms. Our results demonstrate that ferrihydrite–cyanobacteria aggregates settled to the ocean floor either through the formation of uniformly descending concentration fronts or through convective plumes. The sedimentation mechanism depended on both initial Fe(II) concentration and the pH. Correspondingly, two algorithms were developed to characterize the sedimentation velocity. These algorithms tracked the alteration of light intensity from low to high as sediments descended from an initially homogeneous state through a water tank, and as well calculated the average light intensity over time, from which vertical time series were constructed allowing calculation of the sedimentation velocity. Our method not only provides an accurate estimation of the in situ sedimentation velocity of cell–mineral aggregates, but also provides new insights into the physical mechanisms by which the primary sediments composing BIF were deposited.

中文翻译：

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一种研究（生物）化学沉积物沉积的新方法：蓝藻-水铁矿聚集体的沉积速度

通常使用斯托克斯定律计算各种化学沉积物的沉降速度。但是，将其应用于水柱中原位形成的化学沉积物并不理想，因为颗粒性质不能满足支撑斯托克斯定律适用性的许多假设。结果，很难预测古代化学沉积物的沉积速率，例如前寒武纪带状铁形成物（BIF），因为它们的主要沉积物可能包含氢氧化铁的聚集体，例如亚铁酸盐[Fe（OH）3]，以及海洋细菌生物量，包括蓝细菌。在这项工作中，我们使用一种新的实验方法来探讨主要BIF沉积物的机理，由溶解的Fe（II）被O2氧化而形成并与蓝藻Synechococcus sp。同时孵育的细菌沉积到太古代海洋中。具体而言，我们在很宽的初始pH和Fe（II）浓度范围内原位形成聚集体，连续记录每种条件下聚集体的整个沉降过程，然后根据不同的沉降机理在MATLAB中处理数据。我们的研究结果表明，水铁矿-蓝细菌聚集体通过均匀下降的浓度锋面或对流羽流沉淀到海床。沉淀机理取决于初始Fe（II）浓度和pH值。相应地，开发了两种算法来表征沉降速度。这些算法跟踪沉积物通过水箱从最初的均质状态下降时光强从低到高的变化，还可以计算出随时间变化的平均光强，从中可以构建垂直时间序列，从而可以计算出沉积速度。我们的方法不仅可以准确估算细胞-矿物聚集体的原位沉积速度，而且还可以为构成BIF的主要沉积物沉积的物理机理提供新的见解。