Continental shelves are important land–ocean interfaces where natural and anthropogenic processes as well as sediment geochemistry regulate organic matter sequestration. The major objective of this work is to understand spatial variability in reactive iron (rFe) bound organic carbon (OC) during low‐ and high‐flow conditions in the recent sediments of the Mississippi River‐influenced Louisiana shelf in the Northern Gulf of Mexico. Sediment samples were collected from five sites along the river plume during low (August 2016) and high (May 2017) discharge. Reactive iron was significantly higher in May (6.9 ± 1.7 mg g⁻¹_dw) compared to August (4.3 ± 0.9 mg g⁻¹_dw), while OC in August (11.9 ± 3.7 mg g⁻¹_dw) and May (7.8 ± 4.8 mg g⁻¹_dw) was similar. The fraction of OC bound to rFe (Fe‐OC) in the sediment in May (2.4 ± 1.5 mg g⁻¹_dw) was not significantly different from August (3.7 ± 2.7 mg g⁻¹_dw). Molar OC:Fe ratios in the rFe‐associated OC ranged between 0.9 and 6.2, with higher ratios in the deeper sediments (6–20 cm). Our results show that 31.7% ± 6.2% of the total OC in the recent sediments is associated with the rFe phase. Sediment mineralogical composition was characterized using X‐ray diffractometry and Fe‐K edge X‐ray absorption near edge structure spectroscopy. Our findings indicate that most of the iron is present as nonreactive iron in clay mineral smectite and the dominant form of rFe associated with OC is goethite.

中文翻译：

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密西西比河影响的架子沉积物中铁介导的有机物保护

大陆架是重要的陆海界面，自然和人为过程以及沉积物地球化学调节着有机物的固存。这项工作的主要目的是了解在墨西哥湾北部受密西西比河影响的路易斯安那架子的近期沉积物中，在低流量和高流量条件下，活性铁（rFe）结合的有机碳（OC）的空间变异性。在低流量（2016年8月）和高流量（2017年5月）期间，从沿河羽流的五个位置采集沉积物样本。与8月（4.3±0.9 mg g ^-1 _dw）相比，5月的活性铁（6.9±1.7 mg g ^-1 _dw）显着更高，而8月的OC（11.9±3.7 mg g ^-1 _dw））和May（7.8±4.8 mg g ^-1 _dw）相似。5月沉积物中与rFe（Fe-OC）结合的OC分数（2.4±1.5 mg g ^-1 _dw）与8月（3.7±2.7 mg g ^-1 _dw）相比无显着差异）。与rFe相关的OC中的OC：Fe摩尔比在0.9到6.2之间，在较深的沉积物中（6-20 cm）比率更高。我们的结果表明，最近沉积物中总OC的31.7％±6.2％与rFe相有关。沉积物的矿物学组成通过X射线衍射法和Fe-K边缘X射线吸收近边缘结构光谱进行了表征。我们的发现表明，大多数铁以非反应性铁的形式存在于粘土矿物蒙脱石中，与OC相关的rFe的主要形式为针铁矿。