Enrichment effect and environmental control of clay reactive iron in the Changjiang River estuary and East China Sea
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摘要:
铁元素的化学相态分析是深入理解沉积物的来源、环境演化以及铁参与的生物地球化学循环的关键手段，但不同粒级沉积物中铁化学相态研究薄弱，制约了表生铁循环的研究认知。本文选择长江口-东海陆架表层沉积物，通过六步提取法分析沉积物全样及其黏土组分中总铁（FeT）、高活性铁（FeHR）、弱活性铁（FePR）和不活性铁（FeU）的含量，含量均遵循FeHR＞FePR＞FeU。全样中FeT和FeHR的含量与平均粒径、黏土、有机碳和铝含量密切相关，表明富含有机质的黏土矿物易于富集高活性铁；相较于全样，黏土组分中FeHR/FeT比值升高10%，而FePR/FeT比值则降低10%，反映黏土组分对高活性铁的富集效应。河口动力环境基本控制沉积物中Fe的相态分布，长江口最大浑浊带沉积物全样中FeT和FeHR含量较高，且受粒度的影响显著；黏土组分可以显著消除粒度效应的影响，FeT和FeHR被大量截留在最大浑浊带前缘的河口低盐度区域；而在中高盐度的口外区域，Fe的来源相对稳定，主要为富FeHR的长江源和贫FeHR的陆架源沉积物混合。本研究揭示黏土组分在流域-河口-陆架的迁移可能主导了高活性Fe在陆海界面的分布和循环过程，这对深入理解入海颗粒态Fe的源汇过程、地球化学循环及其环境效应有重要参考价值。
Abstract:Chemical speciation analysis of iron (Fe) is a crucial method for understanding sediment provenance, environmental evolution, and the biogeochemical cycling of iron in various environments. However, there are limitations in studying iron speciation, especially in sediments in different grain sizes, which hinders the comprehensive understanding of the iron cycle. In this study, we focused on the surface sediments from the Changjiang River estuary to East China Sea shelf. We employed a six-step extraction method to obtain the concentrations of total Fe (FeT), highly reactive Fe (FeHR), poorly reactive Fe (FePR), and unreactive Fe (FeU) in both bulk sediment samples and their clay fractions. Results show an order of FeHR＞FePR＞FeU in abundance. FeT and FeHR contents in the bulk sample were closely related to the mean grain size and the concentrations of clay, TOC, and Al, indicating that clay minerals rich in organic matter are prone to enrich FeHR. The FeHR/FeT ratio in the clay fraction increased by 10% and the FePR/FeT ratio in the clay fraction decreased by 10% compared to the bulk sample, indicating an enrichment effect of FeHR on clay minerals. The dynamic estuarine environment controlled the distribution of Fe speciation in sediments, with higher FeT and FeHR contents observed in the bulk sediment samples from the turbidity maximum zone of the Changjiang River estuary, which significantly influenced by grain size. The clay fraction could effectively eliminate the influence of grain size, with FeT and FeHR being heavily retained in the low-salinity region at the forefront of the turbidity maximum zone, while in the medium to high-salinity offshore areas, the sources of Fe remained relatively stable, being mainly the mixture of FeHR-rich sediment from the Changjiang River and FeHR-poor sediment from the shelf. This study revealed that the migration of clay fractions from the watershed to the estuary and shelf might dominate the distribution and cycling of highly reactivity Fe at the land-sea interface, and provided important insights into the sources and sinks of particulate Fe in the ocean, geochemical cycling, and their environmental effects.
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Keywords:
- iron speciation /
- clay fraction /
- enrichment effect /
- Changjiang River estuary /
- East China Sea
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图 1 长江口-东海陆架表层沉积物采样站位图（C断面）
Figure 1. Sampling locations of surface sediments in the Changjiang River estuary and East China Sea along the C transect
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图 3 底层水体物理化学性质（a、b）、表层沉积物的粒径组成和平均粒径（c）以及烧失量（LOI）和总有机碳（TOC）含量（d）的空间变化
Figure 3. Spatial variations of bottom water physicochemical properties (a-b), grain size composition and mean grain size (c), and LOI and TOC content (d) of the surface sediments
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图 4 沉积物全样（a）及黏土组分中（b）FeT含量以及FeHR、FePR和FeU贡献比例
Figure 4. Partitioning of FeT, FeHR, FePR, and FeU in the bulk sample (a) and clay fraction (b) of the surface sediments
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图 5 沉积物全样总Fe（FeT）和高活性Fe（FeHR）含量与平均粒径、黏土含量、TOC以及Al含量之间的相关关系
虚线圆代表低盐度和高盐度区域（站位C1至C6、C14至C18），圆外为中盐度区域（站位C6-1至C13），d图中的拟合曲线不包括C1站位。
Figure 5. Comparison of FeT and FeHR content with mean grain size (a), content of clay (b), TOC (c), and Al (d) of the bulk sediments
The dashed ellipses in the figure represent low salinity and high salinity zones (stations C1 to C6 and C14 to C18), while the others represent medium salinity zone (stations C6-1 to C13), and the fitting curve in panel (d) does not include the station C1.
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图 6 沉积物全样和黏土组分中FeHR（a）和FePR（b）与Al含量的相关关系，以及FeHR/FeT（c）和FePR/FeT（d）与FeT含量的相关关系
Figure 6. The correlation between FeHR (a) and FePR (b) with Al content, and between FeHR/FeT (c) and FePR/FeT (d) with FeT in both bulk and clay fraction of the surface sediments
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图 7 长江口-东海陆架表层沉积物FeT（a）和FeHR（b）与Al含量的相关关系
全球河流颗粒物的拟合线来自Poulton和Raiswell[3].
Figure 7. Correlation plot of FeT vs. Al (a) and FeHR vs. Al (b) for the surface sediments in the Changjiang River estuary and East China Sea
Data sources: this study and literature[12-14,22]. The regression of global riverine particulates is from Poulton and Raiswell[3].
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图 8 沉积物全样和黏土组分中FeT（a）、FeHR（b）、FeHR/FeT（c）和FeT/Al（d）沿C断面变化特征
沉积物平均粒径和底层水溶解氧（DO）含量分别绘于图（a、b）和图（c、d）中，蓝色五角星代表长江悬浮物平均值，红色五角星代表东海陆架沉积物平均值，其Fe相态数据来源见表1，灰色区域为最大浑浊带（TMZ），图c中虚线为有氧条件下海洋沉积物FeHR/FeT的上限阈值（0.38）[1,31]，图d中虚线为有氧条件下现代海洋沉积物FeT/Al的范围0.55±0.11（1σ）[32]，蓝色区域为陆源碎屑物质FeT/Al的范围0.47±0.05（1σ）[33]。
Figure 8. Spatial variations of FeT, FeHR, FeHR/FeT, and FeT/Al in both bulk and clay fraction of the surface sediments along the C transect
The mean grain size of sediment is plotted in panel (a) and (b). The dissolved oxygen content (DO) in the bottom water is plotted in panel (c) and (d). The blue star represents suspended sediment of the Changjiang River, and the red star represents East China Sea shelf sediment. The data source of Fe speciation is shown in Table 1. The gray area is turbidity maximum zone (TMZ). The dashed line in panel (c) represents the upper threshold FeHR/FeT ratio of marine sediments under oxic water column conditions (0.38)[1,31]，The dashed line in panel (d) is the range in FeT/Al for average oxic modern marine sediments, 0.55±0.11 (1σ)[32]. The blue area is the range in FeT/Al for terrestrial materials, 0.47±0.05 (1σ)[33].
表 1 长江口-东海陆架表层沉积物全样及黏土组分Fe相态分析结果
Table 1 Fe speciation analyses results of bulk sample and clay fraction of surface sediments in the Changjiang River estuary and East China Sea
样品名称 位置 水深
/m全样 黏土 东经 北纬 FeHR
/%FePR
/%FeU
/%FeT
/%Al
/%FeHR/FeT FeT/Al FeHR
/%FePR
/%FeU
/%FeT
/%Al
/%FeHR/FeT FeT/Al C1 121.10° 31.77° 12.5 2.03 1.03 2.09 5.15 5.01 0.39 1.03 5.07 1.01 1.45 7.54 12.8 0.67 0.59 C2 121.31° 31.61° 18.5 1.34 0.80 0.49 2.64 5.91 0.51 0.45 4.62 1.48 0.84 6.94 13.2 0.67 0.53 C3 121.57° 31.40° 8.0 1.10 0.67 0.42 2.20 5.30 0.50 0.41 5.09 1.73 0.86 7.69 12.3 0.66 0.63 C5 121.75° 31.29° 16.8 1.88 1.97 0.62 4.47 9.50 0.42 0.47 3.25 2.00 0.82 6.07 12.6 0.53 0.48 C6 121.95° 31.12° 8.5 2.07 1.43 0.66 4.16 8.68 0.50 0.48 3.55 1.90 0.84 6.29 12.9 0.56 0.49 C6-1 122.04° 31.07° 6.0 2.08 1.37 0.79 4.25 8.68 0.49 0.49 3.64 1.75 0.73 6.11 12.7 0.59 0.48 C7 122.16° 31.03° 8.7 2.37 1.38 0.78 4.53 9.68 0.52 0.47 3.40 1.33 1.37 6.11 12.8 0.56 0.48 C8 122.25° 31.02° 8.5 1.92 1.40 0.59 3.91 8.56 0.49 0.46 3.81 1.74 0.78 6.32 12.7 0.60 0.50 C9 122.37° 31.00° 10.7 1.88 1.29 0.52 3.69 8.11 0.51 0.46 3.69 1.52 0.95 6.16 12.7 0.60 0.48 C10 122.45° 30.97° 12.0 1.61 1.25 0.55 3.41 7.62 0.47 0.45 3.44 1.03 1.67 6.14 12.6 0.56 0.49 C11 122.62° 30.92° 20.0 1.67 1.41 0.60 3.68 8.09 0.45 0.46 3.36 1.46 1.44 6.26 12.3 0.54 0.51 C12 122.74° 30.94° 22.2 1.95 1.69 0.45 4.09 9.08 0.48 0.45 3.40 1.76 0.83 5.99 12.3 0.57 0.49 C13 122.89° 30.80° 32.7 1.79 1.47 0.55 3.81 8.53 0.47 0.45 3.27 1.82 0.75 5.83 12.2 0.56 0.48 C14 123.26° 30.67° 58.4 1.12 0.88 0.41 2.40 5.84 0.47 0.41 3.06 1.08 1.69 5.83 12.4 0.53 0.47 C15 123.50° 30.51° 56.6 1.09 0.88 0.45 2.42 5.53 0.45 0.44 3.01 1.12 1.51 5.64 12.3 0.53 0.46 C16 124.00° 30.29° 50.0 1.02 0.89 0.48 2.40 5.57 0.43 0.43 2.79 1.87 0.68 5.35 12.1 0.52 0.44 C18 124.49° 30.08° 53.2 1.04 0.86 0.50 2.39 5.79 0.43 0.41 2.93 1.72 0.86 5.51 11.9 0.53 0.46 本研究平均值 – – – 1.64 1.22 0.64 3.51 7.38 0.47 0.48 3.61 1.55 1.06 6.22 12.5 0.58 0.50 标准偏差 – – – 0.44 0.35 0.39 0.92 1.65 0.04 0.14 0.69 0.33 0.36 0.63 0.33 0.05 0.05 长江悬浮物[22] – – – 2.30 0.93 1.92 5.15 9.83 0.45 0.52 – – – – – – – 标准偏差 – – – 0.21 0.20 0.09 0.16 0.92 0.03 0.05 – – – – – – – 全球河流颗粒物[2-3] – – – 2.09 1.21 1.49 4.81 5.82 0.43 0.61 – – – – – – – 标准偏差 – – – 0.08 0.05 0.06 0.19 2.79 0.03 0.17 – – – – – – – 东海陆架沉积物[12-14] 0.99 1.37 0.86 3.23 – 0.30 0.61 标准偏差 0.39 0.29 0.29 0.72 – 0.07 0.10 大陆边缘沉积物[1] – – – 1.03 0.84 1.83 3.69 – 0.28 – – – – – – – – 标准偏差 – – – 0.40 0.26 0.53 0.91 – 0.06 – – – – – – – – 注：“–”代表无数据。 
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