长江口-东海陆架黏土中高活性铁的富集效应及环境控制机制

Enrichment effect and environmental control of clay reactive iron in the Changjiang River estuary and East China Sea

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(Fe_(PR))和不活性铁(Fe_(U))的含量,含量均遵循Fe_(HR)>Fe_(PR)>Fe_(U)。全样中Fe_(T)和Fe_(HR)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Fe_(HR)/Fe_(T)比值升高10%,而Fe_(PR)/Fe_(T)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Fe_(T)和Fe_(HR)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,Fe_(T)和Fe_(HR)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Fe_(HR)的长江源和贫Fe_(HR)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。

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(Fe_(T)),highly reactive Fe(Fe_(HR)),poorly reactive Fe(Fe_(PR)),and unreactive Fe(FeU)in both bulk sediment samples and their clay fractions.Results show an order of Fe_(HR)>Fe_(PR)>FeU in abundance.Fe_(T) and Fe_(HR) 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 Fe_(HR)/Fe_(T) 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 Fe_(T) 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 Fe_(HR)-rich sediment from the Changjiang River and Fe_(HR)-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.