Iron geochemistry in surface sediments of a temperate semi-enclosed bay, North China
Introduction
Iron (Fe) is the most abundant redox-sensitive metal on Earth's surface. Its redox cycling in aquatic environments has profound influences on cycling and fate of carbon, sulfur, phosphorus, and a variety of trace elements (Hamilton-Taylor and Price, 1983, Burdige, 1993, Otero et al., 2009, Taylor and Konhauser, 2011). Solid-phase Fe occurs in various phases in the natural environment: ions adsorbed to mineral surfaces, amorphous or crystalline Fe(II, III) (oxyhydr)oxides, structurally coordinated Fe in silicate minerals, Fe(II, III) phosphates, Fe(II) sulfides, and Fe(II) carbonates (Haese et al., 1997, Whiteley and Pearce, 2003, Hyacinthe and van Cappellen, 2004, Poulton and Canfield, 2005, März et al., 2012). In marine surface sediments underlying oxic water, secondary Fe(III) oxides, hydroxides, and oxyhydroxides (collectively referred to as Fe(III) oxides hereafter) are usually the most important components of reactive Fe pool. But they have a wide spectrum of mineralogy, crystallinity, morphology and chemical composition, and, therefore, display quite variable reactivity in both microbially mediated reduction (Hines et al., 1991, Thamdrup, 2000, Canfield et al., 2005, Hyacinthe et al., 2006) and abiotic reduction by dissolved sulfide. For instance, in abiotic reductions half-lives for amorphous and poorly crystalline Fe oxides are hours or days, but weeks for well crystalline Fe oxides (Canfield, 1989, Poulton et al., 2004, Jones et al., 2009). Intensive cycling of Fe is commonly observed at the redox interface of organic matter (OM)-rich sediments, and usually highly reactive Fe(III) (Fe(III)HR) is preferentially involved in the cycling. Here Fe(III)HR refers to Fe(III)-bearing minerals that are highly reactive towards reaction with free sulfide under anoxic conditions (Canfield, 1989, Canfield et al., 1992, Poulton et al., 2004), and total highly reactive Fe (FeHR) is defined as the sum of Fe(III)HR and Fe(II), assuming that Fe(II) is the product of reduction of previous Fe(III)HR.
Different reactivity and pool sizes of various Fe phases in coastal marine sediments are a combined result of terrestrial inputs and post-depositional diagenesis, which are, in turn, influenced by a multitude of factors, such as sedimentation rate, sediment composition, redox conditions, and quantity and quality of OM (Canfield, 1989, Raiswell and Canfield, 2012). Comprehensive Fe speciation based on chemical extraction and kinetic dissolution are two commonly used methods to differentiate various Fe pools and to deduce Fe(III) oxide reactivity. Concentration, spatial distribution, and (de)coupling of various Fe pools may provide insights into geochemical processes/properties in relation to depositional conditions, such as enrichment/depletion of FeHR, redox conditions of water column and ancient/modern sediments, diagenetic reactions of Fe and sulfur, and even anthropogenic Fe inputs (Lyons and Severmann, 2006, Taylor et al., 2007, Attri et al., 2011, Scholz et al., 2014).
In this contribution, Fe geochemistry in surface sediments of semi-enclosed Jiaozhou Bay was investigated using both selective chemical extraction and kinetic dissolution. One objective of this study is to reveal biogeochemical characteristics of sedimentary Fe and potential impacts of excess inputs of nutrients and industrial/domestic wastewaters on sediment records in the semi-enclosed bay. Another objective is to examine whether FeHR is also enriched in the temperate semi-enclosed bay as in semi-protected settings of the wet-tropical regions (Ku et al., 2008), by comparison with the tropical counterparts and also with the East China Sea (ECS), one of the world's largest shelf seas. The comparison is helpful to reveal whether FeHR enrichment is a common nature of all semi-enclosed bays, regardless of their geographic conditions. This is of geochemical importance because this feature, if proven true, should be considered in global budgets of FeHR (Ku et al., 2008).
Section snippets
Study area and sampling
Semi-enclosed Jiaozhou Bay is surrounded by Qingdao City and linked by a narrow channel (maximum width: 3.1 km) to the Yellow Sea (Fig. 1). This study area has been described in detail elsewhere (Zhu et al., 2012a), and thus only some important aspects are presented here as required for later discussion. Among ∼10 main rivers flowing into Jiaozhou Bay, some in the eastern coast, particularly Haibo River and Licun River, have become the conduits of industrial and domestic wastewater discharges
Selective extractions for Fe speciation
Preweighed wet sediment samples in duplicate were dried at 105 °C until constant weight for gravimetric analysis of water contents. Contents of Fe in all extractions below were expressed in μmol per gram dry weight of sediments (μmol/g).
A combination of several previously used extraction methods (Lovley and Phillips, 1987, Raiswell et al., 1994, Poulton and Canfield, 2005) was employed to quantify five operational Fe phases: (1) acid volatile sulfide (AVS) Fe(II) and carbonate Fe(II) (Fe(II)
FeT and Al
FeT contents in sediments of Jiaozhou Bay were 2.25–3.67 wt.%, with an average of 2.94 ± 0.37% (Table 1), similar to the value (3.1%) reported previously for the sediments (Deng et al., 2010). Aluminum contents were 5.21–6.87%, within the range previously reported (Deng et al., 2010), whereas Al average (6.0 ± 0.39%) was slightly lower than a value previously reported (7.1%) (Deng et al., 2010).
Fe(II)AVS + carb, pyrite-Fe, and AVS
Fe(II)AVS + carb contents in sediments of the bay ranged from 6 to 42.8 μmol/g, with an average of
Assessment on Fe inputs from anthropogenic origin
Average FeT in Jiaozhou Bay surface sediments is in the same range as the China shelf sea (Zhao et al., 1995, Zhu et al., 2012b) and global continental shelf sediments in general (Raiswell and Canfield, 1998). Average FeT/Al ratio (0.49 ± 0.04, range: 0.42–0.57) for the sediments is lower than the average (0.53) for China shelf sea sediments (Zhao et al., 1995) and also lower than that for shale (0.55) (Wedepohl, 1991). The lower FeT/Al ratio is probably due to high outcrop abundance of granite
Summary and conclusions
In surface sediments of Jiaozhou Bay, similar FeT contents to the background for the China shelf sea sediments and a generally good linear coupling of FeT to Al indicate that Fe is mainly of natural weathering source, with no appreciable addition of anthropogenic Fe, despite excess inputs of nutrients and industrial/domestic wastewaters. Among the three Fe(III)HR (i.e., Fe(III)am, Fe(III)pc, and Fe(III)wc), Fe(III)wc is always the predominant pool, followed by Fe(III)pc, and Fe(III)am is only
Acknowledgments
The authors are indebted to H.-B. Wang for field work. This research was jointly supported by the Shandong Province Natural Science Foundation (grant ZR2015DM006 to M.X.Z.), the Taishan Scholars Programme of Shandong Province (to G.P.Y.), and the NSF of China (grants 41576078 and 41076045 to M.X.Z.). We thank the editor and four anonymous reviewers for their critical suggestions and English corrections, which greatly helped to improve the manuscript.
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