Examining bulk and iron-associated organic carbon through depth in margin sea sediments (China) under contrasting depositional settings: Chemical and NEXAFS spectral characterization

doi:10.1016/j.jmarsys.2020.103344

Journal of Marine Systems

Volume 207, July 2020, 103344

https://doi.org/10.1016/j.jmarsys.2020.103344 Get rights and content

Highlights

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Depositional regimes control sources and burial of sedimentary organic carbon (OC).
•
Sediment source, transport history and reworking intensity affect Fe_R contents.
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There is no clear selectivity of OC retention by reactive iron (Fe_R) along depth.
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Fe_R plays limited roles in OC preservation in sediments of east China margin seas.

Abstract

Marine sediments are the largest sink for organic carbon (OC) on Earth, and iron (Fe) oxides play an important role in stabilization of sedimentary OC. However, the roles of Fe oxides in OC stabilization during prolonged burial, for example, up to tens of thousands of years or more are still poorly constrained. In this study, we used traditional chemical extraction and near-edge X-ray absorption fine structure (NEXAFS) spectroscopic technique to characterize bulk OC and Fe-associated OC (Fe-OC) through depth in gravity cores collected from three sites near the Yangtze River Estuary (YRE), in the South Yellow Sea (SYS), and in the middle Okinawa Trough, which have contrasting depositional environments. Results show that depositional environments have exerted quite different influences on sources and burial of sedimentary OC, and thus on OC degradation during prolonged burial at the three sites. Reactive Fe (Fe_R) contents at the three sites are greatly influenced by sediment sources, the history of its transport, and its reworking intensity, with Fe_R contents near the highly dynamic YRE much higher than at the central SYS and the middle Okinawa Trough. The fractions of Fe-OC in total OC (f_Fe-OC) displayed no clear or consistent trends with depth or by site, probably due to the dual roles of Fe redox cycling in OC protection versus its oxidation. As indicated by the f_Fe-OC, reactive Fe plays a limited role in OC preservation in margin sea sediments of East China. A combination of NEXAFS spectra and isotopic compositions of bulk OC and Fe-OC indicates that main OC functionalities have not experienced differential alterations and/or no specific OC moieties have been selectively stabilized/released during prolonged burial in the three contrasting depositional environments.

Introduction

Marine sediments, the largest sink for organic carbon (OC) on Earth, play a key role in carbon budget of the surface Earth (Emerson and Hedges, 1988; Burdige, 2007). Accordingly, OC burial in marine sediments represents the major mechanism that moves carbon from the active surface carbon cycle to the slower geologic carbon cycle (Emerson and Hedges, 1988; Burdige, 2007). In addition, the burial of OC, together with reduced sulfur, plays an important role in controlling the concentrations of atmospheric CO₂ and O₂ and thus in controlling the Earth's climate system on geologic timescales (Berner, 1989). The efficiency of OC burial in marine sediments is controlled by many interwoven factors, including water column depth, source and transport of OC, accumulation rates, hydrodynamic regimes, oxygen exposure time, and OC sorption by mineral matrixes (Keil et al., 1994; Hartnett et al., 1998; Burdige, 2007; Aller, 2014; Hemingway et al., 2019).

The largest pool of OC stored in sediments is intimately associated with clay minerals and metal oxides (Mayer, 1994; Hedges and Keil, 1995; Arnarson and Keil, 2007; Lalonde et al., 2012). Iron (Fe) oxides have high capacity for OC adsorption due to large specific surface area; they also have intense affinity for OC via formation of stable inner-sphere complexes, which play an important role in chemical protection of OC against microbial degradation in soils and sediments (Wagai and Mayer, 2007; Lalonde et al., 2012; Riedel et al., 2013; Salvadó et al., 2015; Shields et al., 2016). The percent fraction (f_Fe-OC) of Fe associated OC (Fe-OC) to sedimentary bulk OC has been reported to be 21.7 ± 7.8%, on average, on global continental margins underlying oxic waters, including the Arctic margin, the St Lawrence estuary and gulf, the Mexican margin, the Eel River basin, the Washington coast and the adjacent Columbia River delta (Lalonde et al., 2012). However, further studies indicate that the fractions are in a wide range from nearly 0 to as high as 42% in various depositional conditions (Salvadó et al., 2015; Shields et al., 2016; Ma et al., 2018; Peter and Sobek, 2018; Sirois et al., 2018; Zhao et al., 2018a; Wang et al., 2019). Many factors such as OC content, Fe reactivity and Fe redox cycling, and mechanisms of OC binding to Fe oxides (adsorption versus co-precipitation) have been invoked to explain the large variability in f_Fe-OC, but no a consensus has been reached (Kleber et al., 2015). Depositional environment is most likely an integrated and primary factor responsible for the variability in f_Fe-OC, since it exerts direct or indirect control on all the other factors above.

Given that Fe oxides are highly redox sensitive, anoxic conditions can lead to reductive dissolution of Fe(III) oxides through biotic and/or abiotic mechanisms and thus concomitant release of Fe-OC. In addition, redox cycling of Fe has dual roles in OC protection versus oxidation (Hall et al., 2018). On the one hand, Fe redox cycling tends to maintain high reactivity of Fe oxides and thus high adsorption capacity and affinity for OC (Cornell and Schwertmann, 2003; Raiswell and Canfield, 2012); on the other hand, Fe dissimilatory reduction oftentimes plays an important role in driving OC oxidation in sediments of continental margins (Canfield et al., 1993), and meanwhile hydroxyl radical, a strong nonselective oxidant of OC produced during Fe²⁺ reoxidation, could also effectively catalyze OC oxidation (Hall and Silver, 2013; Hall et al., 2015). Due to the dual roles, redox fluctuations may exert complicated influences on Fe-OC. As a result, Fe-OC may be only transiently stabilized by Fe oxides in the upper oxic zone. Thus, the amounts of Fe-OC in deeply buried sediments may be dependent on the history of Fe redox cycling in the upper zone and also on progressive reductive dissolution of Fe oxides during prolonged burial. Up to date, there is no a quantitative examination of Fe-OC in deeply buried marine sediments under varying depositional conditions, and thus much still remains unclear concerning the role of Fe oxides in OC stabilization during prolonged burial.

Compositional discriminations, that is, preferential sequestration of specific OC compounds with characteristic isotopic compositions by Fe oxides, have been reported for both synthetic and natural Fe-OC complexes, but no consistent patterns of the discriminations have been observed (Riedel et al., 2013; Chen et al., 2014; Salvadó et al., 2015; Wan et al., 2019; Wang et al., 2019). Mechanisms for the discriminations are complex and poorly understood, but potential factors influencing the discriminations have been proposed, including (i) binding mechanisms of OC on Fe oxides (adsorption versus co-precipitation) (Chen et al., 2014; Shields et al., 2016), (ii) the intensity of Fe redox cycling and its dual roles in OC protection versus oxidation (Hall and Silver, 2013; Hall et al., 2015; Wang et al., 2019), (iii) selective sequestration of OC by Fe oxides versus selective release of Fe-OC during Fe reductive dissolution (Adhikari and Yang, 2015; Adhikari et al., 2016), and (iv) OC sources (Salvadó et al., 2015). Up to date, however, there is no a study on compositional discriminations of Fe-OC in deeply buried sediments in varying depositional conditions. It still remains unknown whether there exist consistent and measurable compositional discriminations of Fe-OC in marine sediments after prolonged burial. This knowledge, nevertheless, is important for better understanding of two classes of mechanism—selectivity and protection—which explain why some OC escapes remineralization in soils and sediments (Hemingway et al., 2019).

Three gravity cores, located near the Yangtze River Estuary in the East China Sea, in the central South Yellow Sea, and in the middle Okinawa Trough (Fig. 1), which have contrasting depositional environments (see 2 Study areas, 3 Material and methods for details), were collected for comparative examination of bulk OC and Fe-OC through depth. We used traditional chemical extraction to quantitatively characterize bulk OC and Fe-OC and their isotopic compositions. We also used synchrotron-based near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, a non-invasive, non-destructive technique that does not require sample pretreatment, to provide a molecular-level insight into the features of bulk OC and Fe-OC. Objectives of the study are to: (i) reveal compositional and isotopic differences of bulk OC and Fe-OC under the three contrasting depositional environments and then disentangle potential factors responsible for the differences; (ii) quantify the role of reactive Fe in OC stabilization through depth; (iii) examine whether there exist compositional discriminations of Fe-OC after prolonged burial.

Section snippets

Study areas

The East China Sea (ECS) is one of the world's largest shelf seas, which is dominated by the Yangtze River (Fig. 1), the world's third largest river in terms of particulate loading (5 × 10⁸ t/yr). The Yangtze River Estuary (YRE) and the adjacent ECS have high sedimentation rates, while the rates decrease rapidly southward along the inner shelf and eastward offshore (DeMaster et al., 1985; Huh and Su, 1999; Liu et al., 2006). For instance, sedimentation rates are typically ~3 cm/yr near the YRE (

Sample collections

Gravity cores for this study were collected during May 2016 and September 2017 at three locations (YRE-4, SYS-12 and OT-8) under contrasting depositional conditions (Fig. 1, Table 1). YRE-4 (30.87°N, 122.50°E, core length 167 cm) is located near the YRE with a water depth of 18.6 m. SYS-12 (35.00°N, 123.00°E, core length 260 cm) is located in the SYS depocenter with a water depth of 73 m. OT-8 (28.81°N, 127.35°E, core length 600 cm) is located in the middle OT with a water depth of 965 m.

Properties of bulk sediments

TOC contents in YRE-4, SYS-12 and OT-8 were in the range of 0.54–0.69% (0.61 ± 0.05%), 0.39–0.84% (0.75 ± 0.13%), and 0.60–1.20% (1.00 ± 0.19%), respectively (Fig. 2a, Supplementary information Table S1). The contents have no clear depth variability. On average, OT-8 had the highest TOC contents, followed by SYS-12 and YRE-4.

δ¹³C_bulk of TOC in YRE-4, SYS-12 and OT-8 ranged from −23.76‰ to −23.37‰ (mean −23.55 ± 0.15‰), −23.33‰ to −22.06‰ (mean −22.46 ± 0.41‰), and −22.21‰ to −21.66‰ (mean

Comparing sources and burial of bulk OC at the three sites

TOC contents, δ¹³C_bulk, and estimates of marine OC fractions derived from δ¹³C values all indicate that YRE-4, SYS-12 and OT-8 are distinctly different in OC sources and burial. The highest fractions of terrestrial OC (~55%) at YRE-4 could be ascribed largely to the proximity of this site to the YRE. Even though the site has high sedimentation rates and receives high fluxes of terrestrial OC from the Yangtze River (DeMaster et al., 1985; Liu et al., 2006; Yao et al., 2015), TOC contents at

Summary and conclusion

Contrasting depositional environments at sites near the YRE, in the SYS, and in the middle OT have exerted distinctly different influences on OC sources, burial, and post-depositional degradation. Sediment near the YRE is low in OC contents and reactivity, and thus OC displays insignificant degradation during prolonged burial, largely due to high fractions of terrestrial OC plus effective OC oxidation in the mobile mud layers. In comparison, sediment in the quiescent central SYS, which is only

Acknowledgments

The crews of Run Jiang and R/V Dong Fang Hong 2 are thanked for their assistances with sampling. The NEXAFS beam time was granted by 4B7B beamline of the Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences. The staff members of the beamline are acknowledged for their help with data collection and processing. The research was jointly supported by the National Natural Science Foundation of China (grants 41776085 and 41576078), the National Key

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Examining bulk and iron-associated organic carbon through depth in margin sea sediments (China) under contrasting depositional settings: Chemical and NEXAFS spectral characterization

Highlights

Abstract

Introduction

Section snippets

Study areas

Sample collections

Properties of bulk sediments

Comparing sources and burial of bulk OC at the three sites

Summary and conclusion

Acknowledgments

Chem. Geol.

Cont. Shelf Res.

Geochim. Cosmochim. Act.

Palaeogeogr. Palaeoclimatol. Palaeoecol.

Geochim. Cosmochim. Act.

Mar. Chem.

Geochim. Cosmochim. Acta

Mar. Environ. Res.

Mar. Pollut. Bull.

Cont. Shelf Res.

Geoderma

Mar. Geol.

Acta Ecol. Sin.

Mar. Geol.

Mar. Chem.

Org. Geochem.

Org. Geochem.

Mar. Geol.

Palaeogeogr. Palaeoclimatol. Palaeoecol.

Deep-Sea Res. Pt. II.

Adv. Agron.

J. Geophys. Res. Biogeosci.

Cont. Shelf Res.

J. Sea Res.

Mar. Pollut. Bull.

J. Mar. Syst.

Chem. Geol.

J. Mar. Syst.

Deep-Sea Res. Pt. A-Oceanog. Res.

Chem. Geol.

Geochim. Cosmochim. Act.

Mar. Geol.

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Paleoceanography

Mar. Micropaleontol.

Mar. Micropaleontol.

Geochim. Cosmochim. Act.

Mar. Geol.

Earth-Sci. Rev.

Chem. Geol.

Cont. Shelf Res.

Appl. Geochem.

Selective stabilization of aliphatic organic carbon by iron oxide

Sci. Rep.

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J. Mar. Res.