Elsevier

Sedimentary Geology

Volume 407, September 2020, 105749
Sedimentary Geology

Sedimentary zonation shift of tidal flats in a meso-tidal estuary

https://doi.org/10.1016/j.sedgeo.2020.105749Get rights and content

Highlights

  • Sediments on tidal flats presents distinct zonal features with frequent zonation shift.

  • Seasonal tidal level fluctuation is responsible for the shifts of sedimentary zonation.

  • The storm controls the embedded downward migration of the sedimentary mixture zone.

Abstract

Understanding the spatial-temporal pattern of sedimentary dynamics on tidal flats is important for examining their ecological function and evolutionary trend. In this study, the dynamic state of sediments across an open-coast tidal flat of the meso-tidal Changjiang Estuary, China, on a monthly scale from December 2014 to June 2016 was examined, based on a high-spatial-resolution sedimentological-topographic survey and a collection of related meteorological-hydrological data. The results revealed a continuously distinct zonation of sediments, which transformed seaward from sand-dominance to silt dominance over a short distance. A zoning, to divide the tidal flat into a sand zone, a mixture zone and a silt zone, was delimited based on the cluster analysis of grain size frequency distributions. The zonation was highly dynamic, with the mixture zone migrating upward during summer and experiencing an uplift of 1 m in its lower boundary over the time period examined. Within each zone, the medium size and sorting coefficient of sediments tended to be smaller in winter than those in summer. Alterations of the mean tidal level likely controlled the zonation shift and sediment variations. A higher mean tidal level indicated a landward shift of the location exhibiting shear stress maxima and the mixture zone had to migrate upward to maintain an appropriate stress for the development of mixed sediments. The impacts of waves were masked under normal conditions, but during storms strong waves dominated an embedded downward displacement of the mixture zone. Characteristics of sediments covering a large region of the tidal flat changed along with zonation shifts, resulting from altered proportions among sandy, mixed and silty sediments. The study highlights that sedimentary zonation shift, under the impacts of seasonal variations in tidal level, can be a major manifestation of tidal flat sedimentary dynamics.

Introduction

The nature of surface sediments on tidal flats is a key factor regulating their organic sorption, pollutant accumulation and significance in global biogeochemical cycling (Costanza et al., 2014; Martinez-Garcia et al., 2015). Additionally, sediment grain size and its distribution hold important implications for sediment transport and erosion/deposition on tidal flats (Gao and Collins, 1994). Knowledge regarding the spatial-temporal pattern of sedimentary dynamics is essential to advance our understanding of the ecological function and evolutionary trend of tidal flats. This is increasingly urgent in the face of climate change and artificial interference, which have resulted in worldwide tidal flat degradation and recession (Kirwan and Megonigal, 2013).

The character of tidal flat sediments rests with the couplings between sediment source, regional hydrodynamics, and biological activity (e.g., Dyer, 1986; Friedrichs et al., 2008; Shynu et al., 2017), and thus can suffer dramatic changes in response to alterations in these processes. For tidal flats neighboring or within estuaries, the composition and quantity of riverine sediments generally play a significant role in sediment variations (e.g., Gratiot and Anthony, 2016). For example, on the Ouse Estuary, England, tidal flats were found to exhibit silty-clayey substrates in winter but sandy surface-sediments in summer, regulated by riverine sediment discharge (Uncles et al., 1998). Some studies have focused on responses of tidal flat sediments to hydrodynamics (e.g., Allen and Duffy, 1998; Alcántara-Carrió et al., 2018). For example, Allen and Duffy (1998) found that the sediments on the tidal flats of the Severn Estuary, UK, grew sandier from spring to autumn, mainly attributable to winter storminess; Shynu et al. (2017) revealed a cyclic formation and disruption of fluid muds induced by seasonally changed waves for the tidal flats of the Alleppey, India; Alcántara-Carrió et al. (2018) pointed out that the alternation of the Subtropical South Atlantic High contributed to the variations in the tidal flat sediments of the Araçá Bay, Brazil. The biological activity of tidal flats, including bioturbation, bio-adhesion, and bio-aggregation, can be independent of sediment supply or hydrodynamics and locally alters sediment texture by changing bed erodibility (Friedrichs et al., 2008). As shown in Andersen et al.'s (2005) study in the Rømø Bight, Denmark, algal biofilms, developed seasonally, led to an increase in erosion threshold and a fining of bed sediments. Sediment grain size variations also show relevance to tidal flat deposition/erosion, with a tendency of sediment fining as deposition occurs (Eisma, 1998), while the actual interactive-mode between sediment grain size and bed elevation was diverse among seasons (Yang et al., 2008). Additionally, sea level eustasy has been reported to bring changes to the long-term morpho-sedimentary dynamics of tidal flats (e.g., van der Wegen et al., 2017), while how seasonal fluctuations can impact the sediments have been rarely mentioned.

Tidal flat sediments are characterized by specific cross-shore distributions (e.g., Dyer, 1986; Fan, 2012). Along a transverse section, the grain size trends of sediments were found to be related to the relative dominance of tides or waves (Eisma, 1998). For example, on the extensive tidal flats in the northern Jiangsu Province, China, in a tide-dominated regime, sediments were discovered to gradually coarsen seaward (Li et al., 2005). As for the strongly wave-exposure tidal flat in the southwestern coast of South Korea, a seaward fining trend, similar to that on beaches, was detected (Yang et al., 2005). An exception was a tidal flat along the coast of Suriname, which showed the finest deposits (dominated by silty clay) and little trend variations (Lefebvre et al., 2004). Additionally, the sediments generally exhibit some transverse differentiation, with large transitions in sediment texture and grain size in some locations (e.g., Eisma, 1998; Chun, 2007). A sedimentary zonation of tidal flats has been put forward (e.g., Fan, 2012), but is less commonly used than the topographic zonation based on specific waterlines (e.g., mean high water level and mean low water level) due to its limitation in global standardization. Accordingly, variations in the spatial patterns of tidal flat sediments, especially possible sedimentary zonation shifts, have been examined in only a few studies (e.g., Chun, 2007; Alcántara-Carrió et al., 2018), which were mostly conducted in a descriptive way. While there are many studies looking at changes in beach sediments, less work has been done to variations in tidal flat sediments with sedimentary zonation shifts.

In this study, the dynamic state of sediments across an open-coast tidal flat in the Changjiang Estuary, China, was examined on a monthly scale, based on an integrated survey of cross-shore sediments and topography and a collection of meteorological and hydrological data. Major objectives of this study were to: (1) explore the spatial pattern alterations of the sediments in terms of zonation shifts; (2) detect the monthly changes in sediment grain size of different zones; and (3) analyze responses of the zonation shifts and grain size variations to external dynamics and internal topographic changes. This would give light to studies on sedimentary dynamics of tidal flats coping with similar regimes.

Section snippets

Study area

The Changjiang River is the largest river in the Eurasian Continent (Fig. 1A), exhibiting a length of 6300 km, with a multi-year average water discharge of 905 × 109 m3/yr and a sediment discharge of 0.43 × 109 t/yr between 1950 and 2000 (Dai et al., 2014). Nearly 47% of the riverine sediment has accumulated in the subaerial delta and estuarine system, favoring the development of tidal flats (Liu et al., 2006). The Nanhui Shoal, located in the south flank of the Changjiang Estuary, has

Sediment observations

A monthly sedimentological survey was carried out over the period December 2014–June 2016. This includes an additional survey on July 15th 2015, i.e. four days after the passage of typhoon Chan-hom, to examine the impacts of this storm. During the survey, bed sediments of the topmost 2 cm were sampled every 12.5 m along the repeat fixed section (Fig. 1C). This sediment thickness corresponds to the active layer, sediments within which could represent timely responses to hydrodynamics (Abuodha,

Cross-shore distribution of sediments

The sediments exhibited three types of grain size frequency distribution (Fig. 2), including a unimodal mode with peak frequency at 2.5–3.0 φ in the land side, a bimodal or unimodal mode exhibiting moderate frequency over grains of 2.5–5.0 φ in middle, and a unimodal mode with a peak at 4.5–5.0 φ on the seaward side. Meanwhile, the sediments were found to transform seaward from sand-dominance (of a sand content >80%) to silt-dominance (of a silt content >65%) over a short distance (dozens of

Sedimentary zonation and its shift

Hydrodynamic sorting, relative to the character of fluvial sediments, seems to control the sediment texture and its distribution here, shown by the sandy-silty flat (Fig. 3) versus the silty-clayey Changjiang sediment input (Gao et al., 2015). This is different from the cases at the coasts of eastern England (Möller et al., 1999) and Guiana (Gratiot and Anthony, 2016). The development of sedimentary zonation under the regulation of hydrodynamic sorting could be explained by a conceptual model

Conclusions

The monthly cross-shore sedimentary dynamics of an open-coast tidal flat in the meso-tidal Changjiang Estuary was examined, based on a suite of meteorological, hydrological, sedimentological and topographic data. Major findings included:

  • 1)

    Despite the fluctuated wind/wave/tidal conditions and dramatic topographic changes, the sediments retained a zonal pattern, with sandy, mixed and silty sediments distributed seaward. The differentiation of bed shear stress, favored by strong wave-exposure,

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This study is supported by the Key projects of intergovernmental science and technology innovation cooperation of the Ministry of Science and Technology in China (2018YFE0109900), International Science and Technology Cooperation Project of Shanghai Science and Technology Commission (19230712400), National Natural Science Foundation of China (41806106), China Postdoctoral Science Foundation (2018M641964) and the Fundamental Research Funds for the Central Universities. We acknowledge Editor

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