2 Laboratory for Marine Mineral Resources, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266071, China;
3 University of Chinese Academy of Sciences, Beijing 100049, China
Laser in-situ scattering and transmissometery (LISST) can be used to measure volume concentrations (VCs) of marine suspended particulate matter (SPM) (Agrawal and Pottsmith, 2000) under undisturbed conditions. In field applications, this technique has been widely used to investigate SPM in studies of modern sedimentary processes. Such LISST measurements have revealed a notable phenomenon—coarse particles ranging from tens to hundreds of microns are widely found in estuarine, coastal (Mikkelsen and Pejrup, 2000; Fettweis et al., 2006; Mikkelsen et al., 2006; Braithwaite et al., 2010; Papenmeier et al., 2014; Many et al., 2016) and deepsea waters (Iversen et al., 2010; Karageorgis et al., 2012).
Previous studies have suggested that these coarse particles, which are strongly controlled by their biogenic matter (Karageorgis et al., 2012; Lee et al., 2016), are aggregates that have coalesced from individual fine particles (Kiriakoulakis et al., 2009; Guo et al., 2010; Iversen et al., 2010). The fine particles are commonly referred to as primary particles (Mikkelsen and Pejrup, 2000; Fettweis et al., 2006). The properties of these coarse particles directly affect sedimentation of SPM, because as fine particles flocculate into aggregates, their settling velocities increase by orders of magnitude (Milligan, 1996; Mikkelsen and Pejrup, 2000). Furthermore, aggregate size and primary particle composition will affect both SPM settling velocity (Markussen and Andersen, 2013; Hurley et al., 2016) and the particulate organic carbon (POC) flux (Ramondenc et al., 2016). In previous studies, SPM samples have been dispersed in laboratory settings to reveal assemblages of in situ SPM (Xia et al., 2004; Williams et al., 2007). However, such studies have primarily focused on estuarine and coastal areas; few studies have focused on waters above continental shelves. In coastal and estuarine environments, fine cohesive particles that consist mainly of mineral particles and terrigenous organic particles (Dupont et al., 1994) form aggregates (Eisma, 1986; Uncles et al., 2006) based on electrochemical attraction (Whitehouse et al., 2000). In contrast, in continental shelf waters, sticky organic substances, e.g., transparent exopolymer particles (TEPs) and fecal pellets, could be responsible for biogenic aggregates (De La Rocha and Passow, 2007). These types of aggregates are markedly different from SPM of coastal and estuarine waters. Continental shelf areas are important conduits for transporting sediments from coastal areas to the open ocean (Dong et al., 2011).
Therefore, detailed information concerning coarse particles in continental shelf waters would contribute to a more thorough understanding of SPM compositions, formation environments, and sedimentation processes. As a case study, we focused on coarse particles within the water column of the continental shelf of the North Yellow Sea—a typical inner shelf environment. The particle size distributions (PSDs) of in situ effective particles, dispersed particles, and inorganic particles were measured to analyze coarse particle structure and composition. Micrographs of SPM on filters illustrate the various types of coarse particles encountered in this study. Finally, changes in particle properties within the water column related to thermal stratification and sediment resuspension are discussed.2 MATERIAL AND METHOD 2.1 Field work and sampling
The research vessel Dongfanghong No. 2 undertook a scientific cruise during the period 8-21 July 2016 in the area of the North Yellow Sea shown in Fig. 1a. In this area, near the Shandong peninsula, water depth is shallow, varying from ~20 m in the south to ~50 m in the north. During the cruise, in situ effective particle size distribution (EPSD) (Williams et al., 2007; Jouon et al., 2008), water temperature, and turbidity were profiled at 10 stations, with depths varying from 21 to 58 m (Fig. 1b). The EPSDs, which are reported in terms of volume concentration (VC, μL/L), were measured using a type C LISST-100X—an autonomous instrument manufactured by Sequoia Scientific, Inc. (Bellevue, WA, USA). The particle size information was derived from laser diffraction measurements, with size-dependent light scatterers resolved into 32 logarithmically spaced size classes ranging from 2.5-500 μm (Agrawal and Pottsmith, 2000). Water temperature (℃) and turbidity profiles were measured using an SBE 9plus11 CTD (Sea-bird Scientific, Inc., Bellevue, WA, USA), with an attached WET labs ECO-NTU turbidity sensor (Sea-bird Scientific, Inc.). Turbidity is an apparent optical property that depends on the size, color, and shape of the scattering particles; it is usually reported in nephelometric turbidity units (NTU) (Mitchell and Mersch, 2010). The sampling frequency of the CTD was 24 Hz, and these data were averaged over each vertical meter.
Throughout the summer, the North Yellow Sea Cold Water Mass (NYSCWM) occurs at the bottom of this area at water depths greater than 50 m (Guan, 1963; Bao et al., 2009). A strong thermocline forms between the NYSCWM and the upper mixed layer; it appears in spring and disappears in autumn (Bao et al., 2009). In the coastal waters of the Shandong peninsula, the hydrodynamic conditions are enhanced by coastal (Naimie et al., 2001; Lu et al., 2011; Pang et al., 2016) and tidal currents (Fang, 1986), which weaken this stratification (Zhao et al., 1994). The stratification has important effects on the vertical exchange of materials (Qiao et al., 2011) and contributes to differences in SPM properties within the water column.
To evaluate the effects of stratification on the properties of SPM, water samples and abiotic measurements were taken simultaneously from standard layers (the sampling depths of each layer at each station are shown in Fig. 4), including the surface layer, middle layer (near the thermocline), and bottom layer. Each water sample was divided into four subsamples: (1) sample-A (2 000 mL) was used for mass concentration (MC) analysis and microscopic observations; (2) sample-B (2 000 mL) was used for POC analysis; (3) sample-C (100 mL) was used for dispersed particle size distribution (DPSD) analysis; and (4) sample-D (100 mL) was used for inorganic particle size distribution (IPSD) analysis. To prevent the organic matter from decomposing, sample-C was fixed with 5 mL of formaldehyde solution aboard the ship (Guinder et al., 2009; Modéran et al., 2012).2.2 Laboratory analysis
Sample-A was filtered through pre-weighed Whatman filters (with a 0.45-μm nominal pore size and a 47-mm diameter; GE Healthcare Life Sciences, Chicago, IL, USA). The dry mass was derived from the difference in mass of the filter with SPM and the blank filter. The MC was calculated as the dry mass per unit volume. Additionally, the SPM collected on these filters was inspected using a Phenix XZJ metallurgical microscope (Phenix Optical Holding Stock Co. Ltd., China) to identify the types of coarse particles present within each layer.
Sample-B was filtered through a pre-combusted (500℃ for 4 h) Whatman GF/F filters (with a 0.7-μm nominal pore size and a 25-mm diameter; GE Healthcare Life Sciences) and frozen at -20℃ immediately after collection (Bauer et al., 2002). The samples collected on these filters were acidified with HCl to remove inorganic carbonates (Le Moigne et al., 2013). After drying under vacuum, the POC was measured using a Vario Isotope Cube (Elementar Analysensysteme GmbH, Germany). Here, POC is expressed as a mass fraction in the form of a percentage.
Sample-C was concentrated by centrifugation and mixed with sodium hexametaphosphate to create 100 mL of solution, with a 0.4% concentration for chemical dispersion (Mikkelsen and Pejrup, 2000; Xia et al., 2004; Hodder and Gilbert, 2007; Williams et al., 2007). It was then further mechanically dispersed in an ultrasonic bath for 5 min to disperse particles (or primary particles). We stirred the sample to mix it thoroughly and placed it into the LISST- 100X(C) sample chamber to measure the DPSD. Sample-D was concentrated by centrifugation. The organic fraction was removed from the sample using an excess of 30% hydrogen peroxide (George et al., 2007; Hodder and Gilbert, 2007) in a water bath (60℃ for 8 h). After rinsing three times with distilled water, the water sample was mixed with sodium hexametaphosphate to create 100 mL of solution, with a 0.4% concentration. It was mechanically dispersed in an ultrasonic bath for 5 min. Finally, it was thoroughly mixed into a homogenous suspension and placed into the LISST-100X(C) sample chamber to measure the IPSD. A background correction of the LISST was performed using distilled water prior to field and laboratory measurements. The VCs of organic particles were derived from the experimental data by subtracting the VCs of the inorganic particles from the VCs of the dispersed particles.
Although dispersion and organic fraction-removing treatments were carried out in previous studies (Mikkelsen and Pejrup, 2000; Xia et al. 2004; George et al., 2007; Hodder and Gilbert, 2007; Williams et al., 2007), no quantitative evaluations of these experimental results were undertaken to validate this method. Thus, we compared the measured MC and dry mass concentration estimated from the experimental data (MCdry). The MCdry was determined using the following equation:
where V is the volume concentration, ρ is the density, and the suffixes, 'org' 'w' and 'in' denote organic particles, and the water in organic particles and dispersed inorganic, respectively. Because the water content in biogenic organic particles is much higher (up to 97%) than most other substances (Marzooghi et al., 2017),
making Eq.1 into: