Major Characteristics of Surface Sediments Within the Shallow Water of the Cat Ba Island, North Vietnam

Abstract

The Cat Ba is one of the largest Islands offshore North Vietnam, which is characterized by an abundance of coral reefs in the East and Southeast of the island. The surface sediments are considered the basic elements for the coral ecosystem development. In this study, the authors present some new results studying pH, Eh, mineral composition, and grain size as the basic information for environmental assessment of this area. The results show that the pH value of the surface sediment varies from 6.90 to 8.09, with an average of 7.24 while the Eh value of the sediment ranges from -121.10 to -48.20mV, an average of -68.39mV, demonstrating a reducing environment. The surface sediments have been classified into 8 size classes: the coarse silt > very coarse silt > medium sand, very fine sand > very coarse sand, fine sand > coarse sand, very fine gravel. Most of the sediments are poorly sorted - very poorly sorted, only a few sedimentary samples are well sorted, moderately sorted, and moderately well sorted. The average mineral composition of the surface sediments consists of: 25% quartz, 17% illite, 16% aragonite, 13% kaolinite, 10% calcite, 5% chlorite, 4% gothite, 3% halite, 2% feldspar and less montmorillonite, and dolomite. These results allowed the researcher to interpret that the sediments have been deposited in a relatively calm environment and the terrigenous sediment sources are dominant over the marine sources. Source marine sediment groups are characterized by coarse grains, high pH, and are rich in carbonate minerals, which have been produced by biological materials. In contrast, the terrigenous sediment group is dominated by fine-grained sediments, rich clay minerals, quartz, and gothite. These fine-grained sediments are commonly distributed in the area and are favorable places for pollutant accumulation.

Grain sizes and minerals in sediments in a part of the Vietnamese Gulf of Tonkin

“…The very ne sands distributed only surface sediments and absent in cores. On the surface, distributed from nearshore (4,11,17,28) to offshore (8,13,15,19,20,21), they were 33.3% (10/30 samples) (Fig. 2).…”

“…Goethite ranged from 3 to 7%, averaging 4.7% in surface sediments, with content 6-7% distributed nearshore (3,7,17,22), and content 5% was both nearshore (1,2,5,6,11,12,14,18,23,30) and offshore (19,20,24), content 3-4% distributed nearshore (4,10,27,28,29) and offshore (8,9,13,15,16,21,25,26). In C1, it was 2-4% with an average of 3.2%, content 3% and 4% were the most common, but content 2% was uncommon.…”

“…Illite was found in both surface and core sediments. Surface sediment content ranged from 3 to 23% and averaged 14.3%, with 21-23% distributed offshore (9,26) and nearshore (10,14), 10-20% distributed nearshore (3,5,7,11,12,17,18,23,27,29,30) and offshore (8,13,15,16,19,20,21,24,25), 3-8% distributed only nearshore (1,2,4,6,22,28). It ranged from 16 to 25% and averaged 19.6% in C1, with content 21-25% distributed from the middle to the bottom core, 18-20% distributed top, middle, and bottom, and 16-17% distributed top and middle.…”

“…The mineral content of sediments decreased with concentration: quartz > illite > aragonite > kaolinite > calcite > chlorite > goethite > halite > feldspar and less montmorillonite and dolomite. Calcite and aragonite, goeth and halite from marine organisms were formed by chemical processes, while quartz, illite, kaolinite, chlorite and montmorillonite from terrigenous sources were formed by rivers, currents, weathering and erosion (Hoang et al, 2020).…”

Grain sizes and minerals in sediments of Gulf of Tonkin, Viet Nam

“…Also, sand and silt are distributed at the mouths of Red River System and Ha Long bays (Duc et al, 2007;Nhon et al, 2016;Nhon et al, 2017), Base on smectite mineral in Ha Long Bay, the sediment supplies have been identi ed from the Red River system and partly from erosion and weathering from the islands, geological system surround (Vuong et al, 2016). In addition, calcite and aragonite derived from marine organisms and goethite have been created by chemical processes (Hoang et al, 2020). The contaminants in sediment have been increasing over time, for instance, the heavy metals have exceeded the ISQGs (As, Cu, Pb, Zn) in embayment, lagoon, and estuary sediments (Anh et al, 2005;Ho et al, 2010;Ho et al, 2013a;Ho et al, 2013b;Tue et al, 2011;Nguyen et al, 2016;Stefania et al, 2012;Quy et al, 2012;Luong et al, 2016;Ha et al, 2016).…”