Distribution of type sediments
Five types of sediments were distributed in the Gulf of Tonkin, the surface sediments included fine sand, very fine sand, very coarse silt, coarse silt and medium silt, the sediment cores were divided into three types, which were very coarse silt, coarse silt and medium silt, and there were no fine and very fine sands.
Fine sand was only in surface sediments, distributed nearshore from a depth of 30 m to the coast (1, 2, 6, 22, 27), offshore only one (24), fine sand was 20% (5/30 samples) (Fig 2). Mean diameter (Md) was 0.130 - 0.244 mm, sorting (S0) was moderately well to poorly sorted (S0 = 1.45 - 3.81), skewness (Sk) was symmetrical to very coarse skewed (Sk = 0,05 - 0.34).
The very fine 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). Md was 0.063 - 0.112 mm, S0 was moderately well to very poorly sorted (S0 = 1.78 - 1 .53), Sk was very fine to coarse skewed (Sk = -0.52 - 0.16).
Very coarse silt was present in both surface and core sediments. Surface sediment distributed nearshore (7, 10, 12, 14) offshore (16, 25), was 20% (6/30 samples) (Fig. 2), Md varied from 0.032 to 0.061 mm, S0 was poorly sorted (S0 = 2.47 – 3.92), Sk was very fine skewed (Sk ranging from -0.61 to -0.38). In C1, very coarse silts were uniformly distributed from 0 to 70 cm, present in 100% (25/25 samples), Md varied from 0.038 to 0.054mm, S0 from 1.79 to 2.41, indicating moderately well to poorly sorted, Sk was - 0.59 to -0.03 showed a very fine to symmetric skewed. In C2, very coarse silt was distributed alternately from the top, middle and bottom of the core, they were 60% (15/25 samples), Md was 0.031 - 0.047 mm, S0 was 2.25 - 2.52 indicated poorly sorted, and Sk was between - 0.70 to 0.07 indicating a very fine to symmetrical skewed.
Coarse silt was present in surface and C2 core sediments, distributed nearshore (3, 5, 18) and offshore (9, 26), were 16,6% (5/30 samples) (Fig. 2), Md was 0.021 - 0.029 mm, S0 was poorly to very poorly sorted (S0 = 2.71 to 4.59), Sk was very fine to fine sknewed (Sk = -0.45 to -0.12). In C2, coarse silt ranged from 4 to 47 cm, was 36% (9/25 samples), Md from 0.021 to 0.030 mm, S0 was poorly sorted (S0 = 2.59 to 3.78), Sk was very fine to symmetrically skewed (Sk = -0.62 to -0.03).
Surface and C2 core sediments contained medium silt. Surface sediments were distributed nearshore (13, 29, 30), it was 10% (3/30 samples) (Fig. 2), Md was 0.013 - 0.015 mm, S0 was very poorly sorted (S0 = 4.00 – 4.73) and Sk was symmetrical skewed (Sk varied from - 0.10 to -0.09). In C2 it was only 4% (1/25 samples) with a length of 38- 41 cm, S0 was poorly sorted (S0 = 3.06), Sk was coarse sknewed (Sk = 0.30).
Distribution of minerals in sediments
Sedimentary minerals included quartz, illite, kaolinite, chlorite, feldspar, goethite, halite, calcite, aragonite, gibbsite, and montmorillonite (Table 1).
Montmorillonite was found in surface sediments in concentrations ranging from 0 to 3%, with 1-3% found in nearshore (1, 5,10) and offshore (9,20), but not in other areas. It did not appear in the cores (Fig. 3a).
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. In C2, content 21- 25% was distributed in the top, middle, and bottom of the core, content 18- 20% was distributed in the top, middle, and bottom of the core, and content 17% was distributed in the top of the core (Fig. 3b).
Table 1. Contents of minerals in sediment
Position
|
Levels
|
Minerals (%)
|
Mont.
|
Illi.
|
Kao.
|
Clo.
|
Qua.
|
Fel.
|
Go.
|
Ha.
|
Py.
|
Gip.
|
Can.
|
Ara.
|
Surface sediment (n=30)
|
Min.
|
0.0
|
3.0
|
3.0
|
2.0
|
26.0
|
3.0
|
3.0
|
0.0
|
0.0
|
0.0
|
0.0
|
0.0
|
Max.
|
3.0
|
23.0
|
18.0
|
7.0
|
79.0
|
16.0
|
7.0
|
3.0
|
0.0
|
0.0
|
11.0
|
5.0
|
Aver.
|
0.3
|
14.3
|
10.0
|
5.0
|
50.5
|
6.9
|
4.7
|
1.8
|
0.0
|
0.0
|
2.7
|
0.2
|
SD.
|
0.8
|
5.0
|
4.7
|
1.3
|
13.5
|
3.7
|
0.8
|
0.8
|
0.0
|
0.0
|
3.1
|
0.9
|
C1 (n=25)
|
Min.
|
0.0
|
16.0
|
12.0
|
5.0
|
32.0
|
3.0
|
2.0
|
0.0
|
0.0
|
1.0
|
0.0
|
0.0
|
Max.
|
0.0
|
25.0
|
19.0
|
7.0
|
48.0
|
7.0
|
4.0
|
2.0
|
4.0
|
3.0
|
3.0
|
7.0
|
Aver.
|
0.0
|
19.6
|
15.0
|
5.4
|
41.3
|
4.8
|
3.2
|
0.8
|
2.6
|
2.0
|
0.4
|
0.4
|
SD.
|
0.0
|
2.5
|
1.5
|
0.6
|
4.1
|
1.1
|
0.8
|
1.0
|
1.0
|
0.5
|
1.0
|
1.6
|
C2 (n=25)
|
Min.
|
0.0
|
17.0
|
6.0
|
5.0
|
35.0
|
3.0
|
3.0
|
0.0
|
2.0
|
0.0
|
0.0
|
0.0
|
Max.
|
0.0
|
25.0
|
16.0
|
7.0
|
51.0
|
13.0
|
4.0
|
2.0
|
4.0
|
2.0
|
0.0
|
0.0
|
Aver.
|
0.0
|
20.5
|
12.8
|
6.0
|
42.7
|
6.2
|
3.6
|
1.0
|
2.8
|
0.4
|
0.0
|
0.0
|
SD.
|
0.0
|
2.1
|
2.1
|
0.5
|
3.7
|
2.5
|
0.5
|
1.0
|
0.7
|
0.8
|
0.0
|
0.0
|
Mont. – Montmorillonite; Illi. – Illite; Kao. – Kaolinite; Clo. - Chlorite; Qua. - Quartz; Fel. – Feldspar; Go. – Goethite; Hal. - Halite; Can. – Calcite; Ara. – Aragonite; Gip. – Gibbsite; Py. – Pyrite; n = samples.
Kaolinite in surface sediments ranged from 3 to 18% and averaged 10.0%, with 15-18% distributed in offshore (9, 8, 26) and nearshore (10, 14), 10-14% distributed in nearshore (3, 5, 7, 11, 12, 17, 23, 30) and offshore (13, 15, 21, 25), and 3-7% distributed in nearshore (1, 2, 4, 6, 18, 22, 27, 28, 29) and offshore (16, 19, 20, 24). In C1, content 17-19% distributed in the top, middle, and bottom of the core, content 15-16% distributed in the top, middle, and bottom of the core, and content 12-14% distributed in the top, middle, and bottom of the core. It ranged from 6 to 16% and averaged 12.8% in the C2, with content 15-16% distributed in the top and bottom of the core, 10-14% distributed most often in the top, middle, and bottom, and 6-11% distributed in the middle and bottom of the core (Fig. 3c).
Chloride content in surface sediments varied between 2-7% and an average of 5.0%, the concentration of 6-7% was distributed near the coast (12, 23, 30) and in the offshore area (9, 13, 21, 25, 26), the concentration of 5% was the largest and distribution near the coast (2, 3, 5, 7, 10, 11, 14, 18, 29, 27) and offshore (8, 15, 16, 19, 20, 24), content 2 - 4% distributed nearshore (1, 4, 6, 22, 28). In C1, it varied from 5-7%, with an average of 5.4 %, 6-7% content was distributed in the top, middle and bottom of the core, and 5-6% content was most distributed from top to bottom of core. In C2, it varied from 5 to 7% and averaged 6.0%, at 5% and 7%, were not more abundant than at 6%. Chloride content in core and surface sediments did not differ significantly between stations and layers (Fig. 3d).
Quartz was most abundant in surface and core sediments. Surface sediments contained 26-79% and on average 50.5%, with content 60-79% extended almost in the nearshore (1, 2, 6, 22, 27, 28), with content 40-60% was from the nearshore (3, 5, 7, 10, 11, 12, 17, 18, 23, 29, 30) to the offshore (8, 16, 19, 20, 24), the content of 26- 40% was mostly distributed in the offshore (9, 13, 15), 21, 25, 26) and near the shore (14). In C1, it was 32- 48% and the average was 41.3%, the content of 45- 48% is divided into upper, middle and lower, the content of 40- 44% was divided into upper, middle and lower, and the content of 32-39% distributed top to bottom. In C2, it was 35-51%, with an average of 42.7%, the content of 45-51% was divided into upper, middle and lower, content 40- 44% from top to bottom and 35-39% from top to middle (Fig. 3e).
Feldspar was surface and two sedimentary cores. In surface sediments, it ranged from 3 to 16%, averaging 6.9%, with 11-16% distributed nearshore (18, 29) and offshore (13, 15, 16, 25), and 5-9% distributed nearshore (1, 2, 3, 5, 7, 11, 17, 22, 23, 27) and offshore (8, 9, 19, 20, 24, 26), with 3-4 % of content distributed nearshore (4, 6, 10, 12, 14, 28, 30) and offshore (21). In C1, distributed 3-7% with an average of 4.8%, content 6-7% was distributed at the top, middle, and bottom of the core, content 5% was most at the top, middle, bottom, and content 3% was some layers at the top, middle and bottom. In C2, it was 3-13% with an average of 6.2%, with content 7-13% distributed between top and middle, and with 3-6% distributed between top, middle and bottom (Fig. 3f).
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. In C2, it was 3- 4%, with an average of 3.6%, with content 4% distributed most from middle to bottom (Fig. 3g).
Halite was 0-3% in surface sediments and on average 1.8%, content of 2-3% was distributed both on nearshore (3, 7, 12, 14, 22, 23, 29, 30) and offshore (8, 9, 13, 15, 19, 20, 24, 25, 26), the content of 1–1.5% was distributed nearshore (1, 2, 4, 5, 6, 10, 11, 18, 28) and offshore (16, 21), nearshore (17, 27) without halide. It ranged from 0 to 2% for C1 and C2, and averaged 0.8 % in C1 and 1% in C2, and the 2% content in both C1 and C2 was most distributed from the middle to the bottom of the core (Fig. 3h).
Calcite and aragonite were carbonate minerals formed from biological materials. In surface sediments, they spread far from the shore and their content was often high, up to 11%, the content of 5-11% distributed almost to offshore (9, 19, 21, 26, 24, 25) and nearshore (14), the content 1-4 % were distributed nearshore (1, 2, 5, 6, 7, 11, 22, 23) and offshore (8, 13, 15, 16, 20), and some stations at nearshore absent calcite and aragonite (3, 4, 10, 12, 17, 18, 27, 28, 29, 30). In C1 had some layers with a content of 3-9% (17.0, 36.5, 42.5, 48.5cm), the remaining layers did not contain calcite and aragonite. In C2, calcite and aragonite were no present (Fig. 3i).
Gibbsite was only in C1 and C2 and the content was less than 4%, it was absent from surface sediments. In C1, it ranged from 1% to 3%, with an average of 2%, with 2% content was common than 3% and 1% content. For C2, it ranged from 0 to 2%, with an average of 0.4%, with contents of 1% and 2% distributed in some layers, most layers did not appear on the gibbsite (Fig. 3j).
Pyrite was present only in the C1 and C2 cores and absent in surface sediments. In C1 ranged from 0 to 4% with an average of 2.6%. Content was common at 2% and spread lower at 3% and 4%. In C2 averaged 2.8% with a range of 2- 4%, with content of 2% and 3% were common. Both C1 and C2 containing pyrite were of high content below the surface layer (Fig. 3k).
Correlation between sediment parameters
In surface sediments, parameters showed negative and positive correlation (Fig. 4a). There was a strong positive correlation between illite and kaolinite and chloride (R = 0.84); moderate correlation (R = 0.50 - 0.57) was between halite and illite, kaolinite, chlorite, Md and Sk, halite and calcite; and there was a weak correlation (R = 0.38 - 0.43) between quartz and Md, Sk, gothite and S0, calcite and chloride (Fig. 6). There was a strong negative correlation between quartz and illite, kaolinite and chloride (R ranging from -0.84 to -0.92); moderate correlations (R ranging from -0.52 to -0.64) were between Md and illite, kaolinite, quartz and halite, calcite, Md and chloride; and weak correlation between halite and Md, S0, Sk, kaolinite and Sk.
In C1 there was no strong positive correlation, a moderate correlation (R = 0.57 - 0.64) was between illite with kaolinite, chloride; there was a weak correlation between kaolinite with chloride, and between S0 with quartz (R = 0.40). There was no strong negative correlation, moderate correlation (R ranging from -0.66 to -0.73) between quartz with illite and kaolinite, between S0 and Sk; there was a weak correlation between chlorites and quartz, goethite, between kaolinite and calcite (Fig. 4b).
In C2, there was a moderate positive correlation between illite and chloride (R = 0.56); there was a weak correlation (R = 0.46 - 0.49) between halite and gothite, and between pyrite with S0. A moderate negative correlation (R from -0.58 to - 0.75) was between Md and S0, Sk and pyrite, between kaolinite and quartz; there was a weak correlation (R ranging from -0.42 to -0.49) between quartz and S0, illite, chlorite, between illite and feldspar (Fig. 4c).
Characteristic sediment groups
Based on grain sizes and minerals, cluster techniques were used, and the surface sediments of the Gulf of Tonkin have been divided into three groups with different characteristics (Table 2, Fig. 5). Group 1 consists of 7 stations located mainly nearshore (1, 2, 4, 6, 22, 27, 28), which was fine sand, poorly sorted and symmetrically skewed, with low content of illite, kaolinite, chlorite felspar, calcite, halite and high content of quartz. Group 2 had 15 stations, which distributed nearshore stations (3, 5, 7, 11, 12, 17, 18, 23, 29, 30) and offshore stations (8, 16, 19, 20, 24); they were very coarse silt, very poorly sorted, was fine in skewness; those of illite, kaolinite, chlorite, halite, feldspar and calcite were higher than group 1, while quartz was lower than group 1. Group 3, with 8 stations, was the most abundant offshore (9, 13, 14, 15, 21, 25, 26) and near the shore (10); they were very coarse silt, poorly sorted, very fine skewed; those of illite, kaolinite, chlorite, feldspar, and calcite and aragonite were higher than groups 1, 2, while the quartz content was the lowest (Table 2). The groups of sedimentary parameters were divided into three groups (Fig. 5), group 1 (Md, S0, Sk, quartz, goethite) represents a strong environment (at zone land-sea strong interaction), group 2 (montmorillonite, illite, kaolinite, chlorite, halite, calcite) represents a quiet environment (at zone land-sea weak interaction), and group 3 (feldspar) represented a mixed environment.
C1 had 3 groups with different characteristics (Table 2). Group 1 had 11 samples (layers) from top to middle of the core, Md was the highest, poorly sorted and fine skewed; those illite, kaolinite and chlorite were higher than groups 2 and 3, while quartz was the lowest. Group 2 had 13 samples (layers) most distributed near the bottom of the core, with some layers near the top and middle; The minerals illite, kaolinite, chlorite and pyrite decreased and were below group 1, while quartz was the highest. Group 3 had only one sample (layer) distribution in the middle core, only the content of calcite, aragonite, pyrite, halite and goethite was higher than groups 1 and 2, and the content of other minerals was lower. Sediment parameters were divided into two groups, group 1 (Md, pyrite, illite, kaolinite, chlorite, gibbsite, feldspar, Sk, aragonite, halite) indicated a quiet environment, group 2 (S0, quartz, goethite, calcite) indicated environment more dynamic.
Table 2. Grain sizes and mineral compositions in groups of sediment
Position
|
Group
|
Number of station
|
Sediment parameters
|
Content (%) of minerals
|
Md (mm)
|
S0
|
Sk
|
Mont.
|
Illi.
|
Kao.
|
Clo.
|
Qua.
|
Fel.
|
Go.
|
Ha.
|
Can.
|
Ara.
|
Py.
|
Gip.
|
Surface
|
1
|
7
|
0.134
|
2.57
|
0.05
|
0
|
7
|
4
|
3
|
70
|
6
|
5
|
1
|
1
|
0
|
-
|
-
|
2
|
15
|
0.059
|
4.42
|
-0.26
|
0
|
15
|
10
|
5
|
50
|
7
|
5
|
2
|
2
|
0
|
-
|
-
|
3
|
8
|
0.054
|
3.37
|
-0.37
|
1
|
19
|
15
|
6
|
35
|
8
|
4
|
2
|
5
|
1
|
-
|
-
|
C1
|
1
|
11
|
0.049
|
2.05
|
-0.26
|
-
|
22
|
16
|
6
|
37
|
5
|
3
|
1
|
0
|
0
|
3
|
2
|
2
|
13
|
0.045
|
2.18
|
-0.33
|
-
|
18
|
14
|
5
|
45
|
5
|
3
|
0
|
0
|
0
|
2
|
2
|
3
|
1
|
0.048
|
2.10
|
-0.24
|
-
|
16
|
14
|
5
|
39
|
4
|
4
|
2
|
2
|
7
|
4
|
1
|
C2
|
1
|
22
|
0.033
|
2.66
|
-0.22
|
-
|
21
|
13
|
6
|
42
|
6
|
4
|
1
|
-
|
-
|
3
|
0
|
2
|
2
|
0.027
|
3.01
|
-0.08
|
-
|
18
|
13
|
6
|
41
|
13
|
3
|
1
|
-
|
-
|
3
|
1
|
3
|
1
|
0.040
|
2.25
|
-0.31
|
-
|
20
|
6
|
6
|
51
|
6
|
4
|
0
|
-
|
-
|
2
|
0
|
Mont. – Montmorillonite; Illi. – Illite; Kao. – Kaolinite; Clo. - Chlorite; Qua. - Quartz; Fel. – Feldspar; Go. – Goethite; Hal. - Halite; Can. – Calcite; Ara. – Aragonite; Gip. – Gibbsite; Py. – Pyrite.
There were 3 sediment groups in C2 (Table 2). Group 1 consisted of 22 samples (layers) distributed from top to bottom of the core; those sediments were very coarse silt, poorly sorted, fine skewed; the content of illite, kaolinite and chlorite was high. Group 2, with 2 samples (layers) distributed near the top (2-4 cm and 6-8 cm), was coarse silt, poorly sorted and symmetrically skewed; the minerals with feldspar, illite, quartz, and gibbsite were different with group 1 and 3. In group 3, only one sample (layer) was near the core end (50-54cm) and had high quartz and low kaolinite content.