SEAWATER
n·alkanol
nglg dry sediment
150�---100
50
0
1 2 3 5
Site
ria
Fa tty acids4-4. Lipid compositions as source indicator of sediments deposited at a coastal system in Sagami Bay
4-4-1. Distributions of the low CPI hydrocarbons in Sagami Bay
The sample sediments, lAC, 2AC, SAC, 6AC and
7
AC, which contained the low CPI values ( <2) of n-alkane were distributed around the bay mouth area. The low CPI values are known in the case of oil pollution because the oil CPI value is almost 1. The CPI values of sedimentary hydrocarbons in the Tokyo Bay are reported to range from approx. 1 to 2 at the bay mouth by Takada (1993) and to range from 2.2 to 2.3 at the bay-head area by Uzaki et al., (1993). The coastal area of the Tokyo Bay is the most important place for oil industry, store and refining in Japan, then the seawater and deposit have been polluted (Uzaki et al., 1993: Takada, 1993). The low CPI values suggest the pollution in Tokyo Bay extend into these five samples in the Sagami Bay. Tokuyama et al., (1996) suggest the sediments of Samples lAC, 2AC, 6AC and 7 AC are derived from Tokyo Bay by the elemental compositions of the clastics (Table12).
In Fig.27
discrimination diagram using major element compositions and in Fig. 28 MORE-normalized spider diagram of Pearce ( 1983) show that elemental compositions can be divided two origins, one is Sakawa River and the other is Tokyo Bay. The Sakawa River type is characterized by high (FeO*+MgO)/(Si02+K 20+Na20) content and disitinct Nb anomaly together with low Rb content. The Tokyo Bay type is characterized by low (FeO*+MgO)/(Si02+K20+Na20) content without Nb anomaly.The CPI values of sample lAC and 2AC are the smallest and those sediments locate the nearest the mouth of the Tokyo Bay. Therefore, the oil pollutions are deduced to be spreading into the Sagami Bay by hypopicnal flow and accumulated by precipitation.
4-4-2. Depositional processes of terrigenous organic matter in Sagami Bay
Samples 2AC, 3AC and 7AC are enriched in terrigenous organic matter, however, it can not be shown the recognizable relation between the distribution patterns of organic compounds and tl1e clastic deposit types. In the Sagami Bay, the depositional processes of the terrigenous organic matter must not be uniform because large amounts of terrigenous
67
organic matter are unevenly distributed in the bay floor.
Terrigenous organic matter are supplied to the sea floor mainly by the two processes;
precipitation of suspended detritus as fecal matter or as absorbed on particles and turbidity currents from river or coastal area enriched in these matter. Site 3 is naturally enriched in terrigenous matter because this site locates on the fan off the Sakawa River, therefore, turbidity currents transport terrigenous matter directly. Although at Site 7 autochthonous input are also abundant, this reason can be explained by the transportation of the organic rich deposits from the Tokyo Bay by turbidity currents. The Tokyo Bay is an eutrophic bay and much terrigenous organic matter is carried into the bay by the several rivers (Ishiwatari, 1988).
In contrast, the higher amount of terrigenous organic matter in sample 2AC, located eastern coastal area, is supplied by rapid precipitation of suspended detritus as a result of zooplankton grazing. Because this site is far from the river mouth and locate on the continental slope, therefore, turbidity current can not be expected. In the Sagami Bay, coastal current rotate clockwise, therefore, the suspended detritus in the hypopycnal flows from the river are transported to the eastern coast side (Otsuka et al., 1973). Also the hypopycnal flow from the Tokyo Bay is assumed one of the sources of terrigenous organic matter at this site. The concentration of the cholesterol in sample 2AC is the second highest and the CPI value is the lowest among the all sites. Those results reflect the higher activity of the zooplankton grazing at this site. Consequently, large amount of terrigenous organic matter is accumulated here.
The concentration of the terrigenous organic mattes in sample 8AC is the lowest among the all samples in Sagami Bay. The northwest bay-head area is reported as one of the most active field of frequent submarine slumping and turbidity currents (Otsuka et al., 1973).
Burrows and tracks of organisms were not observed on this core sediment. Therefore, it suggests the surface sediment, including fresh organic matter, was denuded by submarine slumping or turbidity currents in contrast with Sites 3 and 7. Phytol is more stable against diagenetic degradation than fatty acids (Van Vleet and Quinn, 1979), therefore, it is considered the old deposit contains relatively higher concentration of phytol than fatty acids.
68
4-4-3. Characterization of cold water seepage zone at Site S
Around Site S, cold water seepages were located (Hashimoto and Hotta, 1994), and authigenic carbonates, mainly dolomite, are accumulated at the seepage zone (Masuzawa and Kitano, 1983). Sample SAC are enriched in iron, manganese, magnesium and calcium (Table 12), the cold water seepage accumulates them as dolomite into the deposit.
The fatty acids composition of Sample SAC is affected by predominance of anaerobic bacteria. Around this site the existence of Calyptogena communities are reported by Hashimoto and Hotta (1994), and those communities are demonstrated to be fed by hydrogen sulfide produced by sulfate-reducing bacteria as one of anaerobic bacteria (Masuzawa et al., 1992). The sulfate-reducing bacteria utilize methane dissolved in the seepage water (Masuzawa eta!., 1992), therefore, these bacteria are expected present around seepage zone.
This result suggests preference of anaerobic bacterila metabolisms rather than aerobic bacteria, fatty acid composition represents cold seep community.
The both compositions of the elements and the fatty acids can be the good indicator for discovery of the cold water seepage zone and the special ecosystem, which supported by higher activity of sulfate-reducing bacteria.
Table 12 Elemental compositions of top 3 em thick (5cm at Site l) of sediment cores obtained from the Sagami Bay analyzed by XRF.
lAC 2AC 3AC SAC 6AC 7AC 8AC
Si02 63.6 60.25 59.19 56.97 63.4 61.28 58.78
Ti02 0.71 0.77 0.85 0.77 0.72 0.75 0.9
Al203 14.48 14.74 15.42 14.82 14.05 14.58 16.09
� Fe20, 7.42 7.7 9.57 9.55 7.94 7.8 9.84
.!:: 01) MnO 0.08 0.09 0.1 0.15 0.08 0.13 0.15
"Q)
� MgO 2.73 3.01 3.52 3.85 2.82 2.87 3.91
CaO 5.3 7.14 5.53 8.4 5.07 6.54 5.64
Na20 1.81 1.88 2.1 2.02 1.78 2.02 2.14
K20 1.78 1.64 1.21 1.1 1.71 1.59 1.27
P20s 0.2 0.21 0.37 0.14 0.24 0.18 0.23
Ba 271 211 199 208 333 300 185
Sr 244 290 260 261 243 253 254
Zn 200 221 208 205 236 164 151
v 162 192 220 250 171 217 254
s Cu 89 104 129 115 112 109 115
0.. Zr 105 100 85 77 98 100 89
0..
Cr 82 69 80 107 76 71 88
Rb 59 52 33 28 54 49 35
Ni 38 32 38 27 36 29 35
y 23 25 20 22 22 24 21
Nb 8 5 5 3 7 5 5
('.1
0
"(�, CJ) 0 ('.1
<(
0.4
-0.35
0.3 - From Sakawa River
SACO 3ACO 5ACO 1
0.25
- � -�
- (, Ar.�l6'8o '; .
- - ---�
om Tokyo Bay0.2
0.08 0.1 0.12 0.14 0.16
( FeO*+Mg 0 )/(
Sl02+K20
+Na20)
0.18
Fig. 27 Discrimination diagram using major element compositions of sediment samples from the Sagami Bay.
100 -- - - ---- - - -- -- --- - -- ---- ---- --- - - --- --- - - -- - -- - - - -�---,
a)
10
--o- 1AC
t. 2AC
100 ------ ----- - - ------ ·-----.---.---.---r---.
10
1
b)
• 3AC---•--- SAC
" SAC
... .. ...
\ - -• ....
---"lit":--.... .,._--- ... -. - -----.--------------------------.
Fig. 28 MORE-normalized spider diagram ofPearce (1983). a) Sample lAC, 2AC, 6AC, and 7 AC, b) Sites 3AC, SAC, and SAC. Elemental compositions were determined by XRF method.
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4-5. Summary
4-5-1. Fatty acid as indicator of biological community structure around hydrothermal vents
The sediment samples obtained from the hydrothermal areas in the western Pacific, were enriched in monounsaturated fatty acids in quantity. This characteristic indicates predominance of aerobic bacterial metabolisms in the sediments. The low concentration and/or absence of PUF As also indicate vigorous bacterial activity in the sediments of the hydrothermal areas. While, the presence of some PUF As in the sediments suggests a contribution of animal metabolisms. Significant amount of methyl-branched fatty acids in the sediments is indicating presence of anaerobic bacterial populations.
The total fatty acid concentrations and estimated biomass in the sediments of the hydrothermal areas must be higher than that of usual marine sediment. Especially, the fatty acid concentrations of the sediment samples from DESMOS Caldera and the knoll of the Kaogshima Bay are one order higher. The result is indicating development of large biomass associated with hydrothermal activities. The sediment samples obtained from Iheya Ridge were recognized low concentration of bacterial fatty acids and relatively high concentration of terrigenous material. It would be cause of extensive sedimentation in the Okinawa Trough.
4-5-2. Lipid compositions as indicator of the sediment source and biological community structure in the Sagami Bay
The investigation shows the distributional patterns of the clastic deposit types and the organic compounds are not uniform. Main source of clastic deposits in the Sagami Bay is deduced the Tokyo Bay and also oil pollution is supplied as suspension by hypopycnal flow from the Tokyo Bay. Turbidity currents are main source for accumulation of deposits, although biological activities such as zooplankton grazing enhance accumulation of organic detritus onto seafloor. Turbidity current is the most important process for accumulation of large amounts of terrigenous organic matter into the channel and submarine fan deposits, however, sometimes the currents denude surface deposit contained fresh organic matter.
73