博士 / 國立臺灣大學 / 海洋研究所 / 86 / In this study, sediments, sediment trap particulates, suspended matter and
filtered seawater from off northeastern Taiwan have been analyzed for selecte
d metals by atomic absorption spectrophotometry. The objectives of this work
are: (1) to evaluate the possible sources of sediments in the Southern Okinawa
Trough (SOT); (2) to qualitatively understand the role of an eddy over the so
uthern East China Sea (ECS) shelf-slope region in the dispersal of terrigenous
materials from the shelf to the SOT and in the exchange of marine constituent
s between the shelf and the Kuroshio; and (3) to assess the possible impact of
pollutants (especially heavy metals) of riverine and aeolian sources on trace
metal composition in the water column of SOT.
Analytical results of metals for sediments and trap particulates showed th
at their metal contents were very similar, indicating that their compositions
were the same (except for samples from the trap at 1340 m of Sta. T6) and that
the settling particles should originate from the resuspended surface sediment
s and vice versa. In contrast, the suspended matter was rich in trace metals;
the metal excesses for suspended matter relative to deposited and settling pa
rticles could be due to the processes of particle scavenging (Ni and Cu), biog
enous source (Cd, Cu and Ni) and atmospheric input (Pb and Ni).
Apparent sediment mass fluxes observed from sediment traps revealed an inc
rease with depth and decreases from the Mien-hua Canyon (MHC) toward the troug
h and from the MHC to the North MHC. In terms of overall patterns of mass flu
xes over the trough the sources and transport pathway of the SOT sediments mig
ht be drawn when the present data are combined with those in Chung (1997). It
showed the fluxes on the north slope of the Ilan Shelf were much higher than
those in the canyon and trough. These trends were similar to those of PAl, su
ggesting that lateral transport of sediments from the Ilan Shelf and Ridge to
the trough was probably the major sediment source to the SOT and that the neph
eloid layers were the most important transporting vehicle. Hsu et al. (1998)
found that the horizontal distributions of particulate Al (PAl) in the surface
(5 m) and subsurface (30-50 m) water presented an "Ω-like shape, demonstrati
ng the influence of the cyclonic eddy on the distribution and transport of lit
hogenic particles in the shallow water. They also proposed a conceptual model
describing the eddy as a conduit for the shelf-slope sediments to the SOT. T
ogether with the model, another schematic model established in the study attem
pted to explain the whole picture of sources and transport of the SOT sediment
s.
The results suggest that Mn, Pb and Cd have different sources and biogeoch
emistry. The horizontal distributions of particulate and dissolved Mn (PMn an
d DMn) concentrations in the surface and subsurface water shared a similar fea
ture, i.e. an offshore decrease from the eastern and northern Taiwan. However
, the PMn was driven by advection and removal via settling, whereas DMn was dr
iven by advection-diffusion and particle scavenging via oxidation. Also, the
vertical distributions of PMn and DMn similarly existed a maximum in near-bott
om water on the slope and in middle water in the trough at depths of ~600-1000
m. When the surface sediment on the slope north of the Ilan Ridge resuspende
d, the Mn-rich interstitial water also introduced into ambient seawater and in
volved in nepheloid layers. This might be mainly responsible for the Mn plume
, but the localized diagenetic contributions from the ECS slope sediments shou
ld be taken into account. Additionally, DMn exhibited a surface maximum, whic
h could be attributed to eolian input and photoreduction. The higher DMn in c
oastal surface water, however, could be supplied from riverine sources.
Particulate and dissolved Pb (PPb and DPb) exhibited a contrary distributi
on in the surface and subsurface water. PPb showed a decrease offshore; where
as DPb showed an increase offshore. Also, the shallower water (< 50 m) exhibi
ted an elevated DPb; the deeper water showed a decreasing trend. These results
suggested that the atmospheric input was the major source for Pb and the part
icle scavenging was the most important removal mechanism. The DPb in shallowe
r water averaged 135 ng/l, which was about one order of magnitude higher than
in the open Pacific, reflecting the strength of atmospheric supply (Flegal and
Patterson, 1983). It is also noted that some elevated DPb occurred in the ne
ar-bottom water of the slope (especially beneath the eddy) which could be expl
ained by the regeneration of Pb-bearing biogenic particles at the sediment-sea
water interface.
The vertical distributions of particulate and dissolved Cd (PCd and DCd) s
howed a contrary pattern: PCd was enriched in the surface water and decreased
with depth whereas DCd was depleted in the surface water and increased with de
pth. For the horizontal distributions in the surface and subsurface water, th
ey were characterized by a similar pattern that is a maximum occurring in the
eddy center. These variation trends of PCd and DCd suggested that the nutrien
ts-rich upwelled water was the most predominant source for Cd and that the bio
logical uptake in the euphotic zone and regeneration of organic matter in deep
er water were the processes controlling the cycling of Cd. In the water colum
n of the slope and trough, DCd-maxima (0.6-1.1 nM) existed at depths of 500-10
00 m, which were comparable with the results reported for the Pacific. A pron
ounced high concentration of DCd, however, was observed in near-bottom water b
eneath the eddy, that was related intimately to the regeneration of high level
organic matter of the slope sediments. The phenomenon was consistent with Hs
u et al. (1998)''s conceptual model. Therefore, the overall variation patterns
of PCd and DCd appeared to be linked to the biogeochemistry of Cd and hydrogr
aphy of water.
In this work, the partition between solid and solution for Mn, Pb and Cd h
ave been estimated. The partition coefficients (KD) followed the sequence: Mn
(log [KD] = 6.3(0.4) > Pb (log [KD] = 5.3(0.4) > Cd (log [KD] = 4.2(0.6). Th
e dissolved form for these three metals in the water column was more predomina
nt than particulate form; the proportions of the particulate form accounting f
or the total of these three metals were 37%, 6% and 1%, respectively.
Methane and manganese anomalies can be used to identify hydrothermal plume
s. Besides the documented methane anomalies, the Mn anomalies have also been
detected in the long-term trap particulates (T6) and seawater at a depth of ab
out 1300 m in the SOT. Together with Chen''s data, the ratios of DMn/CH4 were
quite small, probably reflecting the unique tectonic and geological characters
of the SOT.
| Identifer | oai:union.ndltd.org:TW/086NTU00274003 |
| Date | January 1998 |
| Creators | Hsu, Shih-Chieh, 許世傑 |
| Contributors | Jeng Woei-Lih, Lin Fei-Jan, 鄭偉力, 林斐然, --- |
| Source Sets | National Digital Library of Theses and Dissertations in Taiwan |
| Language | zh-TW |
| Detected Language | English |
| Type | 學位論文 ; thesis |
| Format | 268 |
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