Previous studies have shown that the continental shelf and slope area off
northeastern Taiwan is a region where the material exchange processes between the
East China Sea and open ocean are most rigorous under the influences of the
Kuroshio and the monsoons. In order to understand the topographic effects on the
particulate flux and the transport path of the settling particles in this region, this study
has conducted a series of field measurements using time-series sediment traps and
current meters as tools. The temporal and spatial variations on the current, the
particulate flux and the particle-size distribution as well as the activity and flux of
210 Pb and 210 Po are presented in this thesis.
The biological productivity and the runoff of main rivers from Taiwan were not
correlated with the particulate fluxes in our study area. However, the variations of
the particulate flux in the canyon and on the slope areas were clearly affected by the
tidal currents, showing good positive correlation with their amplitudes. The
particulate, 210 Pb and 210 Po fluxes all showed an increase toward the ocean bottom but
a decrease away from the shore, suggesting that the transports of particle and particle
reactive elements were strongly influenced by the lateral transport process in the study
area. Highest fluxes were observed in the Mien-Hua Canyon(MHC) and the lower
slope southwest of the canyon. Intermediate values were observed in the South
Okinawa Trough and the North Mien-Hua Canyon(NMHC). The intercanyon slope
and the shallow traps in the NMHC had lowest values. The particulates and
particle reactive elements were transported out of the shelf or upper slope area
essentially through the MHC toward the slope southwest of the canyon. The NMHC
and the intercanyon area were not major pathways for these materials. The fluxes of
particle, 210 Pb and 210 Po in the study area were generally at least an order of
magnitude greater than those observed from other continental margins in the world,
except for the lowest values at the intercanyon station and the shallow water in the
NMHC. Thus the continental margin off northeastern Taiwan is one of the most
important areas for material exchanges with open oceans in the world.
The trapped particulates were mainly silt with variable sand fractions. High
sand contents were observed in the MHC and the lower slope southwest of the canyon,
the intermediate values were in the outlet of the MHC and the NMHC, and the lowest
in the South Okinawa Trough. This shows that the sand content decreases awayfrom the shore. On the contrary, silt and clay fractions in percent increase away
from the shore. Under the influence of tidal currents, the settling sand particles in
the MHC and its adjacent lower slope were laterally transported from resuspended
sandy sediments of the continental shelf and upper slope area adjoining the head of
the MHC. The fluxes of sand, silt and clay all decreased toward the South Okinawa
Trough but increased toward the bottom. This particle-size distribution pattern also
reflected the effect of lateral transport process in the study area.
Variations of the 210 Pb activity in this study area were generally correlated
inversely with the particulate flux and the sand content, indicating the effect of
dilution of the mass flux and the sand content, but were positively correlated with the
silt and clay contents because they were major 210 Pb scavengers. No apparent
correlations of 210 Po with the mass flux and the three particle-size fractions were
observed, suggesting that the variation of 210 Po activity was not related to the mass
fluxes or the three particle-size fractions.
High 210 Pb activities were found to be associated with high contents of POC, Mn
and Fe. Correlations of 210 Pb with both Mn and Fe indicate that the oxide and
hydroxide precipitates of Mn and Fe supplied from sediments on the continental shelf,
upper slope area and submarine hydrothermal activities might serve as important
mechanisms for enhanced 210 Pb scavenging. The relationship between the 210 Pb
activity and the POC content might reflect either the settling mode of the fine particles
incorporated with fecal pellets or aggregates, or sharing the same source with fine
particles. The variatio ns of 210 Po activity were not correlated with Mn, Fe, major
components or biogenic components, implying that the association of 210 Po with
biogenic components might be masked by the enormous terrigenous materials.
Except for the upper-trap samples collected at the intercanyon slope and the
NMHC, the 210 Po activity was lower than the 210 Pb activity with 210 Po/210 Pb ratios less
than unity. Based on comparisons of 210 Pb and 210 Po with relevant parameters and
references of past studies, we can propose several possible mechanisms to explain
why the ratios were less then unity: 1. 210 Pb was preferentially scavenged by
advecting and settling particulates of terrigenous origin. 210 Po, known to be
biophilic, might remain associated with the suspended organic particulates which
could be dissolved or not settling. Moreover, the oxide and hydroxide precipitates of
Mn and Fe in the study area would further enhance the scavenging of 210 Pb. 2. The
particulates in suspension were favorable for differentiation between 210 Pb and 210 Ponuclides while those settled in sediments would allow them to grow toward the
secular equilibrium. Therefore, the trapped particulates might be in frequent
suspension and short in deposited sediments during their lateral transport. 3. The
210 Pb supplied from the East Asia aerosol in large quantity was accumulated in water
column abd carried by the Kuroshio to the study area. The intrusion and eddies of
Kuroshio off northeastern Taiwan could bring large amount of 210 Pb into the region of
continental shelf and slope, allowing particulates to scavenge 210 Pb more effectively
from the surrounding water.
The upper trap in the NMHC and that in the intercanyon site yield lowest
particulate flux but highest POC content. With less terrigenous material and more
biogenic components in these samples, 210 Po/210 Pb ratios were greater than unity and
variations of 210 Pb activity were hardly correlated with those of POC contents but
were inversely correlated with those of Mn contents. However, the 210 Po activities
of upper trap at the intercanyon site were well positively correlated with POC and PIC
contents, indicating effects of the fresh biogenic particles.
Based on the mass balance calculations of particulate and 210 Pb fluxes, current
data and other related data gathered in this study, we may infer the sedimentation
dynamics and transport path of the settling particles below 300m depth in the study
area. Under the influence of tidal oscillation, terrigenous sediments on the shelf and
the shelf break as well as the upper slope would be resuspended and carried into the
MHC and transported further away. Most of the sand particles would deposit in the
canyon while part of the silt and clay would be transported to the South Okinawa
Trough directly and the rest to the lower slope southwest of the MHC. The
particulate flux at the lower slope southwest of MHC might also have a portion of
particulates swept from the upper slope by the Kuroshio eddies. As the NMHC and
the intercanyon sites are located in the central area of the Kuroshio eddies, the
particulates from the continental shelf and upper slope area would be blocked,
resulting in very low particulate fluxes. High particulate fluxes observed in the
lower NMHC trap should largely be due to resuspension of the sediments from the
banks of the canyon.
| Identifer | oai:union.ndltd.org:NSYSU/oai:NSYSU:etd-0201101-151718 |
| Date | 01 February 2001 |
| Creators | Hung, Gwo-Wei |
| Contributors | Y.-C. Chung, J.-J. Hung, K.-K. Liu, T.-Y. Tang, C.-L. Wei, M.-P. Chen, C.-A. Huh |
| Publisher | NSYSU |
| Source Sets | NSYSU Electronic Thesis and Dissertation Archive |
| Language | Cholon |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://etd.lib.nsysu.edu.tw/ETD-db/ETD-search/view_etd?URN=etd-0201101-151718 |
| Rights | unrestricted, Copyright information available at source archive |
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