Modern (<100 years) sedimentation in the seas around Taiwan
Chih-An Huh
Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan, R.O.C.
Erosion on land leads to sedimentation at sea. Considering that Taiwan’s
erosion rate is by far the highest in the world, it is quite incomprehensible that little is
known about modern sedimentation in the seas around this high-standing, tectonically
active island. Fallout radionuclides produced naturally (7Be and 210Pb) and
artificially (137Cs and 239, 240Pu) can be used as tracers to study sedimentation
dynamics from event to decadal and centennial timescales. My co-workers (listed in
the References) and I have in the past 11 years collected/studied hundreds of box and
gravity cores from S2S dispersal systems in the East China Sea (ECS), the Southern
Okinawa Trough (SOT), the shelf/slope/submarine canyon area off the Gaoping River,
southwestern Taiwan, and the Taiwan Strait (TS). These cores were analyzed for
fallout radionuclides (and other ancillary properties) to derive accumulation rates,
pathways and budgets of sediments. The following points and their evidence will be
highlighted at this workshop.
1. The most pronounced source of sediments in the ECS is the Yangtze River. The
sediment plume out of the river’s mouth is transported alongshore toward the
south, forming an elongated mud wedge whose volume (and sediment chronology)
accounts for most of the sediment exported from the Yangtze River. For more
details about sedimentation in the ECS, please refer to Huh and Su (1999), Su and
Huh (2002) and Liu et al. (2007).
2. Lanyang River off northeastern Taiwan is a major source of sediments trapped in
the SOT. Hemipelagic sediments in much of the SOT provide a wealth of
information about sediment chronology, particle scavenging and teleconnection
(and its speed) between the equatorial Pacific and the western Pacific margins via
the NEC and Kuroshio. In the middle of the SOT, there exists a depocenter
where the sediment strata are characterized by layers of turbidites interbedded in
hemipelagic sediments. The turbidite layers can be correlated not only spatially
but also temporally with the history of major submarine earthquakes in and around
the region, suggesting that they were seismoturbidites. For more details about
sedimentation records in the SOT and their implications, please refer to Lee et al.
(2004) and Huh et al. (2004, 2006).
3. A large number of box cores collected during 2004-2006 from an area of ~3,000
km2 off the Gaoping River, SW Taiwan, were analyzed for fallout radionuclides
(210Pb, 137Cs and 7Be) to elucidate sedimentation rates and processes, and for the
calculation of a sediment budget. The study area is located at an active collision
margin with a narrow shelf and a submarine canyon extending essentially into the
river’s mouth. The results indicate fairly constant hemipelagic sedimentation in
much of the open margin and for most of the time except in the inner shelf and
along the axis of the canyon where sediment transport is more dynamic and is
controlled by tidal current and wave activities constantly, and by fluvial floods or
gravity-driven flows episodically. Sedimentation rates in the study area derived
from 210Pb and constrained by 137Cs vary from 0.04 to 1.5 cm/yr, with the highest
rates (>1 cm/yr) flanking the Gaoping canyon over the upper slope (200-600 m)
and the lowest rates (<0.1 cm/yr) in the distal basin beyond the continental slope.
The depocenter delineated from 210Pb-based sedimentation rates overlaps with the
area covered by a flood layer resulting from super-typhoon Haitang in July 2005.
Such correspondence supports the notion that the processes operating on event
timescale have bearing on the formation of the sediment strata over centennial or
longer timescales. From the distribution of sedimentation rates, sediment
deposited in the study area annually is estimated to be 6.6 MT/yr, accounting for
less than 20% of Gaoping River’s sediment load. The calculated budget, coupled
with the presence of 7Be and non-steady-state distribution of low levels of 210Pb in
sediments along the canyon floor, suggests rapid transport of sediment from
Gaoping rivers’s mountainous watershed via the Gaoping Canyon and adjacent
channels to the abyssal plain and the Manila Trench in the South China Sea. The
results have been written up and submitted to JMS (Huh et al., submitted).
4. Compared with other settings mentioned above, sedimentation in the Taiwan Strait
is more dynamic and variable with time due to much greater supply of sediments
to a shallow and relatively confined accommodation space with energetic tides
and waves. Preliminary data show that the Choshui River is the most
pronounced source of sediments to the TS and that freshly deposited mud in such
a dynamic setting can be frequently and preferentially resuspended and
transported toward the north by the Taiwan Warm Current, leaving sand-sized
sediments behind.
References
Huh, C.-A. and C.-C. Su (1999) Sedimentation dynamics in the East China Sea
elucidated from 210Pb, 137Cs and 239,240Pu. Marine Geology 160(2), 183-196.
Huh, C.-A., C.-C. Su, W.-T. Liang and C.-Y. Ling (2004) Linkages between turbidites
in the southern Okinawa Trough and submarine earthquakes. Geophysical
Research Letters 31, L12304, doi:10.1029/2004GL019731.
Huh, C.-A., C.-C. Su, C.-H. Wang, S.-Y. Lee and I.-T. Lin (2006) Sedimentation in
the Southern Okinawa Trough – rates, turbidites and a sediment budget. Marine
Geology 231, 129-139.
Huh, C.-A., H.-L. Lin, S.-W. Lin and Y.-W. Huang (submitted) Modern accumulation
rates and a budget of sediment off the Kaoping River, SW Taiwan: a tidal and
flood dominated depositional environment around a submarine canyon.
Submitted to Journal of Marine Systems.
Lee, S.-Y., C.-A. Huh, C.-C. Su and C.-F. You (2004) Sedimentation in the Southern
Okinawa Trough: Enhanced particle scavenging and teleconnection between the
equatorial Pacific and western Pacific margins. Deep-Sea Research Part I 51,
1769-1780.
Liu, J.P., K.H. Xu, A.C. Li, J.D. Milliman, D.M. Velozzi, S.B. Xiao and Z.S. Yang
(2007) Flux and fate of Yangtze River sediment delivered to the East China Sea.
Geomorphology 85, 208-224.
Su, C.-C. and C.-A. Huh (2002) 210Pb, 137Cs and 239,240Pu in East China Sea sediments:
sources, pathways and budgets of sediments and radionuclides. Marine Geology
183, 63-178.