RA228 AND RA226 FROFILES FROM THE NORTHERN SOUTH CHINA SEA

1
OS11A-04
RA-228 AND RA-226 FROFILES FROM THE
NORTHERN SOUTH CHINA SEA Hsiu-Chuan Lin, Yu-Chia
Chung and Chi-Ju Lin Institute of Marine Geology
and Chemistry, National Sun Yat-sen University,
Kaohsiung, Taiwan
Introduction The South China Sea (SCS) is a
large semi-enclosed marginal sea to the west of
the tropical Pacific. It is bounded by the
landmass of China and Vietnam, the Philippine
Islands, and Borneo. The northern part of the
SCS includes a large shelf and slope area on the
north and a deep basin on the south (Fig. 1).
The SCS is connected to the western Pacific
through the Luzon Strait where the western
Philippine Sea (WPS) deep water may exchange with
the SCS deep water through a deep passage of
about 2200m depth. The surface water circulation
and hydrography in the SCS are strongly
influenced by the East Asian monsoon system that
generates a cyclonic circulation in winter and an
anti-cyclonic one in summer. The seasonal
changes of the monsoon system induces changes in
the mixed layer thickness, upwelling, primary
production, and associated biogeochemistry. It
is thus important to understand how these
parameters are inter-related in their temporal
and spatial variations, including their effects
on the deep water. In order to learn more
about the characteristics of the Kuroshio
intrusion and the source strength of radium
isotopes from the coastal zone, we carried out
surface water and water column samplings for
Ra-228 and Ra-226 measurements in the northern
SCS and the Luzon Strait areas. The surface
water stations are located along the track
forming an inverse triangle in the northern SCS
and three locations in the Luzon Strait (Fig. 1).
Profiles of Ra-228 and Ra-226 were measured at
Station J and three additional Ra-228 profiles
were measured at Station I, C, and M1. This
poster presents the distributions of surface
water Ra-228 and its vertical profiles. The
surface and vertical distributions of
Ra-228/Ra-226 ratio are also discussed.
Methods Both the Ra-228 and Ra-226 samples
were collected during four R/V Ocean Researcher I
cruises one (ORI-662) was in October, 2002 the
other (ORI-688) was in July, 2003 a third
(ORI-707) was in February, 2004 and a fourth
(ORI-734) was in October, 2004. Ra-226 samples
of about 20L size were measured using a
regenerated radon method adopted by the GEOSECS
program. Ra-228 samples of about 200L surface
water were collected by pumping while the profile
samples were collected by 10 Niskin bottles of
20L size mounted on a CTD rosette. Each Ra-228
sample was filtered through a column of MnO2
impregnated fiber for Ra extraction. The
extracted Ra was removed from the fiber by acid
leaching into a container for gamma counting.
The extraction efficiency of the MnO2 fiber was
estimated to be between 80 and 90. The results
presented here are preliminary because this
factor has yet to be corrected for. The
uncertainty of measurements should be about 20
or less.
Results and discussion The surface water
Ra-228 distribution as measured from October in
2004 (ORI-734, three samples), 2002 (ORI-662) and
2000 (ORI-597, only a few samples) is shown in
Fig. 2. Higher activities (gt28 dpm/100L) are
seen in the northern (shelf and slope) area
lower values (lt24 dpm/100L) are observed in the
southern (deep basin) area the lowest values
appear in the eastern (Luzon Strait) area. Quite
similar distribution (Fig. 3) was obtained in
July, 2003 (ORI-688). Highest activities are
located at Station H near the Pearl River estuary
for both the October and July data, suggesting a
strong Ra-228 source from the Pearl River and its
adjacent coastal zone. However, the location of
the lowest value in October is at Station D,
while that in July is at Station F. The
distribution pattern suggests an inflow of the
WPS surface water of low Ra-228 values, and this
inflow shifts southward with increasing intensity
from July to October, reflecting seasonal
changes in surface water circulation. The
Ra-228/Ra-226 activity ratio indicates that the
values in October (Fig. 4) are lower than those
in July (Fig. 5). The October values are mostly
around 1.5, but the July values are higher,
falling generally between 1.75 and 2.5. The
higher values in July are mostly due to somewhat
higher Ra-228 coupled with lower Ra-226 in July.
Five Ra-228 vertical profiles were measured in
the northern SCS (Fig.6), including deep water
and shallow water stations. These 228Ra profiles
show a similar pattern higher values in the
surface layer fairly uniform at somewhat lower
values below 200m depth. Some profiles show an
increase toward the bottom most likely due to
input from the underlying sediments. The deep
water values around 10 dpm/100L are probably the
highest among all deep water. The Ra-228 and
Ra-226 profiles observed at Station J in October
(ORI-662) and July (ORI-688) are shown in Fig. 7.
Above 1000m depth, the Ra-228 values of July and
October tend to be more variable or scattered.
Below this depth, Ra-228 becomes fairly constant
with depth. The associated Ra-226 profiles show
a general increase toward the bottom. The
associated 226Ra profiles are variable,
especially above 1500m depth, but remain
comparable to those seen in the northwest
Pacific. Within the upper 500m layer, Ra-226 has
about the same values for both cruises, showing a
minimum at about 200m. From 500m to 1500m, the
October Ra-226 values are systematically higher
than the July values by at least 5 dpm/100L.
Ra-226 profiles from both cruises indicate a
sudden drop in activities from 1500m to 2000m.
It is clear that Ra-228 activity in the SCS is
much higher than in the open ocean, but Ra-226
activity has no significant difference between
them. The Ra-228/Ra-226 activity ratio for the
two profiles at Station J is shown in Fig. 8
together with that of the open Atlantic and
Pacific oceans. The ratio is generally greater
than unity above 500m but becomes less than unity
below. The values decrease from 0.5 at about
500m to 0.35 below 3000m depth. These values are
about ten times higher than those of the open
ocean, suggesting a strong Ra-228 input relative
to Ra-226 from the deep SCS. The high Ra-228
activities distributed in the northern SCS
indicate that its source is most likely from the
coastal zones of southern China, including the
Pearl River estuary.
Conclusions The surface water Ra-228
distribution is quite similar in pattern as
observed in October and July, showing a general
increase from the south or southeast toward the
north or northwest. This pattern indicates a
strong source from the coastal zone of southern
China and a dilution effect by the intrusion of
the WPS water. The resulting Ra-228/Ra-226
activity ratio increases generally from about 1.5
to 2.5 toward the west in July. The Ra-228
activities are much higher in the SCS than in the
open ocean, but both tend to show a similar
pattern higher values at the surface and lower
value below. The Ra-226 profiles as measured
from Station J in October and July show similar
activities below 1500m but vary quite
significantly above 1000m depth, but remain
comparable to those of the western North Pacific.
The October profile shows systematically higher
activities than the July one above 1500m probably
due to variable source strength above this depth.
The Ra-228/Ra-226 activity ratio decreases from
greater than unity in the upper layer to about
0.35 in the deep water. This deep water value is
much greater than that in the open oceans where
it is generally less than 0.1. This suggests a
strong input of Ra-228 relative to Ra-226 from
the bottom sediments into the deep water of the
semi-enclosed SCS.