Ching-Ling Wei

Nutrient assimilation, export production and 234Th scavenging in the eastern equatorial Pacific

Abstract Primary productivity, nitrogen uptake using 15N-labeled NH4+ and NO3−, and the distribution and vertical fluxes of POC, PON, chlorophyll, phaeopigments and 234Th have been analysed at four stations in the eastern equatorial Pacific. Two stations were north and two stations were south of the equatorial front. Most analyses show little variation across the front, which is a boundary between cold, nutrient-rich surface water to the south and warm, nutrient-poor surface water to the north. New production and the f- ratio has been calculated using several approaches, including nitrogen uptake rates, export fluxes of POC and PON and the residence time of 234Th. At three of the four stations these independent estimates agree to within a factor of 2. All estimates of the f- ratio (0.1–0.3) are lower than would be predicted from published f vs NO3− concentration or f vs primary production curves. The ammonia concentration may be the controlling factor in this case. The residence time of 234Th is about a factor of three less than the residence time of carbon but agrees well with the residence time of total suspended matter. The residence time of nitrogen is about 50% longer than that of carbon.

Response of Kuroshio upwelling to the onset of the northeast monsoon in the sea north of Taiwan: Observations and a numerical simulation

A cold water anomaly, which manifests upwelling of the subsurface Kuroshio Water, has been frequently observed at the shelf break of the East China Sea to the north of Taiwan. Its response to the onset of northeast monsoon was observed during August–October 1990. The wind direction reversed in mid-September, indicating the onset of northeast monsoon. Shortly thereafter, the sea surface temperature at the center of the cold eddy showed a pulselike sudden drop, and a significant concentration of nitrate (up to 5 μM) appeared in the surface water, suggesting intensification of upwelling. Subsequently, the upper layer of the Kuroshio Water intruded onto the shelf. The general circulation model of the East China Sea previously developed by Chao was used to simulate the overall response of the East China Sea. The numerical simulation reproduced the intensification of upwelling. It also predicted extensive Kuroshio intrusion along the shelf break farther north of Taiwan as well as the temporary intensification of the northeast branch of the Yangtze River outflow. The energy source of this sudden intensification comes from the potential energy released by the Kuroshio as the isopycnals maintaining the Kuroshio rise in response to the wind change.

The behavior of scavenged isotopes in marine anoxic environments: 210Pb and 210Po in the water column of the Black Sea

Abstract Vertical profiles of dissolved and particulate 210Pb and 210Po were determined at two stations in the Black Sea in June 1988. Vertical fluxes of 210Pb and 210Po were also measured in the upper 150 m, using floating sediment traps. The fractionation of 210Pb between dissolved and particulate phases in the Black Sea is strongly influenced by the redox conditions in the water column. Dissolved 210Pb dominates in the oxic zone, while particulate 210Pb is the major form in the deep sulfide-rich anoxic zone. The distribution of 210Pb across the suboxic zone appears to be mainly controlled by redox cycling of manganese and iron. In the sulfide-rich layer coprecipitation of lead with iron sulfide is probably the dominant scavenging mechanism. A simple scavenging model was used to calculate the residence times of dissolved and particulate 210Pb in the oxic, suboxic, and anoxic zones. The residence times of dissolved 210Pb relative to scavenging by particles are 0.5–1, 2–3, and 3.5 years in the oxic, suboxic, and anoxic layers, respectively. The corresponding residence times of particulate 210Pb relative to particle removal processes in the same layers are 0.1, 1.5–2.5, and 8.5 years, respectively. A particle settling velocity of about 40 m y−1 was derived from the 210 Pb 226 Ra disequilibrium in the deep Black Sea. The relatively short residence times of 210Pb support the hypothesis that anoxic basins are important sites for boundary scavenging of 210Pb. The 210Po profiles indicate that biological rather than inorganic particles are the major carrier phases for polonium in the Black sea. Dissolved 210Po is deficient relative to dissolved 210Pb in the euphotic zone, suggesting preferential uptake of 210Po over 210Pb by particles residing in that layer. The residence time of dissolved 210Po, with respect to scavenging by particles in the euphotic zone, is about 200 days. Below the mid-depth of the suboxic zone, 210Po is in excess relative to 210Pb, and is thought to originate from shelf and slope sediments. Based on the magnitude of distribution coefficients (KD), the relative partitioning of lead, polonium, and thorium to particles found in the oxic and anoxic layers of the Black Sea are Po > Th > Pb and Po = Pb > Th, respectively. The dependency of KD on particle concentration suggests that colloidal phases may be important for the scavenging of these radionuclides.

234Th/238U disequilibria in the Black Sea

Profiles of total suspended matter, dissolved 238U, and dissolved and particulate 234Th were determined at three stations in the Black Sea during the R.V. Knorr cruise in June 1988. Sediment traps were deployed on a free-floating mooring line to measure vertical fluxes of total mass and 234Th at two deep stations. This paper presents the first data set of 234Th/238U disequilibria in anoxic environments. Our dissolved238U results are consistent with literature data. The dissolved 238U deviates from the predicted conservative mixing activities below the depth of oxygen depletion. Dissolved 234Th deviates from secular equilibrium activities in the euphotic and suboxic zones. The profiles of dissolved 234Th across the oxic-anoxic interface are similar to those of phosphate, indicating the importance of Fe-Mn oxyhydroxides in controlling the thorium distributions. Particulate 234Th is enriched in the euphotic zone and in the upper layer of the suboxic zone. Comparison of the profiles of 234Th, Fe and Mn suggests that the distribution of 234Th is mainly controlled by the Mn rather than the Fe redox cycle. In general, BS3-2 (western basin) has higher total mass and 234Th fluxes than BS3-6 (central basin). The fluxes of 234Th measured by sediment traps deployed at depths from 40 to 150 m, range from 200 to 800 dpm m2 day−1, with a maximum at 60 m. The residence times of dissolved and particulate 234Th and total suspended materials are calculated using the irreversible scavenging model and the sediment trap flux data. The results suggest that, under these conditions, Th is a good tracer for the total mass flux.

Geochemical behavior of 210Pb and 210Po in the nearshore waters off western Taiwan.

Dissolved and particulate (210)Pb and (210)Po were determined at 15 stations along the coastline off western Taiwan in April 2007. The (210)Pb activities in dissolved and particulate phases fell within a relatively small range of 2.4-5.2 dpm 100 L(-1) and 1.0-3.2 dpm 100 L(-1), respectively. The dissolved and particulate (210)Po activities also fell within a small range of 0.8-3.4 dpm 100 L(-1) and 1.1-2.9 dpm 100 L(-1), respectively. The correlation of the distribution coefficients (K(d)) of (210)Pb and (210)Po with particle concentration in turbid waters are not as evident as in the open ocean. The mass balance calculation shows that the residence times of (210)Pb and (210)Po with respect to particle removal from the nearshore waters ranges from 3 to 15 days and from 14 to 125 days, respectively. The flux of particulate organic carbon was estimated by (210)Po proxy and ranged from 4.8 to 33.7 mmol-C m(-2) d(-1).

Water column distribution of 230Th and 232Th in the Black Sea

Abstract Profiles of 230 Th and 232 Th at a station in the western central Black Sea were determined using a highly sensitive mass spectrometry method. Compared with most open ocean and coastal waters, concentrations of “dissolved” ( μ m) 230 Th and 232 Th in the Black Sea are significantly higher, primarily due to lower scavenging rate in this predominantly anoxic environment. Above the anoxic zone (0–95 m) about 42% of 230 Th and 57% of 232 Th are in the particulate form, compared with 9 and 21%, respectively, in the anoxic zone. The distribution indicates that a sizable fraction of particulate Th is associated with Mn-containing particles at the O 2 H 2 S interface just above the anoxic zone, which is released into solution when such particles are transported across the redox boundary and are dissolved in the anoxic zone. From the partitioning and isotopic composition of Th between dissolved and particulate phases, it is estimated that approximately 20–40% of the dissolved 230 Th in the water column is terrigenous. The residence time dissolved Th in the Black Sea water column is 43–48 years, compared with 6–20 years for the same depth range elsewhere in the world oceans.

Spatial variation of 234Th scavenging in the surface water of the Bashi Channel and the Luzon Strait

Activities of dissolved and particulate234Th were measured to study the spatial variation of scavenging phenomena in the surface water of the Bashi Channel and the Luzon Strait. Using an irreversible scavenging model, the residence times for total, dissolved, and particulate234Th are 28–613, 8–156, and 7–306 days, respectively. Along the PR21 transect of the WOCE project, three major domains can be identified based on the hydrography and the234Th data: the Bashi Channel, the Northern Luzon Strait, and the Southern Luzon Strait. Tight correlation between the scavenging constant and the suspended particle matter (TSM) concentration in the surface water of the Northern Luzon Strait implies that TSM concentration is an important parameter in studying thorium scavenging.

Sediment trap experiments in the water column off southwestern Taiwan:234Th fluxes

The activity of234Th (t1/2=24.1 days) in dissolved, particulate and sediment trap samples was determined in the water column off southwestern Taiwan during 2–4 October, 1993. Vertical234Th fluxes measured by the free-floating sediment traps ranged from 363 to 2290 dpm m−2 d−1 in the upper 450 m. Th-234 fluxes predicted from the irreversible scavenging model concur with those measured by the sediment traps. Comparison of the residence times of particulate234Th and particulate organic carbon showed that their respective values differ by a factor of approximately 2∼3, which suggests organic carbon is preferentially recycled relative to234Th in the euphotic zone.

Removal fluxes of Mn and Fe from the nearshore waters along the west coast of Taiwan

Dissolved and particulate Mn and Fe in the nearshore waters were determined at 27 stations along the west coast of Taiwan during 19-23 November 2004. The latitudinal distributions of Mn and Fe were very similar; however, the concentration in the dissolved phase was lower, whereas that in the particulate phase was higher in the northern regime. The higher percentage of Fe than Mn that was associated with particles resulted in a two-order of magnitude higher particle-water partition coefficient for Fe, K(d)(Fe), than that for Mn, K(d)(Mn). The removal fluxes of Mn and Fe could be estimated by multiplying the particulate (234)Th removal flux with the Mn/(234)Th and Fe/(234)Th ratios in suspended particles, which ranged from 0.1 to 3.8 mmol m(-2) d(-1) for Mn, and from 3.4 to 194.5 mmol m(-2) d(-1) for Fe. Residence times ranged from 1 to 20 days for Mn and Fe were estimated in the nearshore.

Distributions and removal fluxes of trace metals in the water column of the Hung-Tsai Trough off southwestern Taiwan.

Vertical profiles of dissolved and particulate Mn, Fe, Ni, Cu, Zn, Cd, Pb, and (234)Th were determined in the Hung-Tsai Trough off southwestern Taiwan during 19-23 November, 2004. Except in the case of Cd, the distribution coefficient (K(d)) of the trace elements showed a negative correlation with the suspended particle concentration. Based on the average K(d) values, the general sequence of particle affinities for the eight trace elements is, from highest to lowest, Fe>Mn≈Pb>Zn≈Th>Cd≈Cu≈Ni. The trace metal data was coupled with the particle removal flux estimated from (234)Th/(238)U disequilibrium to investigate metal removal by particle sinking from the euphotic layer. The residence time of trace elements with respect to particle removal from the euphotic layer was estimated. A negative correlation between the residence time and the distribution coefficient for the trace metals was found.