Ayako Okubo, 1 Hajime Obata, 1 Yoshiyuki Nozaki, 1,2 Yoshiyuki Yamamoto, 1 and Hideki Minami Introduction. 2. Hydrography and Topography L22306

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1 GEOPHYSICAL RESEARCH LETTERS, VOL. 31, L22306, doi: /2004gl020226, Th in the Andaman Sea: Rapid deep-sea renewal Ayako Okubo, 1 Hajime Obata, 1 Yoshiyuki Nozaki, 1,2 Yoshiyuki Yamamoto, 1 and Hideki Minami 3 Received 21 April 2004; accepted 18 October 2004; published 23 November [1] 230 Th activities were determined in seawater collected from the Bay of Bengal and the Andaman Sea. The Andaman Sea is a semi-closed basin that is isolated from the Bay of Bengal by the Andaman-Nicobar Ridge. The noticeable trend in the vertical distribution of 230 Th activity in the Andaman Sea is its uniformity below 1250 m depth. We applied the scavenging-mixing model to this vertical profile in the Andaman Sea, and estimated the upper limit of renewal time to be 6 yr. We compared 210 Pb/ 226 Ra ratios among deep waters of various oceanic basins to constrain the application of 230 Th as a tracer of deep-water renewal. The 210 Pb/ 226 Ra ratios were almost identical for the deepwater of the Andaman Sea and the inflow water from the Bay of Bengal, suggesting that the deep water in the Andaman Sea is replaced rapidly without bottom scavenging effects on the 210 Pb/ 226 Ra ratio. INDEX TERMS: 4860 Oceanography: Biological and Chemical: Radioactivity and radioisotopes; 4825 Oceanography: Biological and Chemical: Geochemistry; 4808 Oceanography: Biological and Chemical: Chemical tracers. Citation: Okubo, A., H. Obata, Y. Nozaki, Y. Yamamoto, and H. Minami (2004), 230 Th in the Andaman Sea: Rapid deep-sea renewal, Geophys. Res. Lett., 31, L22306, doi: /2004gl Introduction [2] Because of the small water volume in semi-closed basins, abyssal circulation is influenced by the global climate change more clearly than that in oceanic basins. Changes in water circulation are sometimes reflected in the distribution of chemical properties, such as oxygen and nutrients [Gamo et al., 1986; Nozaki et al., 1999]. For better understanding of abyssal circulation, the residence time was calculated in some semi-closed basins, e.g., yr in the Japan Sea [Gamo et al., 2001] and 100 yr in the South China Sea [Broecker et al., 1986]. [3] The Andaman Sea is a semi-closed basin in the northern Indian Ocean. Sediments of the Andaman Sea, which is mainly fed by the Irrawaddy River [Wijayananda and Cronan, 1994] provide a record of the variability of erosion intensity in the Himalayan and Burma ranges [Colin et al., 1999]. Nevertheless, little is known of the biogeochemical cycles in the present Andaman Sea. Deep water in the Andaman Sea has a dissolved oxygen concentration that 1 Marine Inorganic Chemistry Division, Ocean Research Institute, University of Tokyo, Tokyo, Japan. 2 Deceased 4 January Department of Marine Science and Technology, School of Engineering, Hokkaido Tokai University, Sapporo, Japan. Copyright 2004 by the American Geophysical Union /04/2004GL is constant both vertically and horizontally [Sarma and Narvekar, 2001]. That concentration corresponds to that of the inflowing water from Bay of Bengal, suggesting that the deep Andaman Sea is replaced rapidly before detectable dissolved oxygen consumption occurs in its deep water. Nozaki and Alibo [2003] reported that the REE (III) concentrations of deep water in the Andaman Sea are almost uniform, presumably because of rapid vertical mixing. The concentration level is similar to that of ca m depth in the Bay of Bengal. [4] This study provided an estimate of the renewal time in the Andaman Sea using the vertical distribution of 230 Th. [5] Vertical distribution of thorium is well described by the reversible scavenging model [Nozaki et al., 1981; Bacon and Anderson, 1982; Nozaki et al., 1987]. The model is able to predict the vertical profile of 230 Th, increasing linearly from the surface to deep waters. However, concave profiles with higher or lower concentrations in deep waters than those predicted by the reversible scavenging model have often been reported and explained by the scavenging-mixing model [Rutgers van der Loeff and Berger, 1993; Moran et al., 1997, 2001, 2002; Scholten et al., 1995]. In those models, 230 Th vertical profiles in seawater were governed not only by the settling particle dynamics and adsorption/desorption equilibrium but also by lateral transport with water ventilation. In the Weddell Basin, the ventilation time of deep water was estimated to be 35 yr by the scavenging-mixing model [Rutgers van der Loeff and Berger, 1993]. This scavenging-mixing model was also applied to the Labrador Sea [Moran et al., 1997] and the eastern North Atlantic [Vogler et al., 1998]. The present study adopted this model to the deep Andaman Sea. 2. Hydrography and Topography [6] The geographical feature shows the Andaman Sea to be a semi-closed basin that is isolated from the Bay of Bengal by the Andaman-Nicobar Ridge (Figure 1). The main channels between the Bay of Bengal and the Andaman Sea are the Preparis Channel (250 m sill depth), the Ten Degree Channel (800 m), and the Great Passage (<1800 m) [Nozaki and Alibo, 2003]. Vertical profiles of potential temperature, salinity, and dissolved oxygen at the two stations have been shown by Nozaki and Alibo [2003]. Uniform properties of potential temperature, salinity, and dissolved oxygen in the deep water of the Andaman Sea suggest the existence of a vigorous vertical mixing process. Varkey et al. [1996] reviewed the physical oceanography of the Andaman Sea from the meridional distribution of salinity and temperature along 95 E, about 5 14 N; that study also reported that the deep water in that basin has almost uniform composition. Therefore, we infer that the L of5

2 Figure 1. Sampling stations occupied by the R.V. Hakuho- Maru in the Bay of Bengal and the Andaman Sea. See color version of this figure in the HTML. waters at the sampling station (PA-10) are representative of the Andaman Sea. 3. Sampling and Methods [7] The conductivity-temperature-depth (CTD) and hydrographic observation in the Bay of Bengal (PA-9; N, E; 3603 m water depth) and the Andaman Sea (PA-10; N, E; 3780 m water depth) were made on 28 and 31 January, 1997, respectively, during the Piscis Austrinus expedition using R/V Hakuho-Maru [Gamo, 1997]. Large-volume (250 l each) seawater samples were collected from the surface to bottom depth with a PVC large-volume water sampler [Gamo, 1997]. Nozaki et al. [1981] and Nozaki and Nakanishi [1985] have described analytical procedures for Th. 4. Results and Discussion 4.1. Vertical Profile of 230 Th in the Andaman Sea [8] Figure 2 shows vertical profiles of 230 Th in the Bay of Bengal and the Andaman Sea. Table 1 1 lists the data of 230 Th. The surface 230 Th activity is relatively higher than those ( dpm/m 3 ) in the open oceans [Nozaki et al., 1981; Nozaki and Nakanishi, 1985; Bacon et al., 1989]. Nevertheless, the surface activity is the same level as that in the South Atlantic [Rutgers van der Loeff and Berger, 1993]. [9] Below the surface layer, 230 Th activities increase gradually to 1250 m (0.63 dpm/m 3 ), which corresponds with the 230 Th profiles in other oceans. The noticeable trend in the vertical distribution of 230 Th activity in the Andaman Sea is uniformity below 1250 m depth. The average 230 Th activity below 1250 m to the bottom is 0.63 dpm/m 3, which is lower 1 Auxiliary material is available at ftp://ftp.agu.org/apend/gl/ 2004GL than the predicted value of the reversible scavenging model. Nozaki and Yamamoto [2001] used the vertical distribution of 228 Ra and nitrate to estimate the integrated nitrate uptake fluxes in the euphotic zone as 3.6 mmol/m 2 /day (PA-9) and 1.4 mmol/m 2 /day (PA-10). Based on these nitrate uptake fluxes and the Redfield ratio, new productions at PA-9 and PA-10 are calculated to be 286 mgc/m 2 /day and 111 mgc/ m 2 /day, respectively. Because of lower productivity at PA-10, the scavenging effect by settling particle cannot explain the depletion of 230 Th in deep water of the Andaman Sea. We will discuss the depletion of 230 Th activity in the deep water of the Andaman Sea by rapid deep-water renewal Scavenging-Mixing Model in the Andaman Sea [10] Based on hydrographic data at PA-9 and PA-10 [Nozaki and Alibo, 2003], we assume that the 230 Th activity of incoming water is 0.56 dpm/m 3 (1000 m at PA-9), and that the renewal process takes place in the water column below 1250 m depth. For the deep-water column, the balance of the 230 Th activity can be expressed by the following equation as a steady state. C t dt ¼ P lc t SK dc t dz þ ð C i C t Þ ¼ 0 ð1þ t w In that equation, C t represents the total 230 Th activity, P is the in situ production of 230 Th by decay of 234 U(P = dpm/m 3 ), and l is a radioactive decay constant. S (=300 m/y) is the sinking rate of particles, which was calculated by fitting the model-derived 230 Th activity with the measured 230 Th activity in the layer atop 1000 m, and K (=0.34) is the distribution coefficient of 230 Th between solution and particles in seawater, which is calculated from the concentration of suspended particulate matter (auxiliary material, Table 2) and the experimental equation of Luo et al. [1995]. Z is the depth, C i is the 230 Th activity of incoming seawater (0.56 dpm/m 3 ) and t w is the residence time (=renewal time) of the deep water. If we assume that the input concentration C i is constant, then equation (1) can be written as the following. C t ¼ ðc i þ Pt w Þ 1 exp z z 0 SKt w þ C 0 exp z z 0 SKt w ð2þ Figure 2. Vertical profiles of total 230 Th in (a) the Bay of Bengal and (b) the Andaman Sea. Dashed lines indicate the predicted total 230 Th vertical profiles from the reversible scavenging model. 2of5

3 Figure 3. Scavenging-mixing model derived 230 Th profiles in the Andaman Sea with renewal time variation. No renewal line is calculated from the simple reversible scavenging model. Therein, Z 0 is 1250 m and C 0 = Ct (at 1250 m). Figure 3 shows the model-derived 230 Th profiles in the Andaman Sea with variation in renewal time. The model-predicted lines best fit the measured 230 Th distribution with 3 yr of renewal time. Considering the error propagated from 230 Th concentration in the inflowing water (±13%) and the sinking rate (±65%), the upper limit of renewal time is calculated as 6 yr. [11] The high particle-affinity of Th leads one to infer that the loss of 230 Th along the pathway from the Bay of Bengal to the Andaman Sea should be evaluated. Nozaki and Alibo [2003] showed the depletion of Ce in the Andaman Sea relative to the Bay of Bengal, but Ce (III) is oxidized to Ce (IV) via microbial oxidation and scavenged by sinking particle [Moffett, 1990]. As a result of these complex scavenging processes, it will be difficult to compare the behavior of 230 Th with that of redox-sensitive Ce. Considering other particle reactive elements, there is no significant difference of 210 Pb/ 226 Ra between the deep Andaman Sea and incoming waters. We will discuss this point fully in the next section. [12] Based on the model-derived renewal time (=residence time in deep water column exceeding 1250 m of the Andaman Sea) of 6yr, the volume transport of the incoming water can be calculated to 2.4 Sv (10 6 m 3 s 1 ). Because the volume transport of Indian Deep Water is reported to be 6 Sv in the eastern basin of the northern Indian Ocean [Tomczak and Godfrey, 1994], the exchange rate on the Andaman Sea seems to be realistic. [13] Some potential driving forces of the renewal process exist aside from cooling of surface water in the semi-closed basins. In the Sulu Sea, a semi-closed Southeast Asian basin, turbidity currents were proposed as contributing to deep-basin ventilation [Quadfasel et al., 1990]. Geothermal heating is also suggested to induce deep-water mixing in the Sulu Sea [Frische and Quadfasel, 1990]. In the Izu- Ogasawara trench, Nozaki et al. [1997b] used natural radionuclide profiles to estimate the rapid renewal in the trench waters (5.7 yr). Fujio et al. [2000] observed the strong currents in the Izu-Ogasawara trench by direct current measurement, which may contribute to the rapid renewal in the trench water. Further observations are necessary to propose the main driving force of the vigorous mixing process in the deep Andaman Sea Pb/ 226 Ra Activity Ratio [14] For relatively particle-reactive radionuclides such as 231 Pa and 210 Pb, the bottom scavenging effect from the overlying seawater to surface sediments is an important sink [Craig et al., 1973; Bacon et al., 1976; Bacon and Anderson, 1982; Anderson et al., 1983; Nozaki and Yamada, 1987; Bacon and Rutgers van der Loeff, 1989; Nozaki et al., 1997a]. It is also reported that the shorter scavenging residence times of 210 Pb in deep seawater were found in smaller basins [Nozaki et al., 1997a]. Because the specific ratio of surface sediment area to the water mass volume is higher in semi-closed basins such as the Andaman Sea than in oceanic regions, the bottom scavenging may affect the 230 Th distribution in seawater. We will discuss the distributions of 210 Pb and 226 Ra activities and mean residence time of 210 Pb and 230 Th based on a simple box model. Thereby, we will examine the bottom scavenging effect within the Andaman Sea. [15] The 210 Pb activity and 210 Pb/ 226 Ra activity ratio in the deep Andaman Sea were uniform [Obata et al., 2004]. They corresponded to those at the depth of 1000 m at PA-9 ( 210 Pb activity = 7.3 ± 0.6 dpm/100 kg, 210 Pb/ 226 Ra = 0.37 at PA-10 (1490 m-bottom); 210 Pb activity = 6.6 ± 0.5 dpm/ 100 kg, 210 Pb/ 226 Ra = 0.35 at the depth of 1000 m at PA-9 [Obata et al., 2004]), suggesting that deep water in the Andaman Sea is replaced rapidly before the bottom scavenging effects on the deep-sea 210 Pb/ 226 Ra ratio. [16] Nozaki et al. [1997a] calculated the deep-sea 210 Pb scavenging residence time in the Bismarck Sea as t 210Pb,= (R/(1 R))t l, where R is the activity ratio of 210 Pb/ 226 Ra and t l is the mean life of 210 Pb (32 yr). The scavenging residence time for that semi-closed basin is only 8 yr ( 210 Pb/ 226 Ra = 0.20). That period is much shorter than the 90 yr ( 210 Pb/ 226 Ra = 0.74) of the equatorial Pacific, but surface ocean productivity does not differ greatly between these two regions. The short deep-sea 210 Pb scavenging residence time in the Bismarck Sea was attributed to the bottom scavenging effect from the overlying seawater to surface sediments. Compared with other basins, the deep-sea 210 Pb/ 226 Ra ratio in the Andaman Sea ( 210 Pb/ 226 Ra = 0.37, t 210Pb = 19 yr) is comparable with those in other larger volume semi-closed basins, such as the Japan Sea ( 210 Pb/ 226 Ra = 0.32, t 210Pb = 15 yr) and the East China Sea ( 210 Pb/ 226 Ra = 0.32, t 210Pb = 15 yr). An enhanced bottom scavenging resulting from a high specific ratio between seawater and surface sediment, such as that in the Bismarck Sea, was not observed in the Andaman Sea. 3of5

4 [17] The integrated nitrate uptake flux in the euphotic zone in the South China Sea (PA-11; N, E, 4240 m water depth) was calculated to be 2.0 mmol N/m 2 / day [Nozaki and Yamamoto, 2001], which correspond to 156 mg C/m 2 /day. The surface ocean productivity does not differ greatly between PA-10 and PA-11, but the mean deep-sea 210 Pb/ 226 Ra activity ratio was 0.25 at PA-11 [Obata et al., 2004], which was 70% of that in PA-10. Because of the longer residence time (100 y) of the deep water in the South China Sea, 210 Pb may be strongly removed by the bottom scavenging. The scavenging residence time of 230 Th, as calculated by t Th = P h/i, where P is the in situ production of 230 Th and I is the inventory of 230 Th in the water column, h = 3959 m [Vogler et al., 1998], is 23 yr in the deep Andaman Sea. That value is comparable to that of 210 Pb (19 yr). Although Th is more particle-reactive than Pb [Bacon and Anderson, 1982], bottom scavenging in the Andaman Sea does not seem to be significant for 230 Th and 210 Pb. Using the model-predicted renewal time in the Andaman Sea, the buried 230 Th in the sediment corresponds to less than 55% of the production from 234 U. Exported 230 Th from the basin may be buried in higher particle flux and strong bottom scavenging regions. Further investigations are needed for discussion of the deposition of 230 Th that is exported from the Andaman Sea. 5. Conclusion [18] We estimated the upper limit of renewal time in the Andaman Sea to be 6 yr using the scavenging-mixing model with vertical distribution of 230 Th. The 210 Pb/ 226 Ra ratios in the deep-water of the Andaman Sea correspond to the inflow water from the Bay of Bengal, suggesting that the deep water in the Andaman Sea is replaced rapidly without bottom scavenging effects on the 210 Pb/ 226 Ra ratio. [19] Acknowledgments. 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