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High-resolution 14C analyses of annually-banded coral skeletons from Ishigaki Island, Japan: implications for oceanography
Nuclear Instruments and Methods in Physics Research B 223–224 (2004) 455–459 www.elsevier.com/locate/nimb
High-resolution 14C analyses of annually-banded coral skeletons from Ishigaki Island, Japan: implications for oceanography T. Mitsuguchi a,b,*, H. Kitagawa c, E. Matsumoto c, Y. Shibata b, M. Yoneda b, T. Kobayashi b, T. Uchida d, N. Ahagon e a
Japan Society for the Promotion of Science, Tokyo 102-8471, Japan National Institute for Environmental Studies, Tsukuba 305-8506, Japan c Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8601, Japan d Nagoya Institute of Technology, Nagoya 466-8555, Japan Mutsu Institute for Oceanography, Japan Marine Science and Technology Center, Mutsu 035-0022, Japan b
e
Abstract We report high-resolution radiocarbon (D14 C) analyses of annually-banded coral skeletons (Porites lutea) from Ishigaki Island, the Ryukyus, Japan, for the years 1985–1993. Analyses were carried out using an accelerator mass spectrometer (AMS) system at the National Institute for Environmental Studies, Tandem accelerator for Environmental Research and Radiocarbon Analysis (NIES-TERRA). Results provide a D14 C time series showing significant seasonal and interannual variations superimposed on a long-term trend. D14 C has a range of 66&, from a minimum of 79& to a maximum of 145&. The amplitude of the seasonal variation is 15–40& with lower D14 C values in summer, which may indicate monsoon-induced local upwelling. The long-term trend of D14 C is equivalent to a decrease of 30& for the 8 yr from 1985 to 1993. Most of this trend probably reflects the gradual diffusion of bomb-derived 14 C from the atmosphere to the deep ocean via air–sea gas exchange and vertical movement of water masses. These results and ongoing analyses contribute to understanding of the ocean environment and ocean–atmosphere interactions in the far western subtropical Pacific. Ó 2004 Elsevier B.V. All rights reserved. PACS: 92.10.Ei; 92.10.Fj; 92.70.Gt; 93.30.Rp Keywords: Radiocarbon; Coral skeletons; Annual bands; Upwelling; Ocean–atmosphere interaction
1. Introduction Radiocarbon (14 C) is produced both naturally in the upper atmosphere and artificially by atmo* Corresponding author. Address: National Institute for Environmental Studies, Tsukuba 305-8506, Japan. Tel.: +8129-850-2450; fax: +81-29-850-2574. E-mail address: [email protected] (T. Mitsuguchi).
spheric nuclear tests performed in the 1950s and early 1960s. The resulting 14 C is quickly combined with oxygen to form radioactive carbon dioxide (14 CO2 ) and incorporated into the ocean, soil, vegetation, etc. Because the timescale of the global ocean circulation (1500 yr) is not negligible compared with the half-life of 14 C (5730 yr), the dissolved inorganic carbon in seawater varies significantly in 14 C concentration according to region,
0168-583X/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2004.04.086
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T. Mitsuguchi et al. / Nucl. Instr. and Meth. in Phys. Res. B 223–224 (2004) 455–459
depth and water mass. The nuclear tests caused a great excess of 14 C in the atmosphere relative to the ocean, breaking the natural equilibrium of global 14 C distribution. This atmospheric 14 C excess peaked in the mid-1960s and diminished during the following decades. This was primarily due to the relatively rapid CO2 exchange at the air–sea interface, leading to a great excess of 14 C in the surface ocean relative to the deeper ocean. We can take advantage of bomb-induced surfaceocean 14 C excess as a very sensitive indicator of vertical water-mass movements. Hermatypic corals secrete CaCO3 skeletons in the surface ocean, with some species forming annual growth bands in their skeletons at rates of more than 10 mm/yr and occasionally growing to form gigantic colonies containing hundreds of years of coral growth. Thus high-resolution 14 C analysis of annuallybanded coral skeletons by accelerator mass spectrometry (AMS) provides a high-resolution window into past surface-water movements that might be linked with ocean–atmosphere climate systems such as El Ni~ no-Southern Oscillation (ENSO). Previous investigations have analyzed coral specimens from the eastern and central Pacific (e.g. the Galapagos Islands, the Hawaiian Islands, Nauru, Rarotonga) [1–4]. In this paper, we present a high-resolution 14 C time series for recent annual bands (A.D. 1985–1993) of a coral specimen from Ishigaki Island located in the far western subtropical Pacific.
2. Sample and methods In September 1993, a 180-cm-long core (9 cm in diameter) was drilled from the top of a living coral colony of Porites lutea (250 cm in height) on the eastern side of Ishigaki Island, the Ryukyus, Japan 00 00 (24°330 32 N, 124°200 08 E) (Fig. 1). The top of the colony was 2–3 m below the sea surface. The drilling site lies near the fringing reef opening, with continuous flow of ocean water and no adjacent river discharge. The hydrogeology around the drilling site suggests little effect of groundwater that may be depleted in 14 C. In this region, sea surface temperature (SST) varies seasonally from 20 to 30 °C; the wind system is dominated by the seasonally-reversing East Asian monsoon, with SSW-S winds during spring and summer (typically in summer) and NNE-NE winds during autumn and winter (typically in winter) with a mean wind speed of 3.0–6.0 m/s (Fig. 2). The Kuroshio Current, a strong warm ocean current from the eastern side of the Philippines, passes between Ishigaki Island and Taiwan (Fig. 1). The coral core was cut longitudinally into 7mm-thick slabs at the Australian Institute of Marine Science. X-radiography of the slabs revealed clear annual bands which are due to seasonal variation of skeletal density. The skeletal growth rate was 13–17 mm/yr (Fig. 3(A)). It has been demonstrated, through high-resolution analyses of Mg/Ca, Sr/Ca and d18 O (i.e. SST proxies),
Fig. 1. Location map and local map of Ishigaki Island. A core sample of coral skeleton (Porites lutea) was collected on the eastern side of Ishigaki Island. The meteorological data shown in Fig. 2 were observed at stations A (wind speed), B (most frequent wind direction) and C (sea surface temperature).
T. Mitsuguchi et al. / Nucl. Instr. and Meth. in Phys. Res. B 223–224 (2004) 455–459
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Fig. 2. Monthly-averaged wind speed, most frequent wind direction, and sea-surface temperature at Ishigaki Island (Fig. 1). Vertical shaded bars denote periods of southerly winds, interrupted by short periods of northerly winds in 1987 and 1992. The seasonallyreversing wind system is dominated by the East Asian monsoon, with SSW-S winds during spring and summer (typically in summer) and NNE-NE winds during autumn and winter (typically in winter).
that the coral colony forms a narrow high-density band around the annual SST maximum (darker bands in the X-ray positive, Fig. 3(A)) [5]. The core includes more than 100 annual bands back to the 1880s. Along the skeletal growth axis, 60 subsamples were manually picked out with a special needle at 2-mm increments, corresponding to a temporal resolution of 6–8 weeks. The subsampling track covered the annual bands 1985–1993 (Fig. 3(A)). Each subsample (9–10 mg) was ground into fine powder and treated with 2 ml of 10% H2 O2 at 60 °C for 15 min with ultrasonic agitation. Split (4 mg) was used for analyses of d18 O, d13 C, Mg/Ca, Sr/Ca, Na/Ca, etc. The d18 O and d13 C analyses were performed using a Finnigan MAT252 mass spectrometer with an automated carbonate preparation device. Mg/Ca and Sr/Ca ratios were measured by inductively coupled plasma atomic emission spectrometry with a Seiko
SPS-7000A. The remaining split (5–6 mg) was converted to CO2 in a vacuum line by dissolution in H3 PO4 , then to graphite at 630 °C on Fe powder catalyst (1.2 mg) with H2 gas as the reductant [6]. The ‘‘new’’ NIST oxalic acid standard (HOxII) and 14 C-free CaCO3 were converted to graphite as a standard and a blank material, respectively, in the same vacuum line in the same condition. These graphite materials were individually pressed into a target and measured for 14 C/ 12 C and 13 C/12 C ratios using an AMS system at the National Institute for Environmental Studies, Tandem accelerator for Environmental Research and Radiocarbon Analysis (NIES-TERRA). Sample targets were randomized and set into an ion-source carrousel with 40 target chambers, with five samples sandwiched between two HOxII standards. Each target was measured for 10 min per one cycle, which was replicated to nine cycles
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T. Mitsuguchi et al. / Nucl. Instr. and Meth. in Phys. Res. B 223–224 (2004) 455–459
Fig. 3. (A) X-ray positive print of the coral sample (Porites lutea) showing annual density bands. This coral forms a narrow highdensity band (darker band) in summer. (B) Time series of Mg/Ca, d18 O and D14 C along the subsampling track shown in the X-ray photo. Vertical dotted lines denote positions of the annual high-density bands formed around the time of maximum SST in summer. Error bars for the D14 C values correspond to 70% confidence intervals. Generally, D14 C decreases in summer. For the annual bands of 1985, 1987 and 1992, as highlighted by vertical shaded bars, a marked D14 C increase interrupts the usual summer D14 C decrease. The long-term trend of the D14 C time series is shown as a broken line.
for acquisition of at least 100 000 14 C counts. The samples produced 108 000–157 000 counts, except for two samples of 85 000 counts, while
the HOxII standards produced 132 000–163 000 counts. The blanks produced 900 counts, which was taken into account as the background level.
T. Mitsuguchi et al. / Nucl. Instr. and Meth. in Phys. Res. B 223–224 (2004) 455–459
Results are reported as D14 C values according to the standard procedure of [7] (i.e. for geochemical samples with age correction). The D14 C values were calculated using the d13 C values obtained from the AMS measurement.
3. Results and discussion Results for Mg/Ca, d18 O and D14 C are shown in Fig. 3(B). The time series of Mg/Ca and d18 O exhibit clear cyclical variations synchronous with each other, both of which are representative of annual SST cycles. When SST increases, the Mg/ Ca ratio increases and d18 O decreases. This confirms that the coral colony formed a narrow highdensity band around the annual SST maximum in summer (see vertical dotted lines in Fig. 3(B)). The D14 C time series shows significant seasonal and interannual variations superimposed on a longterm trend, with a dynamic range of 66&, from a minimum of 79& to a maximum of 145&. Error bars represent 70% confidence intervals. The amplitude of the seasonal variation is 15–40& with lower D14 C values generally in summer when the Mg/Ca ratio increases, d18 O decreases, and the narrow high-density band is formed. D14 C is lowest around the annual SST maximum, although it is anomalous in 1985, 1987 and 1992, when a marked D14 C increase interrupts the usual summer D14 C decrease (see vertical shaded bars in Fig. 3(B)). The most convincing explanation is that the D14 C decrease in summer may reflect local upwelling of subsurface water depleted in 14 C relative to the surface water. It is very likely that the SSW-S monsoon during spring and summer induces Ekman transport on the eastern side of Ishigaki Island and that the offshore transport of surface water leads to coastal upwelling of 14 Cdepleted subsurface water. Since the Ryukyu Islands are sandwiched between the Ryukyu Trench (<)6000 m) and the Okinawa Trough (<)2000 m), the submarine topography around Ishigaki Island is steep, suggesting a strong likelihood of 14 C-depleted subsurface water. The anomalous summer D14 C increases in 1985, 1987 and 1992 may be due to the usual monsoon-induced upwelling being
459
temporarily interrupted by penetration of a different water mass with higher 14 C concentration. It has been suggested from field observations on the eastern side of Ishigaki Island, that a typhoon induces vertical mixing between surface and subsurface waters [8]. The typhoon-induced vertical mixing might contribute to the seasonal D14 C decrease in our sample. To test these inferences, we need to investigate oceanographic data not only around Ishigaki Island but also over an extended area. The long-term D14 C trend is equivalent to a decrease of 30& from 1985 to 1993 (see broken line in Fig. 3(B)). Most of this trend is probably due to the gradual diffusion of bomb-derived 14 C from the atmosphere to the deep ocean via air–sea gas exchange and vertical movement of water masses. We will extend the 14 C analysis to the older annual bands of this coral specimen to obtain a clearer understanding of ocean environment and ocean–atmosphere interactions in the far western subtropical Pacific.
Acknowledgements We thank Janice M. Lough of the Australian Institute of Marine Science (AIMS) and two anonymous reviewers for their helpful comments. Thanks are also expressed to Peter J. Isdale and Bruce Parker of AIMS for drilling the coral core.
References [1] T.P. Guilderson, D.P. Schrag, Science 281 (1998) 240. [2] T.P. Guilderson, D.P. Schrag, M. Kashgarian, J. Southon, J. Geophys. Res. 103 (C11) (1998) 24641. [3] T.P. Guilderson, D.P. Schrag, E. Goddard, M. Kashgarian, G.M. Wellington, B.K. Linsley, Radiocarbon 42 (2) (2000) 249. [4] E.R.M. Druffel, S. Griffin, T.P. Guilderson, M. Kashgarian, J. Southon, D.P. Schrag, Radiocarbon 43 (1) (2001) 15. [5] T. Mitsuguchi, E. Matsumoto, O. Abe, T. Uchida, P.J. Isdale, Science 274 (1996) 961. [6] H. Kitagawa, T. Masuzawa, T. Nakamura, E. Matsumoto, Radiocarbon 35 (2) (1993) 295. [7] M. Stuiver, H.A. Polach, Radiocarbon 19 (3) (1977) 355. [8] K. Nadaoka, Y. Nihei, R. Kumano, T. Yokobori, T. Omija, K. Wakaki, Coral Reefs 20 (4) (2001) 387.
High-resolution 14C analyses of annually-banded coral skeletons from Ishigaki Island, Japan: implications for oceanography T. Mitsuguchi a,b,*, H. Kitagawa c, E. Matsumoto c, Y. Shibata b, M. Yoneda b, T. Kobayashi b, T. Uchida d, N. Ahagon e a
Japan Society for the Promotion of Science, Tokyo 102-8471, Japan National Institute for Environmental Studies, Tsukuba 305-8506, Japan c Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8601, Japan d Nagoya Institute of Technology, Nagoya 466-8555, Japan Mutsu Institute for Oceanography, Japan Marine Science and Technology Center, Mutsu 035-0022, Japan b
e
Abstract We report high-resolution radiocarbon (D14 C) analyses of annually-banded coral skeletons (Porites lutea) from Ishigaki Island, the Ryukyus, Japan, for the years 1985–1993. Analyses were carried out using an accelerator mass spectrometer (AMS) system at the National Institute for Environmental Studies, Tandem accelerator for Environmental Research and Radiocarbon Analysis (NIES-TERRA). Results provide a D14 C time series showing significant seasonal and interannual variations superimposed on a long-term trend. D14 C has a range of 66&, from a minimum of 79& to a maximum of 145&. The amplitude of the seasonal variation is 15–40& with lower D14 C values in summer, which may indicate monsoon-induced local upwelling. The long-term trend of D14 C is equivalent to a decrease of 30& for the 8 yr from 1985 to 1993. Most of this trend probably reflects the gradual diffusion of bomb-derived 14 C from the atmosphere to the deep ocean via air–sea gas exchange and vertical movement of water masses. These results and ongoing analyses contribute to understanding of the ocean environment and ocean–atmosphere interactions in the far western subtropical Pacific. Ó 2004 Elsevier B.V. All rights reserved. PACS: 92.10.Ei; 92.10.Fj; 92.70.Gt; 93.30.Rp Keywords: Radiocarbon; Coral skeletons; Annual bands; Upwelling; Ocean–atmosphere interaction
1. Introduction Radiocarbon (14 C) is produced both naturally in the upper atmosphere and artificially by atmo* Corresponding author. Address: National Institute for Environmental Studies, Tsukuba 305-8506, Japan. Tel.: +8129-850-2450; fax: +81-29-850-2574. E-mail address: [email protected] (T. Mitsuguchi).
spheric nuclear tests performed in the 1950s and early 1960s. The resulting 14 C is quickly combined with oxygen to form radioactive carbon dioxide (14 CO2 ) and incorporated into the ocean, soil, vegetation, etc. Because the timescale of the global ocean circulation (1500 yr) is not negligible compared with the half-life of 14 C (5730 yr), the dissolved inorganic carbon in seawater varies significantly in 14 C concentration according to region,
0168-583X/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2004.04.086
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T. Mitsuguchi et al. / Nucl. Instr. and Meth. in Phys. Res. B 223–224 (2004) 455–459
depth and water mass. The nuclear tests caused a great excess of 14 C in the atmosphere relative to the ocean, breaking the natural equilibrium of global 14 C distribution. This atmospheric 14 C excess peaked in the mid-1960s and diminished during the following decades. This was primarily due to the relatively rapid CO2 exchange at the air–sea interface, leading to a great excess of 14 C in the surface ocean relative to the deeper ocean. We can take advantage of bomb-induced surfaceocean 14 C excess as a very sensitive indicator of vertical water-mass movements. Hermatypic corals secrete CaCO3 skeletons in the surface ocean, with some species forming annual growth bands in their skeletons at rates of more than 10 mm/yr and occasionally growing to form gigantic colonies containing hundreds of years of coral growth. Thus high-resolution 14 C analysis of annuallybanded coral skeletons by accelerator mass spectrometry (AMS) provides a high-resolution window into past surface-water movements that might be linked with ocean–atmosphere climate systems such as El Ni~ no-Southern Oscillation (ENSO). Previous investigations have analyzed coral specimens from the eastern and central Pacific (e.g. the Galapagos Islands, the Hawaiian Islands, Nauru, Rarotonga) [1–4]. In this paper, we present a high-resolution 14 C time series for recent annual bands (A.D. 1985–1993) of a coral specimen from Ishigaki Island located in the far western subtropical Pacific.
2. Sample and methods In September 1993, a 180-cm-long core (9 cm in diameter) was drilled from the top of a living coral colony of Porites lutea (250 cm in height) on the eastern side of Ishigaki Island, the Ryukyus, Japan 00 00 (24°330 32 N, 124°200 08 E) (Fig. 1). The top of the colony was 2–3 m below the sea surface. The drilling site lies near the fringing reef opening, with continuous flow of ocean water and no adjacent river discharge. The hydrogeology around the drilling site suggests little effect of groundwater that may be depleted in 14 C. In this region, sea surface temperature (SST) varies seasonally from 20 to 30 °C; the wind system is dominated by the seasonally-reversing East Asian monsoon, with SSW-S winds during spring and summer (typically in summer) and NNE-NE winds during autumn and winter (typically in winter) with a mean wind speed of 3.0–6.0 m/s (Fig. 2). The Kuroshio Current, a strong warm ocean current from the eastern side of the Philippines, passes between Ishigaki Island and Taiwan (Fig. 1). The coral core was cut longitudinally into 7mm-thick slabs at the Australian Institute of Marine Science. X-radiography of the slabs revealed clear annual bands which are due to seasonal variation of skeletal density. The skeletal growth rate was 13–17 mm/yr (Fig. 3(A)). It has been demonstrated, through high-resolution analyses of Mg/Ca, Sr/Ca and d18 O (i.e. SST proxies),
Fig. 1. Location map and local map of Ishigaki Island. A core sample of coral skeleton (Porites lutea) was collected on the eastern side of Ishigaki Island. The meteorological data shown in Fig. 2 were observed at stations A (wind speed), B (most frequent wind direction) and C (sea surface temperature).
T. Mitsuguchi et al. / Nucl. Instr. and Meth. in Phys. Res. B 223–224 (2004) 455–459
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Fig. 2. Monthly-averaged wind speed, most frequent wind direction, and sea-surface temperature at Ishigaki Island (Fig. 1). Vertical shaded bars denote periods of southerly winds, interrupted by short periods of northerly winds in 1987 and 1992. The seasonallyreversing wind system is dominated by the East Asian monsoon, with SSW-S winds during spring and summer (typically in summer) and NNE-NE winds during autumn and winter (typically in winter).
that the coral colony forms a narrow high-density band around the annual SST maximum (darker bands in the X-ray positive, Fig. 3(A)) [5]. The core includes more than 100 annual bands back to the 1880s. Along the skeletal growth axis, 60 subsamples were manually picked out with a special needle at 2-mm increments, corresponding to a temporal resolution of 6–8 weeks. The subsampling track covered the annual bands 1985–1993 (Fig. 3(A)). Each subsample (9–10 mg) was ground into fine powder and treated with 2 ml of 10% H2 O2 at 60 °C for 15 min with ultrasonic agitation. Split (4 mg) was used for analyses of d18 O, d13 C, Mg/Ca, Sr/Ca, Na/Ca, etc. The d18 O and d13 C analyses were performed using a Finnigan MAT252 mass spectrometer with an automated carbonate preparation device. Mg/Ca and Sr/Ca ratios were measured by inductively coupled plasma atomic emission spectrometry with a Seiko
SPS-7000A. The remaining split (5–6 mg) was converted to CO2 in a vacuum line by dissolution in H3 PO4 , then to graphite at 630 °C on Fe powder catalyst (1.2 mg) with H2 gas as the reductant [6]. The ‘‘new’’ NIST oxalic acid standard (HOxII) and 14 C-free CaCO3 were converted to graphite as a standard and a blank material, respectively, in the same vacuum line in the same condition. These graphite materials were individually pressed into a target and measured for 14 C/ 12 C and 13 C/12 C ratios using an AMS system at the National Institute for Environmental Studies, Tandem accelerator for Environmental Research and Radiocarbon Analysis (NIES-TERRA). Sample targets were randomized and set into an ion-source carrousel with 40 target chambers, with five samples sandwiched between two HOxII standards. Each target was measured for 10 min per one cycle, which was replicated to nine cycles
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T. Mitsuguchi et al. / Nucl. Instr. and Meth. in Phys. Res. B 223–224 (2004) 455–459
Fig. 3. (A) X-ray positive print of the coral sample (Porites lutea) showing annual density bands. This coral forms a narrow highdensity band (darker band) in summer. (B) Time series of Mg/Ca, d18 O and D14 C along the subsampling track shown in the X-ray photo. Vertical dotted lines denote positions of the annual high-density bands formed around the time of maximum SST in summer. Error bars for the D14 C values correspond to 70% confidence intervals. Generally, D14 C decreases in summer. For the annual bands of 1985, 1987 and 1992, as highlighted by vertical shaded bars, a marked D14 C increase interrupts the usual summer D14 C decrease. The long-term trend of the D14 C time series is shown as a broken line.
for acquisition of at least 100 000 14 C counts. The samples produced 108 000–157 000 counts, except for two samples of 85 000 counts, while
the HOxII standards produced 132 000–163 000 counts. The blanks produced 900 counts, which was taken into account as the background level.
T. Mitsuguchi et al. / Nucl. Instr. and Meth. in Phys. Res. B 223–224 (2004) 455–459
Results are reported as D14 C values according to the standard procedure of [7] (i.e. for geochemical samples with age correction). The D14 C values were calculated using the d13 C values obtained from the AMS measurement.
3. Results and discussion Results for Mg/Ca, d18 O and D14 C are shown in Fig. 3(B). The time series of Mg/Ca and d18 O exhibit clear cyclical variations synchronous with each other, both of which are representative of annual SST cycles. When SST increases, the Mg/ Ca ratio increases and d18 O decreases. This confirms that the coral colony formed a narrow highdensity band around the annual SST maximum in summer (see vertical dotted lines in Fig. 3(B)). The D14 C time series shows significant seasonal and interannual variations superimposed on a longterm trend, with a dynamic range of 66&, from a minimum of 79& to a maximum of 145&. Error bars represent 70% confidence intervals. The amplitude of the seasonal variation is 15–40& with lower D14 C values generally in summer when the Mg/Ca ratio increases, d18 O decreases, and the narrow high-density band is formed. D14 C is lowest around the annual SST maximum, although it is anomalous in 1985, 1987 and 1992, when a marked D14 C increase interrupts the usual summer D14 C decrease (see vertical shaded bars in Fig. 3(B)). The most convincing explanation is that the D14 C decrease in summer may reflect local upwelling of subsurface water depleted in 14 C relative to the surface water. It is very likely that the SSW-S monsoon during spring and summer induces Ekman transport on the eastern side of Ishigaki Island and that the offshore transport of surface water leads to coastal upwelling of 14 Cdepleted subsurface water. Since the Ryukyu Islands are sandwiched between the Ryukyu Trench (<)6000 m) and the Okinawa Trough (<)2000 m), the submarine topography around Ishigaki Island is steep, suggesting a strong likelihood of 14 C-depleted subsurface water. The anomalous summer D14 C increases in 1985, 1987 and 1992 may be due to the usual monsoon-induced upwelling being
459
temporarily interrupted by penetration of a different water mass with higher 14 C concentration. It has been suggested from field observations on the eastern side of Ishigaki Island, that a typhoon induces vertical mixing between surface and subsurface waters [8]. The typhoon-induced vertical mixing might contribute to the seasonal D14 C decrease in our sample. To test these inferences, we need to investigate oceanographic data not only around Ishigaki Island but also over an extended area. The long-term D14 C trend is equivalent to a decrease of 30& from 1985 to 1993 (see broken line in Fig. 3(B)). Most of this trend is probably due to the gradual diffusion of bomb-derived 14 C from the atmosphere to the deep ocean via air–sea gas exchange and vertical movement of water masses. We will extend the 14 C analysis to the older annual bands of this coral specimen to obtain a clearer understanding of ocean environment and ocean–atmosphere interactions in the far western subtropical Pacific.
Acknowledgements We thank Janice M. Lough of the Australian Institute of Marine Science (AIMS) and two anonymous reviewers for their helpful comments. Thanks are also expressed to Peter J. Isdale and Bruce Parker of AIMS for drilling the coral core.
References [1] T.P. Guilderson, D.P. Schrag, Science 281 (1998) 240. [2] T.P. Guilderson, D.P. Schrag, M. Kashgarian, J. Southon, J. Geophys. Res. 103 (C11) (1998) 24641. [3] T.P. Guilderson, D.P. Schrag, E. Goddard, M. Kashgarian, G.M. Wellington, B.K. Linsley, Radiocarbon 42 (2) (2000) 249. [4] E.R.M. Druffel, S. Griffin, T.P. Guilderson, M. Kashgarian, J. Southon, D.P. Schrag, Radiocarbon 43 (1) (2001) 15. [5] T. Mitsuguchi, E. Matsumoto, O. Abe, T. Uchida, P.J. Isdale, Science 274 (1996) 961. [6] H. Kitagawa, T. Masuzawa, T. Nakamura, E. Matsumoto, Radiocarbon 35 (2) (1993) 295. [7] M. Stuiver, H.A. Polach, Radiocarbon 19 (3) (1977) 355. [8] K. Nadaoka, Y. Nihei, R. Kumano, T. Yokobori, T. Omija, K. Wakaki, Coral Reefs 20 (4) (2001) 387.