Isotopic Evidence in Tree Rings for Historical Changes in Atmospheric

Aug 9, 2006 - Little is understood about the usefulness of sulfur isotopic ratios (δ34S) in tree rings because the sulfur content in rings is general...
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Environ. Sci. Technol. 2006, 40, 5750-5754

Isotopic Evidence in Tree Rings for Historical Changes in Atmospheric Sulfur Sources HIDEHISA KAWAMURA* Department of Chemistry and Physics of Condensed Matter, Graduate School of Science, Kyushu University, 6-10-1, Hakozaki, Higashi-ku, Fukuoka 812-0053, Japan NOBUAKI MATSUOKA Institute of Environmental Systems, Kyushu University, 6-10-1, Hakozaki, Higashi-ku, Fukuoka 812-0053, Japan NORIYUKI MOMOSHIMA Department of Science, Faculty of Science, Kumamoto University, 2-39-1, Kurokami, Kumamoto 860-8555, Japan MASAMI KOIKE Environmental Engineering Group, Research Laboratory, Kyushu Electric Power Co., Inc., 2-1-47, Shiobaru, Minami-ku, Fukuoka 815-0032, Japan YOSHIMASA TAKASHIMA Kyushu Environmental Evaluation Association, 1-10-1, Matsukadai, Higashi-ku, Fukuoka 813-0004, Japan

Little is understood about the usefulness of sulfur isotopic ratios (δ34S) in tree rings because the sulfur content in rings is generally insufficient for analysis using conventional methods. We present δ34S values of the water-soluble and the organically bound sulfur fractions in rings of coniferous trees grown in Japan, analyzed using a large-volume oxygen bomb. Comparing the δ34S values of the organically bound fraction in tree rings with past atmospheric sulfur concentrations and with those of their sources, we find clear evidence that the δ34S values of the organically bound fraction in the rings are dependent upon the values of the atmospheric sulfur sources. The evidence suggests that the δ34S values in tree rings are a useful chronological proxy for evaluating possible causes of past atmospheric sulfur pollution.

Introduction Plants absorb sulfur, which is necessary for growth, as watersoluble SO42- from soil via their roots and as atmospheric sulfur via their stomata (1, 2). Since the δ34S values found in plants directly reflect those of the sulfur absorbed, they have proven useful in identifying sulfur sources (1). This analysis, however, has been limited to the foliage, seeds (2-4), and annual rings of certain species of trees, i.e., phreatophytes (5), which have a high sulfur content. Stable isotopic ratios of hydrogen, carbon, and oxygen in tree rings are known useful proxies for climatic reconstruction * Corresponding author phone: +81-92-662-0410; fax: +81-92662-0990; e-mail: [email protected]; present address: Kyushu Environmental Evaluation Association, 1-10-1, Matsukadai, Higashiku, Fukuoka 813-0004, Japan. 5750

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(6-10) and determination of environmental stresses (11, 12). If the δ34S values of tree rings reflect those of the sulfur annually absorbed, they could be a similarly useful chronological proxy for identifying past sulfur sources and assessing historical sulfur pollution. In this study, we investigated sulfur concentrations and δ34S values of the water-soluble and the organically bound sulfur fractions in annual rings of common coniferous trees grown in urban and rural areas on Kyushu Island, Japan.

Experimental Section Sampling. Three disk samples (5 cm thick) taken at breast height were obtained from the following three trees: (1) a 74-year-old Japanese cedar (Cryptomeria japonica) grown in the Fukuoka research forest, Kyushu University, and cut down in April 1997; (2) a 48-year-old Hinoki cypress (Chamaecyparis obtusa) grown in the Kitakyushu municipal forest and cut down in April 1997; and (3) a 54-year-old Japanese cedar grown in the Shiiba research forest, Kyushu University, and cut down in October 1997. As shown in Figure 1, the former two trees were grown in urban areas facing the sea of Japan in the northwest of Kyushu, close to the densely populated and heavily industrialized cities of Fukuoka and Kitakyushu, respectively. The third tree was grown in a rural area surrounded by high mountains in central Kyushu, far not only from the coast but also from anthropogenic sources of sulfur. Sulfur Concentration and δ34S Analysis. We divided sulfur in tree rings into two fractions for determination of sulfur concentrations and δ34S values (13): a water-soluble sulfur (WSS) fraction extracted with distilled water, and an organically bound sulfur (OBS) fraction that is the residue of the water extraction. The tree ring disk samples were first dried at 105 °C for several days. The samples were then divided into half-decade increments from the bark toward the pith, and ground into 20-mesh size using a Willy Mill. About 100 g of each sample was pretreated for analysis. The ground sample was put into a polypropylene bottle together with distilled water (g sample/mL water 1:20) and shaken for 1 h using an agitator. After filtration with an ADVANTEC cellulose filter (No. 5B), the solution and residue were defined as the WSS and OBS fractions, respectively. The OBS fraction was dried in an oven for 1 day. About 8 g of dried sample was put into a 50 mL metal capsule, and set in a combustion metal vessel, large-volume Parr oxygen bomb (No. 1121), with about 10 mL of distilled water containing a few drops of H2O2 solution. The vessel was sealed and pressurized to 21 atm with oxygen. The sample in the capsule was ignited electrically and allowed to cool to room temperature after burning. The combustion gas in the vessel was exhausted by opening the pressure valve. The electrode, capsule, and inside of the vessel were then rinsed with distilled water into a 200 mL glass beaker to recover sulfur as SO42-. Several repetitions of the combustion procedure resulted in a sufficient amount of sulfur for analysis. Finally, the solution was filtered with an ADVANTEC cellulose filter (No. 5C) and the volume was subsequently adjusted to 200 mL in a volumetric flask. Approximately 20 mL of the solution was put into a 20 mL glass vial for determination of SO42concentration by a DIONEX ion chromatograph (2000I). A few drops of HCl solution were then added to the remainder to adjust pH to about 2 or 3, and 5% (w/v) BaCl2 solution was added to precipitate BaSO4. After heating on a hotplate, the BaSO4 was collected on an ADVANTEC mixed cellulose ester membrane filter (plain type, d ) 47 mm with a pore size of 0.45-µm), washed with distilled water, and dried completely. 10.1021/es060321w CCC: $33.50

 2006 American Chemical Society Published on Web 08/09/2006

FIGURE 1. Sampling locations of the coniferous trees from urban and rural areas on Kyushu Island, Japan.

TABLE 1. Sulfur Isotopic Ratios of the Urban Japanese Cedar, Urban Hinoki Cypress, and Rural Japanese Cedar urban Japanese cedar age (year)

FIGURE 2. Half-decade changes in the WSS and OBS concentrations of the urban Japanese cedar, urban Hinoki cypress, and rural Japanese cedar. According to the V2O5-SiO2 method, thermal decomposition of the BaSO4 to SO2 was performed prior to δ34S analysis for the OBS fraction (14). For the WSS fraction, the sample was evaporated, soaked in approximately 1 g of an ADVANTEC cellulose powder (No. E) in a 50 mL metal capsule, and completely dried on a hot plate. The rest of the analytical procedure was conducted in the same manner as that performed for the OBS fraction. Sulfur isotope abundances were measured using a VG Isogas mass spectrometer (SIRA 10) with SO2 prepared by MSS-3 BaSO4 (+3.5‰) as the secondary standard (15). The δ34S values were determined with respect to the Canon Diablo Troilite (CDT) as the international standard. The standard deviation (1σ) of the δ34S measurements was better than 0.2‰. For purposes of discussion, the δ34S values of the OBS and WSS fractions of the tree rings are defined as δ34SOBS and δ34SWSS, respectively.

Results and Discussion Sulfur Concentrations and δ34S Values in Tree Rings. The OBS and WSS concentrations in rings of the three trees are shown in Figure 2. The OBS concentrations of the urban Japanese cedar rings ranged from 63 µg g-1 in the late 1920s, with a gradual increase to a maximum of 170 µg g-1 in the early 1980s followed by some subsequent decline. The OBS

1925-1929 1930-1934 1935-1939 1940-1944 1945-1949 1950-1954 1955-1959 1960-1964 1965-1969 1970-1974 1975-1979 1980-1984 1985-1989 1990-1994 1995-1997

urban Hinoki cypress

rural Japanese cedar

δ34S WSS δ34SOBS δ34S WSS δ34SOBS δ34SWSS δ34SOBS (‰) (‰) (‰) (‰) (‰) (‰) 8.5 7.1 7.1 6.2 5.5 4.5 3.5 3.2 3.7 3.4 3.2 5.8

10.9 10.6 9.7 9.8 9.6 8.9 7.7 6.0 3.6 2.3 2.0 2.0 2.6 3.1 3.7

2.2 2.0 0.5 2.0 2.8 4.3 4.1 7.0

6.2 5.0 3.1 1.2 -0.9 0.8 2.4 3.1 3.6 3.4

4.2 4.4 2.7

3.0 2.6 2.2 2.0 1.7 2.1 1.4 1.4 2.1

2.6 4.1 5.0 4.0

concentrations of the rural Japanese cedar generally exhibited a much narrower range, from 48 µg g-1 in the early 1970s through 72 µg g-1 in the early 1990s, aside from the most recent higher measurement. The OBS concentrations of the urban Hinoki cypress ranged from 98 through 136 µg g-1, and like the urban Japanese cedar, showed higher concentrations as compared to those of the rural Japanese cedar. The WSS concentrations of the rings of all three trees, on the other hand, were far lower than the OBS concentrations. This result indicates that the contribution of the WSS fraction to the total sulfur (OBS fraction + WSS fraction) in the rings is considerably small. Additionally, the OBS and WSS concentrations in the most recent rings of each tree are relatively high, which may be attributable to the contamination of the cambium with high sulfur content. The δ34SOBS and δ34SWSS values of rings of the three trees are shown in Table 1. The δ34SOBS values of the urban Japanese cedar remained at about +10 ‰ before the late 1940s, and they gradually decreased until the late 1970s, reaching a VOL. 40, NO. 18, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Comparison of the δ34S values of tree rings with past atmospheric sulfur concentrations and with those of their sources. (a) Half-decade changes in the δ34SOBS values of the urban Japanese cedar, urban Hinoki cypress, and rural Japanese cedar. Rings before 1924, 1949, and 1954, respectively, are core samples. (b) Recent δ34S values of atmospheric SO2 (average value by 24 measurements) (24), and of SO42- in bulk precipitation (24 measurements) (24), and of SO42- in soil (9 measurements) (13) collected in Fukuoka, and of various sulfur sources (17, 20). (c) Annual changes in atmospheric sulfur oxide concentrations at monitoring stations in Fukuoka from 1966 onward and Kitakyushu from 1959 onward (21, 22). minimum of +2.0 ‰ before increasing once again (Figure 3a). The δ34SOBS values of the urban Hinoki cypress also decreased until the early 1970s, reaching a minimum of -0.9 ‰. The most recent δ34SOBS value of the urban Japanese cedar (+3.7 ‰) was very close to that of the Hinoki cypress (+3.4 ‰), although there was a considerable difference between the minimum values obtained for these two trees. The δ34SOBS values of the rural Japanese cedar, on the other hand, showed a change of only a +1.6 ‰ from the late 1950s through the late 1990s, remaining relatively low throughout this period of time. There is a relationship that should be noted in Table 1 between the δ34SWSS and δ34SOBS values of the tree rings for each half-decade increment. The δ34SWSS values of the urban Japanese cedar and Hinoki cypress are less than the δ34SOBS values before the 1960s, and higher after that, although the δ34SWSS values of the rural Japanese cedar are higher than the δ34SOBS values throughout. It is reported that the δ34SOBS values of plant leaves are about +3.5 ‰ lower than the δ34SWSS values due to a kinetic isotopic fractionation effect during sulfur assimilation from absorbed SO42- to H2S, followed by sulfur compounds synthesis, such as cysteine (1, 13, 16). The inverse relationship found in both urban trees before the 1960s indicates an inter-ring translocation of the WSS fraction. Based on this finding, we will discuss only the δ34SOBS values below. 5752

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Sulfur Isotopic Ratios in Tree Rings and Atmospheric Sulfur Sources. Atmospheric sulfur sources are classified as natural or anthropogenic, each of which has inherent δ34S values (Figure 3b) (1, 17-20). The changes in the δ34SOBS values of the urban Japanese cedar can be clearly explained by changes in the inherent values of the main atmospheric sulfur sources during the following periods: (I) before the 1940s, (II) from the 1950s through the 1970s, and (III) after the 1980s. These changes were also apparent in the case of the urban Hinoki cypress. Atmospheric sulfur oxide concentrations in Fukuoka and Kitakyushu increased with petroleum combustion until 1972 and 1969, respectively, after which they markedly decreased (21, 22) as a result of emission control through enforcement of the Air Pollution Control Law in Japan from 1968 (Figure 3c). It is notable that the δ34SOBS values of both urban trees decreased with increasing sulfur oxide concentrations, reaching a minimum after the sulfur oxide concentrations reached a maximum in both areas during period II. This decrease is clear evidence of the dependency of δ34SOBS values of rings on those of the main atmospheric sulfur sources. The decreasing δ34SOBS values of both urban trees can therefore be attributed to changes in the main sulfur source from high to low δ34S values, the source undoubtedly being anthropogenic sulfur released locally into the atmosphere. Lower values can be explained by the fact that the petroleum

imported from foreign countries (and especially from the Middle East (23)) from the 1950s onward has negative δ34S values (1, 18-20). During the high economic growth periods of the 1950s and 1960s in Japan, a large amount of anthropogenic sulfur with negative δ34S values was released into the atmosphere with increasing petroleum consumption, and as a result, depositions of sulfur with negative δ34S values increased during these periods. Since the contribution of anthropogenic sulfur to the total sulfur absorbed by trees increased annually during period II, the δ34SOBS values of the rings showed a resultant decrease. The difference in the minimum values of the urban Japanese cedar and the Hinoki cypress might therefore be dependent on the amount of sulfur deposition, since a large difference in sulfur oxide concentrations was observed between Fukuoka and Kitakyushu (Figure 3c). The delay of about half a decade in the responses of δ34SOBS values found in the rings to the maximum sulfur oxide concentrations in the atmosphere likely depends on the period required for the mobility of sulfur from the soil surface to the roots after deposition, and for the metabolism of sulfur in the trees after absorption. The δ34SOBS values of rings should show almost no change if there are little changes in the values of atmospheric sulfur sources. Evidence supporting this phenomenon was observed in the δ34SOBS values of the rural Japanese cedar. Sulfur sources in this rural area have apparently remained natural, with no anthropogenic input, and as a result, the δ34SOBS values of the Japanese cedar showed subtle change. This lack of change also seems true in the case of the urban Japanese cedar during period I. The constant δ34SOBS values of about +10 ‰ indicate little change in the values of the atmospheric sulfur sources, suggesting that the sulfur was derived from domestic natural sources due to the apparently low human activity during this period. The δ34SOBS values of the rural Japanese cedar remained relatively low (about +2 ‰) compared to those of both urban trees, despite the lack of anthropogenic sources in this area. This fact can be explained by the different natural sources in urban and rural areas. The lower δ34SOBS values in the rural Japanese cedar can clearly be attributed to the small contribution of sea salt sulfate, a natural source with a high δ34S value of about +21 ‰ (17), because of the geographically interior location of the rural site. The subtle variation (+1.6 ‰) of the δ34SOBS values do not likely depend on an appearance of new sources with inherent δ34S values, but instead on the changes in the ratio of dry deposition to wet deposition of natural sulfur with inherent δ34S values (13). The trend of the δ34SOBS values to the contrary, the OBS concentrations of the both urban trees showed high levels after the early 1950s (Figure 2). It is reported that most of sulfur absorbed as SO42- by plants is utilized to synthesize cysteine, methionine, and proteins, and excess sulfur is stored as glutathione (16). One possible reason of the high OBS concentration of both urban trees may be attributable to the glutathione of the OBS fraction, because much anthropogenic sulfur was obviously supplied during this period. During period III, the release of anthropogenic sulfur was markedly less (Figure 3c). Despite this fact, the δ34SOBS values of both urban trees did not revert back to about +10 ‰ observed before the late 1940s, but instead remained close to +3.5 ‰. As shown in Figure 3b, recent δ34S values of atmospheric SO2, SO42- in bulk precipitation, and watersoluble SO42- in soil collected in Fukuoka, are -1.6 ‰, +6.8‰, and +4.4‰, respectively (13, 24), each lower than +10 ‰. The most recent δ34SOBS values of urban trees are very similar to that of water-soluble SO42- in soil, and are intermediate between those of atmospheric SO2 and SO42in bulk precipitation, suggesting that the trees examined absorb sulfur mainly as water-soluble SO42- from soil via

their roots, which in turn has been derived from dry and wet depositions of atmospheric sulfur. The δ34SOBS values of tree rings in period III, therefore, certainly depend on those of atmospheric sulfur released from sources with values lower than +10 ‰. We speculate that there are two sources with low δ34S values. One source is the result of petroleum combustion since the 1950s. The negative value of atmospheric SO2 indicates that petroleum combustion cannot be ignored as an historical source of SO2 in urban areas. A newer source is derived from the long-range transport of sulfur as SO42from continental Asia to Japan, caused by the sharp increase in anthropogenic sulfur released into the atmosphere in Asia and its transport by seasonal northwest winds. This is a phenomenon that, since the 1980s, has been pointed out in many investigations using air trajectory analysis (25, 26), chemical analysis (27), and isotopic analysis (20, 24, 28-30). Using δ34S values of atmospheric sulfur and the monthly variations observed in Fukuoka, we also found that SO42with lower values than +10 ‰ from continental Asia has been incorporated into the bulk precipitation (24). The δ34SOBS values found in both urban trees during period III, therefore, are deduced to mainly reflect the values of these sources. Results indicate that the δ34SOBS values of tree rings are a useful chronological proxy for identifying past sulfur sources in locations where the various sulfur sources have significantly different δ34S values, as the isotopic values clearly depend on the sources of sulfur. The δ34SOBS values of tree rings, therefore, can play an important role in the evaluation of possible causes of past atmospheric sulfur pollution.

Acknowledgments We thank Y. Hayashi (Department of Chemistry and Physics of Condensed Matter, Graduate School of Science, Kyushu University), Y. Wakiyama, and M. Noto (Kyuden Sangyou Co. Ltd.) for sample preparation and δ34S measurements; Y. Maeda, K. Yoshimura (Department of Chemistry, Faculty of Science, Kyushu University), and S. Osaki (Radioisotope Center, Kyushu University) for discussions; Fukuoka and Shiiba research forests, Kyushu University for sampling and providing meteorological parameters; And D. B. O’Niell, R. A. Culp, and J. E. Noakes (Center for Applied Isotope Studies, The University of Georgia) for comments and improving the manuscript.

Supporting Information Available Ring widths of the urban Japanese cedar, urban Hinoki cypress, and rural Japanese cedar; and meteorological parameters for each sampling site. This material is available free of charge via the Internet at http://pubs.acs.org.

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Received for review February 13, 2006. Revised manuscript received May 23, 2006. Accepted June 26, 2006. ES060321W