Potential uses of stable isotope ratios of Sr, Nd, and Pb

Review

Potential uses of stable isotope ratios of Sr, Nd, and Pbin geological materials for environmental studies

By Takanori NAKANO*1,†

(Communicated by Eitaro WADA, M.J.A.)

Abstract: The ratios of stable isotopes of certain elements in rocks and minerals have strongregional characteristics that are reflected in atmospheric components, in water, and in the livingorganisms that form Earth’s surface environment as well as in agricultural and fishery products.Geologically derived stable isotope ratios can be used as a tracer for the source of many kinds ofsubstances, with current geochemical techniques allowing the precise determination of numerousstable isotope ratios in both natural and manmade objects. This review presents examples of the useof stable isotopes as tracers within diverse dynamic ecosystems, focusing on Sr isotopes but alsoincluding examples of Nd and Pb isotopic analysis, and reviewing the potential of this technique fora wide range of environmental research, including determining the geographic origin of food andarcheological materials.

Keywords: stable isotopes, strontium, neodymium, lead, traceability indexes, environment

1. Introduction

Some 80 of the 92 naturally occurring elementson Earth are stable, with 54 of these having two ormore stable isotopes. The fact that stable isotopesdiffer in mass number but not in atomic numbermeans that the different stable isotopes of a givenelement differ slightly in their physicochemicalbehavior.1)–4) Consequently, the relative abundancesof the various stable isotopes that compose mattercan act as fingerprints that enable the tracing of theorigin (or source) of the elements in a substance.

The stable isotope ratios of individual elementsare affected by two main factors, namely isotopicfractionation and radioactive decay. Isotope fractio-nation occurs during physicochemical processeswhen atoms of an element are involved in chemicalreactions, diffusion, and transformation betweensolid, liquid, and gaseous phases. The degree offractionation is generally dependent on the relativedifference in mass between the stable isotopes of a

given element and the temperature at which thefractionation process occurs. Light elements, such ashydrogen (H), carbon (C), nitrogen (N), oxygen (O),and sulfur (S), are present in a wide variety ofchemical forms and phases. These light elementshave large isotopic fractionations that reflect thelarge relative difference in mass between their variousisotopes. Consequently, the ratios of the stableisotopes of these elements have been used for a widerange of research into the processes that operate inEarth’s surface environment, including ecosystemresearch.1)–4)

Radioactive decay affects certain isotopes at asteady rate, including the decay of radioactiveelements into stable isotopes of other elements. Thishas led to the development of techniques for thedating of rocks and minerals using the radioactiveisotopes of parent elements and the correspondingstable isotopes of their daughter elements, asexemplified by the rubidium–strontium (Rb–Sr),samarium–neodymium (Sm–Nd), uranium–lead(U–Pb), and thorium–lead (Th–Pb) methods ofdating (Table 1), where 87Rb decays into 87Sr bybeta emission, 147Sm decays into 143Nd by alphaemission, and 238U, 235U, and 232Th ultimately decayinto 206Pb, 207Pb, and 208Pb, respectively, by acascade of alpha and beta emissions. These daughter

*1 Research Institute for Humanity and Nature, Kita-ku,Kyoto, Japan.

† Correspondence should be addressed: T. Nakano, Re-search Institute for Humanity and Nature, 457-4 Motoyama,Kamigamo, Kita-ku, Kyoto 602-0878, Japan (e-mail: [email protected]).

Proc. Jpn. Acad., Ser. B 92 (2016)No. 6] 167

doi: 10.2183/pjab.92.167©2016 The Japan Academy

http://dx.doi.org/10.2183/pjab.92.167

elements also include stable isotopes with primordialorigins (86Sr) or radioisotopes with very long half-lives (144Nd and 204Pb). This means that the relativeabundances of isotopes of Sr, Nd, and Pb withinrocks and minerals (87Sr/86Sr, 143Nd/144Nd, 207Pb/204Pb, etc.) are considered to be constant in environ-mental studies on time scales of <104 years, but canvary significantly depending on their age of formationand the ratios of parent and daughter elements (e.g.,Rb/Sr or Sm/Nd). In addition, 206Pb, 207Pb, and208Pb isotopic ratios vary as a result of differing Pb,U, and Th ratios.2),5)

The water, living organisms, and soil of ecosys-tems contain elements of geological origin that inmany cases have undergone isotope fractionationduring chemical transformations and changes ofstate. However, heavy elements such as Sr, Nd, andPb experience only minimal isotopic fractionation,primarily as they have small relative differences inmass between their stable isotopes.2)–4) The mass-dependent isotope fractionation of Sr and Nd, both ofwhich have several non-radiogenic stable isotopes,can be compensated for during analysis, demonstrat-ing that the radiogenic stable isotope ratios of Srand Nd in rocks and minerals (i.e., 87Sr/86Sr and143Nd/144Nd) are directly reflected in the water andliving organisms within ecosystems.2),6)

The fact that the stable isotope ratios of Sr, Nd,and Pb within environmental substances have strongregional variations that are commonly controlled bythe underlying geology means that these elementscan be used as traceability indexes to determine theirorigins. Strontium is geochemically similar to theimportant bioelement calcium (Ca), meaning thatSr stable isotopes can be used as a traceability indexin environmental and ecosystem studies.7)–10) This

paper reviews previous research that has appliedgeologically derived stable isotopes to material flowsin ecosystems, and focuses on the Sr isotopic system,which is widely used in global environmental research,including ecosystem and water cycle studies. Thelocations used in this study are shown in Fig. 1.

2. Stable isotopes of Sr, Nd, and Pb

The factors that control the Sr, Nd, and Pbstable isotopic ratios of geologic materials includethe age of the material, and the ratio of parent todaughter elements (e.g., Rb/Sr, Sm/Nd, U/Pb,Th/Pb), where higher parent:daughter ratios leadto a higher abundance of radiogenic Sr, Nd, or Pb.Alkaline elements such as K and Rb show a positivecorrelation with each other but a negative correlationwith alkaline earth elements such as Ca and Sr. Thisleads to large differences between rock Rb/Sr ratiosand the ratios in soils and sediments produced by theweathering of these rocks, a factor that in turn leadsto wide variations in the 87Sr/86Sr ratios of crustalmaterials.

The ancient granitic rocks that dominate thecontinental crust have elevated Rb/Sr and 87Sr/86Srratios. In contrast, the mantle-derived basalts thatdominate the oceanic crust have low Rb/Sr and 87Sr/86Sr ratios. Volcanic rocks within island arc and con-tinental settings also have low 87Sr/86Sr ratios, againreflecting their derivation from mantle material. Incontrast, Nd isotope ratios (143Nd/144Nd) generallynegatively correlate with Sr isotope ratios, primarilyas the ultramafic rocks within the mantle have higherSm/Nd ratios than rocks within the continentalcrust.2),5)

Economically important metals, including Pb,are mined and utilized on a large scale, with the Pb

Table 1. Parent and daughter isotopes, half-lives, and other daughter stable isotopes of the Rb–Sr, Sm–Nd, U–Pb, andTh–Pb systems

Parent

isotope

Daughter

isotope

Half-life

(years)Other stable isotopes

Isotope ratio corrected for87Sr/86Sr and 143Nd/144Nd

87Rb 87Sr 4.97 # 1010 84Sr, 86Sr, 88Sr 86Sr/88Sr F 0.1194

147Sm 143Nd 1.06 # 1011142Nd, 144Nd*, 145Nd, 146Nd,

148Nd, 150Nd*

146Nd/144Nd F 0.7219

(143Nd/144Nd)CHUR F 0.512638238U 206Pb 4.468 # 109

204Pb*235U 207Pb 7.038 # 108

232Th 208Pb 1.405 # 1010

CHUR F Chondritic Uniform ReservoirHalf-lives are calculated using decay constants.* F super long-lived radioisotope regarded as stable in geological and environmental studies; the half-lives of 144Nd, 150Nd, and 204Pbare 2.29 # 1015, 7.9 # 1018, and >1.4 # 1014 year, respectively, which is far longer than the age of the Earth (4.56 # 109 year).

T. NAKANO [Vol. 92,168

released from metal ores by human activities makingan enormous contribution to the total Pb withinwater and in ecosystems. The fact that metal oresgenerally contain negligible amounts of U and Thmeans that they have Pb isotope ratios that are afunction of their age and the U/Pb and Th/Pb ratiosof the geological materials responsible for oreformation. In addition, the fact that granites containelevated concentrations of U and Th means that theyare also enriched in radiogenic lead, especially whencompared with radiogenic lead-depleted basalt andisland arc volcanic rocks.2),5)

Strontium isotopic ratio measurements involvethe simultaneous measurement of four stable isotopes(84Sr, 86Sr, 87Sr, and 88Sr; Table 1); 87Sr is the onlyone of these isotopes that is radiogenic, and it formsas a result of the beta decay of 87Rb. This means thatthe ratios of non-radiogenic 84Sr, 86Sr, and 88Sr onlychange as a result of isotope fractionation, with theisotopic fractionation of 87Sr relative to 86Sr almosthalf that of 87Sr relative to 88Sr. Setting the 86Sr/88Srratio at a fixed value (0.1194) enables an approachthat allows compensation for changes in the 87Sr/86Srratio due to isotope fractionation and the precisedetermination of the abundance of radiogenic 87Sr ingeological materials.

Neodymium has five stable isotopes (142Nd,143Nd, 145Nd, 146Nd, and 148Nd) and two radioisotopes(144Nd and 150Nd) with half-lives of 2.29 # 1015 and6.7 # 1018 years, respectively. These very long half-lives means that 144Nd and 150Nd are regarded asstable isotopes in both geological and environmentalstudies. The alpha decay of 147Sm forms 143Nd, andNd isotopic comparisons are expressed in terms of the143Nd/144Nd ratio. Neodymium isotopes are correctedin a similar fashion to the correction of 87Sr/86Srmeasurements by setting the 86Sr/88Sr ratio to0.1194, with Earth-derived materials having 143Nd/144Nd ratios that are corrected to a 146Nd/144Nd ratioof 0.7219, the chondritic uniform reservoir (CHUR)value.2),5) The fact that the range of 143Nd/144Ndratios is small compared with that of 87Sr/86Sr ratiosmeans that the 143Nd/144Nd ratio of a sample R(ð143Nd

144NdÞ0R) is often expressed in epsilon notation

(i.e., C0(Nd)), where the Nd isotopic composition ofthe sample is evaluated by comparison with thepresent-day CHUR isotope ratio, as follows:

"0ðNdÞ ¼

�143Nd144Nd

�0

R

��

143Nd144Nd

�0

CHUR�143Nd144Nd

�0

CHUR

8>>>><>>>>:

9>>>>=>>>>;

� 104 ½1�

Fig. 1. Study sites in Japan and Far East Asia; orange indicates desert and loess areas, and dotted lines indicate mountainous areas.

Uses of geological stable isotopes in environmental studiesNo. 6] 169

where the superscript zero refers to the present time(t F 0) and ð143Nd

144NdÞ0CHUR ¼ 0:512638.

Lead has four isotopes, and environmentalstudies use six Pb isotope ratios as traceabilityindices. The relative abundance of Pb isotopes innatural materials is generally in the order 208Pb,206Pb, 207Pb, and 204Pb from highest to lowestabundance. Lead isotope ratios are expressed informs comparing the respective ratios of 206Pb,207Pb, and 208Pb with both primordial and radiogenicorigins to primordially derived 204Pb (206Pb/204Pb,207Pb/204Pb, and 208Pb/204Pb), and the ratios ofthe former three isotopes (208Pb/206Pb, 207Pb/206Pb,208Pb/207Pb, or 206Pb/207Pb).

3. Sr isotopes in freshwater ecosystems

Nearly all of the Earth’s surface water (97.3%)is seawater, which contains elevated concentrationsof Sr (98 ppm) and has an extremely homogenous87Sr/86Sr ratio.2),5) The 87Sr/86Sr ratio of benthicforaminifera collected from the western Pacific Oceanand in corals from shallow seas and dated to thepast 30,000 years is 0.709175 ’ 0.000005,11) with therange in this value less than twice the uncertainty onthese measurements. In contrast, large variations arepresent in freshwater 87Sr/86Sr ratios.12)

3.1. Groundwater and river water. Ground-water, lake water, and river water resources contain22.3%, 1.5%, and 0.016% of the world’s freshwater,respectively. The quality of groundwater and springwater (i.e., groundwater appearing at the Earth’ssurface) is strongly buffered by subterranean rocksand is generally stable. One example of the geo-chemistry of groundwater, from Saijo in south-western Japan, is shown in Fig. 2; the two ground-water sites in this area show small temporalvariations in 87Sr/86Sr ratios (<0.00002), althoughthese variations are several times higher than theuncertainties on the analyses. The permanent mini-mum flow in rivers, representing groundwater ap-pearing at the Earth’s surface, is known to hydrol-ogists as baseflow and has a stable 87Sr/86Sr ratio,indicating that both groundwater and river baseflowhave characteristic 87Sr/86Sr ratios that reflect thegeologic environment of the watershed.

The 87Sr/86Sr ratios of most of the world’s riversrange between 0.703 and 0.730, although some rivershave ratios close to 1.0.12) The 87Sr/86Sr ratios ofmajor continental rivers (average: 0.7119) are higherthan the ratio of seawater, although many Japaneserivers drain areas of volcanic material and as a resulthave lower 87Sr/86Sr ratios.13) Rocks in the interior of

southwestern Japan and the Kanto region have 87Sr/86Sr ratios higher than seawater, including sandstoneand shale units within Jurassic accretionary com-plexes. The sandstone and shale of the ShimantoBelt, a Cretaceous to Paleogene accretionary com-plex on the Pacific side of southwestern Japan, andthe Mesozoic granites of the Chubu and Kinki regionsof Japan also have high 87Sr/86Sr ratios.14) This inturn has generated river water and groundwater inthese areas that typically have higher 87Sr/86Sr ratiosthan seawater. This is further exemplified by theLake Biwa basin in the interior of southwesternJapan, which is generally underlain by Triassic andJurassic sandstone and shale, and Cretaceous graniteand rhyolite units. The water within the majority ofrivers in this basin has 87Sr/86Sr ratios higher thanseawater, primarily as a result of the Sr derived fromthese rocks.15),16) The 87Sr/86Sr ratios of river wateralso change according to variations in the contribu-tions from tributaries with different 87Sr/86Sr ratios.This is also demonstrated by studies of Lake Biwa,where 87Sr/86Sr ratios at a given spot show temporalchanges that are larger than the uncertainty on theSr isotopic analysis (Fig. 3). However, these changesin river water are very small compared with thevariations in 87Sr/86Sr ratios caused by changes inatmospheric precipitation in inland Japan, whichexceed 0.002 in most cases (Fig. 4).

3.2. Lake and marsh ecosystems. Lakes andmarshes represent areas of mixing of river water andgroundwater of variable water quality, yielding waterwith uniform isotope ratios. The 87Sr/86Sr ratios ofwater in freshwater lakes and marshes in Japan havea characteristic and relatively narrow range of values(Fig. 5a). The 87Sr/86Sr ratio of Lake Biwa water ischaracteristically high, reflecting the generally high87Sr/86Sr ratios of the river water within the water-shed. Conversely, the watersheds of Lake Towadaand Lake Suwa are dominated by volcanic materialswith low 87Sr/86Sr ratios, yielding lake waters withsimilarly low 87Sr/86Sr ratios.

The range of geographic variations in lake water87Sr/86Sr ratios also differs depending on the lake. Forexample, the variation in 87Sr/86Sr ratios in LakeTowada is only several times the uncertainty on theanalysis, whereas the variations in Lake Biwa areseveral dozen times the uncertainty on the analysis(Fig. 5b). The factors involved in this variationinclude physical processes (e.g., the velocity of lakewater circulation) and differences in the 87Sr/86Srratios of inflowing river water and groundwater.Caldera lakes such as Lake Towada have low volumes


Fig. 3. Comparison of 87Sr/86Sr ratios in river water flowing intoLake Biwa in the summer of 2003 and the winter of 2004.16)

Water quality data are provided for four areas (north, south,east, and west) that differ according to topography, geology, andthe level of human activity. Sampling sites for baseflowobservations in individual rivers are shown in the geologicalmap in the lower right part of the figure, where red F granite,pink F rhyolite, yellow–green F Triassic–Jurassic sandstone andshale, pale blue F limestone, and white F Pliocene–Quaternarysediments.

Fig. 4. Time series of monthly 87Sr/86Sr ratios in wet precipitation at three locations in Japan (Kawakami, Morioka, and Toyama) and asmall island to the north of Taiwan (Peng–Chia–Yu).28)–30),53) Dotted line (SS) indicates the 87Sr/86Sr ratio of seawater and brownbars (SMC) show the range of 87Sr/86Sr ratios for evaporitic (salinization) minerals in desert sand and loess within northernChina.31),32)

Fig. 2. Temporal variations (a) and frequencies (b) in the 87Sr/86Sr ratios of groundwater at the Uchinuki (deep groundwater)and Kannonsui (shallow groundwater) sites in Saijo, south-western Japan. The variations in 87Sr/86Sr ratios for each watertype are shown as a histogram; note that the variations ingroundwater 87Sr/86Sr values are only a few times larger thanthe range of analytical uncertainty (’0.000005, as indicated bythe bar in the histograms) associated with thermal ionizationmass spectrometer (TIMS) analysis.


of inflowing river water that generate small varia-tions in 87Sr/86Sr ratios and low amounts of Srisotopic heterogeneity. In comparison, the largesurface area of Lake Biwa and the significantvariations in Sr isotopic composition of inflowingriver water have led to the development of Sr isotopicheterogeneity.

The food web in aquatic systems starts withphytoplankton and leads through zooplankton topredators and decomposers. Phytoplankton incorpo-rates carbon dioxide and water containing Sr andother dissolved elements, meaning that phytoplank-ton and other organisms in aquatic environmentsshould have similar 87Sr/86Sr ratios to the ambientwater in which they are hosted. The 87Sr/86Sr ratiosof lakes and marshes coincide with the isotopiccomposition of sedentary organisms such as watergrasses (measured in leaves and stems) and fresh-water snails (measured in shells; Fig. 5). However,closer examination indicates that the variations in87Sr/86Sr ratios of lake and marsh water, and insedentary organisms are greater than the uncertain-ties on these analyses, as shown in an example fromLake Biwa (Fig. 5b). The water and water grassesor mollusk shells have very similar 87Sr/86Sr ratiosthat are generally higher in the North Basin of thelake compared with the South Basin. This finding isconsistent with the higher 87Sr/86Sr ratios (>0.712)of rivers entering the North Basin (west and east

areas in Fig. 3) compared with those entering theSouth Basin. However, some areas, for examplewhere river water enters the lake, have water 87Sr/86Sr ratios that differ from the Sr isotopic composi-tion of organisms within the same area. The fact thatthe 87Sr/86Sr ratios of organisms reflect the compo-sition of the ambient lake water during their growthmeans that this disagreement is ascribed to temporalvariations in the 87Sr/86Sr ratios of the lake water. Inaddition, brackish lakes where freshwater and sea-water mix have water and organism 87Sr/86Sr ratiosthat can vary significantly. This heterogeneity of Srisotope ratios in water and organisms could be usedto trace relationships between material dynamics andthe organisms of these lakes in even greater detail.

The 87Sr/86Sr ratios of bones from great cormor-ants (Phalacrocorax carbo) collected from two differ-ent sites at Lake Biwa are identical to the composi-tion of the lake water, indicating that these birds areeating fish and drinking water from the lake.6) Thefractionation of Sr isotopes occurs during metabolicprocesses, although unlike N or C the mass-depend-ent fractionation of Sr can be corrected during datareduction by normalizing to 86Sr/88Sr. This meansthat the 87Sr/86Sr ratios of water and organisms, bothof which reflect the regional geology of an area, canbe used as a geologic index. The same is true for Ndisotopes, although these isotopes have only limiteduses in environmental and ecological applications

Fig. 5. Spatial variations in 87Sr/86Sr ratios within freshwater lakes and marshes in Japan (a) and in Lake Biwa (b), includingunpublished data.15),16),44)–54) Locations of samples are shown in the inset maps, and the sizes of the squares in (a) denotes the rangeof 87Sr/86Sr ratios. The sample materials analyzed during these studies were leaves and stems for water grasses, and shells for snails.The Lake Biwa data are divided into two basins, with open symbols for the north basin and solid symbols for the south basin.


compared with Sr isotopes as a result of the generallyvery low concentrations of Nd in these environments.

3.3. Application to fish studies. Both marineand land areas are habitats for sedentary and sessileorganisms with 87Sr/86Sr ratios that are identicalto the associated ambient water. However, fish thatmigrate in the aquatic environment, particularlyanadromous fish, have behavioral histories that arereflected by their body tissues. In particular, otolithsthat grow sequentially in fish provide an excellentarchive that can help reconstruct the history of theaquatic environment in which the fish lived.

The Sr/Ca ratio of seawater (0.02) is approx-imately twice that of freshwater. This difference maybe reflected in otoliths and can thereby be used toprovide information on the living environment withinboth marine and land areas.17) However, freshwaterSr/Ca ratios are generally invariant between riversystems, and the Sr/Ca ratios of otoliths change withtemperature and vital effects.18) This means thatotolith Sr/Ca ratios cannot be used to distinguishbetween rivers, for instance when attempting toidentify the habitats or tracking the migrationhistory of freshwater fish. For example, the Sr/Caratios of salmon or trout otoliths from fish migratingupstream will not reveal whether the fish in questionare returning from the sea to their natal streamswhere they were spawned. However, river water(unlike seawater) shows large geographic variationsin 87Sr/86Sr ratios, meaning that differences in 87Sr/86Sr ratios between water and biological material canbe ignored. As such, otolith Sr isotope ratios enablethe reconstruction of the history of a salmon’sbehavior, even to the extent of determining whetherthe fish had returned to its natal stream.19),20)

Freshwater fish otolith 87Sr/86Sr ratios can alsoprovide a good index of water mass movement,primarily as the narrow range of living conditions ofthese fish generates 87Sr/86Sr ratio variations thatare no greater than the Sr isotopic variations insedentary organisms. This is exemplified by three-spined sticklebacks (Gasterosteus aculeatus) livingin springs with small changes in water quality, whichyield 87Sr/86Sr fluctuations no greater than theuncertainty of the analysis.6)

4. Forest ecosystems:atmosphere, plants, and soil

Forest is a complex ecosystem composed ofmacroorganisms (plants and animals) and soil, withthe latter containing air, water, and microorganismsin addition to soil minerals. Mountain forests free of

human inhabitants have plant and soil ecosystemscontaining elements derived from the atmosphereand the underlying geology. Recent deterioration ofecosystems has accompanied changes in the atmos-pheric environment, including warming or pollutionby nitrogen oxides and heavy metals. As such,evaluation of the influence of atmospheric depositionon ecosystems requires the discrimination of elementswith atmospheric and geological origins.

The analysis of the stable isotopic compositionof material derived from rocks and minerals is aneffective method of demonstrating the movement ofelements between plants and soil or within the soil.For example, the decline of forests as a result of acidrain was a serious problem in many countries duringthe late 20th century. One of the causes of forestdecline may be loss of Ca, an essential plant nutrient,from the soil due to leaching by acid rain.21) Both Srand Ca are geochemically similar because they aredivalent elements with similar ionic radii,2) meaningthat Sr can be used as a proxy for Ca. This in turnmeans that Sr isotope ratios enable the quantificationof the relative contributions of Sr and Ca from atmos-pheric deposition and basement rock sources.10),22)

4.1. Yakushima World Natural HeritageSite. The Yakushima World Natural HeritageSite is an island far from the Japanese mainlandthat has experienced minimal local anthropogenicinfluence on the natural environment (Fig. 1). Theisland receives two to five times more precipitation(2,700–8,600mm/year) than the average for Japan(1,700mm/year), but this precipitation containslarge amounts of air pollutants originating from theAsian continent. In fact, precipitation on Yakushimais acidic (annual average pH is 4.7) and contains largeamounts of non-sea salt sulfur (SO4

2!: 22–93 µeqL!1,nssSO4

2!/SO42!: 0.6–0.8) that is derived from trans-

boundary pollution.22) The granite that forms themajority of the island has a small acid neutralizationcapacity, meaning that concern is growing aboutthe effect of this air pollution on the island’secosystem.22),23) Wet precipitation is also rich incomponents derived from sea salt particles thathave a relatively constant 87Sr/86Sr ratio (0.70917–0.70935) throughout the year.22) In contrast, the87Sr/86Sr ratio of stream waters on the island differsmarkedly between watersheds draining granitic andsedimentary rocks, indicating that 70%–80% of theSr2D and Ca2D in stream waters is derived frombasement rocks.24),25)

A detailed study conducted in the graniticYotsuse area of northwestern Yakushima yielded


87Sr/86Sr ratios for soil-exchangeable components(obtained by leaching samples with ammoniumacetate) that were almost the same as those ofseawater regardless of soil depth. The 87Sr/86Sr ratiosin plants reflect the composition of these soil-exchangeable components, but soil minerals havemarkedly differing Sr isotopic compositions (Fig. 6).This result indicates that (1) the Ca and Sr thatplants absorb has a largely marine origin and isadsorbed onto materials in soil before being taken upby plant roots, and (2) the Sr present in soil mineralsis not involved in this exchange. Elsewhere onYakushima, the 87Sr/86Sr ratios in plants is close tothat of seawater, indicating that the Sr from sea saltparticles makes a significant contribution to the Srfound in plants on the island.24),25)

Soil minerals are generally derived from theunderlying rocks, and Japanese soil contains volcanicash and minerals transported through the atmos-phere, including Asian dust derived from desertregions. The fact that forest surface soil in Japan isacidic (pH 3.5–8.1, average pH 5.1)26) means thatsoil minerals are predominantly silicates. In addition,Nd and Sr isotopic analyses can be used to identifythe source of soil minerals, primarily as silicatesderived from local bedrock and from Asian dust havedistinctive Sr and Nd isotopic compositions.

The Sr and Nd isotopic composition of soil in theYotsuse area varies with depth, indicating a greaterabundance of Asian dust-derived silicates near theground surface (Fig. 6). Soils at different depths plotin the mixing zone between silicate minerals withlocal and Asian origins, and the stable isotope ratiosof these two components enable quantification ofthe relative contributions of these two sources. Inparticular, the fine-grained nature of particles in theatmosphere means that a more precise evaluationof soil components can be obtained by measuringdifferent grain-size fractions, with fine-grained min-erals having isotopic compositions more similar toAsian dust and the uppermost layers of soil in thisarea containing up to 980% fine-grained mineral.25)

Smaller grainsize soil minerals have largerspecific surface areas, meaning they play a significantrole in determining soil-exchange capacity. Conse-quently, the predominance of minerals derived fromAsian dust in surficial soils suggests that this dust hasa marked influence on material exchange with plants.The lower soil layers on Yakushima island aredominated by weathered granite, and approximately90% of the Ca, 80% of the Sr, and 70% of the P hasbeen leached from this granitic substrate.25) This

suggests that the supply of essential plant nutrients(e.g., Ca and P) on Yakushima island is stronglydependent on atmospheric deposition.

4.2. Kawakami Experimental Forest Site,Yatsugatake, Japan. The contribution of at-mospheric deposition to forest ecosystems varies withthe volume of rainfall, the properties of the basementrock, and other factors. Soils in Japan contain largeamounts of easily weathered volcanic material. It istherefore assumed that basement-rock-derived com-ponents play a larger role in forest ecosystems inJapan than in similar ecosystems in Europe andEastern North America, which host soils containingless volcanic material. In addition, the volcanoes inJapan are relatively young. Volcanoes that formedapproximately one million years ago produce volcanicminerals that have 87Sr/86Sr ratio variations that aresmaller than the precision of the analyses, indicatingthat the basement rock 87Sr/86Sr ratios in areasdominated by young volcanics (e.g., Japan) can beregarded as constant.

The Kawakami Experimental Forest of theUniversity of Tsukuba is located far from the sea inthe southern Yatsugatake Mountains of centralJapan (Fig. 1). This forest is located in a mountain-ous headwater area of 0.14 km2 with elevations from1,500 to 1,680m above mean sea level and is located inan area dominated by early Pliocene andesite volcanicrocks. The vegetation in this area is dominated bydwarf bamboo (Sasa nipponica) and Mongolian oak(Quercus crispula) that is spaced at about 5mintervals. Although the 87Sr/86Sr ratios of both plantspecies (measured in oak buds and bamboo leaves) arenearly the same in hillside localities, the ratio changeswith distance from the valley bottom (Fig. 7a).Japanese oak buds and bamboo leaves have almostthe same 87Sr/86Sr ratios as soil water in this area(Fig. 7b), and they have the same 87Sr/86Sr ratios asbulk soil NH4Cl and H2O2 leachates.27) This meansthat this area is the same as the soils on Yakushimaisland in that the 87Sr/86Sr ratios of soil-exchangeablecomponents at individual sites are almost identical tothe 87Sr/86Sr ratios of organic matter and soil water.

The bulk soil in the Kawakami ExperimentalForest is dominated by silicate minerals but hashigher 87Sr/86Sr ratios than the underlying andesiticbedrock (0.7039) and associated weathering products(0.7044). This indicates that the Kawakami soil, likethe soils on Yakushima island, contains silicatesderived from Asian dust.27) The variation in bulk soil87Sr/86Sr ratios (0.7051–0.7084) indicates that Asiandust plays a complex role in these soils that is


partially dependent on the location of the soil in thevalley. As is the case on Yakushima, the Sr isotopicdata from the Kawakami Experimental Forestprovides evidence of material exchange betweenwater, organic matter, and exchangeable compo-nents, although Sr and other elements in soil mineralsare not involved in these exchanges.

The concentrations of Sr2D in soil solutions inthe Kawakami Experimental Forest correlate with

both Ca2D and NO3! ions.27) This result suggests that

nitrogen deposition from the atmosphere is linked tothe leaching of components from soil and basementrocks within the forest ecosystem. The soils in thissmall watershed become thinner with increasingheight above the valley floor, and the 87Sr/86Sr ratiosin plants, soil organic matter, and soil water increasewith increasing height on the valley slope, indicatinga positive correlation between decreasing distance

Fig. 7. 87Sr/86Sr ratios in (a) soil water, plants (leaves of dwarf bamboo and young buds of Mongolian oak), bulk soil, and trainmillipedes (unpublished data) from a valley floor, and (b) the relationship between 87Sr/86Sr ratios in soil water and plants, and thebulk soil.27) The range of 87Sr/86Sr ratios in soil water and plants is shown in yellow.

Fig. 6. Vertical soils profiles at the Yotsuse site in northwestern Yakushima showing Sr isotope ratios for exchangeable components(leachate), bulk components, and mineral particles of different sizes (a), and Nd isotope ratios for bulk soil and soil mineral particlesshown as C0(Nd) values (b).25) Open hexagon in (a) denotes the average 87Sr/86Sr ratio of monthly rainwater in Yakushima and openstars in (a) and (b) denote the average 87Sr/86Sr ratio and C0(Nd) value of granite around the Yotsuse site, respectively. Note that theisotope ratios near the surface deviate from those of the granitic substrate and approach those of Chinese loess.


from the valley floor and increasing proportions ofSr derived from basement rocks. This area containsswarms of train millipedes (Parafontaria laminata)that emerge every 8 years; these millipedes congre-gate in the soil until they reach the 8th instar, whenthey leave the soil and migrate. The 87Sr/86Sr ratiosin 8th instar millipedes differ from the 87Sr/86Sr ratiosin plants and soil organic matter at the same spot,indicating that the millipedes feed at different sitesalong the slope (Fig. 7a). However, the 87Sr/86Srratios in millipedes collected from the lower part of aslope are lower than those of millipedes from theupper part of the same slope. This indicates thatindividual millipedes travel no more than a few dozenmeters along the slope, providing clear evidence ofhow the distribution of Sr isotope ratios can provideinsights into the material dynamics of ecosystems,including animals.

5. Interaction between the atmosphericenvironment, and glacierand forest ecosystems

Stable Sr, Nd, and Pb isotopic ratios of at-mospheric aerosols have been used as tools inatmospheric environmental research, primarily asthese ratios differ according to the sources of aerosolswithin marine and terrestrial areas.2),3),10) Thesegeologically sourced stable isotopes are also usefulfor elucidating interactions between the atmosphereand terrestrial ecosystems.

5.1. Effect of Asian dust minerals on theatmospheric environment and ecosystems. The87Sr/86Sr ratios of precipitation in Japan and Taiwanchange seasonally, with values tending to be high inspring and low in summer and autumn.28)–30) Asiandust generated from desert areas in northern China isactive in spring. Both desert soils in northern Chinaand the loess derived from these soils are associatedwith evaporitic minerals that are dominated bycalcium carbonate (CaCO3) with high concentrationsof Ca and Sr but also contain sulfates and chlorides.These evaporitic minerals are soluble in water andweak-acids and have distinct 87Sr/86Sr ratios (0.710–0.713), which are lower than that of associatedsilicates (0.720 ’ 0.003).31),32) In many places, thehigher 87Sr/86Sr ratio of springtime precipitation thanthat of seawater (Fig. 4) is ascribed primarily to theinput of Asian dust from China’s desert regions.

Soil containing evaporitic minerals is alkalinewhereas rainwater is generally acidic, and the CaCO3

in Asian dust has a neutralizing effect on therainwater that transports the dust. Spring rain at

the Kawakami site has a 87Sr/86Sr ratio close to thatof evaporites, and 970% of the Ca in precipitationat the Kawakami site is derived from the CaCO3 inAsian dust.28)

Inland areas located far from the sea, and highmountain regions have low influxes of airborne seasalt particles and greater contributions from materi-als derived from inland regions. High mountainregions of the Asian continent contain a combinationof algae and windblown minerals called cryoconitewithin ablation zones near the lower terminations ofglaciers. The fact that the presence of cryoconitelowers the local albedo, causing an increase in heatabsorption and an acceleration in glacial melting,means that investigating the origin of this material isimportant.33) Large amounts of cryoconite containingcyanobacteria are present in ablation zones withinmountain glaciers in the Tian Shan and Kunlunmountains of China. In contrast, cryoconite is lessabundant on glaciers in the Altai Mountains to thenorth of China and the Himalaya to the south ofChina, both of which also contain green algae insteadof cyanobacteria.

The Sr and Nd isotope ratios of the silicate min-erals that dominate cryoconite vary because the min-eral sources differ from one glacier to another.34),35)

Silicate materials within Chinese glaciers have Sr andNd isotope ratios that are similar to those of desertsoils within western and northwestern China thatproduce Asian dust. In comparison, organic material(e.g., cyanobacteria) has 87Sr/86Sr ratios that arenearly identical to those of evaporitic minerals withindesert soils (Fig. 8). This indicates that the abundantcryoconite present within mountain glaciers in Chinais dominated by dust sourced from nearby deserts.This dust contains evaporitic minerals (mainlyCaCO3 minerals) that form the main source ofnutrients for the cyanobacteria that thrive in alkalineenvironments.

Mount Tateyama (Fig. 1) in central Japan facesthe Sea of Japan and is associated with precipitationthat has seasonal changes in 87Sr/86Sr ratios similarto those recorded in the Kawakami area. However,the 87Sr/86Sr ratio (0.7099) of the Japanese stonepine (Pinus pumila) in this alpine region is higherthan that of either the basement rock or seawater,indicating that these trees uptake a significantamount of Sr derived from evaporitic minerals withinAsian dust.36) Transboundary materials from theAsian continent are rich in ammonia and nitrogenoxides, both of which tend to be deficient in forestenvironments. Asian dust is also considered to be a


source of Ca and P.37) Therefore, airborne pollutantsfrom Far East Asia may act as nutrient sources forforests and other ecosystems, especially in environ-ments such as glaciers and alpine areas.

5.2. Interaction between rain and plants.The 87Sr/86Sr ratios of precipitation in Japan andTaiwan commonly decrease during summer andautumn (Fig. 4) owing to increased amounts ofairborne material derived from cement and roaddust. The 87Sr/86Sr ratio of limestone, a raw materialin cement manufacture, is 0.7080 ’ 0.0012.38) How-ever, the 87Sr/86Sr ratio of precipitation is sometimeslower than that of seawater or limestone; this is eventhe case in forest areas that are distant from urbanareas, indicating that this Sr is derived from othersources. The Earth’s mantle and mantle-derivedvolcanic rocks have low 87Sr/86Sr ratios, withJapanese volcanic rocks having 87Sr/86Sr values of0.703–0.706. The low 87Sr/86Sr ratio (90.7065) ofautumn rain in the Kawakami area suggests that thisrainfall has been affected by contributions from Srultimately derived from volcanic rocks around theprecipitation area.

Clarifying the interaction between precipitationand plants is important in assessing the impact ofatmospheric deposition on ecosystems, including acid

rain. Rainfall in forest areas is termed ‘incidentprecipitation’, with rain passing through forestcanopies termed ‘throughfall’, and rain flowing alongtree trunks termed ‘stem flow’. Research in theKawakami area indicates that stem flow 87Sr/86Srratios are nearly identical to plant 87Sr/86Sr ratios,with neither varying significantly from season toseason.39) The 87Sr/86Sr ratio of throughfall is thesame as that of incident precipitation in winter, whenthere is little or no foliage, and is closely correlatedwith the Sr isotopic ratio of plants in summer whenfoliage is abundant (Fig. 9). The Sr2D concentrationin throughfall strongly correlates with Ca2D andMg2D concentrations in throughfall. This result,combined with the fact that these cations are presentin high concentrations in throughfall that has 87Sr/86Sr ratios close to the Sr isotopic ratios of plants inthis area, indicates that these elements are sourcedfrom plants locally.

Calcium, Sr, and other elements within plantsare eluted in reactions with rainwater, suggestingthat the introduction of aerosol particles derivedfrom plants to the atmosphere may influence thequality of precipitation. In fact, wet precipitationin forests of northern Europe contains plant-derivedCa2D, Sr2D, and other elements, but the presence ofplant-derived aerosols in this region remains un-clear.40) The average annual precipitation in Japanis 1700mm, and evapotranspiration from forests is9800mm, indicating that approximately half of the

Fig. 8. Frequency diagram of 87Sr/86Sr ratios in organic matterin cryoconite on Chinese mountain glaciers (Grigoriev icecap, Urumqi Glacier no. 1, and Miaoergou Glacier in the TienShan, and Qiyi Glacier in the Qillian Shan), and evaporitic(salinization) minerals in desert sand and loess in northernChina.31),32),34),35)

Fig. 9. Seasonal changes in 87Sr/86Sr ratios in wet precipitation,throughfall, stem flow, soil water, and stream water during May(green), August (red), and November (black) in the KawakamiExperimental Forest of central Japan.39) The 87Sr/86Sr ratios ofplant materials (bamboo leaf and oak bud) are almost identicalto the those of soil water. Shaded area indicates the soil column.


precipitation returns indirectly through the plantsin these forests to the atmosphere.41) The formationof microscopic particles on plant surfaces by theevapotranspiration of rain has also been found inatmospheric aerosols, indicating that these plant-derived particles can also influence the water qualityof incident precipitation, as well as throughfall.

Rain 87Sr/86Sr ratios in Japan reach maximumvalues (0.7105) that are similar to the 87Sr/86Sr ratiosof evaporitic (salinization) minerals within Asiandust (Fig. 4). In contrast, the lowest 87Sr/86Sr ratiosare associated with summer and autumn rain but arearea-dependent. For example, areas dominated byvolcanic rocks (e.g., the Kawakami site) have lowrain 87Sr/86Sr ratios and similarly low plant 87Sr/86Srratios. Other areas also record a correlation betweenthe 87Sr/86Sr ratios of summer and autumn precip-itation and plant 87Sr/86Sr ratios, with precipitation87Sr/86Sr ratios in South Korea and Japan correlatingwith plant 87Sr/86Sr ratios in these areas (Fig. 10).The possible influence of plant-derived Sr on precip-itation 87Sr/86Sr ratios has also been identified intropical forests of central Africa.42) The fact thatland surfaces in forest areas are widely covered withvegetation means that Sr isotope ratios can be usedas a traceability index to evaluate the influence ofterrestrial materials on the atmospheric environmentin these regions.

5.3. Links between Pb isotope ratios inprecipitation and ecosystems. The water qualityof precipitation in Japan varies seasonally, reflectingareal differences in source materials such as watervapor and dissolved components. In contrast to Srisotope ratios, precipitation in Japan has deuteriumexcess values (d) that are high in winter and low insummer, reflecting changes in water vapor sources.Nitrogen compounds, sulfur oxides, heavy metals,and other components are also present in higherconcentrations in winter precipitation (snowfall) andin lower concentrations in summer precipitation. Inparticular, the water quality of precipitation on theSea of Japan side of Japan differs markedly betweensummer and winter, primarily as a result of thepresence of pollutants from the Asian continent inwinter.22)

The concentration of heavy metals in theatmosphere has been increasing since the IndustrialRevolution. In particular, a rapid increase in Pbwithin the atmosphere occurred during the periodof high economic growth following Second WorldWar as a result of the use of leaded gasoline,resulting in serious environmental pollution in many

regions.10) The banning of leaded gasoline in the1970s caused a rapid decrease in atmosphericPb concentrations within industrialized nations,although heavy metals (including Pb) are still presentin ash and particulate matter emitted from sourcessuch as fossil-fuel-fired power plants and refuseincinerators.

The stable isotope ratio of atmospheric Pbdiffers greatly from country to country.43) The oresthat are the ultimate source of atmospheric Pb havestable isotope ratios that differ substantially withthe time of ore formation. Ancient Precambrian oredeposits such as those in Canada and Australia aregenerally deficient in radiogenic Pb.2),5) The use ofAustralian ores in Australia and the United Kingdommeans that atmospheric aerosols in these countriesare characterized by low 206Pb/207Pb and 208Pb/207Pb ratios. The Pb isotope ratios of precipitation inJapan also have regional and seasonal differencesthat reflect changes in the relative contributions fromdiffering sources.29) Winter rain in Japan containslarge amounts of Pb derived from China, with rainon the Sea of Japan side of Japan containing Pboriginating in China, whereas rain in northern Japancontains Pb derived from Russia and Mongolia, andrain in urban areas in the Kanto and Setouchi areascontains Pb sourced from Japan.

Although river water Sr isotope ratios stronglyreflect the underlying geology of a region, research inthe mountains of South Korea and the Tsukubaregion of central Japan indicates that river water Pbisotope ratios are identical to the isotopic composi-tion of atmospheric Pb and clearly differ from the Pbisotopic compositions of local rocks or river sediments(Fig. 11). This result is primarily due to feldsparminerals, as plagioclase feldspar is more susceptibleto chemical weathering but contains low concen-trations of Pb, and K-feldspar contains more Pb butis resistant to chemical weathering.2) This minera-logical factor means that mineral-derived Pb isunlikely to be discharged into the environment.

Plants also have Pb isotope ratios that aresimilar to those of atmospheric Pb. The fact thatheavy metal elements are strongly adsorbed onto soilparticle surfaces means that they are difficult todesorb except in acid environments, a fact that iscompounded for Pb as this element is especiallystrongly adsorbed onto soil particles.10) Soil Pbisotope ratios indicate that heavy metal elementsoriginating in the atmosphere accumulate in ex-changeable pools within the soil before being takenup into the ecosystem material cycle. The majority of


heavy metals in aerosols are ultimately derived fromthe manufactured products formed from ores. Thismeans that Pb stable isotopes are a useful monitoringtool to identify atmospheric deposition, primarily asthe source of ore metals utilized in a given nationchanges as a result of responses to socioeconomicchanges that affect the trading of ores.

6. Relationships to human society

6.1. Anthropogenic impacts on land ecosys-tems: the example of Lake Biwa. Human activitiesadd a wide variety of materials into the naturalenvironment, as exemplified by modern agriculture,which uses large amounts of chemical fertilizers andwater, meaning that the environmental impact ofagriculture is reflected in water quality and byorganisms within associated ecosystems. The waterquality in Lake Biwa has changed with the expan-sion of human activities since the 1960s. Researchbased on Sr, N, and S stable isotopes indicatesthat agricultural activity has contributed to thischange.15),16),44)

Intensive rice farming takes place on the easternplain of Lake Biwa, with water quality components ofthe rivers flowing from the mountains (particularlyCa2D, Mg2D, Sr2D, and SO4

2!) increasing in concen-tration within stream waters as they pass through

Fig. 11. Pb isotope ratios of precipitation, stream water, landplants, soil, and rocks in the Tsukuba area of central Japan.Ratios of water and plants cluster closely and differ substantiallyfrom the Pb isotopic compositions of soil and rock.56) Black solidlone indicates the range of Pb isotopic ratios for precipitation,stream water, and land plants, and brown dotted one that forsoil and rock, respectively.

Fig. 10. Frequency diagrams for 87Sr/86Sr ratios for wet precip-itation (black bars) and land plants (green bar) in threemountainous areas in Japan (Kawakami (a), Tsukuba (b), andYakushima (c)) and in the Chonju district of central southernKorea (d). SS F sea salt component, SMC F evaporitic (salini-zation) minerals in desert and loess areas of China that aresoluble in rain, BR F bedrock around the precipitation site. Astatistically significant relationship is observed between 87Sr/86Sr ratios in rain and in plants.55)


this area. Sulfur isotope ratios indicate that increasesin SO4

2! concentrations are the result of the sourcingof sulfur from agricultural fertilizers such as ammoni-um sulfate. Rice paddy areas also produce acids suchas carbonic acid, organic acids, and sulfuric acidduring the decomposition of organic matter and thedissolution of nitrogenous fertilizer, although nearbyrivers are mildly alkaline, indicating that this acidgeneration is neutralized in this area.

The concentrations of divalent ions such asCa2!, Mg2!, and Sr2! in inland waters tend toincrease downstream from river headwaters, primar-ily as the minerals containing these elements aresubjected to chemical weathering during transportwithin the river system. Increases in Ca2!, Mg2!, andSr2! within stream waters flowing through rice paddyareas are caused either by the decomposition ofsoil minerals containing these elements or by thedesorption of these elements from sediment particlesthrough the action of various acids under rice paddyareas.15),16) The eastern plain of Lake Biwa isdominated by Kobiwako Group sediments that arederived from volcanic rocks of intermediate to felsiccomposition. This means that plagioclase is mostlikely the ultimate source of Ca2! and Sr2! in thewater, whereas Mg2! is most likely derived fromminerals such as amphibole and biotite.

Tributaries of the Mississippi River in theUnited States have alkalinity and Ca2! concentra-tions that increase during flow from forest intoagricultural lands.45) This increase in alkalinityindicates that carbon dioxide (CO2) from the at-mosphere is being dissolved in the water, where it isconverted into carbonic acid and bicarbonate ions.The corresponding increase in acid (HD) is thought toaccelerate the increase in Ca2! concentrations withinthe river water. Chemical weathering on land isdriven by acids, including carbonic acid, nitric acid,sulfuric acid, and organic acids. Although humanactivity contributes to the generation of these acids,freshwater is generally mildly alkaline, suggestingthat anthropogenic acid contributions accelerate thechemical weathering of basement rocks. This in turnmeans that the analysis of geologically derived stableisotopes can add a new perspective to the dynamicanalysis of C, N, S, and other elements. Changes inthe N, S, and Sr stable isotopic composition of lakewater can be reconstructed using data obtainedfrom organisms. The analysis of first-year isaza(Gymnogobius isaza), an endemic species of gobyfish in Lake Biwa, yielded /15N values that increasedover time with coincident decreases in /34S values and

87Sr/86Sr ratios as a result of an increase in humanpopulation within the watershed.15) These compar-isons between temporal changes in lake water qualityand geographic variations in the water quality ofcontributing rivers suggest that a recent increase hasoccurred in the contribution of rivers in the easternplain to Lake Biwa.15) The accuracy of this type ofenvironmental diagnosis can be improved by the useof different samples together with stable light andheavy isotopic analysis.

6.2. Isoscapes: application to agriculturaland marine products, and to archeology. TheSr, Pb, and Nd isotope ratios of marine andagricultural products strongly reflect the isotopiccomposition of ambient water, which varies geo-graphically. This means that these isotope ratiosprovide an excellent traceability index because theyreflect the isotopic composition at a precise samplingpoint.46) Bottled mineral water derived from ground-water has a strong regionality in terms of waterquality, and Sr isotope ratios can be utilized as aforensic traceability index for the characterization ofthe production areas of bottled water and wine.47),48)

Maps of stable isotope ratios for water, agricul-tural products, and marine products make it easy toidentify their production areas. These geographicpatterns of isotopic distributions in environmentalmaterials have been called isoscapes. Such isoscapesare useful in conducting global environmental re-search and have become increasingly frequently usedin a wide variety of research.49) This is especiallyevident in Japan, where it has become increasinglyimportant to be able to distinguish between foodproduced in Japan and food produced elsewhere.50),51)

Isoscapes have also been used in archeologicalresearch, with various forms of ritual tooth extrac-tion, perhaps related to coming-of-age ceremonies,identified in human skeletal remains from the Lateand Final Jomon periods of Japan. Research into theYoshigo and Inariyama skeletal remains at a site insoutheastern Aichi Prefecture indicates that severaltypes of tooth extraction were practiced during thesetimes.52) In addition, although the 87Sr/86Sr ratios ofmarine products match seawater ratios, the 87Sr/86Srratios of terrestrial resources show regional varia-tions. Plant 87Sr/86Sr ratios correlate strongly withgeological conditions, reaching high values in thenorthern part of the granitic Ryoke Belt and valuesclose to that of seawater in the coastal zone of Japan(Fig. 12). Plants near the archaeological site have87Sr/86Sr ratios that are almost the same as seawater,indicating that teeth with 87Sr/86Sr ratios differing


from this value are most likely from immigrants,although this research did not show a clear relation-ship between tooth extraction and whether anindividual was a native or an immigrant.52)

Isotope maps of environmental substances suchas water and plants can be used to trace the origin ofthese substances as well as providing a store of basicinformation concerning the life histories of animalsand dynamic interactions between environmentalsubstances. These maps are gaining attention notonly in global environmental research but also inforensic studies aimed at identifying places of foodproduction and the local certification of agriculturaland marine products. Other implementations of suchmaps, including fingerprint maps for criminal inves-tigations and other applications, will also be benefi-cial for society.

7. Conclusions

Stable Sr, Nd, and Pb isotopic ratios show widegeographical variations that reflect changes in thegeological sources of these elements. These elementshave smaller degrees of isotope fractionation thanis the case for light stable isotopes, which enablesthe mass-dependent fractionation of Sr and Nd to beremoved by analysis of multiple isotopes of these

elements. This paper reviewed the application ofthese geologically derived stable isotopes in researchinto Earth environments, focusing mainly on the useof Sr isotopic techniques.

The 87Sr/86Sr ratios of precipitation in Japanshow seasonal and geographic variations that reflectmixing of Sr source materials, which include sea saltparticles, water-soluble minerals within Asian dust,and geological materials at the site of precipitation.Strontium and Nd isotopic analysis indicates thatinsoluble minerals in Chinese desert soils are alsopresent in glaciers in the surrounding mountains,whereas water-soluble minerals are decomposedand their elements are utilized by cyanobacteria.The 87Sr/86Sr ratios of soil and surface waters arerelatively stable over time but vary with the under-lying basement and watershed rocks. This geographicvariation is reflected in plants, agricultural products,aquatic organisms, and human bodies, indicating theusefulness of 87Sr/86Sr ratios as traceability indices.

The integrated use of stable Sr, Nd, and Pbisotopes is an effective approach that allows thetracing of interactions within and between theatmosphere, pedosphere, hydrosphere, and biosphere.Human activities supply huge amounts of carbon,nitrogen, and sulfur into the environment as a result

Fig. 12. Sr isotope ratio map of plants in southeastern Aichi Prefecture, Japan, based on data from Kusaka et al. (2009).52) Circle is thelocation of sampling site. The 87Sr/86Sr ratios were contoured by inverse distance weighting. The 87Sr/86Sr ratios of plants stronglycorrelate with the underlying geology, where granite and gneiss of the Ryoke Belt in the northern Mikawa highlands are associatedwith plants that have high 87Sr/86Sr ratios (>0.710), whereas plants in the Pliocene sediment-dominated eastern area of the Mikawahighlands and the southern Atsumi Peninsula have low 87Sr/86Sr ratios (as low as 0.7070).


of the use of fossil fuels and agricultural and fisheryproducts, generating acids in water that acceleratethe chemical weathering of geologic materials. Thisin turn indicates that combining stable light andheavy elements is a powerful approach that enablesthe development of multiple traceability indiceswithin Earth environments.

Acknowledgements

This report summarizes stable isotope researchundertaken in collaboration with numerous scientistsand students of the University of Tsukuba and theResearch Institute for Humanity and Nature, Kyoto,Japan, and I gratefully acknowledge their participa-tion in this study. This work was supported in partby a Grant-in-Aid for Scientific Research Projectsfrom the Ministry of Education, Culture, Sports,Science and Technology (18201004) and the JST–CREST project, “Development of Multi-tracer Tech-niques for Diverse Functions of Coastal Ecosystems”.

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36) Uehara, Y. (2013) Water and nutrient cycle in alpineregion at Mt. Tateyama. Ph.D. thesis of KyushuUniv. (in Japanese).

37) Chadwick, O.A., Derry, L.A., Vitousek, P.A.,Huebert, B.J. and Hedin, L.O. (1999) Changingsources of nutrients during four million years ofecosystem development. Nature 397, 491–497.

38) Veizer, J. (1989) Strontium isotopes in seawaterthrough time. Annu. Rev. Earth Planet. Sci. 415,141–167.

39) Nakano, T., Tanaka, T., Tsujimura, M. andMatsutani, J. (1993) Strontium isotopes in soil-plant-atmosphere continuum (SPAC). Tracers inHydrology 215, 73–78.

40) Böstrom, K., Böstrom, B. and Andersson, P. (1980)Natural and anthropogenic components in bulkprecipitation at Blido (Archipelago of Stockholm).Water Resour. Res. 25, 1291–1301.

41) Komatsu, H., Maita, E. and Otsuki, K. (2008) Amodel to estimate annual forest evapotranspirationin Japan from mean annual temperature. J.Hydrol. 348, 330–340.

42) Dupre, D., Négrel, P., Seimbille, F. and Allegre, C.J.(1994) 87Sr/86Sr ratio variation during a rainevent. Atmos. Environ. 28, 617–620.

43) Bollhöfer, A. and Rosman, K.J.R. (2001) Isotopicsource signatures for atmospheric lead: theNorthern Hemisphere. Geochim. Cosmochim. Acta65, 1727–1740.

44) Hosono, T., Nakano, T., Igeta, A., Tayasu, I.,Tanaka, T. and Yachi, S. (2007) Impact offertilizer on a small watershed of Lake Biwa: Useof sulfur and strontium isotopes in environmentaldiagnosis. Sci. Total Environ. 384, 342–354.

45) Raymond, P.A. and Jonathan, J.C. (2003) Increasein the export of alkalinity from North America’slargest river. Science 301, 89–91.

46) Kelly, S., Heaton, K. and Hoogewerff, J. (2005)Tracing the geographical origin of food: Theapplication of multi-element and multi-isotopeanalysis. Trends Food Sci. Technol. 16, 555–567.

47) Voerkelius, S., Lorenz, G.D., Rummel, S., Quetel,C.R., Heiss, G., Baxter, M., Brach-Papa, C.,Deters-Itzelsberger, P., Hoelzl, S., Hoogewerff, J.,Ponzevera, E., Van, B.M. and Ueckermann, H.(2010) Strontium isotopic signatures of naturalmineral waters, the reference to a simple geologicalmap and its potential for authentication of food.Food Chem. 118, 933–940.

48) Durante, C., Baschieri, C., Bertacchini, L., Cocchi,M., Sighinolfi, S., Silvestri, M. and Marchett, M.(2013) Geographical traceability based on87Sr/86Sr indicator: A first approach for PDOLambrusco wines from Modena. Food Chem.141, 2779–2787.

49) West, J.B., Bowem, G.L., Dawson, T.E. and Tu,K.P. (2010) Isoscapes: Understanding Movement,Pattern, and Process on Earth Through IsotopeMapping. New York, Springer.

50) Kawasaki, A., Oda, H. and Hirata, T. (2002)Determination of strontium isotope ratio of brownrice for estimating its provenance. Soil Sci. PlantNutr. 48, 635–640.

51) Ariyama, K., Shinozaki, M. and Kawasaki, A. (2012)Determination of the geographic origin of rice by


chemometrics with strontium and lead isotoperatios and multielement concentrations. J. Agric.Food Chem. 60, 1628–1634.

52) Kusaka, S., Ando, A., Nakano, T., Yumoto, T. andIshimaru, E. (2009) Strontium isotope analysis onthe relationship between ritual tooth ablation andmigration among the Jomon people in Japan. J.Arch. Sci. 36, 2289–2297.

53) Morohashi, S. (2004) Geochemistry of rainwater atsix sites in Japan. Master’s thesis, GraduateSchool of Life and Earth Science School, Universityof Tsukuba (in Japanese with English abstract).

54) Nakano, T. and Noda, H. (1991) Strontium isotopicequilibrium of limnetic molluscs with ambientlacustrine water in Uchinuma and Kasumigaura,

Japan. Ann. Rep. Inst. Geosci. Univ. Tsukuba. 17,52–55.

55) Nakano, T., Jeon, S.R., Shindo, J., Fumoto, T.,Okada, N. and Shimada, J. (2001) Sr isotopicsignature of plant-derived Ca in rain. Water AirSoil Pollut. 130, 769–774.

56) Iida, Y. (2004) Geochemistry of stream water in themountainous area of Tsukuba, central Japan.Master’s Thesis, Graduate School of Life andEarth Science School, University of Tsukuba (inJapanese with English abstract).

(Received Apr. 1, 2015; accepted Apr. 13, 2016)

Profile

Takanori Nakano was born in 1950 and graduated from the Tokyo University ofEducation in 1974. He was Research Associate and Assistant Professor, Institute ofGeoscience, University of Tsukuba, 1982–2003 and Professor of Research Institute forHumanity and Nature (RIHN) 2004–2015, and is Emeritus Professor of RIHN andVisiting Professor of Waseda University 2016–.


Documents

Potential uses of stable isotope ratios of Sr, Nd, and Pb