[U. S. Geological Survey]

Chapter 22

Isotopes as Indicators of Environmental Change

James B. Shanley, Elise Pendall, Carol Kendall, Lora R. Stevens, Robert L. Michel,
Patrick J. Phillips, Richard M. Forester, David L. Naftz, Beiling Liu, Libby Stern, Brent B. Wolfe,
C. Page Chamberlain, Steven W. Leavitt, Tim H.E. Heaton, Bernhard Mayer, L. DeWayne Cecil,
W. Berry Lyons, Brian G. Katz, Julio L. Betancourt, Diane M. McKnight, Joel D. Blum,
Thomas W.D. Edwards, Harold R. House, Emi Ito, Ramon O. Aravena and Joseph F. Whelan
Isotope Tracers in Catchment Hydrology (1998), C. Kendall and J. J. McDonnell (Eds.)
Elsevier Science B.V., Amsterdam. pp. 761-816.
 

22.1 Introduction
22.1.1 Direct and proxy records of environmental change
22.2 Recent Environmental Change Indicators
22.2.1 Groundwater dating
22.2.2 Direct use of water isotopes to infer recent global change
22.2.3 Changes in land use deduced from tracer studies
22.2.4 Isotope tracers for tracking migratory patterns of birds
22.2.5 Changes in atmospheric deposition
22.3 Paleo-Climatic Indicators
22.3.1 Groundwater as an archive of paleo-climatic information
22.3.2 Continental glaciers
22.3.3 Clay minerals, oxides, and hydroxides
22.3.4 Pedogenic carbonates
22.3.5 Paleoenvironmental reconstruction from stable isotopes in tree
rings and plant fossils
22.3.6 Lacustrine environments: organics
22.3.7 Lacustrine environments: authigenic carbonates
22.3.8 Lacustrine environments: ostracodes
22.4 New Research Directions
22.5 Summary

References


22.1 Introduction

In addition to providing an understanding of processes within a catchment system, isotopic techniques have been instrumental in providing reconstructions of catchment climate and other environmental indicators at various time scales. Reconstructions on longer time scales (102 to 107 yr) reveal the time frame, rate of change, and magnitude of variation in natural cycles. This framework gives us a perspective on current changes in our environment, and also provides analogs in the past (such as periods of elevated atmospheric CO2) that gives insight about how our environment may respond to anthropogenic changes. Reconstructions on shorter time scales (100 to 103 yr) provide us with a record of how our environment has changed in historic times, and enables analysis of the role of anthropogenic activity in that change.

Global environmental change has been identified as one of the most urgent issues of earth science research as we approach the turn of the millennium (USGCRP, 1992). Global environmental change refers to the collective and cumulative environmental effects of human activity, including greenhouse gas emissions, urbanization, deforestation, changes in agricultural practices, impoundment of surface waters, and decreases in biodiversity. A fundamental challenge in evaluating global environmental change is to determine natural or background long-term conditions and the natural variations in these conditions, so that the signal (anthropogenic effects) can be clearly detected over the noise (natural variability). This task is made all the more difficult by the interrelations and feedback mechanisms among physical, chemical, and biological processes in the environment. The purpose of this chapter is to bring together isotope geochemistry, hydrology, and climatology to look at new ways of applying isotopic tracing techniques to provide information on environmental change.

Isotopic analysis has become a valuable tool in global environmental change research. For example, isotopic compositions of modern precipitation are an important part of many global climate models (GCMs). However, the most powerful application of isotopes is that they provide a record of past environmental conditions (such as temperature, humidity, vegetation, and water source) that can be compared to present-day environmental conditions. Although many climatological studies using isotopes have been conducted on marine sediments or polar ice sheets, these provide only spatially-averaged trends in global climate. Current concern about the influence of human activity on global climate has prompted increased research aimed at reconstructing past climate in continental settings. Because global warming models predict that the warming will be distributed unevenly in space, it is of fundamental importance to establish past climatic trends and fluctuations in different land regions to evaluate how they responded to global climatic forcing.

The discussion in this chapter begins with "recent" environmental change and concludes with paleoclimate change. The section on recent change includes discussions of how isotopic techniques have been used to monitor changes in recharge water compositions, atmospheric deposition of nitrogen and sulfur, land use, and bird migration patterns. The section on paleoclimate change includes discussions of how climate change is assessed through analysis of isotopes in tree rings (to 160 yr), lacustrine carbonates (30 to 12,000 yr), ice cores in continental glaciers (to 2,000 yr), fossil wood and leaf cellulose (to 40,000 yr), secondary soil carbonates (10,000 to 50,000 yr), and clay minerals (to 107 yr). The role of carbon in global warming is discussed more extensively in Chapter 17.

In this chapter, we will examine how specific isotopic approaches are currently being applied in research on global environmental change. Although some of the studies presented were conducted as part of "classic" catchment research, for this chapter we broaden our consideration to include any shallow (near-surface) non-marine environment. Likewise, we include a few non-isotopic techniques (such as CFC-groundwater dating) that are commonly used in conjunction with isotopic studies.

22.1.1 Direct and proxy records of environmental change

Under some conditions, isotopic data furnish a "direct" record of environmental change. For example, the natural variations in the oxygen and hydrogen stable isotopic compositions of precipitation over continental areas reflect local climatic conditions such as storm track, temperature of precipitation, ambient humidity, air-mass source, and degree of rain-out (Chapter 3). Recharge waters derived from precipitation may closely reflect the isotopic compositions of the rain or may be significantly altered by evaporation, isotopic-exchange, and mixing. Hence, the variations in isotopic compositions of rain and recharge waters can be interpreted in terms of climate changes and are commonly viewed as direct indicators of environmental processes or climate. Other isotopes sometimes used as direct indicators include d13C and d18O of bicarbonate, d15N and d18O of nitrate, d34S and d18O of sulfate, and helium isotopes.

Often, isotopic data provide a "proxy" record of environmental change. For example, unaltered ancient waters, solutes, and gases are rarely preserved, but minerals may form in isotopic equilibrium with these waters, and plants and animals incorporate the isotopic compositions of various local waters and solutes into their tissues during growth. Under favorable conditions, paleoenvironmental information can be determined by analyzing ancient animal shells, lake or soil carbonates, plant cellulose, bone phosphate, clay minerals, and oxides in marine and continental sediments for isotopic composition; these compositions preserve an indirect or proxy record of the isotopic compositions of local waters and solutes at the time of formation.

The climatic records preserved in continental settings range from interannual to millennium-scale to geologic epoch-scale. Past climate patterns are deduced from the isotopic compositions of proxy indicators of climate by comparison of the proxy data with the present isotopic distributions of rain, groundwater, and surface water, taking into account known fractionation factors. Regional patterns in differences between the old and modern isotopic compositions reflect changes in climate or other environmental parameters. Advances in the use of proxy data will require a refined understanding of isotopic variations in response to changes in climate parameters, and how these variations are recorded by the proxy. In particular, both the development of modern-day atmospheric climate models and the reconstruction of ancient climatic and hydrologic models based on proxy isotopic records are hampered by limited information about current isotopic patterns in the modern hydrologic cycle.

The first fundamental lesson of geology is: "the present is the key to the past". Accordingly, isotopes cannot be used effectively to decipher paleoclimates without a broad understanding of modern hydrologic systems and the environmental factors that control the isotopic composition of proxy indicators. Ongoing research topics that promote this understanding include: processes affecting the isotopic compositions of meteoric water; the importance of evapotranspirational recycling to the hydrologic cycle; preservation of isotopic compositions of minerals and fossils over long time periods; calibration of low-temperature geothermometers; the relation of carbonate precipitates and lake fossils to meteoric waters; and the effect of inherent environmental variability on paleoclimatic estimates.

22.2 Recent Environmental Change Indicators

Many stable and short-lived radioactive isotopes, as well as non-isotopic tracers, have proved useful in assessing environmental change on historic timescales, from years to decades. In some cases, the tracers themselves are primarily anthropogenic contaminants, as in the case of tritium, 137Cs, and chlorofluorocarbons (CFCs). Radioactive tracers can be used to determine groundwater age, which can then be combined with groundwater chemistry to reproduce a history of the effect of land use practices on aquifer water quality. In another application, characteristic dD/d18O produced by evaporation from seepage lakes (lakes with no surface water outlet) can be used to trace lakewater movement through aquifers; a record of interannual variations in lakewater dD and d18O produced by interannual climate variations is thus preserved along groundwater flow paths (see Chapter 14). Cosmogenic isotopes with short half-lives are briefly introduced here as well; cosmogenic isotopes with long half lives are discussed in Chapters 19 and 20. Groundwater dating is discussed in detail in Chapter 8. We begin with a discussion of how isotopic techniques have been applied in shallow aquifer studies to assess recent environmental change.

22.2.1 Groundwater dating

Groundwater may hold a record of many types of environmental data. Under the right con-ditions (relatively homogeneous aquifer, unreactive aquifer sediments, low hydraulic gradient), water will move slowly, retain its chemical and isotopic characteristics at time of recharge, and recharge in discrete annual packets that can be dated. This ability of groundwater to serve as an archive of water quality, coupled with the possibility to determine the date of recharge by isotopic or other means, constitutes a powerful tool to investigate time trends in water quality and relate them to changes in atmospheric deposition, land use, or land management practices.

A series of tracers can be used to estimate the age of groundwater. The range of ages obtained can vary from a few days for tracers such as 35S and 7Be (Cooper et al., 1991) to hundreds of thousands of years for 36Cl. Some of these tracers (e.g., 35S, 7Be) are produced by cosmic-ray spallation, and can be considered steady-state tracers (i.e., the concentration in any given reservoir should not change with time). Others are introduced exclusively or primarily by anthropogenic processes (e.g., chlorofluorocarbons {CFCs}, 85Kr). These tracers are generally restricted to studying processes occurring on decadal timescales. Another set of tracers is produced by both processes and these are among the most commonly used tracers (e.g., 3H, 14C, 36Cl). When used in conjunction with data on other constituents, these tracers provide estimates on a wide range of age scales. Using the dates obtained from these tracers, information can be obtained on environmental changes occurring within the atmosphere and hydrosphere.

Tritium is probably the most commonly used isotope for groundwater dating because of its ease of collection and analysis. It can be considered a conservative tracer for most hydrologic studies. Tritium has a natural abundance that was overwhelmed by anthropogenic production during nuclear weapons testing of the 1950's and 1960's. Nuclear power plants are an important source of tritium only in localized studies. Due to the nature of its production, tritium has a very complicated input function, and is also subject to major geographic variations. However, a large number of measurements of tritium in precipitation are available that can be used to obtain input functions (IAEA, 1981; Michel, 1989). Concurrent measurements of tritium and its daughter product, 3He, can overcome complications of the input function.

Michel et al. (1994) used tritium to establish ages of waters sampled in an aquifer under Golden Gate Park in San Francisco, California. Ages were assigned to water samples from the aquifer to establish time trends in the quality of waters recharging the aquifer. During the past 40 years, there appeared to be an increase in sulfate, sodium, and chloride, whereas no trend was evident for nitrate. Egboka et al. (1983) used tritium to follow a contaminant plume and obtain dispersion rates at a landfill in Borden, Ontario. Section 22.2.3 has a more detailed discussion on the use of transient tracers to study changes in groundwater quality with time.

Chlorofluorocarbons (CFCs) have been released to the atmosphere through industrial processes since the 1940's (see Chapter 9). They have the advantage of having a global source function that is not subject to geographic effects, and an increase of concentration in the atmosphere that is reasonably well known (Elkins et al., 1993). Complications with the dating technique result from the need to know the temperature of the water when it last equilibrated with the atmosphere, and the possibilities of adsorption and microbial degradation of CFCs in the subsurface. Krypton-85 also has a global source function but is not chemically reactive in the subsurface. Because 85Kr is measured relative to stable krypton, the temperature of equilibration is unimportant. The main drawbacks to the use of 85Kr are that a large volume sample is usually needed, and analysis is difficult.

Chlorine-36 has both a bomb-produced component and a cosmogenically produced component; it can be used to study the movement of chloride and water over a large range of timescales. The recent source function is well known (Phillips et al., 1988), but variations in the production rate over hundreds of thousands of years are possible. Its use is also hampered by the ubiquitous presence of chloride in the environment. Likewise, 14C is primarily used for longer-term studies. Both 36Cl and 14C are discussed in Section 22.3.1.

22.2.2 Direct use of water isotopes to infer recent global change

This section examines two case studies, one in a polar and one in a subtropical setting, in which dD and d18O of water are used directly to deduce recent environmental change. In the first study, d18O and dD measurements in inlet streams to a polar lake were used to quantify the water budget of the lake, and, specifically, to determine changes in lake ice thickness, which reflect recent climate change. Lake Fryxell, a permanently ice-covered lake in Southern Victoria Land, Antarctica, is fed by runoff from more than three glaciers in a watershed area of 230 km2 (Lawrence and Hendy, 1985). The glacial ice, in general, has lighter isotopic compositions than the streamwaters. The d18O of glacial ice ranges from -28.4 to -33, whereas the weighted average d18O of the surface water inflows is -27.3 (Matsubaya et al., 1979; Stuvier et al., 1981). This enrichment indicates that significant evaporation occurs as the glacier ice melts and flows in surface streams.

The isotopic balance of lakes using a standard mass-balance approach has been accomplished by numerous authors using the initial equation of Gonfiantini (1965). However, because of the lack of surface water outflow, groundwater inflow or outflow, and evaporative loss due to the permanent ice cover, the isotopic mass-balance equation for Lake Fryxell can be simplified greatly. At steady-state conditions, using a mean d18O of lake ice (-25.2) from January 1992, the ice accumulation was calculated as 34 cm/yr. However, direct ice measurements at Lake Hoare, nearby in the same valley, indicate an accumulation rate of 110 cm/yr (Wharton et al., 1992). The wide discrepancy in the calculated and measured rates of lake ice accumulation is undoubtedly due to the fact that these lakes are not in steady-state with respect to volume change. Chinn (1993) and Wharton et al. (1993), also through direct measurements, have demonstrated that over the past few decades lake ice thicknesses have decreased and lake volumes have increased. Thus, the direct use of dD and d18O, combined with the determination of water fluxes, has made it possible to document thinning of the ice, and thus infer at least a short-term climatic warming.

The second study took place in the humid subtropical climate of northern Florida, where groundwater flow is vertically downward, a feature that preserves a record of the age and characteristics of recharge in chemical and isotopic signatures of groundwater with depth. The Lake Barco basin has a MAT of 21oC and a median rainfall of 128 cm (Owenby and Ezell, 1992). Close agreement among the mean annual air temperature, measured groundwater temperatures, and the calculated recharge temperature from N2 and Ar data indicates that recharge most likely occurs throughout the year (Katz, 1993). Thus, the isotopic composition of groundwater is an indicator of mean annual temperature fluctuations. By combining the use of stable isotopes (to determine water source and recharge conditions) and CFCs (to determine groundwater age, as discussed in Section 22.2.3), valuable information was obtained on sources and rates of recharge, and changes in weather patterns over the past 40 years.

Groundwater isotopic composition upgradient from Lake Barco resembles rainfall and plots along the global meteoric water line (Craig, 1961), indicating rapid recharge of meteoric water with little or no evapotranspiration (Katz et al., 1995a,b). Groundwater that was recharged in a given year has an isotopic composition that reflects the temperature of recharge and other climate variables. For example, groundwater at shallow depth had the most depleted dD (-24.5) and d18O (-4.40). This groundwater was recharged in 1989 (CFC-dated) when the mean annual air temperature was 19.3oC. In contrast, deeper groundwater was more enriched in dD (-20.0) and d18O (-3.95), and was recharged during 1978, when the mean annual air temperature was 20.9oC. Thus, in this system it appears that annual temperature variations are preserved in the groundwater isotopic profile; this record coupled with CFC dating to pinpoint the year of recharge allows one to reconstruct recent climatic variations.

22.2.3 Changes in land use deduced from tracer studies

Groundwater dating can be a useful tool for understanding the effect of land-use changes on groundwater quality in surficial aquifers (Dunkle et al., 1993). In the late 1980's, concerns about nitrate pollution in the Chesapeake Bay from agricultural activity led to a study of groundwater quality as a function of recharge date on the Delmarva Peninsula. The study used tritium-3He, 85Kr, and CFC dating techniques (Dunkel et al., 1993; Ekwurzel et al., 1994). Groundwater ages calculated from all three methods agreed to within about ± 2 years for most samples (Ekwurzel et al., 1994). The study was conducted at Locust Grove, a small (< 5 km2) catchment. The Locust Grove surficial aquifer has 18-24 m of permeable sands underlain by very low permeability silt-clay sediments. The depth to water ranges from 0 to 6 m. Most of the catchment is in agricultural land use, including pasture, corn, soybeans, and ornamental trees and bushes. Only five percent of the watershed is covered by residential or forested lands, the latter largely restricted to a narrow strip along the stream channel. The proportion of land in agricultural use has changed little since the early 1950's.

The aquifer is well-suited to the application of CFC age dating techniques (Plummer et al., 1993). Groundwater is aerobic, suggesting that microbial degradation of CFCs should not occur. Because the water table is shallow there should be no significant lag in the transport of CFCs. Contamination of CFCs by local sources is not likely given the rural setting (Dunkle et al., 1993). CFC ages were determined from 15 wells along a 2.5 km groundwater flow-path transect from a drainage divide to Chesterville Branch, the first-order stream draining the catchment. At all but two sites, samples were collected at different depths in the surficial aquifer via nested wells. All samples were analyzed for CFCs and a wide range of other chemical constituents, including major ions, nutrients, dissolved oxygen and other dissolved gases, and the isotopes 3H, D, and 18O.

Wells from the transect at Chesterfield Branch were sampled in late 1990 and early 1991. CFC-modeled recharge dates, which have an uncertainty of ± 2 - 3 yr (Reilly et al., 1994), ranged from 1985 to 1987 in samples from the shallowest wells, and from 1949 to 1967 in samples from wells finished near the base of the surficial aquifer. The oldest sample was collected from a deep well in the groundwater discharge area near the stream, where the groundwater flow paths converge. Hydraulic head measurements indicate that groundwater flow is predominantly horizontal, with upward flow important only within a few meters of the stream. A plot of nitrate concentrations as a function of CFC-modeled age indicates that nitrate concentrations in the surficial aquifer at Locust Grove have increased significantly since 1972 (Figure 22.1). Samples younger than 1972 generally have a median nitrate concentration of 13 mg/L, specific conductance > 100 µS/cm, and greater than 70% of the cations as calcium plus magnesium. In contrast, samples older than 1972 have nitrate concentrations between 3 and 5 mg/L, specific conductance less than 100 µS/cm, and lower proportions of cations as calcium plus magnesium. The dominance of calcium-magnesium-nitrate in waters recharged since the mid-1970's is directly linked to increased fertilizer use at Locust Grove during this period (Denver, 1989).

Denitrification can be ruled out as the cause of the differences in nitrate concentrations between the shallow, younger waters and deep, older waters for 3 reasons: 1) In general, both the young and old groundwaters are nearly fully oxygenated, 2) none of the samples from the deep part of the flow system have excess nitrogen gas which would be present from denitrification, and 3) nitrogen isotope data gave no indication of denitrification (Böhlke and Denver, 1995).

It will take many years for improved nutrient management practices at Locust Grove to improve the water quality in the surficial aquifer. The CFC-modeled ages indicate that it can take from 20 to more than 50 years for water to travel from the shallow part of the surficial aquifer to the deep, downgradient parts of the surficial aquifer (Dunkle et al., 1993; Reilly et al., 1994). Thus, even if all nitrate applications ceased immediately, downgradient parts of the surficial aquifer would likely see increases in nitrate concentrations over a 10 to 20 year period, and substantial portions of the surficial aquifer would not experience declines in nitrate concentrations for decades. Studies at the Fairmount watershed in southern Delaware (Dunkle et al., 1993; Denver and Sandstrom, 1991) found traces of herbicide residues in groundwater with a CFC-modeled recharge date of 1963. These studies indicate that CFC dating combined with isotopic techniques can be very useful in understanding the persistence of water quality problems over time in shallow surficial aquifer settings.
 

22.3 Paleo-Climatic Indicators

Thus far we have considered how isotopes can be used to understand primarily current and recent environmental change. We now turn to isotopic techniques that are useful in understanding climate change and environmental change on timescales from centuries to tens of millennia. Long timescale techniques are fundamental to reconstructing past climate and documenting its natural variability. The long-term perspective provides the necessary framework within which anthropogenic induced perturbations can be assessed.
 

22.3.1 Groundwater as an archive of paleo-climatic information

Long-term changes in climate can be detected using isotopic information archived in hydrogeologic environments. Patterns of variation in stable or radiogenic isotopes have been combined with one or more geochemical dating tools to determine both the age or residence time of subsurface water, and the climatic conditions at the time that groundwater was isolated from the atmosphere. Groundwater composition may be affected by the dynamic movement and mixing of water, interactions with surface water, ion exchange, dissolution and precipitation of minerals, and anthropogenic perturbations. Isotopic characterization coupled with radiometric dating is a useful way to document the chronology and interpret the effects of these mechanisms on a groundwater system.

Radioisotopic dating techniques require that decay schemes have half-lives in the same range as the age of the system or material to be dated. Ideally, initial isotopic abundances or ratios should be well known and the system or material to be dated should have been geochemically isolated from the atmosphere at the time of formation until the time of sampling. Groundwater eventually attains geochemical equilibrium with aquifer sediments, thus limiting additional dissolution or precipitation of mineral phases that could make isotopic dating techniques impractical. Variations in initial isotopic concentrations or ratios are sometimes problematic. Plots of disequilibrium with distance along a flow path may be used to extrapolate back to initial compositions (Durrance, 1986). The documentation of all inputs for a given target isotope is essential to differentiating natural variability from anthropogenic influences (Andrews and Fontes, 1992).

For groundwaters in the age range from hundreds to tens of thousands of years, isotopic dating tools include 36Cl, 14C, and uranium-series nuclides. Possible sources of 36Cl include: (1) cosmic-ray spallation of argon and stable 35Cl in the atmosphere, (2) cosmic-ray irradiation of stable 35Cl in the shallow unsaturated zone, (3) solution of matrix chloride in environments where in-situ production of 36Cl is significant, (4) neutron activation of stable 35Cl in the oceans as a result of nuclear-weapons tests in the 1950's-60's, and (5) anthropogenic inputs from nuclear-fuel reprocessing and nuclear-power generation facilities (Cecil et al., 1992). Possible sources of 14C include: (1) neutron interactions, (2) the decay of certain radium isotopes, (3) alpha particle reactions, and (4) anthropogenic sources. For uranium-series dating methods, disequilibrium between uranium and the longer-lived members of the decay series is used for geochronology of materials with ages less than 1 million years (Ivanovich, 1982). Following precipitation of uranium-bearing materials in an aquifer, a considerable amount of time is needed to attain equilibrium among all members of the decay series. The greater the degree of disequilibrium among members of a decay series, the younger the material.

Carbon-14, with its accurately-known atmospheric source function (Suess and Linick, 1990) and 5730-yr half-life, is ideal for studying processes in aquifers where recharge occurs on long timescales (hundreds to tens of thousands of years) and thus may have implications for climatic change. Accordingly, it can be used to study changes in isotopic and chemical characteristics of recharge during the last glacial period. Despite its well-measured atmospheric source function, the 14C concentration of the CO2 that equilibrates with water that recharges aquifers is less well known. The initial bicarbonate in most groundwater originates from soil gas, whose 14C can be different from that of atmospheric carbon dioxide, due to decomposition of organic matter and interaction with soil matrix material (Plummer et al., 1991). Furthermore, several reactions can occur along the flow path of the groundwater that will significantly alter the 14C concentration of the bicarbonate, and these reactions must be taken into account when calculating the age of the water (Wigley et al., 1978). Plummer et al. (1990) successfully circumvented these problems to obtain accurate ages and flow rates in the large regional Madison Aquifer in the central U.S.A. With groundwater ages established, they related variations in d18O and dD of groundwater to specific past climatic events.

The d18O of calcite has provided an uninterrupted 500,000-year climate record at an open fault zone adjacent to a major groundwater discharge area at Devils Hole in south-central Nevada, USA (Winograd et al., 1992). Variations in d18O were documented for a 36-centimeter-long core of vein calcite collected about 30 m below the water table. A 500,000-year paleotemperature record was established by mass-spectrometric measurements of uranium-series isotopes and disequilibrium dating with subsequent correlation of seasonal shifts in the d18O values (Figure 22.4). It was determined that d18O variations measured on the calcite were a reflection of seasonal variations in atmospheric precipitation falling on the recharge areas for the Devils Hole groundwater system. The subsequent interpretation of seasonal variation associated with the geochronology established by uranium-series dating provided minimum ages for climatic events because these ages reflect the time that calcite precipitated from solution and not the time when recharge fell as precipitation 80 to >160 km away from the study site. This study established a radiometrically well-dated paleoclimate record spanning several glacial cycles. The record was used to determine the timing and duration of major climate shifts and is consistent with a paleotemperature record from Vostok, Antarctica for ice-core deuterium (Lorius et al., 1985) and the record of Northern Hemisphere ice volume deduced from d18O of planktonic foraminifera (Imbrie et al., 1989).

Another recent study documented the glacial chronology of the northern Yellowstone Park area using isotopic ratios of C, O, Sr, and U with uranium-series age dates (Sturchio et al., 1992). The isotopic ratios were determined in travertine to establish diagenesis including deposition temperatures and isotope ratios of paleowaters and their solutes. These data were then utilized to make inferences on the evolution of the underlying hydrothermal system and effects of glaciation. Travertine between 15,000 and 50,000 years of age had 234U/238U ratios close to the crustal abundance value of 1.00. Travertine deposits outside this age window, both younger and older, had 234U/238U values generally in the range of present thermal waters, from 1.5 to greater than 3. Sturchio et al. (1992) concluded that the smaller ratios reflected increased dissolution of carbonate aquifer rock in response to increased hydrostatic pressure at depth during full glacial conditions during the Pinedale glaciation 30,000 to 40,000 years before the present.

22.3.2 Continental glaciers

Long-term records of atmospheric deposition preserved in glaciers and ice sheets are useful in recording changes in deposition chemistry and climate (Lorius et al., 1988; Thompson et al., 1988a). Continuous ice cores from the ice sheets of Greenland and Antarctica have provided important, long-term climatic information (Wagenbach, 1989). In the past, ice cores from glaciers in temperate and tropical latitudes have not been used in climate reconstruction because it was believed that meltwater would alter the isotopic composition of deposition preserved in the annual ice layers (Wagenbach, 1989). Árnason (1981) suggested that under favorable conditions, temperate glaciers may be useful in providing records of global change during the past 2,000 years. High altitude sites on glaciers and ice caps from nonpolar locations in Kenya, Peru, China, Canada, Switzerland, and the USA have been sampled for paleoclimate information. Thompson and Hastenrath (1981) and Thompson (1981) found a distinct smoothing of the d18O profiles with depth in two shallow ice cores collected from Lewis Glacier in Kenya. They concluded this smoothing was due to meltwater infiltration.

A 1,500-year climatic record was reconstructed using d18O values, specific conductance, and microparticle concentrations in ice cores from the Quelccaya Ice Cap in Peru (Thompson et al., 1984, 1986, 1988a). Thompson et al. (1988b, 1989) utilized d18O values in ice cores from the Dunde Ice Cap in China to provide long-term and high resolution climate records from the northeastern section of the Tibetan Highlands. A 103-meter snow/ice core from Mount Logan, Canada has provided a 300-year climatic record (Holdsworth and Peak, 1985). Ice cores collected from the Colle Gnifetti site in Switzerland indicate a good correlation of the smoothed d18O values to summer air temperature at a nearby weather station.

The possible existence of historical records of climate in ice cores from glaciers in the continental USA has been evaluated only recently (Naftz, 1993; Naftz et al., 1993, 1996). In 1991, a 160-meter ice core to the bedrock underlying Upper Fremont Glacier in northwestern Wyoming was collected for paleoclimate reconstruction (Naftz and Miller, 1992). The d18O profile (Figure 22.5) was determined from 760 samples equally spaced along the length of the core. From 101.8 m to 150 m, the mean d18O value shifted abruptly to -19.85, 0.95 lighter than the mean core value of -18.90. Using an age-to-depth relation developed from 3H and 14C data (Naftz et al., 1996), this isotopically lighter section of the core corresponds to the approximate time interval of the end of the Little Ice Age (LIA), from the mid-1700's to mid-1800's A.D. (Thompson, 1992). Thus, a low resolution, stable isotope record of climate has been preserved at the Wyoming site.

Closer inspection of the isotopic profile from the core has provided additional paleoclimatic information. The 101.8 to 150-m core interval (Figure 22.5) is characterized by numerous high-amplitude oscillations in d18O values. Without seasonal dust layers to guide sample selection, the 20-cm composite samples in this section of the core could not be consistently attributed to either 100 percent winter or 100 percent summer precipitation. These large oscillations thus may reflect increased seasonality or better preservation of the annual signal as a result of the cooler summer temperatures during the LIA. For example, the isotopically enriched horizons in this interval could have resulted from decreased rates of melting of isotopically enriched summer snowfall expected from cooler summer temperatures during the LIA. In contrast, during the 1990 and 1991 field seasons, summer snowfall at the site melted within 4 days of deposition. The abrupt decrease in the large amplitude oscillations above the 101.8-m depth (Figure 22.5) indicates a sudden termination of the LIA at this site. Selected tree cores collected 2 km from the drill site consistently showed a sustained period of reduced radial growth beginning about A.D. 1790 and continuing until about A.D. 1840 (Naftz et al., 1996). This decrease in radial growth probably reflects cooler summer temperatures that reduced the growing season at these high-altitude sites.

The isotopic response to climate change at Upper Fremont Glacier appears to be linked to that at the Quelccaya Ice Cap in the Peruvian Andes (14 S, 71 W) (Figure 22.5). Although the age-dating resolution of the Upper Fremont Glacier ice-core record is lower than that from the South American site, three distinct climate-related features seem to be preserved in both sets of records. (1) d18O shifts abruptly to more negative values within core segments that were deposited during the LIA. Relative to whole-core d18O averages, the Quelccaya Ice Cap core shows about a -0.7 shift during A.D. 1600 to 1800 (Thompson, 1992), compared to the -0.95 shift in the Upper Fremont Glacier core representing snow deposited during the end of the LIA (Figure 22.5). (2) The small-scale variation in the d18O signal increases during the LIA (Figure 22.5). In the two Quelccaya ice-core records the average annual range of d18O during the LIA (A.D. 1520 to 1880) was twice the average annual range observed after the termination of the LIA (A.D. 1880 to 1980) (Thompson, 1992). Numerous high-amplitude oscillations in d18O values during the LIA were identified in the Upper Fremont Glacier ice-core record (Figure 22.5). (3) Both cores show an abrupt shift from the high-amplitude isotopic variations during the LIA to much lower amplitude isotopic variations characteristic of post-LIA ice. This transition occurs abruptly (probably in about 2 - 3 years) in all three cores. Thus, the linkages of the Upper Fremont Glacier isotopic record to the established paleoclimate record in the Quelccaya ice cores appear to support further the first documentation of the LIA in an ice-core record from a temperate glacier in south-central North America.
 

22.5 Summary

Global environmental change - encompassing a diverse array of issues such as climate change, changes in atmospheric chemistry, increases in erosion rates, land-use shifts, and decreases in biodiversity - has a sweeping and often adverse impact on our environment. Many recent changes are a direct consequence of anthropogenic activities. Isotopic analysis serves as a valuable tool for distinguishing between natural variations in long-term climatic patterns and anthropogenic effects, yielding improved understanding of natural feedback mechanisms and the development of realistic remediation strategies. Although this multi-authored chapter is by no means comprehensive, it is intended to provide a flavor for how isotopic indicators are being applied in investigations of environmental change in continental settings.

Keeping in mind the adage "the present is the key to the past", the chapter begins with examples of isotopic techniques that have been applied to understanding several types of ongoing and recent environmental changes, and finishes with examples of isotopic techniques applied in paleo-environmental studies. Studies of recent environmental change have the advantage that direct records of environmental conditions are obtained from samples of well-constrained age (e.g., CFC-dated groundwater, tree cellulose, etc.), and that isotopic results often can be supported by historic information (e.g., weather records, land use histories). While some paleoenvironmental studies obtain direct records (glacial ice cores, ancient groundwater), most rely on ancient material, such as soil and lacustrine carbonates, and fossil plants, which preserve an indirect or "proxy" record of the isotopic compositions of local waters (d18O, dD) and solutes (often d13C) at the time of formation. The material is dated independently, most often by isotopic techniques as well. Under favorable conditions, long time series of climatic or hydrologic conditions can be reconstructed, thus providing a framework within which recent environmental change can be evaluated. However, the development of modern-day atmospheric climate models and the reconstruction of ancient climatic and hydrologic models based on proxy isotopic records is severely hampered by limited information about current isotopic patterns in the modern hydrologic cycle. This book is an attempt to help remedy this situation.


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