Climatic changes in central and eastern Canada inferred from deep borehole temperature data

Palaeogeography, Palaeoclimatology, Palaeoecology (Global and Planetary Change Section), 98 (1992) 129-141

129

Elsevier Science Publishers B.V., Amsterdam

Climatic changes in central and eastern Canada inferred from deep borehole temperature data Kelin Wang a, Trevor J. Lewis a and Alan M. Jessop b " Pacific Geoscience Center, Geological Suruey of Canada, Sidney, B.C. V8L 4B2, Canada h Institute of Sedimentary and Petroleum Geology, Geological Survey of Canada, Calgary, Alta. T2L 2,4 7, Canada (Received February 25, 1992; revised and accepted June 27, 1992)

ABSTRACT Wang, K., Lewis, T.J. and Jessop, A.M., 1992. Climatic changes in central and eastern Canada inferred from deep borehole temperature data. Palaeogeogr., Palaeoclimatol., Palaeoecol. (Global Planet. Change Sect.), 98: 129-141. We analyzed data from 23 boreholes at 19 sites in central and eastern Canada, for the purpose of estimating ground surface temperature (GST) histories. These boreholes were logged down to at least 550 m depth with thermistor probes. Thermal conductivity measurements had been previously made at small depth intervals for the entire depth ranges of most of the boreholes. The temperature profiles of these boreholes do not indicate water disturbance. We estimated terrain effects for each borehole using a time dependent solid-angle method. The thermal perturbations caused by lakes or deforestation near the borehole sites are insignificant in most cases. However, four of the holes were found to be severely influenced by terrain effects. GSTs estimated from the borehole data less influenced by the terrain effects form two groups. The first group, which are generally from data of better quality, show a cold period near the end of the last century before the recent warming trend; the second show it 80-100 years earlier. We consider the former typical of the climate of the Boreal climatic region of Canada. The difference between the two groups may reflect the spacial variability of the climate. Four GST estimates do not belong to either type, and the reasons are discussed.

Introduction

Ground surface temperature (GST) is controlled by surface air temperature (SAT) and therefore is an indicator of climatic change. A temperature-depth (T-z) profile of 1 km extent measured at present records the GST history of the past several hundred years. Inversion of the T-z data from carefully selected boreholes thus provides information on climatic changes that occurred before meterological SAT records were available. Compared to proxy methods such as isotope (Epstein and Krishnamurthy, 1990) and tree-ring studies (Fritts, 1981), inferring GST from borehole temperatures involves simple physics

Correspondence to." K. Wang, Pacific Geoscience Center, Geological Survey of Canada, Sidney, B.C. V8L 4B2, Canada.

(1-D heat conduction) and hence few assumptions. Since the temperature history is directly determined from temperature data, the results are free of uncertainties that arise in converting the proxies into temperatures. The estimated temperature history, however, is necessarily a smoothed version of the real GST because of the nature of heat diffusion, and the further back we look, the less detail we can resolve (Clow, this issue). This is in an interesting contrast to the method of tree-ring chronology, in which the high frequency (year to year) temperature variations are well resolved, but the low frequency (century scale) components are less well determined because some of them have to be removed to account for the changes in tree growth pattern not caused by climatic changes. Estimation of GSTs from deep (600 m) single boreholes was first made by Beck and Judge

0921-8181/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved

130

(1969). Lachenbruch and Marshall (1986) analyzed the curvature of the upper section of T-z profiles from many boreholes in the Alaskan Arctic, and addressed the issue of global warming. Beltrami and Mareschal (1991) made the first attempt to use borehole data to constrain past climatic changes in a large area (eastern Canada), when they confirmed that a warming of 1-2 K occurred in most regions of this area during the past 100 years. A summary of the previous works in this field up to 1991 can be found in Wang (1992). North America is geographically important for the study of climate trends in the northern hemisphere. Most regions of Canada have relatively uniform climate patterns and, being at middle and higher latitudes, are sensitive to climatic changes (e.g., Hansen and Lebedeff, 1987). The first systematic instrumental SAT observations in Canada were taken in 1768 at the southwest shore of Hudson Bay, but these early measurements contain various uncertainties (Fall and Kingsley, 1984). It was not until 1839 that the first observatory in North America was established in Toronto and high accuracy continuous SAT observations maintained (Hare and Thomas, 1974). In other parts of Canada, regular meteorological SAT measurements started around 1870 and, as summarized by Longley (1953), provide the SAT history for over one hundred years. There has been little study of proxies that give information on the climatic changes in central and eastern Canada before 1870 (Fritts and Lough, 1985). There are many deep boreholes in this vast area that have been measured in the past three decades for geothermal studies, often with thermal conductivity measured on densely sampled cores. Such a combination provides an excellent data set for inferring past climatic changes. Some of these holes in the Canadian Shield have been studied in the past specifically for this purpose (Cermak, 1971; Nielsen and Beck, 1989; Wang, 1992; Wang and Lewis, 1992). In this study, we selected boreholes at 19 sites in central and eastern Canada, and estimated the GST history for these sites using a spectrum inverse method (Wang, 1992). Some of the boreholes we selected have also been studied independently using a

K. WANG E T AL.

different approach by Beltrami et al. (1992, this issue). It is well known that terrain effects such as those due to topography, vegetation, ground water flow, and the presence of nearby water bodies, may produce effects to obscure the anomalies caused by climatic changes (Chisholm and Chapman, 1992; Lewis and Wang, this issue). We have conducted site-specific studies of the local environment for each borehole site, in order to correct for these effects, or at least provide qualitative assessment of the reliability of the results and possible error sources. A brief description of each site is given in the appendix (fortunately, two of us, L. and J., were involved in the logging of most of the boreholes). We strongly recommend the reader to study these descriptions when citing the results. We do not discuss in this paper the difference and correlation between GST and surface air temperatures. Chisholm and Chapman (1992) compared borehole data with short-term SAT records obtained at nearby meterological stations and found good correlations between the two data sets in a small region. Mareschal and Beltrami (1992) and Lewis and Wang (this issue) showed that snow cover caused the average GST to be a few degrees higher than the average surface air temperature. In the following, we first describe the selection of the boreholes, data aquisition, and site studies, then briefly summarize the inverse method, and finally report the results. Selection of boreholes and site studies

The Canadian Geothermal Data Base, with temperature logs from more than 1000 boreholes, is the largest Canadian borehole temperature data set. Initially, the boreholes were selected based on the following criteria: (1) boreholes logged down to at least 550 m depth; (2) temperatures measured with thermal probes that have an inaccuracy < 0.02 K; (3) thermal conductivities determined at small depth intervals, preferably less than 15 m, for a substantial section of the borehole. Boreholes known to suffer from exceptionally large terrain effects, such as a hole in Yellowknife (Lewis and Wang, this issue) were also

131

CLIMATIC C HA NG E S IN C E N T R A L AND EASTERN CANADA FROM D E E P B O R E H O L E T E M P E R A T U R E DATA

discarded at this stage. Less than 40 boreholes passed the screening. Then, the T-z profiles of all of these boreholes were plotted for the identification of water disturbance. Groundwater flow along an aquifer crossing a borehole or along the borehole itself usually leaves very typical signatures on the T - z profile (Drury et al., 1984). Boreholes with such signatures were discarded, because even small water flow may severely distort a climatic signal; an example is given in Lewis and Wang (this issue). Twenty-three boreholes at nineteen sites illustrated in Fig. 1 were selected at this stage. Although caution was excercised to exclude hydrologically disturbed boreholes, disturbances that did not cause thermal signatures discernable on a T - z plot could not be identified. For this reason, some of the boreholes at the remaining nineteen sites might still have been very slightly affected by water flow.

At sites 13, 15, 16 and 53, the boreholes were logged twice, with separations of 16-22 yr between the two logs. In these cases, the two T - z profiles were inverted simultaneously, giving a slightly better resolution in the GSTs. The thermal conductivities of core samples were measured over a period of many years on different, calibrated divided bars. The samples were soaked under vaccuum before measurement, and measurement of individual disks to an accuracy of 5% was expected. The local environment of the borehole sites must be studied, in order to understand the temperature anomalies not caused by climatic changes. A brief description of the geology a n d / o r local conditions of each site is given in the appendix. With boreholes affected by water flow excluded, the terrain effects worth investigating include topography, vegetation, and the

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132

presence of nearby water bodies. The effect of topography in most cases is insignificant. Site 14, Jacknife, has a topographic effect larger than most other sites, but inversion of the T - z data with and without topographic correction using a solid angle method yielded nearly identical GST estimates. This is expected, because over lateral distances comparable to topography scales (1-5 kin) that would influence GST estimation the most, the variation of average surface temperature is usually very small. Many of the boreholes are near shallow lakes. The contrast between the average bottom water temperature and average ground surface temperature may produce large signals in the T - z profiles from most boreholes. We have calculated such effects for each affected borehole using a time dependent solid angle method (Birch, 1950; Lewis and Wang, this issue), with the lake size and locations obtained from the latest version of 1:50,000 topographic maps. Most of the lakes and drainage systems will have been changed during the last glacial period, so we assumed the lakes to be in their present location since approximately 10 kyr ago. These calculations are considered first order estimates because there is no or very little information on the exact temperature difference between bottom waters and land ground. Also for this reason, we considered it unnecessary to make an effort to include the spatial variation of conductivities in such calculations. A unit temperature difference was used for the calculation that allows the effects to be multiplied by an estimated factor. The annually average bottom water temperatures of lakes in Ontario studied by Allis and Garland (1976, 1979) are in the range of 4.5-5.0°C, similar to the annually average surface temperature of the surrounding land. One can argue that these lakes may be representative for these latitudes in eastern and central Canada, and the effects of shallow lakes may be negligible. However, we are not able to investigate further such effects quantitatively at present. We give a rough estimate of the perturbation in the appendix. A few boreholes are located near the boundary between forested areas and cleared fields. The perturbation to the subsurface temperature

K. WANG ET AL.

by the different average surface temperatures is calculated in exactly the same way as for the lake effect. The temperature contrast in this situation ranges from nearly 1 K to 3 K (Bentkowski and Lewis, 1992; Lewis and Wang, this issue). The year when the clearing took place is an important parameter, but remains uncertain. We take 100 yr as a reasonable guess. One noticeable case is site 13, Hearst, where the apparent large warming, also observed by Cermak (1971) and Nielsen and Beck (1989), is partially due to the clearing of forest. Method of inversion

A spectrum inverse method (Wang, 1992), was used to invert the data described above. The physical model used here is a layered rock medium, in which each layer has a uniform thermal conductivity and diffusivity. The forward solution for the transient temperature distribution in the medium is obtained using a discrete Fourier transform, which establishes an analytical relation between the Fourier components of the GST and the subsurface temperatures (Ciauser, 1984; Lindqvist, 1984). Given T-z profiles, the amplitude and phase spectra of the unknown GST are estimated from this relation. Estimation of GST is made by applying the generalized least squares theory of Tarantola and Valette (1982). In the formulation of the inverse problem, constraints to the GST to be estimated are introduced in the form of a priori Gaussian probabilities. The same concept is used to describe uncertainties in the T - z data. Proper use of a priori information ensures the uniqueness and stability of the inverse solution. In our work, the a priori information on the GST, i.e., its being bounded and smooth, is described with a Hamming autocovariance function as described in Wang (1992). In the frequency domain, this function specifies the variance of all Fourier components. There are two important parameters in the autocovariance function, the time domain standard deviation (SD) and the cut-off period (or autocorrelation scale) Pc. The time domain SD specifies that the GST at any time will be within an SD of the a priori value with a proba-

C H M A T I C C H A N G E S IN C E N T R A L A N D E A S T E R N C A N A D A F R O M D E E P B O R E H O L E I ' E M P E R A T U R E D A T A

blity of about 68%. The a priori SD of the variation of GST about its mean value is set to 1.5 K in our calculations, which reflects a belief in the magnitude of the GST variation in the past several thousand years. Using a different value, provided that the SDs of other parameters and of the T-z data remain the same, will change the amplitude of the estimated GST history slightly. The mean GST, the a priori value of which is readily obtained by extrapolating the bottom section of the T-z profile to the ground surface, almost need no subjective constraint because it is always the parameter best resolved by the T-z data; however a value of 0.5 K is arbitrarily used as its a priori SD. Therefore the total a priori SD of the GST is the square root of [(1.5 K) 2 + (0.5 K)2]. The cut-off period determines the smoothness of the GST and filters out any GST variation with period shorter than P~.. This is analogous to smoothing meterological data using a moving average filter. Pc appears to be the most subjective parameter in the inversion, the value of 100 yr used for the results reported in this work is a reasonable guess. Using a smaller Pc allows details in recent GSTs to be resolved; using a larger Pc. enhances signals of the more remote GST at the expenses of reducing resolution of the recent signals. More detailed illustrations of the effects of P, can be found in Wang (1992) and Shen et al. (this issue). Using a different P,. does not change our conclusions, but the exact timing of resolved GSTs, or climatic events may be changed accordingly. For example, in cases where only single temperature logs are available, a Pc of 20 yr results in a later (by about 20 yr) occurence of the cold period around the turn of the century before the current warming seen in the GSTs of many boreholes. Such ambiguity arises because of the non-unique nature of the inverse problem. Repeat temperature logs separated by certain time, 20 years say, constrain the solutions much better. We use the same SD and P, for all the boreholes, so that any errors introduced should be systematic, allowing meaningful comparison of results from different boreholes. The vertical variation of thermal conductivity with depth is accounted for by the layered structure of the medium, the layers being defined by

133

the variation of measured conductivity. The conductivity assigned to each layer is an average or effective value. Small scale variations within the layer are not modeled, but their effect is taken into account by allowing larger SD values for the T-z data. Lateral variation of thermal properties is not considered, the effects of which cause some errors but are likely to be insignificant at the selected sites. The conductivity and diffusivity of each layer are also formulated as parameters, but with better constraints (smaller SD) since their a priori values are based on measured data. These thermal property values are updated as a part of the inverse solution. This approach prevents spurious GST signals which can be caused by the errors in these values. The a priori SD values used for the conductivities of the layers are typically 0.1-0.15 W m -t K 1, depending on the scattering of conductivity data in the layer. Rarely, the thermal conductivity value of a layer had to be guessed; in such cases, the SD was set at 0.2 W m-1 K ~. The a priori values of diffusivities are obtained by assuming a specific thermal capacity value of 2.3 X 10 and J m -s K-~, and the SDs are assigned values between 0.3 and 0.5 x 10-6 m 2 s -I.

T-z data were obtained to a precision or relative accuracy better than 0.005 K. Still, an SD of 0.01 K was used for the T-z data partly to account for the effects of small scale conductivity variations that are not considered in the layered model. The background heat flux value and the mean GST as determined by the inversion for each borehole is defined by the bottom section of the T-z and conductivity profiles. Even for deep boreholes, the bottom section of the T-z profile may still be under the influence of long term climatic changes, and the heat flux and mean surface temperature determined may be in some error. For this reason, we should consider them as reference instead of steady state values (Cermak, 1971). It should be emphasized that the a priori SDs of all the variables, including the parameters and data, in this inversion method have a relative meaning. One can arbitrarily increase or decrease all the SD values by any common factor without altering the estimates of the variables, only

134

K. WANG

changing all the a posteriori SDs by the same factor. An example involving three boreholes is shown in Fig. 2 to illustrate some salient features of the solutions. The three boreholes are located in a very small area near Lac Dufault (Lewis and Beck, 1977), site 53 on the map shown in Fig. 1. Figure 2a shows the T-z and conductivity data, and the layers used in the inversion. Each hole was logged in 1968 and in 1990, and the two T-z profiles were inverted simultaneously. Very similar GSTs (Fig. 2b) have been obtained for these holes, the more recent GST being better resolved. Little information on the details of the earlier climatic changes, such as those that occurred 200 yr ago, is contained in the T-z profiles. GST estimates with an assumed cut-off period of 20 yr are also shown for the purpose of comparison. It can be seen that using different Pc will change the details of the GST estimates (a demonstration of the non-uniqueness of the inverse problem), but not their overall patterns. The inversion always gives T-z profiles that nearly perfectly match the measured data, the difference generally being smaller than the SDs (a)

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of the T-z data. It is thus unnecessary to plot the T-z profiles in the presentation of results. Also, the SDs of the GSTs estimated from the other boreholes, nearly the same as those for these three holes, are not plotted in the following figures. Results

Borehole data from the 19 sites shown in Fig. 1 were inverted using the method outlined above. By using a cut-off period of 20 yr (results not shown in this paper), all boreholes except for the two sites (54 and 96) near the Atlantic, show a temperature optimum between 1940 and 1950, followed by a brief cooling trend until the 1970's. This is in agreement with the meterological records of the northern hemisphere (Hansen and Lebedeff, 1987). The results reported here were obtained with a cut-off period of 100 yr, in order to enhance the earlier climatic signals. A consequence is that the recent details are lost. Purely for the convenience of presentation, we grouped the estimated GSTs into four groups, and within each group, the GSTs are plotted in the order of

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Fig. 2a. Temperature and conductivity (crosses) data from three boreholes at Lac Dufault and the layered structures used in the inversion. Solid lines are temperatures logged in June, 1968, and dotted lines are temperatures logged in May, 1990. b. GSTs estimated from Lac Dufault data, using a cut-off period of 100 yr (solid line) and of 20 yr (dotted line). The dashed lines show the SD of the former.

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Fig. 3a. Lac Dufault type borehole GSTs. b. English River type borehole GSTs. No conductivity avaible for the upper 150 m of hole 149 and upper 300 m of hole 96. c. GSTs that do not belong to either type, yet borehole temperatures are not affected by large terrain effects. No conductivity available for the top 200 m section of hole 126 and 164. Small water flow is suspected in the near surface section of hole 438. d. Incorrect GSTs from boreholes affected by large terrain effects. The arrows indicate estimated mean temperatures.

136

increasing borehole length. The results from all boreholes are reported, including those affected by large terrain effects. All usable boreholes show a warming trend in the past one or two hundred years. A majority of the boreholes show a relatively cold period before the onset of the warming trend.

Lac Dufault type As shown in Fig. 3a, the GSTs of these boreholes are characterized by a cold period near the end of the last century, followed by rapid warming. These boreholes yield the best quality data in terms of the completeness of conductivity measurements and small terrain effects (see appendix). Boreholes at two sites (16, and 53) also have repeat temperature logs separated by 16-22 yr, which provide better constraints for the GST estimates than single logs. In Fig. 3a, and also among estimates from all the boreholes studied so far, the results from site 53, Lac Dufault, are considered the most reliable, because of the quality of data and because the same results were obtained independently from three nearby boreholes located in two different terrains, each with repeat logs separated by 22 years (Fig. 2).

English River type As seen in Fig. 3b, the major recent warming trend started about 100 years earlier than the Lac Dufault type. These boreholes are not affected by large terrain effects. No conductivity measurements are available for the upper 150 m section of the Lynn Lake hole (149) and for the entire upper 300 m of the Sunny Bank hole (96), and as a result the reliability of the GST estimates for these two sites are very poor. In addition, the Sunny Bank hole is drilled in sediments, and the chance of water flow disturbing the temperatures is higher.

Boreholes that do not belong to either of the above two types, yet do not appear to be affected by large terrain effects GSTs from four such boreholes are shown in Fig. 3c. The results for site 126, Mariner, and site

K. W A N G E T AI..

164, Parke, may be in serious question, because no conductivity measurements are available for the upper 200 m section of both boreholes, and they both penetrate sediments, and may have a larger chance of being affected by water flow. The recent GST for site 438, Matagami, obtained using a cut-off period of 20 yr, appears noisy (not shown), and for this reason, we suspect that the top section of the borehole is affected by a small water flow. Site 6, Roberval, is included in this group simply because it does not show a cold period before the major warming trend as in groups A and B. It seems to bear some identity of the Lac Dufault type according to its onset time for the warming.

Boreholes suffering from large terrain effects" The apparent GSTs from these boreholes (Fig. 3d) can be immediately attributed to terrain effects. Holes at sites 13, Hearst, and 5, St. Jerome, are at the boundary of large forested and cleared fields. The effect of the clearing is to produce or amplify the large apparent warming signals. The large difference in the onset time of the apparent warming may have to do with the actual timing of clearing, but we do not have the exact information to assess this problem. Assuming the clearing took place 100 years before temperature logging,

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CLIMATIC C HA NG E S IN C E N T R A L AND EASTERN CANADA FROM D E E P B O R E H O L E T E M P E R A T U R E DATA

we show in Fig. 4 the perturbation to the borehole temperatures by a unit temperature rise caused by the clearing. Also shown in Fig. 4, is the river effect on the temperatures of the English River hole (site 17), with a unit difference in average bottom water temperature and average ground surface temperature, and assuming the river came to existence 10,000 yr ago; the river effect is usually much smaller than the clearing effect. At site 15, Otoskrwin River, there is no conductivity mesurement for the top 60 m of the hole, and the effect of a nearby lake may be significant (see appendix). We tried to make use of the borehole at site 52, Nielsen Island, because of its length and complete high quality conductivity measurements, but the large terrain effect (appendix) makes it unusable for GST estimation. Discussions

We are not aware of any published tree-ring chronology from the study area. However, treering temperature reconstruction has been made at high north latitudes along the tree growth limits in Alaska and Canada (e.g., Jacoby and D'Arrigo, 1989) and in the U.S. and southwestern Canada (e.g., Fritts and Lough, 1985). It may not be exactly pertinent to compare directly our ground surface temperature estimates with the proxy records from other regions, but some correlation with the chronologies from these adjacent regions should be expected. In the reconstructed summer temperatures of Jacoby et al. (1985) and the annual temperature of Jacoby and D'Arrigo (1989), especially the latter, a period around mid19th century is particularly cold. This cold period is centered in between those of the two types of GST observed in this study. The most recent 300 years of reconstructed temperatures from Washington State by Graumlich and Brubaker (1986) and those averaged for the U.S. and southwestern Canada by Fritts and Lough (1985) are very similar to the Lac Dufault type GST. Undoubtedly, there existed a cold period before the warming trend recorded by surface air temperature records, as shown by both the Lac Dufault type and the English River type GSTs. The major difference between the two groups is

137

the timing of the cold period. In the former, the cold period is centered at the turn of the century; but in the latter, it occurs about 100 years earlier. Although the Lac Dufault type results were obtained generally with better quality data, and are greatly substantiated by the agreement between the results from different boreholes at site 53, we cannot rule out the possibility of different regional climate pattern. Two of the holes in the English River group, 96 and 54, are located near the Atlantic Ocean. Three of them (14, 17, and 84) plus the less reliable 13 and 126, which also show the early cold period, form a cluster north of Lake Superior. It seems that the early cold period and onset of warming might have a geographical distribution. The geographical locations of the Lac Dufault type boreholes, with their better data, indicate that this type of GST is typical of the Boreal climatic region of Canada (Hare and Thomas, 1974). Limited early surface air observations also show a temperature minimum around 1890 in North America (Hansen and Lebedeff, 1987; El[saesser et a[., 1986; Jones et al., 1986). The presence of the Little Ice Age two to five hundred years ago is well known, but its duration and time of termination have never been well defined (Grove, 1988). It may not be a single climatic event after all. Due to the decreasing resolution prior to 200 years, we cannot tell from our borehole data whether there was a Little Ice Age before the cold period near the end of the last century, although some GSTs such as those of site 53, Lac Dufault, seem to indicate so. The difference between the Lac Dufault and English River types of GSTs further demonstrates the complexity of the climate system. Tree-ring width data have also shown that the severeness and timing of the Little Ice Age vary from place to place in North America (H.C. Fritts, pers. comm., 1991). The borehole GSTs may reflect the climatic variations in North America and even on a global scale. Onset of the recent (global) warming as estimated by proxy methods ranges from more than 500 yr ago (Epstein and Krishnamurthy, 1990) to 100 yr ago (Cook et al., 1991), and to non-existent (Briffa et al., 1990). Such diversity

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m a y p a r t l y reflect t h e spacial variability o f the e a r t h ' s c l i m a t e system, b u t m a y also be d u e to the u n c e r t a i n t i e s in r e l a t i n g t h e proxies to climatic signals. R e c e n t l y , G r a y b i l l a n d Shiyatov (1992) a n d C o o k et al. (1991) c o n s t r u c t e d t r e e - r i n g c h r o n o l o g i e s in n o r t h e r n R u s s i a a n d T a s m a n i a , respectively. B o t h show r e m a r k a b l e a g r e e m e n t with the L a c D u f a u l t type G S T a n d with the previously r e f e r e n c e d t r e e - r i n g r e c o r d s from high n o r t h l a t i t u d e , W a s h i n g t o n State, a n d the U.S. a n d s o u t h w e s t e r n C a n a d a in that a short cold p e r i o d existed n e a r the e n d o f the 19th c e n t u r y b e f o r e the r e c e n t warming. T h e p r e s e n c e o f the cold p e r i o d b e f o r e the r e c e n t climatic warming, especially if this p e r i o d is as r e c e n t as a r o u n d the t u r n o f the c e n t u r y as shown in the L a c D u f a u l t type G S T s , m a y have significant i m p l i c a t i o n on t h e m a g n i t u d e o f global warming. It shows t h a t the r e c e n t w a r m i n g is p a r t i a l l y a recovery from this cold p e r i o d , as p o i n t e d out by W a n g a n d Lewis (1992).

Conclusions W e s e l e c t e d d e e p b o r e h o l e s f r o m 19 sites in c e n t r a l a n d e a s t e r n C a n a d a for the p u r p o s e of inferring p a s t climatic changes. A f t e r site-specific study of the local e n v i r o n m e n t of t h e b o r e h o l e s , the d a t a w e r e i n v e r t e d using a s p e c t r u m inverse m e t h o d to give t h e most p r o b a b l e e s t i m a t e of G S T histories. T h e b o r e h o l e G S T is a s m o o t h e d version o f the real G S T , a n d the f u r t h e r b a c k we look in time, the less detail o f the G S T history can we tell. W e can d r a w the following conclusions. (1) T h e e s t i m a t e d G S T s o f the last 100 yr a g r e e very well with the m e t e o r o l o g i c a l d a t a in showing the w a r m i n g t r e n d in the first half of the century. (2) T h e m a j o r w a r m i n g t r e n d in the first half of the c e n t u r y o r e a r l i e r was p a r t l y a recovery of surface t e m p e r a t u r e from a relatively cold p e r i o d . (3) G S T s e s t i m a t e d f r o m b o r e h o l e t e m p e r a tures in c e n t r a l a n d e a s t e r n C a n a d a form two m a j o r types. T h e L a c D u f a u l t type, which shows a cold p e r i o d n e a r the e n d of the last century, is b e l i e v e d to b e typical of the B o r e a l climatic region of C a n a d a . T h e English R i v e r type, in which

K. W A N G E T AL.

the cold p e r i o d is a b o u t 8 0 - 1 0 0 yr earlier, is f o u n d m a i n l y in a region n o r t h of L a k e S u p e r i o r a n d at two b o r e h o l e sites n e a r the A t l a n t i c O c e a n . (4) In most cases in the s t u d i e d area, the effect o f n e a r b y lakes on the b o r e h o l e t e m p e r a tures is p r o b a b l y very small, b e c a u s e the a v e r a g e b o t t o m w a t e r t e m p e r a t u r e is similar to the average g r o u n d surface t e m p e r a t u r e , w h e r e a s the c l e a r i n g of forest has b e e n shown to have a very large effect on the b o r e h o l e t e m p e r a t u r e s .

Acknowledgement W e t h a n k P.Y. Shen, H. Beltrami, a n d H.C. F r i t t s for helpful discussions, T. H a m i l t o n , E. Irving, L. R y b a c h , a n d D. G r a y b i l l for reviewing the m a n u s c r i p t a n d m a k i n g v a l u a b l e suggestions. G e o l o g i c a l Survey of C a n a d a c o n t r i b u t i o n No. 13292.

Appendix: brief descriptions of borehole sites 5. St. Jerome (Fou, 1969). The collar is at the bottom of a small, southwest slope on seminary grounds; the immediate area is now well drained. However, this site is on the edge of a large area probably deforested in the past. The calculated correction for a change of 1 K occurring 100 yr ago is 0.8 K near the surface, diminishing to 0.02 K by 150 m depth. The magnitude of such a change is likely to be larger than 1 K, and the large terrain effect makes the hole unsuitable for GST estimation. 6. Roberval (Wright and Garland, 1968). The collar is in a very flat area where bedrock is exposed, near the intersection of two paved roads. The surrounding area appears to be well-drained, but was probably deforested in the past, judging by the abnormal boundaries of the forests nearby. The estimated geometric effect of clearing 100 yr ago is small because the borehole is not at the edge of the cleared area. The average ground surface temperature should have been increased by deforestation, but thet effect may have been balanced by the cooling effect of subsequent snow clearing in this developed area. 12. Kapuskasing (Cermak and Jessop, 1971). The collar is in the flat area east of the small town of Kapuskasing in bush (trees 8-10 m tall). Large cleared fields are at a minimum distance of 500 m away, and may have some small effect on the temperatures. 13. Hearst (Cermak and Jessop, 1971). The collar is on a slightly elevated, bushed area, next to a field probably cleared out of the surrounding forests approximately 100 yr ago. A small, nearby lake and swampy area affect the temperatures only minimally. The largest effect, from the closer clearing, probably varies from over 2 K near the surface to less than 0.02 K by 150 m depth, and greatly amplifies the apparent effect of any general warming.

CLIMATIC

CHANGES

IN C E N T R A L

AND EASTERN

CANADA

FROM DEEP BOREHOLE

14. Jackfish (Jessop and Lewis, 1978). The collar is located in forested, rugged shield terrain just at the edge of the right-of-way of the Trans Canada highway. There are no nearby lakes nor swamps that will effect temperatures. 15. Otoskwin River (Jessop and Lewis, 1978). This region is a forested, flat lying, swampy area. However, the collar is on the upraised bank of the Otoskwin River, near a large lake, where there is little swampy land. The average bottom water temperature, approximately 2 K higher than the average ground temperature, will have produced higher temperatures in the borehole (a maximum of 0.25 K at 15(/m). 16. Minchin Lake (Jessop and Lewis, 1978). The site is in bush between a secondary highway and a large lake. The collar is slightly higher than the highway 50 m distant, and the land slopes slightly downward 200 m to the nearest bay of Minchin Lake. 17. English River (49°46.4N, 91°22.3W; (Jessop and Lewis, 1978). This forested region is flat and covered with numerous lakes. The collar is located 200 m from a major river. The calculated effect of the river may be negligible since the average ground temperature is estimated to be approximately the same as the bottom water temperature. 52. Nielsen Island (Jessop and Judge, 1971). This collar is located on the beach of an island in Hudson's Bay. The effects of the nearby water and local changes in sea level are too great to permit an accurate calculation of their effects on the borehole temperatures. The hole itself has a very large contrast in thermal conductivity as it enters quartz-rich rocks at depth. The terrain effect makes the borehole unusable for GST estimation. 53. Lac Dufault (Lewis and Beck, 1977; Lewis and Wang, this issue). Many boreholes were measured in 1968 in this one region which consists of more weather-resistant volcanic rocks of the Blake River formation surrounding the low-lying Lac Dufault granodiorite. Water flows were found in most boreholes. Twelve of the boreholes were relogged in 1990. Three of the relogged holes showed no sign of water flows: two from the low-lying area, in small treed areas surrounded by cleared fields, and one from the more rugged, elevated area, which was forested. The temperatures from these three holes were analyzed. If the clearing for the fields had an effect on the data from the two boreholes in the low-lying area (22 and 24), it should cause the temperatures at depths greater than 100 m to increase, not decrease. 54. Oldham (Jessop and Judge, 19711. This forested region is hilly, and the collar is approximately 200 m from a small pond of similar dimensions. No terrain correction was necessary. 67-6. Sudbury. This site is right on the forested bank of a lake. However, the calculated effect of the lake should be very small since the bottom temperatures will nearly equal the ground temperature. Climatic changes will be buffered by the lake. 84. Manitouwadge (Jessop and Lewis, 1978). This collar is in a forested, hilly area, covered by snow in winter. There are no bodies of water or swamps nearby, and no corrections appear necessary. 96, Sunnybank (Drury et al., 1987). This well was drilled after a large area was cleared within a forested area, on a rise above a distant river estuary. No corrections were necessary.

TEMPERATURE DATA

139

The rocks penetrated are sediments. The largest problem with this hole is that conductivity m e a s u r e m e n t s are not available for the entire upper 300 m. 126. Mariner (Drury and Taylor, 1987). This is a flat-lying, forested region, but there are no swampy areas nor bodies of water near the drill site. No calculation was made for terrain effects. The largest problem with this hole is that no conductivity m e a s u r e m e n t s are available for the entire upper 200 m. 136. Flin Flon (Sass et al., 1971; Lewis, 1987). There is some ambiguity about the location of this hole (two possible sties), but it is not near the edge of a lake. A small clearing was made in a forested area for the collar. The region is covered with small lakes, and the combined effect of several of the nearest lakes was calculated using the two possible locations. The perturbation to the subsurface temperatures is found negligible. 149. Lynn Lake. This collar is in a low-lying, forested area near some medium-sized lakes. The calculated effect of the lakes is very small, reaching a maximum of + 0.04 K at 400 m depth. No conductivity m e a s u r e m e n t s are available for the upper 150 m. 164. Parke (Drury et al., 1987). The collar is in the middle of a large clearing, made for the drilling operation, in rolling, forested terrain. There are no nearby bodies of water or swamps. The rocks penetrated are sediments. The largest problem with this hole is that no conductivity measurements are available for the entire upper 200 m, 438. Mattagami. The collar is about 1 km from a mine site, in a 10 m wide cut line in bush. There are no nearby water bodies. Small water flow is suspected in the near surface section of the borehole.

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