Present-day and late Wisconsin glacier equilibrium line altitudes (ELA) were compared on a transect from Patagonia along the coastal Cordillera to Alaska, and ...
Palaeogeography, Palaeoclimatology, Palaeoecology, 95 ( 1992): 41-46
4I
Elsevier Science Publishers B.V., Amsterdam
Equilibrium line altitude variations with latitude, today and during the Late Wisconsin Mauri S. Pelto Department of Environmental Science, Nichols College, Dudley, MA 01571, USA (Received October 15, 1991; revised and accepted March 13, 1992)
ABSTRACT Pelto, M.S., 1992. Equilibrium line altitude variations with latitude, today and during the Late Wisconsin. Palaeogeogr.. Palaeoclimatol., Palaeoecol., 95:41-46 Present-day and late Wisconsin glacier equilibrium line altitudes (ELA) were compared on a transect from Patagonia along the coastal Cordillera to Alaska, and a transect from Mexico along the Sierra Madre-Rocky Mountains to Alaska. Late Wisconsin ELA's were determined from cirques occupied during the late Wisconsin, and present day ELA from observations of active cirque glaciers. The mean change in ELA from the late Wisconsin to the present was 735 m, with a standard deviation of 120 m. This indicates that the change in snowline altitude from the present to the late Wisconsin was approximately uniform and not latitudinally dependent.
Introduction Milankovitch insolation cycles have been demonstrated to be a primary driving force causing the onset of ice ages (Hays et al., 1976), though not necessarily the end of an ice age (Denton et al., 1986). Climate and the resultant glacier equilibrium line altitude (ELA) are assumed to fluctuate directly with summer insolation changes (Budd and Smith, 1981). Because summer insolation is latitudinally variable, resulting climatic change from the present to the late Wisconsin should be variable with latititude (Denton et al., 1986). Hence, if summer insolation change is directly a primary climatic variable, then the magnitude of the change in glacier ELA would vary wi[h latitude. To test if this is the case the fluctuation of glacier ELA with latitude from the present to the late Wisconsin is examined for two transects in western North America. Robin (1988, fig. 4) presented results of an earlier study of this kind and Broecker aod D.enton (1989, fig. 6) presented some initial Correspondence to: M.S.Pelto, Department of Environmental Science, Nichols College, Dudley, MA 01571, USA 0031-0182/92/$05.00
results of this study. This paper examines and presents cirque ELA data in greater detail. One transect of ELA versus latitude is for a maritime climatic setting, from 51°S to 62:>N along the coastal Cordillera of the Americas. The second transect is for a continental climatic setting, from 20°N to 62°N along the Sierra Madre-Rocky Mountain-Brooks Range chain of North America. These transects are the longest continuous latitudinal transect of glaciated mountains in the world. Since the majority of both transects were either distant from or climatically upwind of the continental ice sheets, the climatic setting of the transects were not as affected by the presence of continental ice sheets, as were other mountain ranges.
Equilibrium line altitude determination Late Wisconsin ELA determination is based on the elevation of cirque floors that had been occupied during the late Wisconsin. This method has been chosen because it is the only direct measure of paleo ELA available, the methodology used is consistent, and the resulting data set is the only
,~ 1992 - - Elsevier Science Publishers B.V. All rights reserved.
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M.S.PELTO
latitudinally complete data set available. This technique has been discussed in detail by Porter (1964), Miller (1961), Prw6 (1975) and Trenhaile (1976). It can be argued that cirque elevations do not accurately indicate the lowest ELA of an ice age, for several reasons: (1) Cirques take time to develop; (2) Because of wind drift and avalanche accumulation cirques can develop below the local ELA; (3) The elevation of cirques is highly dependent on orientation and geographic location. The elevation of cirque glaciers is somewhat below the regional snowline (Trenhaile, 1976); thus, the reported ELA for cirque glaciers is not necessarily the regional snowline altitude. This study is only concerned with the change in cirque ELA from the present to the late Wisconsin, as a direct measure of the
magnitude of climate change. Thus, as long as the methodology is consistent the fact that the regional snowline is not identified does not cause any error. Cirque pairs have been chosen, one from the late Wisconsin and one from the present, that have the same orientation and geographic location. This methodology eliminates the latter two aforementioned errors. Cirque development does take time, but the climatic conditions of interest are the equilibrium conditions that would lead to cirque growth, not shorter duration conditions. Potential sources of error in using cirque pairs are: (1) Bias introduced if the orientation of the cirque pairs used are not kept constant in comparing late Wisconsin and present day cirque glaciation levels; (2) Error resulting from atypical cirques
341
37
34
eP 3O3
16
I$
21
27 2S
~+ 14
26 24
~Q
ira,
Fig. I. Map indicating the location of cirques used in this study. The numbers correspond to those in Table 1.
EQUILIBRIUM LINE ALTITUDEVARIATIONSWITH LATITUDE.TODAY AND DURING THE LAIE WISCONSIN
when data from only two or three cirques are used; (3) In each study the elevation of the cirque floor is assumed to be the former ELA. To remove the first two sources of bias the orientation of the cirques in each cirque pair is constant, and all data points represent data from at least four cirque pairs. The cirque floor is usually slightly below the ELA. Examination of 37 present day cirque glaciers in British Columbia, Alaska, and Washington indicates that the mean ELA is 30 m + 15 m, above
the cirque floor. The bias in this case is then approximately constant and can be removed. Data for the selected cirques are listed in Table i. Location of the cirque pairs are shown in Fig. 1. Determination of present day ELA is based on compilations of active cirque glacier ELA observations for Canada by Ostrem (1966), United States by Meier et al. (1971), Mexico by White (1962) and South America by Nogami (1976). A comparison of present and Wisconsin ELA
TABLE I The altitude of the ELA in meters above sea level during the late Wisconsin and at present, for specific localities in the America's. References are for late Wisconsin ELA data. Present snowline data for South America is from Nogami, (1976); for Mexico, White (1962); for United States, Meier et al. (1971); and Canada, Ostrem (1966). Maritime transect
I 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 36 37 38
43
Location
Latitude (degree)
Wisconsin snowline
Present snowline
Reference
S. Patagonia Patagonia N. Patagonia Dumuyo Quintero Banos Bolivia Peru Ecuador Columbia Venezuela Costa Rica Guatamala Yosemite Jefferson Ranier Glacier Peak Nootka Baranof Juneau St. Elias Kenai Mexico New Mexico San Juan Front Range Wind River Beartooth Glacier N.P Shuswap Purcells N. Selkirks Columbia Omineca Kluane McKinley Brooks Range
51 S 46 42 36 33 29 19 12 0 5 N 10 I0 15 37 43 47 48 50 57 58 60 62 20 N 35 38 40 43 47 49 52 53 53 54 57 60 64 67
500 600 900 2000 3500 4100 4500 4000 4000 3800 3700 3600 3500 2800 1700 1400 I I00 1000 400 450 300 300 4000 3500 3600 3200 2900 2400 1900 1800 ----1400 1300 1300
1200 500 2000 3000 4300 5000 5200 4700 4800 4700 4700 --3700 2500 2200 1900 1700 1000 1050 50 800 4800 --3800 3600 3100 2600 2400 2300 2300 2500 2100 2300 2000 1900
Mercer, 1968 Mercer, 1968 Nogami, 1976 Nogami, 1976 Hastenrath, 1971 Hastenrath, 1971 Nogami, 1976 Hastenrath, 1971 Mercer, 1968 Hastenrath, 1971 Hastenrath, 1973 Hastenrath, 1973 Hastenrath, 1973 Wahrhaftig, 1965 Scott, 1977 Porter, 1964 Porter, 1964 Halstead, 1968 P~w~, 1975 Miller, 1961 P~w~, 1975 P~w6, 1975 White, 1962 Leopold, 1951 Carrera et al., 1984 Meierding, 1982 Flint, 1971 Porter, 1964 Flint, 1971 Duford and Osborn, 1978
Denton and Stuiver, 1967 P6w~, 1975 P6we, 1975
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M.S.PELTO
(Fig. 2) shows that the variation in ELA with latitude does not have a consistent slope either today or in the late Wisconsin. However, the vertical distance between the two lines, marking
the change in ELA between present and late Wisconsin, is comparatively constant, though not graphically convincing. The mean depression of the ELA from present to the late Wisconsin was
Maritime Transects 6000
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Wisconsin time
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211
2000
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(b) 1000
..... 10
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1000 70
Latitude (degrees N) Fig. 2 (a) Comparsion of present and late Wisconsin cirque glacier ELA along a maritime climatic transect from Patagonia to Alaska. Data sources and locations are noted in Table I. The numbers between data pairs are from Table I and Fig. I indicating the location of each data pair. The numbers are located vertically above or below the respective data points; (b) Comparison of present and late Wisconsin cirque glacier ELA along a continental transect from Mexico to Alaska. Data sources and locations are noted in Table 1. The numbers between data pairs are from Table I and Fig. I indicating the location of each data pair. The numbers are located vertically above or below the respective data points.
I.Qt!ILII~.RII'M L I N t & [ _ [ l ' l l I ) l
V A R I A I I O N S WI1H I . A I I I t
I)F. I O I ) A Y A N D I)1, RING I H I [ & l l
735 m. with a standard deviation of 120 m. The correlation coefficient between present day and late Wisconsin ELA is 0.99. indicating the similarity in change regardless of latitude. This high correlation coefficient allows estimation of paleo snowlines from present day snowlines with a rcasonable degree of confidence in the American Cordillera. Of the 29 locations where both late Wisconsin and present day ELA data are available, the change is beteen 600 and 900 m in all but two cases. This, together with the high correlation coefficient and low standard deviation, demonstrates the approximately uniform change in ELA between the present and the late Wisconsin. The uniform change in present to late Wisconsin cirque glacier ELA is not necessarily synchronous, this is the crucial question that must be addressed next. Only a few of the cirques examined in this study have been dated: however, in all cases where dating is available thc dates arc approximately synchronous at 14,500 13,000 B.P. for cirque dcglaciation (Table 2). The timing of cirque occupation is weak: however, the available data suggest that the unifbrm change in ELA was synchronous (Broeckcr and Denton, 1989).
Interpretation of results The uniformity of the ELA change suggest that temperature changc is the driving mechanism, since precipitation changes are generally erratic in comparison (Porter, 1964: Nogami, 1976). The anticipated error of using cirque floors results in an TABLE 2 Radiocarbon dates for late Wisconsin cirque deglaciation. [.ocation
Latitude Age
Source
Patagonia Peru Columbia Columbia San Juan Front Range Yellowstone Cascades Kluane Alaska Range Brooks Range
42 12 S 4N 5 38 40 44 48 60 63 67
Mercer, 1984 Mercerand Palacios, 1977 Helmens, 1988 Herd and Nacser, 1974 Carrerra et al., 1984 Mad~')le,1981) Richmond, 1976 Porter, 1976 Denton and Stuiver, 1967 Hamilton, 1982 Hamilton. 1982
14,355 14,010 14.660 13.800 14,130
14,000 14,200 14,000 13,600 13.270 13,100
~,IS('ONSIN
45
approximately - 30 m correction to the mean ELA change of 735 m, yielding 705 m. Further evidence for a uniform change in ELA is the 700 m drop in ELA between the present and late Wisconsin in New Zealand (Porter, 1983). A drop of 705 m in EI,A is correlated with a 4.2 C drop in temperature, if the standard mountain lapse rate of 0.6 C 100 m is used (Porter. 1964). Certainly the lapse rate will vary depending on locality and with timc, so this calculation is merely uscd as a best estimatc in the absence of more direct data. In areas wherc continental ice sheets were important in determining climatic conditions, the change in ELA will not neccesarily be consistent with the change fout|d in this study. The summer insolation at 65 N at the last Milankovitch minimum is equal to the present summer insolation at 71"N (Milankovitch, 194 1). This 6 latitude shift in summer insolation matches the 5 7 shift in latitudc of ELA in Fig. 2 from the present to the late Wisconsin. This similarity is possibly .just a coincidence. As Mercer (1985) noted, the interhcmispheric synchronous nature of glaciations is "'the fly in the ointment of the Milankovitch theory". Now it appears that not only might the late Wisconsin alpine glaciations be synchronous in the American Cordillera, but the change in ELA from the present is of approximately the same magnitude at all latitudes.
Acknowledgements Particular thanks go to Terence Hughes and George Denton whose interest in this topic spurred this research. And in particular to Terence Hughes for his continuing encouragement and editing of this manuscript. Also I appreciated the marly helpful comments of P.T. Davis and G u n n a r Ostrem.This work was partly supported by Batellc. Pacific Northwest Laboratories, subcontract B-R7112-A-E.
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