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Late Pleistocene Glacial Equilibrium-Line Altitudes in the Colorado Front Range: a Comparison of Methods

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Late Pleistocene Glacial Equilibrium-Line Altitudes in the Colorado Front Range: a Comparison of Methods QUATERNARY RESEARCH l&289-310 (1982) Late Pleistocene Glacial Equilibrium-Line Altitudes in the Colorado Front Range: A Comparison of Methods THOMAS C. MEIERDING Department of Geography and Center For Climatic Research, University of Delaware, Newark, Delaware 19711 Received July 6, 1982 Six methods for approximating late Pleistocene (Pinedale) equilibrium-line altitudes (ELAs) are compared for rapidity of data collection and error (RMSE) from first-order trend surfaces, using the Colorado Front Range. Trend surfaces computed from rapidly applied techniques, such as glacia- tion threshold, median altitude of small reconstructed glaciers, and altitude of lowest cirque floors have relatively high RMSEs @I- 186 m) because they are subjectively derived and are based on small glaciers sensitive to microclimatic variability. Surfaces computed for accumulation-area ratios (AARs) and toe-to-headwall altitude ratios (THARs) of large reconstructed glaciers show that an AAR of 0.65 and a THAR of 0.40 have the lowest RMSEs (about 80 m) and provide the same mean ELA estimate (about 3160 m) as that of the more subjectively derived maximum altitudes of Pinedale lateral moraines (RMSE = 149 m). Second-order trend surfaces demonstrate low ELAs in the latitudinal center of the Front Range, perhaps due to higher winter accumulation there. The mountains do not presently reach the ELA for large glaciers, and small Front Range cirque glaciers are not comparable to small glaciers existing during Pinedale time. Therefore, Pleistocene ELA depression and consequent temperature depression cannot reliably be ascertained from the calcu- lated ELA surfaces. INTRODUCTION existed in alpine regions (Charlesworth, Glacial equilibrium-line altitudes (ELAs) 1957; ostrem, 1966; Flint, 1971; Andrews, have been widely used to infer present and 1975). These indices have not been rigor- Pleistocene climatic conditions. Many ously investigated in a single area to see studies in arctic and alpine areas demon- which best represents past ELAs or to de- strate that the regional trends of modem termine how much error is associated with and Pleistocene ELAs relate to modern each index. Methodological comparison precipitation patterns, which are in turn in- studies by Andrews (1975), Gross et al. fluenced by atmospheric circulation, oro- (1977), and Hawkins (1980) are limited graphic effects, and distance from the sea either by small sample size, by lack of an (Porter, 1964, 1975a, 1975b, 1977; Qstrem, accuracy statement, or by choice of indices 1964, 1966; Wahrhaftig and Birman, 1965; that relate to modern rather that past Grosval’d and Kotlyakov, 1969; Peterson ELAs. Commonly used ELA determination and Robinson, 1969; PCwe and Reger, 1972; techniques will be evaluated in the Col- Andrews and Miller, 1972; Miller et al., orado Front Range for rapidity and subjec- 1975; Trenhaile, 1975; Hawkins, 1980). In tivity of data collection, and for magnitude addition, Pleistocene air temperatures have of local error from regional ELA trends. often been inferred by applying modern These techniques include altitudes of rem- lapse rates to the vertical distance between nant cirque floors, altitude of the glaciation present and Pleistocene ELAs (Peterson et threshold, maximum altitude of lateral al., 1979; Table 1). moraines, median altitude of reconstructed Equilibrium-line altitudes of past glaciers glaciers, and area1 relationships between cannot be directly measured, so a number accumulation and ablation zones of recon- of indices have been developed as surro- strutted glaciers. The “best” method of gates for ELAs, particularly those which representing regional ELAs will then be 289 0033-5894/82/060289-22$02.00/O Copyright 0 1982 by the University of Washington. All rights of reproduction in any form reserved. S E T A T e S n r o u i t l 1 1 s D a 4 9 7 1 1 5 6 9 s r C - - - - E “ e e 0 ( 8 9 4 r T p I 1 p N m e e d U T N R E T S E 0 0 0 0 W 0 0 0 0 n 2 3 5 0 o E 0 i 0 1 1 1 1 e 0 0 0 0 0 0 0 0 0 0 s l 5 H 0 d 0 5 0 0 0 7 0 0 0 0 5 s 9 5 T u 0 - - 2 5 4 2 6 0 3 8 5 6 - m - t - e ( 1 i 2 0 0 1 1 1 1 1 r 0 0 t l M 1 0 0 N p 0 0 I > A > 9 X 8 e 9 9 d E G A T S L A I C e e m m A l m n n g g L o o a n n o o o i n n n n n n i i r r n G r t t o o f f c f y o o o o i i a a o i i i i t t t i a a a h h i t t l s s s s s a a c c i i n v n n n n s s s s e e g s i i c c A o A A t o o o o e e e e r g g i e i i i i r r r r a a E L r L L c l l s e e e s s s s p p p p d N d p E s E v v E g p a g s s s s n e e e e o E n o e e e e e e d d f f f f f f f o r d d d d h f C o h r r r r d i t i t o o t p o o t o o o o o p p p p O o e e h h u c e u T e e e e t t i b M d S b d m d d d e e i h r r r r I i r r d d r n n n n n n n p t E l l n o o o o t o m m r L o o o o o o o s o o o o o o o s i i i i i i i o i i l l l l i o l P f f f f h h s s s s s s s e e e f s d e e R d - - - - - s s s s s m s s s s s s n n n s n n i i i t t e e e e R R A i i e l l l e e l l l l o e e e e e e e e t t - - - r r - - u u u u r r r r r r r E u A A r H e e e e e e n n h h e e T q q q q p p p p p p p q p T b b A A t g t r r r r e e e a a r e e A i i i i e e e e e e e l l i e r r r r r L C C T D P D D C C P T D D D D T T D C T R O F N O I S S , E , k R k r r s P a n a E i a P i D P a t n , , r n o o o s o s k l l o u d d s c a r c n i . f e i . i i a a o a a , n n a l x i x d r r S a S s n n E o t a e a M P e o o , a n l l l n z o o R n i i i i s c s . 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