Middle Pleistocene age of the Nome River glaciation, northwestern Alaska

QUATERNARY

RESEARCH

36, 271-293 (WI)

Middle Pleistocene Age of the Nome River Glaciation, Northwestern Alaska DARRELL S. KAUFMAN,*

ROBERT C. WALTER,*'? JULIE BRIGHAM-GRETTE,~ DAVID M. HOPKINS§

AND

*Center for Geochronological Research, INSTAAR, and Department of Geological Sciences, University of Colorado, Boulder, Colorado 80309-0450; fGeochronology Center, Institute of Human Origins, 2453 Ridge Road, Berkeley, California 94709; #Department of Geology and Geography, University of Massachusetts, Amherst, Massachusetts 01003; and Mlaska Quaternary Center and Department of Geology and Geophysics, University of Alaska, Fairbanks, Alaska 99775-1200 Received June 27, 1990 During the middle Pleistocene Nome River glaciation of northwestern Alaska, glaciers covered an area an order of magnitude more extensive than during any subsequent glacial intervals. The age of the Nome River glaciation is constrained by laser-fusion 40Ar/39Ar analyses of basaltic lava that overlies Nome River drift at Minnie Creek, central Seward Peninsula, that average 470,000 -+ 190,000 yr (kla). Milligram-size subsamples of the lava were dated to identify and eliminate extraneous 40Ar enrichments that rendered the mean of conventional K-Ar dates on larger bulk samples of the same flow too old (700,000 ? 570,000 yr). While the 40Ar/39Ar analyses provide a minimum limiting age for the Nome River glaciation, maximum ages are provided by a provisional K-Ar date on a basaltic lava flow that underlies the Nome River drift at nearby Lava Creek, by paleomagnetic determinations of the drift itself at and near the type locality, and by amino acid epimerization analysis of molluscan fossils from nearshore sediments of the Anvilian marine transgression that underlie Nome River drift on the coastal plain at Nome. Taken together, the new age data indicate that the glaciation took place between 580,000 and 280,000 yr ago. The altitude of the Anvilian deposits suggests that eustatic sea level during the Anvilian transgression rose at least as high as and probably higher than during the last interglacial transgression; by correlation with the marine oxygen-isotope record, the transgression probably dates to stage 11 at 410,000 yr, and the Nome River glaciation is younger still. Analyses of floor altitudes of presumed Nome River cirques indicate that the Nome River regional snowline depression was at least twice that of the maximum late Wisconsin. The cause of the enhanced snowline lowering appears to be related to greater availability of moisture in northwestern Alaska during the middle Pleistocene. 0 1991 University of Washington.

INTRODUCTION

time that precipitation and temperature regimes were conducive to the build up of significant volumes of ice in northwestern Alaska. In marked contrast to the Nome River glaciation, subsequent glacial advances on Seward Peninsula were restricted to the highest mountain valleys (intramontane glaciations of Kaufman, 199 1)) apparently the products of a succession of extremely cold and arid climates. To understand the changes in paleoclimatic conditions that gave rise to the sequence of extensive early and middle Pleistocene glaciations contrasting with restricted younger glacier advances, the glacial record must be linked with more complete records of pa-

One of the most striking aspects of the Quaternary glacial record in northwestern Alaska is the presence of at least two early and middle Pleistocene drift sheets that are much more extensive than the restricted glacial advances of the last 100,000 to 200,000 yr. During the Nome River glaciation, the most recent of the extensive glaciations, ice capped the uplands of Seward Peninsula and extended just beyond the present-day shoreline and covered an area approximately an order of magnitude more extensive than it did during any later Pleistocene glaciation (Fig. 1). This was the last 271

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8 1991 by the University of Washington. of reproduction in any form reserved.

278

KAUFMAN

ET AL. 15a I

hukchi

Outermost drift and approximate

Sea

extent of ice during the Nome River glaciation

Alaska (after Hamilton, FIG. 1. Reconstructed extent of Pleistocene glacier ice in northwestern 1986; and Hopkins, 1973). Intramontane glaciations (Kaufman, 1991) include late Pleistocene, penultimate, and in some places, older advances.

leoenvironmental change, in particular, with the record of oceanic and atmospheric circulation preserved in deep-sea sediments. The link can only be accomplished by placing the terrestrial deposits in a firm geochronological framework. The presence on Seward Peninsula of basaltic lavas and fossiliferous marine sediments in demonstrable stratigraphic relations with drift of the Nome River glaciation provides a unique opportunity to constrain the timing of a remarkable paleoclimatic event in northwestern Alaska. Drift of the Nome River glaciation is notched by, and is therefore older than, a high marine shoreline formed during the last interglaciation (Pelukian; Hopkins, 1967). A more closely confining age for the Nome River glaciation was apparently provided by conventional K-Ar analysis of a basaltic lava flow near the mouth of Minnie Creek located in the Bendeleben Mountains of central Seward Peninsula (Fig. 1). Here, a normally magnetized basaltic lava that

flowed over a moraine ascribed to the Nome River glaciation yielded an age of 810,000 2 90,000 yr on phenocrystic plagioclase (Kaufman and Hopkins, 1985, 1986; Table 1). This determination seemed to be corroborated by a whole-rock age of 797,000 2 59,000 yr (UACGL-87083; D. Turner, University of Alaska, written communication, 1987; Table 1). Based on these minimum-limiting radiometric ages together with a variety of relative-age criteria, Kaufman and Hopkins (1986) assigned the Nome River glaciation to the early Pleistocene. In this study, we report the results of 40Ar/39Ar laser-fusion analyses conducted on milligram-quantity bulk subsamples of the Minnie Creek lava that indicate the presence of extraneous 40Ar that rendered conventional K-Ar dates for larger bulk samples of the same flow too old. A younger age for the Nome River glaciation is further supported by a new K-Ar age determination on one of the lava flows at Lava

ALASKAN TABLE Sample No. 84AKn106C 84AKn 106C 87AHP26 84AKn106 84AKn106D 84AKn106A 84AKn106A

1. SUMMARY

Material” WR WR WR Plag WR WR WR

GLACIAL

OF POTASSIUM-ARGON MINNIE

CHRONOLOGY

DATING RESULTS CREEK BASALTS

Labb

K (%)

Weight (9)

IHOGC IHOGC UA USGS IHOGC IHOGC IHOGC

0.42 0.42 0.398 0.128 0.45 0.47 0.47

5.0136 5.0499 4.169 4.7457 5.0579 7.1032

279

FOR THREE

40Ar*C

(mole/g) 5.221 5.235 5.51 1.79 1.357 8.169 1.965

x x x x x x x

lo-” 10-l’ lo-l3 lo-l3 lo-‘2 IOmJ4 10-14

LABORATOFUES

ON

40Ar* (%I

Aged (myr)

ZlU (myr)

9.1 6.8 4.0 2.5 1.3 0.9 0.3

0.717 0.719 0.797 0.810 1.748 0.100 0.024

0.11 0.13 0.06 0.09 0.57 0.16 0.11

Note. Mean, including all analyses: 702,000 f 568,000 yr, n = 7: mean, excluding data in italics: 761,000 * 50,000 yr, n = 4. LIPlag = Phenocrystic plagioclase separate; WR = whole rock. b UA = University of Alaska, Geophysical Institute; USGS = U.S. Geological Survey, Branch of Alaskan Geology; IHOGC = Institute of Human Origins Geochronology Center. ’ 40Ar* = radiogenic 40Ar. ’ Decay constants: A, + A,’ = 0.581 x lo-” yr-‘; A, = 4.962 x lo-” yr-‘; 1 = 5.543 x lo-” yr-‘; and 40K/K total = 1.167 x 10m4.

Creek (Fig. l), where K-Ar whole-rock analysis of normally magnetized basaltic lava that predates the Nome River glaciation yielded a preliminary age of 564,000 * 66,000 yr (University of Alaska, Cooperative Geochronology Laboratory (UACGL) determination 87086; D. Turner, written communication, 1987). These new results, coupled with amino acid epimerization and paleomagnetic criteria, demonstrate that the Nome River glaciation and the subsequent shift from extensive to restricted glacial advances, is considerably younger than 800,000 yr. SITE DESCRIPTIONS Postglacial Lava at Minnie Creek

Hopkins (1963, p. 58) first recognized the presence of a lava flow overlying part of a moraine of Nome River age on the northern flank of the Bendeleben Mountains in his study of the Imuruk Lake lava plateau. He assigned the small (20 km2) flow at the mouth of the Minnie Creek valley to the Gosling Volcanics, a series of basaltic lavas that display a moderate degree of postemplacement surticial modification. The surface of the flow is composed of angular,

frost-rived blocks impregnated with and covered by a 5- to lo-cm-thick, discontinuous mantle of loess. The right bank of Minnie Creek, where the river is diverted westward by the lava, exposes the margin of the flow nearly 8 km from its source to the northeast (Fig. 2). The Minnie Creek site was visited by Kaufman in 1984 and again by P. E. Calkin (State University of New York, Buffalo) and the authors in 1987 to confirm the morphostratigraphic relations. The lava obliterated the eastern segment of a prominent morainal loop at the mouth of Minnie Creek valley and was ponded behind the proximal flank of the moraine. No erratics are found on the lava surface either up valley or down valley from the moraine loop, and basaltic clasts are absent within the moraine segment preserved on the left bank of Minnie Creek. These morphostratigraphic relations unambiguously demonstrate that the eruption of the Minnie Creek lava postdates the moraine. Correlation of the moraine at Minnie Creek with the type moraine of the Nome River glaciation at Nome is based on relative-age criteria including rock-weathering and moraine-morphologic characteristics (Kaufman and Hopkins, 1986), as well as

280

KAUFMAN

ET AL.

II 65’ 27

65 20

Basaltic lava flows

L

EXPLANATION

Lost Jim basalt (Holocene)

Alluvium (Quaternary)

Gosling Volcanics (Pleistocene)

Drift of the Nome River glaciation(Pleistocene)

Imuruk Volcanics (Pleistocene and Tertiary) 2 0 1 Kilometers Contour interval 200 ft (6lm)

of the Nome River or glaciation (Pleistocene) Bedrock (Paleozoic); includes surficial mantle

Surfcial geology of the Minnie Creek area (after Kaufman, 1986; Hopkins, 1963; and this study); map location shown in Figure 1. FIG.

2.

the distinctive position that the moraines share within the sequence of glacial deposits. Hopkins (1953) named the Nome River glaciation for the broad, smooth morainal ridges mantled by a continuous cover of loess that flank the mouth of the Nome River and correlated these features with the innermost of the prominent but subdued moraines found circling the mouths of welldeveloped glacial troughs throughout southwestern Seward Peninsula. The mo-

rainal loop at Minnie Creek, like the type Nome River moraine, is the innermost of the extensive moraines. Interbedded Glacial and Marine Deposits, Nome Coastal Plain The stratigraphic relations between the glacial and marine deposits that mantle the coastal plain at Nome are best expressed near the lower course of the Snake River (Fig. 3). Present-day exposures there pro-

ALASKAN

II

br

GLACIAL

CHRONOLOGY

EXPLANATION Drift of the Nome River glaciation (Pleistocene)

Marine sand and gravel of the Pelukian transgression (Pleistocene)

Drift of a pre-Nome River glaciation (Plio-Pleistocene)

Estuarine sandy silt of the Pelukian transgression (Pleistocene)

Alluvium (Quaternary)

Marine sand and gravel of the Anvilian and Beringian transgressions (Plio-Pleistocene)

Bedrock (Paleozoic)

281

Paleomagnetic sample site

FIG. 3. Simplified surticial-geologic map of the Nome coastal plain; inset shows the stratigraphic relations between glacial and marine deposits near the mouth of Snake River (after Hopkins et al.. 1960). Map location shown in Figure 1.

vide the type localities for the Nome River glaciation and the Pelukian transgression. In dredge pits actively mined in this area during the early 196Os, Hopkins (1967) documented the presence of three interglacial marine units, each containing a distinctive molluscan fauna, separated by two glacial drifts. The younger of the two drifts represents the Nome River glaciation. The drift is composed of boulder-rich, erraticbearing diamicton resting on fossiliferous sorted sand and gravel deposited during the Anvilian marine transgression. The Nome River drift is overlain by well-sorted and

rounded sand and gravel deposited seaward from a rather continuous shoreline scat-p, the base of which lies at an altitude of 10 to 12 m. This upper marine unit constitutes the type locality of the Pelukian transgression, which is correlated with the interval of highest eustatic sea level during the last interglaciation (- 125,000 yr; Hopkins, 1967). Outwash and slack-water deposits laid down during the retreat phase of the Nome River glaciation are well exposed in borrow pits near the mouth of Hastings Creek above the elevation of the Pelukian shore-

282

KAUFMAN

line (Fig. 3). The section largely consists of well-stratified sand and gravel cut by rare ice-wedge pseudomorphs and interbedded with layers of diamicton thought to be resedimented debris flows (cf. Lawson, 1988). GEOCHRONOLOGY K-Ar and 40Ar139Ar Analyses, Creek Basalt

Minnie

Three samples of the Minnie Creek basalt from a 10-m* outcrop of a single alkalineolivine basaltic lava flow (Fig. 2) were selected for K-Ar and 40Ar/39Ar dating: (1) 84AKn106A is massive and fine grained, with phenocrystic olivine, and an intergranular, diktytaxitic texture; (2) 84AKn106C is highly vesicular, with a glassy to hyaloophitic texture; and (3) 84AKn106D is vesicular, with a very fine-grained, intergranular texture. They are all dark gray, with no visible signs of internal alteration. Each sample was crushed and sieved (48-60 mesh), then washed sequentially in dilute HCl, HF, and boiled in distilled H,O, then dried and resieved. K-Ar analyses. Conventional K-Ar dating was performed at the Institute of Human Origins Geochronology Center laboratory to compare with results from previously determined conventional K-Ar ages for the Minnie Creek basalt. The samples were split into two aliquots: one for K analysis by flame photometry, the other for Ar mass spectrometry using a “Reynoldstype” all-glass mass spectrometer. Approximately 5 g of bulk sample were fused for each Ar extraction using a standard RF induction furnace. Overall, five new K-Ar analyses of subsamples of the Minnie Creek basalt combined with two previous measurements produced widely varied results, with ages that range from 20,000 + 110,000 to 1,700,OOO 2 570,000 yr and average 700,000 f 570,000 yr (flu; Table 1). This broad range can in part be attributed to analytical error in measuring small volumes of radio-

ET AL.

genie 40Ar (40Ar*) in the young basalt, and in part to the wide variation in its atmospheric Ar content. The samples are arranged in Table 1 from highest to lowest percentage of 4oAr* content, which is proportional to the quality of the age determination. Samples 84AKn106A and D contain much lower percentages of 40Ar* (0.31.3%) compared to the other samples (2.59.1%) . The corresponding K-Ar ages of the three subsamples of A and D range more broadly than the other four from sample C, forming outliers at either end of the agefrequency distribution (Fig. 4). The four K-Ar ages that compose the central cluster overlap with one another at &la; they include a phenocrystic plagioclase separate (84AKn106) and three whole-rock analyses performed at three different laboratories. Eliminating the three lowest-quality analyses from the data set results in a mean K-Ar age of 760,000 + 50,000 yr for the remaining four, tightly grouped analyses. 40Ar/39Ar analyses. Fourteen-milligramsize whole-rock splits of the three samples of Minnie Creek basalt were analyzed by the laser-fusion 40Ar/39Ar method. For this procedure, samples were irradiated with fast neutrons at the Los Alamos National Laboratory reactor for a short duration (- 15 min) to minimize the interference of Ca-derived 36Ar, and with Cd shielding to reduce the effects of low-energy neutrons. Fish Canyon (27.84 myr) and Bishop Tuff (0.74 myr) sanidines were used as neutronfluence monitors. After irradiation, single feldspar grains of the standards and approximately 6-9 mg of whole-rock basalt (ca. 100 grains, 48-60 mesh diameter) were loaded into small pits (2 mm wide x 3 mm deep) drilled into a Cu disc. The “minibulk” subsamples were fused using a defocused beam from a 6-W continuous Ar-ion laser. An all-metal MAP 215 mass spectrometer was used for Ar mass spectrometry. Advantages of the laser-fusion 40Ar/39Ar method over the conventional K-Ar technique include the following: (1) Dates are

ALASKAN

GLACIAL

based entirely on measurements of Arisotopic ratios in a single sample and do not require a separate determination of K content. (2) Contamination by atmospheric Ar is minimized because sample volume is small and because only the sample is heated during fusion; thus, the percentage of 40Ar* content is typically higher than for conventional K-Ar analyses. (3) Sampling small portions of the rock facilitates the testing of sample homogeneity. (4) The method provides data for the construction of an Arisotope correlation diagram, thus providing a means of evaluating the initial 40Ar/36Ar ratio of the sample. The ability to evaluate sample homogeneity and initial 40Ar/36Ar ratio is particularly important when dealing with rocks that potentially contain extraneous Ar. Because the Minnie Creek basalt erupted through Cretaceous plutonic and lower Paleozoic metasedimentary rocks (Till et al., 1986), the possibility that it contains inherited 40Ar derived from the country rock must be taken into account. The laser-fusion 40Ar/39Ar results form an age spectrum that can be used to identify discrete populations of isotopic compositions that may be ascribed to disparate origins such as primary and inherited phases. The step-heating 40Ar/39Ar technique has also been used to detect the presence of extraneous 40Ar in other rocks (e.g., Lanphere and Dalrymple, 1976); however, this technique cannot unambiguously detect extraneous 40Ar unless it is locked in a phase that releases its Ar at a substantially different temperature than the primary Ar component (e.g., Faure, 1986; McDougall and Harrison, 1988). Similar to the K-Ar results, the 40Ar/39Ar results show an overall wide variation in the isotopic composition of the 16mg-size subsamples with corresponding ages ranging from 120,000 * 380,000 to 1,380,OOO ? 210,000 yr (Table 2; Fig. 4). The mean of the 14 40Ar/39Ar ages is 600,000 + 340,000 yr and overlaps at +lSE with the mean K-Ar age of 700,000 ? 570,000 yr. Given the broad uncertainties, the mean ages pro-

CHRONOLOGY

283

duced by the two techniques are indistinguishable at +0.33SE. A t test used to test the significance of the difference between the means derived from the two techniques indicates that there is insufficient evidence to reject the null hypothesis that they are equal (p = 0.41). Other comparisons have also shown that the two techniques produce equivalent results (e.g., Hall and York, 1978). Also similar to the K-Ar results, the 40Ar/39Ar ages ar e most uniform in sample 84AKnl06C, whereas subsamples of A and D show considerably greater scatter. Although they are statistically indistinguishable, the mean age calculated from six 40Ar/ 39Ar measurements of sample 84AKn106C is 490,000 + 140,000 yr, compared to 680,000 t: 450,000 and 680,000 + 480,000 yr for samples A and D, respectively. The cause of the greater variation in samples 84AKn106A and D is unclear, but it appears to be related to heterogeneities in the isotopic composition of the basalt. Thus, selecting the single most isotopically uniform sample (84AKn106C) would result in an age of 490,000 + 140,000 yr for the Minnie Creek basalt. An alternative means of selecting the geologically meaningful results from the suite of 14 dates involves an evaluation of the age-frequency distribution (Fig. 4). Of the 14 40Ar/39Ar analyses, 2 produced ages that are well outside the +20 range of the remaining 12. We consider this skewed distribution to represent a composite of 2 isotopically distinct phases: a primary population of 12 subsamples that represent the true age of the Minnie Creek basalt, and the 2 older outliers that we attribute to contamination by extraneous 40Ar. The mean age of the 12 subsamples that compose the primary 40Ar/39Ar population is 480,000 ? 190,000 yr, which is indistinguishable from the mean age calculated for sample 84AKn106C alone. Yet another means of evaluating and selecting the 40Ar/39Ar data is through isochron -~ ~~~svstematics. ~, - ~~~~. A plot of 36Ar140Ar ver-

284 TABLE

KAUFMAN 2. @AI-/~~A~

LASER-FUSION

DATA

ET AL.

AND SUMMARY

STATISTICS

IHOGC Lab. #

Sample ID

40Ar/39Ar

37Ar139Ar

36Ar139Ar

1870-01 1870-02 1870-03 1872-01 1872-02 1872-03 1872-04 1872-05 1872-06 1875-01 1875-02 1875-03

84AKn106A 84AKn106A 84AKn106A 84AKn106C 84AKn106C 84AKn106C 84AKn106C 84AKn106C 84AKn106C 84AKn106D 84AKn106D 84AKn106D

17.6140 17.8987 16.3593 6.9019 7.8472 5.4801 6.5012 9.9828 8.0037 54.4626 30.8622 9.4393

1870-04 1875-04

84AKn106A 84AKn106D

14.9412

5.6912 5.1623 6.3031 8.4502 7.4074 8.1692 7.6311 7.5668 7.4308 6.9871 7.1880 7.2600 5.6483

12.0683

7.2140

0.0518 0.0606 0.0504 0.0188 0.0220 0.0137 0.0196 0.0330 0.0231 0.1807 0.1005 0.0301 0.0384 0.0269

Summary statistics, excluding Mean Weighted-mean ageb Standard deviation Weighted uncertaintyb Standard error of meanb % error’ n

data in italics

(samples

40Ar*/39Ar 2.7931 0.4047 1.9879 2.0334 1.9486 2.1162 1.3275 0.8623 1.7882 1.6429 1.7483 1.1396 4.0747

4.7212

CREEK

MAr* (%I

Age“ (myr)

15.8 2.3 12.1 29.3 24.7 38.4 20.3 8.6 22.2 3.0 5.6 12.0 27.2 38.9

0.819 0.119 0.583 0.5% 0.571 0.620 0.389 0.253 0.524 0.481 0.512 0.334

BASALT

1.194 1.384

?lU (wr) 0.240 0.384 0.329 0.175 0.149 0.214 0.114 0.148 0.125 0.233 0.196 0.147 0.257 0.208

1870-04 and 1875-04) 483,000 yr 468,000 yr 186,000 yr 49,000 yr 54,000 yr 10.5% 12

LI Constants as in Table 1; J = 0.0001625; approximate b From Taylor (1982); excludes data in italics (1870-04 ’ (Weighted uncertainty/weighted mean) x 100.

sus 39ArlNAr produces a straight line with an intercept equal to the initial 40Ar/36Ar ratio, and whose X-intercept (40Ar*/39Ar) is a function of sample age. Deviation from this behavior indicates a systematic error, such as the presence of excess or inherited Ar, which would produce initial 40Ar136Ar values significantly different than 295 5. An inverse isotope correlation diagram was constructed using all 14 laser-fusion analyses to test how well the data conform to “ideal” 40Ar/39Ar behavior (e.g., Dah-ymple et al., 1988; McDougall and Harrison, 1988). A linear regression calculated by taking into consideration the correlated errors in x and y (York, 1969) shows that although the initial 40Ar/36Ar ratio is not significantly different from atmospheric, the mean square of weighted deviates (MSWD) is 3.5, suggesting that the tit is inappropriate and that there are nonrandom errors asso-

FOR THE MINNIE

mass of each sample and 1875-04).

is 1 to 3 mg.

ciated with the data (Table 3). MSWD is a reduced x2 variable (x2/n-2) which, by definition, equals 1 if the observed population meets the expected distribution (McIntyre et al., 1966; York, 1969). A second isochron correlation diagram was constructed, this time excluding the 2 analyses that produced dates considerably older than the other 12 (Fig. 5; Table 2). The linear regression for these 12 analyses shows that (1) the initial 40Ar/36Ar ratio is 2% + 4; (2) the isochron age is 466,000 + 55,000 yr, which is indistinguishable from the weightedmean age of the 12 analyses (Table 2); and (3) the MSWD is 1, indicating that the scatter of the data about the line is attributed to random error, probably an analytical uncertainty associated with measuring small volumes of &Ar* in a young basalt. Thus, the 40Ar/39Ar ages derived from three independent criteria are indistinguish-

ALASKAN

GLACIAL

$4 6 3 2 lE

2

, Ob 0;

Oh 0:s 0.; Age

1:O 13 14 lb

ii

(myr)

FIG. 4. Frequency distribution of conventional K-Ar and laser-fusion 40Ar/39Ar age determinations on the Minnie Creek basalt.

able. The age of the most isotopically uniform sample (490,000 2 140,000 yr), the age of the 12 primary-population subsamples (480,000 + 190,000 yr), and the isochron age (466,000 + 55,000 yr) all overlap well within ?lSE, and their means differ by no more than 5%. We have elected to use the 12 primary-population subsamples to calculate the best age of the Minnie Creek basalt. By maximizing the available data, the most accurate measure of the central tendency can be obtained. The appropriate measure is provided by the weighted mean, which takes into account the uncertainty associated with each analysis (Taylor, 1982); the weighted mean of the primary population is 470,000 yr (Table 2). The appropriate statistic used to quantify the dispersion of a data set of unequal quality is given by the TABLE

3. RESULTS OF THE

A~SOTOPE

tl

MSWD”

12 14

1.05 3.51

D MSWD

Initial 40Arl’6Ar 296.2 296.4

REGRESSION

*la 3.8 3.8

Isochron age (myr)

+lU (myr)

0.466 0.535

0.055 0.087

= mean square of weighted deviates.

CHRONOLOGY

285

weighted uncertainty (Taylor, 1982), which for the Minnie Creek basalt is ~50,000 yr. However, because the mean age is based on rather broad-ranging analytical results, and because of the demonstrated sample inhomogeneities, we adopt the standard deviation (2 190,000 yr) as a reasonable and conservative measure of the uncertainty. These results demonstrate that, through the use of laser-fusion 40Ar/39Ar dating of milligram-size basalt samples, extraneous 40Ar enrichments can be recognized if the domains of this contamination are discrete and inhomogeneously dispersed. In the case of the Minnie Creek basalt, this procedure results in an age estimate of 470,000 + 190,000 yr. Although this value overlaps with the mean K-Ar age of 700,000 2 570,000 yr on larger bulk samples, the 40Ar/ 39Ar results suggest that the mean of the conventional K-Ar is erroneously old due to the presence of extraneous 40Ar. Paleomagnetic Determinations

Oriented samples of Nome River drift were collected from the type locality of the Nome River glaciation near the mouth of the Snake River (Fig. 3) and at Hastings Creek to determine the polarity of the magnetic field at the time of deposition. At the type section, five replicate samples of basal till were taken from an outcrop area covering
286

KAUFMAN

’ 0.00

.

’

’ 0.05

’

ET

’

.

AL.

’ 0.10

’

3

’

’ 0.15

’

.

’

0.20

39Ar/ 4uAr FIG. 5. Inverse 40Ar/39Ar correlation text and given in Table 3.

diagram with linear regression. Results are explained in the

ranged from 35” to 77” with a moderately steep median value of 62” (Table 4). The positive inclinations measured in all 12 samples indicate that Earth’s magnetic field during the Nome River glaciation was normal, as it is today. Previous analyses on lava from Lava Creek (Huston et al. 1990) and Minnie Creek (Kaufman and Hopkins, 1986) demonstrated that both flows are also normally magnetized.

marine transgression and for 22 shells of these 2 genera from Pelukian deposits (Table 5). To avoid taxonomic differences in the rate of epimerization, particularly during the initial stages of epimerization, results from the two genera are considered separately. The aIle/Ile ratios were determined from electronically calculated detection-peak heights measured in the total (acid-hydrolysate) amino acid population.

Amino Acid Epimerization Analyses

Results of amino acid epimerization analyses are used to constrain the timing of the Anvilian high-sea-level interval which predates the Nome River glaciation. The procedure for amino acid analysis involves the separation of the amino acid o-alloisoleutine (aIle) from its diasteriomer L-isoleutine (Ile) using ion-exchange liquid chromatography. The ratio of these two amino acids (aIle/Ile) is a measure of the extent of isoleucine epimerization and provides a value that is proportional to both the age of the fossil and the integrated temperature since deposition (see Miller and BrighamGrette, 1989 for a recent review). Amino acid analyses have been completed for 21 shells of the molluscan genera Hiatella and Mya collected from deposits of the Anvilian

TABLE

4. MAGNETIC SAMPLES

Sample” Snake R-l Snake R-2 Snake R-3 Snake R-4 Snake R-5 Hastings Cr-1 Hastings Cr-2 Hastings Cr-3 Hastings Cr-4 Hastings Cr-5 Hastings Cr-6 Hastings Cr-7

ORIENTATIONS OF NOME RIVER

Inclination (“I 69 68 71 77 89 36 66 67 54 77 62 35

OF REPLICATE DRIFT

Declination (“I 147 14 325 42 175 -

a Snake River and Hastings Creek samples were demagnetized alternating magnetic fields of 150 and 200 +e, respectively; sample-site locations shown in Figure 3.

ALASKAN

GLACIAL

TABLE

5. EXTENT OF ISOLEUCINE EPIMERIZATION @Be/Be) IN THE TOTAL AMINO ACID FRACTION OF ISOLEUCINE IN THE MOLLUSCAN GENERA Hiatella AND Mya aIle/Ile Age

Mya Pelukian Anvilian Hiatella Pelukian Anvilian

(total)

Number of samples

Number of analyses

Mean

-tlU

0.041 0.101

0.007 0.007

16 8

53 28

0.055

0.010 0.015

6 13

19 39

0.120

a Values for the interlaboratory comparison standards IlICL-A, B, and C (Wehmiller, 1984) measured in the INSTAAR Laboratory are 0.16, 0.52, and 1.10, respectively (weighted means of values reported in Miller and Mangerud, 1986).

Each sample was analyzed at least three times; the between-run analytical reproducibility, as measured by the coefficient of variation (U/X), was typically between 1 and 5%. The mean value of replicate runs was used to calculate the group means and standard deviations listed in Table 5. The equation describing the relationship between time, temperature, and aIle/Ile in the total amino acid fraction in the genus Mya given by Miller (1985, Eq. 14.7) is t = (ln((1 + aIle/Ile)/((l - 0.77)aIle/Ile)) -0.0194)/(1.77(1Oexp(16.45 -(6141/T)))) (1) where t time in years and T is temperature in “K. Because the rate of epimerization is strongly temperature dependent, the postdepositional temperature history experienced by the fossil must be known to calculate an age based on an aIle/Ile ratio. Although this is not possible using currently available proxy data, reasonable limits to the thermal history can be constructed and used to constrain the probable age range of the deposits. The complex temperature history experienced by a fossil is expressed as the effective diagenetic temperature (EDT), which is a measure of the integrated effect

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of postdepositional thermal conditions on the rate of epimerization. The EDT calculated for fossil Mya from the Pelukian (125,000 yr) deposits using the mean aIle/Ile ratio of 0.041 is -7.5”C. This is 4°C lower than present mean annual air temperature at Nome. As a first and best approximation for the age of the Anvilian transgression, we assume that the temperature history experienced by the Anvilian fossils is not significantly different than the average for the last glacial/interglacial cycle. Applying the mean EDT (-7.5”C) to the mean aIle/Ile ratio measured in the Anvilian Mya fossils (0.101) yields an age estimate of 400,000 yr for the Anvilian marine transgression. Because the temperature history experienced by the Pelukian deposits may not be strictly representative of the Anvilian deposits, and because the rate of epimerization increases exponentially with temperature, it is important to consider a range of temperature reconstructions on the calculated age of the Anvilian deposits. The ? la error about the mean Pelukian aIle/Ile ratio (0.034 to 0.048) was used to calculate a reasonable range of EDTs for the Anvilian deposits. This range (- 9.0” to - 6.3”C) falls within ? 1.5”C of the mean EDT calculated for the late Quaternary. Applying the range of calculated Pelukian EDTs to the range of measured Anvilian aIle/Ile ratios, including the +la error, results in an age range of 290,000 to 580,000 yr (Fig. 6). Thus, the amino acid age estimate for the Anvilian transgression is expressed as 400,000 + 180,000/- 110,000 yr. DISCUSSION Age of the Nome River Glaciation and the Anvilian Marine Transgression

Taken together, the convergence of results from a variety of geochronologic methods allows the age of the Nome River glaciation to be placed securely within the middle Pleistocene. The new 39Ar/40Ar date on the lava flow at Minnie Creek indicates

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200 0 -12

-10

-6

-2 EtiGT

0

("cy4

FIG. 6. Relation between amino acid age estimate and effective diagenetic temperature (EDT) based on the mean Anvilian aIle/Ile ratio in Mya (bold line). The width of the confidence band (narrow lines) is constrained by the -tlo uncertainty about the mean Anvilian aIle/Ile ratio. The shaded area represents the probable time/temperature range for the Anvilian deposits. It is delimited by the vertically dashed lines, which constrain the EDT range calculated based on an age of 125,000 yr and the alo uncertainty in the Pelukian aIle/Ile ratios. The open circle represents the “best” age estimate (400,000 yr) based on the mean late Quatemary EDT and the mean Anvilian aIle/Ile.

that the underlying moraine of Nome River age is older than 470,000 + 190,000 yr. Because Nome River till was deposited during an interval of normal magnetic polarity, and because the amino acid data preclude an age as old as Jaramillo for the underlying Anvilian marine deposits, the glaciation can be no older than the beginning of the Bruhnes normal-polarity chron, thus no older than -730,000 yr. The new conventional K-Ar date on lava underlying Nome River drift in Lava Creek valley indicates that the Nome River glaciation took place some time after 564,000 + 66,000 yr. However, until more thorough geochronological studies are complete, we consider this a provisional age. Although provisional, the demonstrated potential for extraneous 40Ar enrichments in basalts of the area indicates that this age is a maximum estimate, and thus a valid maximum-limiting age for the glaciation. The maximum-limiting K-Ar date falls within the age range (400,000 + 180,000- 110,000 yr) calculated for the pre-Nome River, Anvilian transgression us-

ET AL.

ing amino acid epimerization data, together with reasonable temperature reconstructions. Thus, taking into consideration the analytical uncertainties, the age of the Nome River glaciation lies between 580,000 yr (the maximum amino acid age estimate for the Anvilian transgression) and 280,000 yr (the minimum 40Ar/39Ar age estimate for the Minnie Creek basalt taken with a broad estimate of the uncertainty). The oxygen-isotopic record from deepsea sediment cores offers an independent means of evaluating the age of the Nome River glaciation. aI80 fluctuations in foraminimeral tests are chiefly controlled by the growth and decay of continental ice sheets which, in turn, modulate sea-level oscillations. Thus, variations in S’*O reflect globally integrated changes in ice volume and are insensitive to fluctuations in extent of localized mountain glaciers, such as those of the Nome River glaciation. In fact, the build up of an ice sheet over northern North America may actually inhibit the growth of ice over much of Alaska, having the direct effect of diverting the westerlies southward (e.g., Broccoli and Manabe, 1987) as well as the indirect effect of drawing down sea level to expose vast areas of continental shelf, displacing available moisture. Rather than correlating the Nome River glaciation to a period of global ice buildup (i.e., isotope maximum), we prefer to use the deep-sea 6180 record as a proxy for sea-level fluctuations, an effect registered along the northern shore of Bering Sea equally as around the shores of the world’s oceans. Therefore, the 6180 record, combined with consideration of shoreline altitudes, provides an independent age assessment for the Anvilian marine transgression, and thus, the maximum age for the Nome River glaciation. In an attempt to refine the oxygenisotope record as a proxy for sea level, Shackleton (1987) combined highresolution records from both benthic and planktonic foraminifera to deconvolve the effects of temperature and ice volume on

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the oxygen-isotope signal. His analysis suggests that sea level was never as high as present during the interval between oxygen-isotope stages 23 and 11 (900,000 to 410,000 yr) during which time isotopic values remained significantly heavier than present. Therefore, the oldest emerged shoreline formed on slowly uplifting coasts during the last 900,000 yr is expected to correlate with stage 11 at 410,000 yr. The suggestion that the Nome coastal plain has undergone only minor uplift since the middle Pleistocene is supported by the elevation of the last interglacial (Pelukian) shoreline which lies at 7 to 10 m, within the range suggested to have been reached by eustatic sea level during this interval (6 m, Bloom et al., 1974, to 15 m, Hollin and Hearty, 1990). Assuming that the maximum amount of uplift experienced by the Pelukian (125,000 yr) shoreline is 4 m (the difference between maximum shoreline altitude and the minimum eustatic sea-level estimate), and by extrapolating this uplift rate to stage 11 time, suggests that the Anvilian deposits have undergone no more than 13 m of uplift. The Anvilian deposits on the Nome coastal plain extend up to an altitude

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of 22 m (Hopkins, 1%7), and can be traced over 30 km without recognizable change in altitude (Hopkins et al., 1960). This coincides with the elevation of shoreline features of the Wainwrightian transgression (23 m) in northern Alaska (Carter et al., 1986) that is correlated with the Anvilian transgression (Kaufman et al., 1989; Kaufman, 1991). Thus, we conclude that during the Anvilian transgression, eustatic sea level rose at least as high and probably higher than during the last interglacial (Pelukian) transgression, and that, by correlation with the oxygen-isotope record, this high stand probably correlates with stage 11 at 410,000 yr. Figure 7 summarizes the age constraints on the Nome River glaciation; it illustrates our preferred age alternative predicated on the assumption that the Anvilian transgression corresponds to stage 11. This alternative is supported by the mean amino acid age estimate, as well as shared evidences of substantially warmer-than-present water temperatures in the diatom record of the North Pacific (Sachs, 1973) during stage 11, and in the molluscan fauna of the Anvilian beds at Nome (Hopkins, 1967). We recog-

Prefered age

I

I’“” -1’0I II >“I1

H Minnie Creek basalt bme River glaciatic Anvilian traosgressio

NLava

Creek basalt

FIG. 7. Summary diagram showing stratigraphic correlations between Nome River drift at dated sites on southwestern Seward Peninsula (NCP = Nome coastal plain; MC = Minnie Creek; LC = Lava Creek), along with the deep-sea oxygen-isotope record (from Imbrie et al., 1984; units in standard deviations; odd numbers are interglacial oxygen-isotope stages). Dates are in thousands of years; N = normal magnetic polarity. The preferred age assignment for the Nome River glaciation hinges on the correlation of the Anvilian transgression with stage 11. See text for complete explanation.

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nize, however, that correlations between the Anvilian transgression and other isotopic minima (i.e., stages 13, 11, or 9) are within the tolerance of the uncertainties associated with the age constraints. Correlations with Other Sites around Northern Bering Sea Glacier ice thought to be correlative with the Nome River glaciation overran marine sediments on Baldwin Peninsula south of Kotzebue and on the northwest coast of St. Lawrence Island (Fig. 1). Amino acid analyses on molluscan shells and paleomagnetic determinations on silty marine sediments of the glaciotectonized Cape Blossom Formation exposed on Baldwin Peninsula (Huston et af., 1990) indicate that this marine unit is no older than and most likely coeval with the Anvilian marine transgression, as defined by the type locality at Nome (Kaufman et al., 1989; Kaufman, 1991). The Cape Blossom Formation is overlain by glaciomarine and glacial drift of the Hotham Inlet Formation, a sedimentary sequence that records the advance of an extensive valley-glacier system from the western Brooks Range into Kotzebue Sound (Huston et al., 1990). Similar to the southwestern Seward Peninsula, the mountains of the western Brooks Range supported more restricted advances subsequent to this extensive glacial expansion. Field work by Brigham-Grette and Hopkins in 1988 confirmed the finding of Hopkins et al. (1972) that during the middle Pleistocene, glaciers advanced from the mountains of Chukchi Peninsula, northeastern Siberia, crossed Anadayr Strait, and terminated on St. Lawrence Xsland (Fig. 1). Although the details will be provided elsewhere, molluscan shells collected from marine sediments overlain by shelly drift near the former ice margin have aIle/ Ile ratios similar to those in shells from the Anvilian deposits at Nome. Because longterm EDTs at Nome and St. Lawrence Island are probably similar (current mean annual temperatures are equivalent), the

ET AL.

amino acid data can be used to correlate the marine beds on St. Lawrence Island with the Anvilian transgression. Future work on the Chukchi Peninsula is planned to test whether the overlying drift is in fact correlative with the Nome River glaciation on Seward Peninsula. Paleoclimatic Significance River Glaciation

of the Nome

Although the difference in the areal extent of ice is striking, a more accurate appraisal of the paleoclimatic contrast represented by the Nome River glaciation and later glacial advances in the Bering Sea region is based on estimates of reconstructed regional snowline depression. Amphitheater forms showing concordance of floor altitudes and preferential northward orientation are common throughout the rolling uplands between the Kigluaik Mountains and the southern coast, beyond and below the limits of any late Pleistocene ice advance. They are interpreted to be ancient cirques last occupied by ice during the Nome River glaciation (Kaufman and Hopkins, 1986). The altitudes of the cirque floors provide an indication of the amount of snowline lowering during Nome River time relative to the late Pleistocene depression. The average altitude of cirque floors active during the Mount Osborn glaciation (late Wisconsin) is about 500 m; those active during the Nome River glaciation lie at 200 m. Based on scanty data from active cirque glaciers in the Kigluaik Mountains, modern regional snowline appears to lie at about 670 m (Kaufman and Hopkins, 1986). During the 1986 balance year, mass-balance studies indicated that the equilibrium line on the largest of these cirque glaciers lies at 750 m (Przybyl, 1988). Based on these two values, modern snowline altitude is near 700 m; thus, the altitudinal difference between modem snowline and cirque floors of Nome River age is more than twice as large as the difference between modem snowline and cirque floors of Mount Osbom age. Assuming that the difference between paleo-

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GLACIAL

snowline and cirque-floor altitude is equivalent for both glaciations, then this value can be used as a proxy measure for the relative difference in paleosnowline depression. If the basins are in fact Nome River cirques, then regional snowline was apparently depressed at least twice as much during the Nome River glaciation as during the last glacial maximum. This conclusion is consistent with that of PBwC (1975, p. 21), who found that “Illinoian” (considered to include Nome River time) snowline lay approximately 500 to 600 m below present snowline and about 150 to 250 m below Wisconsin snowline. He noted (as did Hamilton and Thorson, 1983) that in western Alaska, glaciers were vastly more extensive during the earlier advance than during Wisconsinan time, and that the difference decreases eastward. The cause of the enhanced snowline lowering during Nome River time remains unknown, but appears to be related to greater availability of moisture in the northern Bering Sea region. A variety of paleoclimatic evidence demonstrates that the environ,ment of Beringia during the last glacial maximum was extremely cold and arid (Hopkins, 1982; Carter, 1981). Glacier expansion during this interval must have been moisture limited. Hopkins (1973) first proposed that a seasonally open water body over the Bering-Chukchi shelf could have provided the moisture source necessary to generate the great mass of ancient glacier ice. The recent discovery of glacial marine sediments in the Kotzebue Sound area that were probably deposited during the Nome River glaciation and concurrent with eustatically high sea level (Brigham-Grette and Hopkins, 1989; Huston et al., 1990), lends support to Hopkins’ hypothesis. Enhanced snowline lowering during Nome River time may haveresulted from a rare combination of simultaneous high eustatic sea level and lower-than-present, highlatitude insolation that has not occurred since. Alternatively, changes in the extent or duration of sea-ice cover over the Arctic

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Ocean or orographic effects related to uplift of the Alaska Range and Coast Ranges may have altered atmospheric boundary conditions so as to limit the penetration of southerly storm tracks to northwestern Alaska following the Nome River glaciation. ACKNOWLEDGMENTS We are indebted to N. Shew, D. Turner, and F. Wilson for providing the initial K-Ar dates on the Minme Creek and Lava Creek basalts; to G. Miller for providing access to the INSTAAR amino acid laboratory; E. Larson for overseeing the paleomagnetic analyses of the Snake River samples; M. Huston for analyses of the Hastings Creek samples; and T. Becker and A. Jaouni for the new K-Ar analyses and sample preparation. J. Aronson, D. Carter, B. Dalrymple, S. Forman, P. Managa, G. Miller, S. Porter, R. Thorson, and C. Waythomas offered helpful comments on an earlier manuscript. Financial support was provided by P. Calkin (NSF grant DPP-8412897) and A. Till (USGS) for visits to Minnie Creek; NSF grant DPP87147 to Brigham-Grette and Hopkins for work around northern Bering Sea; National Geographic Society to Hopkins and his students for restudy of Lava Creek valley in 1989; Geological Society of America for offsetting costs of Ar/Ar analyses. This work is part of Ph.D. research supported by the U.S. Minerals Management Service through contract 1412-001-30460 to Kaufman.

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Przybyl, B. J. (1988). “The Regimen of Grand Union Glacier and the Glacial Geology of the Northeastern Kigluaik Mountains, Seward Peninsula, Alaska.” Unpublished M.Sc. thesis, State University of New York, Buffalo. Sachs, H. M. (1973). Late Pleistocene history of the North Pacific: Evidence from a quantitative study of radiolaria in core V21-173. Quaternaty Research 3, 89-98. Shackleton, N. J. (1987). Oxygen isotopes, ice volume and sea level. Quaternary Science Reviews 6, 1831%. Taylor, J. R. (1982). “An Introduction to Error Analysis: The Study of Uncertainties in Physical Mea-

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surements.” Oxford University Press, London/New York. Till, A. B., Dumoulin, J. A., Gamble, B. M., Kaufman, D. S., and Carroll, P. I. (1986). Preliminary geologic map and fossil data from Solomon, Bendeleben, and southern Kotzebue quadrangles, Alaska. U.S. Geologicai Survey Open-File Report 86-276. Wehmiller, J. F. (1984). Interlaboratory comparison of amino acid enantiomeric ratios in fossil Pleistocene mollusks. Quaternary Research 22, 109-120. York, D. (1969). Least squares fitting of a straight line with correlated errors. Earth and Planetary Science Letters 5, 32%324.