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Latest Pleistocene increase in wind intensity recorded in eolian sediments from central Alaska
QUATERNARY
RESEARCH
34, 160-168
Latest Pleistocene
(1990)
Increase Sediments
NANCY BIGELOW,*
in Wind Intensity Recorded from Central Alaska
in Eolian
JIM BEGOT,? AND ROGER POWERS*
*Department of Anthropology, University of Alaska, Fairbanks, Alaska 99775: and fDepartment and Geophysics and University of Alaska Museum, University of Alaska, Fairbanks, Alaska
of Geology 99775
Received March 23, 1989 A brief increase in wind intensity between ca. 11,100 and 10,700 yr B.P. is recorded by a sharp increase in sediment grain size at eolian sections along the Nenana River in central Alaska. This occurred at the same time as the Younger Dryas climatic reversal in northern Europe and an increase in the vigor of atmospheric circulation recorded by Greenland ice cores. Climatic fluctuations in high latitude areas during Younger Dryas time may reflect variations in the COZ content of the atmosphere. D 1990 University of Washington.
ducing windy conditions in mountain passes and valleys to the north, and depositing sand and loess on bluffs and terraces. Deposits of eolian sediments in the Nenana River Valley and other valleys north of the Alaska Range are well-positioned to record variations in wind intensity and storminess in central Alaska due to climatic changes (Thorson and Bender, 1985). The sections in the Nenana River Valley consist primarily of silts, described in the field as loess, with intercalated sandy deposits and organic-rich paleosols. Paleosols are associated with more fine-grained sediments, suggesting that they are formed during warm intervals characterized by low wind intensity. Sediments containing abundant sand are restricted to the upper parts of the sections, except for a prominent sand layer found near the base of sections which has been radiocarbon dated as late Pleistocene in age.
INTRODUCTION
Eolian sediments can accumulate in sheltered environments for thousands of years and in some cases contain relatively continuous records of environmental change. Detailed studies of eolian sediments accumulating on bluffs along the Nenana River in central Alaska reveal systematic patterns of grain-size changes, which we interpret as proxy records of shifts in wind intensity and storminess due to climatic change. New radiocarbon dates from eolian sediments at the Walker Road archaeological site, together with radiocarbon dates from the Dry Creek site (Thorson and Hamilton, 1977), provide chronologic control and allow correlation between postglacial stratigraphic units in the Nenana Valley (Fig. 1). A similar sequence of stratigraphic units suggesting that the sediments record regional environmental changes can be identified at both the Dry Creek and the Walker Road sites, as well as at other sites we have examined in the Nenana River and Teklanika River areas north of the Alaska Range (Powers and Hoffecker, 1989). The Alaska Range constitutes a high topographic barrier that often separates polar air masses from warmer air to the south. In the spring and summer, storms from the Gulf of Alaska cross the Alaska Range, pro-
PROXY RECORD OF AN INCREASE IN WIND INTENSITY CA. 11,000 YR B.P.
A prominent sand layer (Sl) found near the base of the Dry Creek section is bracketed by radiocarbon dates of 11,120 ? 85 and 10,690 ? 250 yr B.P. (Fig. 2 and Table 1). A thermoluminescence date of 11,350 ? 1000 yr B.P. has also been reported for silts 160
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ALASKAN
YOUNGER
DRYAS
REVERSAL
A Archaeological
KILOMETERS
1. Map
showing
site
o Town
0-o
FIG.
161
location
of eolian
under Sl (Table 1). Such sand layers can form only when winds are strong enough to entrain and sweep sand from the underlying Pleistocene sediments or from the bed of Dry Creek up a bluff standing 25 m above the creek (Thorson and Hamilton, 1977). At other times eolian deposition is dominated by silts. The sand layer therefore records a brief period of high wind intensity. Layer Sl at Dry Creek consists of more than 90% well-sorted sand and is the coarsest deposit at the Dry Creek archaeological site. It was “deposited from an actively moving sand sheet during a time when strong surface winds swept the bluff,” and can be identified in other eolian sections on Pleistocene terraces in the Nenana River Valley (Thorson and Hamilton, 1977, p. 160). It contains sand as large as 0.5 phi in
sections
along
the Nenana
River,
Alaska.
diameter and has a modal diameter of 2.G 2.5 phi. It is much coarser than the surrounding loess layers, whose modal diameter is 3.5 phi. No primary sedimentary structures are visible in the loess or sand (Thorson and Hamilton, 1977). The change from predominant deposition of loess from dust clouds to sand from a sand sheet and back to loess reflects a significant environmental change. The contacts between Sl and L3 and L2 above and below it are sharp, suggesting that environmental changes occurred rapidly. The radiocarbon dates suggest that layer Sl formed during an interval lasting approximately 400 yr. Radiocarbon dates collected through the Dry Creek section show that eolian sediments have accumulated at this location
0
FIG.
DRY CREEK 2. Comparative lithostratigraphy,
Unit
50
75
25
25
50
Sand 50
(Xl 75 loo
Immature Tundra
so11
Subarctic Brown soil
KEY
WALKER
ROAD
granulometry, and chronology for eolian sections at Dry Creek and Walker Road.
25
.‘: L5 bottom ..; . . : .‘,‘_ .L ,.;: .; ,‘: : . L4 ,:;,:,. I, .:: ::. : : : ; ‘: ,. .. :‘: .. ,..,‘. L3 .’ . :
_‘,.
100
Sand (So 50 75 100 : ,. :.: ‘... .‘. :,.;; :.. . . ., .,. :; ‘,.‘. 54 : ., ,. ,.. :.:.,.y ‘,‘.‘... . 25
ALASKAN
TABLE
1. RADIOMETRIC
YOUNGER
DRYAS
DATES FROM DRY CREEK AND WALKER ROAD, CENTRAL ALASKA
Age (yr B.P.)
Material dated
Provenience
Dry creek SI-1933A SI-1933B SI-2333 SI-2332 SI- 1934 SI-1937 SI-2331 SI-1935C SI-1935B SI-2115 SI-1935A SI-1544 SI-2328 SI-2329 SI-1936 SI-1938 SI-1561 SI-2880 ITL-6 1
Modem 375 2 40 1145 f 60 3430 -t- 75 3655 f 60 4670 f 95 6270 k 110 6900 +- 95 8355 k 190 8600 k 460 10,600 + 580 19,050 + 1500 7985 ‘- 105 9340 * 195 12,080 -c 1025 23,930 k 9300 10,690 k 250 11,120 2 85 11,350 + 1000
Charcoal Peat and roots Charcoal Charcoal Charcoal Charcoal Charcoal Charcoal Charcoal Charcoal Charcoal Charcoal Charcoal Charcoal Charcoal Charcoal Charcoal Charcoal Loess
Paleosol 4b Paleosol 4b Paleosol 4b Paleosol 4a Paleosol 4a Paleosol 4a Paleosol 3 Paleosol 3 Paleoso13 Paleosol 3 Paleosol 3 Paleosol 3 Paleosol 2? Paleosol 2 Paleosol 2 Paleosol 2 Paleosol 1 Loess 2 Loess 1
Walker road AA-1693 GX-12875 AA-1692 AA-1683 AA-1681 AA-2264 S-11254
3816 4415 8720 11,010 11,170 11,300 11,820
Charcoal Charcoal Charcoal Charcoal Charcoal Charcoal Charcoal
Paleosol 2 Paleoso12 Paleosol 1 Loess 2 Loess 2 Loess 1 Loess 1
Lab No.
Note.
163
REVERSAL
-c 79 * 95 2 250 rt 230 2 180 +- 120 ? 200
Sources: Thorson and Hamilton,
Comments
Forest fire Forest fire Forest fire
Coal contamination? Small sample size Test pit, correlation not clear Large counting error Small sample size Cultural remains Cultural remains Thermoluminescence date AMS date AMS date AMS date, cultural remains AMS date, cultural remains AMS date, cultural remains Cultural remains
1977: Powers and Hoffecker, 1989.
since the latest Pleistocene. However, several age reversals are present in the series of radiocarbon dates, possibly because of contamination by coal dust from nearby Tertiary coals (Thorson and Hamilton, 1977). In order to minimize possible problems due to contamination, 14C accelerator mass spectrometry (AMS) dating was used where appropriate in an ongoing geoarchaeologic study of the Walker Road site (Fig. 1). A similar sequence of loess, paleosol, and sand horizons is present at this locality, although a thick Pleistocene cover sand or colluvium is also locally preserved at the bottom of the section about 60 m from the bluff face. At the bluff face, a relatively coarse sand layer intercalated between two loess units occurs near the base of the sec-
tion (Fig. 2). This layer of sand, which is correlated with Sl at Dry Creek, has a modal grain size of 3.0 phi and lies between loess layers with modal grain sizes of 5.5 phi. This sand is massive with abrupt upper and lower contacts. The sand layers at Walker Road, like those at Dry Creek, must have formed during periods of very strong winds when sand was blown up a high bluff to the site of the eolian deposits, while the loess horizons record deposition predominantly from silt-rich dust clouds. Three AMS dates and one conventional radiocarbon date obtained on organic material collected from immediately beneath the lowest sand layer at Walker Road (Table 1) confirm a correlation with layer Sl from the Dry Creek site and support the validity of the older conventional radiocar-
164
BIGELOW,
BEGfiT,
bon dates on S 1. Although the conventional radiocarbon date at Walker Road is slightly older than the three AMS dates, the internal consistency and excellent agreement of the radiocarbon dates from Walker Road with those obtained at Dry Creek suggest that these dates are reliable. The dating from both sections suggests that wind intensity in the Nenana River canyon jumped to a significantly higher level between approximately 11,100 and 10,700 yr B.P. Sl has been recognized at one other Nenana Valley locality, the Panguingue Creek site (Fig. 1). The stratigraphy at this locale is similar to the stratigraphies at Dry Creek and Walker Road. A l-m-thick Pleistocene cover sand or colluvium resembling that at Walker Road lies at the bottom of the section with a loess unit overlying it. This loess unit (L2) is in turn covered by a lo-cm-thick sand horizon (Maxwell, 1987). We correlate the sand above L2 with Sl at Dry Creek and Walker Road. An AMS date of 10,180 ? 180 yr B.P. (AA-1686) on organic material just above the sand layer places it within the right chronological framework. In addition, the sand is similar to the Dry Creek Sl with a modal diameter of 2.0 phi and is surrounded by loesses with modal diameters of 4.0 and 4.5 phi. In addition to being present in eolian sediments in the Nenana Valley, layer Sl has been identified in the Teklanika Valley some 30-40 km to the west. At the Owl Ridge site, sand layer Sl is as much as 10 cm thick, with bracketing radiocarbon dates of 9325 5 305 yr B.P. (GX-6283) on peat and organic matter above and 11340 + 150 yr B.P. (B-11209) on charcoal below it (Phippen, 1988). Layer Sl has been recognized in archaeological sites over an area of approximately 800 km2 in central Alaska. POSSIBLE YOUNGER
CORRELATION WITH THE DRYAS CLIMATE EVENT
The period of high wind intensity and storminess recorded by coarse sediment layers in eolian sections of latest Pleistocene age in central Alaska is approximately
AND
POWERS
the same age as Younger Dryas cooling reflected by many sorts of proxy climatic records in Europe and around the northern Atlantic Ocean (Broecker et al., 1988). Ice core records of stable isotope changes in precipitation from Greenland document Younger Dryas cooling, and suggest that the Younger Dryas event lasted less than 1000 yr and ended abruptly by 10,700 yr ago (Dansgaard, 1987; Paterson and Hammer, 1987; Dansgaard et al., 1989). An age of 10,750 + 50/-150 yr B.P. has also been suggested for the end of the Younger Dryas interval on the basis of the revised Swedish varve chronology (Stromberg, 1985). A high-resolution pollen study by Lowe (1981) indicates that the Younger Dryas event in Wales began about 11,160 ‘-c 90 yr B.P., a value in good agreement with dates from elsewhere in the British Isles and Scandinavia (Mangerud et al., 1974). Some studies suggest the Younger DryasPreboreal transition in northern European pollen cores occurs at about lO,OOO-10,150 yr B.P. (Anderson, 1981), although in some sequences the transition appears to occur as early as 10,500 yr B.P. (Lowe, 1981). It has been suggested that the Younger Dryas Stade lasted from 11,OOO-10,500 yr B.P., after which “marked thermal improvement” occurred (Lowe and Gray, 1980). Numerous radiocarbon dates associated with moraines deposited during readvances of the Scandinavian Ice Sheet in northern Europe fall between 11,300 and 10,000 yr B.P. (Anderson, 1981). In general, the age of the termination of the Younger Dryas event in ice cores seems to be slightly earlier than that recorded by radiocarbon dates from pollen cores or moraines. This may reflect a systematic error in radiocarbon dates of ice-core chronologies, or it may be due to a more sensitive and rapid response to environmental changes in ice cores than in pollen cores or glaciers. Nevertheless, the timing and the duration of the increase in wind intensity recorded in eolian sections in central Alaska seems to be coincident with the
ALASKAN
YOUNGER
Younger Dryas climatic event of northern Europe. A marked increase in atmospheric circulation intensity during the Younger Dryas is indicated by an increase in dust particle concentration in ice cores. In the Dye 3 core, high-resolution studies show that the decrease in oxygen-isotope values during Younger Dryas time is mirrored by a simultaneous increase in dust content to values six times greater than those of the Holocene background (Hammer et al., 1985; Paterson and Hammer, 1987; Dansgaard et al., 1989). The particulate matter in Greenland ice cores is thought to be largely derived from low- and mid-latitude deserts (Harvey, 1988), so the increase in ice core dust must represent a change in atmospheric circulation intensity over a very large area. A late Pleistocene increase in dust content and an oxygen isotope decrease in the Dome C Antarctic ice core may also be correlated with the Younger Dryas climatic event (Lorius et al., 1979; Petit et al., 1981; Briat et al., 1982; Jouzel et al., 1987).
The well-documented general increase in atmospheric circulation intensity recorded for the younger Dryas by the ice-core data may be directly related to the local increase in wind intensity and storminess in central Alaska which is recorded in eolian deposits in sections along the Nenana River. Possibly a brief expansion of ice and snow cover also occurred at this time in the Alaska Range (see below), producing high katabatic winds (Thorson and Bender, 1985). EVIDENCE FOR HIGH-LATITUDE CLIMATIC CHANGE IN THE NORTHWEST PACIFIC OCEAN AND ALASKA DURING YOUNGER DRYAS TIME
Recent high-resolution studies of isotopic variations in several species of foraminifera and micropaleontological studies of temperature-sensitive coiling ratios of N. pachyderma in northwest Pacific marine core CH84-14, combined with AMS dating
DRYAS
REVERSAL
16.5
of hand-picked foraminiferal samples, show that “a drastic climatic deterioration is well marked in both climatic records culminating at approximately 10,550 yr B.P., at the same time as the Younger Dryas in Europe and in the north Atlantic” (Kallel et al., 1988, p. 372). The AMS dating indicates that the surface waters of the northwest Pacific ocean began to cool ca. 11,100 yr B.P. and then to warm again ca. 10,400 yr B.P.; these dates are in good agreement with those from our eolian sections in central Alaska. Ice-rafted detritus also appears at core site CH84-14 between about 11,100 and 10,550 yr B.P., indicating southward motion of the Polar Front in the Pacific Ocean (Kallel et al., 1988). The discovery that surface waters of the northwestern Pacific Ocean became colder at the same time as the Younger Dryas stade in northern Europe suggests that this cold event may have had a widespread impact on high-latitude oceanic and terrestrial regions. The northwest Pacific Ocean strongly influences the climate of Alaska, in somewhat the same way that the northern Atlantic influences the climate of northwest Europe. It seems unlikely that a significant southward shift of the Polar Front and cooling of the waters of the northwest Pacific Ocean ca. 11,000 yr B.P., as documented by Kallel et al. (1988>, could have occurred without a concomitant effect on the climate and environment of Alaska. We have presented evidence for a jump in wind intensity in central Alaska during the latest Pleistocene. Some additional data suggest that other parts of Alaska may have been affected by environmental changes during this period, although the chronologic control is poorer than that from the eolian sections. For instance, Ten Brink and Waythomas (1985, Fig. 3) suggested that glaciers in the Alaska Range briefly expanded at the end of the Pleistocene at a time broadly correlative with the Younger Dryas event. Moraines and glacial drift from what may be a brief latest Pleistocene glacier advance have also been described from the
166
BIGELOW,
BEGl?T,
Kigluaik Mountains of the Seward Peninsula (Kaufman and Hopkins, 1986), the Yukon-Tanana upland (Weber, 1986), and southeast Alaska (Mann, 1986). A climatic event corresponding to the time period of the Younger Dryas has not been identified by Alaskan palynologists. However, some pollen curves may reflect a transient climatic change ca. 11,000 yr B.P. For instance, pollen accumulation rates drop by 50% at Niliq Lake in northwest Alaska at ca. 11,000 yr B.P. and Pop&s pollen, known to be especially sensitive to summer warmth, virtually disappears between 11,000 and 10,000 yr B.P. from the Joe Lake site in the Brooks Range (Anderson, 1988). Bet& pollen also drops from approximately 55 to 30% of total pollen at Niliq Lake and from 60 to 45% at Joe Lake. Decreases in pollen accumulation rate and Beth pollen influx at ca. 11 ,OO&lO,OOO yr B.P. also occurred at Squirrel Lake and Kaiyak Lake in northwest Alaska (Anderson, 1985). Populus intlux rates also drop at ca. 10,500 yr B.P. at Rupert Lake and Headwaters Lake in the central Brooks Range of Alaska (Brubaker et al., 1983). A brief but dramatic drop of about 40% in total pollen production occurred in latest Pleistocene time at Doll Creek in the northern Yukon (Ritchie, 1982, Fig. 13). These data may reflect some type of widespread environmental change between 11,000 and 10,000 yr B.P. in northern and central Alaska, at about the same time as the increase in wind intensity recorded at archaeological sites in central Alaska. PROPOSED CAUSAL MECHANISM FOR A HIGH-LATITUDE CLIMATIC EVENT CA. 11,000 YR B.P.
We suggest that CO2 forcing produced broadly synchronous climatic changes in high latitudes during Younger Dryas time. Some ice-core data suggest that worldwide atmospheric CO, content quickly changed during the Younger Dryas reversal from approximately 300 to 240-250 ppmv (Oeschger et al., 1984; Paterson and Hammer,
AND
POWERS
1987), a decrease about 40-50% as large as that which occurred at the time of the culmination of worldwide glaciation and cooling during the late Wisconsin. Computer modeling of changes in atmospheric CO, content as large as those which occurred during Younger Dryas time shows that climatic effects would likely be widespread and that high-latitude regions, like central and northern Alaska, would be particularly sensitive to CO* forcing (Hansen et al., 1984). While reductions in atmospheric CO, during the time of the Younger Dryas stade would directly cause reductions in temperatures, particularly in high-latitude areas, changes in other sorts of climatic parameters should also have occurred. A jump in wind intensity may have been one manifestation of CO2 climate forcing ca. 11,000 yr B.P. in central Alaska, perhaps reflecting higher latitudinal temperature gradients. A profound realignment of the circulation regime of the northern Atlantic Ocean may be responsible for the very large Younger Dryas temperature reversal in northern Europe (Broecker et al., 1985, 1988), but decreases in “Greenhouse” warming of the earth due to reductions in atmospheric CO, content would likely result in broadly synchronous climatic changes in other areas. A fundamental test of the CO, model for cooling during the Younger Dryas stade is therefore the presence or absence of synchronous climatic changes in high-latitude regions of the northern and southern hemispheres (Harvey, 1989). Heusser and Rabassa (1987) have documented a pollen record of decreased temperatures from Tierra de1 Fuego, South America, where cooling began after about 11,850 yr B.P. and continued until 10,510 yr B.P. Heusser (1989) suggests that cooling may have occurred in some high-latitude areas of the southern hemisphere at about the same time as the Younger Dryas stade in northern Europe, citing evidence of late Pleistocene glacier readvances in New Zealand (Burrows et al., 1976; Burrows, 1979) and late Pleistocene isotopic shifts
ALASKAN
YOUNGER
suggestive of cooling in several Antarctic ice cores (Johnsen et al., 1972; Lorius et al., 1979; Jouzel et al., 1987). Harvey (1989, p. 137) suggests that, taken together, there is growing evidence of a worldwide cooling at the time of the Younger Dryas climatic event, particularly at high latitudes of the southern hemisphere. Latest Pleistocene environmental changes in Alaska and the North Pacific Ocean may also reflect cooling and environmental changes during Younger Dryas time. We suggest that some records of highlatitude climatic change in Alaska are broadly consistent with a transient decrease in “Greenhouse” warming due to the documented drop in atmospheric CO, content, although local climatic conditions undoubtedly varied widely. SUMMARY
AND CONCLUSIONS
Wind intensity can change rapidly in response to climatic changes. Eolian deposits, in some cases, contain sensitive, semicontinuous proxy climatic records. Eolian sections from central Alaska record a brief episode of increased wind intensity and storminess that occurred from ca. 11,100 to 10,700 yr B.P., a time broadly synchronous with the Younger Dryas climatic event in northern Europe. Widespread and rapid environmental changes may have occurred in Alaska in latest Pleistocene time due to transient variations in atmospheric CO, content. Retrodictions based on the earth’s orbital geometry suggest that the maximum summer insolation in high latitudes of the northern hemisphere occurred about 11,000 yr B.P. (Berger, 1978), almost exactly at the time of the Younger Dryas event. The Younger Dryas cooling in northern Europe and synchronous records of environmental change in high-latitude marine and terrestrial sites from the northwest Pacific Ocean and Alaska show that terrestrial climate does not respond in a simple or linear way to astronomical forcing.
DRYAS
167
REVERSAL
ACKNOWLEDGMENTS We thank the University of Alaska Museum Geist fund and the National Geographic Society for supporting research at the Walker Road site. Thanks also to the Mineral Industry Research Lab at the University of Alaska for the use of their X-ray sedigraph. Careful reviews by R. M. Thorson and S. C. Porter are gratefully acknowledged.
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Brubaker, L. B., Garfinkel, H. L., and Edwards, M. E. (1983). A late Wisconsin and Holocene vegetation history from the central Brooks Range: Implications for Alaskan paleocology. Quaternary Research 20, 194-214. Burrows, C. J. (1979). A chronology for cool-climate episodes in the Southern Hemisphere 12,000-1000 yr B.P. Paleogeography, Paiaeoclimatology, Palaeoecology 21, 281-347. Burrows, C. J., Chinn, T., and Kelly, M. (1976). Glacial activity in New Zealand near the PleistoceneHolocene boundary in the light of new radiocarbon dates. Boreas 5, 57-60. Dansgaard, W. (1987). Ice core evidence of abrupt climatic changes. In “Abrupt Climatic Change: Evidence and Implications” (W. H. Berger and L. D. Labeyrie, Eds.), pp. 223-233. Reidel, Dordrecht. Dansgaard, W., White, J. W. C., and Johnsen, S. J. (1989). The abrupt termination of the Younger Dryas climate event. Nature (London) 339, 532-534. Hammer, C. U., Clausen, H. B., Dansgaard, W.. Nef-
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tel, A., Kristinsdottir, P., and Johnson, E. (1985). Continuous impurity analysis along the Dye 3 deep core. In “Geophysical Monograph 33, Greenland Ice Core: Geophysics, Geochemistry, and the Environment” (C. C. Langway, Jr., H. Oeschger, and W. Dansgaard, Eds.), pp. 90-94. American Geophysical Union, Washington, DC. Hansen, J., Lacis, A., Rind, D., Russell, G., Stone, P., Fung, I., Ruedy, R., and Lemer, J. (1984). Climate sensitivity: Analysis of feedback mechanisms. In “Geophysical Monograph 29, Climate Processes and Climate Sensitivity” (J. Hansen and T. Takahashi, Eds.), pp. 130-163. American Geophysical Union, Washington, DC. Harvey, L. D. D. (1988). Climatic impact of ice-age aerosols. Nature (London) 334, 333-335. Harvey, L. D. D. (1989). Modelling the Younger Dryas. Quaternary Science Reviews 8, 137-149. Heusser, C. J. (1989). Late Quaternary vegetation and climate of southern Tierra de1 Fuego. Quaternary Research 31, 396406. Heusser, C. J., and Rabassa, J. (1987). Cold climatic episode of Younger Dryas age in Tierra de1 Fuego. Nature (London) 328, 609-611. Johnsen, S. J., Dansgaard, W., Clausen, H. B., and Landgway, C. C. (1972). Oxygen isotope profiles through the Antarctic and Greenland ice sheets. Nuture (London) 235, 429-434. Jouzel, J., Lorius, C., Merlivat, L., and Petit, J.-R. (1987a). Abrupt climatic changes: The Antarctic ice record during the late Pleistocene. In “Abrupt Climatic Change: Evidence and Implications” (W. H. Berger and L. D. Labeyrie. Eds.). pp. 235-245. Reidel, Dordrecht. Jouzel, J., Lorius, C., Petit, J. R., Genthon, C., Barkov, N. I., Kotlyakov, V. M.. and Petrov., V. M. (1987b). Vostok ice core: A continuous isotopic temperature record over the last climatic cycle (160,000 years). Nature (London) 329, 403-408. Kaufman, D. S., and Hopkins, D. M. (1986). Glacial history of the Seward Peninsula. In “Glaciation in Alaska: The Geologic Record” (T. D. Hamilton, K. M. Reed, and R. M. Thorson, Eds.), pp. 51-77. Alaska Geological Society, Anchorage. Kallel, N., Labeyrie, L. D., Arnold, M., Okada, H.. Dudley, W. C., and Duplessy, J.-C. (1988). Evidence of cooling during the Younger Dryas in the western North Pacific. Oceanologica Acra 11, 369375. Lorius, C., Merlivat, L., Jouzel. J., and Pourchet, M. (1979). A 30,000-yr isotope climatic record from Antarctic ice. Nafure (London) 280, 644-648. Lowe, S. (1981). Radiocarbon dating and stratigraphic resolution in Welsh lateglacial chronology. Nature (London) 293, 210-212. Lowe, S., and Gray, J. (1980). The stratigraphic subdivision of the Lateglacial of NW Europe: A discussion. In “Studies in the Lateglacial of North-west Europe” (J. Lowe, J. Gray, and J. Robinson, Eds.), pp. 157-176. Pergamon, Oxford.
AND POWERS Mangerud, J., Anderson, S. T., Berlund, B. E., and Donner, J. J. (1974). Quatemary stratigraphy of Norden, a proposal for terminology and classification. Boreas 3, loP-126. Mann, D. H. (1986). Wisconsin and Holocene glaciation of southeast Alaska. In “Glaciation in Alaska: The Geologic Record” (T. D. Hamilton, K. M. Reed, and R. M. Thorson, Eds.), pp. 237-265. Alaska Geological Society, Anchorage. Maxwell, H. E. (1987). “Archaeology and Panguingue Creek: A Late Pleistocene/Early Holocene-aged Site in Central Alaska.” M.A. thesis, University of Alaska, Fairbanks. Oeschger, H., Beer, J., Siegenthaler, U., Stauffer, B., Dansgaard, W., and Langway, C. C. (1984). Late glacial climate history from ice cores. In “Geophysical Monograph 29, Climate Process and Climate Sensitivity” (J. E. Hansen and T. Takahashi, Eds.), pp. 299-306. American Geophysical Union, Washington, DC. Paterson, W. S. B., and Hammer, C. U. (1987). Ice core and other glaciological data. In “North America and Adjacent Oceans during the last Deglaciation” (W. F. Ruddiman and H. E. Wright, Eds.), pp. 91-109. Geol. Sot. Amer., Boulder. Petit, J., Briat, M., and Royer, A. (1981). Ice age aerosol content from East Antarctic ice core samples and past wind strength. Nature (London) 293, 391394. Phippen, P. G. (1988). “Archaeology at Owl Ridge: A Pleistocene-Holocene boundary age site in central Alaska.” M.A. thesis, University of Alaska, Fairbanks. Powers, W. R., and Hoffecker, J. F. (1989). late Pleistocene settlement in the Nenana Valley, central Alaska. American Antiquity 54, 263-287. Ritchie, J. C. (1982). The modem and late-Quatemary vegetation of the Dal1 Creek area, north Yukon, Canada. The New Phytologist, 90, 563-603. Stromberg, B. (1985). Revision of the late-glacial Swedish varve chronology. Boreas 14, 101-105. Ten Brink, N. W., and Waythomas, C. F. (1985). Late Wisconsin glacial chronology of the north-central Alaska Range: A regional synthesis and its implications for early human settlements. In “National Geographic Society Research Reports” (Winfield Swanson, Ed.), pp. 15-32. National Geographic Society, Washington, DC. Thorson, R. M., and Bender, G. (1985). Eolian deflation by ancient katabatic winds: A late Quaternary example from the north Alaska Range. Geological Society of America Bulletin 96, 702-709. Thorson, R. M., and Hamilton, T. D. (1977). Geology of the Dry Creek Site; a stratified early man site in interior Alaska. Quaternary Research 7, 149-176. Weber, F. R. (1986). Glacial Geology of the YukonTanana Upland. In “Glaciation in Alaska: The Geologic Record” (T. D. Hamilton, K. M. Reed, and R. M. Thorson, Eds.), pp. 79-98. Alaska Geological Society, Anchorage.
RESEARCH
34, 160-168
Latest Pleistocene
(1990)
Increase Sediments
NANCY BIGELOW,*
in Wind Intensity Recorded from Central Alaska
in Eolian
JIM BEGOT,? AND ROGER POWERS*
*Department of Anthropology, University of Alaska, Fairbanks, Alaska 99775: and fDepartment and Geophysics and University of Alaska Museum, University of Alaska, Fairbanks, Alaska
of Geology 99775
Received March 23, 1989 A brief increase in wind intensity between ca. 11,100 and 10,700 yr B.P. is recorded by a sharp increase in sediment grain size at eolian sections along the Nenana River in central Alaska. This occurred at the same time as the Younger Dryas climatic reversal in northern Europe and an increase in the vigor of atmospheric circulation recorded by Greenland ice cores. Climatic fluctuations in high latitude areas during Younger Dryas time may reflect variations in the COZ content of the atmosphere. D 1990 University of Washington.
ducing windy conditions in mountain passes and valleys to the north, and depositing sand and loess on bluffs and terraces. Deposits of eolian sediments in the Nenana River Valley and other valleys north of the Alaska Range are well-positioned to record variations in wind intensity and storminess in central Alaska due to climatic changes (Thorson and Bender, 1985). The sections in the Nenana River Valley consist primarily of silts, described in the field as loess, with intercalated sandy deposits and organic-rich paleosols. Paleosols are associated with more fine-grained sediments, suggesting that they are formed during warm intervals characterized by low wind intensity. Sediments containing abundant sand are restricted to the upper parts of the sections, except for a prominent sand layer found near the base of sections which has been radiocarbon dated as late Pleistocene in age.
INTRODUCTION
Eolian sediments can accumulate in sheltered environments for thousands of years and in some cases contain relatively continuous records of environmental change. Detailed studies of eolian sediments accumulating on bluffs along the Nenana River in central Alaska reveal systematic patterns of grain-size changes, which we interpret as proxy records of shifts in wind intensity and storminess due to climatic change. New radiocarbon dates from eolian sediments at the Walker Road archaeological site, together with radiocarbon dates from the Dry Creek site (Thorson and Hamilton, 1977), provide chronologic control and allow correlation between postglacial stratigraphic units in the Nenana Valley (Fig. 1). A similar sequence of stratigraphic units suggesting that the sediments record regional environmental changes can be identified at both the Dry Creek and the Walker Road sites, as well as at other sites we have examined in the Nenana River and Teklanika River areas north of the Alaska Range (Powers and Hoffecker, 1989). The Alaska Range constitutes a high topographic barrier that often separates polar air masses from warmer air to the south. In the spring and summer, storms from the Gulf of Alaska cross the Alaska Range, pro-
PROXY RECORD OF AN INCREASE IN WIND INTENSITY CA. 11,000 YR B.P.
A prominent sand layer (Sl) found near the base of the Dry Creek section is bracketed by radiocarbon dates of 11,120 ? 85 and 10,690 ? 250 yr B.P. (Fig. 2 and Table 1). A thermoluminescence date of 11,350 ? 1000 yr B.P. has also been reported for silts 160
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A Archaeological
KILOMETERS
1. Map
showing
site
o Town
0-o
FIG.
161
location
of eolian
under Sl (Table 1). Such sand layers can form only when winds are strong enough to entrain and sweep sand from the underlying Pleistocene sediments or from the bed of Dry Creek up a bluff standing 25 m above the creek (Thorson and Hamilton, 1977). At other times eolian deposition is dominated by silts. The sand layer therefore records a brief period of high wind intensity. Layer Sl at Dry Creek consists of more than 90% well-sorted sand and is the coarsest deposit at the Dry Creek archaeological site. It was “deposited from an actively moving sand sheet during a time when strong surface winds swept the bluff,” and can be identified in other eolian sections on Pleistocene terraces in the Nenana River Valley (Thorson and Hamilton, 1977, p. 160). It contains sand as large as 0.5 phi in
sections
along
the Nenana
River,
Alaska.
diameter and has a modal diameter of 2.G 2.5 phi. It is much coarser than the surrounding loess layers, whose modal diameter is 3.5 phi. No primary sedimentary structures are visible in the loess or sand (Thorson and Hamilton, 1977). The change from predominant deposition of loess from dust clouds to sand from a sand sheet and back to loess reflects a significant environmental change. The contacts between Sl and L3 and L2 above and below it are sharp, suggesting that environmental changes occurred rapidly. The radiocarbon dates suggest that layer Sl formed during an interval lasting approximately 400 yr. Radiocarbon dates collected through the Dry Creek section show that eolian sediments have accumulated at this location
0
FIG.
DRY CREEK 2. Comparative lithostratigraphy,
Unit
50
75
25
25
50
Sand 50
(Xl 75 loo
Immature Tundra
so11
Subarctic Brown soil
KEY
WALKER
ROAD
granulometry, and chronology for eolian sections at Dry Creek and Walker Road.
25
.‘: L5 bottom ..; . . : .‘,‘_ .L ,.;: .; ,‘: : . L4 ,:;,:,. I, .:: ::. : : : ; ‘: ,. .. :‘: .. ,..,‘. L3 .’ . :
_‘,.
100
Sand (So 50 75 100 : ,. :.: ‘... .‘. :,.;; :.. . . ., .,. :; ‘,.‘. 54 : ., ,. ,.. :.:.,.y ‘,‘.‘... . 25
ALASKAN
TABLE
1. RADIOMETRIC
YOUNGER
DRYAS
DATES FROM DRY CREEK AND WALKER ROAD, CENTRAL ALASKA
Age (yr B.P.)
Material dated
Provenience
Dry creek SI-1933A SI-1933B SI-2333 SI-2332 SI- 1934 SI-1937 SI-2331 SI-1935C SI-1935B SI-2115 SI-1935A SI-1544 SI-2328 SI-2329 SI-1936 SI-1938 SI-1561 SI-2880 ITL-6 1
Modem 375 2 40 1145 f 60 3430 -t- 75 3655 f 60 4670 f 95 6270 k 110 6900 +- 95 8355 k 190 8600 k 460 10,600 + 580 19,050 + 1500 7985 ‘- 105 9340 * 195 12,080 -c 1025 23,930 k 9300 10,690 k 250 11,120 2 85 11,350 + 1000
Charcoal Peat and roots Charcoal Charcoal Charcoal Charcoal Charcoal Charcoal Charcoal Charcoal Charcoal Charcoal Charcoal Charcoal Charcoal Charcoal Charcoal Charcoal Loess
Paleosol 4b Paleosol 4b Paleosol 4b Paleosol 4a Paleosol 4a Paleosol 4a Paleosol 3 Paleosol 3 Paleoso13 Paleosol 3 Paleosol 3 Paleosol 3 Paleosol 2? Paleosol 2 Paleosol 2 Paleosol 2 Paleosol 1 Loess 2 Loess 1
Walker road AA-1693 GX-12875 AA-1692 AA-1683 AA-1681 AA-2264 S-11254
3816 4415 8720 11,010 11,170 11,300 11,820
Charcoal Charcoal Charcoal Charcoal Charcoal Charcoal Charcoal
Paleosol 2 Paleoso12 Paleosol 1 Loess 2 Loess 2 Loess 1 Loess 1
Lab No.
Note.
163
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-c 79 * 95 2 250 rt 230 2 180 +- 120 ? 200
Sources: Thorson and Hamilton,
Comments
Forest fire Forest fire Forest fire
Coal contamination? Small sample size Test pit, correlation not clear Large counting error Small sample size Cultural remains Cultural remains Thermoluminescence date AMS date AMS date AMS date, cultural remains AMS date, cultural remains AMS date, cultural remains Cultural remains
1977: Powers and Hoffecker, 1989.
since the latest Pleistocene. However, several age reversals are present in the series of radiocarbon dates, possibly because of contamination by coal dust from nearby Tertiary coals (Thorson and Hamilton, 1977). In order to minimize possible problems due to contamination, 14C accelerator mass spectrometry (AMS) dating was used where appropriate in an ongoing geoarchaeologic study of the Walker Road site (Fig. 1). A similar sequence of loess, paleosol, and sand horizons is present at this locality, although a thick Pleistocene cover sand or colluvium is also locally preserved at the bottom of the section about 60 m from the bluff face. At the bluff face, a relatively coarse sand layer intercalated between two loess units occurs near the base of the sec-
tion (Fig. 2). This layer of sand, which is correlated with Sl at Dry Creek, has a modal grain size of 3.0 phi and lies between loess layers with modal grain sizes of 5.5 phi. This sand is massive with abrupt upper and lower contacts. The sand layers at Walker Road, like those at Dry Creek, must have formed during periods of very strong winds when sand was blown up a high bluff to the site of the eolian deposits, while the loess horizons record deposition predominantly from silt-rich dust clouds. Three AMS dates and one conventional radiocarbon date obtained on organic material collected from immediately beneath the lowest sand layer at Walker Road (Table 1) confirm a correlation with layer Sl from the Dry Creek site and support the validity of the older conventional radiocar-
164
BIGELOW,
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bon dates on S 1. Although the conventional radiocarbon date at Walker Road is slightly older than the three AMS dates, the internal consistency and excellent agreement of the radiocarbon dates from Walker Road with those obtained at Dry Creek suggest that these dates are reliable. The dating from both sections suggests that wind intensity in the Nenana River canyon jumped to a significantly higher level between approximately 11,100 and 10,700 yr B.P. Sl has been recognized at one other Nenana Valley locality, the Panguingue Creek site (Fig. 1). The stratigraphy at this locale is similar to the stratigraphies at Dry Creek and Walker Road. A l-m-thick Pleistocene cover sand or colluvium resembling that at Walker Road lies at the bottom of the section with a loess unit overlying it. This loess unit (L2) is in turn covered by a lo-cm-thick sand horizon (Maxwell, 1987). We correlate the sand above L2 with Sl at Dry Creek and Walker Road. An AMS date of 10,180 ? 180 yr B.P. (AA-1686) on organic material just above the sand layer places it within the right chronological framework. In addition, the sand is similar to the Dry Creek Sl with a modal diameter of 2.0 phi and is surrounded by loesses with modal diameters of 4.0 and 4.5 phi. In addition to being present in eolian sediments in the Nenana Valley, layer Sl has been identified in the Teklanika Valley some 30-40 km to the west. At the Owl Ridge site, sand layer Sl is as much as 10 cm thick, with bracketing radiocarbon dates of 9325 5 305 yr B.P. (GX-6283) on peat and organic matter above and 11340 + 150 yr B.P. (B-11209) on charcoal below it (Phippen, 1988). Layer Sl has been recognized in archaeological sites over an area of approximately 800 km2 in central Alaska. POSSIBLE YOUNGER
CORRELATION WITH THE DRYAS CLIMATE EVENT
The period of high wind intensity and storminess recorded by coarse sediment layers in eolian sections of latest Pleistocene age in central Alaska is approximately
AND
POWERS
the same age as Younger Dryas cooling reflected by many sorts of proxy climatic records in Europe and around the northern Atlantic Ocean (Broecker et al., 1988). Ice core records of stable isotope changes in precipitation from Greenland document Younger Dryas cooling, and suggest that the Younger Dryas event lasted less than 1000 yr and ended abruptly by 10,700 yr ago (Dansgaard, 1987; Paterson and Hammer, 1987; Dansgaard et al., 1989). An age of 10,750 + 50/-150 yr B.P. has also been suggested for the end of the Younger Dryas interval on the basis of the revised Swedish varve chronology (Stromberg, 1985). A high-resolution pollen study by Lowe (1981) indicates that the Younger Dryas event in Wales began about 11,160 ‘-c 90 yr B.P., a value in good agreement with dates from elsewhere in the British Isles and Scandinavia (Mangerud et al., 1974). Some studies suggest the Younger DryasPreboreal transition in northern European pollen cores occurs at about lO,OOO-10,150 yr B.P. (Anderson, 1981), although in some sequences the transition appears to occur as early as 10,500 yr B.P. (Lowe, 1981). It has been suggested that the Younger Dryas Stade lasted from 11,OOO-10,500 yr B.P., after which “marked thermal improvement” occurred (Lowe and Gray, 1980). Numerous radiocarbon dates associated with moraines deposited during readvances of the Scandinavian Ice Sheet in northern Europe fall between 11,300 and 10,000 yr B.P. (Anderson, 1981). In general, the age of the termination of the Younger Dryas event in ice cores seems to be slightly earlier than that recorded by radiocarbon dates from pollen cores or moraines. This may reflect a systematic error in radiocarbon dates of ice-core chronologies, or it may be due to a more sensitive and rapid response to environmental changes in ice cores than in pollen cores or glaciers. Nevertheless, the timing and the duration of the increase in wind intensity recorded in eolian sections in central Alaska seems to be coincident with the
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Younger Dryas climatic event of northern Europe. A marked increase in atmospheric circulation intensity during the Younger Dryas is indicated by an increase in dust particle concentration in ice cores. In the Dye 3 core, high-resolution studies show that the decrease in oxygen-isotope values during Younger Dryas time is mirrored by a simultaneous increase in dust content to values six times greater than those of the Holocene background (Hammer et al., 1985; Paterson and Hammer, 1987; Dansgaard et al., 1989). The particulate matter in Greenland ice cores is thought to be largely derived from low- and mid-latitude deserts (Harvey, 1988), so the increase in ice core dust must represent a change in atmospheric circulation intensity over a very large area. A late Pleistocene increase in dust content and an oxygen isotope decrease in the Dome C Antarctic ice core may also be correlated with the Younger Dryas climatic event (Lorius et al., 1979; Petit et al., 1981; Briat et al., 1982; Jouzel et al., 1987).
The well-documented general increase in atmospheric circulation intensity recorded for the younger Dryas by the ice-core data may be directly related to the local increase in wind intensity and storminess in central Alaska which is recorded in eolian deposits in sections along the Nenana River. Possibly a brief expansion of ice and snow cover also occurred at this time in the Alaska Range (see below), producing high katabatic winds (Thorson and Bender, 1985). EVIDENCE FOR HIGH-LATITUDE CLIMATIC CHANGE IN THE NORTHWEST PACIFIC OCEAN AND ALASKA DURING YOUNGER DRYAS TIME
Recent high-resolution studies of isotopic variations in several species of foraminifera and micropaleontological studies of temperature-sensitive coiling ratios of N. pachyderma in northwest Pacific marine core CH84-14, combined with AMS dating
DRYAS
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16.5
of hand-picked foraminiferal samples, show that “a drastic climatic deterioration is well marked in both climatic records culminating at approximately 10,550 yr B.P., at the same time as the Younger Dryas in Europe and in the north Atlantic” (Kallel et al., 1988, p. 372). The AMS dating indicates that the surface waters of the northwest Pacific ocean began to cool ca. 11,100 yr B.P. and then to warm again ca. 10,400 yr B.P.; these dates are in good agreement with those from our eolian sections in central Alaska. Ice-rafted detritus also appears at core site CH84-14 between about 11,100 and 10,550 yr B.P., indicating southward motion of the Polar Front in the Pacific Ocean (Kallel et al., 1988). The discovery that surface waters of the northwestern Pacific Ocean became colder at the same time as the Younger Dryas stade in northern Europe suggests that this cold event may have had a widespread impact on high-latitude oceanic and terrestrial regions. The northwest Pacific Ocean strongly influences the climate of Alaska, in somewhat the same way that the northern Atlantic influences the climate of northwest Europe. It seems unlikely that a significant southward shift of the Polar Front and cooling of the waters of the northwest Pacific Ocean ca. 11,000 yr B.P., as documented by Kallel et al. (1988>, could have occurred without a concomitant effect on the climate and environment of Alaska. We have presented evidence for a jump in wind intensity in central Alaska during the latest Pleistocene. Some additional data suggest that other parts of Alaska may have been affected by environmental changes during this period, although the chronologic control is poorer than that from the eolian sections. For instance, Ten Brink and Waythomas (1985, Fig. 3) suggested that glaciers in the Alaska Range briefly expanded at the end of the Pleistocene at a time broadly correlative with the Younger Dryas event. Moraines and glacial drift from what may be a brief latest Pleistocene glacier advance have also been described from the
166
BIGELOW,
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Kigluaik Mountains of the Seward Peninsula (Kaufman and Hopkins, 1986), the Yukon-Tanana upland (Weber, 1986), and southeast Alaska (Mann, 1986). A climatic event corresponding to the time period of the Younger Dryas has not been identified by Alaskan palynologists. However, some pollen curves may reflect a transient climatic change ca. 11,000 yr B.P. For instance, pollen accumulation rates drop by 50% at Niliq Lake in northwest Alaska at ca. 11,000 yr B.P. and Pop&s pollen, known to be especially sensitive to summer warmth, virtually disappears between 11,000 and 10,000 yr B.P. from the Joe Lake site in the Brooks Range (Anderson, 1988). Bet& pollen also drops from approximately 55 to 30% of total pollen at Niliq Lake and from 60 to 45% at Joe Lake. Decreases in pollen accumulation rate and Beth pollen influx at ca. 11 ,OO&lO,OOO yr B.P. also occurred at Squirrel Lake and Kaiyak Lake in northwest Alaska (Anderson, 1985). Populus intlux rates also drop at ca. 10,500 yr B.P. at Rupert Lake and Headwaters Lake in the central Brooks Range of Alaska (Brubaker et al., 1983). A brief but dramatic drop of about 40% in total pollen production occurred in latest Pleistocene time at Doll Creek in the northern Yukon (Ritchie, 1982, Fig. 13). These data may reflect some type of widespread environmental change between 11,000 and 10,000 yr B.P. in northern and central Alaska, at about the same time as the increase in wind intensity recorded at archaeological sites in central Alaska. PROPOSED CAUSAL MECHANISM FOR A HIGH-LATITUDE CLIMATIC EVENT CA. 11,000 YR B.P.
We suggest that CO2 forcing produced broadly synchronous climatic changes in high latitudes during Younger Dryas time. Some ice-core data suggest that worldwide atmospheric CO, content quickly changed during the Younger Dryas reversal from approximately 300 to 240-250 ppmv (Oeschger et al., 1984; Paterson and Hammer,
AND
POWERS
1987), a decrease about 40-50% as large as that which occurred at the time of the culmination of worldwide glaciation and cooling during the late Wisconsin. Computer modeling of changes in atmospheric CO, content as large as those which occurred during Younger Dryas time shows that climatic effects would likely be widespread and that high-latitude regions, like central and northern Alaska, would be particularly sensitive to CO* forcing (Hansen et al., 1984). While reductions in atmospheric CO, during the time of the Younger Dryas stade would directly cause reductions in temperatures, particularly in high-latitude areas, changes in other sorts of climatic parameters should also have occurred. A jump in wind intensity may have been one manifestation of CO2 climate forcing ca. 11,000 yr B.P. in central Alaska, perhaps reflecting higher latitudinal temperature gradients. A profound realignment of the circulation regime of the northern Atlantic Ocean may be responsible for the very large Younger Dryas temperature reversal in northern Europe (Broecker et al., 1985, 1988), but decreases in “Greenhouse” warming of the earth due to reductions in atmospheric CO, content would likely result in broadly synchronous climatic changes in other areas. A fundamental test of the CO, model for cooling during the Younger Dryas stade is therefore the presence or absence of synchronous climatic changes in high-latitude regions of the northern and southern hemispheres (Harvey, 1989). Heusser and Rabassa (1987) have documented a pollen record of decreased temperatures from Tierra de1 Fuego, South America, where cooling began after about 11,850 yr B.P. and continued until 10,510 yr B.P. Heusser (1989) suggests that cooling may have occurred in some high-latitude areas of the southern hemisphere at about the same time as the Younger Dryas stade in northern Europe, citing evidence of late Pleistocene glacier readvances in New Zealand (Burrows et al., 1976; Burrows, 1979) and late Pleistocene isotopic shifts
ALASKAN
YOUNGER
suggestive of cooling in several Antarctic ice cores (Johnsen et al., 1972; Lorius et al., 1979; Jouzel et al., 1987). Harvey (1989, p. 137) suggests that, taken together, there is growing evidence of a worldwide cooling at the time of the Younger Dryas climatic event, particularly at high latitudes of the southern hemisphere. Latest Pleistocene environmental changes in Alaska and the North Pacific Ocean may also reflect cooling and environmental changes during Younger Dryas time. We suggest that some records of highlatitude climatic change in Alaska are broadly consistent with a transient decrease in “Greenhouse” warming due to the documented drop in atmospheric CO, content, although local climatic conditions undoubtedly varied widely. SUMMARY
AND CONCLUSIONS
Wind intensity can change rapidly in response to climatic changes. Eolian deposits, in some cases, contain sensitive, semicontinuous proxy climatic records. Eolian sections from central Alaska record a brief episode of increased wind intensity and storminess that occurred from ca. 11,100 to 10,700 yr B.P., a time broadly synchronous with the Younger Dryas climatic event in northern Europe. Widespread and rapid environmental changes may have occurred in Alaska in latest Pleistocene time due to transient variations in atmospheric CO, content. Retrodictions based on the earth’s orbital geometry suggest that the maximum summer insolation in high latitudes of the northern hemisphere occurred about 11,000 yr B.P. (Berger, 1978), almost exactly at the time of the Younger Dryas event. The Younger Dryas cooling in northern Europe and synchronous records of environmental change in high-latitude marine and terrestrial sites from the northwest Pacific Ocean and Alaska show that terrestrial climate does not respond in a simple or linear way to astronomical forcing.
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ACKNOWLEDGMENTS We thank the University of Alaska Museum Geist fund and the National Geographic Society for supporting research at the Walker Road site. Thanks also to the Mineral Industry Research Lab at the University of Alaska for the use of their X-ray sedigraph. Careful reviews by R. M. Thorson and S. C. Porter are gratefully acknowledged.
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