Climate Dynamics (1992) 6:179-184 aimm¢

Dynamics © Springer-Verlag 1992

Glacial/Holocene change

Fossil-pollen evidence for abrupt climate changes during the past 18000 years in eastern North America*

Eric C Grimm 1 and George L Jacobson Jr 2

i Illinois State Museum, Research and Collections Center, 1920 South 101/2 Street, Springfield, IL 62703, USA 2 Department of Plant Biology and Institute for Quaternary Studies, University of Maine, Orono, ME 04469, USA

Received June 27, 1991/Accepted August 19, 1991

Abstract. A quantitative measure of the rate at which fossil-pollen abundances changed over the last 18000 years at 18 sites spread across eastern North America distinguishes local from regionally synchronous changes. Abrupt regional changes occurred at most sites in late-glacial time (at =13700, ~12300, and

10000 radiocarbon yr BP) and during the last 1000 years. The record of abrupt late-glacial vegetation changes in eastern North America correlates well with abrupt global changes in ice-sheet volume, mountain snow-lines, North Atlantic deep-water production, at- mospheric CO2, and atmospheric dust, although the pa- lynological signal varies from site to site. Changes in vegetation during most of the Holocene, although lo- cally significant, were not regionally synchronous. The analysis reveals non-alpine evidence for Neoglacial/ Little Ice Age climate change during the last 1000 years, which was the only time during the Holocene when cli- mate change was of sufficient magnitude to cause a synchronous vegetational response throughout the sub- continent. During the two millennia preceding this widespread synchronous change, the rate of change at all sites was low and the average rate of change was the lowest of the Holocene.

Introduction

Plant populations have undergone large shifts in abun- dance and distribution during the late Quaternary, pri- marily in response to climate changes (Davis 1981, 1986; Huntley and Birks 1983; Webb 1986; Jacobson et al. 1987). Broad-scale climate change affects different kinds of vegetation in different regions, causing differ- ent responses. The geographic expression of broadly synchronous changes therefore varies considerably.

* Contribution to Clima Locarno - Past and Present Climate Dy- namics; Conference September 1990, Swiss Academy of Sciences - National Climate Program Offprint requests to: EC Grimm

Moreover, at individual sites, some vegetation changes may be local or site-specific, whereas others may be re- gionally synchronous. We have developed a method for identifying abrupt, regionally synchronous changes in fossil-pollen stratigraphies, independent of vegetation type and direction of climate change. Regionally synch- ronous palynological changes are proxy evidence for times of major, broad-scale, changes in climate. In this paper we further discuss the method, evaluate the site to site variability of results from eastern North Ameri- ca, and discuss their climatic and ecological implica- tions.

The method

The rate of change is the critical variable for identifying regionally synchronous palynological changes. The ap- propriate parameter for measuring rate of change in pollen-frequency data is a dissimilarity coefficient be- tween pollen samples. The rate of change is then the dissimilarity coefficient (or distance) between two sam- ples in a time series divided by the time interval be- tween the samples. We used the chord distance dissimi- larity coefficient, which has proven especially satisfac- tory for pollen-frequency data (Overpeck et al. 1985). For each pollen time-series (site) used in this study, the pollen data were first smoothed, and then pollen sam- ples were interpolated at 200-year intervals. The dissim- ilarity between adjacent 200-year samples divided by 2 is the rate of palynological change per century.

The pollen curves from individual sites must be smoothed because of short-term sample-to-sample var- iation owing to counting error, sedimentation phenom- ena, short-term vegetation fluctuations, and year-to- year vagaries in pollen production. The objective of this study was to evaluate rates of change on the temporal scale of a few hundred years. The data were smoothed at the original sample points, and then samples equally spaced in time were interpolated between the nearest samples on either side of 200-year intervals. For exam- ple, if Quercus had smoothed values of 30% at 190 y

180

and 40% at 215 y, the new sample at 200 y was assigned an interpolated value of 34% Quercus.

Because typical time series of pollen samples are not equally spaced in time, smoothing with either a con- stant number of samples or a constant time proved un- satisfactory. The former incorporates variable spans of time, whereas the latter includes variable degrees of noise. We therefore used an algorithm that represents a trade-off between these factors. The filtering method is a modification of a 5-sample moving average, with successively more samples included in the average from either side of each smoothing point (the original sample locations or depths) until 150 y from the point (there- fore a maximum time window of 300 y) or a total of 9 samples are exceeded. We experimented with different minimum and maximum numbers of samples and dif- ferent maximum time windows to include in the aver- age. With the time precision of the pollen data availa- ble, the values chosen proved most satisfactory. Analy- sis of smoothing was made by ordinating together (with correspondence analysis) the smoothed and un- smoothed data from selected sites. The overall degree of smoothing could be judged by the trace of the smoothed samples through the original samples in the ordination. The data for this analysis require relatively close-interval sampling and reliable chronology.

From the COHMAP data set at Brown University, we selected 18 sites in eastern North America with available data meeting these requirements (Jacobson et al. 1987) (Fig. 1). The numbers of radiocarbon dates per site are 5 (5 sites), 6 (4 sites), 10 (3 sites), 11 (2 sites), 13 (1 site), 15 (1 site), 16 (1 site), and 30 (1 site). In several cases a correction of the dates for contamination by an- cient carbon was made based on a radiocarbon date at the rise of Ambrosia pollen, which marks the time of European settlement. In a few cases a correction of dates of low-organic matter silts in the late-glacial sec-

Fig. 1. Locations of sites. Letters indicate sites specifically dis- cussed in text: A Anderson Pond, Tennessee; G Gould Pond, Maine; T Tannersville, Pennsylvania; W Wolsfeld Lake, Minne- sota

Grimm and Jacobson: Abrupt climate changes, eastern USA

tion was made based on more reliable dates of the same pollen stratigraphic unit from nearby sites or from other geological evidence. These corrections are not based on correlation with other of the 18 sites used in this analysis, which obviously would produce synch- ronous palynological changes. The site chronologies coincide with those made by T. Webb for many other analyses of pollen data in the Brown database. Our analysis would obviously benefit from tighter radiocar- bon control and closer-interval counts. We selected sites with the best data available from the Brown data- base, and the results are highly informative.

For individual sites the rate-of-change graphs show periods of rapid palynological change (with large chord distances), periods of more gradual change, and times of relative constancy. The same 200-year intervals were used for all 18 sites (0, 200, 400, ...) and the rates are graphed at the midpoints for these intervals (100, 300, ...). A regional average rate of change is calculated by averaging all the values for each 200-year time interval from all the sites in the region. The average curve smooths out site-specific changes, revealing times of synchronous change in a large geographic region. Synchronous palynological changes occurring at differ- ent sites in a large region will vary in nature, depending on the regional differences in climate and vegetation. For example, an abrupt warming might instigate a change from spruce forest to deciduous forest at more southern sites, but tundra to spruce forest at more northern sites, thus decreasing spruce some places while increasing it others. Futhermore, synchronous changes in different regions could be in opposite direc- tions, cooling in one region and warming in another. The analysis does not reveal the qualitative nature of climate change; it identifies times of rapid change re- gardless of the direction. Moreover, the average for a large region will not identify time-transgressive change; it reveals abrupt, synchronous, geographically wide- spread change. Time-transgressive changes could be identified only with many more sites enabling calcula- tion of averages for narrow latitudinal or longitudinal zones.

Overpeck (1987) used a method similar to ours, but instead of averaging curves he plotted the number of "events" exceeding one standard deviation within each sequence. We repeated our analysis with this method, which gives results broadly comparable to those de- scribed next.

Times of widespread abrupt change

Individual sites show considerable variation in rates of palynological change (Fig. 2), but in the subcontinent of eastern North America, times of significant synch- ronous change occurred during the late-glacial (= 14000-9000 BP), with peaks at about 13700, 12300, and 10000 BP (radiocarbon years before present), and during the last 1000 years (Fig. 3). Separate regional av- erages for the upper Midwest and northeastern United States/southeastern Canada exhibit these same peaks (Jacobson et al. 1987).

Grimm and Jacobson: Abrupt climate changes, eastern USA 181

0"151~ WOLSFELD LAKE, MINNESOTA

01°t/A

0.05

0.00 0 2 4 6 5 10 12 14 15 18

0.20q GOULD POND, MAINE

~ o.10

oo 0.05

o 2 4 6 8 lO 12 14 15 18

0"15 1 TANNERSVILLE BOG, PENNSYLVANIA

O 0.10

0.05

0 . 0 0 . . . . . . . . . . . . . . . . . .

0 2 4 6 8 10 12 14 16 18

0.15~ ANDERSON POND, TENNESSEE

0"10 t 0.05 ,

0.00 0 2 4 6 8 10 12 14 16 18

Radiocarbon years BP (x 1000 )

Fig. 2. Rate-of-change graphs for Wolsfeld Lake, Gould Pond, Tannersville, and Anderson Pond. Height on the vertical axis in- dicates rate of change in chord distance per 100 years, which is the distance between interpolated points placed at 200 year inter- vals on the smoothed pollen curves divided by 2

10000 BP. Farther south, the older peaks are larger. The data from Anderson Pond, Tennessee (Delcourt 1979), change the most at 12500 to 12700 BP, with the 10000 BP peak evident but much less important.

The major peak at 10000 BP is produced by sites that lie mainly between about 40°-50° N latitude, an accessible area with abundant lakes of glacial origin ideal for pollen analysis. The earlier peaks at 12300 and 13700 BP occur at sites farther south and at older sites north of the glacial margin. The greater magnitude of the 10 000 BP peak on the average curve may conse- quently result from the geographic bias of the site ar- ray, although similar major changes appear to be pres- ent in palynological records from northwestern North America (Barnosky et al. 1987) and Europe (Huntley 1990). The relative importance of the earlier peaks should be further validated by additional sites, espe- cially in the southern United States, when they become available. The present data suggest latitudinal synch- rony of changes but perhaps with the relative magni- tude of change varying with latitude. However, more southern sites are required to test this hypothesis.

Climatic and ecological implications

Late-glacial

0.25 -

0,20

0.15 ! ca

0 0 . 1 0 0

0.05

=5 o.oo- 13

Rate of Palynological Change at Individual Sites

2 4 6 B 10 12 14 16 18

O 0.10-

o

0.00 : , , : ,:;: : , ; , ; , 0 2 4 6 8 10 12 14 16 18

Radiocarbon years BP (x 1000)

Fig. 3. Rate of palynological change for the 18 individual sites (top graph) and average rate of change (bottom graph). The top graph shows the variability among sites. Most of the sites do not span the full 18 000 year period

Individual sites typically show rapid changes at two or more of these times, but their relative magnitudes vary among sites (Fig. 2). For example, at Gould Pond, Maine (Jacobson et al. 1987), the late-glacial Picea (spruce) decline was precipitous at 10000 BP, whereas at Wolsfeld Lake, Minnesota (Grimm 1983), the corre- lative Picea decline began somewhat earlier and was gradual. The pollen data from Tannersville, Pennsyl- vania (Watts 1979), change most at about 13 500 and

Widespread synchrony of vegetation change suggests significant reorganization in patterns of atmospheric circulation. The time of widespread, regional change in pollen assemblages 14000-9000 y ago corresponds with abrupt global changes in ice-sheet volume, moun- tain snowlines, North Atlantic deep-water production, atmospheric CO2, and atmospheric dust (Broecker and Denton 1989) (Fig. 4). COHMAP simulation experi- ments with the community climate model of the US

INTERGLACIAL OCEAN-ATMOSPHERE

REORGANIZATION

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ~ \ ~ Ir . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . ~ ~ ' " ' , . . . . . .

YOUNGER DRYAS

. . . . Atmospheric dust & mountain snowlines ~ \ ' . . . . - - " ~ - - - - ' -

......... Atmospho.o co= " ~ - ~ ' - 2 " 2 2 2" 2 - - North Atlantic deep water production & basin temperature

- - -- lee sheets, sea level, & marine &leO GLACIAL

0 2 4 6 8 10 12 14 16 18 Radiocarbon years BP (x 1OO0)

Fig. 4. Average rates of change for 18 sites in eastern North America (this paper) in relation to important global changes in oceans and atmosphere [after the generalized figure of Broecker and Denton (1989)]

182

National Center for Atmospheric Research predict ma- jor shifts in circulation patterns during late-glacial time, as the Laurentide ice sheet thinned and retreated (COHMAP 1988). Webb et al. (1987) show how these circulation changes are revealed in the vegetation changes recorded by pollen data from eastern North America.

With somewhat greater temporal resolution, the rate of palynological change correlates with stepwise shifts in global ice volume, North Atlantic sea-surface tem- peratures, and planktonic foraminifera t~lso records (Bard et al. 1987; Mix 1987). The general pattern is rapid wanning from ~14 000-12 000 BP and again from ~10 000-9000 BP. The older step encompasses the 13700 BP and 12300 BP peaks in our analysis, which may have greater temporal resolution because of generally much higher sedimentation rates in lacustrine environments compared to marine. Thus, the palyno- logical and North Atlantic faunal and surface 6180 re- cords indicate stepwise climate changes that are con- temporaneous within the chronological precision of the records.

The nature of changes in vegetation and climate dur- ing deglaciation still remains an open question. North American reconstructions usually have not included a late-glacial cooling event corresponding to the North Atlantic/Northwest European Younger Dryas ( =10 600-10 000 BP, Cwynar and Watts 1989; Dans- gaard et al. 1989). For example, in a synthesis of data from about 50 radiocarbon-dated late-glacial sites in northern New England and adjacent Canada, Davis and Jacobson (1985) concluded that vegetation changes from 14000-9000 BP were monotonic and indicative of warming. However, recent studies indicate that a late- glacial cooling event may have occurred at about 10600-10000 BP in eastern maritime Canada (Mott et al. 1986; Rind et al. 1986) and Ohio (Shane 1987). In any case, the major interval of palynological change peaking at 10000 BP incorporates the Younger Dryas period. The peak coincides with the end of the Younger Dryas, which terminated abruptly over 20-50 y, as evi- denced by detailed oxygen isotope data from the Greenland ice core Dye 3 and the sediments of Lake Gerzen, Switzerland (Dansgaard et al. 1989). In terms of rates of change, a rapid Younger Dryas cooling fol- lowed by rapid wanning would be identified as an overall time of rapid change regardless of the opposite directions of change. The pollen data used for this study do not have sufficient temporal resolution to dis- tinguish as particular times of rapid change the onset and close of the Younger Dryas period, if they indeed existed in eastern North America.

Another possibility suggested by the rates of palyno- logical change is that the late-glacial changes in some areas of North America were synchronous with (and re- lated to) the classic North European/North Atlantic changes, but different in direction. Thus, reversals to colder climate in some areas may have coincided with shifts towards warmer climate in others. In fact, if the Younger Dryas was caused by an influx of meltwater from the shrinking Laurentide ice sheet (Ruddiman

Grimm and Jacobson: Abrupt climate changes, eastern USA

1987; Wright 1989), rapid warming in at least some parts of continental North America might be expected. If that were the case, many palynological records would indicate monotonically increasing temperature, and the Younger Dryas period would be indicated by only changes in the rates of palynological change, not the directions of change. Such geographic differences in direction of climate change exist in synoptic recon- structions for other time periods (see COHMAP 1988, Huntley and Prentice 1988) and are consistent with general circulation models (GCM) of hemispheric cir- culation. The commonality is synchronous abrupt changes. The analysis presented in this paper can iden- tify those times and facilitate intercontinental compari- sons of vegetation changes.

The independent geological, palynological, GCM, and ~ l so evidence thus show that the period from 14000-9000 BP was a time of major synchronous change, with apparent stepwise shifts in atmospheric chemistry, atmospheric circulation, North Atlantic sea- surface temperatures, 51so, and vegetation. Plant and animal communities in North America were greatly reorganized during this time (Jacobson et al. 1987; Gra- ham and Mead 1987; Webb et al. 1987), human popula- tions expanded dramatically, and much of the Pleisto- cene megafauna became extinct (Martin and Klein 1984; Grayson 1987). The relationships of cause and ef- fect are probably considerably intertwined.

Holocene

The average rate of palynological change is relatively constant during most of the Holocene (Fig. 3). Howev- er, vegetation was not necessarily constant locally. Indi- vidual sites show considerable site-specific change, but the shifts were not synchronous throughout eastern North America. Most sites show relatively little change during the period of 7000-5000 BP. The significant changes during this time result from time-transgressive expansions of various tree populations in the East and movement of the prairie/forest border in the Midwest. Gradual climate change was the probable cause for these vegetation changes (Webb 1986; Webb et al. 1987; Webb et al. 1983; Bartlein et al. 1984), but the climate change was evidently too slow or too small to cause synchronous subcontinental responses in vegeta- tion. An alternative hypothesis for time-transgressive expansions of tree populations in response to changing climate is that of migrational lag following climate change at the end of the Pleistocene (Davis 1976, 1981; Huntley and Birks 1983). Our analysis does not support one of these hypotheses over the other. Either delayed migration or gradually changing climate could cause time-transgressive expansions of tree populations.

A major synchronous event in the northeastern Uni- ted States and southeastern Canada was the Tsuga (hemlock) decline at about 4800 BP. Disease epidemic or insect infestation (and not climate change) probably caused the decline, because only Tsuga was affected, the palynological event occurred only within the range

Grimm and Jacobson: Abrupt climate changes, eastern USA

of Tsuga, and it occurred universally within that range (Davis 1981; Webb 1982; Allison et al. 1986). Although the Tsuga decline is a significant event at several sites in New England and eastern Great Lakes (Jacobson and Grimm 1988), it produces only a small peak on the average curve for eastern North America (Fig. 3).

By 3000 BP the vegetation became relatively stabil- ized over all of eastern North America, and the two millennia from 3000-1000 BP is a period of relative constancy throughout. Vegetation was changing, but only slowly, at all sites in our sample. Then, however, major changes were initiated at many sites beginning about 1000 BP.

Deforestation, agriculture, and other intensive land disturbance accompanying European settlement caused large changes in vegetation. However, at many sites ma- jor vegetation changes began well before European contact, and the change in the average curve during the last millennium has both European cultural and pre- Columbian components. At Gould Pond, Maine, for example, major vegetation shifts began about 1000 BP, with declines of Fagus and Tsuga and the first expan- sion of Picea since the Pleistocene (Jacobson et al. 1987). The pollen sequence from Wolsfeld Lake, Min- nesota, records the rapid development of the "Big Woods" (a mesic hardwoods forest dominated by Ul- mus, Acer, and Tilia) from Quercus woodland begin- ning about 300 BP, 200 years before European settle- ment. This change was the largest since deglaciation, exceeded only by cultural deforestation in the late nine- teenth century. Clark's (1988) detailed charcoal ana- lyses from northern Minnesota show a marked decrease in fire frequency about A.D. 1600. These recent changes generally correspond in time with the Neogla- cial and Little Ice Age climate changes, which initiated glacier advances in mountainous regions throughout the world. In both North America (Davis 1988; Osborn and Luckman 1988; Calkin 1988; Porter 1981) and Eu- rope (Karl6n 1988; R6thlisberger et al. 1980; Hantke 1987), many glaciers in high mountainous regions grew to their greatest sizes since the Pleistocene. Studies by Gajewski (1987, 1988) indicate cooling in non-mountai- nous eastern North America as well. Many kinds of evi- dence show widespread cooling in Europe (Lamb 1977, 1982, 1984). The major qualitative response indicated by the pollen is that of cooling, and the timing varies from about 1000-300 BP. Whether additional climate fluctuations, the Viking warm period for example, can be distinguished within the last millenium awaits more detailed analysis of data with greater temporal preci- sion. The overall curve for rate of palynological change shows widespread vegetation response in a low-alti- tude, non-alpine region. Possibly, the widespread, abrupt changes of the past 1000 years signal the end of the Holocene interglacial.

The "natural" vegetation of eastern North America just before European settlement was an ephemeral state undergoing rapid adjustment to climate change. Under- standing of modern vegetation dynamics must involve this rapid natural change, and predictions of vegetation change in response to greenhouse warming or other hu-

183

man-caused climatic influences must take into account the trajectory of climate in the absence of these ef- fects.

Acknowledgements. We thank M. B. Davis, L. J. Maher, Jr., M. Pelto, M. Prentice, W. F. Ruddiman, T. Webb III, H. E. Wright, Jr., and two anonymous reviewers for helpful suggestions and critically reading earlier versions of the manuscript. This work was supported by NSF grants BSR 8910672 and BSR 8907752. We are most grateful to the individuals who made their pollen data available to us or to COHMAP.

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