science for a changing world

QUANTITATIVE PALEOCLIMATIC RECONSTRUCTIONS FROM

LATE PLEISTOCENE PLANT MACROFOSSILS OF

THE YUCCA MOUNTAIN REGION

by Robert S. Thompson1 , Katherine H. Anderson2, and Patrick J. Bardein3

Open-File Report 99-338

1999

This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards (or with the North American Stratigraphic Code). Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

U.S. DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY

^Denver, Coloradory^Institute of Arctic and Alpine Research, University of Colorado, Boulder, Colorado 'Department of Geography, University of Oregon, Eugene, Oregon

INTRODUCTION

Plant macro fossil assemblages recovered from packrat (Neotoma) middens of late

Pleistocene age from the present-day Mojave Desert of southern Nevada contain plant

species that today live at higher elevations and/or farther north than the midden collection

sites. Previous reconstructions of late Pleistocene climates from packrat midden

assemblages in this region (Spaulding, 1985) assessed the minimum climatic differences

from today by estimating the present-day climatic differences between the fossil midden

sites and the nearest current occurrences of key plant species recovered from the

Pleistocene middens. From this approach Spaulding (1985) concluded that although late

Pleistocene temperatures were considerably below those of today, only modest increases

in precipitation (relative to today) were necessary for these plant species to survive in the

current Mojave Desert during the late Pleistocene.

Spaulding's approach provided "state-of-the-art" results from an intensive careful

examination of the best data available at the time. However, data and techniques

developed since the mid-1980s suggest that there are two possible short-comings to this

approach: 1) the use of lowest elevational and (frequently) most southerly occurrences of

key plant species results in minimal estimates of the differences between Pleistocene and

present-day climates, and 2) the instrumental climate data set available to Spaulding was

limited in duration, non-standard in its method of collection, and indicated a modern

climate wetter than the long-term historic mean, which resulted in relatively small apparent

differences between late Pleistocene and present-day mean annual precipitation levels. In

this report we use a more standard (close to the long-term mean) modern calibration

period and a modern plant distribution data set that permits us to identify modern

analogues for the Pleistocene vegetation. This reexamination permits a more robust

reconstruction of the past climate, and results in estimates of mean annual temperature for

the glacial maximum at Yucca Mountain that are 1.0° to 1.4° C warmer than those of

Spaulding, and estimates of mean annual precipitation that are 60 mm or more higher than

his.

METHODS

In this report we use present-day climatic and vegetational data from across North

America to estimate past temperature and precipitation values from plant macro fossil

assemblages preserved in ancient packrat middens from southern Nevada. Our approach

involves: 1) compiling a comprehensive list of plant macro fossil assemblages from late

Pleistocene packrat middens of the region, 2) employing a ~25 km grid of present-day

climate and plant distributions in North America (Thompson and others, 1999), and 3)

applying numerical analysis that compared the North American data with the packrat

middens to produce estimates of past climates. The following text describes each of these

aspects in greater detail.

Plant Macro fossil Assemblages from Packrat Middens.

Previous studies (e.g. Spaulding, 1985; Wigand and others, 1995) recovered

numerous packrat middens from southern Nevada dating to the most recent period of

continental glaciation during the late Pleistocene (-40,000 to 12,000 yr B.P. [-40 to 12

ka]). This report focuses on quantitative paleoclimatic interpretations of the plant

macrofossil assemblages reported by Spaulding and Wigand, with particular emphasis on

reconstructions of the climate from the last glacial maximum (LGM, -18 ka). In addition

to this interval, we also estimated the past climates for each of four intervals of the late

Pleistocene (35 -30 ka, 27 -23 ka, 20.5 -18 ka, and 14-11.5 ka) that a panel of scientists

from the Desert Research Institute (DRI), Dames and Moore (D&M), DOE, University of

Colorado (CU) and the USGS selected as key times in the paleoclimatic history of the

Yucca Mountain region (Figure 1). These time periods were selected based on previous

paleoclimatic studies involving packrat middens, ostracodes, and other data sets that

suggested that the climatic characteristics of each period were different. Initially, the

period from 35-30 ka was thought to be the wettest interval of the late Pleistocene, and

the 20.5 to 18 ka the coldest.

Presence-absence information on all of the plant species identified from 39 packrat

middens in this region (Figure 1) were compiled by DRI and USGS researchers. The

presence-absence approach places equal weight on the occurrences of all species,

Figu

re 1

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rather than placing more (or less) emphasis based on the apparent abundance of a given

species. This approach seems warranted, as there is no strong evidence that the

abundance of a given plant species in a packrat midden assemblage reflects the actual

abundance on the landscape surrounding the midden.

The late Pleistocene packrat middens from the Yucca Mountain region were

collected from sites where today the plant cover is dominated by creosote bush (Larrea

divaricatd) and other plants adapted to the hot and dry current Mojave Desert. In

contrast, packrat midden data indicate that during the late Pleistocene these sites hosted

plants that today grow farther north and/or at higher elevations (including limber pine

[Pinus flexilis], white fir [Abies concolor], Utah juniper [Juniperus osteosperma}, big

sagebrush [Artemisia tridentata], and shadscale [Atriplex confertifolia]; Figure 2).

Collectively the modern distributions of these plant species imply that the late Pleistocene

climate was cooler and wetter than that of today.

Present-Day Climate and Plant Distributions.

As part of other research (Thompson and others, 1999), we estimated the present-

day climate of North America for each of 31,363 points on a 25-km equal-area grid

covering the continent (Bartlein et al, 1994). We then digitized maps of the current

distributions (from published sources, including: Benson and Darrow, 1981; Critchfield

and Little, 1966; Little, 1971, 1976; and Yang, 1970) of more than 300 major trees and

shrubs and aligned these distributions with the 25-km grid. This procedure provides the

basis for direct comparisons between the distributions of plant species and climatic

parameters and is the foundation for the paleoclimatic reconstructions in this report. In

addition to the plant taxa presented in Thompson and others (1999), the panel of DRI,

D&M, DOE, and USGS scientists determined that additional maps (not available from

previous publications) were required for the modern ranges of twelve additional plant taxa

that were common in late Pleistocene packrat middens from the Yucca Mountain region

(Table 1). USGS and DRI personnel worked together to compile this information, and

USGS contractors digitized the maps and entered this information into the dataset for

paleoclimatic analysis.

Figure 2. The present-day distributions of four key plant species (Pinus flexilis [limber pine], Juniperus osteosperma [Utah juniper], Abies concolor [white fir], and Atriplex confertifolia [shadscale]) recovered from late Pleistocene packrat middens in the Yucca Mountain region.

Pinus flexilis Juniperus osteosperma

Abies concolor Atriplex confertifolia

Table 1. Plant species for which maps were compiled by the USGS and/or DRI and digitized by the USGS.

Scientific Name Common NameAmbrosia dumosa white bur sageAtriplex canescens four-wing saltbushCercocarpus intricatus little-leaf mountain mahoganyChamaebatiaria millifollium fernbushChrysothamnus nauseosus rabbit brushColegyne ramosissima blackbrushEphedra nevadensis boundary ephedraEphedra viridis green ephedraFallugia paradoxa Apache phimePurshia tridentata antelope brushSymphoricarpos longiflorus snowberrySymphoricarpos oreophilus snowberry

Table 2. Present-day and Late Glacial Maximum (LGM) climatic estimates forapproximately 5000 ft (1524 m) elevation in the Yucca Mountain area (based on plant macrofossils from packrat middens).

Annual Annual Temperature Precipitation

PRESENT DAYThis study 13.4° C 125mmSpaulding, 1985 13.5° C 189 mm

LAST GLACIAL MAXIMUMThis study 7.9 to 8.5° C 266 to 321 mmSpaulding, 1985 6.5 to 7.5° C 246 to 265 mm

Estimation of Climatic Parameters from Vegetation Data.

The inventory of fossil plant remains recovered from each packrat midden was

compared with the plant list from each North American grid point to identify those grid

points whose present-day vegetation is similar to the late Pleistocene vegetation found in

the packrat midden (these modern sites are hereafter referred to as "analogues" for the

Pleistocene packrat middens). The modem temperature and precipitation values at the

analogue sites provide estimates for past climates in the region surrounding Yucca

Mountain. We used the latter information to identify climatic patterns for selected past

time periods by mapping the paleoclimatic estimates from multiple packrat midden fossil

localities, and by plotting them against the geographic locations and elevations of the

packrat midden sample sites. Climatic values for the late Pleistocene of Yucca Mountain

were estimated by using the climate-elevation relations derived from this approach.

Modern analogue techniques, methods that identify sites where plants found in

fossil assemblages are living today, are widely used to reconstruct past climates in eastern

North America and Europe (for examples, see Overpeck and others, 1985 and Guiot,

1990). The fossil and the gridded modem vegetation data in this study are both presence-

absence data. In order to obtain a measure of similarity between modern and fossil

presence-absence data, we used the binary Jaccard matching coefficient (Jaccard, 1908;

Schweitzer, 1994) to compare each packrat midden plant assemblage with each gridpoint

within the modern data set. The Jaccard coefficient provides a measure of how similar

each modern gridpoint assemblage is to the fossil assemblage, with a value of 0.0

indicating no shared species between the modern and fossil assemblages and a value 1.0

indicating that the two assemblages have exactly the same list of species.

The Jaccard matching coefficient identifies modern sites that have vegetation that

resembles (to varying degrees as measured by this coefficient) the Pleistocene vegetation

recorded in the packrat midden samples. Some of these potential analogues may include

some of the taxa in the fossil assemblage, whereas other potential analogues may lack

these but include other taxa present in the fossil assemblage. To provide paleoclimatic

estimates from fossil plant assemblages under these circumstances, we used weighted

averages of climatic parameters from a large number of possible analogues for each fossil

sample. To accomplish this, we had to make three decisions: 1) what is the threshold

value for the Jaccard coefficient below which we do not consider the modern vegetation

analogue to be similar enough to the fossil vegetation to be included in the analysis? 2)

how many possible analogues above a threshold value should be included in each

paleoclimatic estimate?, and 3) how should we weight the climatic information from each

analogue so that the information from the least similar modern samples (in comparison

with the fossil assemblage under consideration) does not overshadow the information from

the most similar modern samples? There are no firm guidelines for any of these questions,

so we experimented with different values for each of these three parameters and then

objectively judged the results by comparing climates at a suite of modern gridpoints with

those estimated by our method (see below).

Based on our experiments, we discarded potential modern analogues with Jaccard

coefficients less than 0.3, and then selected the 200 analogues with the highest Jaccard

coefficients above that value (if there were fewer than 200 analogues above that value, we

used all of them). As higher Jaccard coefficients imply greater similarity between the

modern and fossil vegetation than do low coefficients, we weighted the climatic data from

the analogues by the ratio of the cubes of their Jaccard coefficients. Hence the climatic

data from a modern sample with a Jaccard coefficient of 0.9 would be weighted

approximately six times the data from a sample with a coefficient of 0.5 (0.93 = 0.73; 0.53

= 0.13; 0.73/0.13 = 5.62). This ensures that the modern sites with the greatest similarity

contribute the most information to a given paleoclimatic estimate, while also allowing a

broad examination of the potential climates that could account for the fossil vegetation.

We used the method described above with the present-day vegetation to estimate the

modern climate at each of 80 grid points from the western interior of the United States

where weather stations occur near the grid points (Figure 3). Regression analysis of the

observed versus estimated January, July, and annual temperature at the grid points yielded

r2 values of 0.84 to 0.91 for temperature, indicating that, at least in the present-day

situation, this method provides very reliable estimates of temperature. The comparisons of

observed and estimated modern January, July, and annual precipitation provided r2 values

of 0.70 to 0.79 for precipitation and 0.71 to 0.80 for the log of precipitation. This

Figure 3. Comparison of observed (x-axis) and estimated (y-axis) present-day climatenormals for January and My temperature and precipitation based on the gridded vegetation at 80 grid points in the interior of the western United States (r2 values in parentheses indicate the comparison between observed climate at weather stations near the grid points and the predicted value for the grid points).

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indicates that our method produces somewhat less robust, but still acceptable, estimates of

precipitation.

ANALOGUE-BASED PALEOCLIMATIC RECONSTRUCTIONS

Modern Analogues for Pleistocene Vegetation and Climatic Differences from Today.

Using the method described above, the fossil data from packrat middens were

compared to our North American grid to locate the best modern analogues for the late

Pleistocene vegetation. Figure 4 provides an example of the geographic and climatic

spread of the modern analogues (before averaging) for a 13,740 yr B.P. packrat midden

from southern Nevada. As seen in this figure, most of the modern analogues for this

sample (and for other late Pleistocene middens from the Yucca Mountain region) lie in the

steppe and woodland regions of the central Great Basin, and to a lesser extent, the

Colorado Plateau. The climates of these analogue sites are cooler and wetter than the

modern climates at this fossil-midden site (and at other midden sites).

We estimated the modern temperature and precipitation at each packrat midden site

using the same techniques we used to assign climatic values to our North American grid

points (Thompson and others, 1999). "Anomalies" for each climatic parameter at each

site were then calculated by taking the difference between the estimated modern climatic

parameters at the packrat collection site and the late Pleistocene estimates.

RECONSTRUCTION OF LATE PLEISTOCENE CLIMATES

Reconstructed Climates Through Time. The proposed site of the nuclear waste

repository at approximately 5000 ft (1524 m) elevation at Yucca Mountain was apparently

at or near the lower elevational limits of Finns flexilis (Figure 5), indicating that relatively

moist forest environments did not extend below this elevation during the last ~40,000

years. However, climate did vary through the late Pleistocene, and to quantify these

changes we analyzed suites of packrat midden assemblages grouped by the

10

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time periods selected by the advisory panel (discussed above). Comparisons between time

periods may be misleading, as the number of samples and the elevational range vary

greatly between periods. Setting aside these potential problems, the analogue-based

reconstructions suggest that the paleoclimate from 35 to 30 ka was approximately 4° C

colder than today in the Yucca Mountain region, and mean annual precipitation was one

and one-half times greater than today (Figure 6). The climate apparently cooled and

became wetter through 27 to 23 ka, culminating in the coldest period during the late

Wisconsin during the Last Glacial Maximum (LGM; 20.5 to 18 ka). Climatic conditions

warmed somewhat by 14 to 11.5 ka, which was the period of the highest level of

reconstructed precipitation during the late Pleistocene. In the following text we

concentrate on reconstructions of the LGM (Table 2) when the estimated climate was the

coldest and mean annual precipitation was significantly greater than that of the present-

day.

Reconstructed Climates Versus Elevation. Maps of the reconstructed climate reveal

differences in the size of the anomalies (difference between modern and reconstructed past

climate parameters) within short distances, apparently due to the influence of the high

physiographic relief of southern Nevada. For the LGM, there are very strong relationships

between elevation and the amplitude of the temperature difference from today (r2 = 0.97

for January, 0.96 for July, and 0.98 for annual temperature). There are weaker, but still

strong relations for precipitation anomalies versus elevation (r2 = 0.51 for January, 0.82

for July, and 0.88 for annual precipitation) for the late glacial period (14 to 11.5 ka).

These paleoclimatic reconstructions imply that during the late Pleistocene the gradients of

decreasing temperature and increasing precipitation with rising elevation were more

gradual than those of today in southern Nevada. Subsequent analyses of packrat midden

assemblages from farther south in the Mojave Desert to as far north as the Bonneville

basin indicate that these apparent shallower-than-modern relations between increasing

elevation with decreasing temperature and increasing precipitation occurred during the

LGM throughout this region.

Compilations of paleoclimatic proxy data and numerical model simulations of past

14

climates (Thompson and others, 1993) both provide support for the hypothesis that the

westerlies were displaced southward from their modern position into the southwestern

United States during the late Pleistocene. The persistence through the year of a strong

pole-to-equator temperature gradient at that time also caused the westerlies to be much

stronger during the summer season than they are today. These factors would have led to

more frequent cyclonic storms in the Yucca Mountain region throughout the year,

enhanced cloudiness, and a replacement of convective precipitation regime by frontal

storms that provide precipitation across the elevational range. This change in storm

patterns would have reduced the rate of change (relative to today) of decreasing

temperature with rising elevation and the generally wetter climate would also lower the

temperature lapse rate.

Paleoclimatic Inferences from Missing Plant Species During the Late Pleistocene. As

discussed above, the late Pleistocene occurrences of limber pine (Pinus flexilis), Utah

juniper (Juniperus osteosperma), and other montane and steppe plants in modern desert

environments suggest cooler than modern temperatures, and greater moisture availability

than today (Spaulding, 1985). Spruce (Picea engelmannii, P. pungens), lodgepole pine

(Pinus contorta), subalpine fir (Abies lasiocarpa), prostrate juniper (Juniperus communis)

and other boreal plants have not been found in packrat middens in southern Nevada,

suggesting that the late Pleistocene climate was not cold or wet enough for these boreal

plants. On the other hand, Pleistocene-age middens from the Yucca Mountain area lack

evidence of ponderosa pine (Pinus ponderosa\ Douglas-fir (Pseudotsuga menziesii\ and

other montane trees that live today in the mountains of southern Nevada. Their absence,

and the rarity of single-needle pinyon pine (Pinus monophylld), may suggest that

Pleistocene climates were too cold for those plants. Similarly, the absence of the now

widespread creosote bush (Larrea divaricata) suggests that late Pleistocene temperatures

were much colder than those of today (Spaulding, 1985). In this section we address the

question: do our paleoclimatic estimates (if correct) provide an adequate explanation for

the absence of these forest and desert plants?

The present-day climatic and plant distributional data in Thompson and others

15

(1999) provide the basis for examining the apparent climatic meaning of the late

Pleistocene absences of key plant species in the Yucca Mountain Region. As illustrated in

Figure 7, our reconstructed LGM January temperatures were apparently too cold for the

survival ofLarrea divaricata (in accordance with Spaulding's [1985] interpretation) or

Juniperus californica. Conversely, winter temperatures were apparently too warm for

Piceapungens, Picea engelmannii, Abies lasiocarpa, Juniperus communis, and Pinus

flexilis (which, as illustrated in Figure 5, was apparently near its lower elevational limits

here during the LGM). The subsequent figures provide similar illustrations of the

reconstructed climates compared with the apparent tolerances of these species in respect

to July and annual temperature (Figures 8 and 9), and to January, July, and annual

precipitation (Figures 10 to 12). These figures collectively indicate that many of the

'absent' taxa presently live under climatic conditions that differ in at least one aspect of

seasonal or annual temperature or precipitation from the analogue-based reconstruction of

the LGM climate.

Table 3 summarizes the overall patterns observed in these figures, and as seen

here, both temperature and precipitation were important in excluding many of these taxa.

However, the reasons for the exclusions of Pinus edulis, Pinus longaeva, and Pinus

monophylla are unclear. On the other hand, all three of these species lived in or near

southern Nevada during parts of the late Pleistocene, so it is not surprising that the

reconstructed climates would apparently permit their growth near Yucca Mountain.

EVALUATION OF CLIMATIC ANOMALIES FOR THE LGM

As discussed above, Spaulding (1985) estimated climatic anomalies for the late

Pleistocene based on the present-day southern-most and/or lowest elevational occurrences

of key plant species that were recovered from packrat middens in the Yucca Mountain

region. Tables 2 and 4 illustrate the differences between Spaulding's and our

paleoclimatic reconstructions for the LGM (interpolated to the 5000 ft [1524 m] elevation

of the proposed repository). Although our results indicate somewhat warmer and wetter

conditions than Spaulding's, the absolute differences between the two approaches are not

16

Figu

re 7

. Pr

esen

t-da

y co

rres

pond

ence

s be

twee

n m

ean

Janu

ary

tem

pera

ture

and

the

dist

ribu

tion

of se

lect

ed tr

ee a

nd s

hrub

spe

cies

fro

m w

este

rn N

orth

A

mer

ica

(dat

a ar

e th

e 10

% to

90%

dis

trib

utio

nal l

imits

fro

m T

hom

pson

and

oth

ers,

199

9).

The

17 s

peci

es o

n th

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ft ar

e sp

ecie

s th

at d

o no

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cur i

n la

te P

leis

toce

ne p

ackr

at m

idde

n as

sem

blag

es f

rom

the

Yuc

ca M

ount

ain

regi

on (

alth

ough

Jun

iper

us s

copu

loru

m, P

inus

mon

ophy

lla,

Pin

us lo

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nd P

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enzi

esii

and

have

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red

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e br

oade

r so

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aste

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a /s

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tern

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tah

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). Th

e 4

spec

ies

on th

e ri

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ave

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vere

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le m

idde

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es in

the

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ca M

ount

ain

regi

on (a

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gh

Pin

us fl

exili

s w

as a

ppar

ently

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r nea

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est l

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tion

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e pr

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posi

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e gr

ay b

and

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nge

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atic

est

imat

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or th

e La

st G

laci

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rom

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ern

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ary

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re 8

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re 9

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re 1

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n an

nual

pre

cipi

tatio

n an

d th

e di

stri

butio

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sele

cted

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and

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ub s

peci

es f

rom

wes

tern

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th A

mer

ica

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cap

tion

for F

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e 7

for

a m

ore

com

plet

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Pinus

ponderosa

Picea

engelmannii

Abies lasiocarpa

CO | S +£ 2 C

s 1

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magn

ifi

Pseudotsuga menx

CD (

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Table 3. Climatic factors that may have excluded selected taxa from the Yucca Mountain region during the late Pleistocene.

" 'Abies concolor

Abies lasiocarpa

Abies magnified

Juniperus californicaJuniperus communis^Juniperus scopulorum

Larrea divaricata

°icea engelmannii

Picea pungens

Pinus albicaulis

3inus contortaDinus edulis

*Pinus flexilis

^Pinus longaeva

*Pinus monophylla

Pinus ponderosa*Pseudotsuga menziesii

Tsuga heterophyllarsuga mertensiana

January

H

CH

CHHH

H

KEY:H = too hot for species, C = too cold, D

TEMPERATUREJuly

H

MH

CHHHH

H

MHH

= marginal, W =

Annual

H

CH

CHHHH

H

M

PRECIPITATIONJanuary July

DDM W

DM

D MD

D MD

DD D

too wet, M = marginal

AnnualDDD

MM

DDDD

D

DDDD

* = present near Yucca Mountain during part of the late PleistoceneA = present regionally during the late Pleistocene

Januaryleason for exclusion:

too warm or dry 6too cold or wet 2

TEMPERATUREJuly

Plants for whom reason of exclusion unclear: Pinus edulis,

AnnualPRECIPITATION

January July

10 8 6 8121 1

Pinus longaeva, Pinus monophylla

Annual

140

23

Tab

le 4

. R

econ

stru

cted

LG

M c

limat

es a

nd c

limat

ic an

omal

ies b

ased

on

the

pres

ent-

day

clim

ate

estim

ates

use

d in

this

repo

rt c

ontr

aste

d w

ith th

e pr

esen

t-da

y va

lues

use

d by

Spa

uldi

ng(1

985)

.

Mea

n A

nnua

l Tem

pera

ture

Spau

ldin

g T

his

Rep

ort

Mea

n A

nnua

l Pre

cipi

tatio

nSp

auld

ing

Thi

s Rep

ort

Est

imat

ed

Val

ues

MO

DE

RN

°C 13.5

13

.4

mm

18

9 12

5

Rec

onst

ruct

ed

Val

ues

LG

M

LG

M

Min

umum

M

axim

um

°C

°C6.

5 7.

5 7.

9 8.

5

mm

m

m

246

265

266

321

Mul

tipl

ier

for A

nnua

l Pre

cipi

tati

onSp

auld

ing

Thi

s Rep

ort

Ano

mal

ies b

ased

on

Spau

ldin

g M

oder

n V

alue

s

LG

M

LG

M

Min

umum

M

axim

um

°C

°C7

6 5.

6 5

mm

m

m57

76

77

13

2

Mod

. X

Mod

. X

1.3

1.4

2.1

2.6

Ano

mal

ies b

ased

on

This

Rep

ort

Mod

em V

alue

s

LG

M

LG

M

Min

umum

M

axim

um

°C

°C6.

9 5.

9 5.

5 4.

9

mm

m

m

121

140

141

196

Mod

. X

Mod

. X

2.0

2.1

1.4

1.7

Mod

. X =

Mod

ern

mea

n an

nual

pre

cipi

tatio

n m

ultip

liedb

y th

is n

umbe

r

great for either mean annual temperature (6.5° C versus 7.5 ° C) nor for mean annual

precipitation (246 - 265 mm versus 266 - 321 mm). As discussed in greater detail below,

the differences between these estimates are substantially less than the variability observed

during the historic period.

Importance of Historical Climatic Baseline Data. Although the absolute differences

between our estimates and those of Spaulding (1985) are small, the calculated anomalies

for precipitation are large. As shown in Table 4, Spaulding's anomalies for LGM mean

annual precipitation (based on Beatly's historic climate data) are 57 to 76 mm, which

indicates precipitation at levels 1.3 to 1.4 times historic values. In contrast, our LGM

mean annual precipitation anomalies (based on our historic climate normals) are 141 to

196 mm, which implies LGM precipitation at levels 2.1 to 2.6 times historic values. These

comparisons point out the importance of the modern climatic baseline data in the

assessment of the differences between present-day and late Pleistocene climates. As

discussed above, our baseline is based on the climate "normals" for the period 1951 to

1980, with the great majority of our data obtained from U.S. Weather Service calculations

of these "normal" values. Thus these data provide a baseline that is continental in scale,

that is based on three decades of records, and that conforms to the U.S. Weather Service

standards. However, this data set does not provide detailed coverage in proximity to

Yucca Mountain. In contrast, Spaulding (1985) employed a modern baseline derived from

data on the Nevada Test Site collected by Beatly (1975, 1976) for the period 1963 to

1972. This dataset provides more local data, but was collected with non-standard

methods (which may over-estimate precipitation) for only a decade.

For the analyses in this report we interpolated the modern climate estimates to the

elevation of the proposed repository at 5000 ft (1524 m). Although the two data sets

have very similar mean annual temperature estimates for this elevation (Tables 2 and 4)

Spaulding's estimates of modem mean annual precipitation are significantly higher than

ours. The National Oceanic and Atmospheric Administration (NOAA) provides historic

climatic data by divisions within each state (NCDC, 1994; Karl and others, 1986), and

25

(unfortunately) Yucca Mountain is located on the boundary between Nevada Divisions 3

and 4. These data indicate that annual temperature varied relatively little between 1950

and 1972 (the combined period of the Spaulding and our baseline data; Figure 13).

However, precipitation increased almost monotonically over this interval, with the result

that our baseline (1951 to 1980) for annual precipitation fells near the long-term historic

mean, whereas that employed by Spaulding (1963 to 1972) covers a much wetter period

(and, in addition, Beatly's measurements may overestimate precipitation, Figure 14). The

differences in precipitation estimates in the modern baselines employed by Spaulding

(1985) and in this report greatly influence the calculated anomalies for the LGM (Table 4).

In fact, the differences in these anomalies is influenced more by the differences in the

modern baseline data than by differences in the estimation of late Pleistocene climates

(Figure 15).

Comparison of Reconstructed LGM Climates With Historical Climatic Variations.

As discussed above, Yucca Mountain is located on the boundary between NOAA Nevada

Climatic Divisions 3 and 4. Over the past century, historic variations in precipitation in

both of these divisions have reached levels (on an annual basis) as high as those

reconstructed for the LGM (Figure 16). However, temperatures have remained above the

LGM estimates throughout the historic period, although rare years in Nevada Division 3

have approached the mean annual temperature reconstruction for the LGM at Yucca

Mountain (Figure 16). These individual years are labeled on the scatterplot on Figure 17,

where it can be seen that in many years between 1897 and 1917 in Nevada Division 3 the

annual temperatures approached the mean annual temperatures reconstructed for Yucca

Mountain for the LGM. Several of those years (particularly 1901, 1904, 1905, 1906, and

1913) also had precipitation levels comparable to those reconstructed for the LGM.

These years include both El Nino and La Nina years (Cayan and Webb, 1992), although

the amplitude of the difference between these patterns was apparently smaller than in

recent years. The first ~15 years of this century are estimated to have been among the

coldest and wettest of the last 400 years in the Intermountain basins and Southwest

Deserts (Fritts and Shao, 1992, p. 279-280), although precipitation levels of similar

26

Figu

re 1

3. H

isto

ric v

aria

tions

in a

nnua

l tem

pera

ture

dep

icte

d as

10-

year

runn

ing

mea

ns.

Dat

a ar

e fr

om N

OA

A c

ompi

latio

ns o

f ins

trum

enta

l wea

ther

da

ta fo

r Nev

ada

regi

ons

3 an

d 4

(see

inse

t map

). Th

e m

odem

mea

n an

nual

tem

pera

ture

est

imat

es fo

r Yuc

ca M

ount

ain

(500

0 ft,

152

4 m

el

evat

ion)

from

Spa

uldi

ng (1

985)

and

this

repo

rt ar

e sh

own

on th

e rig

ht.

The

mea

n an

nual

tem

pera

ture

reco

nstru

ctio

ns fo

r the

Las

t Gla

cial

M

axim

um (L

GM

) fro

m S

paul

ding

(198

5) a

nd th

is re

port

are

show

n as

gra

y ba

nds

on th

e lo

wer

par

t of t

he fi

gure

.

K)

-4

20 18 16O 2

140>

Q

.

0> (U 3

c1

2 10 8

10-y

ear r

unni

ng m

ean

Nev

ada

Div

isio

n 4

Nev

ada

Div

isio

n 3

Mod

em M

AT

(Spa

uldi

ng,

1985

) M

M

Mod

em M

AT

(Thi

s R

epor

t)

1890

1910

1930

1950

YE

AR

1970

1990

Figu

re 1

4. H

isto

ric v

aria

tions

in a

nnua

l pre

cipi

tatio

n de

pict

ed a

s 10

-yea

r run

ning

mea

ns.

Dat

a ar

e fr

om N

OA

A c

ompi

latio

ns o

fin

stru

men

tal w

eath

er d

ata

for N

evad

a re

gion

s 3

and

4 (s

ee in

set m

ap).

The

mod

ern

mea

n an

nual

pre

cipi

tatio

n es

timat

es fo

r Y

ucca

Mou

ntai

n (5

000

ft, 1

524

m e

leva

tion)

from

Spa

uldi

ng (1

985)

and

this

repo

rt ar

e sh

own

on th

e rig

ht.

The

mea

n an

nual

pr

ecip

itatio

n re

cons

truct

ions

for t

he L

ast G

laci

al M

axim

um (L

GM

) fro

m S

paul

ding

(198

5) a

nd th

is re

port

are

show

n as

gra

y ba

nds

on th

e up

per p

art o

f the

figu

re.

K> 00

350

300

10-y

ear r

unni

ng m

ean

1890

LGM

Pre

cipi

tatio

n (t

his

repo

rt)

1910

1930

1950

YE

AR

1970

1990

LOI«

Pr«c

iplta

tlon

(Spa

uldi

ng, 1

986)

Nev

ada

Div

isio

n 3

Nev

ada

Div

isio

n 4

. M

oder

n M

AP

(Spa

uldi

ng,

1985

)

Mod

ern

MAP

(T

his

Rep

ort)

Figu

re 1

5. C

ompa

rison

s of

estim

ates

of t

he m

oder

n an

d La

st G

laci

al M

axim

um (

LGM

) clim

ates

at Y

ucca

Mou

ntai

n.

Spau

ldin

g (1

985)

in

ferr

ed a

sm

alle

r diff

eren

ce in

pre

cipi

tatio

n be

twee

n to

day

and

the

LGM

than

in th

is re

port,

in p

art b

ecau

se h

is e

stim

ate

of

mod

ern

prec

ipita

tion

is h

ighe

r tha

n th

e on

e us

ed h

ere.

to sO

14 -

c3

12 ~

*H I

^

8 6 - 0

Mod

ern

Clim

ate

Estim

ates

fo

r Yuc

ca M

ount

ain

O

This

R

epor

t

Mod

ern

and

LG

M C

limat

eE

stim

ates

are

for

5000

ft(1

524

m)

elev

atio

n at

Yucc

a M

ount

ain

Spa

uldi

ng

(198

5)

LGM

Clim

ate

Estim

ates

for

Yuc

ca M

ount

ain

100

200

300

Mea

n A

nnua

l Pre

cipi

tatio

n (m

m)

Figure 16. Annual precipitation and annual temperature for the period from 1895 to 1998 for Nevada regions 3 and 4 (see inset map on Figure 13 for the definition of these NOAA regions) characterized by the number of years when precipitation or temperature fell within given ranges. The analogue-based estimates for the Last Glacial Maximum (LGM) are shown as vertical bars. Over the past century there have been years in both regions when precipitation was as great as that reconstructed for the LGM. There have also been a few years when temperatures in region 3 were cool enough to approach those estimated for the LGM.

Mountain Today

200 Precipitation (mm)

300

YuccaMcxntainToday

I/i / i

8 10 12 14 16 Temperature (°C)

INV4 I I IIII\ \

18 20

30

Figu

re 1

7. C

ompa

rison

of h

isto

ric c

limat

es in

NO

AA

Nev

ada

regi

ons

3 (f

illed

circ

les)

and

4 (o

pen

circ

les)

com

pare

d w

ith

mod

ern

and

LGM

clim

ate

estim

ates

for

Yuc

ca M

ount

ain.

Se

lect

ed y

ears

in re

gion

3 a

re la

bele

d. A

s sh

own

here

, the

re w

ere

seve

ral y

ears

in th

e fir

st p

art o

f thi

s ce

ntur

y (n

otab

ly:

1901

,190

4,19

05,1

906,

1913

) whe

n th

e cl

imat

e of

re

gion

3 w

as a

s w

et, a

nd n

early

as

cold

, as

the

LGM

reco

nstru

ctio

ns (a

t lea

st a

s vie

wed

on

an a

nnua

l bas

is -

the

seas

onal

clim

ates

cou

ld s

till b

e qu

ite d

iffer

ent t

han

thos

e of

the

LGM

).

U)

20

n

18 16 14 12 10

8

His

toric

Ann

ual

Pre

cipi

tatio

n vs

. Ann

ual T

empe

ratu

reo

o

o °o

o

o 0

ftof

lO

° <b

°o%

0

oO

Q

O

o N

V4

N

V3

p 0

o o0

o

o.

Q0

0

O

O

«

O

°00°S

b

O f

t °

0

%

<*>

°

o Jt

Fo

° °

° °

o °

°

Mod

em

Mod

ern

(this

rep

ort)

(Spa

uldi

ng)

o o

**s.

*

*

>?

»

.

1898 r

,0V

1916

191?

-1982

.1898

.19

11

1912

0191

1

(Spa

uldi

ng)

1978

LGM

(th

is r

epor

t)

5010

015

0 20

0 25

0

Ann

ual P

reci

pita

tion

(mm

)30

035

040

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amplitudes have been common in Nevada Division 3 through the latter half of the

Holocene (Hughes and Graumlich, 1996). Examination of the synoptic climatology of the

early part of this century in the Southwest could provide insights into the nature of

Pleistocene climates, but is outside the scope of this report.

Although individual years in the historic period had conditions that approached

those reconstructed for the LGM, on a decadal basis the climate of Nevada Division 3 has

remained warmer and (generally) drier than that of the LGM (Figure 18). The range of

inter-decadal variability over the past century has been ~2° C and ~97 mm in Nevada

Divisions 3 and 4, exceeding the differences in LGM estimates between Spaulding (1985)

and this report.

Potential influences of changes in atmospheric chemistry on plant communities.

Atmospheric carbon dioxide concentrations during the late Pleistocene were greatly

reduced relative to pre-industrial Holocene levels, and during the LGM they were as low

as ~ 190 ppmv (versus ~280 ppmv during the Holocene, Bamola and others, 1987). In

addition to its effects on global climates, this change in atmospheric chemistry may have

had influences on the physiology of plants, their elevational distributions, and the

sensitivity of plant species and communities to climatic variations (Ehleringer and others,

1997). Woodward (1987) demonstrated that stomatal density decreased during the last

200 years in a variety of species due to the increased availability of carbon dioxide in the

atmosphere following the industrial revolution. Van de Water and others (1994)

demonstrated that fossil needles of Pinus flexilis in the Southwestern United States had

17% greater stomatal density during the LGM than at 12,000 years ago (by which time

atmospheric carbon dioxide had increased 30% over LGM levels). By their calculations,

water-use efficiency for this species would have been ~15% lower at the LGM than at 12

ka, assuming constant temperature and humidity.

Ecophysiological modeling by Jolly and Haxeltine (1997) suggests that much of

the lowering of upper treelines in East Africa during the LGM could be directly caused by

the lower concentrations of atmospheric carbon dioxide. They also found that some

vegetational assemblages increased their climatic tolerances under lower levels of carbon

33

dioxide. At this point we have no way to assess how well this concept applies to the

Yucca Mountain region, but the stability through time of the limber pine/Utah juniper

woodland (as other indicators of climate changed through time) could conceivably be due

to changes in climatic sensitivity of vegetation related to changes in atmospheric

chemistry. Using the African model, it is possible that the apparent changes in lapse rates

for temperature could be due (at least in part) to an expanded range of climatic tolerance

of woodland vegetation (relative to today). In any case, the increased stomatal density

should have decreased water-use efficiency for most plants during the LGM, and hence

they would require greater-than-modern levels of precipitation to sustain themselves.

Hence, our estimates of late Pleistocene precipitation may be too low, especially for the

LGM.

PRIMARY CONCLUSIONS.

1) For 5000 ft (1524 m) at the Yucca Mountain repository we estimate that mean annual

temperature (MAT) and precipitation (MAP) differed from our modern baseline data

as follows:

35 to 30 ka: MAT = -4° C colder than today, MAP = 1.5X modern levels;

27 to 23 ka: MAT = -5° C colder than today, MAP = 2.2X modern levels;

20.5 to 18 ka: MAT = -8° C colder than today, MAP = 2.4X modern levels;

14 to 11.5 ka: MAT = -5.5° C colder than today, MAP = 2.6X modern levels

2) The selection of historic baseline data has strong influence on the perceived differences

between Pleistocene and "modern" climates. The baseline data used by Spaulding

(1985) was significantly wetter than either the baseline employed in this report or the

long-term historic mean. Consequently, Spaulding's estimates of the difference

between late Pleistocene and "modern" precipitation levels is much lower than ours.

3) It is difficult to assess the potential effects of decreased atmospheric carbon dioxide

concentrations during the late Pleistocene on the physiology of various plant species.

However, it is plausible that nearly all species required higher levels of moisture than

they do under higher concentrations of carbon dioxide during the 20th century.

Consequently, our estimates of late Pleistocene precipitation levels may be too low.

34

4) During the early part of this century there were several years when the climate of the

region adjoining Yucca Mountain on the north approached the annual temperature and

moisture conditions reconstructed for the LGM at Yucca Mountain. Inter-decadal

variability in the historic climate record is greater than the differences in the LGM

reconstructions of Spaulding (1985) and this report.

35

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38