Construction of the Casa Grande by Sequential High Precision C-14 Dating

Austin Long Department of Geosciences, University of Arizona, Tucson, Arizona 85721 John Andresen National Park Service, Casa Grande Ruins National Monument, P.O. Box 518, Coolidge, Arizona 85228 Jeffrey Klein Department of Physics, University of Pennsylvania, Philadelphia, Pennsylvania 19104-3859

Two composite radiocarbon dates on wood beams from a unique, prehistoric building in southern Arizona are the result of growth series subsampling of the beams and statistical evaluation of radiocarbon dates on the subsamples. This procedure avoids some of the problems caused by secular variations in the bio- spheric 14C activity and by the dating of specimens grown over a range of years. This procedure yields results similar to more precise tree-ring dates from other sites in the southwestern United States. The procedure illustrated here is applicable to any archaeological tree specimen consisting of at least the last 40 growth rings.

INTRODUCTION

The Hohokam, a prehistoric farming cul- ture of southern Arizona, is well known for its extensive irrigation systems and unique archi- tectural style. This remarkable culture has been the object of archaelogical study for over a century, but the dating of its phases and major events is still difficult and the focus of current research (Schiffer, 1982). This report provides precise dating for one important event, the construction of the Casa Grande, and discusses a new method of analyzing ra- diocarbon dates which allows greater preci- sion in determining dates of timber cutting.

Radiocarbon dates on wood and charcoal from archaeological sites are nearly always averages of the growth periods represented by whole samples, making it difficult in some cases to relate such dates to the prehistoric human activity under study. Often, the outer portion of a wood sample is burned away or deteriorated, further removing the terminal date of the sample from the time of associated human activity. Rarely can one provide an accurate and precise date on the last year of growth for a piece of wood, even when the wood is well preserved (Dean, 1978).

Calibrated radiocarbon dates usually have a greater uncertainty in terms of actual calen-

dric years than do conventional radiocarbon dates because of established secular varia- tions in the ‘*C levels in atmospheric carbon dioxide. For example, A.D. 1300 to 1400 (Fig- ure 11, a time of known Hohokam building activity, shows a significant fluctuation in 14C

activity. Single, even highly precise, 14C dates in this time range yield ambiguous calibra- tion results.

PROCEDURE

We report here composite radiocarbon dates on the growth periods of two trees used as construction beams in the massive, prehis- toric Hohokam structure known as the Casa Grande, or Big House, now the main ruin at Casa Grande Ruins National Monument in southern Arizona. All dates reported here are from conventional beta-decay counting, as dis- tinguished from accelerator mass spectrome- try dates. Arizona accelerator date lab num- bers are preceded by the “AA-” prefix. Al- though precision of these dates does not match that of conventional tree-ring dates from sites in the southwestern United States, the dates are the most precise radiocarbon dates yet reported from the Hohokam area and come from a unique archaeological context. The

Geoarchaeology: An International Journal, Vol. 2, No. 3, 217-222(1987) 01987 by John Wiley & Sons, Inc. CCC 0883-6353/87/030217-06$04.00

CASA GRANDE

I 1

CALENDRIC DATE (AD)

Figure 1. Fluctuations in carbon-14 activity registered in the calibration schemes of Klein etal. (19821, the area within the dashed lines, and of Stuiver (19821, solid line.

Case Grande, standing at its original 11 m, is the largest known Hohokam building. It rep- resents the peak in Hohokam architectural development, and it may be the tallest build- ing attempted by Hohokam architects.

One beam segment from the Casa Grande is white fir (Abies concolor, Casa Grande Ruins National Monument catalog number 219- 21991, has 48 rings, and crossdates with three other white fir specimens in the Casa Grande Ruins National Monument collection (speci- men numbers 73-1204, 73-1771, and 548- 1056). All four were cut down in the same year but cannot be assigned calendar dates by con- ventional tree-ring methods (W. Robinson, personal communication, 1985).

Another beam segment from the Casa Grande isjuniper (Juniperus sp., catalognum- ber 232-2227), has approximately 72 rings, and does not crossdate with any other beams in the collection, or with any master tree-ring chronology. Juniper is especially difficult to date dendrochronologically owing to its miss- ing and false rings, and crossdating between species is virtually impossible. Both speci- mens dated in this study are the butts of main supports for the Casa Grande’s floors (Wilcox and Shenk, 1977, Table 6).

For radiocarbon processing, each specimen

was cut into concentric subsamples according to ring distance for the outermost, or termi- nal, growth ring. The white fir was cut into five subsamples, the outer subsample repre- senting the last eight years of growth, and the inner four each representing ten years of growth. The radiocarbon age on a subsample approximates the age of the mid-ring for that subsample.

The juniper was similarly cut into four sub- samples, each one representing approximate- ly 18 years of growth of the tree from which it was made. Unlike the white fir specimen, in which each year equals exactly one year of growth, the juniper specimen might have lo- cally absent (“missing”) and false annual (“extra”) rings. A necessary assumption for the calculations explained below is that, on the average, the number of “missing” rings equals the number of “extra” rings in each juniper subsample. This assumption intro- duces an unquantifiable uncertainty factor in- to the juniper data.

Chemical processing of each radiocarbon sample consisted of toluene - alcohol solvent extraction, then chlorite bleaching to produce cellulose. Weights of cellulose were mostly in the 12 to 13 gram range, except for A-2390, which was five grams. The combustion prod- uct carbon dioxide was counted in gas propor- tional counters of 2.5 liters volume at three atmospheres pressure, with backgrounds ranging from four to five counts per minute.

Table I gives the nine Arizona laboratory numbers for the subsamples, ring counts, con- ventional 5568-year half-life ages with their one sigma counting uncertainty, and Cali- brated ages based on two independent schemes (Klein et al., 1982; Stuiver, 1982). The Klein et al. calibrated dates are in the form of 95% confidence intervals, which in length are roughly four times one standard deviation. Stuiver’s calibration scheme (Stuiver, 1982) has a higher resolution than the longer one of Klein et al. (1982) and consequently cali- brated dates based on the former can have two or more disconnected ranges. This results from the reversals illustrated for the time range considered here in Figure 1, which shows both calibration graphs superimposed.

21 8 VOL. 2. NO. 3

D z

- f rri n f

Tab

le I.

Nin

e su

bsam

ples

ofw

ood

from

two

beam

s wit

h ra

dioc

arbo

n da

tes a

nd d

endr

o-ca

libr

ated

ages

usi

ng tw

o ca

libr

atio

n sc

hem

es.

Tex

t exp

lain

s der

ivat

ion

of p

lott

ed p

oint

s, so

me

of w

hich

(*I a

re n

ot m

idpo

ints

of r

ange

. Cut

ting

dat

e es

tim

ates

are

inte

rcep

ts a

t rin

g nu

mbe

r 1 w

ith

coun

ting

sta

tist

ics

unce

rtai

nty.

Sta

ndar

d de

viat

ion

of t

he m

ean

is i

n pa

rent

hese

s.

Con

vent

iona

l H

igh-

reso

luti

on

95%

con

fide

nce

14C

dat

e w

ith

67%

con

fide

nce

cali

brat

ed r

ange

K

lein

et a

l.

Rin

g co

unt

Subs

ampl

e L

ab n

o.

(fro

mou

tsid

e)

mid

-rin

g

stan

dard

de

viat

ion

(yr b

efor

e pr

esen

t)

Whi

te F

ir

rang

e (S

tuiv

er

Hig

h-re

solu

tion

ca

libr

atio

n pl

otte

d po

int

(cal

endr

ic

(cal

endr

ic

year

s)

year

s)

(Kle

in e

t a2.-

cali

brat

ion)

(c

alen

dric

ye

ars)

plot

ted

poin

t 67

% co

nfid

ence

(c

alen

dric

ye

ars)

A-2

208

1-8

4

A-2

207

9-18

14

A

-220

6 19

- 28

24

A

-220

5 29

- 38

34

A

-220

4 39

-48

44

630 f 1

9

674 f 1

9 63

4 t 2

6 68

4 f 1

9 69

3 f 3

1

AD

130

0-13

20

AD

137

0-13

90

AD

128

5-12

95

AD

129

0-13

95

AD

128

5-12

95

AD

129

0-12

95

AD

131

0 +-

10

AD

129

0 f 5

A

D 1

292 * 3

A

D 1

290 f 5

A

D 1

288

+- 8

AD

126

0-14

00

AD

125

0-13

40

AD

126

5 - 14

00

AD

125

0-13

40

AD

124

5-13

55

AD

133

5 f 3

5

AD

130

0 f 2

3 A

D 1

330

? 3

4 A

D 1

295 f 2

3 A

D 1

295 f 2

5

cutt

ing

date

est

imat

es

AD

131

6 ?

2 (i

= 5

) A

D 1

331-

f 12

(s

= 7

)

A-2

387

1-18

9

725

2 2

1 A

D 1

265-

1285

* A

D 1

280 * 1

0 A

D 1

235-

1325

A

D 1

285 f 2

5 A

-238

8 19

-36

27

725 f 1

9 A

D 1

265-

1285

* A

D 1

280 f 1

0 A

D 1

235-

1325

A

D 1

285 * 2

5 A

-238

9 37

-54

45

743

2 1

7 A

D 1

260-

1280

A

D 1

270 * 1

0 A

D 1

230-

1320

A

D 1

275 f 2

5 A

-239

0 55

-72

63

772 f 5

5 A

D 1

250-

1285

* A

D 1

260 f 1

8 A

D 1

210-

1300

A

D 1

260 f 4

5

AD

1310

-f 14

(s

= 6

)

Juni

per

cutt

ing

date

est

imat

es

AD

130

5 f 6

Cs

= 7

)

CASA GRANDE

DISCUSSION

The “calibrated range” dates in Table I, at least in principle, correct for uncertainties in offset, or differences between some radiocar- bon ages and calendar ages. No calibration scheme, however, can completely eliminate statistical uncertainty in counting radiocar- bon decay (or in counting atoms in the case of direct detection). We describe below a tech- nique for reducing this counting uncertainty for special situations such as segments of dates of known relative age.

We have plotted calibrated dates for each of the specimens versus tree-ring number (num- bered from the outside) using both calibration schemes (Figure 2). In the case of the high resolution calibration where two calibrated ranges resulted, we use only the older range because the younger range is inconsistent with the other precision calibrated dates.

For purposes of regression, it is convenient to choose a point within the interval which represents the most probable value. This point, however, might not be the midpoint of the interval because of curve inflections within the interval (Figure 1). In the case of the high resolution calibration, the calibrated points

plotted for regression are those corresponding to the most probable 14C activity (conven- tional date). This determination was made from a plot connecting our point estimates from the Stuiver calibration scheme (Stuiver, 1982). In the case of Klein et al. calibration (Klein et al., 1982), the plotted point corres- ponds to the center of the subtended range on the calibrated scale. The diagonal lines are the least squares best fit line constrained to a 45 degree slope. The estimate of the cutting date is where the best fit line intersects the terminal ring on the outer perimeter of each beam. The four estimates arise from the use of two beams and two calibration schemes.

If the radiocarbon dating and annual ring determination uncertainties were zero, the calibrated dates from the subsamples of each beam when plotted against mid-ring dates for each sample would fall along a 45 degree an- gle line. Figure 2 shows that the series of dates do not fall neatly along a 45 degree angle line. We assume this failure is largely the result of the statistical uncertainty in the measurement of each date, given as standard deviations in Table I. Other contributors to

A n a Y

W s n 0 a !3 a

K

z

0

1340

1320

1300

1280

1260

t I 1 I 1 I I I I 1 I I - - + a b

- -

-

- -

- -

0 10 20 30 40 50 0 10 20 3 0 4 0 50 80

TREE-RING NUMBER Figure 2. Calibrated carbon-14 dates vs. tree-ring number for white fir (a) and juniper (b) specimens using the Stuiver scheme (0 ) and the Klein et al. scheme (+ ). Diagonal lines are least-squares best fit, constrained to 45”. Solid lines represent (o ) , dashed lines (+ ). The intercepts at the “zeroth” tree ring are the four estimates of cutting dates for the two beams. See Figure 3 for presentation of statistical uncertainties of intercepts.

220 VOL. 2, NO. 3

CASA GRANDE

nonexactness of fit are inherent uncertainties in the calibration schemes and the possibility of missing or extra rings.

The four cutting-date intercepts (Table I, Figure 3) are in remarkable agreement con- sidering the various uncertainties. The only outlier is the Klein et al. result on the white fir. This seeming anomaly may be caused by the calibration ambiguity at A.D. 1310 and 1380. We estimated the later date as unlikely using the Stuiver scheme. The clear advan- tage to this sequential sample analysis ap- proach is that one may choose between two or more possible calibrated ranges. Thus the most likely cutting date is closer to the time indi- cated by the other three dates, i.e., in the first two decades of the 14th century A.D.

The student’s t-test on the high resolution estimates of cutting dates on the two beams suggest a 30% probability that the cutting dates were the same year. In addition to 14C statistical uncertainties, the tenuous assump- tion that each juniper ring represents one year may contribute to the difference between these results. A menu of cutting date calculations (Figure 3) illustrates their essential statisti-

1340

1320 0 K P

1300

cal agreement. Our choice for the most reli- able cutting date conclusion is the one based on the Stuiver calibration of the white fir specimen. The most likely time of cutting is the decade A.D. 1310 to 1320.

Several lines of evidence suggest that the roughly 600 beams used as floor and roof sup- ports in the Casa Grande were freshly cut for that structure, transported quickly, and then promptly used in construction (Wilcox and Shenk, 1977). If this inference is correct, then our radiocarbon study indicates an early 14th century time of construction for the Casa Grande because the building, with floor and roof supports embedded in the walls, is un- likely to have been built before A.D. 1310 and was probably built by or soon after A.D. 1320. The possibility remains that some beams placed in the Casa Grande were re-used from earlier structures, a possibility not evaluated here. The expense and destruction required by our approach to beam dating prevents ana- lyzing a larger number of beam samples, which might distinguish between fresh and used beams, if the latter were employed at all.

We hope this communication will help the

a b c d e f g h i

Figure 3. Cutting date estimates inferred from sequential carbon-14 dates on two beams calibrated by two schemes: vertical bars are one standard deviation (see text), circles (a,c) = white fir, triangles (b,d) = juniper, open symbols (a,b) = Stuiver sheme, filled symbols (c,d) = Klein et al. scheme. Also illustrated are the weighted averages, with standard deviations, of both beams using the Stuiver scheme (el and the Klein et al. scheme (r), and the weighted averages of both schemes for the white fir ( g ) and the juniper (h). On the far right (i) is the weighted average ofthe four intercepts (a,b,c,d).

GEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL 221

CASA GRANDE

archaeologist consider the trade-off between the increased sample destruction and labora- tory expense for a certain degree of increase in dating precision. Recent developments in tan- dem accelerator mass spectrometry (TAMS) radiocarbon analysis have brought precision with milligram-sized samples within the 1% range. With continued improvements expected in the next few years, this approach to high accuracy archaelogical 14C dating will be pos- sible without significant specimen destruction.

We thank Keith Anderson, Western Archaeological and Conservation Center, Tucson, for help in arranging funding for the laboratory work under National Park Service contract PX 8100-0-0076. NSF grants BNS- 8211864 and BNS-8505083 also supported this study. We are grateful for helpful comments on this manu- script from William Robinson, Laboratory of Tree-Ring Research, J.J. Reid, Department of Anthropology, and Paul Fish, Arizona State Museum, all of the University of Arizona, Tucson. Richard Howard, Casa Grande Ruins

National Monument, provided helpful comments and criticisms throughout various stages of this project.

REFERENCES

Dean, J.S. (1978). Independent Dating in Archaeologi- cal Analysis, in M.B. Schiffer, Ed., Archaeological Method and Theory, Vol. 1 , pp. 257-314. New York: Academic Press.

Klein, J., Lerman, J.C., Damon, P.E., and Ralph, E.K. (1982). Calibration of radiocarbon dates: Tables based on the concensus data of the workshop on Cali- brating the radiocarbon time scale. Radiocarbon 24,

Schiffer, M.B. (1982). Hohokam Chronology: An Essay on History and Method, in R. McGuire and M.B. Schiffer, Eds., Hohokam and Patayan, pp. 229-344. New York: Academic Press.

Stuiver, M. (1982). A high-precision calibration of the AD radiocarbon time scale. Radiocarbon 24, 1-26.

Wilcox, D.R., and Shenk, L.O. (1977). The Architecture of the Casa Grande and its Interpretation. Archaelogi- cal Series No. 115. Tucson: Arizona State Museum.

103- 150.

Received December 15, 1986 Accepted for publication February 18, 1987

222 VOL. 2, NO. 3