DATING BULK SOIL VS. IDENTIFIED ORGANICS AT ARCHAEOLOGICAL SITES

Kathryn Puseman, Paleo Research Labs, Golden, Colorado
and
Ralph E. Klinger, Bureau of Reclamation, Denver, Colorado

INTRODUCTION

Not only is it important to recover a specific type of material for dating, it is important to identify the material being dated. The separation and identification process must be performed under strict conditions of cleanliness to prevent contamination. Identification of charcoal and other charred plant material prior to radiocarbon analysis provides the opportunity to choose the material that would yield the best date possible. For example, a mixed charcoal sample might not yield as good a date as a single identified species. Identification of material is a recommended pretreatment strategy. Paleoenvironmental data and information concerning plant resources available to and utilized by the site occupants also can be obtained by identifying charcoal and other charred plant material prior to radiocarbon dating.

DISCUSSION

Although bulk soil samples commonly are used for conventional radiocarbon analysis by archaeologists and other researchers, they are very low in the "recommended sample material for radiocarbon dating" order. Trumbore (2000:43) notes that "¹⁴C dating is applicable to organic matter formed from photosynthetically fixed carbon within the last 50,000 to 60,000 years." When determining a potential usage for radiocarbon dating, both the cultural/contextual and the biophysical/biochemical characteristics must be considered for a particular sample type. Charcoal and charred organic material (including macrofossils and bone) are considered the most reliable type of sample material for radiocarbon analysis. When an insufficient amount of charcoal or charred organic material is available for dating using conventional decay-counting methods, an attempt is made to obtain a date on the bulk organic matter found in a soil. "Radiocarbon activity of soil organic fractions is extremely variable and the usefulness of using such values to infer age in archaeological applications is generally quite limited except under special conditions" (Taylor 1987:62). One of these conditions includes using bulk samples collected from buried soils that are beyond the range of bioturbation. This would limit the input of organic material and restrict the potential for contamination.

Bulk soil samples are not recommended for radiocarbon analysis because a soil sample can incorporate either old or modern carbon depending on environmental conditions, the type of material, and the degree to which the sample is closed to contamination. Older material can be eroded and reworked or incorporated into younger deposits. Soils also are noted to receive continual input of new carbon (Birkeland 1999; Hsieh 1992, 1993). Younger material is commonly introduced through bioturbation such as insect, earthworm, or burrowing mammal activity. Seeds, leaves, and grasses often are carried into the subsurface as foodstuffs and bedding, and these animals introduce fecal material into the soil. In addition, some seeds have special features that allow them to work their way deep into the ground. Erodium cicutarium (filaree) has a corkscrew-shaped awn that drives the seed into the ground. This species was introduced into central California in the late eighteenth century by Spanish missionaries. Erodium seeds have been found more than a meter below the ground surface in central California archaeological sites known to be several thousand years old (G. J. West, personal communication, 1997).

Because the organic matter in soils is a mixture of materials of different ages and because the proportions of old and modern carbon incorporated into subsurface deposits are unknown, radiocarbon dates obtained from bulk soil samples represent composite ages (e.g. an average age for all of the organics in the sample). Depending on the number of factors that control the accumulation and decay of organic matter in a given deposit, the proportions of young to old carbon can be highly variable and large uncertainties in the measured ages become inherent. Because of these large uncertainties, bulk ages are questionable at best, and measured ages might not accurately represent the true age of a deposit. Contamination of a bulk sample with younger carbon has a greater effect on the resulting age than does contamination with older carbon (Polach et al. 1981, Rosholt et al. 1991). Studies by Andrews and Miller (1980) demonstrate that addition of only 5 percent modern carbon into a sample can give a true age of 20,000 years an apparent age of 16,500 years, and give a true age of 5,000 years an apparent age of 4,650 years. When 20 percent modern carbon is introduced, a true age of 10,000 years gives an apparent age of about 7,000 years (Figure 1).

The identification of specific material to be dated is particularly advantageous and allows the researcher or archaeologist to know precisely what material is submitted for radiocarbon analysis. When dating organic material, charcoal and other charred plant remains that have been specifically identified can help resolve issues concerning stratigraphic relationships between the sample and the stratum from which it was collected. For example, in fluvial deposits the identification of local riparian flora versus distant or exotic species can be particularly helpful in interpreting the deposition context. More accurate dates also can be obtained by submitting only specific types of charcoal or other charred plant material for dating. It would be preferential to date a local species rather than a foreign one, to date a single species rather than a mixture of several types, and to date the plant type with the shortest life span, such as dating a charred corn kernel rather than wood charcoal, or dating charcoal from a shorter-lived shrub rather than a longer-lived tree. Identification of the material also is a recommended pretreatment strategy prior to radiocarbon analysis. Taylor (1987:41) notes that "whenever possible, the proper scientific nomenclature for species of plant and animal sample material should be obtained even if the fragmentary nature of the sample permits only genus or even family level designations."

\When the amount of identified material for radiocarbon analysis is small (less than one or two grams for charcoal), the accelerator mass spectrometry (AMS) method is used for obtaining a date. Conventional radiocarbon analysis is based on the production, distribution, and decay of ¹⁴C, the radioactive isotope of carbon, and involves measuring the residual content of ¹⁴C present in a sample. A very small fraction of the ¹⁴C atoms present in a sample actually are measured. Alternatively, AMS radiocarbon analysis involves acceleration of ¹⁴C atoms in the form of ions to higher energies in particle accelerators, separation of ¹⁴C ions from other isotopes and molecules, then counting the individual ¹⁴C ions present. AMS radiocarbon analysis can be done using a much smaller amount of material than conventional radiocarbon analysis--as little as 5 milligrams of charcoal. The AMS methodology produces a more precise date than conventional radiocarbon analysis due to smaller analytic laboratory errors. For this reason, some researchers will choose AMS radiocarbon analysis even when enough material is present for a conventional date. Similar precision can be achieved with extended counting (measurement) time during conventional radiocarbon analysis, although it still requires a greater amount of the material being dated than the AMS method.

One example illustrates why taxonomic identification of macroflora is important before radiocarbon dates are determined. The case involves nine charcoal radiocarbon samples from features in a unit pueblo in northern New Mexico (Roomblock S at site LA 20266). Sediment from these same features was analyzed for macrofloral remains; however, the charcoal submitted for radiocarbon analysis was recovered prior to submission of the macrofloral samples for flotation. The radiocarbon dates obtained from these charcoal samples ranged in age from the middle of the Pueblo I period to after abandonment of the area at the end of the Pueblo III period and were not considered useful in determining the time of occupation. These charcoal samples were not identified prior to submission for conventional radiocarbon analysis. The macrofloral record from the same features that were dated contained a variety of remains (Table 2, Table 3). Many of the samples contained charred corn cupule and/or kernel that could have been submitted for AMS radiocarbon analysis. The charcoal record often consisted of a variety of charcoal types, including longer-lived pine and juniper, as well as shorter-lived shrubs such as sagebrush.

Saltbush (Atriplex) charcoal also was present in these samples. Most woody plants have a C3 photosynthetic pathway, except for saltbush, which has a C4 photosynthetic pathway. The tissues of C4 plants have slightly more ¹³C in their tissues than C3 plants have. The average d¹³C value for C3 plants is , -24â PDB, whereas C4 plants have an average d¹³C value of -14.5â PDB. The presence of saltbush charcoal in a mixed charcoal assemblage will change the d¹³C value by several per mil depending upon the percent of saltbush present. Consequently, to correctly determine the carbon isotope composition of the C3/C4 plant mixture, the d¹³C values must be measured directly on the gas resulting from the combustion of the mixed species sample. The amount of correction will depend on how much saltbush charcoal is present. When obtaining conventional radiocarbon dates, it is even more crucial to obtain this value because the error produced is doubled. When dating a mixture of charcoal, especially if saltbush is known to be present or expected to be present and if conventional radiocarbon analysis is used, the d¹³C value should always be determined at the time of the radiocarbon analysis to correct the radiocarbon age reported for sample (Darden Hood at Beta Analytic, personal communication, September 14, 2000; Tom Stafford at Stafford Research Laboratories, personal communication, April 2001). The d¹³C value is automatically determined when samples are submitted for AMS radiocarbon analysis, but not when samples are submitted for conventional radiocarbon analysis--it must be requested. The radiocarbon dates obtained from Roomblock S at site LA 20266 might have been more useful if the charcoal had been identified prior to submission for radiocarbon analysis to determine the best charcoal type to date, if the d¹³C value had been determined, and/or if the charred corn had been submitted for AMS radiocarbon analysis rather than charcoal for conventional radiocarbon analysis.

Identification of charcoal and other charred organic material prior to submission for radiocarbon analysis also can provide paleoenvironmental data and/or information concerning use of individual plants. Assuming that subsurface disturbance is not too great, charred organic material from non-cultural deposits most likely represents plants growing in the area that were burned in a past fire. Charred plant material from cultural contexts most likely represents resources utilized by the site occupants. Identification of this material provides information concerning plants available to the occupants at the time of occupation, as well as specific plant resources utilized.


SUMMARY

In archaeological applications and other areas of research, it is best to submit identified material, especially charcoal and other charred organic material, for radiocarbon analysis rather than bulk soil samples. Bulk soil samples can contain reworked older material and/or introduced younger material; therefore, bulk soil samples might not accurately date the deposit. Wood charcoal and charred organic material, including bone, are believed to be the most reliable types of samples for radiocarbon analysis. Determining the d¹³C value, using the AMS methodology, and/or using extended counting produces a more precise date. The material to be dated should be identified prior to submission for radiocarbon analysis. This can aid in determining the best material for dating. It also can provide information concerning plants present in the past environment and resources available to and utilized by the site occupants.

Sample			Charred		Uncharred		Weights/
No.	Identification	Part	W	F	W	F	Comments
LC1-3-4	Liters Floated						2.30 L
86-97	Light Fraction Weight						24.52 g
cmbs	FLORAL REMAINS:
	Poaceae (Grass family)	Caryopsis	2
	Rosa (Wild rose)	Seed	5	2
	Fruity tissue			1
	Unidentified	Seed	2
	Chenopodium (Goosefoot)	Seed			3	6
	Taraxacum (Dandelion)	Seed			2
	Modern rootlets					X	Numerous
	Sclerotia				X		Few
	CHARCOAL/WOOD:
	Alnus (Alder)	Charcoal		19			0.13 g
	Artemisia (Sagebrush)	Charcoal		1			0.01 g
	Rosa (Wild rose)	Charcoal		4			0.02 g
	Salix (Willow)	Charcoal		11			0.07 g
	Unidentified > 2 mm	Charcoal		X			0.13 g
	NON-FLORAL REMAINS:
	Animal tooth enamel					1
	Bone					6
	Insect chitin					13
	Mollusk shell > 1 mm				1	116	0.24 g
	Rock/Gravel					X	Present

W = Whole F = Fragment

X = Presence noted in sample g = grams

From Puseman 1997

Sample			Charred		Uncharred		Weights/
No.	Identification	Part	W	F	W	F	Comments
37	Liters Floated						4.80 L
Feature	Light Fraction Weight						32.19 g
37	FLORAL REMAINS:
Room 2	Pinus	Bark scale		X			Few
	Zea mays 2mm	Cupule	4	13			0.13 g
	Zea mays < 2mm	Cupule		X			Few
	Zea mays	Kernel		5			0.01 g
	Chenopodium	Seed			2	1
	Euphorbia	Seed				1
	Rootlets					X	Numerous
	CHARCOAL/WOOD:
	Total charcoal > 2 mm						2.05 g
	Amelanchier	Charcoal		3			0.13 g
	Artemisia	Charcoal		1			0.01 g
	Atriplex	Charcoal		1			0.01 g
	Cercocarpus	Charcoal		1			<0.01 g
	Juniperus	Charcoal		4			0.02 g
	Pinus	Charcoal		1			0.02 g
	Quercus	Charcoal		2			0.03 g
	Salicaceae	Charcoal		15			0.18 g
	Salix	Charcoal		12			0.10 g
	NON-FLORAL REMAINS:
	Insect	Chitin				206
	Insect	Larva			1	1
	Worm casts				X	X	Moderate
40	Liters Floated						6.00 L
Feature	Light Fraction Weight						47.65 g
40	FLORAL REMAINS:
Room 6	Cheno-am	Embryo		5
	Chenopodium	Seed	4	7	1	1
	Sphaeralcea	Seed			1
	Rootlets					X	Numerous
40	CHARCOAL/WOOD:
Feature	Total charcoal > 2 mm						0.59 g
40	Ephedra	Charcoal		3			<0.01 g
Room 6	Juniperus	Charcoal		26			0.13 g
	Pinus	Charcoal		9			0.02 g
	Rhus	Charcoal		4			0.01 g
	Rosaceae	Charcoal		2			0.01 g
	Amelanchier	Charcoal		2			<0.01 g
	cf. Cowania	Charcoal		3			0.01 g
	Sarcobatus	Charcoal		1			<0.01 g
	Juniperus	Wood				2pc	0.11 g
	Pinus	Wood				2pc	0.01 g
	NON-FLORAL REMAINS:
	Calcined bone			3
	Insect	Chitin				15
	Insect	Puparia				22
	Sand					X	Moderate
	Worm casts					X	Few
34	Liters Floated						4.00 L
Feature	Light Fraction Weight						47.02 g
34	FLORAL REMAINS:
Room 7	Cheno-am	Embryo	1
	Pinus	Bark scale		X			Numerous
	Yucca	Seed	3	5
	Zea mays	Cupule		1
	Zea mays	Kernel		7			0.02 g
	Vitrified tissue			X			Few
	Amaranthus	Seed			1
	Chenopodium	Seed			15	8
	Erodium	Seed				1
	Euphorbia	Seed			2	1
	Rootlets					X	Moderate
34	CHARCOAL/WOOD:
Feature	Total charcoal > 2 mm						20.72 g
34	Artemisia	Charcoal		2			<0.01 g
Room 7	Atriplex	Charcoal		1			0.04 g
	Juniperus	Charcoal		30			3.20 g
	Juniperus	Charcoal		1pc			0.90 g
	Pinus	Charcoal		8			0.48 g
	Salicaceae	Charcoal		4			0.52 g
	NON-FLORAL REMAINS:
	Bone					2
	Insect	Chitin				28
	Ant				3
22	Liters Floated						4.00 L
Feature	Light Fraction Weight						21.95 g
22	FLORAL REMAINS:
Room	Cheno-am	Embryo	42
11	Chenopodium	Seed	21	14	12	13
	Pinus	Bark scale		X			Few
	Zea mays 2mm	Cupule	2	9			0.07 g
	Zea mays	Kernel		4			0.012 g
	Vitrified tissue			X			Few
	Portulaca	Seed			2	2
22	CHARCOAL/WOOD:
Feature	Total charcoal > 2 mm						2.58 g
22	Amelanchier	Charcoal		3			0.05 g
Room	Artemisia	Charcoal		16			0.06 g
11	Atriplex	Charcoal		2			0.02 g
	Cercocarpus	Charcoal		8			0.14 g
	Juniperus	Charcoal		24			0.17 g
	Pinus	Charcoal		3			0.04 g
	Pinus ponderosa-type	Charcoal		1			0.02 g
	Quercus	Charcoal		2			0.06 g
	Salicaceae	Charcoal		1			0.03 g
	NON-FLORAL REMAINS:
	Bone 2mm		3			1
	Bone < 2mm		X	X		X	Few
	Rodent fecal pellet		1
	Insect	Chitin				10
	Worm casts				X	X	Few
28.2	Liters Floated						1.60 L
Feature	Light Fraction Weight						5.13 g
28	FLORAL REMAINS:
Room	Chenopodium	Seed	1		5	13
11	Pinus	Bark scale		X			Few
	Zea mays	Kernel		1			<0.01 g
	Vitrified tissue 1mm			X			Moderate
	Echinocereus	Seed			1
	Portulaca	Seed			1
	Rootlets					X	Numerous
	CHARCOAL/WOOD:
	Total charcoal > 2 mm						0.22 g
	Juniperus	Charcoal		10			<0.01 g
	Pinus	Charcoal		1			0.22 g
	Salicaceae	Charcoal		1			<0.01 g
	NON-FLORAL REMAINS:
	Insect	Chitin				12
61	Liters Floated						8.00 L
Feature	Light Fraction Weight						37.69 g
61	FLORAL REMAINS:
Kiva 1	Cactaceae	Areole		9			0.01 g
	Sclerocactus	Seed	2
	Cheno-am	Embryo	3	1	117
	Amaranthus	Seed	1	2	7	4
	Atriplex	Fruit		1
	Chenopodium	Seed	5	2	4	7
	Juniperus	Leaf		2
	Pinus	Bark scale		X			Few
	Pinus	Cone scale		1			<0.01 g
	Portulaca	Seed	7	5	48	23
	Zea mays	Cupule	1	3			0.02 g
	Zea mays	Kernel	1	2			0.08 g
	Euphorbia	Seed			17	3
	Mammillaria	Seed			1
	Sphaeralcea	Seed			2
	Rootlets					X	Numerous
	CHARCOAL/WOOD:
	Total charcoal > 2 mm						2.62 g
	Artemisia	Charcoal		12			0.03 g
	Atriplex	Charcoal		2			0.02 g
	Cercocarpus (many small twigs)	Charcoal		28			0.40 g
	Juniperus	Charcoal		12			0.10 g
	Pinus	Charcoal		1			0.01 g
	Rosaceae	Charcoal		8			0.03 g
	Salicaceae (several small twigs)	Charcoal		7			0.03 g
	Unidentified bark	Charcoal		3			0.02 g
	NON-FLORAL REMAINS:
	Bone			2		X	Few
	Insect	Chitin				X	Few
	Sand/Silt					X	Abundant
69	Liters Floated						4.00 L
Feature	Light Fraction Weight						26.31 g
69	FLORAL REMAINS:
Kiva 2	Cactaceae	Areole	1	4			0.01 g
	Pinus	Bark scale		X			Few
	Rhus	Seed		3			<0.01 g
	Unidentified	Seed	1
	Amaranthus	Seed				13
	Chenopodium	Seed			116	36
	Echinocereus	Seed			1	1
	Rootlets					X	Numerous
	CHARCOAL/WOOD:
	Total charcoal > 2 mm						7.77 g
	Amelanchier	Charcoal		1			0.03 g
	Artemisia	Charcoal		2			0.03 g
	Atriplex	Charcoal		1			0.09 g
	Cercocarpus	Charcoal		6			0.25 g
	Juniperus	Charcoal		39			1.73 g
	Pinus	Charcoal		2			0.20 g
	Pinus ponderosa-type	Charcoal		1			0.02 g
	Quercus	Charcoal		5			0.34 g
	Total wood 2mm						0.42 g
	Juniperus	Wood				14pc	0.41 g
	Salicaceae	Wood				1pc	0.02 g
	NON-FLORAL REMAINS:
	Insect	Chitin				80
	Sand					X	Moderate
68	Liters Floated						4.00 L
Feature	Light Fraction Weight						22.47 g
68	FLORAL REMAINS:
Plaza	Pinus	Bark scale		X			Few
	Chenopodium	Seed			236	29
	Echinocereus	Seed			1
	Erodium	Seed			4	3
	Euphorbia	Seed				34
	Rootlets					X	Numerous
68	CHARCOAL/WOOD:
Feature	Total charcoal > 2 mm						0.54 g
68	Artemisia	Charcoal		5			0.01 g
Plaza	Juniperus	Charcoal		23			0.20 g
	Pinus	Charcoal		5			0.02 g
	Rosaceae	Charcoal		6			0.03 g
	Salicaceae	Charcoal		2
	NON-FLORAL REMAINS:
	Insect	Chitin				67
	Snail shell				1
	Worm casts				X	X	Few

W = Whole

F = Fragment

X = Presence noted in sample

g = grams

* = Estimated frequency

pc = Partially charred

Scientific Name	Common Name
FLORAL REMAINS:
Cactaceae	Cactus family
Echinocereus	Hedgehog cactus, Strawberry cactus
Mammillaria	Nipple cactus, Fishhook cactus
Sclerocactus	Pineapple cactus, Devil-claw
Cheno-am	Includes goosefoot and amaranth families
Amaranthus	Pigweed, Amaranth
Atriplex	Saltbush, Shadscale
Chenopodium	Goosefoot
Erodium	Storksbill
Euphorbia	Spurge
Pinus	Pine
Portulaca	Purslane
Rhus	Sumac, Skunkbush, Squawberry
Sphaeralcea	Globe mallow
Yucca	Yucca, Soapweed
CULTIGENS:
Zea mays	Maize, Corn
CHARCOAL/WOOD:
Artemisia	Sagebrush
Atriplex	Saltbush, Shadscale
Ephedra	Ephedra, Mormon tea
Juniperus	Juniper
Pinus	Pine
Pinus ponderosa-type	Ponderosa pine
Quercus	Oak
Rhus	Sumac, Skunkbush, Squawberry
Rosaceae	Rose family
Amelanchier	Juneberry, Serviceberry
Cercocarpus	Mountain mahogany
cf. Cowania	Cliffrose
Salicaceae	Willow Family
Salix	Willow
Sarcobatus	Greasewood

Figure 1. Curve showing the effects of varying degrees of contamination on Apparent Radiocarbon Age (data from Andrews and Miller, 1980).

Andrews, J. T. and G. H. Miller
1980 Dating Quaternary deposits more than 10,000 years old. Chapter 18 IN Timescales in Geomorphology, edited by R. A. Cullingford, D. A. Davidson, and J. Lewin. John Wiley & Sons, Ltd.

Birkeland, P. W.
1999 Soils and Geomorphology. Oxford University Press, New York.

Hsieh, Y. P.
1992 Pool size and mean age of stable soils organic carbon in cropland. Soil Science Society of America Journal 56:460-464.

1993 Radiocarbon signatures of turnover rates in active soil organic carbon pools. Soil Science Society of America Journal 57:1020-1022.

Matthews, J. A.
1980 Some problems and implication of 14C dates from a podzol buried beneath an end moraine at Haugabreen, southern Norway. Geografiska Annaler 62A:185-208.

Polach, H., J. Golson, and J. Head
1981 Radiocarbon Dating: A Guide for Archaeologists on the Collection and Submission of Samples and Age-Reporting Practices. In Australian Field Archaeology: A Guide to Techniques. Australian Institute of Aboriginal Studies, Canberra, Australia, p. 145-152.

Puseman, Kathryn
1997 Examination of Bulk Soil and Detrital Charcoal From Along Lost Creek, Northeast Utah. Ms. on file with the U.S. Bureau of Reclamation, Denver Federal Center, Denver, Colorado.


Rosholt, J. N., S. M. Colman, M. Stuiver, P. E. Damon, C. W. Naeser, N. D. Naeser, B. J. Szabo, D. J. Muhs, J. C. Liddicoat, S. L. Forman, M. N. Machette, and K. L. Pierce
1991 Dating methods applicable to the Quaternary. In Dating Methods Applicable to the Quaternary, edited by R. B. Morrison, pp. 45-74. The Geology of North America, vol. K-2, Geological Society of America, Boulder, Colorado.

Taylor, R. E.
1987 Radiocarbon Dating An Archaeological Perspective. Academic Press, Inc., Orlando.

Trappe, James M.
1962 Fungus Associates of Ectotrophic Mycorrhizae. In The Botanical Review. U.S. Department of Agriculture, Washington, D.C.