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Interpreting De Distributions of Single-grain Data

James K. FeathersUniversity of Washington

Work funded by National Science Foundation and South Carolina Institute of Archaeology and Anthropology

De distributions of single-grain data are often characterized by high over-dispersion

-- higher than expected for single-age samples-- interpretation not always well understood

Three case studies:1) Miracema do Tocantins – Brazil2) Sumidouro – Brazil3) Carolina bays – South Carolina

Luminescence Protocol1. Single-grain dating of 180-212 µm

quartz2. Dose recovery:

a. Test of procedures b. Provides minimum over-dispersion

expected for single-aged samplec. Provides over-dispersion value for

distribution models3. Over-dispersion beyond that in the

natural samples is accounted for by three main possibilitiesa. Heterogeneous beta dose rateb. Partial bleachingc. Post-depositional mixing

Potential of beta dose heterogeneity was evaluated using the simulation model of Mayya et al. (2006).

1. Divide different components of De by scaled dose rate (which depends on the 40K concentration and the dose rate from U and Th) to see if age distribution evens out.

2. In none of the cases considered here did this prove to be a significant factor in accounting for high over-dispersion

Partial bleaching investigated using LM-OSL3. Most grains in all three cases were

dominated by fast component4. Those grains that appeared to have a

contribution from slower components did not significantly differ in De from fast component grains.

Post-depositional mixing is most likely cause of the high over-dispersion

Miracema do Tocantins, Brazil

Bueno, L., Feathers, J., De Blasis, P., 2012The formation process of a paleoindian open-air site in central Brazil: integrating lithic analysis, radiocarbon and luminescence dating. Journal of Archaeological Science, available on line

Two lithic horizons with distinctive technology and composition, but blurring

C-14 dates in two clusters 1. 6-7 ka 2. 10-12 ka

Five OSL samples collected at Miracema do Tocantins

Lab number Sample # Test unit Depth (cm)

Location

UW823 OSL 1 N1606 E1063 west wall 161 Lower lithic horizon UW824 OSL 2 N1026 E1006 east wall 204 Lower lithic horizon UW825 OSL 3 N1037 E1011 east wall 261 Lower lithic horizon UW826 OSL 4 N1037 E1011 east wall 175 Between two lithic horizons UW827 OSL 5 N1037 E1011 east wall 84 Above upper lithic horizon

Dose Recovery ratio: 1.01 ± 0.04 Over-dispersion: 18%

Sample N Central age De (Gy) σb (%) Minimum age De (Gy)

UW823 89 6.7±0.3 39±4 4.3±0.4

UW824 121 5.7±0.2 38±3 3.8±0.2

UW825 86 6.4±0.4 50±5 3.6±0.4

UW826 92 4.6±0.2 44±4 2.8±0.4

UW827 82 2.0±0.2 66±7 0.9±0.2

Dose rate about 0.47 Gy/ka for all samples

Sample Component 1 Component 2 Component 3

De (Gy) % De (Gy) % De (Gy) %

UW823 4.2±0.5 30.1 8.2±0.5 69.9

UW824 4.9±0.3 77.7 9.8±1.3 22.3

UW825 1.9±0.3 6.5 6.1±0.3 73.5 12.9±1.5 20.0

UW826 2.8±0.5 26.6 4.9±0.4 63.4 12.1±1.4 10.0

UW827 1.4±0.1 68.3 4.5±0.3 31.7

Finite Mixture Model

UW824 38% UW825 50%

Interpretation1. Small lithics have moved up,

presumably sand grains have too

2. If the mechanism is wind reworking, this should, unless only wind at night, bleach the sand grainsa. Minimum age model

would not provide depositional age but rather the age of last reworking

b. Original depositional age should be something older

3. What about bioturbationa. Little direct evidenceb. But might explain the

varied structure from sample to sample

c. If bringing up grains without exposure, then central age will include older grains and thus be older than the depositional age

SumidouroPaleoindian site with artifacts distributed in three discrete levels.

Araujo, A. G. M., Strauss, A. M, Feathers, J. K., Paisani, J. C., Schrage, T. J., 2012. Paleoindian open-air sites in tropical settings: a case study in formation processes, dating methods, and paleoenvironmental models. Geoarchaeology,, in review

Unit B1 1. Colluvial deposition on lakeshore slope – do archaeological layers represent discrete occupations or are they artifact of post-depositional changes?a. Complex stratigraphy – large

differences between units just a few meters a part

b. Suggests arm of lake once extended into the site area. Also gullies

2. Lithic refitting, granulometric analysis and micromorphology all suggest overall integrity to the deposits but some mixinga. Abundant evidence of bioturbation,

but it localized and irregularb. Some areas show influence of

wave action – more mixing and reworking here

3. Charcoal does not correlate with lithics, probably has natural origina. C-14 dating shows some of it

widely displacedb. But for most part age increases

with depth

12 luminescence samples

Dose rate ~ 2 Gy/ka

Dose recovery ratio: 1.09 ± 0.04, (Some grains dominated by slower components)

Over-dispersion from dose recovery : 15 ± 3%

Lab # Unit Depth (cm)

N De (Gy)Central

age

Over-Dispersio

n (%)

# FMMcompone

nts

UW1392 B1 27 165 3.4±0.2 64.1 4UW1391 B1 70 145 8.0±0.3 37.6 3UW1390 B1 137 316 16.9±0.5 39.4 2UW1389 B1 160 229 19.9±0.7 45.7 4UW1388 B1 196 196 24.6±0.8 34.5 3UW2238 B1 250 70 25.6±2.0 57.3 3UW2236 B1 274 19 10.8±3.4 128 3UW2234 S6 105 76 16.2±1.4 68.2 3UW2237 S6 171 94 35.9±2.6 62.4 3UW2235 S6 275 57 26.8±2.1 48.6 3UW1885 S4 295 196 24.6±0.8 34.5 2UW1886 S7 70 86 10.6±0.7 50.3 3

UW1388 34.5% UW1389 45.7%

UW1392 64.1%

UW1885 34.5%

UW2234 68.2%

UW1390 39.4%

Sample Depth (cm) Central age Largest component

Calibrated 14C range

UW1392 27 1.8±0.2 1.4±0.2 (45%)  

UW1391 70 4.1±0.3 3.5±0.3 (65%)  

UW1390 137 10.1±0.7 7.7±0.5 (62%) 5.7-6.0

UW1389 160 9.3±0.7 10.1±0.9 (62%) 9.3-9.5

UW1388 196 12.4±0.9 14.8±1.1 (69%)  

UW2238 250 14.3±1.5 18.5±1.5 (81%)  

UW2236 274 5.4±1.7 15.7±2.4 (44%)  

Sample Unit Depth (cm) Central Age Largest component

UW2234 S6 105 6.9±0.7 5.2±0.4 (50%)

UW2237 S6 171 16.0±1.5 26.4±2.1 (52%)

UW2235 S6 275 11.5±1.1 10.6±1.1 (58%)

UW1886 S7 70 5.7±0.5 5.3±0.5 (66%)

UW1885 S4 295 28.1±2.4 33.6±2.7 (81%)

Ages from central age model for B1 not in stratigraphic order. They are for the largest FMM component. Agreement with C-14 only approximate.Might be expected where general integrity but some mixing.

Ages for S6 out of stratigraphic order using either central age or largest component. This is unit inferred to be affected by wave action so strong mixing. Ages for S7 and S4 samples are reasonable given stratigraphic position

Carolina BaysCarolina Bays are oriented, shallow upland ponds on Atlantic Coastal Plain

Origins enigmatic but were locations for prehistoric settlement

Three bays with archaeological sites sampled for luminescence

Paper in preparation by Chris Moore et al., Savannah River Archaeological Research Program,. University of South Carolina

13 OSL samples from three bays

Evidence of bioturbation, including prehistoric dug pits, but sedimentary evidence suggests overall integrity

Dose Recovery Ratio: 1.02±0.2Over-dispersion from dose recovery: 16%

Sample N Central age De

(Gy)

σb (%) #FMMCompon

ents

Minimum age De (Gy)

Flamingo BayUW2076 283 10.5±0.4 58±3 3 4.9±0.3UW2077 125 14.1±0.7 44±4 3 8.0±0.6UW2078 114 18.5±1.0 47±4 3 10.1±0.9UW2079 104 21.4±0.9 34±4 2 14.9±1.3UW2080 68 21.8±1.3 42±5 2 12.6±1.4

Frierson BayUW2139 192 4.4±0.2 54±4 4 2.3±0.1UW2140 40 7.9±0.6 41±7 2 4.7±0.7UW2141 12 20.7±5.5 85±19 2 5.2±1.8

Johns BayUW2142 79 9.0±0.6 58±6 3 4.4±0.5UW2143 122 11.6±0.6 51±4 4 5.6±0.5UW2144 117 15.6±0.6 36±4 3 10.2±0.8UW2145 80 24.2±0.9 24±3 1 19.8±1.8UW2146 223 34.6±1.1 36±3 2 23.4±1.2

UW2078 47% UW2079 34%

UW2142 58% UW2145 24%

Sample Depth (cm) Age (ka)Central age

model

Age (ka)Minimum age

model

IndependentAge (ka)

Flamingo Bay

UW2076 35 10.9 ± 0.9 5.0 ± 0.5 C-14 7.9

UW2077 50 16.1 ± 1.5 9.2 ±1.0 C-14 9.6

UW2078 65 21.1 ± 1.9 11.5 ± 1.3 Kirk point ~10.4

UW2079 80 22.2 ± 1.9 15.5 ± 1.8  

UW2080 100 22.8 ± 1.5 13.1 ± 1.7  

Frierson Bay

UW2139 50 7.6 ± 0.8 4.0± 0.4  

UW2140 66 10.8 ± 1.3 6.4 ± 1.1 Middle Archaic artifacts ~ 6-9

UW2141 80 27.7 ± 7.7 7.0 ± 2.5  

Johns Bay

UW2142 60 14.5 ± 1.7 7.0 ± 1.1  

UW2143 80 19.3 ± 2.0 9.3 ± 1.2 Early Archaic artifacts ~10-8

UW2144 100 25.4 ± 2.6 16.7 ± 2.0  

UW2145 180 33.6 ± 3.2 27.5 ± 3.5  

UW2146 270 39.7 ± 3.0 27.0 ± 2.3  

Both central age and minimum age in right stratigraphic order, not the case for largest FMM component. Partially bleached? Reworking bringing older grains up without solar resetting?