(Freeman, New York, 1993). 22. W~ththe use of vapor pressure osmometry the n- 26 N. Go, M. Go, H. A. Scheraga, Proc. fdat Acad. Sci. 3. J D. Bryngeson J. N. Onuchc. N. D. Socc~,P. G. termolecular assocation constants K, of 1 to 6 could U.S.A. 59, 1030 (1968). Wolynes, Proteins 21, 167 (1995); K. A. Dill et a/. , be reaby measured in acetontrie at 37°C. K, n- 27. W. C St,A. Tempczyk, R. C. Hawey, T. Hendrck- Protein Sci. 4, 561 (1995): L. A. Mirny. V. Abkevch. creases with n: For example, K,(2) = 8 = 3 M-', son, J. Am Chem. Soc. 112, 61 27 (1990); Macro- E. I. Shakhnovch, Folding Design 1, 221 (1996). Ka(4)= 49 = 14 M-' , and K,(6) = 1 186 ? 143 M-' Model, v5.0 (1996) [F. Mohamadi etal., J. Comput. 4. J W. Bryson etal., Science 270, 935 (1995); D. A. If varation in K,(6) over small changes in temperature Chem. 11 , 440 (1990)] Dolg~kh.M. P. K~rpichn~kov.0. B. Ptitsyn, V. V. IS neglected, only -3% of 6 IS expected to be in- 28. The authors acknowledge support from the Critical Chemeris, Mol. Biol. 30, 149 (1996). volved in bimolecular aggregates at 10 p.M and Research Initiatives Program of the Univers~tyof i- 5. L. Pauling, R. B. Corey, H. R. Branson, Proc. Nail. 25% In each case, K, was determined w~tha model nois, NSF grant CHE 94-961 05 (to J.S.M.),and NSF Acad. Sci. U.S.A. 37, 205 (1951). that assumes K, IS the same for the d~merand all grant CHE-93-01474 (to J.G.S.). This work was 6. C. M. Venkatchalam and G. N. Ramachandran, higher-order aggregates. completed while P.G.W. was a Scholar-in-Resl- Annu. Rev. Biochem. 38, 45 (1969); B Honig and 23. J. C. Nelson, J. K. Young, J S. Moore, J Org. Chem. dence at the Fogarty Internatonal Center at NH.The F. E. Cohen, Folding Design 1, R17 (1996). 61, 81 60 (1996) NMR studies were funded n part from the W. M. 7. J.-M. Lehn, Angew. Chem lnt. Ed. Engl 29, 1304 24. W. L Jorgensen and J. Tirado-Rives, J. Am. Chem. Keck Foundation, NH(grant PHs 1 SlO RR10444- (1990); G. M. Whtesides, J. P. Mathas, C. T. Seto, Soc. 110, 1657 (1988). Ol), and NSF (grant CHE 96-10502). Science 254, 1312 (1991); D. S. Lawrence, T. Jiang, 25. K. Okuyama, T Hasegawa, M. to, N. M~kam~,J. M. Levett, Chem. Rev. 95, 2229 (1995). Phys. Chem. 88, 171 1 (1984). 8 May 1997; accepted 24 July 1997 8. M Hagihara, N. J Anthony, T J. Stout, J. Clardy, S. L. Schrerber, J. Am. Chem. Soc. 114, 6568 (1992); G. P.' Dado and S. H. Geman, /bid. 116, 1054 (1994); A B. Sm~thet a/. . ibid., p. 9947; Y. A Complex in at 5400-5000 Hamuro, S J. Geib, A. D. Hamilton, ibid. 118, 7529 (1996): D. H. Appela, L. A. Chrstianson, I. L. Kare, Years Before the Present D. R. Powell, S. H. Geman, ibid.. p. 13071; D. Seebach etal. , Helv. Chim. Acia 79, 2043 (1996). 9. W. Kauzmann, Adv Prot. Chem. 14, 1 (1959); M. Joe W. Saunders,* Rolfe D. Mandel, Roger T. Saucier, Bixon, H. A. Scheraga. S. L~fson,Biopolymers 1.419 E. Thurman Allen, C. T. Hallmark, Jay K. Johnson, (1963); S. Padmanabhan and R. L. Baldw~n,J Mol. Biol. 241, 706 (1994); Z Luthey-Schulten, B. E. Edwin H. Jackson, Charles M. Allen, Gary L. Stringer, Ramirez, P. G Wolynes, J. Phys. Chem. 99, 2177 11 9951: J. G. Saven and P. G Wovnes. J. Mol. Biol. Douglas S. Frink, James K. Feathers, Stephen Williams, 257, i99 (1996). Kristen J. Gremillion, Malcolm F. Vidrine, Reca Jones 10. A:S. Yanq and B. Hon~q,J. Mol. Biol. 252, 351 (1995). 11. R. S. Lokey and B. L. Iverson, Nature 375, 303 An 1I-mound site in Louisiana predates other known mound complexes with earthen 11 995) $ ---, enclosures in by 1900 years. Radiometric, luminescence, artifactual, 12. D. Poland and H A. Scheraga, Theory of Helix-Coil Transitions in Biopolymers (Academc Press, New geomorphic, and pedogenic data date the site to over 5000 calendar years before York, 1970). present. Evidence suggests that the site was occupied by hunter-gatherers who sea- 13. A modified verslon of the force fed of Jorgensen and sonally exploited aquatic resources and collected plant species that later became the first Tirado-Rives (2a)was ~lsed,whlch included a 0.6 domesticates in eastern North America. kce/mo barrer to torsona rotaton about the triple bond as inferred from gas-phase spectroscopy (25).

The hex-co free energy d~fferenceAG = GhslIx- G,, was estimated using Eqs. 1 and 2, GheX= Gs3s,,-ltcr - RT In z*,, (1 Natlve American have been rec- resent early evidence for organized society

Gzcl= GScl,,E,,c, - RT In z,, - RT In a, ,,,,, (2) ogn~zedand studled in the eastern Un~ted in North America. Most of the earthen where GhelIxand G,, are the free energles of the States for Inore than a century. They rep- mounds and enclosures in the east date to heca and random coil parts of conformat~onspace <2500 calendar years before present (B.P.) (10, 26). In each case, the solvent-accessible surface area of an eriergy-m~nmizedIieicai or extended con- J. W. Saunders, G. L. Stringer, R Jones, Department oi (1). In the 1950s, the recognition of prece- formaton was determned, and sem-empirical rea- Geosc~ences.Northeast Louis~anaUnivers~tv Monroe. ramic mounds and earthen enclosures from tlons were used to map this area to a free energy of LA 71 209, USA. earlier times came first at the sovatlon, G,, #,,,,, (27). For both GhelIxand G,,, the R. D. Mandel, Departnient of Geography, University of vibrational partiton function^^ ,was calculated with a Kansas, Lawrence, KS 66045-21 21. USA site in Louisiana, dating to 3500 calendar normal mode analys~sabout the local m~nimum,In R. T. Saucier, 4325 Winchester Road, V~cksburg,MS years B.P. (2, 3). By the 1970s, four mound G,,, a,,.,on correctsz*,, to include full rotatonal tor- 391 80-8969, USA. sites in Louisiana and one in Florida had son about the acetylene brdges, and these torson E. T. Allen, Natural Resources Consenlat~onSenwe, angles were taken to be independent of one another. 1605 Arzona Street, Monroe, LA 71 202-3697, USA. been dated to >5000 calendar years B.P. Both z ,,, and G,,,,,,,,, were essentlaiy identlca for C. T. Hallmark, Soil and Crop Sclences Department, Tex- (Middle Archaic), but the data were not any of the extended, planar local minma chosen to as A&M University, College Staton, TX 77843-2474, conclusive and the antiquity of the sites represent the nonhelcal ensemble. USA. remained in doubt 14. J. Zhang, D. J. Pesak. J. L. Ludwick, J. S Moore. J. K. Johnson. Department of Sociology and Anthropoi- (4, 5). J. Am Chem. Soc. 116, 4227 (1994). ogy, Univers~tyof M~sslssipp~.Un~versity, MS 38677, In the 1990s, four additional mound sites ;5. V. A. Bloomfed, D. M Crothers, 1. Tnoco, Physical USA. in Louisiana (6-8) and two in Florida (8) Chemistry of Nuclelc Acids (Harper & Row. New E. H. Jackson, Department of Sociology and Anthropo- have been identified as Middle Archaic in York, 1974). ogy, Univers~tyof Southern Missssppi, Box 5074, Hat- 16. C. R. Cantor and P. R Scli~mmel,Biophysical tesburg, MS, 39406-5074, USA. age. Collectively, the Middle Archaic Chemistry (Freeman, New York, 1980). C M. Allen, Department of Biology Northeast Louisana mound sites provide 56 radiometric dates 17, l,,,il,,, of the octadecamer 9 sample did not Univers~ty,Monroe, LA 71 209, USA. that establish the antiquity of earthen change upon dilution by a factor of 50. For 9 In D. S. Frink, Archaeology Consulting Team, Inc., Post mounds in the southeast. Of these sites, acetontre, E (290 nm) was only 12% less than the Off~ceBox 145, Essex Junction, VT 05453, USA. extrapolated value based on short cha~nlengths, J. K Feathers, TL Dating, DH-05 Anthropology, Univer- Watson Brake in northeast Louisiana is the which makes I,,, a useful Internal reference. sitv of Washinnton. Seattle. WA 981 95. USA. largest, most co~nplex,and most securely 18. J M. Scholtz. H. Alan, E. J. York, J. M. Sterward, s.' Wlams, Fast Office Box 22354, Santa Fe, NM dated site. Its mounds and connecting R. L. Badwn, Biopolymers 31. 1463 (1991). 87502-2354, USA. 11 19 S. J. Perkns, Biol. IWagn. Reson. 4. 193 (1982). K. J. Gremlllon, Department of Anthropology, Ohio State ridges forin an oval-shaped earthen enclo- 20. A. S. Shetty, J. Zhang, J. S. Moore, J. Am. Chem. Univers~ty.244 Lord Ha. 214 West 17 venue, Coum- sure 280 m in diameter (Fig. 1). The largest Soc. 118, 1019 (1996). bus. OH 4321 0-1 364, USA. M. F. Vidrne. Division of Scences. Louisiana State Un- mound (Gentry Mound) is 7.5 m high; the 21, 6, = [I6 1(6)]/I I(?),where S is the chemical shift, l(6) vers~ty at Eun~ce,Post Off~ceBox 1129, Eun~ce,LA other mounds measure between 4.5 and 1 In IS the spectral ntensty, and each sum extends only 70535, USA over the aromatlc region of the spectrum. typically 6 in height, and the connecting ridges aver- = 7.2 to 8.3 -To whom correspondence sho~lldbe addressed. age 1 In in height. Here we present evi-

1796 SCIENCE VOL. 277 19 SEPTEMBER 1997 www.scienc dence that Watson Brake predates the (clay coatings) in the Bt horizons. The younger-than-expected age (I3). Dates on large-scale earthworks of Poverty Point by argillans have variable thickness (100 to four charcoal scatters from submound and 1900 years, making it the earliest such hu- 300 p~n)and are composed of ~nicrola~ni- subridge A horizons (mounds A, B, and C; man construction so far recognized in the nated clav. Extensive leaching is indicated ridge KIA) range between 5880 and 5450 New World. by low concentrations of &changeable calibrated (cal.) calendar years B.P. Dates Watson Brake is constructed on the edge bases (Ca, Mg, Na, and K) and pH of 5.1 to for soil hu~natesfrom two subinound A of a low, flat Pleistocene terrace (Mid-Wis- 3.9 to great depths. Base saturation ranges horizons (~noundsA and D) are 5285 and consin stage) overlooking the Holoce~le from 10 to 5 1% in the Bt horizons. Iron has 5450 cal. calendar years B.P. These dates floodplain (<12,000 calendar years B.P.) of leached from the A and E horizons (0.3 to suggest that mound construction at Watson the (9, 10). Before 7000 0.5%) and illuviated into the Bt horizons Brake began between 5400 and 5300 calen- calendar years B.P. (lo), meander belts (0.9 to 1.1%). dar years B.P. formed by the paleo-Arkansas River pro- The strongly developed soils on the Six charcoal samples from buried A ho- vided gravel and sand shoal channels in the mounds and ridges at Watson Brake may be rizons on successive stages of inound B and Ouachita Valley, and swamp and small- ~artiallvattributed to the nature of the stage I of ridge KIA yielded radiocarbon stream habitats formed in backwater areas material used to construct these features. ages between 5590 and 5290 cal. calendar near Watson Brake. These conditions per- Emplacement of weathered (precondi- years B.P., and humates from four buried A sisted until about 4800 calendar years B.P., tioned) material during the final stage of horizons in mounds A, C, and D range in vvhen a diversion of the Arkansas River into mound and ridge" construction would favor age between 4870 and 4520 cal. calendar the present course of the Ouachita River rapid pedogenesis. However, the formation years B.P. In addition, humates from a pit- caused rapid alluviation near Watson Brake of thick Bt horizons that meet argillic cri- in the surface of stage I, mound C (lo), decreasing the extent of the swamp teria is time-dependent because weathering, dates to 4826 cal. calendar years B.P. On and small-stream habitats. This event may clay formation, and translocation are slow average, the humate ages are 700 years coi~lcidewith abandonme~ltof the site. processes (12). Hence, we suggest that the younger than the charcoal ages. The hu- We verified the cultural origin of each Ultisols and Alfisols developed in the mate ages appear to be too recent because of mound and ridge- with eight" test units and mounds and ridges are products of thou- postburial input of organic carbon. Soil or- soil cores. Mounds and ridges along the sands of years of pedogenesis. ganics are susceptible to contamination by edge- of the Pleistoce~leterrace were con- We obtained 19 radiocarbon assavs fro~n modern rootlets, huinic acids, and other strutted in multiple stages and on pre- buried '4 horizons on the premound and sources of young carbon, which can yield mound or ridge ~niddens( 1 1 ). Mounds and preridge surfaces as well as from the surfaces anomalously young radiocarbon ages ( 14, ridges set back from the terrace edge were of s~~ccessivestages of mound and ridge 15). constructed in single stages and on the construction (Table 1). Two of the dates Independent means of determining the truncated Bt horizon of the Pleistocene ter- are considered anornal'ous because of their antiquity of Watson Brake include lu~nines- race (the A horizon was removed before mound or ridge construction). This removal suggests that ~noundand ridge placement on the north and east sides followed the natural topography, whereas mounds and ridges on the west and south sides of the enclosure were placed to complete the en- closure. The extensive weathering of fill in the single- and multiple-stage mounds and ridges suggests that all of the earthen struc- tures are contemporaneous. The physical, chemical, and ~norpholog- ical properties of soils that developed in the i~loundsand ridges indicate that the earth- en architecture is of great antiquity. These soils are strongly weathered, with well-ex- pressed A-E-Bt horizonation (Ultisols and Alfisols). A typical profile consists of a 1-m solu~nwith a fine sandy loam ochric epipe- don (A horizon),, , a fine sandv loam to loamy fine sand alhic (E) horizon, and a reddish, clay-enriched argillic (Bt) horizon. The Bt horizolls are 40 to 80 cm thick and have fine and medium subangular-blockJl structure. Clay translocation, which leads to the developinent of the Bt horizons, is prono~~ncedas indicated by (i) clay deple- tion of the overlying A and E horizons, (ii) enrichment of the Bt horizons in clay con- I C--- tent relative to overlyi~lghorizons and un- Magnetic 0 50 100 m derlying unweathered mound fill, (iii) high north ratios of fine clay to total clay in the Bt Fig. 1. Watson Brake topographic map. Black squares Indicate test un~ts.Test unlt 5 is an extension of horizons, and (iv) the presence of argillans test unlt 4 in mound C: thus there are only seven squares on the map. Contour intervai = 0.5 m.

?ceinag.org SCIENCE \'OL.277 19 SEPTEXIBER 199i 1797 cence dating of mound and ridge sediments mounds and ridges. None of the Poverty dominate and fish are the most abundant (1 6). We examined samples from the buried Point archetypes (20, 21) have been recov- food remains. Main channel species were A horizons of stage I, mound B and stage I, ered from Watson Brake. Projectile points preferred, including freshwater drum (Sci- ridge KIA. It was hypothesized that the from Watson Brake are Middle to Late Ar- aenidae), catfish (Ictaluridae), and suckers formation of A horizons on the intermedi- chaic in age (22). Lithic material is local (Catostomidae). Additional aquatic foods ate stages had provided adequate time for gravel, in contrast to Late Archaic and Pov- include 17 mussel species (Unionidae), bleaching of sediments through recycling to erty Point sites, where nonlocal material is one auuatic snail (thousands of Cambe- the surface by pedoturbation. Thermolumi- common (23, 24). production at loma si.), turtle, and duck. Deer, turkky, nescence analysis of the slowly bleaching Watson Brake is casual, with minimal plat- raccoon, opossum, squirrel, rabbit, dog, component in quartz produced geological form preparation and no platform rejuvena- and rodent remains also were recovered rather than archaeolo~ical" ages, - which sue-" tion. At Poverty Point, blade cores have from the . gests that bleaching was incomplete. Opti- more formal platform preparation and blade Charred seeds from the same midden de- cally stimulated luminescence (OSL) of the removal (25). Watson Brake blades fre- posits represent three species of the complex rapi'dly bleaching component 'using single quently were transformed into microdrills for of weedy annuals (29). These include goose- aliquot analysis (17) yielded an age of bead production, averaging 9.9 by 2.7 by 2.2 foot (Chenopodium berlandieri), knotweed 4003 i 444 calendar years B.P. for ridge mm. This is similar to the Middle (Polygonum spp.), and possibly marshelder KIA and a maximum age of 5538 ? 936 Archaic assemblages from east central Loui- (lua annua). The morphology of the goose- calendar years B.P. for mound B. The OSL siana (26) as well as from the Keenan Cache foot seeds is consistent with var. boscianum, a signal for the ridge sample appeared well (27) and the Slate Site (28) in Mississippi. taxon uncommon in the area today. None of bleached in antiquity, but variation in Local gravel was used for stones the seeds exhibit morphological flatures as- equivalent dose from aliquot to aliquot sug- at Watson Brake. In addition, fired earthen sociated with evolution under cultivation. gested that bleaching of the mound B sam- objects (function unknown) occur in a vari- However, their presence may reflect the ear- ule was incomulete. ety of undecorated shapes (cuboidal, rectan- ly development of ecological relationships The mean residence time of three gular, spherical, and cylindrical); block forms that eventually led to . mounds and two ridge segments was calcu- are the most common. Similar fired earthen The fauna and flora suggest that Watson lated with the experimental dating method objects have been recovered from French- Brake was occupied seasonally. Fish otoliths of oxidizable carbon ratio (OCR). The man's Bend Mounds, a Middle Archaic site indicate that most fish were caught in OCR procedure megsures biochemical deg- 35 km northeast of Watson Brake (7). spring to early summer and fall. The spring radation of organic carbon within the site- There, mound deposits with fired earthen to early summer peak probably corresponds spec~ficenvironmental context (18, 19). blocks date to 5570 (7) and 5290 cal. calen- wlth the spawnlng of the freshwater drum Analysis of 200 soil samples suggests that dar years B.P. (@69637,4560i 140 correct- (Aplodznotus grunnlens). The plant specles the sublnound soils were buried with mound ed. uncalibrated 14C vears B.P.. charcoal seed In the summer through fall, also sug- fill, thereby arresting biochemical degrada- scatter), which is about the same age as that gesting seasonal occupation of the site. tion. at -5180 i 155 calendar vears B.P. of Watson Brake snecimens. Undecorated Increases in terrestrial specles coupled and that mound fill pedogenesis' began at blocks have not beel; found at Poverty Point. with decreases in mussel and slackwater fish -5010 i 150 calendar vears B.P. Over 175.000 nieces of bone were re- between the earlier nre~noundmidden and Artifactual data support a pre-Poverty covered from mound B, stage I and its the later stage I midden of mound B may Point (>3500 years B.P.) origin for the sublnound surface. Aquatic species pre- reflect a change in the local environment. The decline in the main channel, gravel/ sand shoal habitats, backwater swamps, and Table 1. 14C dates from Watson Brake. Rg, ridge; Md, mound small-stream habitats near Watson Brake may have resulted in a shift from aquatic Calibration resources to terrestrial species, eventually Age In curve Dendrocal~brated Provenience Material Lab radiocarbon intercept 20 age range leadine" to site abandonment. number" years B.P.? (years (years B.P.)S: Geomorphic, pedogenic, radiometric, lu- B.P.): minescence, and artifactual data have es- tablished the Middle Archaic age of Rg WA, subr~dge Charcoal scatter p72670 5070 -t 110 5883 6000 to 5593 Watson Brake. Faunal and floral data show Rg WA, stage I Charcoal scatter p72669 4840 i 170 5591 5931 to 5242 that the site was constructed and occupied Rg WA, stage I Charcoal piece p66045 4610 i 90 5309 5492 to 5029 Md B, submound Charcoal scatter p82009 4960 i 120 5703 5941 to 5455 by hunter-gatherers who seasonally exploit- Md B, stage I Charcoal scatter p80792 4660 i 1 10 5442 5606 to 5034 ed riverine animals and plants. Planned i Md B, stage I1 Charcoal scatter p72671 461 0 90 5309 5492 to 5029 large-scaleu earthworks such as Watson Md B, stage I1 Charcoal scatter p72512 4860 i 100 5596 5761 to 5442 Brake were previously considered to be be- Md B, stage ll Charcoal scatter p72672 4550 i 1 10 5288 5468 to 4872 yond the leadership and organizational Md A, stage I Humates TX9003 4361 i 71 4870 5081 to 4822 skills of seasonally mobile hunter-gatherers. Md A, submound Charcoal scatter p95000 4690 i 130 5450 5665 to 4987 Md A submound Humates TX9004 4540 i 58 5285 5326 to 5021 Poverty Point was considered the exceu- Md C, stage I hearth Organic fill p93880 4220 2 60 4826 4779 to 4567 tion, and its extensive trade was cited as Md C, stage I Humates TX9002 4200 i 63 4822 4860 to 4563 evidence for sophisticated socioeconomic Md C, submound Charcoal scatter p95002 4700 i 90 5452 561 3 to 5253 organization (20). Our data imply that less Md D, stage Ill Humates TX9005 4046 i 49 451 7 4645 to 4406 complex mound building societies flour- Md D, stage I Humates TX9007 4329 i 64 4864 5054 to 481 5 ished in the southeast more than 1900 years Md D, submound Humates TX9006 4702 i 53 5452 5497 to 5307 before Poverty Point. Furthermore, not only 'p Beta Analytic; TX. Radiocarbon Laboratow, University of Texas tuncalibrated 13C/i2C-c~rrected14C age of did these Middle Archaic societies establish specimens in '?C years B.P. (tlu), $Intercept between the conventional '"C age and the dendrocal~brated calendar tme scale, In calendar years B.P. (Radiocarbon Cabraton Program 1993, rev. 3.0 3c, M. Stuiver and P M. mon~unentalarchitecture in the southeast, Remer) $Two-sigma dendrocalbrated age range for specimens, in calendar years B.P but they also may have initiated ecological

1798 SCIEYCE \'OL.277 19 SEPTEMBER 1997 wvvn.sciencemag.org relationshi~sthat led to the eventual do- Reversible Nanocontraction and Dilatation in a mestication of weedy annuals In eastern North America. Solid Induced by Polarized Light

REFERENCES AND NOTES P. Krecmer, A. M. Moulin, R. J. Stephenson, T. Rayment,

1. B. D. Smth, ~nAdvances in World Archaeology, F. M. E. Welland, S. R. Elliott* Wendori and A. Close, Eds, (Academ~cPress, New York, 19861, vo. 5. pp. 1-92. Reversible, controllable optical nanocontraction and dilatation in a chalcogenide glass 2. J. A. Ford. Science 122. 550 (1955). film was induced by polarized light, and a direct correlation of this optomechanical effect 3. J. L. Gibson, in Archaeology of the Mid-Holocene Southeast K. E. Sassaman and D. G. Anderson, with the reversible optical-induced optical anisotropy (dichroism) also exhibited by the Eds. fUnlv. of Florida Press Ga~nesv~lle.FL. 19961. chalcogenide glass was observed. A microscopic model of the photoinduced, reversible pp. 288-305. structural phenomenon responsible for the optomechanical behavior is presented. The 4. J. L. G~bsonand J. R. Shenkel, in Middle Woodland Settlement and Ceremoniaiism in the Mid-South and ability to induce an anisotropic optomechanical effect could form the basis of a number LowerMississippiValley, R. C. Mainfort Jr., Ed. (Archae- of applications, including polarized light-dependent optical nanoactuators, optome- ological Report No. 22. M~ss~ss~ppDepartment of Ar- chanical diaphragm micropumps, and even motors driven by polarized light. chives and History Jackson, MS, 1988) pp. 7-18. 5. M. Russo, Southeast. Archaeol. 13, 93 (1994). 6. J. W. Saunders and T. Allen, Am. Antiq. 59, 471 (1994). 7. , R. T. Saucier, Southeast. Archaeol. 13, 134 So far, only piezoelectric and electrostric- nanocontraction parallel to the direction of (19941. 8 M. Russo, In Archaeology of the Mid-Holocene tive positioning devices have the potential the electric vector of the polarized light and Southeast. K. E. Sassaman and D. G. Anderson. to meet the accuracy demands in nanotech- nanodilatation along the axis orthogonal to Eds. (Univ, of Florida Press, Gainesvile FL, 1996), nology, which require movement and mea- the electric vector of the light. This behav- pp. 259-287. surement with nanometer-scale precision ior can be interpreted in terms of a net- 9. R. T. Sauc~erand A. R. Fleetwood, Geol. Soc. Am. Buli. 81, 869 (1970). (1). Here we show that certain alnorphous work-related mechanism for this reversible 10. R. T. Sauc~erGeomorphology and Quaternan] Geo- selniconducting materials exhibit an opto- optomechanical and PA effect. logic History of the Lower Mississippi Valley (U.S. mechanical effect that is solely dependent A direct method for the determination Army Engineer Waterways Experiment Station, V~cksburg,MS, 199,4) vo. I. on the absorption of polarized light. This of stress in thin films involves the bending 11. Ridge B/C is the exception. effect could be exploited to make devices of a lnicrobealn (cantilever), in which a 12. P. W. Birkeland, Soils and Geomorphoiogy (Oxford that would supplement or substitute the change in the stress of a film deposited on Univ. Press, New York, 19841 13. Charcoal from the mound D submound suriace dat- presently available range of electric field- one side will cause the film-cantilever struc- ed to 3824 i 120 cal. calendar years B.P. (P95003). dependent piezoelectric devices. ture to bend to minimize its stored strain Humate ages from the same hor~zonand buried A Reversible photoinduced anisotropy energy. If, for instance, the tensile stress in horizons above the sample were >700 years older. (PA) can be induced by polarized light the film deposited on the top surface of the Charred bone from augerlng of stage I, mound B dated to 4140 i 60 cal. calendar years B.P. (so-called vectoral phenomenon) in chal- cantilever increases, the film tends to con- (872331). Subsequent charcoal samples from cogenide glasses (2, 3). By using this effect, tract and the cantilever bends up. mound B, stage I and overlyng stage I were 1000 a previously optically isotropic chalco- We focused a probe laser onto the end of years older. 14. C. W. Mart~nand W. C. Johnson, Quat. Res. 43,232 genide glass sample can be made linearly or a cantilever and measured the bending of (1995). circularly dichroic or birefringent after the the beam by the movement of the reflected 15. Y. Wang, R. Amundson, S. Trumbore, ibid. 45, 282 absorption of linearly or circularly polarized laser spot on a position-sensitive photo- (19961. 16. J. K. Feathers, Quat. Geochronol. 16, 333 (1997). light, respectively. Current views on the diode; the deflection of the cantilever is 17. G. A. T. Duller, Radiat. Meas. 24, 21 7 (1995) structural origin of PA can be classified into linearly dependent on the current output. 18. D. S. Frnk, n Pedological Perspectives in Archaeolog- two groups. The first group ascribes PA to a We used commercially available V-shaped ical Research, M. Coli~ns,Ed (Sol Scence Soc~etyof variety of relatively isolated atomic events atomic force microscope microcantilevers. America Spec~alPubcation 44, Madison Wl, 1995). 19. , Archaeol. East. Nodh Am. 20, 67 (19921. that occur at short length scales, mainly on They are fabricated from silicon nitride 20. C. H. Webb, "The Poverty Pont Culture" No. 17 the basis of the spatial redistribution of with typical dimensions of 200 p.m in (School of Geoscence, Lou~sanaState Unlversty, covalent bonds (2), directional changes in length, 20 p.m in width, and 0.6 p.m in Baton Rouge, LA, 1982). 21. A Poverty Pont Object was found in level I, TU 7, the electric dipole moment arising from thickness, and the bottom surface is coated r~dgeB/C. defect sites (4),or lone-pair electron orbit- with a thin layer of gold (-20 nm thick) 22. F. Schambach, thes~s,Haward Unversity (1970). als (5) inherent in chalcogenide glasses. that increases the optical reflectivity for the 23. M. D. Jeter and H. E. Jackson, La. Archaeol. 17,133 (1994). The second group invokes the orientation probe laser beam. A thin amorphous 24. J. L. G~bson,ibid., p. 251. of structural elements that interact at long- AsjoSe,, film (250 nm thick) was evaporat- 25. C. H. Webb and J. L. G~bson,Geosci. Man 22, 85 er length scales, that is, pseudocrystal-like ed on to the top surface of the cantilever. (19811. structures (6), or the cooperative effect of As demonstrated previously (7, 8), a com- 26. J. L. G~bson,Bull. Tex. Archaeol. Soc. 38, 1 (19681. 27. J Connaway, La. Archaeoi. 8, 57 (19811. local anisotropic events resulting in a global posite cantilever is sensitive to thermal 28. J K. Johnson, Southeast. Archaeol. 12, 59 (1993). distortion of the amorphous network (5). changes, for example, when heated by an- 29. B. D. Smth, Rivers of Change: Essays on Early Ag- Our investigations show that upon irra- other laser beam incident on the top sur- riculture in Eastern North America (Smithsonian In- sttution Press Washington, DC, 19931, diation with polarized light, thin amor- face. This phenomenon, the bimetallic ef- 30. M. Stuver and P. M. Reimer, Radiocarbon Cabra- phous films of AsjOSe,, exhibit reversible fect, results froln the differential thermal tion Program 1993, rev. 3.0.3~(Quaternary Isotope expansion of the two bonded materials. The Lab, Un~versityof Washington, Seattle, WA). P. Krecmer, T. Rayment, S. R. Ell~ott,Department of total stress change in a deposited film is 31 Funders included the National Geographc Soc~ety Chem~stry,~nlversjty of Cambr~dge, Lensf~eld Road, (two grants) the National Park Service, and the State therefore the of two cilnponents: the Cambr~dgeCB2 1EW, UK. of Lou~sana.The Gentry family, W~llametteIndus- and stresses' The A. M. Moul~n,R. J. Stephenson, M. Welland Depart- tries, Inc., and the Archaeolog~calConservancy a- E. ment of Englneerng, University of Cambrdge, Trumping- stress is related to differences in the struc- lowed access to the slte. ton Street Cambrdge CB2 IPZ, UK. ture, which, for this experiment, was mod- 27 May 1997; accepted 4 August 1997 "To whom correspondence should be addressed. ified by a change in polarization of the light

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