Report

Coral 230Th Dating of the Imposition of a Ritual Control Hierarchy in Precontact Hawaii

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Science  07 Jan 2005:
Vol. 307, Issue 5706, pp. 102-104
DOI: 10.1126/science.1105432

Abstract

In proto-historic Hawaii (1500–1795 A.D.), as in many other evolving polities, temples functioned as centers for control over production and the extraction of surplus food and goods. Thorium-230 dates (uncertainty ± ∼10 years) on branch coral dedicatory offerings from temples in the Kahikinui district (Maui) indicate that its temple system was constructed within 60 years, far more rapidly than indicated by radiocarbon dating. Introduction of the temple system in 1580–1640 A.D. coincided with predatory expansion and consolidation of the Maui polity to form an incipient archaic state.

Complex chiefdoms and the early archaic states that emerged from them typically exercised power through the imposition of a control hierarchy with three to four administrative levels. Linked to religious ideology and temple ritual (1, 2), these hierarchies are reflected archaeologically in monumental architecture, particularly temples and palaces (3). The Hawaiian Islands offer a model locality for investigating the transition from a chiefdom to an archaic state because of their isolation from outside contact after ∼1200–1400 A.D. (4, 5). During the late expansion to proto-historic periods of Hawaiian prehistory (∼1500–1800 A.D.), several sociopolitical changes took place, including (i) the emergence of endogamous classes and of the ideology of kingship, (ii) the replacement of kinship-based land control with a territorial system, (iii) the imposition of corvée labor and the collection of surplus food and goods as tribute, (iv) agricultural and aquacultural intensification, and (v) the imposition of ritualized controls on production (69). The rate at which these changes took place has been uncertain.

Hawaiian temples were constructed on monumental-scale platforms and terraces of dry-laid basalt. The temple system corresponded to a hierarchy of major gods associated with agricultural production or war (10). Agricultural temples (ranging in size from 50 to 2000 m2) often marked territorial land divisions. War temples (up to ∼10,000 m2) are more dispersed (6).

Dating the construction of temples has depended on radiocarbon (14C) dating of wood charcoal. Dated Maui Island temples (11, 12) increased in size between ∼1400 and 1650 A.D., corresponding to a period of archipelago-wide population growth (13) and the intensification of irrigated and dryland agricultural systems (14). 14C dating in the past 500 years, however, is confounded by irregular fluctuations in atmospheric 14C concentrations, leading to uncertainties of 40 to >250 years (15, 16). Moreover, spuriously old 14C ages can result from burning of wood that significantly predates temple building.

We report a refined chronology of construction for temples on Maui and Molokai Islands obtained via 238U-234U-230Th dating (230Th dating) of branch corals used as dedicatory offerings. This approach offers substantial advantages over 14C dating. 230Th ages of materials that remain a closed system and are free of initial 230Th depend only on their 230Th/238U and 234U/238U ratios and the decay constants of 230Th, 234U, and 238U: The isotope ratios may be accurately determined by mass spectrometry, and the decay constants are known to ∼0.3% or better (17). Corrections for initial 230Th in corals are generally minor.

The ancient Hawaiians regarded branch coral (several species in the genus Pocillopora) as an appropriate offering material for placement on coastal fishing shrines and at inland agricultural temples (Fig. 1, A and B). The preservation of delicate surface structures (verrucae) on these corals and the lack of evidence of abrasion or erosion indicate that the corals were collected as live specimens before their placement at temples (Fig. 1C); the measured age of the sample therefore corresponds to the target event to be dated: the placement of a dedicatory offering (18, 19). That corals were not simply deposited at temples after construction is indicated by their incorporation into walls and platform fill.

Fig. 1.

Branch coral from Kahikinui archaeological sites. (A) The arrow points to a branch coral offering in situ at temple site KIP-405; a shell of Cellana exarata is adjacent to the coral. (B) Entire coral head, with branch tips removed by ancient Hawaiians, from site MAW-255. (C) Dated sample KIP-405. The base is dated 1601 ± 7 A.D.; the tip, 1608 ± 7 A.D. The preservation of branch shape and delicate surface structures (verrucae) indicates that living coral was collected from the sea bottom.

We focused on temples within the ancient district of Kahikinui, on southeast Maui Island, where 30 temple foundations are preserved, with basal foundations ranging from 60 to 1400 m2 in area (20). Most were agricultural temples, located in areas of productive soils for dryland agriculture (21). We selected branch coral offerings from seven of these temples, representing different temple size classes and both coastal and inland temples (Table 1). We also dated corals from a boundary temple at Kawela, Molokai Island, which was at various times incorporated into the competing Maui and Oahu polities. The Kawela site is interpreted as a Hale o Lono, or a temple dedicated to the deity Lono and linked to the annual Makahiki rite of tribute collection, one of the means of extracting agricultural surplus that was used by the chiefly classes (22). We also analyzed one living coral to refine our estimate of initial 230Th (23). The time span of prehistoric Hawaiian occupation in Kahikinui has been established as 1400–1800 A.D. through 14C dating of 159 samples of wood charcoal from agricultural, habitation, and ceremonial contexts (supporting online text and fig. S1).

Table 1.

Temple sites with dated branch coral samples.

Location Site no. Basal area (m2) Architectural form
Kahikinui, Maui AUW-11 30 Walled enclosure
Kahikinui, Maui KIP-273 174 Notched enclosure and platform
Kahikinui, Maui KIP-275 120 Platform
Kahikinui, Maui KIP-405 400 Notched enclosure and terrace
Kahikinui, Maui KIP-414 433 Notched enclosure
Kahikinui, Maui KIP-1010 1400 Double notched walled enclosures
Kahikinui, Maui MAW-255 150 Terrace complex
Kawela, Molokai 90 Walled enclosure and platform

To determine whether temple construction and dedication could be more accurately dated, nine corals from archaeological sites (Table 1) and one living coral from Maui were analyzed for U and Th isotopes. The archaeological corals consist of broken branches, basal fragments of coral colonies, and an intact coral colony (Table 2). Many samples have distinct upper and lower surfaces, apparently reflecting long exposure in a constant orientation to dust infall, rain, and sunlight. The samples are identified as Pocillopora meandrina, except for one of the Molokai corals, which is probably P. damicornis. P. meandrina is a common coral species on Hawaiian reef slopes at shallow depths and grows at a rate of ∼1 cm/year. Individual colonies can be as large as 30 to 40 cm (24).

Table 2.

230Th dates for archaeological and modern corals (95% confidence interval errors). Uncorrected dates assume no 230Thnr, whereas corrected dates assume a (230Thnr/232Th) activity ratio of 2.5 ± 1.2 determined from analysis of a modern Kahikinui coral. Kawela 1-A and Kawela 1-B are replicate samples obtained by splitting 2 g of coral. 232Th abundances are corrected for a total procedural blank of 29 ± 15 pg; the blank (230Th/232Th) activity ratio is ≤3.7. Modern coral was collected in July 2002 and analyzed in December 2003. Isotopic ratios are activity ratios; decay constants used are those of (30). Full analytical data are available in the supporting online material (table S1).

Sample Coral habit 232Th (pg/g) (230Th/ 232Th)act Uncorrected date (A.D.) Corrected date (A.D.)
Kawela 1-A Branch fragment 570 67.6 1557 ± 7 1575 ± 12
Kawela 1-B Branch fragment 543 71.2 1552 ± 5 1569 ± 10
Kawela 2 Branch tip 297 106.4 1553 ± 6 1565 ± 8
AUW-11 Branch fragment 284 119.9 1629 ± 4 1638 ± 6
KIP-273 Branch tip 231 139.2 1610 ± 6 1618 ± 7
KIP-275 Branch fragment 194 133.5 1617 ± 5 1625 ± 6
MAW 255 Branch tip 350 108.1 1619 ± 5 1629 ± 7
KIP-405 Branch tip 360 91.7 1596 ± 4 1608 ± 7
KIP-405 8 cm from branch tip 249 147.2 1594 ± 6 1601 ± 7
KIP-414 Colony-base fragment 76 306.7 1569 ± 5 1574 ± 6
KIP-1010 Colony-base fragment 115 241.6 1574 ± 10 1580 ± 10
Modern Branch tip 336 3.1 1992 ± 1 2002 ± 5

When shallow-water corals form, they are profoundly depleted in 230Th relative to its ultimate parent, 238U; therefore, they are generally highly suitable for 230Th dating (25). The 232Th concentrations of corals analyzed for this study are, however, about an order of magnitude higher than 232Th concentrations reported for many reef-building corals (25). The presence of 232Th indicates that small amounts of nonradiogenic 230Th (230Thnr) (230Th not generated by in situ U decay) are also present. Such 230Thnr can affect calculated ages for corals that are less than a few thousand years old (26). The effect of 230Thnr can be corrected by using 232Th (half-life t1/2 ∼ 14 Gy) as an index isotope if the 230Thnr/232Th ratio can be estimated or determined from a sample of known age. Sources of 230Thnr in the Hawaiian corals might include silicate detritus derived from basalt or eolian dust (27) and Th from seawater that is either dissolved or adsorbed on suspended particles. Expected 230Thnr/232Th atomic ratios in such materials range from ∼4 × 10–6 in silicates (assuming secular equilibrium and a typical crustal Th/U ratio) to ∼1 × 10–5 in shallow Pacific sea-water (28). Other sources of Th and combinations of sources are also possible.

To estimate the 230Thnr/232Th ratio, we analyzed a sample from the outer centimeter of a living coral collected adjacent to the Maui study area. Given typical P. meandrina growth rates of ∼1 cm/year, this sample is expected to be about 0.5 years old. Adopting that age (and subtracting the 230Th formed between collection and analysis), we calculate for the modern coral a 230Thnr/232Th atomic ratio of 1.4 ± 0.2 × 10–5, or equivalently a 230Thnr/232Th activity ratio of 2.5 ± 0.4. This value falls near the middle of the range of 230Thnr/232Th values found in an extensive study of living and young fossil corals in the central Pacific (26).

The 232Th concentration of the modern coral sample (336 pg/g) is similar to or greater than the 232Th concentrations of the other dated Maui corals; thus, there is no evidence of 232Th addition to the fossil corals during their subaerial exposure. Accordingly, we used the 230Thnr/232Th ratio determined from the modern coral to correct for 230Thnr in the fossil corals. The largest 230Thnr correction applied (to sample Kawela 1-A) was 18 years; the median correction for all fossils corals was 9 years. To encompass possible variability in 230Thnr, we assigned an uncertainty of ±50% to the 230Thnr/232Th ratio applied to fossil corals. The resulting 230Thnr correction adds <1 to 5 years to the calculated age errors, depending on the 232Th content of each sample.

Calculated dates for the archaeological corals fall in a narrow range from 1565 ± 8 to 1638 ± 6 A.D. (Table 2). Moreover, dates on colony-base fragments may overestimate the harvest dates by as much as 30 to 40 years: the estimated longevity of individual P. meandrina colonies. If only the dates of coral branches and branch fragments from Kahikinui are considered, the age range is restricted to 1608 ± 7 to 1638 ± 6 A.D. Thus, we infer that all of the dated Kahikinui corals were placed as dedicatory offerings in an interval of ∼60 years and perhaps in as little as 30 years—possibly within the span of a single generation of Hawaiians. Branches from the Kawela site on Molokai yield two analytically indistinguishable dates, 1571 ± 8 A.D. (weighted mean of two analyses of Kawela 1) and 1565 ± 8 A.D., which are slightly older than the Kahikinui branches.

Our sample of Kahikinui corals includes those from both small and mid-sized temples, as well as the largest temple structure (KIP-1010) in the district. Accordingly, the timing of intensive temple construction, reflecting a fundamental change in the sociopolitical structure of the district—the imposition of a ritual control hierarchy—occurred rapidly. The ages from Molokai indicate penecontemporaneous or perhaps slightly earlier construction of similar temples outside of Kahikinui.

Chiefly genealogies and oral traditions collected in the 19th century (9) provide detailed information about Hawaiian political history. For Maui Island, these traditions indicate that two formerly independent chiefdoms were brought under the control of a single leader during the reign of Pi`ilani, dated to approximately 1570–1600 A.D. Pi`ilani's grandson, Kamalalawalu (∼1610–1630 A.D.), extended the Maui polity by taking over the nearby island of Lanai (and probably also smaller Kahoolawe Island). These conquests, coinciding with our dates for intensive temple construction, expanded the Maui polity in size from about 940 km2 to more than 2360 km2, the kind of territorial expansion predicted with the formation of an archaic state (29). A parallel process of sociopolitical change apparently occurred on Hawaii Island at the same time, under the reign of Liloa and his son `Umi-a-Liloa (9).

The temples provide tangible archaeological evidence of the speed with which a fundamental sociopolitical transition occurred in proto-historic Hawaii. The surprising swiftness of the transition revealed by the new dates raises the possibility that similar transitions elsewhere may have been equally abrupt.

Supporting Online Material

www.sciencemag.org/cgi/content/full/307/5706/102/DC1

Materials and Methods

Figs. S1 and S2

Table S1

References

References and Notes

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