5.1 A ¹⁴C chronology

Seven ¹⁴C dates on procellariid bone from sinkhole sites 1710-1 and 9659-1 (Table 5)¹¹ provide the data needed to construct an absolute chronology for the stratigraphic sequences of Kalaeloa sinkholes. Davis designed the dating project carefully, selecting bone samples for dating from each of three depositional units present at the two sites. Because of this, the dating calibration can integrate relative stratigraphic information by use of a Bayesian statistical framework (Buck et al. 1996, 1992, 1991). In this framework, information on relative ages of dated events is used to constrain the calibrated ages of dated samples; the calibrated age of a sample will always be younger than the calibrated age of a sample recovered from a stratigraphically older deposit, regardless of the relative ¹⁴C ages of the two samples. Thus, samples that yield inverted ¹⁴C ages are restored to their correct relative ages, as this relationship is defined stratigraphically. Typically, addition of stratigraphic information to the calibration procedure improves the archaeological interpretibility of results. In addition, adoption of a Bayesian framework provides a way to obtain age estimates for events that were not directly dated, which is useful in this case because it is possible to estimate ages of depositional unit boundaries. Under the assumption that changes in depositional modes were penecontemporaneous across the region, age estimates for depositional unit boundaries derived from sites 1710-1 and 9659-1 can be extrapolated to depositional sequences of sinkhole sites that were not dated. A primary objective of the Bayesian ¹⁴C calibration reported here is to estimate calendar ages of transitions from one depositional unit to the next.

Lab. no. Site Depth Dep. unit CRA 13C Event Beta-11192 9659-1 10-20 transported 920±100 -17.6 1 Beta-11193 9659-1 20-30 collapse 1130±100 -17.5 2 Beta-11194 9659-1 30-40 collapse 1370±100 -19.0 3 Beta-11188 1710-1 16-26 collapse 1090±100 -23.2 4 Beta-11189 1710-1 26-36 collapse 1260±100 -13.8 5 Beta-11190 1710-1 36-46 collapse 1730±100 -19.8 6 Beta-11191 1710-1 56-65 basal 2320±100 -24.7 7 Table 5. 14C dates on procellariid bone

The analysis requires three assumptions: 1) sediment deposition in sinkholes is continuous - there are no hiatuses between or within depositional units; 2) there is no significant hiatus between the death of the bird whose bone was dated and deposition of the bone in the sinkhole; and 3) post-depositional movement of dated bones through the stratigraphic column was not sufficient to change their positions relative to depositional unit boundaries. We believe that these assumptions are reasonable at this stage of analysis, but do not believe that they are universally valid. In our view, further study of the dynamics of sediment deposition in sinkholes is clearly warranted.

Given the stratigraphic and ¹⁴C information, and assumptions listed above, it is possible to formulate a model of the relationships among depositional units and unknown calendar ages of events represented by seven ¹⁴C dates. Following standard practice, we indicate the lower boundary of depositional unit i (i = I, II, III) as _i and the upper boundary as _i. Let _j denote the unknown calendar date BP of event j (j = 1.7). Then archaeological and ¹⁴C information from the two Kalaeloa sinkholes can be expressed in the form of the following inequalities.

(1)

(2)

This model was implemented using the OxCal software package (Ramsey 1995). Seven ¹⁴C determinations associated with the _i (Table 5) were calibrated with a marine curve (Stuiver and Braziunas 1993) using a r value of 110±80 established for ocean waters surrounding the Hawaiian Islands (Dye 1994b).

Dye & Tuggle Event Davis Christensen 95.4% h.p.d. 1 A.D. 1265-1490 A.D. 1030 A.D. 1420-1880 2 A.D. 1215-1410 A.D. 820 A.D. 1150-1510 3 A.D. 1030-1325 A.D. 580 A.D.890-1340 4 A.D. 1255-1415 A.D. 860 A.D. 1200-1540 5 A.D. 1055-1350 A.D. 690 A.D. 1010-1410 6 A.D. 790-1215 A.D. 200 A.D.580-1060 7 A.D. 445-855 370 B.C. 200 B.C.-A.D. 450 Table 6 Estimated ages of dated events. Sources: Davis (1990); Christensen (1995).

Estimates of the calendar ages of the dated events are listed in Table 6 as 2 highest posterior density regions, along with calendar ages reported by Davis (1990) and Christensen (1995). The estimates yielded by Bayesian analysis are younger by 200-700 years than age estimates reported by Christensen (1995), as expected. They are, however, very close to the results yielded by Davis' calibration procedure, with two important exceptions. The two exceptions are ₇, at the early end of the sequence, and ₁ at the late end of the sequence. The Bayesian estimate for the age of ₇ is 400-600 years earlier than Davis' estimate, and the estimate for ₁ is 200-400 years later. These differences have the effect of doubling the estimated duration of the interval between the earliest and latest events in the sequence, transforming the 400-1,000 year sequence posited by Davis, to one that spans 1,000-2,000 years.

Figure 3 Estimated ages of depositional unit boundaries. Left, boundary of basal diagenetic and structural collapse deposits; right, boundary of structural collapse and transported sediment deposits. Solid lines above the x-axis indicate 67% and 95.4% highest posterior density regions.

The 95.4% highest posterior density region yielded by Bayesian calibration for the estimated age of the boundary of basal diagenetic and structural collapse deposits is 50 B.C.-A.D. 950 (Fig. 3), an interval that spans current estimates of the date of initial Polynesian colonization of the islands. The date of colonization has become a point of contention, over which roughly two schools of thought have formed. There is an argument for "early" colonization dating to the A.D. 100-400 range (e.g. Kirch 1985; Hunt and Holsen 1991) and an argument for a "late" colonization, as late as A.D. 600-1000 or even A.D. 800-900 (e.g. Spriggs and Anderson 1993; Athens et al. 1997). Events in the basal diagenetic deposit, exemplified by ₇, are likely to have occurred either very early in the Polynesia era, or before Polynesian colonization of the islands. Although the early colonization estimate is coeval with the boundary of basal diagenetic and structural collapse deposits, it is unlikely that Polynesians would have settled or farmed this marginal region soon after colonization (Tuggle 1997) and it is safe to say that events in basal diagenetic deposits pre-date significant Polynesian activities at Kalaeloa. It is not possible with the data at hand to estimate with confidence when basal diagenetic sediments were first deposited. This event pre-dates event ₇, however, and a reasonable inference is that deposition of basal diagenetic deposits began more than 2,000 years ago.

The 95.4% highest posterior density region for the estimated age of the boundary of structural collapse and transported sediment deposits is A.D. 1320-1740, an interval that encompasses the last four centuries of the pre-Contact era (Fig. 3). The structural collapse deposits represent at least the first half of the Polynesian era, but given uncertainties in the date of Polynesian colonization and in the age estimate of the depositional unit boundary, these deposits might encompass nearly the whole of the era. Events in transported sediment deposits either took place late in the pre-Contact era, or in the period after Contact. The bone dated for event ₁, whose estimated age falls late in the Polynesian period, was collected from the bottom half of the transported sediment deposit at site 9659-1 and is consistent with this assessment.