Establishing a Numerical Chronology for Monument Construction at Easter Island

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1 Abstract This research will establish a chronology of monument construction on Easter Island in the southeastern Pacific Ocean based on surface exposure dating. The new chronology will address the timing of statue and ceremonial platform construction and will apply the chronology to unresolved issues related to the initiation of monument building, periods of monument construction, and the end of monument production. The precise timing of archaeological events on Easter Island has eluded archaeologists since the late 1800s. It was not until the advent and development of radiocarbon dating (Libby et al., 1949) that numerical age measurements of organic materials (e.g. charcoal, bones) was made possible and first utilized at Easter Island during the Norwegian Archaeological Expedition (Heyerdahl and Ferdon, 1961). More recently, Hunt and Lipo (2006) published a collection of radiocarbon dates from the 1950s to present and critically evaluated the materials collected for radiocarbon dating, which were used to interpret Easter Island colonization. Of 53 published and newly collected radiocarbon dates, Hunt and Lipo (2006) chose to reject 36 (~70%) of the collected samples and corresponding ages. Several of the dates rejected by Hunt and Lipo (2006) were limiting radiocarbon dates of monument construction. Our research will provide an unprecedented chronology of monument development at Easter Island that will offer answers to questions associated with the timing of island colonization, development of the "monument culture", and resource depletion for monument construction. This will prove geographically useful in establishing a more precise chronology of Eastern Polynesian colonization across the Pacific Qualifications XXXXXXXXXXX s qualifications to undertake this research include 18 years experience using cosmogenic nuclides for dating rock surfaces, experience measuring cosmogenic 3He with mass spectrometer, recent funding at Oregon State University for a Noble Gas Mass Spectrometry lab (co-pi with XXXXXXXXXX) that will allow for "in-house" 3He measurements, 20+ papers as first or co-author on various cosmogenic nuclide applications, and field work related to this subject in Antarctica, Norway, Labrador, and Greenland. This project was intiated by XXXXXXXXXXXXX and it will compose part of his Ph.D. dissertation at Oregon State University. XXXXXXXXX's qualifications include nearly five years of geologic interpretation and mapping experience, field experience collecting rock samples for cosmogenic dating (10Be, 3He), experience measuring cosmogenic nuclides (10Be), and field work related to cosmogenic dating throughout western North America Project description 1. Introduction: Easter Island (Rapa Nui, Chile) is a remote isle in the southeastern Pacific (27 09 S, W) (Figure 1) that is internationally recognized for the large, humanconstructed monuments that reside on the island. Two fundamental questions regarding the construction of the massive statues (moai), ceremonial platforms (ahu), and other structures on Rapa Nui are: 1) when were these monoliths carved from the local bedrock and, 2) how long did it take to produce the number observed today? Early explorers to the island first posed these questions, along with many others, in the 1700s, but without a way to directly date the monuments in question further understanding of the timing of their construction had to wait until the advent of numerical dating with radiocarbon in the mid-1900s (Libby et al., 1949). One of the first systematic attempts to develop numerical constraints on the construction of

2 the monuments, as well as the initial colonization of the island, was during the Norwegian Archeological Expedition (Heyerdahl and Ferdon, 1961). Since the Norwegian expedition, several other research groups have produced multiple radiocarbon and obsidian hydration dates to help constrain the timing of both island colonization and ahu/moai construction (e.g., (Skjolsvold, 1993; Martinsson-Wallin and Wallin, 1994; Skjolsvold, 1994; Steadman et al., 1994; Martinsson-Wallin and Wallin, 2000). A compilation of over 100 radiocarbon dates that ranged from 410 B.C. to 0 A.D from settlement sites, building structures, statue sites, and agricultural sites was assembled by Martisson-Wallin and Crockford (2002). In their compilation paper, Martisson-Wallin and Crockford concluded that the most likely timing of Rapa Nui colonization was between A.D. and that ahu and maoi related structures were created shortly thereafter and as late as A.D. More recently, Hunt and Lipo (2006) reexamined a collection of radiocarbon dates from the 1950s to the present and critically evaluated the materials collected for radiocarbon dating, which were used to interpret Rapa Nui colonization and statue construction. Following Spriggs and Anderson s (1993) criteria for rejecting erroneous or contaminated dates, Hunt and Lipo (2006) selected 45 published radiocarbon dates together with 8 newly collected dates and chose to reject 36 (~70%) of the ages. Thirteen of the dates rejected by Hunt and Lipo (2006) were limiting radiocarbon dates of monument construction. Based on their reanalysis, they concluded that colonization of Rapa Nui was much later than previously thought, approximately 1200 A.D. Though this rejection criteria has not been unanimously accepted (Gibbons, 2006), it does cast doubt on the previously perceived chronology of colonization and subsequent monument building on the island and leaves the question of precisely when colonization occurred on the island and monument construction ensued. Making no assumptions about which radiocarbon dates are "wrong" or "right", the timing of colonization and monument construction could be as early as B.C. from the oldest charcoal date found on the island, nearly 1400 years younger than the "accepted" timing of colonization. It is this chronologic matter that we will address using cosmogenic nuclide dating. Developing a radiocarbon chronology requires careful sampling techniques because many intrinsic complications with radiocarbon can produce dating errors of 100s 1000s of years. The most common problems with radiocarbon dating come from contamination with modern and inorganic carbon, ocean reservoir effects, fractionation effects, and complications associated with the calibration of radiocarbon years to calendar years (Bradley, 1999). These effects were noted by Spriggs and Anderson (1993) and utilized by Hunt and Lipo (2006) to reject the large population of radiocarbon dates noted previously. We propose to employ a numerical dating method that is independent of radiocarbon dating uncertainties to test the timing of monument construction on Rapa Nui. Our research will develop a numerical chronology based on cosmogenic (3He and 36Cl) surface exposure dating of the monuments that will address the timing of statue and ceremonial platform construction. We will apply the chronology to unresolved issues related to the initiation of monument building, periods of monument construction, and the end of monument production. Surface exposure dating with cosmogenic nuclide has recently proven successful in obtaining high precision, Holocene dates for multiple geologic problems (Laughlin et al., 1994; Licciardi et al., 1999; Stone et al., 2003; Licciardi et al., 2004; Kerschner et al., 2006) and has recently provided numerical dates of 300 ± 30 years for glacial deposits in New Zealand (Schaefer, 2007). In addition, current work here at Oregon State University is developing a cosmogenic chronology with 10Be for landforms that are postulated to be years old. It is our belief that cosmogenic dating has been underutilized in the archeological community, despite its

3 enormous potential in developing precise numerical dates, and that with increased analytical precision in accelerator mass spectrometry (AMS) measurements and increased understanding of the dating limitations it will become a new dating tool for archeologists as it recently has for the geological community. 2. Research Questions: The focus of this research at Rapa Nui is developing a cosmogenic chronology that is independent of radiocarbon dating. This chronology will answer the following questions or at least provide more insight into the problems. 1) When was the initiation of monument construction at Rap Nui, are there identifiable hiatuses in monument construction due to famine or war, and when did monument construction cease? 2) Was island colonization much earlier (~300 B.C.) or later (~1200 A.D.) based on the cosmogenic dates from the monuments and associated structures? 3) Assuming a late colonization as proposed by Hunt and Lipo (2006), how quickly was the monument culture established on the island and does this cast doubt or favor on the proposed theory that the early Rapa Nui peoples depleted their resources to the brink of civil collapse as alleged most recently by Diamond (2005)? 4) Can cosmogenic dating provide precise, accurate dates for monument construction at Rapa Nui and can this dating method be utilized to date other architectural structures associated with other preexisting societies (e.g., Maya, Inca)? 3. Research Strategy Cosmogenic Dating with 3He and 36Cl: We will constrain the age of monument construction at Rapa Nui by using surface exposure dating with cosmogenic nuclides. Cosmogenic nuclides are produced when galactic cosmic rays collide with atoms in the earth s atmosphere, creating a secondary cascades of particles (neutrons, muons) that collide with primary target atoms in rocks and sediments at the earth s surface to produce terrestrial cosmogenic nuclides (Gosse and Phillips, 2001). By measuring the amount of cosmogenic nuclides in a rock and knowing the production rate, one can calculate the total time (exposure age) since the rock became exposed to the atmosphere. 3He is produced in minerals largely by spallation due to the collision of high-energy cosmic rays with target nuclei in minerals, and is well suited for dating minerals common in basic volcanic rocks (olivine and pyroxene) (Gosse and Phillips, 2001). Olivine and pyroxene bearing volcanic rocks are common on the island (Bandy, 1937) and we selected multiple quarry and ahu sites on Rapa Nui that are conducive for surface exposure dating with 3He. A second cosmogenic nuclide, 36Cl, will be utilized for rock types with low abundances of olivine and pyroxene but with abundant phenocrysts of plagioclase, which are ideal for spallogenically produced 36Cl (Gosse and Phillips, 2001). We will establish the timing of monument construction at each site by measuring the amount of 3He and 36Cl in rocks carved for ahu and maoi manufacturing using appropriate, published 3He and 36Cl production rates (Phillips et al., 1996; Gosse and Phillips, 2001; Licciardi et al., 2001). Sampling and Analytical Protocols: The principle investigator and collaborators have extensive experience in sampling rocks for cosmogenic dating (Brook et al., 1996; Licciardi et al., 1999; Licciardi et al., 2001; Clark et al., 2003; Licciardi et al., 2004; Rinterknecht et al., 2006). Sampling boulders for 3He and 36Cl dating will follow established procedures that account for topographic shielding, rock geometry and orientation, sample thickness, vegetation cover, and erosion (Gosse and Phillips, 2001). Sampling large populations of rocks allows for better statistical precision and identification of outliers in a sample group. Therefore, we will sample ahu platforms and other associated rock

4 "carvings" for cosmogenic dating to gain a complete understanding of monument construction across the island. We will concentrate our sampling efforts both at the origin of the ahu and maoi (i.e. quarries) as well as on the ahu and maoi themselves. Sites that we have chosen are the quarries on Rano Raraku where the ahu and moai were constructed and where unweathered sculpting of the bedrock still exists (Figure 2). The quarries are particularly attractive because the large amount of material removed created fresh surfaces not previously exposed to cosmic radiation. We will also focus our sampling at several ahu sites that are proximal and distal to the quarry allowing for a range of monuments to be sampled. To protect the structure, aesthetic appeal, and general nature of the monuments, we propose to collect our samples using a diamond bit drill that will core small diameter (1 2 in.) holes that are in. long in the rocks. These drill cores will be taken at strategic locations that are conducive for cosmogenic dating (e.g., orientation and thickness considerations) but also preserve the unique exterior nature of the monuments themselves. To do this we will take horizontal drill cores as close to the surface of the rocks as possible, back fill all of the holes with cement or mortar, and fill the exterior of the hole with a cut section from the core that we collect. Care will be taken at all sites to limit our impact and the necessary requests to the Chilean government are underway. Initial sample preparation (crushing, sieving, mineral separation) will be done at facilities at Oregon State University, Department of Geosciences, where all necessary equipment is available (jaw crusher, disk mill, Franz magnetic separator, binocular microscopes, heavy liquids, balances, etc.). Noble gas analysis for 3He will be conducted at the new OSU Noble Gas Laboratory using the Nu Noblesse mass spectrometer and associated noble gas extraction line, which will be installed in October Major and trace element measurements needed for 36Cl measurements will be made at OSU using the College of Ocean and Atmospheric Sciences ICP Emission Spectrometer, which we routinely use for this purpose, and 36Cl AMS measurements will be made at PRIME Labs in Purdue, Indiana. 4. Work Plan Year 1. Data Collection and Analysis: Sample ahu, quarries, and other sites for 3He and 36Cl at Rapa Nui. Process rocks for 3He and 36Cl targets and make initial Accelerator Mass Spectrometer (AMS) and Noble Gas Mass Spectrometer (NG-MS) measurements during academic year. Begin preparing manuscript(s) for publication. Year 2. Analysis and Communication: Complete processing of rocks for 3He and 36Cl targets and make final AMS and NG-MS measurements. Complete manuscript for publication and present results at Geological Society of America and the Society for American Archeology annual meetings. 5. Intellectual Merit and Broader Impacts: This research will provide an unprecedented chronology of monument development at Easter Island that will offer answers to questions associated with the timing of island colonization, development of the "monument culture", and possible resource depletion for monument construction. This will also prove geographically useful in establishing a more precise chronology of Eastern Polynesian colonization across the Pacific. In addition to establishing an absolute monument chronology, this research will present archeologists with a dating method that has not been widely utilized in the field to date despite its enormous potential in providing precise, numerical dates for surface exposures initiated by past civilizations. Lastly, this research will aid with

5 XXXXXXX s PhD. dissertation and overall academic experience. XXXXXXX developed this project and proposal, and will participate in all aspects of the work.