δ18O analysis of Atactodea striata: evaluating a proxy for sea-surface temperature and shellfish foraging from a prehistoric rockshelter in Palau, Micronesia

Journal of Archaeological Science: Reports 4 (2015) 477–486

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δ18O analysis of Atactodea striata: evaluating a proxy for sea-surface temperature and shellfish foraging from a prehistoric rockshelter in Palau, Micronesia Nicholas P. Jew ⁎, Scott F. Fitzpatrick Department of Anthropology, 1218 Condon Hall, University of Oregon, Eugene, Oregon, United States

a r t i c l e

i n f o

Article history: Received 9 June 2015 Received in revised form 24 August 2015 Accepted 5 October 2015 Available online 23 October 2015 Keywords: Shell midden sclerochronology Caroline Islands Pacific Sea-surface temperature Paleoecology

a b s t r a c t In this research we examined the isotopic signatures of sequential growth increments from 10 modern and 11 archaeological Atactodea striata shells (21 shells with a total number of 112 samples). Modern shells and recorded sea surface temperature measurements (SSTs) were used to evaluate the geochemistry of A. striata as a suitable candidate for recording ambient SST. Pairing oxygen isotopes and recorded SST of modern samples with xray diffraction (XRD) allows the identification of the biomineralogical composition of A. striata and provides the necessary information to select the most appropriate carbonate temperature equation to convert oxygen isotope values to estimated SST. This SST conversion was then applied to isotopic data from 11 shells recovered from a ~1700 year old component at the Chelechol ra Orrak site in Palau, Micronesia. We discuss the biomineralogical composition of A. striata, modern and prehistoric sea-surface temperature variation, and the importance of using modern shellfish analogues in conjunction with archaeological samples for paleoenvironmental reconstructions. Isotope values from modern A. striata, when converted using Grossman and Kus' (1986) temperature conversion equation, was found to be similar to measured SST. Estimated SST from archaeological shells showed that the mean SST at 1700 cal. BP were similar to modern values, but SST ranges may have been slightly greater. © 2015 Elsevier Ltd. All rights reserved.

1. Introduction The sclerochronological study of mollusk hard tissues has played an important role in reconstructing modern and ancient nearshore environments throughout the world (e.g., Andrus, 2011, 2012; Gordilla et al., 2014; Leng and Lewis, 2014; Prendergast and Stevens, 2013; Schöne and Gillikin, 2013). As mollusks grow and precipitate calcium carbonate (CaCO3), they provide a sequential record of ambient sea-surface temperature (SST) throughout their lifespan although some species may cease growing for periods of time. This biochemical record, particularly for those that continue to grow continuously to some extent, has been instrumental in helping researchers understand the ecology of both modern and ancient marine landscapes and environmental change in a variety of contexts (see Andrus, 2011; Ford et al., 2010; Leng and Lewis, 2014; Rhoads and Lutz, 1980). Stable oxygen isotope studies (18O/16O reported as δ18O) in particular, have provided a wealth of information in reconstructing ancient or paleo sea-surface temperatures using shellfish recovered from archaeological assemblages (e.g., Andrus, 2011, 2012; Andrus and Thompson, 2011; Leng and Lewis, 2014; Reitz et al., 2012; Thompson and Andrus, 2013). ⁎ Corresponding author. E-mail addresses: [email protected] (N.P. Jew), smfi[email protected] (S.F. Fitzpatrick).

http://dx.doi.org/10.1016/j.jasrep.2015.10.015 2352-409X/© 2015 Elsevier Ltd. All rights reserved.

Ontogenetic studies of various shellfish species combined with nearshore environmental information such as seasonal SST variability, for instance, have also provided contextual information for determining the season of shellfish harvesting (e.g., Andrus, 2012; Ford et al., 2010, Jew et al., 2013, 2014; Kennett, 2005; Killingley, 1981; Mannino et al., 2007; Rick et al., 2006; Shackleton, 1969, 1973), which allows researchers to combine additional archaeological evidence to examine human sedentism, subsistence resource availability, and reconstruct various aspects of ancient nearshore environments. For each new species used in sclerochronological studies, several issues must be considered. These include ontogenetic growth variation, growth cessation, biomineralogical composition, vital effects, and a species' ability to accurately record ambient SSTs (Andrus, 2011:2983; see also Campana, 1999; Eerkens et al., 2013; Ford et al., 2010; Jew et al., 2013; Jones et al., 2005; Richardson, 2001; Schöne, 2008). In shell midden sclerochronology, where δ18O is used to estimate paleoenvironmental SST, it is extremely important to have a modern analog of the species to evaluate whether a specific gastropod or bivalve is a reliable recorder of their ambient SST, and if so, what the most appropriate paleo SST equation may be (e.g., Böhm et al., 2000; Craig, 1965; Epstein et al., 1951, 1953; Grossman and Ku, 1986; Horibe and Oba, 1972; Kim and O'Neil, 1997; Killingley, 1981). Several studies have combined modern species stable isotope calibration studies with ecology to better understand species variability (e.g., Burman and Schmitz, 2005;

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subsequent research suggesting that over the course of site use, subsistence strategies changed; fishing gradually declined whilst mollusks were harvested more intensively, and possibly sustainably (Giovas et al., 2010). Samples for analysis were selected from Level 7, 60–70 cm below surface (cmbs) in Layer 7 in Test Unit 1 (1.0 × 1.0 m), which was excavated to a depth of 110 cmbs in 2000. A total of 18 radiocarbon dates (14C) from this unit (Fitzpatrick, 2003a) span the sequence of occupation. The basal layer of this is radiocarbon dated to between ca. 29002700 cal BP (see Fitzpatrick and Kataoka, 2005:3).

Carre et al., 2005; Glassow et al., 2012; Kennett, 2005; Leng et al., 1999). To assess whether a given species can serve as a prehistoric SST proxy in oceanic nearshore environments with relatively consistent salinity, modern monitoring, recording, and collection of select shellfish species and measurement of ambient SST at the time of collection can be compared to the shell's growth and ecology (see Andrus, 2011). Ideally, samples should be collected from nearshore environments adjacent to the archaeological site. It is important, however, to recognize that oxygen isotopes measured in a species can be influenced by local and regional (especially in the tropics) environmental variation through space and time, including salinity, upwelling, rainfall, monsoons, or other freshwater inputs, as well as the availability of a particular species. As such, each mollusk taxon should be evaluated on a case-by-case basis. The Palauan archipelago in western Micronesia is an ideal location to examine these issues. Archaeological evidence here demonstrates a long history of human occupation that extends back for at least 3000 years (Clark, 2005; Clark et al., 2006; Fitzpatrick, 2003a; Liston, 2005), with peoples exploiting a wide range of marine resources for the bulk of their protein such as fish and shellfish, particularly in nearshore and adjacent coral reefs (Fitzpatrick and Donaldson, 2007; Fitzpatrick and Kataoka, 2005; Fitzpatrick et al., 2011; Ono and Clark, 2012). The intensity and diversity of marine resource exploitation, especially shellfish species, is evident in several archaeological deposits that contain thousands of individuals and dozens of taxa. At Uchularois Cave, for example, more than 40,000 minimum number of individuals (MNI) from 74 shellfish taxa were reported (see Masse, 1989; Masse et al., 2006). Similarly, at Chelechol ra Orrak, nearly 100 shellfish species have been identified from anthropogenic deposits, with data showing an increase in shellfish exploitation commensurate with a decline in fishing over the last 1500–2000 years (Fitzpatrick et al., 2011; Giovas et al., 2015). The sheer richness of prehistoric shellfish exploitation in Palau provides an excellent opportunity to explore a range of different species to assess the potential of the oxygen isotope values in their shells to serve as proxies for reconstructing paleo SST. Despite the diverse taxa represented in Palauan assemblages, there has been a dearth of studies assessing whether certain species are suitable candidates for reconstructing ancient marine landscapes in the Pacific Islands generally. In this study, we evaluate the efficacy of A. striata, one of the most ubiquitous mollusk species found in prehistoric sites in Palau, as a proxy to infer SST recorded in the shells for nearshore waters adjacent to Chelechol ra Orrak. Our analyses show that peoples who occupied the site collected these shells in a range of SSTs. Mean SST during this period appears similar to modern, but with slightly higher annual ranges. Our results suggest that: 1) A. striata's shell is comprised of aragonite; 2) A. striata are suitable candidates for recording ambient SST; and 3) this is one method that can allow researchers to evaluate key species in preliminary analyses to confirm whether the select shellfish taxon can be used for palaeoenvironmental reconstruction.

A. striata (Gmelin, 1791), referred to as the striate beach or surf clam, is geographically distributed in the Indo-Pacific from East Africa, including Madagascar and the Red Sea, to eastern Polynesia, up to northern Japan, and south to central Queensland in Australia (Poutiers, 1998). A. striata are small, burrowing short-lived bivalves found in the intertidal zone along sandy beaches in low to medium energy wave environments (Baron and Clavier, 1994). They congregate in high population densities and prefer to occupy medium to coarse substrate beaches (Baron, 1992; Baron and Clavier, 1994). They are sedentary filter feeders with relatively robust shell morphology, a poorly defined umbo, pronounced lateral teeth and grooves, and sculptured concentric ridges along the exterior of the shell (Fig. 2, Lamprell and Whitehead, 1992; Paulay, 2000). A. striata typically grows up to 25 mm in length, but are known to reach 40 mm (Poutiers, 1998). Several factors influence the growth of A. striata, including changes in salinity, ambient SST, age, gender, and reproduction (Baron, 1992; Baron and Clavier, 1994; Lamprell and Whitehead, 1992). In New Caledonia, for instance, A. striata go through several stages of maturation. They begin spawning between November and April when they are 22 mm in length (Baron, 1992). Reproduction appears to stop from May through July (Baron, 1992). These phases may cause fluctuations in growth rates whereby A. striata will expend more energy towards reproduction instead of growth, but will to continue to grow at a slower rate. Like most short-lived small mollusk species, A. striata commonly live between 1 to 3 years. Since reproduction usually begins when shells reach 22 mm in length, we can assume that smaller shells are expending more energy towards growth. Current data on A. striata, including average lifespan of 18 months (1.5 years) and average growth of 25 mm, approximates 1.4 mm of growth per month excluding variation in seasonal grow for smaller shells (b22 mm) that have not reached reproduction phases. Growth rates vary between 1 to 2 mm per month under various environmental conditions (Baron, 1992). For Palau, medium coarsegrained sand substrates, relatively warm SST, and low-medium wave energy beaches provide ideal conditions for this species.

2. Background

4. Methods

2.1. Chelechol ra Orrak site

We analyzed 112 isotopic samples from 21 shells, including modern and archaeological specimens that ranged in size from 21.6 to 14.7 mm in length (see Appendix A). We also submitted two modern shells for XRD analyses to determine their biomineralogical composition. In preparation for isotopic analysis, each shell was cleaned with a brush using distilled water and inspected under a low-powered microscope for an intact terminal edge. Each shell was etched for approximately one minute using hydrochloric acid (0.5 M) to remove foreign substances and diagenetically altered carbonate (see Bailey et al., 1983; Culleton et al., 2006; McCrea, 1950; Robbins and Rick, 2007), microscopically inspected, and if necessary, re-etched. Calcium carbonate samples were removed from the surface of each shell using a Sherline 5410 Micromill, vertically milling each sample to approximately 0.5 to 1 mm in depth with a carbide drill bit (1 mm thick) while maintaining a low rpm to avoid heating the extracted powder samples (see

The site of Chelechol ra Orrak (“beach of Orrak”) is located on the western edge of the small island of Orrak situated just off the southeast coast of Babeldaob (Fig. 1). Orrak is just one of hundreds of uplifted limestone islands that comprise a major portion of the southern Palauan archipelago and are commonly referred to as the “Rock Islands”. Intensive archaeological investigation of the site began in 2000 to examine Yapese stone money quarrying (Fitzpatrick, 2003b). Early human burials were also discovered dating back to between ca. 2900 to 1700 BP (Fitzpatrick, 2003a; Nelson and Fitzpatrick, 2006), overlain by dense occupational refuse that included pottery, dozens of other artifacts made from bone, shell, and stone, and a rich faunal assemblage. The latter was comprised primarily of mollusks and finfish (Fitzpatrick and Kataoka, 2005; Fitzpatrick et al., 2011; Giovas et al., 2015), with

3. Atactodea striata: growth and ecology

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Fig. 1. Map of western Oceania (A) and Palau (B), including the approximate location of Chelechol ra Orrak.

Robbins and Rick, 2007:29). To avoid cross-contamination, drill bits were sonicated in distilled water and allowed to dry between drilling sessions. The samples were then cleaned with compressed air and drill bits were baked overnight at 150 °C to burn off any trace residue from previous sessions. We selected 11 whole A. striata shells from Chelechol ra Orrak, Unit 1 (level 7) between 50 and 60 cmbs (see Fitzpatrick et al., 2011). For each

Fig. 2. Photo of modern A. striata shells (Mod-6) collected from intertidal zone near Chelechol ra Orrak. Note the milling holes (terminal edge is missing because it snapped after removal) along the shell in 1 mm intervals (total scale is 5 mm).

shell, powdered samples were taken in 2 mm intervals starting with the terminal edge sampling along the lateral margin towards the hinge. Five powdered samples were removed from each shell totaling ~8 mm (four powder samples and four 1 mm intervals). Because maturation begins in shells that are typically larger than 22 mm in length, we selected relatively small shells for our analyses, which are likely only a year in age based on growth rates, size, and reproduction strategies (see Baron, 1992). Based on the growth (1.4 mm per month) and ecology of the species, isotopic sampling of shells in 2 mm intervals, including the 1 mm carbide drill bit for 8 mm of total growth, should provide an extended sequence to identify general trends in ambient SST for approximately six months. Estimated SST values were adjusted using Grossman and Ku's (1986:66, mollusks equation c, see below) and plotted with linear trend lines to identify sequential increases or decreases in SST. Eleven samples were analyzed from one shell 20.8 mm in length, providing a 20 mm growth sequence that was used to reconstruct prehistoric SST and compared to modern SST. All CaCO3 samples were analyzed at the University of Oregon's Geological Sciences Stable Isotope Laboratory. Samples were loaded into exetainers, placed in an autosampler, and flushed with helium. Samples were then reacted with several drops of orthophosphoric acid (H3PO4, 100% concentration), producing carbon dioxide (CO2). δ18O were measured using a Thermo-Finnigan MAT253 isotope ratio mass spectrometer with continuous helium flow. All isotopic values are reported in δ-notation in per mil (‰) units relative to the Vienna PeeDee Belemnite (VPDB) standard using the formula:

δ18 O ¼ ððR sample−R standardÞ=R standardÞ  1000

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Fig. 3. X-ray diffraction spectra of two modern A. striata shells, illustrating matching peaks with aragonite (aragonite, CaCO3, 00-005-0453).

where R represents the heavy/light ratio for the abundance of two isotopes. A positive δ value contains greater enrichment in heavy isotope compared to the standard while negative δ values reflect the depletion of heavy isotopes (Wefer and Berger, 1991). Analytical precision for oxygen isotope ratios was ± 0.05‰ (1σ), based on repeated measurements of the international standard NBS-19. The isotopic values and estimated SSTs, including minimum, maximum, and mean values are illustrated in the tables and figures below. To identify the most appropriate δ18O to SST equation, ~ 100 live modern A. striata were collected from the intertidal zone near the site in mid-November 2014. Ambient SST for the live samples was between 28.0 and 29.5 °C. Ten modern shells less than 22 mm in length were sampled for δ18O analysis. Three modern A. striata shells were profiled in 2 mm intervals (including the 1 mm drill bit thickness) starting at the terminal growth increment for a total of 18 mm of sequential growth per shell. Seven modern shells were sampled starting with the terminal edge plus two additional sampled growth increments in 2 mm intervals for a total of 4 mm of growth. Two modern shells were sampled and submitted for energy dispersive XRD analyses at the x-Ray Diffraction Laboratory located at the University of Oregon. Powder X-ray diffraction data were collected on a Rigaku Ultima IV diffractometer using CuKα-radiation (1.54 Å). Data were collected using 2θ/θ scan mode, in the range 20 to 60° for 2θ angles, with a step of 0.02°. Scanning methods were adapted after Ni and Ratner (2008), where polymorphs were identified using 40 kV and 40 mA at a scanning at 2° per minute. Phase analysis was carried out inside Rigaku PDXL software package based on powder diffraction database ICDD PDF-2 Release 2009 (http://www.icdd.com/products/pdf2. htm). The peaks for each shell were compared against calcite and aragonite sample standards to identify the biomineralogical composition of A. striata shells. This identification was essential for determining the appropriate carbonate-SST equation to use. It is important to note that XRD analyses can also be used in mollusk species to detect diagenesis in archaeological samples. Two A. striata shells selected for δ18O (Orr-7 and Orr-11) were also sampled for accelerator mass spectrometry 14C dating. We used Oxcal's

4.2 and calibrated our radiocarbon samples using Marine13 (see Bronk Ramsey, 2014; Reimer et al., 2013). Orr-7 produced a 14C age of 2203 ± 21 (D-AMS-009262) or between 1870 and 1720 cal. BP (2-sigma) and Orr-11 produced a 14C of 2040 ± 21 (D-AMS-009263) or between 1685 and 1535 cal. BP. The average calibrated 14C average for the two dates acquired from the submitted two A. striata shells are 1700 cal. BP. 5. Results Phase analysis showed that X-ray powder diffraction for both samples provide a good match for orthorhombic phase of CaCO3, aragonite (DB card number 00-005-0453, ICDD PDF2009) (Fig. 3). This is not surprising given that aragonite compositions have also been determined for bivalves such as Saxidomos purpuratus and other shellfish species (Yang et al., 2011). It is important to recognize, however, that some shellfish are comprised of a mixture of aragonite and calcite, which can make selecting a δ18O to SST conversion equation difficult. A. striata wavelength peaks match those of pure aragonite. This means that calcite-SST equations cannot be used for these shells and that aragonite equations should be applied to A. striata. Fifty one modern δ18O values (Table 1 and 2, Appendix A) range between −2.7‰(VPDB) and −1.2‰(VPDB), with a mean of −1.7‰(VPDB). The reported terminal edge values for all 10 modern shells range between −2.2‰(VPDB) and −1.3‰(VPDB) with a mean of −1.6‰(VPDB). Modern intrashell variation range between − 0.2‰(VPDB) and − 1.0‰(VPDB). The number of samples per shell reflect this variation where longer sequences overall contain more variation than those with fewer reported samples. The 61 prehistoric A. striata δ18O values range between − 2.1‰(VPDB) and − 0.6‰(VPDB) with a mean of − 1.3‰(VPDB). The prehistoric values appear to be slightly more enriched in δ18O than modern values. 6. Discussion and conclusion Based on XRD results and the aragonite composition of A. striata, we use Grossman and Ku's (1986:66, mollusks equation c) formula

N.P. Jew, S.F. Fitzpatrick / Journal of Archaeological Science: Reports 4 (2015) 477–486 Table 1 Descriptive statistics for A. striata shells, including δ18O values for means (x), minimum (min), maximum (max), standard deviation (s), and variance (σ2). Modern (Mod-) samples are from the intertidal zone near the site and archaeological samples (Orr-) are from the 1700 cal. BP component at Chelechol ra Orrak. δ18O (VPDB) ID#

n

Min

Max

x

s

σ2

Mod-1 Mod-2 Mod-3 Mod-4 Mod-5 Mod-6 Mod-7 Mod-8 Mod-9 Mod-10 Orr-1 Orr-2 Orr-3 Orr-4 Orr-5 Orr-6 Orr-7 Orr-8 Orr-9 Orr-10 Orr-11

3 3 3 3 3 10 10 3 10 3 5 5 5 5 5 5 5 5 5 5 11

−1.9 −2.2 −1.5 −1.7 −2.2 −2.2 −2.7 −1.6 −2.1 −1.6 −1.8 −1.5 −1.5 −1.3 −1.4 −1.4 −2.1 −2.1 −1.9 −1.9 −1.9

−1.2 −1.2 −1.3 −1.4 −1.5 −1.3 −1.3 −1.3 −1.3 −1.2 −1.2 −1.0 −0.8 −0.9 −0.7 −1.0 −1.5 −1.2 −0.7 −1.2 −0.6

−1.6 −1.7 −1.4 −1.6 −1.8 −1.8 −1.9 −1.5 −1.8 −1.5 −1.5 −1.2 −1.3 −1.1 −1.0 −1.2 −1.8 −1.5 −1.3 −1.6 −1.4

0.4 0.5 0.1 0.2 0.4 0.3 0.4 0.2 0.3 0.2 0.3 0.2 0.3 0.2 0.3 0.2 0.3 0.3 0.5 0.3 0.4

0.1 0.3 0.0 0.0 0.1 0.1 0.1 0.0 0.1 0.1 0.1 0.1 0.1 0.0 0.1 0.0 0.1 0.1 0.2 0.1 0.2

for converting δ18O values to SST estimates where:   T ð CÞ ¼ 21:8−4:69 δ18 Oaragonite −δw The selected δ18O water (δw) for modern (0.0‰) and the 1700 cal. BP component (0.05‰) were adjusted using the reconstructed curve from Fairbanks' (1989:639) SST averages for the Pacific Ocean. Grossman and Ku's, 1986 mollusk equation c formula provided the most accurate pairing between modern isotopic ranges from − 2.2‰(VPDB) and −1.3‰(VPDB) and measured SST for the intertidal zone. All three modern shells profiles have a terminal edge estimated water value between 28 to 29 °C, showing a linear trend decreasing through time (Fig. 4). These linear trends are consistent with reported modern average SST trajectories for the Palauan region (Fig. 5) although ambient water temperature is slightly cooler than reported averages for mid-November. Sea-surface temperatures around Palau are relatively consistent throughout the year, fluctuating between approximately 28 to 31 °C. Additionally, the last 30 years of recorded salinity for the region has been relatively consistent between 33.6 and 34.8 psu (see Osborne et al., 2014:721). Linear trends throughout an annual cycle include seasonal SST changes with an increase from the coldest SST between January to March from 28 and 29 °C followed by fluctuations in temperatures from April through June, ranging between ~29 to 30 °C, and further oscillation from July through September between 28 to 29.5 °C. Finally, there is a gradual decrease from September to December from ~30 to 29 °C. The overall linear trajectories from our profiled modern shell values are similar to those reported from Colin (2000) for Malakal Harbor where November is part of an overall decrease in SST (Fig. 5). The 61 oxygen isotope analysis from archaeological A. striata shells were between −2.1‰(VPDB) and −0.6‰(VPDB). This equates to estimated SST between 25.2 °C and 31.8 °C with a mean value of −1.3‰(VPDB) or 28.4 °C (Table 3). The mean SST for archaeological shell samples are similar to modern SST for the Palau region and also estimated SST from modern A. striata shells. The highest amount of SST variation for 8 mm of sequential growth was observed in Orr-9 with a range of 6 °C. High ranges were also observed in the modern shell Mod-7, containing a range of 6.5 °C, although the profile included an additional 10 mm of growth. Archaeological shell Orr-6 contained the lowest

481

observed range of only 1.9 °C. The archaeological samples contained greater SST ranges in 8 mm of sequential incremental growth than the reported modern seasonal SST distributions. Several distinguishable patterns can be observed from the δ18O and estimated SST in the archaeological samples. Seven shells were harvested in SST below 29 °C (−1.5‰(VPDB)) and half of the archaeological samples contained maximum SST values above 30 °C (−1.4‰(VPDB)) while the other half fell below this temperature (Tables 2 and 3). Linear trajectories observed from sequential growth for each shell demonstrates that seven were harvested from warm to cool SST while three were harvested from cool to warm SST (see Table 2 and Fig. 6). Our analyses of A. striata shells from both modern and archaeological contexts offers insight for future studies related to shell midden sclerochronology and understanding Late Holocene nearshore ecology in the northern Rock Islands of Palau. Our analysis demonstrates how using isotopic and SST data from modern mollusk analogs from the local areas to the archaeological site is essential to evaluate a species' ability to record their ambient SST in the oxygen isotope ratios of their shells. Our results and pairing between modern terminal edge values and measured ambient SST during time of collection suggest that oxygen isotope values from A. striata are a suitable proxy for estimating modern and prehistoric SST. Previous reviews of shell midden sclerochronological studies (e.g., Andrus, 2011; Leng and Lewis, 2014) have emphasized the importance of understanding the modern ecology of a shellfish species by conducting proxy validation to evaluate a species' ability to record ambient SST in their shell geochemistry. Several studies (e.g., Grossman, 2012; Grossman and Ku, 1986; Horibe and Oba, 1972; Killingley, 1981; Kim and O'Neil, 1997) have addressed the fractionation behavior of inorganic and biogenic calcite and aragonite where each conversion formula produces slightly different estimated SST. There have also been studies looking at vital effects or the kinetic fractionation to better understand offsets from the isotopic equilibrium in carbonates (e.g., Adkins et al., 2003; Böhm et al., 2000; Spero et al., 1997; Weber and Woodhead, 1972). Ideally, modern species and their ecology should be studied over extended periods of time to understand intrashell variability through ontogenetic growth, particularly for studies involving incremental shell-growth analyses. For preliminary assessment of isotopic analyses in marine and coastal settings, however, our method of sampling multiple shells harvested at the same time and location, documenting SST at the time of collection, and correlating the isotopic data of the terminal edges with the recorded SST, can help researchers in assessing preliminary suitability of various species before more extensive analyses are performed. This method will provide an isotopic average of terminal edge values from modern samples, which allows researchers to select the closest SST estimate equation and thus increases the likelihood of accurately identifying ancient paleo-SST. Because mollusk growth rates vary, with calcium carbonate recording hourly, daily (see Goodwin et al., 2001), weekly, or monthly SST, the terminal edge and recorded SST provides one of the most suitable and efficient methods for pairing approximate SST and isotopic values. The employed sampling method will depend on a combination of isotopic sampling resolution, species ecology, and growth rates. For example, higher sampling resolution (i.e., trenching) of a shell may provide daily or weekly growth rates of a species. However, without modern analog data, biomineralogical composition of the shell, or understanding how a species grows and precipitates calcium carbonate, using various sampling methods or conversion equations can alter estimated SST values by several degrees and provide a false range of SST for shells sampled from archaeological assemblages. For A. striata collected from the western shores of Orrak Island, one of the most appropriate δ18O to SST formula for aragonite is Grossman and Ku's (1986) equation c for mollusks, where the difference between average modern isotopic terminal edges and recorded SST upon collection varied by only around 1 °C.

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Fig. 4. Modern estimated SST (after Grossman and Ku, 1986, T (°C) = 21.8 − 4.69 (δ18Oaragonite − δw)) and δ18O, including linear trend lines for A. striata collected from nearshore waters off the coast of Palau.

Fig. 5. Modern SST averaged from January to December 2000 from Malakal Harbor (~12 km from Orrak). Adapted after Colin (2000).

Our estimated prehistoric SST from isotopic samples of A. striata shells, suggest that nearshore waters off of western Orrak Island in Palau approximately 1700 cal. BP was on average, similar to modern. This assessment is further supported by general similarities between the available modern mollusk species and those reported in the archaeological assemblages (S. Fitzpatrick, personal observation). The ranges of isotope values suggest that there is slightly greater SST variation (ca. 2 °C) around 1700 cal. BP compared to the present. Modern sampling of three sequential growth increments in 2 mm intervals of seven shells show average fluctuations between ~ 1 and 2 °C with a maximum of 4.3 °C (Mod-2A–C) and a minimum of 0.6 °C (Mod-3A–C). The archaeological samples vary on average between 3 and 3.5 °C, with a maximum of 6 °C (Orr-9A–D) and minimum of 1.9 °C (Orr-6A–D). If A. striata are sensitive to short-term episodic events such as storms and monsoons—which can dramatically change recorded SST over a short period of precipitating calcium carbonate—increasing the number of shells or decreasing the distance between sampling intervals for both modern and archaeological samples may provide higher-resolution

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483

Table 2 Stable oxygen isotope values and estimated sea-surface reconstructions (adjusted using Grossman and Ku's (1986) equation where T (°C) = 21.8 − 4.69 (δ18Oaragonite − δw)) for modern and prehistoric A. striata shells from Chelechol ra Orrak. Results are ordered starting with the terminal edge (0 mm) outward towards the hinge in 2 mm intervals. Sample ID

δ18O (VPDB)

SST °C

Distance

Length (mm)

0

2

4

6

8

10

12

14

16

18

20

0

2

4

6

8

10

12

14

16

18

20

Mod-1 Mod-2 Mod-3 Mod-4 Mod-5 Mod-6 Mod-7 Mod-8 Mod-9 Mod-10 Orr-1 Orr-2 Orr-3 Orr-4 Orr-5 Orr-6 Orr-7 Orr-8 Orr-9 Orr-10 Orr-11

17.5 16.7 14.9 17.0 17.3 16.2 17.4 16.5 15.7 16.2 20.1 20.2 19.1 17.4 19.7 20.7 17.6 20.4 21.0 19.3 19.6

−1.9 −2.2 −1.3 −1.4 −2.2 −1.3 −1.3 −1.6 −1.5 −1.6 −1.3 −1.4 −1.4 −0.9 −1.3 −1.0 −1.5 −1.5 −0.7 −1.5 −1.5

−1.2 −1.2 −1.4 −1.7 −1.6 −1.7 −1.8 −1.3 −1.4 −1.6 −1.2 −1.5 −1.3 −1.3 −0.7 −1.2 −1.7 −1.3 −1.2 −1.8 −0.6

−1.6 −1.7 −1.5 −1.6 −1.5 −1.6 −1.7 −1.6 −1.3 −1.2 −1.7 −1.1 −1.5 −1.1 −1.4 −1.1 −1.6 −1.2 −1.4 −1.4 −1.2

– – – – – −1.4 −1.7 – −1.6 – −1.7 −1.0 −0.8 −1.1 −0.8 −1.3 −2.1 −1.4 −1.5 −1.2 −0.8

– – – – – −2.0 −2.1 – −1.9 – −1.8 −1.1 −1.5 −0.9 −0.9 −1.4 −2.1 −2.1 −1.9 −1.9 −1.0

– – – – – −2.1 −2.7 – −1.8 – – – – – – – – – – – −1.7

– – – – – −2.2 −1.9 – −1.9 – – – – – – – – – – – −1.7

– – – – – −2.2 −1.6 – −2.1 – – – – – – – – – – – −1.8

– – – – – −1.9 −1.8 – −2.0 – – – – – – – – – – – −1.9

– – – – – −1.7 −1.9 – −2.0 – – – – – – – – – – – −1.4

– – – – – – – – – – – – – – – – – – – – −1.8

30.8 31.9 28.0 28.4 32.3 27.8 28.1 29.2 28.9 29.3 28.1 28.7 28.8 26.3 28.0 26.6 29.2 29.2 25.2 29.1 28.9

27.6 27.6 28.6 29.9 29.5 29.7 30.0 28.1 28.6 29.4 27.5 29.2 27.9 28.3 25.5 27.6 29.9 28.3 27.7 30.6 25.0

29.5 29.7 28.6 29.3 28.8 29.2 29.9 29.4 27.9 27.6 30.0 27.2 29.1 27.4 28.4 27.3 29.4 27.7 28.6 28.7 27.7

– – – – – 28.4 29.9 – 29.2 – 30.1 26.6 25.9 27.0 25.6 28.2 32.0 28.6 29.1 27.6 25.7

– – – – – 31.0 31.6 – 30.9 – 30.6 27.2 28.9 26.5 26.2 28.5 31.8 31.8 31.2 30.7 26.9

– – – – – 31.5 34.6 – 30.2 – – – – – – – – – – – 29.9

– – – – – 32.2 30.9 – 30.7 – – – – – – – – – – – 30.1

– – – – – 32.2 29.3 – 31.5 – – – – – – – – – – – 30.4

– – – – – 30.8 30.0 – 31.1 – – – – – – – – – – – 30.9

– – – – – 29.8 30.6 – 31.4 – – – – – – – – – – – 28.8

– – – – – – – – – – – – – – – – – – – – 30.4

data to identify whether the observed SST variation is a product of the mollusk growth behavior, variation in ambient SST, actual short-term episodic events, or a combination of these or other factors. Diagenesis of archaeological shells may also contribute to the greater variation observed in archaeological samples; however, etching the surface of the shell prior to sampling can minimize the amount of sampled diagenetically altered carbonate (see Culleton et al., 2006). Current evidence suggests that there was substantially less SST variation in

Table 3 Estimated SST (adjusted using Grossman and Ku (1986), T (°C) = 21.8 − 4.69 (δ18Oaragonite − δw)) for all samples, including count (n), minimum (min), maximum (max), mean (x) standard deviation (s), and variance (σ2) for A. striata shells sampled from modern intertidal zones near the site and archaeological assemblage from Chelechol ra Orrak. SST °C ID

n

Min

Max

x

s

σ2

Mod-1 Mod-2 Mod-3 Mod-4 Mod-5 Mod-6 Mod-7 Mod-8 Mod-9 Mod-10 Orr-1 Orr-2 Orr-3 Orr-4 Orr-5 Orr-6 Orr-7 Orr-8 Orr-9 Orr-10 Orr-11

3 3 3 3 3 10 10 3 10 3 5 5 5 5 5 5 5 5 5 5 11

27.8 27.8 28.2 28.7 29.1 28.0 28.3 28.3 28.1 27.7 27.5 26.6 25.9 26.3 25.5 26.6 29.2 27.7 25.2 27.6 24.9

31.3 32.5 28.9 30.2 32.9 32.8 35.5 29.7 32.1 29.7 30.6 29.2 29.1 28.3 28.4 28.5 32.0 31.8 31.2 30.7 30.8

29.6 30.1 28.7 29.5 30.6 30.7 31.0 29.2 30.4 29.0 29.2 27.8 28.1 27.1 26.7 27.6 30.4 29.1 28.4 29.3 28.6

1.8 2.4 0.4 0.8 2.0 1.7 1.9 0.8 1.4 1.1 1.4 1.1 1.3 0.8 1.4 0.8 1.4 1.6 2.2 1.3 2.0

3.1 5.5 0.2 0.6 4.1 2.8 3.6 0.6 2.0 1.3 1.9 1.2 1.7 0.6 1.9 0.6 1.8 2.6 4.7 1.8 4.0

modern shells compared to prehistoric samples. Therefore, the higher variation in SST for the 11 shells collected from the archaeological assemblage are likely, to some extent, to be the result of greater variation in nearshore SST. Tropical environments present relatively constant SST with minor oscillations where shellfish thriving in warm waters such as A. striata are present year-round. However, given the minimal changes in SST throughout the year and the current resolution of estimating SST from oxygen isotope values discerning beyond average SST reconstruction such as identifying season of capture may be difficult. In contrast, in regions where seasonal fluctuations in SST dramatically change, mollusks that favor certain environmental conditions may be present or absent at certain times of the year or substantially slow down or stop growth. Two exceptions, which may also explain higher or lower SST variability over extended periods of time in longlived shellfish species, are La Niña and El Niño Southern Oscillation events (e.g., Bruno et al., 2001). However, short-lived species such as A. striata may not provide long enough sequences to identify these events. Reported oxygen isotope results of terminal edges for 11 archaeological shell samples suggest these shells were collected in ambient SST between 26 and 29 °C. Several shells contain linear increases or decreases of SST in their incremental growth patterns. Orr-1, for instance (Fig. 6), shows an overall decrease in SST between 30.6 and 27.5 °C and temperature ranges of shell Orr-6 also shows a continual decrease from 28.5 °C to 26.6 °C. Future studies can increase the resolution of sampling to gain a better understanding of SST fluctuations of A. striata, which may permit a more precise season of capture identification. The waters surrounding the Palauan archipelago contain a diverse array of marine ecological zones that provided a rich foraging environment for prehistoric settlers. Based on archaeological evidence, it is clear that shellfish was an essential component in the diets of early Palauans (Clark, 2005; Giovas et al., 2015; Ono and Clark, 2012). There are hundreds of edible shellfish species in the region and many have the potential to serve as proxies for ancient nearshore environmental reconstructions. At Chelechol ra Orrak, for instance, almost 100 shellfish

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Fig. 6. δ18O and estimated SST including linear regression lines for A. striata shells recovered from Chelechol ra Orrak.

taxa have been identified in limited excavation, including more than 16,000 A. striata shells. Sclerochronology is continuing to provide an array of analytical methods of various mollusks for researchers to reconstruct environments associated with archaeological contexts. To ensure that proper conversion equations are implemented and that a mollusk species can accurately record their ambient SST, preliminary assessments of each species' biomineralogical composition and their local environments should be conducted on a case-by-case basis using modern analogs to increase the likelihood of accurately estimating paleo SST in archaeological contexts.

Acknowledgments We thank Ilya Bindeman and Jim Palandri at the Stable Oxygen Isotope Laboratory at the University of Oregon for their help in processing the samples and Lev Zakharov at the XRD Laboratory at the University of Oregon for help in running and identifying the biomineralogical composition of our shells. Staff from the Palau Bureau of Arts and Culture has provided much needed logistical and field assistance during past projects at Chelechol ra Orrak. Thanks also go to the anonymous reviewers who gave useful comments and editorial suggestions that helped to improve various aspects of this manuscript.

Appendix A. Atactodea striata shell oxygen stable isotope values in SMOW and VPDB (s.d., b.05), distance from the terminal edge (DTE), and shell length

Shell-ID #

δ18O (SMOW)

δ18O (VPDB)

Mod-1A Mod-1B Mod-1C Mod-2A Mod-2B Mod-2C Mod-3A Mod-3B Mod-3C Mod-4A Mod-4B Mod-4C Mod-5A Mod-5B Mod-5C Mod-6A Mod-6B Mod-6C Mod-6D Mod-6E

28.9 29.6 29.2 28.6 29.6 29.1 29.5 29.4 29.4 29.4 29.1 29.2 28.6 29.2 29.3 29.5 29.1 29.2 29.4 28.8

−1.9 −1.2 −1.6 −2.2 −1.2 −1.7 −1.3 −1.4 −1.5 −1.4 −1.7 −1.6 −2.2 −1.6 −1.5 −1.3 −1.7 −1.6 −1.4 −2.0

DTE (mm) 0 2 4 0 2 4 0 2 4 0 2 4 0 2 4 0 2 4 6 8

Shell length (mm) 16.7

16.5

14.7

17.1

17.4

16.6

N.P. Jew, S.F. Fitzpatrick / Journal of Archaeological Science: Reports 4 (2015) 477–486 Appendix A (continued) (continued) 18

485

Appendix A (continued) (continued) 18

Shell-ID #

δ O (SMOW)

δ O (VPDB)

DTE (mm)

Mod-6F Mod-6G Mod-6H Mod-6I Mod-6J Mod-7A Mod-7B Mod-7C Mod-7D Mod-7E Mod-7F Mod-7G Mod-7H Mod-7I Mod-7J Mod-8A Mod-8B Mod-8C Mod-9A Mod-9B Mod-9C Mod-9D Mod-9E Mod-9F Mod-9G Mod-9H Mod-9I Mod-9J Mod-10A Mod-10B Mod-10C Orr-1A Orr-1B Orr -1C Orr-1D Orr-1E Orr-2A Orr-2B Orr-2C Orr-2D Orr-2E Orr-3A Orr-3B Orr-3C Orr-3D Orr-3E Orr-4A Orr-4B Orr-4C Orr-4D Orr-4E Orr-5A Orr-5B Orr-5C Orr-5D Orr-5E Orr-6A Orr-6B Orr-6C Orr-6D Orr-6E Orr-7A Orr-7B Orr-7C Orr-7D Orr-7E Orr-8A Orr-8B Orr-8C Orr-8D Orr-8E Orr-9A Orr-9B Orr-9C Orr-9D

28.7 28.6 28.6 28.9 29.1 29.5 29.1 29.1 29.1 28.7 28.1 28.9 29.2 29.1 28.9 29.2 29.5 29.2 29.3 29.4 29.5 29.2 28.9 29.0 28.9 28.7 28.8 28.7 29.2 29.2 29.6 29.5 29.7 29.1 29.1 29.0 29.4 29.3 29.7 29.9 29.7 29.4 29.6 29.3 30.0 29.4 29.9 29.5 29.7 29.8 29.9 29.6 30.1 29.5 30.1 29.9 29.9 29.6 29.7 29.5 29.4 29.3 29.1 29.2 28.7 28.7 29.3 29.5 29.6 29.4 28.7 30.2 29.6 29.4 29.3

−2.1 −2.2 −2.2 −1.9 −1.7 −1.3 −1.8 −1.7 −1.7 −2.1 −2.7 −1.9 −1.6 −1.8 −1.9 −1.6 −1.3 −1.6 −1.5 −1.4 −1.3 −1.6 −1.9 −1.8 −1.9 −2.1 −2.0 −2.0 −1.6 −1.6 −1.2 −1.3 −1.2 −1.7 −1.7 −1.8 −1.4 −1.5 −1.1 −1.0 −1.1 −1.4 −1.3 −1.5 −0.8 −1.5 −0.9 −1.3 −1.1 −1.1 −0.9 −1.3 −0.7 −1.4 −0.8 −0.9 −1.0 −1.2 −1.1 −1.3 −1.4 −1.5 −1.7 −1.6 −2.1 −2.1 −1.5 −1.3 −1.2 −1.4 −2.1 −0.7 −1.2 −1.4 −1.5

10 12 14 16 18 0 2 4 6 8 10 12 14 16 18 0 2 4 0 2 4 6 8 10 12 14 16 18 0 2 4 0 2 4 6 8 0 2 4 6 8 0 2 4 6 8 0 2 4 6 8 0 2 4 6 8 0 2 4 6 8 0 2 4 6 8 0 2 4 6 8 0 2 4 6

Shell length (mm)

17.8

18.9

Shell-ID #

δ18O (SMOW)

δ18O (VPDB)

DTE (mm)

Orr-9E Orr-10A Orr-10B Orr-10C Orr-10D Orr-10E Orr-11A Orr-11B Orr-11C Orr-11D Orr-11E Orr-11F Orr-11G Orr-11H Orr-11I Orr-11J Orr-11K

28.9 29.3 29.0 29.4 29.6 28.9 29.3 30.2 29.6 30.1 29.8 29.1 29.1 29.0 28.9 29.4 29.0

−1.9 −1.5 −1.8 −1.4 −1.2 −1.9 −1.5 −0.6 −1.2 −0.8 −1.0 −1.7 −1.7 −1.8 −1.9 −1.4 −1.8

8 0 2 4 6 8 0 2 4 6 8 10 12 14 16 18 20

Shell length (mm) 19.2

20.8

16.2

References

16.4

20.4

20.6

19.4

17.1

19.1

21.6

17.4

19.4

18.9

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