δ18O analysis of Donax denticulatus: Evaluating a proxy for sea surface temperature and nearshore paleoenvironmental reconstructions for the northern Caribbean

Journal of Archaeological Science: Reports 8 (2016) 216–223

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δ18O analysis of Donax denticulatus: Evaluating a proxy for sea surface temperature and nearshore paleoenvironmental reconstructions for the northern Caribbean Nicholas P. Jew a,⁎, Scott M. Fitzpatrick a, Kelsey J. Sullivan b a b

Department of Anthropology, 1218 Condon Hall, University of Oregon, Eugene, OR 97403-1218, United States Department of Anthropology, 555 E. Pine Knoll Drive, Northern Arizona University, Flagstaff, AZ 86011-5200, United States

a r t i c l e

i n f o

Article history: Received 21 March 2016 Received in revised form 3 June 2016 Accepted 9 June 2016

Keywords: Stable oxygen isotopes Nearshore sea-surface temperature reconstruction Nevis Lesser Antilles

a b s t r a c t To identify oxygen isotope (δ18O) values and associated sea-surface temperature (SST) in the northern Caribbean, 82 δ18O values from 3 modern and 11 prehistoric Donax denticulatus shells were sampled. First, we conducted X-ray diffraction (XRD) on several modern shells to confirm aragonite composition. After identifying the biomineralogical composition of the shell, we applied several δ18O-to-SST conversion equations and selected the most appropriate formula based on pairing modern isotopic data with SST during the time of collection. We then converted prehistoric isotopic values to SST estimates to reconstruct paleo-SST for nearshore waters off the island of Nevis in the northern Lesser Antilles. Our results indicate that prehistoric isotopic signatures are slightly more depleted than modern signatures, with estimated SST similar to modern conditions for the region. Our study demonstrates the potential for Donax denticulatus to serve as a reliable proxy for recording ambient SST and reconstructing local paleoecology of nearshore environments, important for examining a host of issues related to prehistoric settlement and island adaptations. © 2016 Elsevier Ltd. All rights reserved.

1. Introduction Stable oxygen isotope (δ18O) analysis is a well-established method used as a proxy to estimate sea-surface temperature (SST), seasonality of shellfish harvest, and reconstruct the nearshore ecology of a given area (Ambrose et al., 2016; Andrus, 2011, 2012; Bailey et al., 1983, 2008; Burchell et al., 2013; Culleton et al., 2009; Eerkens et al., 2010; Epstein et al., 1951, 1953; Jew et al., 2013, 2014; Killingley, 1981; Shackleton, 1973; Thompson and Andrus, 2013). When mollusks grow, they precipitate calcium carbonate (CaCO3) in discrete growth bands within their shells that provide a proxy record of ambient SST (Epstein et al., 1951, 1953; Ford et al., 2010; Killingley and Berger, 1979; Wanamaker et al., 2007, 2008). Sampling CaCO3 (calcite or aragonite) layers along the shell's growth axis provides a sequential record of SST changes throughout a mollusk's lifespan, with the terminal edge recording SST at the time of death or collection (Killingley, 1981). Identifying changes in enriched or depleted δ18O values can be used to estimate SST for inter/subtidal zones and identify an approximate season of harvest and average SST (Andrus, 2012; Culleton et al., 2009; Shackleton, 1969, 1973), important factors for examining long-term adaptations to island marine environments. ⁎ Corresponding author. E-mail address: [email protected] (N.P. Jew).

http://dx.doi.org/10.1016/j.jasrep.2016.06.018 2352-409X/© 2016 Elsevier Ltd. All rights reserved.

There have been several studies evaluating isotopic and skeletal geochemistry of Donax as proxies for reliably recording ambient SSTs, including Donax variabilis (Jones et al., 2005), Donax gouldii (Hatch and Schellenberg, 2010), Donax deltoides (Godfrey, 1988), Donax serra Röding (Jerardino et al., 2014), and others. While some were shown to be problematic, most results suggest Donax spp. are reliable indicators of their ambient SST throughout their lifespan. Along with hundreds of shellfish species, Donax denticulatus was an important subsistence resource for prehistoric occupants on the island of Nevis in the northern Lesser Antilles; however, virtually nothing is known related to the environmental conditions associated with the site. Moreover, there is paucity of nearshore paleoecological records for much of the Caribbean, leaving critical gaps in the environmental contexts for Pre-Columbian archaeological sites. D. denticulatus is abundant in archaeological sites found on Nevis and provides an excellent opportunity to evaluate this species ability to serve as a proxy to reconstruct local nearshore ecologies. In this report, we investigated the efficacy of using D. denticulatus to serve as a proxy for reconstructing paleo SST in the Caribbean. First, modern shells were collected from Grand Anse Bay in Grenada (southern Lesser Antilles) where δ18O analysis and modern reported SST provided analogs for pairing oxygen isotope values and SST equations. We then identified the biomineralogical composition of D. denticulatus shells using X-ray diffraction (XRD) on CaCO3 powder samples. This in

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turn, confirmed the shell's composition (calcite or aragonite) to determine which isotope-to-paleo SST conversion equation was appropriate for pairing δ18O values to associated SST for modern samples. We then applied the selected formula to δ18O samples from archaeological D. denticulatus specimens. Two shells were radiocarbon (14C) dated to ~1050 cal BP from a dense midden assemblage recovered at the Late Ceramic Age site of Coconut Walk on Nevis (Kaye et al., 2011). Here, we present our results for modern and prehistoric δ18O samples, including estimated SSTs to evaluate the species' ability to serve as proxy for paleoenvironmental reconstruction. 2. Background Nevis is located along the northeastern fringe of the Lesser Antilles in the Eastern Caribbean (Fig. 1). Along with St. Kitts, the island is the smaller (93 km2) of the two that make up the St. Kitts and Nevis Federation. The site of Coconut Walk is a Pre-Columbian site situated along the east coast of the island that was first recorded by Samuel Wilson (2006) during his surveys in the 1980s. The site was initially excavated by the British television show Time Team in the late 1990s, and research continued in 2010 by one of the authors (SMF) and colleagues (Kaye et al., 2011). Results demonstrated that the site was occupied over at least a 700 year period between ca. 760 and 1440 CE, a period that encompasses much of the Late Ceramic Age (ca. 500/600–1400 CE), with residential areas surrounded by dense midden deposits extending to around 40 cm in depth. During excavation of a 5 × 5 m trench (2273), which was subdivided into 25 1 × 1 m units to enhance spatial and vertical control, a rich artifactual and faunal assemblage was recovered that

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included pottery, dozens of other artifacts made from shell, stone, and bone, a human burial, vertebrate remains (primarily finfish, but also terrestrial animals), and more than 63,000 mollusk individuals, including at least 78 individual taxa (Poteate and Fitzpatrick, 2013). Shellfish were dominated by a variety of gastropods (n = 66) followed by bivalves (n = 11). D. denticulatus comprised 98% of the bivalve species present and have a reported minimum number of individuals (MNI) of 2367 from excavated units in Trench 2273 from the Coconut Walk site (see Poteate and Fitzpatrick, 2013). Two D. denticulatus shells sampled for δ18O analysis were submitted to DirectAMS for accelerator mass spectrometry (AMS) radiocarbon 14C dating and calibrated using the OxCal 4.2 Marine13 curve (see Bronk Ramsey, 2014). D. denticulatus shell (Nev-11) was 14C dated to 1541 ± 33 RCYBP (D-AMS 007668), or between ~990 and 1190 cal BP (2σ). A second shell (Nev-7) was 14C dated to 1464 ± 24 RCYBP (DAMS 007667) or between ~930 and 1080 cal BP. These results are slightly earlier than those previously reported, ranging between 1350 ± 40 and 570 ± 30 RCYBP, but still fit within the expected date range for a Late Ceramic Age occupation (see Giovas et al., 2013). 2.1. Donax denticulatus growth and ecology Donax denticulatus (Linnaeus 1758), also referred to as the common Caribbean Donax clam and locally as “chip chips”, belong to the Donacidae family and are short-lived bivalves found from the Caribbean to Northern Brazil. D. denticulatus live in wash zones, areas containing high wave energy sandy beaches where they burrow up to 200 mm (20 cm) in depth and are found in sands submerged up to 1 m in tidal

Fig. 1. Map of the Caribbean Sea showing the location of Nevis in the Northern Lesser Antilles.

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Fig. 2. Donax denticulatus shell (22 mm in length) inset shows incremental growth bands, radial grooves, and black microscopic pinpoints.

waters (Marcano et al., 2003; Wade, 1967). D. denticulatus commonly migrate along beaches synchronously with high/low tides, expending energy to maintain zonation in the tidal waters. They are highly resilient to physical stressors, including strong wave action, sand abrasion, or substrate variation (Mori, 1938, 1950; Loesch, 1957; Sastre, 1984). These clams can be found as either isolates or in large congregations up to 20,000 m2 (Coe, 1953, 1955; Sastre, 1984). Reproduction rates of the clam vary by geographic region. Sastre (1984), for instance, recorded year-round reproduction in wash zones around Puerto Rico and off the shores of Margarita Island north of Venezuela. In contrast, Marcano et al. (2003) noted reproduction most frequently between October and February. Despite the geographic distribution and reproductive variability of this species, D. denticulatus and other Donax spp. have been recovered from other Caribbean archaeological assemblages and contributed to prehistoric human subsistence activities (e.g., see Giovas et al., 2013; Keegan et al., 2003; Poteate and Fitzpatrick, 2013). Several factors influence D. denticulatus growth rates, such as surface exposure, energy expenditure from burrowing, reproduction, and changes in salinity and SST (Sastre, 1984). D. denticulatus live between one and two years and grow up to 30 mm in length, an estimated 1.0 to 2.5 mm per month for juveniles with growth rates decreasing during adulthood as part of senescence (see Marcano et al., 2003; Sastre, 1984). D. denticulatus shells are wedge-shaped and exterior layers contain radial grooves of microscopic punctae (Fig. 2). As with other mollusk species, D. denticulatus growth bands provide a sequential record of ambient SST through the absorption of enriched or depleted oxygen isotopes. 3. Materials and methods We analyzed 82 isotopic signatures from 14 shells, including three modern and 11 archaeological specimens that ranged in size from 18 to 25 mm in length. For isotopic and geochemical analysis, each shell was soaked and gently brushed using deionized water and inspected under a low powered microscope for an intact terminal edge. The exterior of each shell was etched 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. Samples were removed using a 0.05 mm carbide drill bit from the surface of each shell at

~0.5 mm deep using a Sherline 5410 Micromill while maintaining a low rpm to avoid heating the calcite (see Robbins and Rick, 2007:29). To avoid cross contamination, drill bits were sonicated in distilled water and allowed to dry between drilling sessions, shells were cleaned with compressed air, and drill bits were baked overnight at 400 °C to burn off any trace CaCO3 from previous sessions. Three modern D. denticulatus shells were sampled in 1 mm intervals starting from the terminal edge, including five powdered samples, each from two shells totaling 8 mm of growth per shell (inclusive of the 1 mm thickness of the drill bit). One modern shell 25 mm in length was extensively profiled for a total of ~18 mm of incremental growth. We selected 11 whole prehistoric D. denticulatus shells from the Coconut Walk site, trench 2273, square 7/planum 2 between 20 and 30 cmbs (see Giovas et al., 2013; Poteate and Fitzpatrick, 2013). For each prehistoric shell, samples were taken in 2 mm intervals (including drill bit thickness) starting with the terminal edge sampling along the lateral margin toward the hinge. Five samples were removed from each shell totaling ~ 8 mm of incremental growth. Twelve powdered samples were removed from one prehistoric shell (Nev-11) 25 mm in length, providing a 22 mm growth sequence that was used to reconstruct prehistoric SST for at least one full annual cycle. All powder samples were analyzed at the University of Oregon Geological Sciences Stable Isotope Laboratory where they 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 values were measured using a Finnigan MAT253 isotope ratio mass spectrometer with continuous helium flow. All isotopic values are reported in δnotation in per mil (‰) units relative to the Vienna PeeDee Belemnite (VPDB) standard using the formula: δ18 O ¼



Rsample −Rstandard =Rstandard  1000

where R represents the heavy/light ratio for the abundance of two isotopes. A positive δ value represents a more enriched heavy isotope compared to the standard where negative δ values are associated with the depletion of heavy isotopes (Wefer and Berger, 1991). Precision of oxygen and carbon isotopic ratios was ±0.1‰ (1σ), calibrated repeatedly against international standard NBS-19. For all isotopic and estimated SST measurements, we provide descriptive statistics, including mean, minimum, and maximum values.

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Fig. 3. X-ray diffraction spectra of three modern Donax denticulatus shells, illustrating matching peaks with aragonite (aragonite, CaCO3, 00-005-0453).

To identify the most appropriate δ18O to SST calculation equation, three modern D. denticulatus were collected from the intertidal zone on the island of Grenada in May 2014 where SST was between 27 and 28 °C. Three modern Donax 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 the 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 powders sampled from each shell, which is essential for selecting either the aragonite or calcite conversion equation.

4. Results Phase analysis illustrated that the X-ray powder diffraction for all three modern shell samples provide a good match for orthorhombic phase of CaCO3, aragonite (DB card number 00-005-0453, ICDD [PDF2009]). It is important to recognize, however, that the compositions of some shellfish are a mixture between aragonite and calcite, which can make selecting a δ18O to SST conversion equation difficult. However, D. denticulatus wavelength peaks match those of pure aragonite and removes several equations based on calcite, allowing for a more precise matching between fractionation equations based on known biomineralogical compositions (Fig. 3). For modern δ18O samples from D. denticulatus shells (Tables 1 and 2), our 20 isotopic values range between − 2.5‰(VPDB) and −0.9‰(VPDB), with a mean of −1.7‰(VPDB). Modern intrashell variation range between as little as 0.6‰(VPDB) and a maximum of 1.6‰(VPDB) although we realize that more samples may increase the variation of our

Table 1 Reported δ18O means (x̅), minimum (min), maximum (max), standard deviation (s), and variance (σ2) for D. denticulatus shells sampled modern intertidal areas and from the archaeological assemblage at Coconut Walk. δ18O(VPDB) ID

n

Min

Max

x̅

s

σ2

Mod-don1 Mod-don2 Mod-don3 Nev-1 Nev-2 Nev-3 Nev-4 Nev-5 Nev-6 Nev-7 Nev-8 Nev-9 Nev-10 Nev-11

5 5 10 5 5 5 5 5 5 5 5 5 5 12

−2.5 −1.9 −1.9 −1.5 −1.8 −2.3 −3.2 −2.1 −2.2 −1.9 −1.9 −1.9 −1.8 −1.7

−0.9 −1.7 −1.3 −1.3 −1.0 −1.8 −2.0 −1.0 −1.4 −1.5 −1.2 −1.2 −1.3 −1.0

−2.1 −1.6 −1.6 −1.3 −1.4 −2.1 −2.4 −1.4 −1.9 −1.7 −1.5 −1.6 −1.5 −1.4

0.3 0.3 0.2 0.3 0.3 0.2 0.5 0.5 0.3 0.2 0.3 0.3 0.2 0.2

0.10 0.08 0.05 0.07 0.10 0.04 0.23 0.23 0.10 0.03 0.09 0.08 0.04 0.04

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Table 2 Stable oxygen isotope for all modern (Grenada) and prehistoric (Nevis) D. denticulatus shells from the Coconut Walk site. Results are ordered starting with the terminal edge (0 mm) toward the hinge in 2 mm growth increments. δ18O(VPDB) Lab ID

0

2

4

6

8

10

12

14

16

18

20

22

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

−1.7 −1.7 −1.7 −0.9 −1.0 −1.8 −3.2 −2.1 −1.4 −1.9 −1.9 −1.2 −1.8 −1.6

−1.9 −1.8 −1.9 −1.5 −1.8 −2.1 −2.3 −1.6 −2.0 −1.5 −1.7 −1.5 −1.4 −1.3

−2.5 −1.9 −1.6 −1.1 −1.2 −2.3 −2.0 −1.0 −2.1 −1.8 −1.4 −1.6 −1.7 −1.7

−2.2 −1.3 −1.9 −1.5 −1.5 −2.0 −2.1 −1.1 −2.0 −1.6 −1.3 −1.9 −1.5 −1.3

−2.3 −1.3 −1.9 −1.3 −1.5 −2.2 −2.3 −1.0 −2.2 −1.5 −1.2 −1.8 −1.3 −1.0

– – −1.6 – – – – – – – – – – −1.5

– – −1.5 – – – – – – – – – – −1.5

– – −1.4 – – – – – – – – – – −1.4

– – −1.5 – – – – – – – – – – −1.5

– – −1.3 – – – – – – – – – – −1.5

– – – – – – – – – – – – – −1.1

– – – – – – – – – – – – – −1.5

samples. For individual shells, sequential variation between adjacent growth intervals varied from 0.1‰(VPDB) to up to 0.4‰(VPDB). Interestingly, our terminal edge values, representing the most recent deposition of calcium carbonate and point of death for all three modern shells, were − 1.7‰(VPDB). This information, combined with the biomineralogical composition, provided the necessary information for

pairing isotopic and SST values for the region, which provided an analog for estimating paleo SST for the island of Nevis. The 62 prehistoric D. denticulatus shell δ18O values range between −3.2‰(VPDB) and −0.9‰(VPDB) with a mean (x̅) value of −1.6‰(VPDB) and standard deviation (s) of ± 0.44‰(VPDB) (Table 1). For individual shells, Nev-4 contained the largest variation of 1.2‰(VPDB) and Nev-1

Table 3 Calcium carbonate (CaCO3) equations for aragonite illustrating the differences in SST conversions for D. denticulatus. We applied SMOW correction of 0.9‰ for modern samples (see Schmidt et al., 1999) and 0.05‰ (see Fairbanks, 1989) noted in the equations represented as either δw or δ18Ow. Reference (aragonite)

δ18O to sea-surface temperature conversion equation

Grossman and Ku (1986) mollusk c (E1) Grossman and Ku (1986) equation 1 (E2) Hudson and Anderson (1989) (E3) Kim et al. (2007) (E4)

T (°C) = 21.8–4.69(δ18Oaragonite − δw − 0.2) T (°C) = 20.6–4.34(δ18Oaragonite − δw − 0.2) T (°C) = 19.7–4.69(δ18Oaragonite − δw) T (°C) = 17.88 (±0.13)(103 T(K)−1) − 32.42 (±0.46)

Sample ID

δ18O(VPDB)

Estimated sea-surface temperature (°C) E1

E2

E3

E4

Nev-11a Nev-11b Nev-11c Nev-11d Nev-11e Nev-11f Nev-11g Nev-11h Nev-11i Nev-11j Nev-11k Nev-11l Mod-3a Mod-3b Mod-3c Mod-3d Mod-3e Mod-3f Mod-3g Mod-3h Mod-3i Mod-3j Mod-1a Mod-1b Mod-1c Mod-1d Mod-1e Mod-2a Mod-2b Mod-2c Mod-2d Mod-2e

−1.6 −1.3 −1.7 −1.3 −1.0 −1.5 −1.5 −1.4 −1.5 −1.5 −1.1 −1.5 −1.7 −1.9 −1.6 −1.9 −1.9 −1.6 −1.5 −1.4 −1.5 −1.3 −1.7 −1.9 −2.5 −2.2 −2.3 −1.7 −1.8 −1.9 −1.3 −1.3

28.4 27.1 29.1 27.3 25.7 28.1 27.9 27.4 28.1 28.0 26.5 28.0 33.0 34.0 32.8 34.0 33.8 32.4 32.1 31.9 32.2 31.4 33.2 34.2 36.7 35.4 35.9 33.1 33.3 34.0 31.0 31.1

26.7 25.5 27.3 25.7 24.2 26.4 26.3 25.8 26.4 26.3 24.9 26.3 27.2 28.2 27.1 28.2 28.0 26.8 26.4 26.2 26.5 25.8 27.5 28.3 30.7 29.5 29.9 27.3 27.6 28.2 25.5 25.5

27.3 25.9 27.9 26.2 24.6 26.9 26.8 26.3 26.9 26.8 25.3 26.8 31.8 32.9 31.6 32.9 32.6 31.3 30.9 30.7 31.0 30.2 32.0 33.0 35.6 34.3 34.7 31.9 32.2 32.8 29.9 29.9

26.6 25.1 27.1 25.1 23.7 26.1 26.1 25.6 26.1 26.1 24.1 26.1 31.5 32.5 31.0 32.5 32.5 31.0 30.5 29.9 30.5 29.4 31.5 32.5 35.7 34.1 34.7 31.5 32.0 32.5 29.4 29.4

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had only 0.2‰(VPDB). The differences between sequential growth increments for each shell varied (Table 2), where several adjacent measurements varied by only 0.1‰(VPDB) (e.g., Nev-5, 6, 7, 8, 9 and 11). In contrast, one adjacent measurement between varied over 1.0‰(VPDB) (Nev-4). 5. Discussions and conclusions Since the 1950s, several fractionation/conversion equations have been developed to estimate SST from δ18O values sampled from various mollusks (see, Grossman, 2012; 183–184). Modern analogs and identification of the biomineralogical composition of shells are crucial for pairing the isotopic and SST values. To illustrate the variation in formulas, we applied several aragonite equations to our modern and archaeological shell profiles (Table 3). The selected δ18O value for standard modern ocean water (SMOW) (δw) 0.9‰, which was estimated after NASA δ18O values for the region (see Schmidt et al., 1999). The modern value for this general region ranged higher due to evaporation. For Grossman and Ku's (1986) equations, we modified the equations δw value by further subtracting an additional 0.2‰. The differences between the estimated SST using various equations can vary up to 5 °C, thus illustrating the importance of identifying the biomineralogical composition and modern ambient SST. Comparing the terminal edge values for each aragonite-to-SST equation with the recorded ambient water temperatures for the three modern samples (terminal edge deposition at −1.7‰(VPDB)) and recorded ambient water temperature between 27 and 28 °C at the time of collection, the closest conversion between values were from Grossman and Ku's (1986:66, equation 1, E2 in Table 3) formula for converting δ18O values to sea-surface temperature where:   T ð CÞ ¼ 20:6−4:34 δ18 Oaragonite −δw It is important to note, however, that extensive monitoring of modern shellfish species in the local area near the site for an extended period of time will increase the resolution of pairing δ18O and SST values, which may result in the selection of a different temperature conversion equation. This Grossman and Ku (1986) equation was further adjusted by subtracting 0.2‰ for equation 1. We ran an analysis of variance between modern reported SST for Nevis against the estimated SST for all modern samples using Grossman and Ku's (1986) equation 1 (E2) and found no statistically significant differences (df = 19, F = 0.313, p = 0.919, α.05). For the prehistoric shells and associated δ18O values from the 1050 cal BP component, we adjusted for the change in SMOW by

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0.05‰ after Fairbanks' (1989:639) Barbados sea level curve, where SMOW during this time was less enriched based on curve estimates. Grossman and Ku's, 1986 mollusk equation c provided the most accurate pairing between modern isotopic values and recorded SST for the waters where they were collected. All terminal edge estimated SST values of the three modern shells were 27.0 ± 0.5 °C. These values are consistent with reported modern average SST ranges near southwest Grenada (where modern samples were collected and reported) during the month of collection in May between 27.5 °C (see Fig. 4, NOAA, 2016). Ten year modern SST averages between 1985 and 1995 reported by the National Oceanic and Atmospheric Administration (NOAA) for the Caribbean Sea near the northern lesser Antilles range from ~25 to 29 °C (Fig. 4), with increasing SSTs (27–29 °C) between late June to early September and decreasing (29–27 °C) from late September to early December, with further declines from late December to early March and an increase from late March to early June. Isotopic estimates by Sepulcre et al. (2009, 2011) for the northern Caribbean Sea using plankton foraminifera Globigerinoides ruber show modern δ18O values between approximately −2.1‰ to −0.5‰ and mean (x̅) modern SST estimates between 27 and 28 °C, closely aligning with NOAA SST averages of 27.4 °C. Additionally, annual recorded salinity for the northern Caribbean is relatively consistent between 34 and 36 psu (NOAA, 2016), with larger fluctuation to the south where higher freshwater input from the Amazon and Orinoco Rivers and various tributaries, can significantly increase or decrease salinity throughout the year. The majority of estimated SST from the prehistoric D. denticulatus shells are similar to the range of reported modern SST for the region (Fig. 5). Only one of our prehistoric samples, Nev-4, presented high variation, where isotopic signatures and estimated SST for 8 mm of growth varied by 6 °C, which may indicate storms, increased surface exposure for a short period, or dramatic short-term changes in SST. The inhabitants of the Coconut Walk site most likely used D. denticulatus as a supplement to other major protein-rich food resources that included various gastropods, finfish, seabirds, crustaceans, and some terrestrial vertebrates. Recent research has demonstrated that shellfish were a major subsistence resource at Coconut walk, illustrated by the relatively high frequency of mollusks recovered from the site (Giovas et al., 2013; Poteate and Fitzpatrick, 2013). Nerita tessellata, the most abundant gastropod recovered from the site (MNI = 37,591), for instance, appear to increase in size through time despite heavy predation, and they remained a major source of subsistence throughout the site's seven centuries of occupation (Giovas et al., 2013). The MNI for D. denticulatus at the Coconut Walk site (n = 2367) is the highest for any reported bivalve recovered, followed by

Fig. 4. National Oceanic and Atmospheric Administration (NOAA) reported annual sea surface temperature (solid line) and salinity (dashed line) for the northern Caribbean near the islands of Nevis and St. Kitts. Adapted after NOAA (2016) and WWF (2014).

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Fig. 5. Estimated SST (after Grossman and Ku, 1986 equation 1: T (°C) = 20.6–4.34(δ18Oaragonite − δw) and means including isotopic values for sampled shells from the Coconut Walk site and Moderns SST ranges for Nevis.

Codakia orbicularis (n = 193, see Poteate and Fitzpatrick, 2013:3697), illustrating their relative abundance in nearshore waters around Nevis and/or food preference by local inhabitants. Stable oxygen isotopes of mollusk species are greatly contributing to our understanding of nearshore environments and SST reconstructions around the world. This study is part of a larger initiative to expand isotopic studies to mollusks commonly found in archaeological assemblages in island environments (see Jew and Fitzpatrick, 2015). As more researchers incorporate shellfish growth and ecology into their sampling methods, we are beginning to understand the isotopic and seasonal variation that exists during a mollusk's lifespan. Our results suggest that δ18O values from shells recovered from a ~1050 cal BP component from the Late Ceramic Age Pre-Columbian site of Coconut Walk are relatively depleted; however, paleoreconstructions suggest that nearshore waters off the coast of southeast Nevis were comparable in SST range to today. The overall range of SST for sampled D. denticulatus at the Coconut Walk site fall within a range between 24 and 30 °C, with the highest SST range (upwards of 34 °C) of water temperature skewed by Nev-4, which is most likely an outlier. Future investigations, including increased resolution of isotopic sampling of modern and prehistoric D. denticulatus and annual monitoring of the nearshore ecology adjacent to archaeological sites such as Coconut walk, can provide baseline information between the species and local ecology to further investigate whether seasonality of harvest can be identified for species inhabiting regions containing narrow seasonal SST parameters. Ongoing analysis of vertebrate remains from the site, once coupled with the mollusk assemblage, should also allow us to examine whether there were any distinct shifts in marine resource harvesting around 1000 BP that might be explained by local climatic changes. Overall, our study adds to the growing literature of potential candidates to serve as proxies for SST reconstruction in paleo nearshore environments where current research suggest that D. denticulatus can reliably record ambient SSTs throughout their lifespan in nearshore environments. Acknowledgements We thank the Nevis Historical and Conservation Society, particularly Evelyn Henville and Paul Diamond, who helped facilitate the 2010 archaeological research project. Thanks to Michiel Kappers and Quetta Kaye who helped to co-direct the project, Aaron Poteate for his assistance in preparing samples for isotopic analyses, Angus Martin (Grenada National Museum) for providing specimens for modern analysis, Jim Palandri at the UO Stable Isotope Laboratory for processing our samples, and Lev Zakharov at the XRD Laboratory at the University of Oregon for running and identifying the biomineralogical composition of our shells. We thank reviewers for their comments, which helped to improve various parts of the manuscript.

References Ambrose Jr., W.G., Locke V, W.L., Fisher, J.L., Hamilton, N.D., Levitt, J., 2016. Harvest of the soft-shell clam (Mya arenaria) by Malaga Island, Maine, residents from 1865 to 1912 occurred primarily in the Fall and Winter based on incremental growth assessment. Journal of Island and Coastal Archaeology 11 (1), 50–67. http://dx.doi.org/10.1080/ 15564894.2015.1052864. Andrus, C.F.T., 2011. Shell midden sclerochronology. Quat. Sci. Rev. 30 (21), 2892–2905. Andrus, C.F.T., 2012. Mollusks as oxygen-isotope season-of-capture proxies in southeastern United States archaeology. In: Reitz, E., Quitmyer, I., Thomas, D. (Eds.), Seasonality and Human Mobility Along the Georgia Bight. Scientific Publications of the American Museum of Natural History, NY, pp. 123–132. Bailey, G., Deith, M., Shackleton, N., 1983. Oxygen isotope analysis and seasonality determinations: limits and potential of a new technique. Am. Antiq. 48 (2), 390–398. Bailey, G., Barrett, J., Craig, O., Milner, N., 2008. Historical ecology of the North Sea Basin: an archaeological perspective and some problems of methodology. In: Rick, T.C., Erlandson, J.M. (Eds.), Human Impacts on Ancient Marine Ecosystems: A Global Perspective. University of California Press, Berkeley, pp. 215–242. Bronk Ramsey, C., 2014. https://c14.arch.ox.ac.uk/oxcal/OxCal.html. Burchell, M., Hallmann, N., Martindale, A., Cannon, A., Schöne, B.R., 2013. Seasonality and intensity of shellfish harvesting on the north coast of British Columbia. Journal of Island and Coastal Archaeology 8 (2), 152–169. Coe, W.R., 1953. Resurgent populations of littoral marine invertebrates and their dependence on ocean currents and tidal currents. Ecology 34, 225–229. Coe, W.R., 1955. Ecology of the bean clam, Donax gouldi on the coast of Southern California. Ecology 36, 512–515. Culleton, B.J., Kennett, D.J., Ingram, B., Erlandson, J., 2006. Intrashell radiocarbon variability in marine mollusks. Radiocarbon 48 (3), 387–400. Culleton, B.J., Kennett, D.J., Jones, T., 2009. Oxygen isotope seasonality in a temperate estuarine shell midden: a case study from CA-ALA-17 on the San Francisco Bay, California. J. Archaeol. Sci. 36 (7), 1354–1363. Eerkens, J., Rosenthal, J., Stevens, N.E., Cannon, A., Brown, E.L., Spero, H.J., 2010. Stable isotope provenance analysis of Olivella shell beads from the Los Angeles Basin and San Nicolas Island. Journal of Island and Coastal Archaeology 5, 105–119. Epstein, S., Buchsbaun, R., Lowenstam, H., Urey, H., 1951. Carbonate-water isotopic temperature scale. Bull. Geol. Soc. Am. 62, 417–426. Epstein, S., Buchsbaun, R., Lowenstam, H., Urey, H., 1953. Revised carbonate–water isotopic temperature scale. Bull. Geol. Soc. Am. 64, 1315–1326. Fairbanks, R.G., 1989. A 17,000-year glaci-eustatic sea level record: influence of glacialmelting rates on the Younger Dryas event and deep-ocean circulation. Nature 342, 637–642. Ford, H.L., Schellenberg, S.A., Becker, B.J., Deutschman, D.L., Dyck, K.A., Koch, P.L., 2010. Evaluating the skeletal chemistry of Mytillus californianus as a temperature proxy: effects of microenvironment and ontogeny. Paleoceanography 25, PA1203. Giovas, C.M., Clark, M., Fitzpatrick, S.M., Stone, J., 2013. Intensifying collection and size increase of the tessellated nerite snail (Nerita tessellata) at the Coconut Walk site, Nevis, northern Lesser Antilles, AD 890–1440. J. Archaeol. Sci. 40, 4024–4038. Godfrey, M.C.S., 1988. Oxygen isotope analysis: a means for determining the seasonal gathering of the pip (Donax deltoids) by Aborigines in prehistoric Australia. Archaeol. Ocean. 23 (1), 17–21. Grossman, E.L., 2012. Oxygen isotope stratigraphy. In: Gradstien, F.M., Ogg, J.G., Schmitz, M., Ogg, G. (Eds.), The Geologic Timescale. Elsevier, pp. 195–220. Grossman, E., Ku, T., 1986. Oxygen and carbon isotope fractionation in biogenic aragonite: temperature effects. Chem. Geol. 59, 59–74. Hatch, M.B., Schellenberg, S.A., 2010. Donax do and don't tell: the relationship of isotopic and elemental variations to environmental conditions in the shell chemistry of a common intertidal bivalve. American Geophysical Union, Fall Meeting 2010, #PP11A-1407. Hudson, J.D., Anderson, T.F., 1989. Ocean temperatures and isotopic compositions through time. Trans. Roy. Soc. Edinb. Earth Sci. 80, 183–192. http://www.aoml.noaa.gov/phod/cyclone/data/ca.html, Accessed September 2016

N.P. Jew et al. / Journal of Archaeological Science: Reports 8 (2016) 216–223 Jerardino, A., Navarro, R., Galimberti, M., 2014. Changing collecting strategies of the clam Donax serra Röding (Bivalvia: Donacidae) during the Pleistocene at Pinnacle Point, South Africa. J. Hum. Evol. 68, 58–67. Jew, N.P., Fitzpatrick, S.M., 2015. 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, 477–487. Jew, N.P., Erlandson, J., Watts, J., White, F., 2013. Shellfish, seasonality, and stable isotope sampling: δ18O analysis of mussel shells from an 8800 year old shell midden on California's Channel Islands. Journal of Island and Coastal Archaeology 8 (2), 1–26. Jew, N.P., Erlandson, J., Rick, T., Reeder-Myers, L., 2014. Oxygen isotope analysis of California mussel shells: seasonality and human sedentism at an 8,200 year old shell midden on Santa Rosa Island, California. Anthropological and Archaeological Sciences 6 (3), 293–303. Jones, D., Quitmyer, I., Andrus, C.F.T., 2005. Oxygen isotopic evidence for greater seasonality in Holocene shells of Donax variabilis from Florida. Palaeogeogr. Palaeoclimatol. Palaeoecol. 228, 96–108. Kaye, Q., Fitzpatrick, S.M., Kappers, M., Thompson, V., 2011. Archaeological investigations at Coconut Walk, Nevis, West Indies, 1st July – 4th August, 2010: beyond Time Team. Papers from the Institute of Archaeology 20, pp. 137–147. Keegan, W.F., Portell, R.W., Slapcinsky, J., 2003. Changes in invertebrate taxa at two preColumbian sites in southwestern Jamaica, AD 800–1500. J. Archaeol. Sci. 30 (12), 1607–1617. Killingley, J.S., 1981. Seasonality of mollusk collecting determined from O-18 profiles of shells. Am. Antiq. 46 (1), 152–158. Killingley, J.S., Berger, W.H., 1979. Stable isotopes in a mollusk shell: detection of upwelling events. Science 205, 186–188. Kim, S.T., O'Neil, J.R., Hillaire-Marcel, C., Mucci, A., 2007. Oxygen isotope fractionation between synthetic aragonite and water: influence of temperature and Mg2+ concentration. Geochim. Cosmochim. Acta 71, 4704–4715. Loesch, H.C., 1957. Studies on the ecology of two species of Donax on Mustang Islands, Texas. Limnol. Oceanogr. 11, 429–431. Marcano, J.S., Prieto, A., Larez, A., Salazar, H., 2003. Crecimiento de Donax denticulatus (Linne1758) (Bivalvıa: Donacidae) en la ense-nada La Guardia, isla de Margarita, Venezuela. Zootec. Trop. 21, 237–259. McCrea, J.M., 1950. On the isotopic chemistry of carbonates and a paleotemperature scale. J. Chem. Phys. 18, 849–857. Mori, S., 1938. Characteristic tidal rhythmic migration of a mussel, Donax semignosus dunker, and the experimental analysis of its behavior at the flood tide. Zoological Magazine Tokio 50, 1–12. Mori, S., 1950. Characteristic tidal rhythmic migration of a mussel, Donax semigranosis Dkr. and the experimental analysis of its behavior. Dobutsugaku Zasshi 59, 88–89. Ni, M., Ratner, B.D., 2008. Differentiation of Calcium Carbonate Polymorphs by Surface Analysis Techniques-An XPS and TPPF-SIMS Study. National Institute of Health, pp. 1356–1361 40(10).

223

Poteate, A.S., Fitzpatrick, S.M., 2013. Testing the efficacy and reliability of common zooarchaeological sampling strategies: a case study from the Caribbean. J. Archaeol. Sci. 40, 3696–3705. Robbins, J., Rick, T., 2007. The analysis of stable isotopes from California coastal archaeological sites: implications for understanding human cultural developments and environmental change. Proceedings of the Society for California Archaeology 20, 29–33. Sastre, M.P., 1984. Relationships between environmental factors and Donax denticulatus populations in Puerto Rico. Estuar. Coast. Shelf Sci. 19, 217–230. Schmidt, G.A., Bigg, G.R., Rohling, E.J., 1999. Global Seawater Oxygen-18 Database - v1.21. http://data.giss.nasa.gov/o18data/. Sepulcre, S., Tachikawa, K., Vidal, L., Thouveny, N., Bard, E., 2009. Preservation state of metastable magnesium calcite in periplatform sediment from the Caribbean Sea over the last million years. Geoscience 10, Q11013. http://dx.doi.org/10.1029/ 2009GC002779. Sepulcre, S., Vidal, L., Tachikawa, K., Rostek, F., Bard, E., 2011. Sea-surface salinity variations in the northern Carribean Sea across the Mid-Pleistocene transition. Clim. Past 7, 75–90. Shackleton, N.J., 1969. Marine molluska in archaeology. In: Brothwell, D., Higgs, E.S. (Eds.), Science in Archaeology. Thames and Hudson, New York, pp. 407–414. Shackleton, N.J., 1973. Oxygen isotope analysis as a means of determining season of occupation of prehistoric middens. Archaeometry 15 (1), 133–141. Thompson, V.D., Andrus, C.F.T., 2013. Using oxygen isotope sclerochronology to evaluate the role of small islands among the Guale (AD 1325 to 1700) of the Georgia Coast, USA. Journal of Island and Coastal Archaeology 8 (2), 190–209. Wade, B.A., 1967. Studies on the biology of the West Indian beach clam Donax denticulatus (Linne) life history. Bull. Mar. Sci. 17, 149–174. Wanamaker, A.D., Krutz, K.J., Borns, H.W., Introne, D.S., Feindel, S., 2007. Experimental determination of salinity, temperature, growth, and metabolic effects on shell isotope chemistry of Mytilus edulis collected from Maine and Greenland. Paleoceanography 22, PA 2217. Wanamaker Jr., A.D., Dreutz, K.J., Wilson, T., Borns Jr., H.W., Introne, D.S., 2008. Experimentally determined Mg/Ca and Sr/Ca ratios in juvenile bivalve calcite for Mytilus edulis: implications for paleotemperature reconstructions. Geo-Mar. Lett. 28 (5–6), 359–368. Wefer, G., Berger, W.H., 1991. Isotope paleontology: growth and composition of extant calcareous species. Mar. Geol. 100, 200–204. Wilson, S.M., 2006. The Prehistory of Nevis, A Small Island in the Lesser Antilles. Yale University Press, New Haven, CT. World Wildlife Fund, 2014. Hyperlink: http://assets.panda.org/img/original/monthly_ salinity_caribbean.gif Accessed September 2015.