Determination of sea surface temperatures using oxygen isotope ratios from Phorcus lineatus (Da Costa,1778) in northern Spain: Implications for paleoclimate and archaeological studies

Abstract

Changes in oxygen isotope ratios from shell carbonates are mainly dependent on sea surface temperature, which enables the estimation of temperatures during periods of shell growth and helps to determine the season of the year when the mollusk died. The marine topshell Phorcus lineatus (Da Costa, 1778) is commonly found in Holocene archaeological deposits of Atlantic Europe and is one of the most abundant subsistence resources utilized during the Mesolithic in northern Spain. Before applying isotopic techniques to ancient samples, calibration of the past isotopic data and its variability must be performed through the study of modern specimens to test their potential as paleoclimate proxy and their suitability for determining the collection season. Although previous studies performed in the region highlighted the existing relationship between sea surface temperatures and isotopic signatures, no systematic works have been done so far. In this paper, calibration of modern P. lineatus shells from northern Spain was carried out using δ¹⁸O analysis. The results showed (1) the existence of a robust inverse correlation between instrumental temperatures (T_meas) and δ¹⁸O_shell (R² > 0.9), accompanied by the lack of significant dependence from δ¹⁸O_water variations (R² = 0.06); (2) the existence of conditions of (or close to) isotopic equilibrium during the formation of the aragonite in the P. lineatus shells; and (3) that using mean annual δ¹⁸O_water values, past temperatures could be calculated with a maximum uncertainty of ±3°C. Moreover, results suggested that P. lineatus generally grew without substantial slow/cessation throughout the year, reflecting the four annual seasons. Therefore, our study not only confirms the potential of oxygen isotope analysis on P. lineatus for paleoclimate reconstruction and archaeological studies highlighted in previous studies but also shows for the first time that the aragonite of those shells grew under conditions of isotopic equilibrium, opening new avenues for future research.

Keywords

marine environment mollusks oxygen stable isotopes paleoclimate sea surface temperature shell midden

Introduction

As mollusk shells are abundant in many coastal archaeological sites, the analysis of their oxygen isotopic ratios can provide not only paleoenvironmental inferences (Emiliani et al., 1964; Lécuyer et al., 2012; Rye and Sommer, 1980; Schöne et al., 2004; Wang et al., 2012) but also interesting information about human behavior. The European Mesolithic is characterized by the formation of shell middens in the Atlantic and Mediterranean coastal areas (Colonese et al., 2011b; Gutiérrez-Zugasti et al., 2011). This situation is the result of increased exploitation of marine environments in the late Pleistocene and early Holocene (Álvarez-Fernández, 2011; Colonese et al., 2011b; Gutiérrez-Zugasti, 2011; Gutiérrez-Zugasti et al., 2013; Mannino and Thomas, 2001). One of the most exploited species during this period in Atlantic Europe is the topshell Phorcus lineatus (Da Costa, 1778). Therefore, the analyses of oxygen stable isotopes on shells are useful to infer information concerning the behavior of the last hunter-fisher-gatherers who inhabited and exploited coastal areas. These analyses are especially pertinent to investigating topics like settlement patterns, shell midden formation, or determining the seasonality of shell collection (Andrus, 2011; Colonese et al., 2009, 2011a; Culleton et al., 2009; Jones et al., 2008; Kimball et al., 2009; Mannino and Thomas, 2007; Mannino et al., 2003, 2007, 2011).

Despite the enormous potential of stable isotopes in archaeological studies, their use is still very limited in some areas of Atlantic Europe and in many cases hampered by different aspects which are inherent to the method and the studied materials (e.g. intra-specific variability, changes in growth patterns, vital effects, etc.). In particular, the inference of accurate paleoclimate data is not a straightforward task. One of the main challenges is calibrating the past isotopic data and its variability. For this purpose, the study of modern analogues, that is, the study of shells of living mollusks, accompanied by the monitoring of present-day conditions of seawater, is the common approach. Calibration of the past isotopic data through the study of modern shells was performed on the topshell P. lineatus by Deith (1983), Deith and Shackleton (1986), and Mannino et al. (2003). Despite some limitations related with the sampling methods and the study of present-day conditions of the seawater, the results obtained by these pioneering studies were significant and made possible a first approach to the use of this species as an indicator of the season of collection in archaeological studies.

In this paper, we test the ability of P. lineatus shells from northern Spain as a paleoclimate proxy through the study of oxygen isotope ratios on modern samples. This study includes a tighter control of variables than in previous research (Deith, 1983; Deith and Shackleton, 1986; Mannino et al., 2003) by including (1) a larger sample size, (2) higher sampling resolution, and (3) seawater monitoring. We then correlate isotopic results with modern environmental parameters (sea surface temperature, salinity and oxygen isotope composition of water). This is followed by a discussion of the implications of isotopic studies on P. lineatus to paleothermometry and archaeological research. The calibration of this proxy is crucial to investigating paleoenvironmental reconstruction and its implications on archaeological interpretation of hunter-fisher-gatherer societies during the late Pleistocene–Holocene transition in northern Spain and across Atlantic Europe.

The study area: Geographical and environmental conditions

The northern coast of Spain (also known as Cantabrian Coast) is located in the Iberian Peninsula (Figure 1). The climate is oceanic, humid, and temperate, with mild winters and summers. The mean annual ambient temperature is approximately 15–16°C. The coldest month is January with an average temperature of 9–10°C and the warmest month is August with 20–22°C. The mean annual rainfall exceeds 1200 mm and shows a marked seasonality, with the driest period coinciding with summer months (Source: National Meteorology Agency, http://www.aemet.es). The climatic characteristics are defined by geographic elements, since the sea and especially the Gulf Stream cause the temperature to be higher than expected for this latitude (~43°N). The higher rainfall is a result of the Foehn effect because the mountains prevent the clouds from crossing inland to the Meseta in north-central Spain (Rasilla, 1999).

Figure 1.

Location of the study area in northern Spain.

The Cantabrian Sea (southern Bay of Biscay) represents a boundary between subtropical and boreal conditions in the Eastern Atlantic. Sea surface temperatures follow a seasonal warming and cooling pattern, ranging from ~22°C to ~12°C in the central part of the region (data from the Spanish Institute of Oceanography). Hydrographic conditions throughout the year follow a regular pattern characterized by winter mixing and summer stratification. Wind-induced upwelling events, which are characterized by low temperatures, high salinity, and nutrient concentrations, have been observed to occur mainly in summer (Álvarez et al., 2011; Lavín et al., 1998). The water related to these upwelling events in the region is generally Eastern North Atlantic Central Water (ENACW), which is a cold and salty water mass. However, some authors have also detected winter upwelling events associated with the Iberian Poleward Current (Gil et al., 2002) and with shelf bottom seawater (see Álvarez et al., 2011 and references therein).

Material and methods

Ecology and biology of ‘Phorcus lineatus’ (Da Costa, 1778)

The ecology and biology of the topshell P. lineatus have been widely studied (Figure 2a). It inhabits the rocky substrate on the high and medium zone of the intertidal area, and it has been reported to move according to tide and weather changes, avoiding unfavorable conditions of salinity and oxygen concentration such as those present in rock pools (Crothers, 2001). Its geographic distribution ranges from southern Britain to southern Morocco (Donald et al., 2012; Kendall, 1987). The nourishment of this toothed topshell consists of algae detritus (Daguzan, 1991). Previous studies investigating the longevity and size of this species showed that P. lineatus is able to live up to 10 years or even more (Crothers, 1994), its length rarely exceeds 35 mm, and the annual growth rate, measured along the center of the whorl, is 15–26 mm (Gaillard, 1965; Regis, 1972). The formation of growth checks (varices) on the external layer is reflective of slow/cessation growth during the winter months caused by lower temperatures. However, these rings can only be seen on P. lineatus in their northern distribution areas (Williamson and Kendall, 1981). Previous studies have identified the microstructure of P. lineatus using scanning electron microscopy (SEM) and x-ray diffraction (XRD). These methods have shown that they have an outer calcite layer and an inner aragonite layer (Mannino and Thomas, 2007; Mannino et al., 2003).

Figure 2.

(a) Modern specimen of Phorcus lineatus (Da Costa, 1778). (b) View showing the direction of growth and the cutting axis for thin sections. (c) Cross section showing shell structure. Four layers were identified: periostracum, prismatic calcite, foliated calcite, and aragonite. (d) Sampling method used to extract the carbonate from the inner part of the shell aperture. (e) Sampling method used to extract the carbonate sequentially along the whorl (45 samples per shell were taken).

P. lineatus becomes adult and sexually mature when its length reaches 9–10 mm, during the second year after its birth (Bode et al., 1986; Desai, 1966; Williams, 1965). The reproductive cycle has been investigated on different zones of the distribution area, such as the shores of Wales (Desai, 1966; Garwood and Kendall, 1985; Williams, 1965), England (Underwood, 1972), France (Daguzan, 1991; Gaillard, 1963), and Spain (Bode et al., 1986; Lombas et al., 1984). The conclusions obtained by these investigations were quite heterogeneous. Most evidently, differences in reproductive times were argued to be caused by variability in sea temperature among the latitudinal zones. Research developed on the Asturian shore (northern Spain), very close to our study area, concluded that this species is sexually mature when its length is larger than 9–10 mm. Its spawning or breeding stage occurs from June–July to September, and gonadal development occurs from November to May–June (Bode et al., 1986; Lombas et al., 1984).

Methodology

Modern samples of P. lineatus were collected from October 2011 to October 2012 in the medium intertidal zone on Langre beach, located on the Cantabrian shore (northern Spain) (Figure 1). No quadrates were established for collection, but shells were always collected from the same rocky area (rock pools were avoided). During this period, we performed 20 visits to the beach and captured 240 specimens, giving priority to the collection of medium-sized shells. However, for our oxygen isotope analyses we only used 46 shells collected on 16 visits. We analyzed three shells per collection day, except for October 2011 and October 2012 when we only used two shells per visit. To clean the shells, they were submerged on H₂O₂ (30%) and H₂O de-ionized (70%) solution for 48 h, they were dried at ambient temperature, then cleaned in an ultrasonic bath for 5 min, and finally dried at ambient temperature again. The shell structure was investigated using thin sections and petrographic microscope. Two shells collected in October 2012 were consolidated using epoxy resin to prevent breakage during the cutting process, and then cut along the growth direction (Figure 2b). The resulting cross sections were mounted on glass slides, polished up to 100 microns, and viewed under the petrographic microscope at different magnifications.

Carbonate samples were taken manually on the aragonite layer following the methodology applied by other authors (Colonese et al., 2009; Mannino and Thomas, 2007, 2009; Mannino et al., 2003, 2007, 2008; Prendergast et al., 2013) to allow comparison between studies. We used a dentist microdrill (0.5 mm drill bit) coupled to a stereoscopic microscope. Two sampling strategies were accomplished, in order to get two different types of data sets: (1) microsamples extracted from the inner part of the shell aperture edge, corresponding to the last portion of shell growth and thus they can be compared with the environmental parameters that prevailed immediately before the sampling stage. A total of 44 shells were sampled using this method and analyzed for oxygen isotopes, to get the complete record of 1 year (Figure 2d). (2) Sequential microsampling along the shell growth: two shells collected in October 2012 (LANO-5 and LANO-52) were analyzed through long sequences of 45 samples per shell taken along the whorl (Figure 2e) to establish intra-annual isotopic variations. The first two samples were taken from inside the edge of the shell aperture, to thus avoid breaking the edge when removing the outer layers (i.e. periostracum and calcite layer). The remaining samples were taken sequentially along the whorl, but from outside the shell. The outer layers were removed using a Dremmel microdrill and 2 mm drill bit. The sampling spots were separated apart by 0.3 mm, and the carbonate recovered weighed approximately 150–200 µg.

A total of 134 carbonate samples were taken for oxygen isotope analysis. Samples were run in an IRMS Thermo Scientific MAT 253 coupled to Kiel device at the University of Michigan (USA). Isotopic ratios were calibrated against the NBS-19 standard and are reported as δ¹⁸O (‰) relative to the Vienna Pee Dee Belemnite (VPDB) standard. The analytical error of the instrument was better than ±0.08‰.

Instrumental data on sea surface temperature (T_meas) from September 2011 to October 2012 were provided by the Aquaculture Facility of Santander’s Oceanographic Center (Cantabria, north of Spain) (Figure 1), which belongs to the Spanish Oceanographic Institute. The mean annual T_meas was 15.9°C, February was the coldest month (11.8°C), and August was the warmest month (21.5°C). Mean T_meas on the 10 days prior to shell collection were used for comparison with δ¹⁸O values and calculated temperatures, while daily T_meas were used for time calibration of values from shells sampled sequentially (Figure 3a).

Figure 3.

(a) Instrumental data on daily and at the time of shell collection (mean of the 10 days prior to each shell collection event) seawater temperature (T_meas) from the Aquaculture Facility of Santander’s Oceanographic Centre. (b) Salinity (S_meas) and isotopic composition of the water (δ¹⁸O_water) at Langre Beach from October 2011 to October 2012 (recorded at the time of shell collection).

In order to characterize the seawater chemistry (δ¹⁸O_water) and properties (salinity), water samples were obtained on each visit to Langre beach, at the same time of modern P. lineatus collection. The water samples were collected in airtight polyethylene bottles (HDPE) with no headspace, so no air bubbles got trapped inside the bottle, and then stored in a standard refrigerator at ~4°C. The samples were analyzed using an IRMS Thermo Delta V coupled to a Gas Bench II at Cornell University (USA). To calibrate the instrument, both standard samples and international samples from the Atomic Energy International Association were analyzed. The results were reported in relation to the international standard, Vienna Standard Mean Ocean Water (VSMOW ‰) and δ¹⁸O (‰) units. The analytical precision of the instrument was 0.17‰. Salinity (S_meas) was measured using a conductivity meter WTW Cond 330i at the University of Cantabria (Spain), and results were presented in practical salinity units (PSU).

Results

Morphometrics and mineralogy of ‘Phorcus lineatus’

The mean length of the 46 shells used for isotopic analysis was 16.3 mm, and the mean diameter (or width) was 17.1 mm (see Appendix 1). The minimum length recorded was 13.9 mm, which according to the information published by Bode et al. (1986) and Lombas et al. (1984) indicates that all specimens were adults and sexually mature. The study of thin sections from modern P. lineatus showed that the shell structure is composed of three layers, which are parallel to the surface (Figure 2c). The two calcite layers located underneath the periostracum present prismatic and foliated crystallization, respectively (Taylor and Reid, 1990). The most internal layer, which is directly in contact with the soft parts of the mollusk, has an aragonite composition, and it is much wider than the other two layers. Major growth checks corresponding to annual slow/cessation growth periods were not identified in the outer surface of the shells, even when using substantial magnification. Growth lines and increments were visible in the calcite and aragonite layers.

Seawater: δ¹⁸O, salinity, and temperature

The oxygen isotope ratios from water samples (δ¹⁸O_water) collected on Langre beach revealed small variations throughout the year, and the lack of a well-defined seasonal pattern. The minimum and maximum δ¹⁸O_water values were 0.55‰ (April) and 1.19‰ (July), respectively, while the mean was 0.89‰ and the range was 0.64‰ (Figure 3b; see also the Appendix 1). Lower values can be attributed to some influx of meteoric waters during the period of greater rainfall, whereas the highest values correspond to summer months, when conditions are drier and greater evaporation is expected. Absolute values were lower than those reported for Mediterranean locations, such as Gibraltar (Max = 1.67‰, Min = 0.99‰, see Ferguson et al., 2011) and Malta (Max = 1.6‰, Min = 1.1‰, see Prendergast et al., 2013), but the range was very similar (0.68‰ and 0.5‰, respectively). Although in general terms the δ¹⁸O_water variability recorded in our study can be considered as quite homogeneous throughout the year (especially when compared with variability from locations with heavy seasonal influence of freshwater, such as estuaries), the annual range should not be ignored since the difference between using the maximum or minimum δ¹⁸O_water annual value when calculating temperatures can be ~3°C (using 0.2‰ as the average value expected for the fractionation factor variability in relation to temperature, and corresponding to 1°C).

Salinity (S_meas) data showed minor and noisy variability throughout the year with values ranging between 34.0 (April) and 36.9 (June) PSU. Although no seasonal pattern can be clearly recognized in the S_meas data, the fact that the lowest values correspond to April and May would confirm a small input of meteoric waters during those months (Figure 3b).

Sea surface temperature (T_meas) through the considered time interval (October 2011 to October 2012) ranged in the Langre area between ~21°C in August and September and 11.7°C in February, and showed a marked seasonality (Figure 3a). The comparison of T_meas with δ¹⁸O_water and S_meas revealed no clear correlation patterns. In fact, the coefficients of determination indicated the existence of a weak correlation both between T_meas and δ¹⁸O_water (R² = 0.17) and between T_meas and S_meas (R² = 0.18).

In summary, the low variability of both δ¹⁸O_water and S_meas suggests that P. lineatus shells collected in Langre beach grew in normal marine conditions, with only minor changes in the hydric balance through the year.

Oxygen isotope ratios from ‘P. lineatus’ shells

A total of 44 shells were selected for the analysis of oxygen isotope ratios (δ¹⁸O_shell) in the aperture zone. δ¹⁸O_shell data for each sampling date showed remarkably homogeneous values, with an average standard deviation of 0.1‰ (Table 1). On the contrary, the complete time series showed a marked variability (Figure 4a), with values ranging from 2.46‰ to 0.31‰ (annual range of 2.15‰, see Appendix 1), while the averaged values for each collection event ranged from 2.39‰ to 0.44‰ (annual range of 1.95‰, see Table 1). The δ¹⁸O_shell series outlines a robust seasonal pattern, with an increasing trend from August to December and a decreasing trend from January to July. Major inflexions thus occur during summer (July–August) and winter (January–February), respectively, showing the lowest and the highest δ¹⁸O_shell values.

Table 1.

Measured δ¹⁸O_shell, predicted δ¹⁸O_shell, and difference between both for shell aperture samples collected between October 2011 and October 2012 in Langre Beach.

Collection date	Measured δ¹⁸O_shell (‰)	Standard deviation	Predicted δ¹⁸O_shell (‰)	Difference between measured and predicted δ¹⁸O_shell (‰)
15/10/2011	0.61	0.03	0.57	0.03
12/11/2011	1.01	0.05	0.80	0.21
25/11/2011	1.37	0.05	1.20	0.17
24/12/2011	1.55	0.10	1.63	−0.09
12/01/2012	1.89	0.06	1.72	0.16
10/02/2012	2.03	0.12	2.02	0.01
23/02/2012	2.39	0.06	2.18	0.21
25/03/2012	1.96	0.09	1.97	−0.01
22/04/2012	1.72	0.11	1.45	0.26
05/05/2012	1.51	0.22	1.32	0.19
03/06/2012	1.01	0.15	1.23	−0.22
21/06/2012	0.90	0.04	0.89	0.01
22/07/2012	0.51	0.09	0.73	−0.21
05/08/2012	0.49	0.12	0.30	0.19
02/09/2012	0.44	0.09	0.12	0.32
01/10/2012	0.89	0.09	0.64	0.24
Mean	1.27	0.09	1.17	0.16
Maximum	2.39	0.22	2.18	0.32
Minimum	0.44	0.03	0.12	−0.22
Range	1.95	0.18	2.06	0.54

Figure 4.

(a) δ¹⁸O_shell values from shell aperture samples per collection event. (b and c) Sequential δ¹⁸O_shell values from shells LANO-5 and LANO-52. Results show a clear sinusoidal pattern related to seasonal variations.

The shells of two specimens of P. lineatus (LANO-5 and LANO-52) collected on October 2012 were sampled sequentially (following the direction of growth from the aperture to the apex) for oxygen isotopes analysis. The resulting data series exhibited a substantial variation of δ¹⁸O_shell and well-defined patterns (Figure 4b and c). The maximum and minimum δ¹⁸O_shell values of both samples (LANO-5: 2.35‰, −0.06‰; LANO-52: 1.90‰, 0.18‰) as well as the range of variability were comparable with those obtained from the shell apertures.

Interpretation and discussion

δ¹⁸O_shell, T_meas, δ¹⁸O_water, and isotopic equilibrium

Several factors determine the oxygen isotopic composition of carbonates: (1) the fractionation index of the carbonate mineral (in this case, aragonite), (2) the temperature (lower temperatures inducing greater fractionation), (3) the oxygen isotopic composition of the solution (in this case, the seawater) from which the carbonate precipitates (higher δ¹⁸O_water inducing larger δ¹⁸O_shell), and (4) a range of kinetic (biochemical or physicochemical) factors which break the isotopic equilibrium.

The δ¹⁸O_shell data presented in the previous section for the shell apertures exhibited a good inverse correlation with seawater temperatures (T_meas). A linear correlation between both variables showed a very good coefficient of determination, R² = 0.91 (Figure 5a and b). This coefficient increases to 0.93 for a second-order polynomial adjustment. On the contrary, the same data showed a lack of correlation when plotted against the δ¹⁸O_water (R² = 0.06), suggesting a very secondary role of this variable in the values of δ¹⁸O_shell in our study case.

Figure 5.

(a) Mean δ¹⁸O_shell values from shell aperture samples compared with predicted δ¹⁸O_shell values calculated using instrumental seawater temperature (T_meas), δ¹⁸O_water, and Eq. (1). Error bars were calculated using 1σ SD of the three samples measured by collection event plus the replication precision error of the mass spectrometers used for δ¹⁸O_shell and δ¹⁸O_water. Results show a sinusoidal pattern related to seasonal variations in temperature and strong correlation between variables. (b) Correlation between δ¹⁸O_shell and instrumental seawater temperature (T_meas). (c) Correlation between measured and predicted δ¹⁸O_shell.

These δ¹⁸O_shell–T_meas inverse correlation coefficients were higher than that obtained by Mannino et al. (2003, R² = 0.64) for P. lineatus from different areas of Atlantic Europe, and similar to those published by Schifano and Censi (1983, R² = 0.87), Mannino et al. (2008, R² = 0.78–0.90), Colonese et al. (2009, R² = 0.91), and Prendergast et al. (2013, R² = 0.90–0.96) for Phorcus turbinatus from Mediterranean shores. Maximum and minimum δ¹⁸O and the annual range are in agreement with results obtained by Mannino et al. (2003) and Deith (1983) for shells collected in northern Spain.

An interesting point is the possibility of analyzing whether conditions of isotopic equilibrium prevailed during the formation of the biogenic aragonite in the P. lineatus shells. In order to test for isotopic equilibrium, seawater temperatures and δ¹⁸O_water measured in the same place where the P. lineatus were collected were used to calculate theoretical values of δ¹⁸O_shell (or predicted values). For that purpose, we adopted the water–aragonite fractionation factor obtained by Kim et al. (2007) from laboratory precipitation of synthetic aragonite:

1000 \ln α = 17.88 * (10^{3} / T) - 31.14

(1)

where T is the temperature measured in Kelvin and α is the fractionation between water and aragonite described by the equation:

α = (1000 + δ^{18} O_{shell} (SMOW ‰)) / (1000 + δ^{18} O_{water} (SMOW ‰))

(2)

Predicted values of δ¹⁸O_shell are listed in Table 1 and compared with measured values of our study in Figure 5a and c. As can be observed, differences between predicted and measured values are significantly very low (lower than 0.2‰ in average). This value is close to or within the instrumental errors, and thus, we can conclude that the aragonite of the shells of P. lineatus precipitated in conditions of (or close to) isotopic equilibrium. This aspect has a critical importance for using ancient shells of this taxon in paleoclimate studies.

Measured (T_meas) versus calculated (T_δ18O) temperatures: Implications for paleothermometry

The fact that the aragonite of P. lineatus shells was precipitated under conditions of isotopic equilibrium could open a range of possibilities not only for paleoclimate analyses but also for malacological studies through the analysis of growth patterns. Theoretical water temperatures through a gastropod life can be inferred from δ¹⁸O_shell, assuming the two key aspects described in the previous section: isotopic equilibrium conditions (and roughly stable δ¹⁸O_water) and adopting the water–aragonite fractionation factor of Kim et al. (2007).

Remarkably, calculated T_δ18O from shell aperture samples ranged from 10.5°C to 21.1°C (annual range = 10.6°C, see Appendix 1), while mean calculated T_δ18O for each collection event ranged from 10.8°C to 20.5°C (annual range = 9.7°C, see Table 2). This exhibited a similar pattern to that of instrumental temperatures (T_meas). In fact, T_δ18O and T_meas showed a strong correlation (R² = 0.94), and average calendar temperatures for the 16 collection events are within the range of standard deviation of calculated temperatures (Figure 6). A mean offset of 0.7°C (maximum offset = 1.4°C) between both variables was recorded (Table 2). In a similar way, the maximum and minimum calculated T_δ18O values from shells sampled sequentially (LANO-5 = 21.8°C, 10.5°C; LANO-52 = 20.6°C, 12.5°C), and the annual ranges (LANO-5 = 11.3°C; LANO-52 = 8.1°C) were in good agreement with data from T_meas and also with T_δ18O values from shell aperture samples.

Table 2.

Difference between instrumental data (T_meas) and calculated temperatures (T_δ18O) obtained using Kim et al. (2007) equation, and comparison with results provided by Grossman and Ku (1986) equation.

Collection date	T_meas (°C)	T_δ18O (°C) using Eq. (1) by Kim et al. (2007)	Standard deviation	Difference between T_meas and T_δ18O (°C), Kim et al. (2007)	T_δ18O (°C) using Eq. (1) by Grossman and Ku (1986)	Standard deviation	Difference between T_meas and T_δ18O (°C), Grossman and Ku (1986)
15/10/2011	18.8	17.8	0.2	1.0	19.9	0.1	−1.1
12/11/2011	16.8	15.7	0.2	1.1	18.0	0.2	−1.2
25/11/2011	16.2	15.4	0.2	0.8	17.7	0.2	−1.5
24/12/2011	14.1	14.0	0.5	0.0	16.4	0.5	−2.4
12/01/2012	13.7	12.9	0.3	0.8	15.4	0.3	−1.7
10/02/2012	12.0	11.8	0.5	0.2	14.3	0.5	−2.3
23/02/2012	11.7	10.8	0.3	0.9	13.4	0.3	−1.7
25/03/2012	12.7	12.9	0.4	−0.1	15.3	0.4	−2.6
22/04/2012	13.1	11.8	0.5	1.2	14.4	0.5	−1.3
05/05/2012	13.3	13.0	1.0	0.4	15.4	0.9	−2.1
03/06/2012	15.0	16.4	0.7	−1.4	18.6	0.6	−3.6
21/06/2012	17.5	17.5	0.2	0.0	19.7	0.2	−2.2
22/07/2012	19.5	20.5	0.4	−1.0	22.4	0.4	−2.9
05/08/2012	20.9	20.1	0.6	0.7	22.1	0.5	−1.2
02/09/2012	21.2	20.0	0.4	1.2	21.9	0.4	−0.7
01/10/2012	17.9	18.0	0.4	0.0	20.1	0.4	−2.2
Mean	15.9	15.5	0.4	0.7	17.8	0.4	−1.9
Maximum	21.2	20.5	1.0	1.2	22.4	0.9	−0.7
Minimum	11.7	10.8	0.2	−1.4	13.4	0.1	−3.6
Range	9.5	9.7	0.8	2.6	9.0	0.8	2.9

Figure 6.

Mean calculated seawater temperature (T_δ18O) using δ¹⁸O_shell, δ¹⁸O_water, and Eq. (1) compared with instrumental seawater temperature (T_meas). Error bars were calculated using 1σ SD of the three samples measured by collection event plus the replication precision error of the mass spectrometers used for δ¹⁸O_shell and δ¹⁸O_water. Both variables are highly correlated. Instrumental temperatures recorded at the time of collection are in the range of the standard deviation of calculated temperatures.

Previous studies on paleothermometry carried out by other authors on shell aperture samples from P. turbinatus identified noticeable temperature overestimation or underestimation relative to instrumental data. Mannino et al. (2008) indicated that the highest offsets for the three shores where they collected shells were between 3.1°C and 8.7°C, Colonese et al. (2009) presented a maximum offsets of 2.8°C, and Prendergast et al. (2013) showed an offset of 3.8°C. It should be noted that all these works calculated their temperatures from δ¹⁸O_shell using the empirical equation proposed by Grossman and Ku (1986) on the basis of biogenic aragonite (including foraminifers, mollusks, and other organisms) from North American coasts. In our study, calculations on shell aperture and sequential samples using the equation of Grossman and Ku (1986) (and the subsequent correction by Dettman et al., 1999) also produced larger offsets, with calculated temperatures usually exceeding measured ones (averaging a difference of 1.9°C, and yielding a maximum offset of 3.6°C, see Table 2).

Despite the good correlation and minor offsets obtained from our dataset in temperature calculation by using Kim et al. (2007) equation, the effect of δ¹⁸O_water when calculating past temperatures should not be discounted. Although changes in sea surface temperature appear as a major controlling factor of the δ¹⁸O_shell in our study case, the variability in the isotopic composition of the seawater recorded in Langre Beach (annual range = 0.64‰) deserves some attention. We tested the impact of seawater composition in calculated temperatures from shell aperture samples using different δ¹⁸O_water values: monthly, mean annual, and maximum and minimum annual. Using mean annual rather than monthly values produced a difference between calculated temperatures of ±1.4°C, while comparison of temperatures obtained using maximum and minimum δ¹⁸O_water values showed a difference of ~3°C (Table 3). Adding the mean standard deviation (±0.5°C) and the analytical error (±1.1°C), past temperatures could be calculated with a maximum uncertainty of ±3°C. This suggests that P. lineatus δ¹⁸O_shell can be successfully used to reconstruct seasonal temperature ranges in northern Spain assuming the occurrence of similar seasonal ranges of δ¹⁸O_water in the past. Prendergast et al. (2013) found similar δ¹⁸O_water ranges and similar uncertainty in P. turbinatus shells from the Mediterranean.

Table 3.

Differences in calculated temperatures (T_δ18O) using different δ¹⁸O_water values: monthly, mean annual, and maximum and minimum annual.

Collection date	T_δ18O (°C) monthly δ¹⁸O_water	T_δ18O (°C) mean annual δ¹⁸O_water	Difference in T_δ18O (°C) using monthly and mean annual δ¹⁸O_water	T_δ18O (°C) maximum annual δ¹⁸O_water	T_δ18O (°C) minimum annual δ¹⁸O_water	Difference in T_δ18O (°C) using maximum and minimum annual δ¹⁸O_water
15-10-11	17.8	18.6	−0.8	20.0	17.0	3.0
12-11-11	15.7	16.7	−1.0	18.1	15.1	3.0
25-11-11	15.4	15.0	0.4	16.4	13.4	3.0
24-12-11	14.0	14.2	−0.1	15.6	12.6	3.0
12-01-12	12.9	12.6	0.3	14.0	11.1	2.9
10-02-12	11.8	12.0	−0.2	13.3	10.4	2.9
23-02-12	10.8	10.3	0.4	11.7	8.8	2.9
25-03-12	12.9	12.3	0.6	13.7	10.7	2.9
22-04-12	11.8	13.4	−1.6	14.8	11.8	2.9
05-05-12	13.0	14.4	−1.4	15.7	12.8	3.0
03-06-12	16.4	16.7	−0.3	18.1	15.1	3.0
21-06-12	17.5	17.2	0.3	18.6	15.6	3.0
22-07-12	20.5	19.0	1.4	20.5	17.4	3.1
05-08-12	20.1	19.1	1.0	20.6	17.5	3.1
02-09-12	20.0	19.4	0.6	20.8	17.8	3.1
01-10-12	18.0	17.3	0.7	18.7	15.7	3.0
Max	20.5	19.4	1.1	20.8	17.8	3.1
Min	10.8	10.3	0.4	11.7	8.8	2.9
Mean	15.5	15.5	0.0	16.9	13.9	3.0
Range	9.7	9.0	0.6	9.1	8.9	0.2

Growth patterns

Mollusks slow down or stop precipitation of calcium carbonate to form their shells throughout their life for a range of environmental and physiological reasons, such as changes in ambient temperature, the tidal cycles, the reproduction cycle, the effect of thermal extremes, or the so-called vital effects (Goodwin et al., 2001; Lazareth et al., 2006; Mannino et al., 2008; Schöne, 2008; Schöne et al., 2002, 2003). Environmental conditions are not recorded during growth cessation, and for that reason, understanding the shell growth patterns is important for the interpretation of environmental and cultural information obtained from geochemical data.

As a first approach, the possibilities of reconstructing growth patterns in single shells from δ¹⁸O_shell were explored. For that purpose, the calculated temperatures from shells sampled sequentially were then compared with the instrumental record of sea surface temperatures of the last few years for time calibration (Figure 7a and b). Once the time calibration was performed (R² = 0.96 and 0.91 for LANO-5 and LANO-52, respectively), the seasonal amount of growth (mm) was estimated (Table 4). In general terms, growth rates were higher (LANO-5) and similar (LANO-52) to those documented in northerly locations by biological studies (Gaillard, 1963; Regis, 1972). In particular, results suggested that LANO-5 grew throughout the year, but growth was higher in summer months. Growth patterns on LANO-52 were more heterogeneous. Although higher growth rates occurred also in summer, information for the rest of the year showed great variability, with minimum growth in spring and especially winter, when growth cessation has been identified. Therefore, environmental parameters were not recorded in LANO-52 during the time of growth cessation, and the effect in our calculated temperature (T_δ18O) data series was translated into higher winter temperatures and smaller annual ranges than those recorded in LANO-5. In addition, while LANO-5 exhibited considerable growth rates in winter, LANO-52 showed much lower rates, and even growth interruption for some time during the cold season. Given the marked differences in growth rates exhibited between both shells, further research is clearly needed to establish detailed growth patterns on P. lineatus.

Figure 7.

(a) LANO-5 and (b) LANO-52 calendar aligned reconstructed temperatures (T_δ18O) and correlation with instrumental data (T_meas). Time calibration was performed using δ¹⁸O_shell, δ¹⁸O_water and Eq (1).

Table 4.

Seasonal growth patterns (mm of growth per season) from sequentially sampled shells LANO-5 and LANO-52.

	Growth (mm) per season
	Summer (2012/2011)	Spring (2012)	Winter (2011/2012)	Autumn (2011)
LANO-5	11.2	6.4	9.6	8
LANO-52	7.2/17.6	2.4	1.6	8

Implications for archaeological studies

P. lineatus is one of the most abundant species in Mesolithic archaeological sites, being an important resource for hunter-fisher-gatherers during this period in different places of Atlantic Europe, such as northern Spain (Gutiérrez-Zugasti, 2009) and southern England (Mannino and Thomas, 2001). Analysis of stable isotope ratios on P. lineatus has been applied to archaeological research in those regions of Atlantic Europe in the past. Deith (1983, and see also Deith and Shackleton, 1986) studied modern and archaeological shells which produced consistent results in terms of season of collection, although a few modern shells indicated a slower growth because of ontogeny and produced not so consistent results. Mannino et al. (2003) concluded that shells collected on the English shore (where temperatures are colder than in northern Spain) are exposed to longer periods of growth cessation, and even so they showed good seasonal resolution in terms of collection season. Our research has shown that P. lineatus is a good proxy to calculate temperatures throughout the life of the mollusks using the equation formulated by Kim et al. (2007). The δ¹⁸O values from both shell aperture and sequentially sampled shells reflected correctly the intra-annual variation of temperature, as well as the collection season. Unlike the results provided by Deith, our results indicate a good correlation between shell aperture isotopic values and collection season in all cases, probably because of the use of higher resolution sampling methods and small–middle sized shells. Therefore, the results presented here show that P. lineatus is a good proxy for determining collection season, able to provide significant data for archaeological research since it enables the reconstruction of settlement and exploitation patterns, and helps to increase the information on subsistence and social strategies of hunter-fisher-gatherers.

Conclusion

Oxygen stable isotope analyses on aragonite samples from modern specimens of P. lineatus collected in northern Spain showed a strong inverse correlation between instrumental temperatures (T_meas) and δ¹⁸O_shell. Moreover, the monthly variation in δ¹⁸O_water was not correlated to δ¹⁸O_shell, suggesting that the influence of the isotopic composition of seawater in temperature calculation in our study case was limited. This information along with the comparison between measured and predicted δ¹⁸O_shell values has shown that P. lineatus grew under conditions of (or close to) isotopic equilibrium with the surrounding environment. Calculated temperatures (T_δ18O) using the equation by Kim et al. (2007) showed high correlation and were in good agreement with instrumental data in both shell aperture and sequential samples. A much smaller offset than in other studies that used the equation by Grossman and Ku (1986) has been recorded. Although the δ¹⁸O_water annual range recorded at Langre Beach was relatively small, a comparison of temperatures calculated using different δ¹⁸O_water values revealed that past temperatures could be calculated with a maximum uncertainty of ±3°C when using mean annual δ¹⁸O_water values. A preliminary study on growth rates showed that P. lineatus grew throughout the year, although the seasonal growth rate was not constant, with higher rates occurring in summer. Growth interruption during some time in winter was also identified in one shell (LANO-52).

Thus, the study of δ¹⁸O_shell paleothermometry has shown the potential of P. lineatus to be used not only as a reliable paleoclimate proxy in Atlantic Europe but also as a good indicator for the season of collection. Important implications are derived from these results, since environmental reconstruction is crucial to the interpretation of prehistoric lifeways, while establishing the collection season is critical for a better understanding of the settlement patterns and subsistence strategies of past human groups.

Footnotes

Appendix

Appendix 1

Collection date	δ¹⁸O_water (‰)	Sample	Size (mm)		Measured δ¹⁸O_shell (‰)	Calculated T_δ18O (°C)
			Length	Width
15/10/2011	0.72	LANO-7	15.5	16.2	0.57	17.9
15/10/2011		LANO-8	15	16.5	0.64	17.6
12/11/2011	0.67	LANO-9	17.3	17.9	1.07	15.4
12/11/2011		LANO-10	16	18	0.99	15.7
12/11/2011		LANO-11	16.1	18	0.96	15.9
25/11/2011	0.97	LANO-12	15.5	15.5	1.31	15.7
25/11/2011		LANO-13	15.3	16	1.37	15.4
25/11/2011		LANO-14	16	16.1	1.44	15.1
24/12/2011	0.86	LANO-15	17	17.3	1.43	14.6
24/12/2011		LANO-16	16.7	17.8	1.53	14.1
24/12/2011		LANO-17	15.8	15.5	1.68	13.4
12/01/2012	0.95	LANO-18	17.2	17	1.81	13.2
12/01/2012		LANO-19	17.5	18	1.95	12.6
12/01/2012		LANO-20	16.5	17	1.89	12.9
10/02/2012	0.85	LANO-21	18	17.8	1.95	12.1
10/02/2012		LANO-22	16	16.5	2.20	11.0
10/02/2012		LANO-23	15	16.5	1.94	12.2
23/02/2012	0.99	LANO-24	14.3	15.8	2.39	10.8
23/02/2012		LANO-25	15	16.8	2.32	11.1
23/02/2012		LANO-26	15.9	17	2.46	10.5
25/03/2012	1.02	LANO-27	15.2	16.5	1.83	13.5
25/03/2012		LANO-28	15.5	16.3	2.01	12.6
25/03/2012		LANO-29	15.1	16.5	2.04	12.5
22/04/2012	0.55	LANO-30	16	16.5	1.59	12.4
22/04/2012		LANO-31	16	16.5	1.70	11.9
22/04/2012		LANO-32	15.8	16.5	1.86	11.2
05/05/2012	0.59	LANO-33	15.5	17	1.50	13.0
05/05/2012		LANO-34	17.5	17.5	1.78	11.8
05/05/2012		LANO-35	17	17.5	1.25	14.2
03/06/2012	0.83	LANO-36	18	17	1.22	15.4
03/06/2012		LANO-37	16.4	17.5	0.92	16.8
03/06/2012		LANO-38	16	17.3	0.90	16.9
21/06/2012	0.95	LANO-39	17	17.5	0.94	17.3
21/06/2012		LANO-40	16	17.5	0.90	17.5
21/06/2012		LANO-41	15.1	16.3	0.85	17.7
22/07/2012	1.19	LANO-42	16.2	17.5	0.55	20.3
22/07/2012		LANO-43	19	19	0.39	21.1
22/07/2012		LANO-44	18.1	19	0.60	20.0
05/08/2012	1.10	LANO-45	17.1	18	0.62	19.5
05/08/2012		LANO-46	17	18	0.53	20.0
05/08/2012		LANO-47	17.1	18.5	0.33	20.9
02/09/2012	1.02	LANO-48	16.5	17.1	0.52	19.6
02/09/2012		LANO-49	17	17	0.48	19.8
02/09/2012		LANO-50	17.3	18.2	0.31	20.6
01/10/2012	1.04	LANO-5	13.9	15.7	0.79	18.4
01/10/2012		LANO-52	15.4	16.4	0.98	17.5
Mean	0.89		16.3	17.1	1.29	15.4
Maximum	1.19		19.0	19.0	2.46	21.1
Minimum	0.55		13.9	15.5	0.31	10.5
Range	0.64		5.1	3.5	2.15	10.6

Acknowledgements

We must thank the Fishing Activity Service of the Cantabrian Government for the authorization to collect the modern Phorcus lineatus, the Aquaculture Facility of Santander’s Oceanographic Centre for providing the information related to sea surface temperatures, and José Ramón Mira Soto for his help measuring salinity. We also thank John Rissetto for correcting the English and two anonymous reviewers for their useful comments. The material and resources were provided by the Instituto Internacional de Investigaciones Prehistóricas de Cantabria (IIIPC), the University of Cantabria, and the Complutense University of Madrid (CEI Moncloa).

Funding

This research was performed as part of the projects ‘The human response to the global climatic change in a littoral zone: the case of the transition to the Holocene in the Cantabrian coast (10,000–5000 cal. BC) (HAR2010-22115-C02-01)’ and ‘Tracing Climatic Abrupt Change Events and Their Social Impact during the Late Pleistocene and Early Holocene (15–7 ky calBP) (HAR2013-46802-P)’ funded by the Spanish Ministry of Economy and Competitiveness. Two of the authors (IG-Z and AG-E) were supported by a contract from the Juan de la Cierva program funded by the Spanish Government and a predoctoral grant from the Basque Country government, respectively.

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Determination of sea surface temperatures using oxygen isotope ratios from Phorcus lineatus (Da Costa,1778) in northern Spain: Implications for paleoclimate and archaeological studies

Abstract

Keywords

Introduction

The study area: Geographical and environmental conditions

Material and methods

Ecology and biology of ‘Phorcus lineatus’ (Da Costa, 1778)

Methodology

Results

Morphometrics and mineralogy of ‘Phorcus lineatus’

Seawater: δ18O, salinity, and temperature

Oxygen isotope ratios from ‘P. lineatus’ shells

Interpretation and discussion

δ18Oshell, Tmeas, δ18Owater, and isotopic equilibrium

Measured (Tmeas) versus calculated (Tδ18O) temperatures: Implications for paleothermometry

Growth patterns

Implications for archaeological studies

Conclusion

Footnotes

Appendix

Acknowledgements

Funding

References

Seawater: δ¹⁸O, salinity, and temperature

δ¹⁸O_shell, T_meas, δ¹⁸O_water, and isotopic equilibrium

Measured (T_meas) versus calculated (T_δ18O) temperatures: Implications for paleothermometry