Galaxea, Journal of Coral Reef Studies 14: 63-72（2012）

Abstract The Mg/Ca and Sr/Ca ratios were determined for Porites lutea specimens that were collected from Koh Chueak, Surat Thani Province, Thailand, using inductively coupled plasma optical emission spectrometry. The ele­mental analyses were performed from the surface to the interior of the coral specimen along its growth axis at 1­mm intervals, generating the near­monthly resolved time series for the Mg/Ca and Sr/Ca ratios. The ratios demonstrate clear annual cycles, which are assumed to reflect variations in the sea surface temperature (SST). Comparing with the annual maximum and minimum values of SST, the following Mg/Ca-SST and Sr/Ca-SST relations were determined: Mg/Ca (mmol/mol)＝−1.72＋0.193×SST and Sr/Ca (mmol/mol)＝11.83−0.098× SST. These relations are different than those of previously published studies. The reason for this difference is unclear; however, it may be related to vital effects that have been observed for geochemical proxies in coral skeletons and to differences in chemical composition of seawater and in sampling procedure.

Keywords Coral, Mg/Ca ratio, Sr/Ca ratio, Surat Thani province, Thailand

Introduction

Massive corals incorporate a variety of trace elements in their skeletons, and the contents of the elements vary with the marine environment in which the corals were formed (Smith et al. 1979). Mg and Sr have long residence times in seawater that result in relatively constant Mg/Ca and Sr/Ca ratios (Swart 1981; Beck et al. 1992). In ad­dition, the coral skeletons provide information regarding the paleoclimate through the evaluation of Sr/Ca and Mg/Ca ratios as proxies for the sea surface temperature (SST) (e.g. Shen et al. 1996; Wei et al. 2000; Cardinal et al. 2001; Sun et al. 2005; Yu et al. 2005; Ayling et al. 2006; Cahyarini et al. 2008; Liu 2008). Sr/Ca ratio is widely accepted as an excellent paleothermometer, whereas Mg/Ca ratio is still unclear. Subsequent works from other locations showed the contention made by Amiel et al (1973) that 20% to 30% of the magnesium is located in exchangeable sites or associated with the organic com­ponent of the skeleton (Watanabe et al. 2001). The re­liability of the Mg/Ca thermometer is therefore greatly questionable (Fallon et al. 1999; Mitsuguchi et al. 2001; Marshall 2002; Mitsuguchi et al. 2003). However, pre­

Mg/Ca and Sr/Ca ratios in a coral from Koh Chueak, Surat Thani, Thailand

Wararat SIRIANANSAKUL1, Nathsuda PUMIJUMNONG1, *, Takehiro MITSUGUCHI2, Sumaitt PUTCHAKARN3, and Narin BOONTANON1

1 Faculty of Environment and Resource Studies, Mahidol University, Nakhon Pathom 73170, Thailand2 Institute for Carbonate Geochemistry, 5-5 Ninomiya­cho, Chuo­ku, Kobe 651-0093, Japan3 Institute of Marine Science, Bang Saen, Chonburi 20131, Thailand

* Corresponding author: N. PumijumnongE­mail: grnpm@mahidol.ac.th, nathsuda@gmail.com

Communicated by Yasufumi Iryu (Geology and Physical Geography Editor)

Original paper

Sirianansakul et al.: Mg/Ca and Sr/Ca ratios in a coral from Thailand64

vious studies indicated that Mg/Ca had a good positive relation with SST. Mitsuguchi et al. (1996) derived the Mg/Ca thermometry based on the investigation of Mg and Ca with ICP­AES. Wei et al. (2000) implied that coral Mg/Ca from Hainan Island, South China Sea, is a valid SST proxy with a precision better than ±0.5℃.

For paleoclimate reconstruction, it is necessary to ex­tend the SST record back several years to produce a database for the SST reconstruction. Massive corals, such as Porites sp., live up to 300-500 years or more, and be­cause their growth rate is commonly in the order of 1-2 cm/yr (Knutson et al. 1972; Buddemeier et al. 1974; Dodge and Thomson 1974; Mitsuguchi et al. 2003), it is possible to retrieve high­resolution (weekly to monthly) SST records from their Mg/Ca and Sr/Ca ratios, whereas the instrumental record is very imprecise at this time scale. In Thailand, SST data have been rarely recorded, and few paleoclimatic studies have been performed. The aim of this study is to study the correlation between the Mg/Ca and Sr/Ca ratios of coral skeletons and the SST of Koh Chueak, Surat Thani Province, Thailand.

Materials and methods

Study siteThe study site was located in Koh Chueak (9º25′N,

99º40′E) in the northeastern region of Surat Thani Pro­vince, 〜10 km, 〜27 km, and 40 km from the Amphur Donsak coast, Koh Samui, and the Tapi estuary, re spec­tively (Fig. 1). The climate data were not recorded at Koh Chueak but were obtained at a nearby meteorological station (Fig. 1). The climate records from the Thai Meteo­rological Department showed that the annual mean air temperature in the region averaged 27.9±0.3℃ (1σ 1981- 2010). The annual mean precipitation averages were 1,945 mm (1981-2010). The investigated coral reefs were found in the West and South Koh Chueak. Coral reefs grow to heights of approximately 1.5-5 m, and Porites sp. corals are the most commonly found constituents in these reefs. Within the study region, there is limited coral growth because of the anchoring of boats and the trawling of fishermen, which increases the turbidity of the seawater and damages coral reefs. The chemical and physical pro­

perties of the water, such as the SST, pH, dissolved oxygen (DO), salinity, and conductivity, were measured in 18 Au­gust 2009: SST, 27.8℃; pH, 8.0; DO, 5.40 mg/l; salinity, 29.2 psu; and conductivity, 42.1 ms/cm. The SST data were derived from the 4­week averages of the NOAA SST data (NOAA NCEP EMC CMB GLOBAL Reyn_Smith Olv2 weekly SST) for a 1×1 degree grid (9.5 N, 99.5 E). The monthly mean SST from January 2004 to August 2009 shows a seasonality (Fig. 2). The mean minimum SST in January was 27.7±0.4℃, whereas the mean maxi­mum SST in April was 30.3±0.1℃.

Coral sampleIn August 2009, the massive Porites lutea colonies with

a size of ＞30 cm in diameter was collected at a depth of approximately 2 m using a hammer and a chisel. The coral sample was rinsed and soaked in distilled water at room temperature in plastic boxes for 24 hr and was dried in clean plastic boxes for several weeks. The sample was cut along its longitudinal axis using a circular rock saw to produce 5­ to 7­mm­thick slabs. To remove surface con­tamination, the coral slab was cleaned ultrasonically using a large amount of distilled/deionized water and were dried in an oven at 60℃ for 24 hr (Al­Rousan et al. 2007). The coral slab was X­rayed with Philips Optimus at Mahidol University’s Veterinary Medical Center before analyses. Exposures were made at 46 kV, 2.0 mA for 2 sec, with a source­to­film distance of 100 cm to reveal the extension rates. Each couple of high­/low­density band represents an annual extension rate, and the upper was assigned to the date of collection (2009). The extension rates were directly measured along the major growth axes from the computed radiography.

Analytical methodsThe chemical treatment and analysis were basically

followed by a previous study (Mitsuguchi et al. 2003). For the analysis of the Mg/Ca and Sr/Ca ratios, microsamples of P. lutea, each weighing 2-3 mg, were collected manu­ally at 1.0 mm intervals using an etching needle; the cross­section of the microsampling plane measured approxi­mately 1.5×1.5 mm. The samples were treated sequen­tially with 1 ml of distilled/deionized (DDW) water at room temperature (20-25℃), 1 ml of 4 mM HNO3 at room

Sirianansakul et al.: Mg/Ca and Sr/Ca ratios in a coral from Thailand 65

temperature (20-25℃) and 1 ml of 30% H2O2 at 60℃. The treatments were performed in acid­cleaned 5­ml glass vials and were subjected to ultrasonic agitation for 15 min. At the end of each treatment step, the vials were cen­trifuged (15 cm radius at 2,000 rpm) for 10 min, and the supernatants were carefully removed by siphoning using a micropipette. The treated microsamples were dried in a vacuum desiccator at 60℃ for at least 6 hr without a dry­ing agent (Mitsugushi et al. 2003). The microsamples (1-2 mg) were weighed and dissolved in 4 ml of 0.5 M

HNO3 on a microbalance to standardize the Ca concent­ration of the sample solution to 95 ppm±10%. The Mg, Sr, and Ca concentrations in the solutions were measured using inductively coupled plasma optical emission spec­trometry (ICP­OES) at Faculty of Science, Silpakorn Uni­versity, with Agilent 710 ICP­OES. Reproducibilities (re­lative standard deviation) of the Mg/Ca and Sr/Ca meas­urements were estimated to be 1.0% and 0.5%, re spec­tively.

A linear least­squares regression was used to analyze

Fig. 1　Map of Koh Chueak showing the sample collection sites. The black circle and black rectangle represent a sampling site and a nearby meteorological station, respectively

Sirianansakul et al.: Mg/Ca and Sr/Ca ratios in a coral from Thailand66

the correlation of the Mg/Ca and Sr/Ca ratios with the SST.

Results

Skeleton extension ratesAccording to the X­radiograph, the annual extension

rate increments range from 12.5 to 19.5 mm. The mean annual extension rate of 14.7 mm/yr is similar to the coral extension rates from the Pulau Tioman, Malaysia (Lee and Mohamed 2009), and Timor, Indonesia (Cahyarini et al. 2008), which grew at rates of approximately 14-20 mm/yr. The average sampling interval for the subsamples was about 1.0 mm, which corresponded to the temporal resolution of the data that vary approximately from 3 to 4 weeks. Dark (high density) and light (low density) bands were shown within the annual extension in the X­radio­graph (Fig. 3).

Mg/Ca and Sr/Ca variationsThe 4­weekly average SST values between 2004 and

2009 revealed that the maximum SST for each year are roughly identical, ranging from approximately 30.3℃ to 30.8℃. The highest SSTs occurred in 2004 and 2006, and the SST generally reached a maximum from April to June each year. The SST generally reached a minimum in Janu­ary or December (Fig. 4), in the range of approximately 27.0℃-28.0℃, and the lowest SST occurred in 2009.

In this study, sixty­five P. lutea subsamples were anal­yzed from a coral. The Mg/Ca and Sr/Ca ratios exhibited seasonal variations, ranging from 3.53 to 4.30 mmol/mol and from 8.74 to 9.14 mmol/mol, respectively (Fig. 4). The sampling positions were converted to times by as­suming that the winter and summer SST extremes cor­responded to the extremes of Mg/Ca and Sr/Ca ratios in a year because previous studies documented that the ratios and SST show a significantly positive and negative cor­relation, respectively (e.g. Beck et al. 1992; McCulloch et al. 1994). X­radiography showed that low values of Mg/Ca and high Sr/Ca values correspond to the high­density bands that were deposited during the winter months (November­February) and high values of Mg/Ca and low

Fig. 2　Monthly mean sea surface temperature with stand­ard deviation between 2004 and 2009 from NOAA NCEP EMC CMB GLOBAL Reyn_SmithOlv2 weekly SST

Fig. 3　X­radiograph positive print of P. lutea coral. The sixty­five sampling profiles for Mg/Ca and Sr/Ca ratios are indicated by white dashed lines

Sirianansakul et al.: Mg/Ca and Sr/Ca ratios in a coral from Thailand 67

Sr/Ca values to the low­density bands that were deposited during the summer months (March­October). Similar to a previous article (Mitsuguchi et al. 2003) on Porites coral

from Ryukyu Islands, all the profiles have narrow troughs in winter and broad peaks in summer, suggesting the coral grew slowly in winter and rapidly in summer.

Previous studies have confirmed that the Mg/Ca and Sr/Ca ratios are predominantly controlled by the SST and are suitable for the reconstruction of SST for the past (e.g. Beck et al. 1992; McCulloch et al. 1994; Shen et al. 1996; Mitsuguchi et al. 1996; Alibert and Mcculloch 1997; Swart et al. 2002). To confirm a characteristic of the var­iations observed in the Mg/Ca and Sr/Ca ratios, a scatter plot of the two variables was analyzed using a linear regression. Because there was a difference in the timing of annual maximum and minimum of the Mg/Ca and Sr/Ca ratios (Fig. 4), the scatter plot showed a weak correlation between the Mg/Ca and Sr/Ca ratios (Fig. 5), yielding a regression line with a value of r＝0.61.

Calibrations of coral Mg/Ca-SST and Sr/Ca-SSTTo calibrate the Mg/Ca and Sr/Ca ratios against the

SST, a least­squares regression analysis was performed using the annual maximum and minimum values in this

Fig. 4　The variations in (A) 4­weekly average SST values for the period 2004-2009, (B) the Mg/Ca and (C) Sr/Ca ratios in the coral skeleton

Fig. 5　Mg/Ca­Sr/Ca scatter plot for the coral skeleton

Sirianansakul et al.: Mg/Ca and Sr/Ca ratios in a coral from Thailand68

study. This calibration procedure is followed by a number of studies (e.g. Gagan et al. 1998; Cardinal et al. 2001; Sun et al. 2004; Deng et al. 2009) that have established calibrations using not only the extreme values but also the other interpolated values, based on the assumption that the extension rate of coral skeletal is constant between the extreme values (e.g. Mitsuguchi et al. 1996; Shen et al. 1996). However, this assumption could be applied in very limited cases because the skeletal extension rate generally

changes in response to seasonal environmental fluctua­tions; therefore, only the extreme values were used to calibrate the Mg/Ca and Sr/Ca thermometers.

The following calibration equations for the Mg/Ca and Sr/Ca ratios and SST were obtained (Fig. 6):

Mg/Ca (mmol/mol)＝−1.72＋0.193×SST; r＝0.97, p＜0.001

Sr/Ca (mmol/mol)＝11.83−0.098×SST; r＝0.96, p ＜0.001

Discussion

Several studies calibrated coral Mg/Ca and Sr/Ca ther­mometers for Porites corals from the South China Sea, and a literature survey of the Mg/Ca and Sr/Ca versus SST calibrations is shown in Table 1. The slopes of these cali­brations exhibit large variations. Comparing the results of this study with previous studies, the slopes for the Mg/Ca and Sr/Ca calibrations obtained in this study are quite different from others (Fig. 7). These variations can be ex­plained by differences in the minor element composition of the seawater (e.g. Shen et al. 1996; Sun et al. 2005). Variations in the minor element content of seawater are important for controlling the intake of minor elements into coral skeletons. The variation in skeletal Sr/Ca ratios is due mainly to variations in the Sr composition, as sug­gested by Enmar et al. (2000). Sun et al. (2005) indicated that variations in seawater Ca concentration do not sig­nificantly affect the Ca content of coral skeletons. Sea sur­face salinity (SSS) is an important parameter in climate studies and change in SSS can often be related to change in precipitation/evaporation systems (Maes et al. 2002). Schifano (1982) indicated that the Mg/Ca ratios in sea­water may change with salinity because Mg contents of seawater vary with salinity. The Thai Meteorological De­partment revealed that the trend of maximum precipitation records (AD 2004-2009) occurred in October­November corresponded with the minimum SSS in these months (Fig.8). Therefore, the change in SSS may cause a vari­ation in Mg/Ca and Sr/Ca. Furthermore, a number of ad­ditional factors affect element ratios, such as the growth rate, chemical treatments, and environmental factors (e.g. YoshioKa et al. 1985; Marshall and McCulloch 2002;

Fig. 6　Linear least­squares regressions of SST vs. (A) Mg/Ca and (B) Sr/Ca ratios for the coral skeleton

Sirianansakul et al.: Mg/Ca and Sr/Ca ratios in a coral from Thailand 69

Cohen and Hart 2004). Previous studies indicated that the growth rate is not an

important factor influencing the Mg/Ca and Sr/Ca ratios in Porites corals (e.g. Wei et al. 2000; Mitsuguchi et al. 2003; Allison and Finch 2004). On the other hand, some investigations documented that biological/metabolic ef­fect and microenvironmental variations have a large in­fluence on the Mg/Ca ratio (e.g. Fallon et al. 2003; Inoue et al. 2007; Reynaud et al. 2007). Mitsuguchi et al. (2001)

and Mitsuguchi and Kawakami (2012) reported that, in addition to chemical treatment, distilled deionized water (DDW) resulted in small decreases in the Mg/Ca ratio, which increased following treatment with H2O2 and HNO3, whereas the Sr/Ca ratio exhibited little variation through­out the sequence of treatments.

In this study, other factors that generate the discrepancy of the Mg/Ca­ and Sr/Ca­SST calibrations cannot be iden­tified because of no sufficient data or information re­

Table 1　Summary of Mg/Ca and Sr/Ca vs. SST calibrations for the genus Porites corals in the South China Sea

Fig. 7　Calibrations for SST vs. (A) Mg/Ca and (B) Sr/Ca ratios for the genus Porites corals in the South China Sea

Sirianansakul et al.: Mg/Ca and Sr/Ca ratios in a coral from Thailand70

garding the sea­surface conditions around Koh Chueak. The analytical uncertainties and sampling procedure may be partially attributed to the errors in the Mg/Ca and Sr/Ca.

Conclusions

The Mg/Ca and Sr/Ca ratios (AD 2004-2009) in an­nually banded P. lutea specimens collected from Koh Chueak in the Gulf of Thailand were analyzed in terms of their relation with the SST. The Mg/Ca and Sr/Ca time series exhibited an annual cycle, which may primarily reflect the seasonal SST variation in Koh Chueak. The Mg/Ca and Sr/Ca ratios in the coral skeleton were cor­related with the SST; however, the Mg/Ca­SST and Sr/Ca­SST calibrations largely differed from those of pre­vious studies. The specific reasons for the discrepancy re­main unknown. Further studies are needed to evaluate the biological effects and differences in the minor element composition of the seawater or in the sampling procedures. For future studies, the retrieval of longer coral cores from the area is recommended to establish a more accurate SST reconstruction.

Acknowledgments

The authors thank their colleagues in Mahidol Uni ver­sity that helped with the fieldwork and X­radiograph the coral slab. This work was supported by the Mahidol Uni­versity 2008-2010 budget and the 60th Year Supreme Reign of his Majesty King Bhumibol Adulyadej Scholar­ship granted by the Faculty of Graduate Studies, Academic Year 2009, Mahidol University. The manuscript has bene­fited from the constructive criticism of anonymous re­viewers.

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Received: 17 May 2012Accepted: 8 September 2012

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