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ITB J. Eng.Sci. Vol. 42, No. 1, 2010, 65-72 65
Received June 17th, 2009, Revised January 6th, 2010, Accepted for publication January 10th, 2010.
Geochemical Tracer in Coral as a Sea Surface
Temperature Proxy: Records from Jukung Coral
S.Y. Cahyarini1 & J. Zinke2
1Puslit Geoteknologi LIPI, Bandung, Indonesia
2Dept. Paleoclim. & Paleoecology Vrije Universiteit Amsterdam, The Netherlands
Abstract. Sr/Ca has been analyzed from coral core from the Seribu Islands reef
complex i.e Jukung Island. SST from ERSST dataset and air temperature
measured at Jakarta is used for Sr/Ca calibration. The results show that Jukung
Island coral Sr/Ca correlates better with SST than with air temperature. A
comparison between the Sr/Ca records with the Nino 3.4 index shows that
Jukung coral sites indicate warming sea surface temperatures during the
beginning of El Nino events followed by cooling temperatures at the end of El
Nino years.
Keywords: corals; El Nino; Sr/Ca; SST.
1 Introduction
Sea surface temperature (SSTs) is one of the most important climatic
parameters. The instrumental SST dataset available is rarely covered till back
hundreds to thousand years. Geochemical proxy data (e.g. content in coral,
sediment etc) overcome this problem. Many previous studies show that Sr/Ca
content in corals is a promising temperature proxy (e.g. [1]; [2]; [3]). It is
believed that Sr/Ca is influenced by SST only. The ratio of the incorporation of
Sr to Ca is controlled by two factors: (1) The Sr/Ca activity of the ocean water,
(2) the Sr/Ca distribution coefficient between aragonite and seawater. When a
compatible trace component substitutes for lattice calcium in aragonite (a
guest/host substitution) the concentration of that trace element can be predicted
by its distribution coefficient (DSr):
/
/coral
sr
seawater
Sr Ca
DSr Ca
The distribution coefficient strongly depends on the temperature of the seawater
where the coral grows [4,5]. Several studies confirm that the value of DSr has
remained constant (e.g [6]; [7]). The Sr/Ca ratios of seawater are constant. Thus,
variations of the distribution coefficient, and hence variations in coral Sr/Ca, are
determined by ambient water temperature. Thus, coral Sr/Ca can be used to
reconstruct temperature.
66 S.Y. Cahyarini & J. Zinke
Linear regression analysis of Sr/Ca measurements against SST is commonly
applied to quantify the coral Sr/Ca - SST relationship (e.g. [8]; [9], [10]).
Ideally, one should use a continuous time series of SST directly from the site
where the coral grew. However, the limited local SST measurements available
have forced most studies to use grid-SST from various sources.
In this study Sr/Ca ratios of corals from the Seribu Islands reef complex i.e.
Jukung Island is analyzed. Sr/Ca is calibrated with SST from Extended
Reconstructed Sea Surface Temperature (ERSST) dataset [11,12] and air
temperature measured at Jakarta station. The hypothesis of this study is that the
offshore Jukung corals will correlate with the sea surface temperature in
seasonal scale.
2 Material and Method
2.1 Corals
Massive Porites coral cores were drilled across the Seribu Islands reef complex,
i.e Jukung, Island (S 534’01”, E 10631’38”) in September 2005. The coral
was drilled vertically. The Jukung (JU) Island core is 2.23 m in length and was
drilled at 2 m depth. A pneumatic drill powered by scuba air pressure is used in
this study. The drill bit is a diamond-tipped steel tube 3.6 cm in diameter and
30 cm long. By using extension rods of 1 m length, it is possible to recover
cores of up to 5 m in length [13].
Coral core was cut in a thickness of 4 mm. Slabs were rinsed several times in
an ultrasonic bath for about 15 minutes and dried with compressed air after each
step. The clean slabs were dried overnight at 40˚C and X-rayed using 35 kvp
for 12 minutes to reveal the annual banding. Slabs were subsampled manually
using a hand-held drill with a drilling bit of 1 mm along the growth axis at ± 1
mm interval to get a monthly resolution. The sample powders were splitted for
Sr/Ca and other geochemical analysis.
2.2 Historical data: Sea Surface Temperature (SST) and Air
Temperature (AT)
In this study historical data such as sea surface temperature (SST) and air
Temperature (AT) is used for calibration. SST data is obtained from Extended
Reconstructed Sea Surface Temperature (ERSST) database version 2 [11,12].
The ERSST dataset is available for the period 1854 till the present time. The
local measurements of air temperature at Jakarta weather station is used in this
study (from June 1992 to September 2005) which is obtained from Badan
Meteorologi, Klimatologi dan Geofisika (BMKG). Figure 1 shows both dataset
Geochemical Tracer in Coral as a Sea Surface Temp. Proxy 67
for the period 1992-2005. The Extended Reconstructed Sea Surface
Temperature (ERSST) data are averaged over a grid of 5°-6°S, 105°-106°E.
2.3 Sr/Ca Preparation and Analysis Method
Sr/Ca ratios are measured in an Inductively Coupled Plasma Optical Emission
Spectro-photometer (ICP-OES) at the Geological Institute of the University of
Kiel following a combination of the techniques described in detail by [14] and
[15]. The ~ 0.5 mg coral powder is dissolved in 1 ml HNO3 2%. The working
solutions were prepared by a serial dilution of the sample solution with HNO3
2% to get a concentration of about 8 ppm Ca. The standard solution was
prepared by dilution of 1ml from a stock solution (0.52 gram of coral powder
from a Mayotte coral in 250 ml HNO3 2% with 2 ml HNO3 2%). The relative
standard deviation (RSD) of multiple measurements on the same day and on
different days is about ±0.15%.In this session Sr/Ca of the coral core tops and
272 powder samples at 2 mm resolution is analyzed.
Figure 1 (Top) Monthly variation of sea surface temperature (source ERSST
dataset) and air temperature (source BMG). (bottom) Measured Sr/Ca from
Jukung coral in monthly resolution.
2.4 Chronology Development Method
The preliminary chronology was developed using the annual density banding
observed in the X-Ray’s. A monthly resolved time series using the anchor point
68 S.Y. Cahyarini & J. Zinke
method by assigning the minima and maxima in coral Sr/Ca to maxima and
minima in SST, respectively (e.g. [3]) is developed. The uncertainty of the
chronology development based on the anchor point method is about 1-2 months
in any given year. The result of the chronology development based on Sr/Ca
measurements is result in 1968 to 2005 time window (see [16]).
3 Sea Surface Temperature at Seribu Islands Waters
Based on the monthly mean SST for the grid-box covering the Seribu Islands
(average of grid 5-6°S, 105-106°E) two warming peaks are observed in this
region i.e. in May and November. The warmer temperature is about 29.61°C
(May) and 28.98°C (November). The coolest SST is observed in February
(28.59°C) and September (28.20°C).
4 Results and Discussions
4.1 Historical data
Comparison between SST (ERSST) averaged for the grid between 5°S-6°S,
105°E-106°E with single SST (ERSST) datasets from Jukung Island coordinates
does not show significant differences. However, the correlation of ERSST with
air temperature measured at Jakarta is low (R=0.42). Both air temperature and
ERSST is used for calibration with coral Sr/Ca records.
4.2 Coral Chronology
Based on chronology using anchor point method, where minimum/maximum
coral Sr/Ca is matched with the maximum/minimum SST it is obtained a time
window covering May 1968 to September 2005 (Figure 2).
4.3 Sr/Ca vs. Sea Surface Temperature Calibration
The result of Sr/Ca analysis shows the seasonal variation of Sr/Ca (Figure 1).
The average value of the Sr/Ca ratios is 8.593 mmol/mol. Table 1 shows
statistics for Sr/Ca content in Jukung Island coral.
Coral Sr/Ca is calibrated with temperature using linear regression. Decreasing
coral Sr/Ca corresponds to increasing temperature. Calibration of Sr/Ca from
the Jukung Island record (further mention Sr/CaJu) shows a good correlation
with ERSST (R= 0.53) and lower correlation with air temperature (R= 0.39).
The regression slope of Sr/CaJu vs ERSST is 0.05 mmol/mol/°C. For the whole
period of the Jukung coral i.e. 1968-2005 the correlation coefficient of Sr/CaJu
vs ERSST is R = 0.49, the regression slope is 0.04 mmol/mol/°C. The
calibration slope of Jukung Sr/Ca records is in the range of calibration slopes
Geochemical Tracer in Coral as a Sea Surface Temp. Proxy 69
reported for Sr/Ca vs temperature in the literature (i.e. -0.04- -0.08
mmol/mol/°C, [5]; [2,15];[17];[18]; [7]; [19]).
Table 1 Descriptive statistics of Jukung coral Sr/Ca analysis.
Descriptive Statistic
Mean
8.593
Standard Deviation
0.047
Minimum
8.446
Maximum
8.723
Count
272
Figure 2 Calibration of Sr/Ca and SST in (Top) monthly and (Bottom) annual
mean scale.
In the annual mean calibration the correlation between Sr/CaJu with SST is low
R= 0.37, however the calibration slope ( -0.042 mmol/mol/°C) is in the range of
the agreement slope for Sr/Ca vs. SST.
Based on the calibration results, it is suggested that the corals from Jukung
Island record SST clearly in seasonal and annual mean scale. Figure 2 shows
calibration chart of Sr/Ca vs. SST.
70 S.Y. Cahyarini & J. Zinke
4.4 Sr/Ca signal during El Nino years
ENSO is the climate phenomena, which is characterized by the SST gradient
anomaly between eastern and western Pacific. This climate phenomena has
global influence around the world including Indonesia. The El Nino 1997/98
cause mortality of the Seribu corals due to bleaching [20]. To indicate the
influenced of ENSO event in the Seribu waters region till back to 40 yrs, the
coral Sr/CaJu is correlated with the Nino 3.4 index. The result is shown in Figure
4. The high negative correlation between Sr/CaJu and Nino 3.4 index is found
during boreal winter. This indicates that during the El Nino event which is
strong during the boreal winter the Seribu Islands waters show decreasing
temperature while increasing temperature shown in the eastern Pacific
characterized (Figure 3).
Figure 3 Correlation between Sr/Ca and Nino 3.4 index in 3 month seasonal
average.
5 Summary
The Sr/Ca from the core tops of corals from the Seribu Islands reef complex
shows correlation with sea surface temperature. A correlation with the El Niño
events which shows high negative correlation between coral Sr/Ca and the Nino
3.4 index during boreal winter is observed. The correlation is changes from one
season to another season. Longer records of Sr/Ca coral are still required to
understand the long-term history of SST changes across the Seribu island reef
complex and to improve statistical time series analysis. Besides, analysis of
longer proxy records which represent the inshore to offshore corals from Seribu
islands is required to more understand the impact of the Jakarta coastal city to
the ocean waters ecosystem in the Jakarta bay.
Acknowledgements
We acknowledge funding through the Koninklijke Nederlandse Akademie van
Wetenschappen (KNAW-SPIN) and International Foundation of Sciences (IFS)
Geochemical Tracer in Coral as a Sea Surface Temp. Proxy 71
grant no A4605-1 for SYC. The project was also supported by the LIPI
Competitive Research Grant 2007 No: 11.03/SK/KPPI/ /2007 for SYC. Thank
to Dudi Prayudi, Samsuardi, Yayat Sudrajat for their assistant during the field
work at the Seribu islands. Thanks to the reviewers for their comments which
improve this manuscript.
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