Kiritimati Coral Isotope Data: Readme file
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                   NOAA Paleoclimatology Program
                               and
              World Data Center A - for Paleoclimatology
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NOTE: PLEASE CITE ORIGINAL REFERENCE WHEN USING THIS DATA!!!!!

NAME OF DATA SET: Kiritimati Coral Isotope Data 
LAST UPDATE: 10/98 (Original Receipt by WDCA Paleo)  

CONTRIBUTORS: Michael Evans, Richard G. Fairbanks, and James L. Rubenstone,
Lamont-Doherty Earth Observatory 

IGBP PAGES/WDCA CONTRIBUTION SERIES NUMBER:  1998-035

SUGGESTED DATA CITATION: Evans, M., Fairbanks, R.G., and Rubenstone, J.L., 1998,
Kiritimati Coral Isotope Data, IGBP PAGES/World Data Center-A for Paleoclimatology 
Data Contribution Series # 1998-035.  NOAA/NGDC Paleoclimatology Program, 
Boulder CO, USA.

ORIGINAL REFERENCES: 
1.  Evans, M.N., R.G. Fairbanks and J.L. Rubenstone, A Proxy Index of ENSO 
Teleconnections, Nature 394:732-733, 1998.

2.  Evans, M.N., R.G. Fairbanks and J.L. Rubenstone, The Thermal 
Oceanographic Signal of ENSO Constructed from a Kiritimati Island Coral, J. 
Geophys.  Res., 1998, in review; preprint available at 
http://rainbow.ldeo.columbia.edu/~mevans/preprints/thermal_signal/

GEOGRAPHIC REGION: Eastern equatorial Pacific 
PERIOD OF RECORD: 1938 to 1993

FUNDING SOURCES: National Oceanographic and Atmospheric Administration and 
National Aeronautical and Space Administration, USA

LIST OF FILES: Readme.Kiritimati.txt (this file), 
ASCII data files kiritimati1.txt and kiritimati2.txt.

DESCRIPTION: 
Kiritimati Coral Isotope Data
Kiritimati (Christmas) Island, Republic of Kiribati
LATITUDE: 2.0 N
LONGITUDE: 157.3 W
BATHYMETRY: 9 mbsl (meters below sea level)


FILE1: kiritimati1.txt
VARIABLE NAMES:

COLUMN 1: Time
COLUMN 2: o18
COLUMN 3: c13

o18 is shorthand for delta 18O, the deviation of the 18O/16O ratio in a 
sample from a known standard ratio, expressed in per mil units:

o18 = delta 18O = (Rsamp/Rstd - 1)*1000 [per mil units]

where R = 18O/16O.

c13 is defined analogously:

c18 = delta 13C = (Rsamp/Rstd - 1)*1000 [per mil units]

where R = 13C/12C.

VARIABLE UNITS:

COLUMN 1: years, data interpolated to monthly resolution
COLUMN 2: per mil, relative to the Pee Dee Belemnite standard 
COLUMN 3: per mil, relative to the Pee Dee Belemnite standard

DATA PRECISION (1 STANDARD DEVIATION):

COLUMN 1: 1938-1945: +/- 0.5 year; 1946-1955: +/- 0.7 year; 1956-1965: +/- 
0.8 year; 1966-1975: +/- 0.8 year; 1976-1985: +/- 0.8 year; 1986-1993: +/- 
0.7 year.
COLUMN 2: +/- 0.09 per mil
COLUMN 3: +/- 0.06 per mil


FILE2: kiritimati2.txt

VARIABLE NAMES:

COLUMN 1: Time
COLUMN 2: c13
COLUMN 3: o18
COLUMN 4: o18 anomaly
COLUMN 5: Sr/Ca ratio
COLUMN 6: Sr/Ca ratio anomaly

VARIABLE UNITS:

COLUMN 1: years, data interpolated to monthly resolution
COLUMN 2: per mil, relative to the Pee Dee Belemnite standard 
COLUMN 3: per mil, relative to the Pee Dee Belemnite standard 
COLUMN 4: per mil, relative to the Pee Dee Belemnite standard, anomaly 
relative to the 1981-1986 mean
COLUMN 5: mmol/mol 
COLUMN 6: mmol/mol, anomaly relative to the 1981-1986 mean

DATA PRECISION (1 standard deviation):

COLUMN 1: +/- 0.8 year
COLUMN 2: +/- 0.09 per mil
COLUMN 3: +/- 0.06 per mil
COLUMN 4: +/- 0.09 per mil
COLUMN 5: +/- 0.015 mmol/mol
COLUMN 6: +/- 0.015 mmol/mol

DATA FORMAT: ASCII text tab-delimited files, kiritimati1.txt and kiritimati2.txt.  

ADDITIONAL INFORMATION:

The following is excerpted from Evans et al.,  J.Geophys.  Res., in review, 
1998 (please see this paper for references made in this readme file):

Proxy climate data derived from the isotopic composition and chemistry of 
aragonite formed by massive reef corals provide a supplementary data set 
for the study of the tropical near-surface climate [Cole et al., 1993; 
Dunbar et al., 1994; Linsley et al., 1994; Tudhope et al., 1995; Charles et 
al., 1997].  Given constant sea water o18 and biological processes, 
variability in the oxygen isotopic composition (o18) of coralline aragonite 
is a function of the sea surface temperature in which the coral secreted 
its skeleton [Epstein et al., 1953; Weber and Woodhead, 1972; McConnaughey, 
1989].  However, coral o18 also records changes in the o18 of sea water 
[Cole et al., 1993; Linsley et al., 1994; McCulloch et al., 1994] if 
freshwater flux variance is significant.

The Strontium:Calcium (Sr/Ca) ratio in corals has been shown to be a 
function of sea surface temperature as well, with minimal sea water Sr/Ca 
influence [Weber, 1973; Schneider and Smith, 1979; Beck et al., 1992; 
DeVilliers et al., 1994; Shen et al., 1996].  Application of these two 
proxy measurements in tandem has been used to distinguish sea water o18 and 
sea surface temperature anomalies recorded on the Great Barrier Reef during 
the 1982-3 El Nino-Southern Oscillation (ENSO) event [McCulloch et al., 
1994].

We made measurements of stable isotopic composition (o18 and c13) and Sr/Ca 
on a coral collected live from Kiritimati (Christmas) Island (157.3 W, 2 
N), in the Republic of Kiribati.  Kiritimati lies at the eastern edge of 
the western Pacific warm pool and just west of the NINO3 box (Fig.  1).  
The seasonal range of sea surface temperature (SST) is less than 1.2 
degrees C. Because the seasonal cycle is modest, ENSO-induced warm and cold 
phase sea surface temperature anomalies of +/-3 degrees C are prominent 
[Bjerknes, 1969; Wyrtki, 1975; Wyrtki, 1985; Philander, 1990].  By 
contrast, ENSO-driven precipitation anomalies are moderate in magnitude and 
brief in duration, creating a minor change in sea water o18 due to enhanced 
rainfall [Ropelewski and Halpert, 1987] and eastward advection of lower 
salinity water [Picaut et al., 1996].  Thus, Kiritimati is an ideal 
location from which to monitor the thermal signal associated with the full 
ENSO cycle and should record features similar to those in the NINO3 region.

Data and Methods: Stable isotope data

Coral PP7-3 was collected live and cored at South West Point, Kiritimati 
Island, at 9 meters depth in March 1994.  The coral has been identified as 
Porites sp., probably P. australiensis, possibly P. lobata [D. Potts, UCSC, 
pers.  comm.].  The core was slabbed along the growth axis and 
X-radiographed for densitometry study; slabs were then ultrasonically 
cleaned in de-ionized water and dried at 50 degrees C prior to sampling.  
Samples for oxygen and carbon isotope analyses were drilled from the slab 
at 0.5 mm intervals starting from just below the most recent growth and 
extending to a maximum depth of 1004 mm.  A precise age model was 
recoverable for the upper 839.5 mm.  Samples were drilled from lines chosen 
parallel to the axis of maximum growth to avoid non-temporal (i.e.  
potential differential growth-related) isotopic effects [McConnaughey, 
1989].  X-ray diffraction analyses of drilled and chipped coral aragonite 
verify that samples were 100% aragonite after drilling.  Sequential 
isotopic analyses were made using a Finnigan MAT-251 gas source mass 
spectrometer coupled to a Carousel-48 automated sample preparation device.  
Measurement precisions (1-sigma) were +/-0.06 per mil and +/- 0.04 per mil 
for o18 and c13 analyses, based on analysis of a lab standard; sample 
replication over the 1981-1987 period (see section on Sr/Ca data below) 
gives an external precision of +/- 0.09 per mil in the o18 time series 
shown in Fig.  2.  All stable isotope measurements are reported relative to 
the PDB standard.

We use the strongly periodic c13 record to provide an age model for time 
series analysis of the o18 data.  The carbon isotopic composition of 
coralline aragonite varies with seasonal changes in insolation, 
reproductive activity, and changes in the c13 of [sigma CO2] of sea water 
[Fairbanks and Dodge, 1979, Patzold, 1984, Gagan et al., 1994, Gagan et 
al., 1996].  The carbon isotope record from coral PP7-3 (Fig.  2) shows a 
series of regular c13 enrichments, which correspond to thin low density 
bands.  These enrichments are probably too large to be explained entirely 
by variability of c13 of sea water [ sigma CO2] [Swart et al., 1996] or to 
be solely related to seasonal insolation variability [Fairbanks and Dodge, 
1979].  However, a model which invokes the drawdown of carbon-12 within the 
coral polyp by production of gametes during a brief period of each year, a 
process called mass spawning [Babcock et al., 1986, Gagan et al., 1994; 
Gagan et al., 1996], may explain the annual enrichments we observe in the 
carbon isotopic record.  There are no published accounts of the timing of 
mass spawning events in the central Pacific.  We assume that such events 
take place in April of each year, at the onset of the April-September 
insolation maximum at Kiritimati.  Intermittently in the PP7-3 record, 
single years do not display a c13 enrichment, perhaps due to sub-optimal 
coral reproductive conditions.  In these cases we use the same growth rate 
as the previous and subsequent years to continue the age model, an 
assumption consistent with an age model based on densitometric observations 
(Figure 2, top) and relatively constant extension rate (Figure 4).  By 
these criteria, the length of record extends from September 1993 back to 
April 1938 (Figs.  2,4).  The high (0.5mm) sampling resolution enabled us 
to obtain 25-35 samples per annual band and achieve a chronology with 
biweekly to monthly resolution.  The chronology we construct is in good 
agreement with annual age determinations based on X-radiography of the 
sampled slab, and with an age model based on a constant growth rate 
assumption (Fig.  4).

Uncertainty in such an age model might be expected to vary with time before 
present; therefore a single error estimate for the entire time period is 
unsatisfactory.  Since the timing of mass spawning at Kiritimati has not 
been observed, we conservatively assume an initial age model error of +/- 
1/2 year, and we estimate the age model error as a function of the absence 
of the annual \delc\ chronometer.  The mean extension rate is 14 mm/yr, 
with a standard deviation of 1.5 mm/yr (11%).  Therefore, we estimate age 
model uncertainty for each year within the preceding five decades as (0.5 + 
2*n*sigma(growth rate) years, where n is the number of years in that decade 
in which the c13 enrichment is absent.  These errors range from +/-0.5 to 
+/-1 year per year of age model (Fig.  4).  We believe that uncertainty in 
the age model is well represented by these estimates.  Initially we derived 
cumulative age model error estimates, but it was clear by this model that 
these error estimates were too large in the earlier half of the record, and 
too small in the more recent half.  A better error representation was 
obtained with the error model below, which acknowledges that periods of 
consistently periodic c13 pulses provides better age control than periods 
with more intermittent reproductive activity.  Subsequent intercomparison 
of the Kiritimati o18 record with the historical SST analysis by [Kaplan et 
al.  1998] suggests our age model estimates are probably appropriate.

Data and Methods: Sr/Ca data

Sr/Ca may be the preferred proxy for the purpose of SST reconstructions, 
since unlike o18, Sr/Ca variability is solely a function of SST. However, 
since analysis of \delo\ is more easily automated, requires less sample 
preparation and is 10-20 times faster than measurement of Sr/Ca by thermal 
ionization mass spectrometry (TIMS), we use limited, high precision TIMS 
Sr/Ca measurements to confirm the interpretation of the Kiritimati o18 time 
series as primarily driven by regional SST anomaly.  Based on the carbon 
isotope age model, we resampled the 1981-1987 period (Fig.  3A).  Samples 
were drilled at 1 mm intervals in a transect parallel to the line chosen 
for the high resolution stable isotopic measurements described above.  The 
samples created were split into two portions: one for replicate stable 
isotope analysis and one for Sr/Ca analysis.  Stable isotope and Sr/Ca 
sample sizes were 70-120 micrograms and 130-250 micrograms, respectively.  
The replicate stable isotope measurements were used to verify the 
chronology for the Sr/Ca data and to assess the reproducibility of the 
oxygen isotope anomaly record.  Stable isotope measurements were made using 
the previously described procedures.  Sr and Ca concentrations were 
measured using the isotope dilution technique on a VG Sector thermal 
ionization mass spectrometer, using enriched Sr-84 and Ca-43 spikes.  
In-run isotopic fractionation was corrected using measured Sr-86/Sr-88 and 
Ca-42/Ca-44 and an exponential fractionation law [Russell et al., 1978].  
Duplicate or triplicate runs were made on each Sr/Ca sample and averaged.  
The standard error (2-sigma) on the Sr/Ca measurements is less than +/-0.03 
mmol/mol, based on internal precision, duplicate analyses, and replicates; 
this translates into an SST error estimate of less than +/-0.5 degrees C.