Global Environmental Change in the Ocean and on Land, Eds., M. Shiyomi et al., pp. 205–227.
© by TERRAPUB, 2004.
Climate Reconstructions from Annually Banded Corals
Thomas FELIS and Jürgen P ÄTZOLD
DFG Forschungszentrum Ozeanränder (DFG Research Center Ocean Margins),
Universität Bremen, 28359 Bremen, Germany
Abstract. Instrumental observations are too short to resolve the full range of
natural climatic and environmental change. Massive, annually banded corals
from the tropical and subtropical oceans however, provide a means to reconstruct
climatic conditions prior to the instrumental record. Isotopic and elemental
tracers, incorporated into the carbonate skeletons of these corals during growth,
provide proxies of past environmental variability of the surface ocean. Here we
give an overview of the most frequently used coral based proxies reflecting
variations in sea surface temperature (Sr/Ca, U/Ca, δ18O), hydrologic balance
(δ18O), ocean circulation and upwelling (∆14C), and terrestrial runoff (Ba/Ca)
at seasonal resolution. We present coral based reconstructions of climatic and
environmental change for the last few centuries, the last millennium, the
Holocene, and the Pleistocene; with a focus on seasonal, interannual, and
decadal scale climate variability. Finally, future directions in coral-based
paleoclimatology are discussed.
Keywords: coral paleoclimatology, interannual variability, ENSO, AO/NAO,
δ18O, Sr/Ca
1. INTRODUCTION
Instrumental climate records are too short to resolve the full range of seasonal,
interannual, and decadal-scale natural climate variability. Banded corals, tree
rings, ice cores, and varved sediments provide paleoclimatic archives which can
be used to reconstruct past climate variability in the pre-instrumental period in
annual resolution. These proxy climate indicators provide paleoclimatic records
which are important for the assessment of perturbations to the natural climate
variability by anthropogenic forcing, for climate predictability and for a better
understanding of the dominant modes of the global climate system, e.g., the El
Niño-Southern Oscillation (ENSO) phenomenon of tropical Pacific origin, the
Asian and African monsoon, the Arctic Oscillation (AO)/North Atlantic Oscillation
(NAO), the Pacific Decadal Oscillation (PDO), and the mechanisms of decadal
climate variability. These natural modes have important socio-economic effects
owing to their large-scale modulation of droughts, floods, storms, snowfall, or
fish stocks.
Massive “stony” (scleractinian) corals from the modern and fossil reefs of
the tropical and subtropical oceans provide an important archive of past climate
205
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and ocean variability. These corals build skeletons of aragonite (CaCO3) and
grow at rates of millimetres to centimetres per year. During growth, annual
density bands are produced in the skeleton that can be used for the development
of chronologies. As corals grow they incorporate isotopic and elemental tracers
reflecting the environmental conditions in the ambient seawater during skeleton
precipitation, e.g., water temperature, hydrologic balance (evaporation,
precipitation, runoff), and ocean circulation. Compared to other paleoclimatic
archives corals provide a clear seasonal resolution. Modern corals from living
reefs provide continuous climate records extending several centuries back from
the present. Well-preserved fossil corals from emerged or submerged reef
terraces provide information on climate variability during time-windows
throughout the late Quaternary. The most commonly used corals in
paleoclimatology are those of the genus Porites which are ideally suited for subannual sampling owing to their dense skeletons and rapid growth rates (about 1
cm per year).
Most reef-building (hermatypic) corals live in the upper ~40 m of the ocean
where there is sufficient light for the photosynthetic activity of a coral’s
endosymbiotic algae (zooxanthellae). Furthermore the development of coral
reefs is restricted to warm water temperatures. Most corals are located in regions
where mean annual temperatures are not below 24°C and/or mean winter minimum
temperatures are not below 18°C, i.e., roughly equatorwards of 23–24° latitude.
Therefore recent coral-based paleoclimatic research has focused mainly on the
tropics providing important implications on past variability of the ENSO
phenomenon and decadal tropical climate variability (e.g., Cole et al., 1993,
2000; Charles et al., 1997; Urban et al., 2000; Cobb et al., 2001). However, some
ocean currents transport warmer tropical waters to higher latitudes leading to
coral growth also at some rare subtropical/mid-latitude locations. These locations
have provided the opportunity to study coral records from up to ~29°S in the
southeastern Indian Ocean (Kuhnert et al., 1999); ~32°N in the North Atlantic
(Pätzold and Wefer, 1992; Kuhnert et al., 2002), and ~28–29°N in the northern
Red Sea (Felis et al., 1998; Felis et al., 2000; Rimbu et al., 2001, 2003). These
paleoclimatic records were generated from living corals covering the past
centuries. In contrast, fossil corals have revealed important aspects on climate
variability during time-windows of up to several decades throughout the last
millennium (Kuhnert et al., 2002; Cobb et al., 2003a), the Holocene (Beck et al.,
1997; Gagan et al., 1998; Corrège et al., 2000; Moustafa et al., 2000; Tudhope et
al., 2001; Abram et al., 2003), and the last interglacial warm period (Hughen et
al., 1999; Suzuki et al., 2001; Tudhope et al., 2001).
The paper is organized as follows. In Section 2 the construction of the age
model for coral chronologies is described. In Section 3 the main climate tracers
which are used in coral-based paleoclimatology are introduced. In Section 4
important coral-based climate reconstructions are presented. Finally, in Section
5 recent directions in coral-based paleoclimatology are discussed. This paper is
an updated version of two previously published review papers on the same topic
(Felis and Pätzold, 2003, 2004). Its main intention is to provide scientists from
Climate Reconstructions from Annually Banded Corals
207
other fields (e.g., paleoclimatologists working on other climate archives, climate
modellers) with basic information for a more efficient interpretation of coralbased climate reconstructions. Therefore, only the most commonly used isotopic
and elemental tracers in coral-based climatology are described in detail. Criteria
for the coral-based reconstructions presented here are their length and paleoclimatic
implication as well as a seasonal resolution.
2. ANNUAL DENSITY BANDING AND AGE MODEL
As corals grow, new skeleton is generated within the living tissue layer
which always remains as a thin band (of several millimetres width) at the
outermost surface of a colony. Centuries-old coral colonies can become several
meters high considering typical growth rates for Porites of about 1 cm per year.
Such large living corals are usually sampled by drilling a core vertically from the
top to the bottom of a colony along the major axis of growth.
In general, coral skeletons reveal a pattern of alternating bands of high- and
low density, with each year being represented by a pair of such bands. The density
variations result from changes in a coral’s rate of calcification and/or linear
extension. The preliminary age model of a coral chronology is usually based on
counting the annual density-band pairs, which are revealed by X-radiography.
The counting starts at the top of a coral core within the tissue layer, whose age is
known from the date of collection of the core, provided a living colony was
drilled. The preliminary age model based on banding is then refined using the
seasonal cyclicity of isotopic or elemental tracers in the coral skeleton that
reflects the seasonal cycle of temperature or light. Sometimes, seasonal cycles in
measured variations of density (Lough and Barnes, 1997) or luminescence (Isdale
et al., 1998) are also used for the development of an age model. Furthermore,
characteristic patterns of distinct luminescent lines in coral skeletons associated
with freshwater influx at some locations were used to improve the chronological
control of coral records and to develop a master chronology (Hendy et al., 2003).
Well-preserved fossil corals can be dated either using radiocarbon or the
230
Th/234U method. The 230Th/234U method is applied to date corals which are
older than 30,000 to 45,000 years which is the limit of the 14C method. Furthermore,
230
Th/234U ages can be considered as absolute ages compared to 14C ages, which
are influenced by ocean reservoir effects and variations in the 14C level of the
atmosphere (Yokoyama and Esat, 2004). Because of this, the 230Th/234U method
was also applied to date young fossil corals from the last millennium (Edwards
et al., 1988; Kuhnert et al., 2002; Cobb et al., 2003a, b). The dating of a fossil
coral provides a floating chronology for which top and bottom ages are known
only approximately.
3. CLIMATE TRACERS IN CORALS
3.1 Annual growth rates
Coral skeletal growth is associated with two variables, linear extension and
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T. FELIS and J. PÄTZOLD
calcification. The annual extension rates of corals can be inferred from the annual
density-band pattern and can provide a paleoclimatic tracer. Coral extension rates
can reflect several environmental parameters such as temperature, nutrient or
food availability, water transparency, and sediment input. A centuries-long
record of coral extension from Bermuda in the North Atlantic was shown to
reflect temperature variability, probably as a result of enhanced coral extension
during periods of increased wind-induced vertical mixing resulting in cooler but
nutrient richer surface waters (Pätzold and Wefer, 1992; Pätzold et al., 1999).
However, due to the dependence on several environmental factors extension rate
variations in most corals are difficult to interpret in climatic terms.
The average annual calcification rate of several centuries-long coral records
from the Great Barrier Reef (Australia) was shown to provide a proxy for sea
surface temperature (SST) for the period 1746–1982 (Lough and Barnes, 1997).
One of these records extends back to the year 1479. The density variations were
measured by gamma densitometry and provided data for average annual density
and annual extension rate. These were used to estimate average annual calcification.
The calcification rate is the product of the linear extension rate and the average
density at which skeleton was deposited in making that extension. Interestingly,
the annual calcification rate was more strongly correlated with SST variations
compared to the annual extension rate (Lough and Barnes, 1997).
3.2 Oxygen isotopes
The ratios of the isotopic species of oxygen (18O/16O) incorporated in coral
skeletons during growth, reported as δ18O, are primarily influenced both by the
temperature and the δ18O of the ambient seawater during skeleton precipitation.
In localities where one of these two environmental factors dominates the other,
coral δ 18O records can therefore provide information either on variations in water
temperature or in δ18O of the seawater with the latter being related to the
hydrologic balance.
As temperature increases, there is a decrease in the δ 18O (depletion in 18O)
of the coral skeletal aragonite. Near-weekly resolution calibrations of Porites
coral δ18O variability suggest a temperature dependence of 0.18‰ per 1°C
(Gagan et al., 1994). This ratio is supported by a regression of several long
annually-averaged Indo-Pacific Porites coral δ18O records against local SST
anomaly which indicates a ratio of 0.19‰/°C (Evans et al., 2000).
Variations in the δ18O of the seawater can result from evaporation (enrichment
18
in O), precipitation (enrichment in 16O), or runoff (enrichment in 16O), i.e., such
variations reflect the hydrologic balance. Water mass transport also can play a
role. High evaporation results in both, an increase in the δ18O of the seawater and
a higher salinity; high precipitation or runoff have opposing effects.
Coral skeletons are depleted in 18O with respect to isotopic equilibrium with
the ambient seawater, i.e., compared to inorganically precipitated aragonite. This
isotopic disequilibrium or vital effect is most likely biologically mediated
(McConnaughey, 1989; Adkins et al., 2003). The isotopic disequilibrium is
Climate Reconstructions from Annually Banded Corals
209
assumed to be constant over time along the major growth axis of an individual
coral colony, where growth and calcification rates are at their maximum
(McConnaughey, 1989; Guilderson and Schrag, 1999; Linsley et al., 1999).
However, long-term trends reported in some coral δ18O based studies are consistent
in sign but too large to be explained by observed changes in SST, seawater
salinity, or rainfall over contemporaneous intervals, as emphasized by Evans et
al. (2000). Therefore, it has been speculated that at least some of the lowfrequency variability and long-term trends observed in individual coral δ 18O
records may originate not directly from climatic influences but from variations in
the mean disequilibrium offset through time as a result of complex biological or
ecological processes (Evans et al., 2000).
The mean isotopic disequilibrium offset from seawater can vary significantly
for individual corals living at the same location (Guilderson and Schrag, 1999;
Linsley et al., 1999; Felis et al., 2003). Therefore, caution is required when past
mean climate conditions are estimated based on comparison of mean δ18O values
of modern and fossil corals even from the same area. Coral δ18O provides an
excellent proxy for variability but should not be considered as an absolute proxy
for SST and/or δ18O of seawater.
3.3 Stable carbon isotopes
The environmental interpretation of the ratios of the stable isotopic species
of carbon (13C/12C) incorporated in coral skeletons, reported as coral δ13C, is
complicated because of interactions with physiological processes such as symbiont
photosynthesis and respiration (e.g., Grottoli and Wellington, 1999). Therefore
the application of the coral δ13C signal as a climate proxy has been hampered,
despite the fact that δ18O and δ13C values are routinely produced together by mass
spectrometry.
Several coral records indicate a long-term trend towards lower δ13C (depletion
13
in C) during the past centuries (e.g., Nozaki et al., 1978; Pätzold, 1986; Quinn
et al., 1998). This trend most likely reflects the δ13C of the dissolved inorganic
carbon (DIC) of the seawater and is usually attributed to a corresponding trend in
the δ13C of atmospheric CO2 owing to an increased anthropogenic release of 13Cdepleted CO2. However, the slopes of the trends differ among coral records even
for individual corals from the same location (e.g., Guilderson and Schrag, 1999).
On the seasonal timescale coral δ13C variations are thought to be mainly
controlled by the photosynthetic activity of the coral’s endosymbiotic algae, and
are therefore attributed to the seasonal light cycle, cloudiness or water column
transparency (Fairbanks and Dodge, 1979; Pätzold, 1984; McConnaughey, 1989;
Wellington and Dunbar, 1995). Periods of higher photosynthesis lead to higher
δ13C values in coral skeletons (Fairbanks and Dodge, 1979; Swart, 1983;
McConnaughey, 1989). Therefore, the seasonal cyclicity in coral δ13C is sometimes
used for age model improvements of corals records where a clear cyclicity in
coral δ18O is absent due to local environmental conditions (e.g., Cole et al., 1993;
Guilderson and Schrag, 1999; Urban et al., 2000).
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At some locations, strong coral δ13 C signals with paleoceanographic
implications can emerge. For example, a coral δ13C record from the northern Red
Sea documents interannual events of extraordinarily large plankton blooms
caused by deep vertical water mass mixing in certain winters. This was attributed
to changes in the coral’s food uptake, i.e., increased heterotrophic feeding on
zooplankton during the periods of high plankton availability (Felis et al., 1998).
3.4 The strontium/calcium (and uranium/calcium) ratio
Sr/Ca ratios in corals were shown to provide a promising proxy for water
temperature variability (Beck et al., 1992; McCulloch et al., 1994; Alibert and
McCulloch, 1997; Gagan et al., 1998). As temperature increases, there is a
decrease in the Sr/Ca ratio of the coral skeletal aragonite. Apparently the coral Sr/
Ca-temperature calibrations do not vary significantly for individual corals living
in the same area (Alibert and McCulloch, 1997), but this has to be confirmed for
other locations. However, there are still differences in the temperature calibrations
between studies from different sites. The average of several coral Sr/Ca calibrations
suggests a temperature dependence of 0.062 mmol/mol per 1°C (Gagan et al.,
2000) and a more recent compilation shows a similar average slope (0.0597
mmol/mol per 1°C) of the calibration equations (Marshall and McCulloch, 2002).
However, it is not known whether this coral Sr/Ca-temperature relationship is
generally valid for Porites. Other temperature-sensitive trace elements
incorporated in coral skeletons are Mg (Mitsuguchi et al., 1996) and U (Min et al.,
1995; Shen and Dunbar, 1995). Recent studies revealed that U/Ca ratios in corals
could provide a temperature proxy comparable in accuracy to Sr/Ca (Corrège et
al., 2000; Corrège et al., 2001; Hendy et al., 2002) whereas Mg/Ca ratios
probably do not (Schrag, 1999).
Sr/Ca ratios in coral skeletons show a vital effect that produces disequilibrium
with the ambient seawater compared to inorganically precipitated aragonite. This
vital effect was suggested to be partly biologically controlled (de Villiers et al.,
1994) but can be probably neglected along the major growth axis of individual
corals (Alibert and McCulloch, 1997). However, coral Sr/Ca ratios are influenced
by the Sr/Ca ratio of the ambient seawater during skeleton precipitation. Due to
the long residence times of Sr and Ca in the ocean (millions of years) the Sr/Ca
ratios of seawater are supposed to be constant on glacial-interglacial timescales
(e.g., Guilderson et al., 1994). In contrast, some works indicate that seawater Sr/
Ca ratios can vary significantly between sites (de Villiers et al., 1994) as well as
at the same location over the annual cycle (Shen et al., 1996). Seawater Sr/Ca
ratios were shown to be correlated with nutrient variability, e.g., at upwelling
sites (de Villiers et al., 1994). Furthermore it was suggested that during glacial
conditions the Sr/Ca ratio of seawater could have been significantly different due
to the weathering and dissolution of Sr-enriched aragonitic carbonates exposed
on the continental shelves (Stoll and Schrag, 1998). All this is critical to the
applicability of coral Sr/Ca ratios as an absolute proxy of past SST.
Climate Reconstructions from Annually Banded Corals
211
Combined determinations of δ18O and Sr/Ca in corals can provide information
on δ18O seawater as well as temperature variability, through removing the
temperature component of the coral δ18O variations which is derived from the
coral Sr/Ca signal (McCulloch et al., 1994; Gagan et al., 1998; Hendy et al., 2002;
Ren et al., 2002). If the relationship between δ18O of seawater and salinity is
constant over time, the double-tracer technique of coupled δ18O and Sr/Ca
measurements in corals can be used to reconstruct past variations in ocean surface
salinity.
3.5 Radiocarbon
Radiocarbon (14C) incorporated in coral skeletons during growth reflects the
C content of the DIC of the ambient seawater during skeleton precipitation and
provides a useful tracer for ocean circulation and upwelling. The 14C content of
the surface ocean is controlled by the 14C level of the atmosphere (equilibration
time about a decade) and by mixing with waters which have a different 14C
signature (e.g., Rodgers et al., 1997). The latter can result from changes in the
depth of the mixed layer or thermocline or changes in the rate of vertical mixing
and upwelling which brings radiocarbon-depleted waters to the surface. Another
factor is the horizontal advection of surface waters with a different 14C signature
from other oceanic source regions (e.g., Druffel and Griffin, 1993). The 14C
content is reported as ∆14C (‰), which is the 14C/12C ratio relative to a standard.
The atmospheric testing of nuclear weapons in the 1950s and early 1960s
increased the ∆14C of the surface ocean. During this time of rapidly increasing
bomb 14C in the atmosphere air-sea exchange was the primary controller on
surface ocean ∆14C. This increased the natural ∆14C gradient between surface and
subsurface waters and makes recent coral ∆ 14C records from the surface ocean
very sensitive to changes in upwelling, but also to changes in the horizontal
advection of water masses which upwelled elsewhere (e.g., Guilderson and
Schrag, 1998; Guilderson et al., 1998). Because the rates of biological processes
on ∆14C DIC as well as radioactive decay are negligible relative to surface water
dynamics and the timeframe of interest, respectively, ∆14C in surface waters is a
quasi-conservative, passive tracer for advection (Guilderson et al., 1998;
Guilderson et al., 2000).
14
3.6 The barium/calcium ratio
Ba/Ca ratios in corals can provide records of suspended sediment loads and
hence in the nutrients entering reef areas as a result of river floods (McCulloch
et al., 2003). Ba is desorbed from fine-grained suspended particles (clays) in the
low-salinity region of the estuarine mixing zone. Thereafter Ba behaves as an
essentially conservative dissolved tracer, being advected with the flood plume
and incorporated into the coral skeleton in proportion to the ambient seawater
concentration (McCulloch et al., 2003). Earlier studies used coral Ba/Ca to
reconstruct the variability in upwelling of nutrient rich waters in the equatorial
Pacific (Lea et al., 1989; Shen et al., 1992).
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T. FELIS and J. PÄTZOLD
4. CLIMATE RECORDS FROM CORALS
4.1 Last centuries
Current paleoclimatic records which are based on δ18O, Sr/Ca, U/Ca, or ∆14C
determinations in modern corals do not extend back beyond the mid-16th century.
This is due to the fact that most still growing massive corals which can be found
in the modern reefs are not older than about 200 to 400 years. Furthermore, these
centuries-old coral colonies are usually quite rare in most reef environments.
Most long coral-based paleoclimatic records are the result of extensive surveys
to discover the biggest colony of an area.
The generation of century-long stable isotope records in annual resolution
started in the 1980s with a coral from the Philippines (Pätzold, 1986). The longest
coral stable isotope or trace element time series available are a 347-year record
from Galápagos (Ecuador) (Dunbar et al., 1994) and a composite 420-year record
from the Great Barrier Reef (Australia) (Hendy et al., 2002) with annual and 5yearly resolution, respectively. Most recent studies exploit the clear seasonal
resolution that corals can provide. However, of the published coral records which
are currently archived in the World Data Center for Paleoclimatology only 8 have
seasonal or higher resolution beyond 1850 (Fig. 1). The records in Fig. 1 are based
on δ18O with the exception of the Rarotonga record which is based on Sr/Ca
(Linsley et al., 2000). The latter was generated by applying a newly developed
method for rapid analysis of high-precision Sr/Ca ratios in corals (Schrag, 1999).
Subsequently, a δ 18O time series was generated from the Rarotonga record (Ren
et al., 2002) which is not shown in Fig. 1. These coral records cover a latitudinal
range of ~28°N to ~29°S in the Indian and Pacific Oceans (Fig. 2). Most of these
records are correlated with local and regional climate variability but also reflect
aspects of large-scale climate phenomena. Many of them indicate decadal-scale
climate variability.
The bimonthly resolution Ras Umm Sidd coral δ18O record from the northern
Red Sea (Sinai, Egypt) (Felis et al., 2000) extends back to 1750 and is correlated
with instrumental observations of Middle East climate which in turn is related to
large-scale Northern Hemisphere atmospheric variability. On interannual to
multidecadal timescales the coral δ18O variations apparently reflect varying
proportions of both SST and δ18O seawater variability. The narrow northern Red
Sea represents a relatively continental setting which is strongly influenced by
mid-latitude climate and is very sensitive to atmospheric processes due to a
relatively weak stratification of the water column. Northern Red Sea SST and
coral δ18O variability are controlled by a high-pressure anomaly over the
Mediterranean Sea associated with the AO/NAO which favours an anti-cyclonic
flow of surface winds in the eastern Mediterranean, resulting in an advection of
relatively cold air from southeastern Europe (Rimbu et al., 2001). Colder and
more arid conditions in the northern Red Sea correspond to a high index state of
the AO/NAO and vice versa (Fig. 3) (Felis et al., 2000; Rimbu et al., 2001).
Strong correlations between the coral record and instrumental indices of AO/
Climate Reconstructions from Annually Banded Corals
1650
1700
1750
1800
1850
1900
1950
-4.0
2000
1 Ras Umm Sidd
Northern Red Sea (Egypt)
N
(Felis et al., 2000)
-3.4
warmer/wetter
213
-2.8
-6.4
2 Secas Island
Eastern Pacific Ocean (Panama)
-5.8
VPDB)
(Linsley et al., 1994)
-5.2
-5.2
(
3 Maiana
Western Pacific Ocean (Kiribati)
18O
-4.6
Coral
(Urban et al., 2000)
-4.0
-5.3
cooler/drier
4 Mahe
Western Indian Ocean (Seychelles)
-4.7
(Charles et al., 1997)
-4.1
-5.5
5 Espiritu Santo Island
Western Pacific Ocean (Vanuatu)
-4.9
warmer/wetter
6 Rarotonga
Central Pacific Ocean (Cook Islands)
26
(Linsley et al., 2000)
23
-4.8
7 Amedee Lighthouse
Western Pacific Ocean (New Caledonia)
-4.2
(Quinn et al., 1998)
-3.6
-4.5
8 Houtman Abrolhos Islands
cooler/drier
Coral
18O
(
VPDB)
Sr/Ca SST ( C)
(Quinn et al., 1996)
-4.3
29
Eastern Indian Ocean (Australia)
-3.9
(Kuhnert et al., 1999)
-3.3
1650
1700
1750
1800
1850
1900
1950
2000
Year
Fig. 1. Coral δ 18O and Sr/Ca records in seasonal or higher resolution extending back beyond 1850
which are archived at the World Data Center for Paleoclimatology (http://www.ngdc.noaa.gov/
paleo/corals.html). Thick white lines represent 3-year running means.
214
T. FELIS and J. PÄTZOLD
1
2
3
4
8
5
7
6
Fig. 2. Map showing locations of long high-resolution coral records as shown in Fig. 1.
NAO, North Pacific climate variability, and ENSO are evident at interannual
periods of 5–6 years (Felis et al., 2000). Further dominant oscillations in the coral
time series with periods of about ~70 years and 22–23 years are probably related
to variations in the North Atlantic thermohaline circulation (Delworth and Mann,
2000) and in the PDO (Mantua et al., 1997; Biondi et al., 2001), respectively.
Large-scale Northern Hemisphere atmospheric variability is most strongly
documented in the winter time series (January–February) of the Ras Umm Sidd
coral δ18O record (Rimbu et al., 2001), which contains also important information
on the non-stationarity in the ENSO teleconnections over Europe and the Middle
East during the pre-instrumental period (Rimbu et al., 2003).
The Secas Island coral δ 18O record (10 samples/year) from the eastern
Pacific Ocean (Panama) (Linsley et al., 1994) primarily reflects δ18O seawater
variability which is controlled by changes in precipitation. The precipitation
pattern in this part of Central America is related to seasonal and interannual
variability in the latitudinal position of the Intertropical Convergence Zone. The
coral time series extending back to 1707 is dominated by strong decadal variability.
Coral δ18O records from the western equatorial Pacific primarily reflect δ 18O
seawater variability mainly driven by changes in precipitation with minor
contributions from relatively small changes in SST. During El Niño events
increased precipitation associated with the eastward migration of the Indonesian
Low and advection of fresher and slightly warmer surface waters, resulting from
the eastward expansion of the western Pacific warm and fresh pool, combine to
generate strong coral δ18O anomalies. Relatively cool and dry conditions during
La Niña events produce coral δ18O anomalies of opposite sign. Therefore corals
from this region are excellent recorders of ENSO variability (Cole et al., 1993;
Urban et al., 2000).
The bimonthly resolution Maiana coral δ18O record from the western
equatorial Pacific (Kiribati) (Urban et al., 2000) is strongly correlated with
instrumental indices of ENSO variability (Fig. 4). This coral-based reconstruction
of ENSO extending back to 1840 suggests that variability in the tropical Pacific
is linked to the region’s mean climate. Cooler and drier background conditions
during the mid to late 19th century when anthropogenic greenhouse forcing was
absent were accompanied by prominent decadal variability and weak interannual
Climate Reconstructions from Annually Banded Corals
VPDB)
-3.0
1950
1960
1970
Northern Red Sea coral
1980
1990
18O
Coral
18O
-2.8
(
1940
215
-3.2
-3.4
3
0
-1
NAO index
NAO index
2
1
-2
Temperature ( C)
-3
7
6
5
4
3
2
1
0
-1
-2
1940
Central England temperature
1950
1960
1970
1980
1990
Year
Fig. 3. The winter Ras Umm Sidd coral δ 18O record from the northern Red Sea (Felis et al., 2000)
and the winter index of the North Atlantic Oscillation (NAO) (Jones et al., 1997) show common
variability. A similar variability is indicated by the winter time series of Central England temperatures
(Parker et al., 1992) suggesting that both Central England temperatures and northern Red Sea coral
δ 18O are influenced by the NAO. Coral δ 18O and Central England temperatures are for January–
February; the NAO index is for December–March. Thick lines represent 3-year running means.
variability. During a gradual transition towards warmer and wetter conditions in
the early 20th century variability with a period of ~2.9 years intensified. Between
1920 and 1955 2–4 year variability was attenuated. With an abrupt shift towards
warmer and wetter conditions in 1976 variability with a period of about 4 years
becomes prominent. The results suggest that changes in the tropical Pacific mean
climate and its variability have occurred during periods of natural as well as
anthropogenic climate forcing.
The monthly resolution Mahe coral δ 18O record from the western equatorial
Indian Ocean (Seychelles) was suggested to primarily reflect variations in
regional SST (Charles et al., 1997). Interannual variability in the coral time series
is correlated with Pacific climate records suggesting a consistent influence of
ENSO on Indian Ocean SSTs for over a century. However, the coral time series
extending back to 1846 is dominated by strong decadal variability which was
suggested to be characteristic of the Asian monsoon system, implying important
interactions between tropical and mid-latitude climate variability. This has been
216
T. FELIS and J. PÄTZOLD
VPDB)
-5.3
(
1950
-5.5
-4.9
1960
1970
Tropical Pacific coral
1980
1990
18O
18O
-4.7
Coral
-5.1
-4.3
-4.5
-4.1
2
1
0
-1
-2
SST anomaly ( C)
3
-3
1950
1960
1970
1980
1990
Year
Fig. 4. The Maiana coral δ 18O record from the western equatorial Pacific and the Niño 3.4 sea surface
temperature (SST) index, a commonly used measure of the state of the El Niño-Southern Oscillation,
are strongly correlated (Urban et al., 2000). SSTs are from Kaplan et al. (1998).
questioned recently by presenting evidence for a tropical Pacific forcing of
decadal SST variability in the western equatorial Indian Ocean inferred from an
annual-resolution coral δ 18O record from Malindi (Kenya) (Cole et al., 2000).
A seasonally-resolved coral δ18O record from Espirito Santo Island (Vanuatu)
in the western South Pacific (Quinn et al., 1996) documents the combined effects
of SST and rainfall-induced salinity changes since the year 1807. The coral time
series is dominated by decadal variability but unfortunately has several gaps.
The monthly resolution Rarotonga coral Sr/Ca record from the central
subtropical South Pacific (Cook Islands) (Linsley et al., 2000) was suggested to
provide a proxy for regional SST variability. The coral time series extending back
to 1726 is dominated by decadal variability. Several of the largest-scale SST
variations at Rarotonga a coherent with SST regime shifts in the North Pacific,
as indicated by the index of the PDO over the past 100 years. Because of this
hemispheric symmetry it was suggested that tropical forcing may play an
important role in the decadal variability which is observed in the Pacific Ocean
(Linsley et al., 2000; Evans et al., 2001).
The seasonal resolution Amedee Lighthouse coral δ18O record from the
western South Pacific (New Caledonia) (Quinn et al., 1998) is correlated with
variations in local and regional SST. The coral time series extending back to 1657
shows prominent decadal fluctuations, especially in the early 18th and early 19th
century. Interannual-scale cooling events in the coral record coincide within 1
year with known volcanic eruptions (Crowley et al., 1997), e.g., 1808 (unknown
source), 1813–1821 (several eruptions including Tambora 1815), 1835
(Coseguina), 1883 (Krakatau), and 1963 (Agung).
Climate Reconstructions from Annually Banded Corals
1950
1960
1970
1980
1990
217
2000
200
Rarotonga coral
14C
Coral
14C
( )
150
100
Nauru coral
50
0
Galápagos coral
14C
14C
-100
1950
1960
1970
1980
1990
3
2
1
0
-1
-2
-3
2000
SST anomaly ( C)
-50
Year
Fig. 5. Coral based ∆ 14C records from different locations in the Pacific Ocean. Rarotonga,
subtropical South Pacific (Guilderson et al., 2000); Nauru, western equatorial Pacific (Guilderson
et al., 1998); Galápagos, eastern equatorial Pacific (Guilderson and Schrag, 1998). The long-term
trend in the coral ∆ 14C records reflects the uptake of bomb 14C in the ocean. Interannual variability
in the equatorial records reflects changes in upwelling and surface circulation which in turn are
related to the El Niño-Southern Oscillation (ENSO) phenomenon. The Niño 3.4 sea surface
temperature index, a commonly used measure of the state of ENSO, is also shown. SSTs are from
Kaplan et al. (1998).
The bimonthly resolution Houtman Abrolhos Islands coral δ 18O record from
the eastern subtropical Indian Ocean (Australia) (Kuhnert et al., 1999) is correlated
to local SST variability. The location is influenced by the Leeuwin Current which
is coupled to the Indonesian throughflow. The coral time series extending back
to 1795 shows prominent pentadal and decadal variability.
A coral Sr/Ca, U/Ca, and δ 18O record from the Great Barrier Reef (Australia)
with 5-yearly resolution provides information on SST and salinity variations in
the southwestern Pacific since the year 1565 (Hendy et al., 2002). The composite
420-year coral time series indicates that SST and salinity were higher in the 18th
century than in the 20th century. An abrupt freshening accompanied by a cooling
is observed after 1870. These findings are supported by other coral-based
reconstructions from the southwestern Pacific (Druffel and Griffin, 1993; Quinn
et al., 1996, 1998; Linsley et al., 2000). It was suggested that the reconstructed
higher salinities during the period 1565–1870 were due to the combined effect of
advection and wind-induced evaporation as a result of a stronger latitudinal
temperature gradient and intensified circulation during the so-called Little Ice
Age (Hendy et al., 2002).
218
T. FELIS and J. PÄTZOLD
The longest coral-based ∆14C time series available is a 323-year record from
the Great Barrier Reef which has a biannual resolution (Druffel and Griffin,
1993). More recent studies on Pacific corals provide seasonal to higher resolution
records which give information on the seasonal to interannual variability in the
surface circulation and thermocline structure in the Pacific basin (Fig. 5).
The Galápagos coral ∆14C record from the eastern equatorial Pacific (Ecuador)
(Guilderson and Schrag, 1998) documents the seasonal upwelling of low ∆14C
subsurface waters. The interannual variability of the coral time series is dominated
by ENSO. During El Niño events the depth of the thermocline increases and the
upwelling of low ∆14C water is reduced. The coral record shows that ∆14C values
during the upwelling season increased abruptly after the El Niño event of 1976.
This suggests a reduction in the contribution of deeper, lower ∆14C water to the
upwelling since 1976, and together with a simultaneously occurring shift in
upwelling season SSTs was interpreted as a shift in the vertical thermal structure
of the eastern tropical Pacific towards a deepened thermocline.
The Nauru coral ∆14C record (Guilderson et al., 1998) documents the
interannual redistribution of surface waters in the western equatorial Pacific
which is the result of mixing between waters of subtropical origin (higher ∆14C)
and water upwelled in the eastern equatorial Pacific (lower ∆14C) then advected
zonally by equatorial currents. The interannual variability in the coral time series
is dominated by ENSO. During El Niño events coral ∆14C values increase,
reflecting the reduction of low ∆14C water upwelling in the eastern Pacific and the
invasion of high ∆14C subtropical water into the western equatorial Pacific.
A coral Ba/Ca record provides information on the sediment flux to the inner
Great Barrier Reef (Australia) as a result of river floods for the period 1750–1998
(McCulloch et al., 2003). The coral time series has an approximately weekly
resolution and was generated by using laser ablation inductively coupled plasma
mass spectrometry specifically adapted for coral studies. Following the beginning
of European settlement a substantially increase in the delivery of sediments is
recorded after about 1870. This was attributed to land-use practices associated
with European settlement leading to degradation of the semi-arid river catchments.
4.2 Last millennium
Because modern corals are usually not growing for more than 200 to 400
years, fossil corals provide an opportunity to reconstruct climate variability
during time-windows throughout the last millennium. However, fossil corals
from the early- and mid-last millennium that provide records of several decades
or more are quite rare. One possible reason could be that, in the living reefs, most
dead corals are rapidly affected by bioerosion.
A seasonal-resolution coral δ18O and Sr/Ca record from Bermuda in the
subtropical North Atlantic provides information on regional SST variability for
an 84-year time window during the 16th century (Kuhnert et al., 2002). This
record was generated from a dead coral which has been subsequently overgrown
by a living coral. The dead section was dated with the 230Th/234U method and
covers the period 1520–1603 (±15 years). An oscillation in this 16th-century
Climate Reconstructions from Annually Banded Corals
219
coral δ 18O time series with a period of ~7.8 years is also observed in regional
SSTs during the last century where it is associated with the AO/NAO. The results
suggest that similarly to present-day the AO/NAO was influencing Bermuda
SSTs at interannual to decadal timescales during this period of the Little Ice Age.
Monthly resolved coral δ18O records from Palmyra Island provide information
on climate variability in the central tropical Pacific and ENSO for time windows
during the last 1100 years (Cobb et al., 2001, 2003a). Next to a record of a modern
coral these records were generated from storm-derived deposits of fossil corals
collected from ocean-facing beaches. The individual fossil corals provide time
windows of 30–90 years. Based on high-precision 230Th/234U dates (Cobb et al.,
2003b) and high coral-to-coral reproducibility the splicing of overlapping coral
δ18O sequences provided four time windows of up to 150 years starting in the year
928. The coral records indicate relatively cold and dry mean climate conditions
during the 10th century and increasingly warmer and wetter conditions in the 20th
century. However, the behaviour of ENSO is poorly correlated with these
estimates of mean climate, with the most intense ENSO activity occurring during
the mid-17th century. The Palmyra coral records imply that the majority of ENSO
variability over the last millennium may have arisen from dynamics internal to
the ENSO system itself (Cobb et al., 2003a).
4.3 Holocene
Well-preserved fossil corals can provide records of climate variability
comparable in resolution and quality to those derived from modern corals for
time-windows throughout the Holocene. However, finding a fossil Holocene
coral that provides a century-long record is even more difficult than finding a
comparable modern coral in the living reefs. Fossil corals are usually collected
from uplifted or submerged reefs mostly by drilling. The chance to recover a long
core along the major growth axis of a single coral by drilling into a fossil reef flat
is rather small. Direct collection or coring of individual fossil colonies in the field
might provide a better solution but is possible only at some rare locations.
The nearly monthly resolved Eilat coral δ18O records from the northern Red
Sea (Israel) show an increased seasonality for time-windows during the MidHolocene compared to modern corals (Moustafa et al., 2000). The records of up
to 18 years length were derived from corals which grew around 6000 to 4500 14C
years before present (BP). The results suggest an increased SST seasonality and/
or changes in the seasonal cycle of evaporation and/or precipitation relative to
present-day during periods of the mid-Holocene.
Monthly-resolved coral Sr/Ca records from the southwest tropical Pacific
(Vanuatu) provide information on the temperature variability during timewindows of up to 6 years during the early- to mid-Holocene (Beck et al., 1997).
The corals grew around 10,300 to 4,200 calendar years BP and the Sr/Ca signal
suggests that SSTs during the early Holocene in this part of the tropics (~16°S)
were about 6.5°C cooler than today, but increased abruptly during the following
1500 years. However, this result needs verification.
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T. FELIS and J. PÄTZOLD
A coral from Tasmaloum (Vanuatu) which grew around 4150 calendar years
BP provides a monthly resolved 47-year record of SST variability based on coral
Sr/Ca and U/Ca (Corrège et al., 2000). Composite coral Sr/Ca and U/Ca derived
temperatures suggests that SSTs in the southwest tropical Pacific during the midHolocene were comparable to modern SSTs with respect to the mean as well as
typical ENSO variability. However, the variability in the seasonal amplitude as
well as interannual SST variability during the mid-Holocene were considerably
stronger than today. The coral time series shows several prominent interannual
cooling events occurring at decadal-scale intervals as well as a decadal-scale
modulation of the seasonal cycle. The results could be interpreted in a way that
phase shifts in the ENSO mode similar to today also occurred during the midHolocene but probably with stronger exchanges between the tropics and the
extratropics.
Combined δ 18O and Sr/Ca records in fortnightly resolution derived from
Great Barrier Reef corals (Australia) provide important information on the
temperature and surface-ocean hydrologic balance during the mid-Holocene
(Gagan et al., 1998). Coral Sr/Ca ratios indicate that the tropical western Pacific
was 1°C warmer ~5350 calendar years BP ago. The residual coral δ18O signal as
derived from the difference between the Sr/Ca and δ18O records indicates that the
δ18O of the surface water was 0.5‰ higher relative to modern seawater. The
results suggest that the higher temperatures increased the evaporation resulting
in higher δ 18O seawater. This δ 18O seawater anomaly may have been sustained by
transport of part of the additional water vapor to extratropical latitudes.
Fortnightly to weekly resolved coral Sr/Ca- and δ 18 O-based SST
reconstructions from western Sumatra (Indonesia) document anomalous upwelling
events during periods of the mid-Holocene associated with the so-called Indian
Ocean Dipole (IOD). The reconstructed mid-Holocene IOD events exceed the
strong 1997 event in magnitude (Abram et al., 2003). However, coral reef
mortality as observed during this modern event is not documented for the past
7000 years, as inferred from the absence of growth unconformities in a suite of
48 coral cores.
4.4 Pleistocene
Because the growth of reef-building corals is restricted to the surface ocean
the dating of fossil corals from uplifted or submerged Pleistocene reefs can
provide a record of sea-level fluctuations (Bard et al., 1990; Gallup et al., 1994;
Chappell et al., 1996; Stirling et al., 2001; Yokoyama et al., 2001; Lambeck et
al., 2002; Cutler et al., 2003). Furthermore, well-preserved fossil corals can
provide records of climate variability during time-windows of the Pleistocene
comparable in resolution and quality to those derived from modern corals. Next
to the difficulties in finding a long-lived fossil colony, corals from uplifted
Pleistocene reef terraces are often affected by diagenesis making them not
suitable for paleoclimatic reconstructions based on stable isotopes or trace
elements.
Climate Reconstructions from Annually Banded Corals
221
Coral δ18O and Sr/Ca records derived from cores recovered from the
submerged Barbados offshore reefs indicate that SSTs in the western equatorial
Atlantic were 4.5–5°C colder than present values during the late glacial and the
last glacial maximum (LGM) 18,000 to 24,000 years ago (Guilderson et al., 1994,
2001), a finding in conflict with the CLIMAP reconstructions (CLIMAP Project,
1976, 1981).
A suite of fossil corals from the uplifted reef terraces of the Huon Peninsula
(Papua New Guinea) provides information on ENSO-related rainfall and SST
variations in the western Pacific Warm Pool for time windows of several decades
during the last 130,000 years (Tudhope et al., 2001). The bimonthly to monthly
resolved coral δ18O records suggest that ENSO was active at interannual timescales
even during periods of high latitude glaciation and substantially lower global sea
level (38,000–42,000, 85,000, 112,000, and 130,000 years ago). The coral
records suggest that ENSO variability was relatively weak during the midHolocene but strongest during the 20th century with respect to the past 130,000
years. A combined effect of ENSO dampening during glacial conditions and
ENSO forcing by precessional orbital variations was suggested to explain the
observed changes in amplitude (Tudhope et al., 2001). Coral Sr/Ca values of one
of these corals indicate that SSTs were 6°C colder during the penultimate
deglaciation than either last interglacial or present-day temperatures in this
region of the tropics (McCulloch et al., 1999). This result again raises the
question whether the tropics underwent significant cooling during glacial periods.
However, the coral Sr/Ca values may overestimate the cooling by 1–3°C (Tudhope
et al., 2001) due to proposed glacial-interglacial changes in oceanic Sr/Ca (Stoll
and Schrag, 1998).
Monthly resolved coral δ 18O and Sr/Ca records derived from a fossil colony
collected on an uplifted reef terrace on Bunaken Island, North Sulawesi (Indonesia),
reflect interannual variability in precipitation and SST in the western equatorial
Pacific during the last interglacial period 124,000 years ago (Hughen et al.,
1999). The 65-year long coral time series reveals ENSO variability similar to the
modern instrumental record. This indicates that ENSO was robust during the last
interglacial, a time when global climate was slightly warmer than Holocene.
However, changes in ENSO magnitude and frequency after 1976 appear different
with respect to the earlier instrumental and last interglacial records. The results
were interpreted to support the hypothesis that ENSO behaviour in recent decades
is anomalous with respect to natural variability.
A coral δ18O record of a fossil colony from Ryukyu Islands (Japan) in the
northwest Pacific indicates an increased seasonality during the last interglacial
(127,000 years ago) relative to modern corals from the region (Suzuki et al.,
2001), most likely resulting from SST changes. This increased seasonality, which
is also observed in the δ 13C data of the 10-year long coral time series, was
attributed to an enhanced seasonal cycle of insolation on the Northern Hemisphere
at that time, resulting from changes in the Earth’s orbital parameters.
It was suggested that the atmospheric ∆14C is linked to abrupt changes in
oceanic ventilation which occurred during the last deglaciation, and that these
222
T. FELIS and J. PÄTZOLD
variations can be reconstructed from fossil corals cross-dated by radiocarbon and
the 230Th/234U method (Bard, 1998). The ∆14C of fossil corals from Huon
Peninsula (Papua New Guinea) was used to reconstruct changes in atmospheric
radiocarbon during periods of the Pleistocene (Yokoyama et al., 2000; Yokoyama
and Esat, 2004). Peaks of coral ∆14C which are related to episodes of sea-level rise
and reef growth at Huon Peninsula appear to be synchronous with Heinrich events
and concentrations of ice-rafted debris in North Atlantic sediments. It was
suggested that the connection factor between these events is the interruption of
the North Atlantic thermohaline circulation following periodic partial disruptions
of the Laurentide ice sheet.
5. DIRECTIONS IN CORAL-BASED PALEOCLIMATOLOGY
More multicentury coral records in at least seasonal resolution are necessary
to provide a more complete picture on global patterns of seasonal, interannual,
and decadal-scale climate variability during the last millennium. Apparently a
bimonthly sampling resolution for long coral cores seems to be a good compromise
with respect to significant detection of climate variability on these timescales as
well as laboratory expenditure (e.g., Felis et al., 2000; Urban et al., 2000).
Applying the double tracer technique of combined δ18O and Sr/Ca (or U/Ca)
determinations to multicentury coral records will provide insights into the
variability of temperature and hydrologic balance at key locations of the global
climate system during the past millennium (e.g., Linsley et al., 2004).
Most multicentury coral records are from sites in the equatorial or subtropical
southern Pacific, and have provided the opportunity for a coral δ 18O-based
reconstruction of SST fields in the Pacific basin for the period 1607–1990 (Evans
et al., 2002). The network of multicentury coral records in the Indian and
especially the Atlantic Ocean is still sparse but work is in progress. Recent work
from the northernmost Red Sea, one of the rare subtropical/mid-latitude locations
of coral growth has shown the potential of multicentury coral records to provide
information on past AO/NAO-related atmospheric variability over the Northern
Hemisphere (Felis et al., 2000; Rimbu et al., 2001, 2003).
The advantage of coral records compared to other natural climate archives
is a clear seasonal resolution. The disadvantage is that records generated from
living corals usually extend back for a few centuries only, with a maximum of 420
years reported for a climate reconstruction from the Great Barrier Reef (Hendy
et al., 2002). One approach to solve this problem is splicing together records of
fossil corals from the last millennium (Cobb et al., 2003a), but this will probably
be possible only at some rare sites. Another approach is to use slow-growing
corals that have the potential to provide continuous climate records spanning up
to 1000 years (Watanabe et al., 2003). However, ongoing work on such slowgrowing corals in the Atlantic Ocean, where fast-growing corals are relatively
rare, has shown that the extraction of climatic information can be complicated.
Decades- to century-long records of fossil Holocene and Pleistocene corals
can provide information on seasonal to interannual climate variability for time
Climate Reconstructions from Annually Banded Corals
223
windows during glacial-interglacial cycles (Tudhope et al., 2001). This enables
the assessment of natural variability of the climate system on these timescales
during periods with boundary conditions and forcings different from present-day,
and is critical for evaluating and improving climate models (Trenberth and OttoBliesner, 2003).
Acknowledgements—We thank A. Suzuki and Y. Yokoyama for valuable comments and
W. H. Berger and H. Fischer for comments on an earlier version of the manuscript. RCOM
contribution No. 0123.
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