effects may not have been uniform throughout the
tropical oceans.
Second, this cool phase was followed by a general
warming/freshening of the global tropical ocean
beginning during the nineteenth century (Figure 5).
This overall warming/freshening trend is observed in
seven of the nine records. The timing of the onset of
this warming/freshening is consistent with the onset
of industrialization and the consequent increases in
greenhouse gases due to increased emissions from
fossil fuel consumption. If the shift in d
18
O were
solely due to increases in SST, it would be equivalent
to an increase of 0.3–2.01C since 1800. Instrumental
data indicate that the tropics only warmed by
B0.51C since 1850. The influence of SSS on d
18 O is
probably responsible for the difference and needs to
be taken into account when interpreting d
18
O records. Although Sr/Ca ratios are thought to be unaffected by SSS, only a few shorter coral records are
currently published. Until recently, Sr/Ca measurements were very time-consuming. With recently
developed technology, the use of Sr/Ca as a paleothermometer proxy should increase. Two main
limitations exist with the correct interpretation of
decadal and long-term d
18 O trends: (1) an interdecadal cycle of unknown origin is commonly
identified in long coral d
18
O records; and (2) longterm trends of increasing d
18
O are observed in some
coral while other coral d
18 O records show a decreasing trend. Whether these trends are due to
biological processes or are the result of gradual
environmental changes (i.e., global warming) is
unclear.
Finally, evidence of increased fossil fuel emission
into the atmosphere can be seen in the general decrease in D
14 C from 1850 to 1955 in shallow corals
from the Atlantic and Pacific Oceans. This phenomenon, referred to as the Suess Effect, is mainly
the result of
14 C-free CO 2 produced from combusted
fossil fuel entering the atmosphere, the oceans and
eventually, the coral skeleton (post-1950, coral D
14
C
values skyrocketed as a result of
14 C produced by
thermonuclear bombs effectively swamping out the
Suess effect).
Fossil Corals
Fossil corals provide windows into past climate.
Records covering several decades to centuries offer
the opportunity to compare the same three components (seasonal, interannual-to-decadal, and long
term) in climate in the distant past to the present. A
3.0 million-year-old south-western Florida coral reveals a seasonal d
18
O derived temperature pattern
similar to today but B3.51C cooler. A North Sulawesi, Indonesian coral indicates that 124 000 years
BP the variability in ENSO was similar to modern
ENSO frequency from 1856 to 1976. However,
the shift in ENSO frequency observed in modern
records after 1976 is not found in the fossil coral
record nor in pre-1976 instrumental records. This
suggests that the current state of ENSO frequency is
outside of the natural range of ENSO variability.
Perhaps anthropogenic effects are having an effect
on ENSO frequency. Finally, long-term changes in
climate can also be reconstructed from fossil coral
records. A series of coral records from Vanuatu indicate that B10 300 years BP the south-western
tropical Pacific was 6.51C cooler than today followed by a rapid rise in temperature over the subsequent 15 000 years. This rapid rise in temperature
lags the post-Younger Dryas warming of the Atlantic
by B3000 years suggesting that the mechanism for
deglacial climate change may not have been globally
uniform. How seasonal, decadal (ENSO) and longterm climate changes varied in the distant past
throughout the tropics can be addressed using fossil
coral records and can offer us a better idea of the
natural variability in tropical climate over geologic
time.
Deep-sea Corals
Deep sea corals do deposit calcium carbonate exoskeleton but do not contain endosymbiotic zooxanthellae and are not colonial. Their isotopic and
trace mineral composition reflects variation in ambient conditions on the seafloor. Although this does
not directly reflect changes in climate on the surface,
ocean circulation patterns are tightly coupled with
atmospheric climatic conditions. Understanding the
history of deep and intermediate water circulation
lends itself to a better understanding of climate. For
example, the origin of the Younger Dryas cooling
event (13 000 to 11 700 years BP) has recently been
attributed to a cessation or slowing of North Atlantic
deep water formation and subsequent reduction in
heat flux. Isotopic evidence from deep-sea corals
suggests that profound changes in intermediatewater circulation also occurred during the Younger
Dryas. Other studies of deep-sea corals show rapid
changes in deep ocean circulation on decadal-tocentennial timescales at other intervals during the
last deglaciation. Reconstructing intermediate and
deep ocean circulation patterns and their relationship
to climate using isotopic, trace element and minor
element records in deep sea coral promises to be an
expanding line of paleoclimate research.
430 PAST CLIMATE FROM CORALS
tropical oceans.
Second, this cool phase was followed by a general
warming/freshening of the global tropical ocean
beginning during the nineteenth century (Figure 5).
This overall warming/freshening trend is observed in
seven of the nine records. The timing of the onset of
this warming/freshening is consistent with the onset
of industrialization and the consequent increases in
greenhouse gases due to increased emissions from
fossil fuel consumption. If the shift in d
18
O were
solely due to increases in SST, it would be equivalent
to an increase of 0.3–2.01C since 1800. Instrumental
data indicate that the tropics only warmed by
B0.51C since 1850. The influence of SSS on d
18 O is
probably responsible for the difference and needs to
be taken into account when interpreting d
18
O records. Although Sr/Ca ratios are thought to be unaffected by SSS, only a few shorter coral records are
currently published. Until recently, Sr/Ca measurements were very time-consuming. With recently
developed technology, the use of Sr/Ca as a paleothermometer proxy should increase. Two main
limitations exist with the correct interpretation of
decadal and long-term d
18 O trends: (1) an interdecadal cycle of unknown origin is commonly
identified in long coral d
18
O records; and (2) longterm trends of increasing d
18
O are observed in some
coral while other coral d
18 O records show a decreasing trend. Whether these trends are due to
biological processes or are the result of gradual
environmental changes (i.e., global warming) is
unclear.
Finally, evidence of increased fossil fuel emission
into the atmosphere can be seen in the general decrease in D
14 C from 1850 to 1955 in shallow corals
from the Atlantic and Pacific Oceans. This phenomenon, referred to as the Suess Effect, is mainly
the result of
14 C-free CO 2 produced from combusted
fossil fuel entering the atmosphere, the oceans and
eventually, the coral skeleton (post-1950, coral D
14
C
values skyrocketed as a result of
14 C produced by
thermonuclear bombs effectively swamping out the
Suess effect).
Fossil Corals
Fossil corals provide windows into past climate.
Records covering several decades to centuries offer
the opportunity to compare the same three components (seasonal, interannual-to-decadal, and long
term) in climate in the distant past to the present. A
3.0 million-year-old south-western Florida coral reveals a seasonal d
18
O derived temperature pattern
similar to today but B3.51C cooler. A North Sulawesi, Indonesian coral indicates that 124 000 years
BP the variability in ENSO was similar to modern
ENSO frequency from 1856 to 1976. However,
the shift in ENSO frequency observed in modern
records after 1976 is not found in the fossil coral
record nor in pre-1976 instrumental records. This
suggests that the current state of ENSO frequency is
outside of the natural range of ENSO variability.
Perhaps anthropogenic effects are having an effect
on ENSO frequency. Finally, long-term changes in
climate can also be reconstructed from fossil coral
records. A series of coral records from Vanuatu indicate that B10 300 years BP the south-western
tropical Pacific was 6.51C cooler than today followed by a rapid rise in temperature over the subsequent 15 000 years. This rapid rise in temperature
lags the post-Younger Dryas warming of the Atlantic
by B3000 years suggesting that the mechanism for
deglacial climate change may not have been globally
uniform. How seasonal, decadal (ENSO) and longterm climate changes varied in the distant past
throughout the tropics can be addressed using fossil
coral records and can offer us a better idea of the
natural variability in tropical climate over geologic
time.
Deep-sea Corals
Deep sea corals do deposit calcium carbonate exoskeleton but do not contain endosymbiotic zooxanthellae and are not colonial. Their isotopic and
trace mineral composition reflects variation in ambient conditions on the seafloor. Although this does
not directly reflect changes in climate on the surface,
ocean circulation patterns are tightly coupled with
atmospheric climatic conditions. Understanding the
history of deep and intermediate water circulation
lends itself to a better understanding of climate. For
example, the origin of the Younger Dryas cooling
event (13 000 to 11 700 years BP) has recently been
attributed to a cessation or slowing of North Atlantic
deep water formation and subsequent reduction in
heat flux. Isotopic evidence from deep-sea corals
suggests that profound changes in intermediatewater circulation also occurred during the Younger
Dryas. Other studies of deep-sea corals show rapid
changes in deep ocean circulation on decadal-tocentennial timescales at other intervals during the
last deglaciation. Reconstructing intermediate and
deep ocean circulation patterns and their relationship
to climate using isotopic, trace element and minor
element records in deep sea coral promises to be an
expanding line of paleoclimate research.
430 PAST CLIMATE FROM CORALS