Rb. Dunbar et al., EASTERN PACIFIC SEA-SURFACE TEMPERATURE SINCE 1600-AD - THE DELTA-O-18 RECORD OF CLIMATE VARIABILITY IN GALAPAGOS CORALS, Paleoceanography, 9(2), 1994, pp. 291-315
We measured stable oxygen isotope ratios and skeletal growth rates in
the massive corals Pavona clavus and P. gigantea from the west coast o
f Isabela Island, Galapagos, to assess interannual to decadal climate
variability in the eastern Pacific. Comparisons of instrumental data s
ets show that sea surface temperatures (SST) in the Galapagos region a
re representative of a broad portion of the eastern equatorial Pacific
. The site is especially well-suited for long-term studies of the El N
ino/Southern Oscillation (ENSO) phenomenon, as it lies within the east
ern Pacific ''center of action'' for thermal anomalies associated with
ENSO. The P. gigantea isotope record is nearly monthly in resolution,
spans the period 1961-1982, and shows strong correlation with a Galap
agos instrumental SST record (r = -0.90 for annual averages). Cross-sp
ectral analysis shows that SST can explain greater than 80% of the var
iance in deltaO-18 at both the annual cycle and within the high-freque
ncy portion of the ENSO band (3-5 years). The P. clavus record is annu
al in resolution, extends from 1587 to 1953 A.D., and was obtained fro
m a 10-m diameter colony preserved within the Urvina Bay uplift. Becau
se seawater deltaO-18 variations in the region are very small, we inte
rpret the Urvina Bay coral deltaO-18 record in terms of annual average
SST. The isotopic record appears to be a very good, but not perfect,
indicator of ENSO events and shows good correspondence with the histor
ical ENSO reconstruction of Quinn et al. (1987). A number of low delta
O-18 excursions that we observe during the 17th and 18th centuries ver
y likely represent ENSO events that are missing from the historical ta
bulations. Most interannual deltaO-18 variations between 1607 and 1953
A.D. represent annual average temperature excursions of 1-degrees to
2.5-degrees-C. During the Little Ice Age, the annual deltaO-18 series
correlates well with many North American tree ring records and shows l
ow temperatures during the early 1600s and early 1800s, and relatively
warmer conditions during the 1700s. Unlike most northern hemisphere t
ree ring and instrumental records, we see no evidence at this site for
warming between 1880 and 1940 but rather observe a slight cooling (<1
-degrees-C). Oscillatory modes within the ENSO frequency band dominate
the 347-year deltaO-18 time series, accounting for >28% of the total
variance. The main ENSO mode is centered at 4.6 years and accounts for
12% of the total variance. Additional significant oscillations occur
at periods of 3.3, 6, 8, 11, 17, 22, and 34 years. Both annual growth
rate and deltaO-18 show variance at periods equivalent to the solar an
d solar magnetic periods (e.g., 11 and 22 years, respectively). In add
ition, the amplitude of the 11-year deltaO-18 cycle generally varies w
ith the amplitude of the solar cycle, supporting previous suggestions
that the solar cycle may modulate interannual to decadal climate varia
bility in the tropics. The dominant oscillatory modes, both within the
ENSO and interdecadal frequency bands, shift to shorter periods from
the early to middle 1700s and again from the middle to late 1800s. Thi
s may reflect major reorganizations within the tropical ocean-atmosphe
re system and suggests that tropical Pacific climate variability is li
nked across timescales ranging from years to decades.