NOAA/WDS Paleoclimatology - Barbados Coral Oxygen Isotopes

STUDY NOTES: Coral-based delta18O time-series derived from measurements made on submerged or drowned coral reef sequence. The core samples used have been described elsewhere (cf. Fairbanks, 1989; Guilderson et al., 1994). Measurements were made on three different species of corals: the reef-crest Acropora palmata and the mixed or buttress zone species Montastrea annularis and Porites asteroides. Linear growth rates of individual corals were determined by using the low density to high-density band distance as an approximate annual chronometer. To minimize growth-rate kinetic induced effects, we only analyzed specimens with similar linear growth rates and from the same relative position within the corals. Typical linear extension rates were 5mm/year for all specimens analyzed and ranged from 3 to 7 mm/year. Two to four parallel transects encompassing 4-8 years per transect were extracted using a micro dental drill along the main vertical growth axis. To minimize the potential for skeletal inhomogeneities, we attempted to only sample theca (walls), except for the P. asteroides specimens whos e skeletal architecture is too fine for theca sampling. A. palmata specimens were only sampled along the upper growing surface as deduced from the skeletal architecture observed in x-radiographs. We used x-ray diffraction analysis of drilled and ground specimens to confirm that the aragonite was pristine as indicated by the absence of any calcite peaks (detection limit 0.5 weight percent). The sampling protocol did not convert aragonite to calcite. Living specimens collected over the last ~20 years were treated in a similar fashion to the offshore cores. Samples (100-200µg) were reacted at 90°C in a Carousel-48 device and analyzed on a Finnigan MAT 251 isotope ratio mass spectrometer. Calibration is through secondary standards calibrated via NBS (now NIST SRM) standards 16, 17, 19, and 20. Analytical precision of concurrently analyzed standards is better than ±0.05‰ (1-sigma) for both oxygen and carbon. Data is reported in d-notation relative to Pee Dee Belemnite using the defined values of NBS-20 (d18O -4.14‰ PDB, d13C -1.06‰ PDB). We have not corrected the absolute values of our results for the "acid-alpha" difference between aragonite (samples) and calcite (standards). Radiocarbon and 230Th/234U chronological control is provided by previously reported radiocarbon and Th/U TIMS measurements (Fairbanks 1989; 1990; Bard et al., 1993), supplemented by additional measurements (Fairbanks et al., in prep.). The majority of samples through the deglaciation have been dated with both techniques. M. annularis specimens older than 15.7 14C kyrs have yet to be Th/U dated. Absolute ages for these specimens are interpolated using a least squares regression through the appropriate coral data (Fairbanks et al., in prep.). Ice volume residual Dd18O is calculated in a similar fashion to Guilderson et al., (1994). In specimens older than the last glacial maximum as determined by the A. palmata Barbados sea level record, the residual is calculated using an ice volume component of 1.1‰.
ABSTRACT SUPPLIED BY ORIGINATOR: We have generated a detailed oxygen isotope time-series from the fossil coral reefs from offshore Barbados. The Barbados coral-based record is a unique paleoceanographic data set with an equivalent sedimentation rate in excess of 600 cm/kyr where not only is the annual signal uniquely preserved but seasonality as well. Oxygen isotope values during the late glacial and LGM (16–20 14C kyrs; 19–24 calendar kyrs) are ~2.3‰ heavier than corresponding living specimens, and indicate a regional cooling on the order of 4.5°C. There is also an isotopic expression of the Bølling-Pre-Boreal climate oscillation, with values reflecting a cooling during the Younger Dryas. Seasonality, a key diagnostic of the state of the climate system, remained the same or slightly less than present and indicates that the observed coolings were a change in the mean state of the western tropical Atlantic. Pan-tropic cooling during the last glaciation is best explained by a change in the radiative balance of the tropics. Variable tropical sea surface temperatures during climate oscillations such as the Younger Dryas challenge the paradigm that climate change only cascades from variations in North Atlantic deep water production.