DOI: 10.25921/wbae-7x06
title: NOAA/WDS Paleoclimatology - Kurrimine Beach, Brook Island, Britomart Reef, Great Palm Island, Lodestone Reef, Pandora Reef, Havannah Island - del18O, Sr/Ca, and U/Ca Data

description: A 420-year history of strontium/calcium, uranium/calcium, and oxygen isotope ratios in eight coral cores from the Great Barrier Reef, Australia, indicates that sea surface temperature and salinity were higher in the 18th century than in the 20th century. An abrupt freshening after 1870 occurred simultaneously throughout the southwestern Pacific, coinciding with cooling tropical temperatures. Higher salinities between 1565 and 1870 are best explained by a combination of advection and wind-induced evaporation resulting from a strong latitudinal temperature gradient and intensified circulation. The global Little Ice Age glacial expansion may have been driven, in part, by greater poleward transport of water vapor from the tropical Pacific.
description: Coral Sr/Ca, U/Ca and delta-18O records were replicated from eight massive Porites colonies, spanning 120 to 420 years of continuous growth. The cores were collected at seven reefs, 12 to 120 km apart, from the central Great Barrier Reef, Australia and drilled between 1984 and 1988. The use of multiple cores allowed us to test the fidelity of individual tracers over century time scales and to establish regional-scale proxy climate signals. A 'master' reconstruction for each of the three proxies was calculated by normalizing all records relative to the longest continuous record, and then taking the average of all eight cores to construct the delta-18O composite record, while the U/Ca and Sr/Ca composite records were calculated from the average of seven cores. A confidence envelope was calculated using 95% confidence intervals for each pentannual period. A 373-year chronology for eight, multi-century, Porites coral cores was developed using cross-dating techniques adapted from dendrochronology.  Characteristic patterns of distinct luminescent (fluorescent) lines within the coral skeletons were matched between coral cores. Skeleton-plots of luminescent banding were produced for each core and combined into a master chronology back to 1615. Density banding in x-radiographs were counted to provide an independent check on the age model. This method controlled the dating difficulties caused by growth orientation, skeletal discontinuities and breaks between core sections. For further details on age model development see Hendy, Gagan and Lough 'Chronological control of coral records using luminescent lines and evidence for non-stationary ENSO teleconnections in northeastern Australia' (submitted to 'The Holocene' in 2001). Five-year bulk increments were sampled for equivalent periods across all eight cores from 1565-1570 to 1980-1985 (Austral winter to winter).  A fine powder was milled from 2 mm sampling grooves along the center of the maximum growth axis. A preliminary cleaning cut, along the sample track, removed the surface 1 mm. Duplicate samples of 150-200 µg were analyzed for d18O by an individual-carbonate (Kiel) device attached to an automated Finnigan MAT 251 mass spectrometer (mean standard deviation (sd) between duplicates was 0.04 permil, n = 1035). Calibration to the Vienna Peedee belemnite (V-PDB) is through the National Bureau of Standards NBS-19, which was analyzed 6 times in each 24 hour run with an average sd of 0.04‰ over the 3-year period (n = 281). From a solution of 1 mg of coral powder dissolved in 5 ml of 0.2 M HCl, an aliquot of 50 µl was removed for Sr/Ca determination by isotope dilution using spikes of 43Ca and 84Sr and analyzed on a Finnigan MAT 261 TIMS.  Average Sr/Ca sd between replicated coral samples is equivalent to 0.15°C (sd = 4.6 µmol/mol, total n = 550; internal laboratory standard, sd = 3 µmol/mol, n = 21). U/Ca analysis was undertaken by isotope dilution on the remaining coral solution, using a mixed spike including 45Sc (as a proxy for 45Ca) and 235U, and analyzed on a Fisons PQ II ICP-MS. U/Ca analyses were internally calibrated to a laboratory coral standard run at either end of a 7-sample block.  Replication of an internal laboratory reference coral gave a sd of 5 nmol/mol (equivalent to 0.2°C, n = 56). The 420-year history of strontium/calcium, uranium/calcium, and oxygen isotope ratios in eight coral cores indicates that sea surface temperature and salinity were higher in the 18th century than the 20th century. An abrupt freshening after 1870 occurred simultaneously throughout the southwestern Pacific, coinciding with cooling tropical temperatures.  Higher salinities between 1565 and 1870 are best explained by a combination of advection and wind-induced evaporation resulting from a strong latitudinal temperature gradient and intensified circulation. The global Little Ice Age glacial expansion may have been driven, in part, by greater poleward transport of water vapor from the tropical Pacific. UPDATE: June 2017 with d18O_resid data. For d18O anomaly residual propagation of total uncertainties gives an average 95% CI of ±0.15° from 1700 to 1780 and ±0.11° from 1780 to 1985