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Zaunbrecher, LK, Cobb KM, Beck JW, Charles CD, Druffel ERM, Fairbanks RG, Griffin S, Sayani HR.  2010.  Coral records of central tropical Pacific radiocarbon variability during the last millennium. Paleoceanography. 25   10.1029/2009pa001788   AbstractWebsite

The relationship between decadal to centennial changes in ocean circulation and climate is difficult to discern using the sparse and discontinuous instrumental record of climate and, as such, represents a large uncertainty in coupled ocean-atmosphere general circulation models. We present new modern and fossil coral radiocarbon (Delta(14)C) records from Palmyra (6 degrees N, 162 degrees W) and Christmas (2 degrees N, 157 degrees W) islands to constrain central tropical Pacific ocean circulation changes during the last millennium. Seasonally to annually resolved coral Delta(14)C measurements from the 10th, 12th-17th, and 20th centuries do not contain significant interannual to decadal-scale variations, despite large changes in coral delta(18)O on these timescales. A centennial-scale increase in coral radiocarbon from the Medieval Climate Anomaly (similar to 900-1200 AD) to the Little Ice Age (similar to 1500-1800) can be largely explained by changes in the atmospheric Delta(14)C, as determined with a box model of Palmyra mixed layer Delta(14)C. However, large 12th century depletions in Palmyra coral Delta(14)C may reflect as much as a 100% increase in upwelling rates and/or a significant decrease in the Delta(14)C of higher-latitude source waters reaching the equatorial Pacific during this time. SEM photos reveal evidence for minor dissolution and addition of secondary aragonite in the fossil corals, but our results suggest that coral Delta(14)C is only compromised after moderate to severe diagenesis for these relatively young fossil corals.