Department of Earth System Science

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 (Δ¹⁴C) records from Palmyra (6°N, 162°W) and Christmas (2°N, 157°W) islands to constrain central tropical Pacific ocean circulation changes during the last millennium. Seasonally to annually resolved coral Δ¹⁴C measurements from the 10th, 12th–17th, and 20th centuries do not contain significant interannual to decadal-scale variations, despite large changes in coral δ ¹⁸O on these timescales. A centennial-scale increase in coral radiocarbon from the Medieval Climate Anomaly (∼900–1200 AD) to the Little Ice Age (∼1500–1800) can be largely explained by changes in the atmospheric Δ¹⁴C, as determined with a box model of Palmyra mixed layer Δ¹⁴C. However, large 12th century depletions in Palmyra coral Δ¹⁴C may reflect as much as a 100% increase in upwelling rates and/or a significant decrease in the Δ¹⁴C 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 Δ¹⁴C is only compromised after moderate to severe diagenesis for these relatively young fossil corals.