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.

On behalf of the journal, AGU, and the scientific community, the Editors would like to sincerely thank those who reviewed manuscripts for Geophysical Research Letters in 2018. The hours reading and commenting on manuscripts not only improves the manuscripts but also increases the scientific rigor of future research in the field. We particularly appreciate the timely reviews, in light of the demands imposed by the rapid review process at Geophysical Research Letters. With the revival of the “major revisions” decisions, we appreciate the reviewers' efforts on multiple versions of some manuscripts. Many of those listed below went beyond and reviewed three or more manuscripts for our journal, and those are indicated in italics. In total, 4,484 referees contributed to 7,557 individual reviews in journal. Thank you again. We look forward to the coming year of exciting advances in the field and communicating those advances to our community and to the broader public.