Coral geochemical and chronological methods
The Palmyra corals were cross-sectioned into 1cm-thick slabs and subsequently X-rayed
at UCSD's Thornton Hospital. 100-200µg samples were drilled at 1mm intervals with a 1mmdiameter low-speed drill and analyzed on a Finnigan MAT 252 stable isotope mass spectrometer
equipped with a Fairbanks carbonate preparation device. The sample oxygen and carbon isotopic
ratios are reported as per mil deviations from the PDB standard according to the convention:


 18O‰  


 O O
 O O
18
16
18
16
 
 
sample
  1  1000
 
PDB 

We used an internal laboratory standard made of ground, homogenized Porites coral to
monitor the long-term precision of the 18O measurements, which we report as ±0.05‰ (1,
N=550).
In general, the coral 18O depth series were converted into chronologies using two
constraints: 1) a ~0.2‰ seasonal cycle, and 2) growth rates that averaged 18mm/yr. By
comparing subannual features of the modern coral 18O record with the instrumental record of
climate for the period 1981-1998, we determined that the lowest 18O value of a given year
occurs on August 26th ± 57 (1) days, while the highest 18O value occurs on January 15th ± 43
(1) days. We used these two tie-points to construct chronologies for the modern and fossil coral
18O records. In the rare years that a seasonal 18O cycle was poorly resolved in the fossil corals,
we used growth rate constraints to determine which points to choose as the chronological tiepoints, as growth rates in the corals were relatively stable at 18mm/yr. Ages were assigned
between tie-points by linear interpolation for an average of 18 samples/yr. All coral 18O data in
this manuscript are presented at monthly resolution (12 samples/yr).