The potential origins and palaeoenvironmental implications of high temporal resolution d18O heterogeneity in coral skeletons

d18O was determined at high spatial resolution (beam diameter ~ 30µm) by secondary ion mass spectrometry (SIMS) across 1-2 year sections of 2 modern Porites lobata coral skeletons from Hawaii. We observe large (>2‰) cyclical d18O variations that typically cover skeletal distances equivalent to periods of ~20-30 days. These variations do not reflect seawater temperature or composition and we conclude that skeletal dδ18O is principally controlled by other processes. Calcification site pH in one coral record was estimated from previous SIMS measurements of skeletal d11B. We model predicted skeletal d18O as a function of calcification site pH, DIC residence time at the site and DIC source (reflecting the inputs of seawater and molecular CO2 to the site). We assume that oxygen isotopic equilibration proceeds at the rates observed in seawater and that only the aqueous carbonate ion in incorporated into the precipitating aragonite. We reproduce successfully the observed skeletal d18O range by assuming that DIC is rapidly utilised at the calcification site (within one hour) and that ~80% of the skeletal carbonate is derived from seawater. If carbonic anhydrase catalyses the reversible hydration of CO2 at the calcification site, then oxygen isotopic equilibration times may be substantially reduced and a larger proportion of the skeletal carbonate could be derived from molecular CO2. Seasonal skeletal d18O variations are most pronounced in the skeleton deposited from late autumn to winter (and coincide with the high density skeletal bands) and are dampened in skeleton deposited from spring to summer. We observed no annual pattern in sea surface temperature or photosynthetically active radiation variability which could potentially correlate with the coral d18O. At present we are unable to resolve an environmental cue to drive seasonal patterns of short term skeletal d18O heterogeneity.