High temporal resolution Mg/Ca and Ba/Ca records in modern Porites lobata corals

Secondary ion mass spectrometry (SIMS) was used to determine Mg/Ca and Ba/Ca variations across two skeletal structures (the fasciculi and the centers of calcification, COCs) in two Porites lobata corals of different linear extension rates, from Oahu, Hawaii. Both corals exhibit large Mg and Ba heterogeneity, which is not related to sea surface temperature (SST) or composition. Higher concentrations of Ba occur in the fasciculi and higher Mg in the COCs of the slowly extending coral compared to the fast growing coral. Mg and Ba concentrations are significantly higher in the COCs of both corals compared to the fasciculi. Trace element variations may reflect changes in the calcifying fluid composition or changes in skeletal trace element incorporation. Regressions between Mg, Ba, and Sr (reported in a prior publication) indicate that the skeletal incorporation of each element is affected by a different factor or combination of factors. Smoothed SIMS records of Ba/Ca variations across the fasciculi of the fast growing coral follow the seasonal trend predicted from SST and expected seawater chemistry variations. Smoothed SIMS Mg/Ca records do not correlate with SST, indicating that SIMS Mg/Ca records cannot be used to estimate past SSTs.