δ18O comparisons of coral cores in the western tropical Pacific, Palau

Certain corals secrete calcium carbonate exoskeletons that can record changes in ocean water temperature and salinity when tested for δ18O by mass spectrometry. Records from these corals provide insight into the chemistry of the seas for times when instrumental records were not available. The composition of δ18O in coral calcium carbonate reflects both the temperature of the water and the δ18O composition of the water the corals were growing in. Calcium carbonate that is relatively high in 18O indicates that the corals were growing in ocean water with cooler temperatures and/ or higher salinity due to decreased precipitation or increased evaporation. On the other hand, a decrease in 18O indicates warmer and/ or less saline waters. Broadly speaking, changes in evaporation and precipitation are linked to El Niño/La Niña and Southern Oscillation cycles (ENSO) in the tropical Pacific. Palau, an archipelago in the western tropical Pacific, is greatly affected during El Niño years by abnormally cool waters and decreased rainfall. The opposite conditions are common during La Niña. In 2000, two coral cores were collected in a lagoon in Ngaragebal, Palau by Japanese researchers (7°24.30.3” N, 134°26’53.1”E; Iijima et. Al, 2003). Later, in 2008, two different cores of the same species were collected in the same lagoon in Ngaragebal, Palau by a separate group of scientists (7°24.386”N, 134°26.115”E). Here I compare overlapping δ18O data from the two research groups to create a correlation coefficient which will reveal how closely related the data sets are. Due to it’s location inside a lagoon, unlike other coral collection sites which may be more exposed to open ocean conditions, data from Ngaragebal could be capturing a local signal. However, if data from multiple sites around Palau are similar, this gives us confidence in our interpretations of long term trends. Also, by comparing Ngaragebel data from two different research groups, we can test for the possibility that there are differences in the way the data was measured in the two laboratories. It is thus important to ensure that the Ngaragebal corals from different scientists show reproducible results before drawing conclusions.