The purpose of this study was to initiate reliable, correlative, modern and ancient baseline proxy data for Guam’s past hydrologic and climatic conditions using geochemical records accumulated in live coral from its coastal water and in speleothems from coastal caves. Two coastal marine sites were studied: (1) Gabgab Beach, which is well inside Guam’s Apra Harbor on the west-central coast; and (2) Haputo Bay, which is 25 km to the northeast on the northwest coast, and faces the open ocean across a platform reef. Coral cores were extracted at each site. Cores were drilled with a novel instrument designed especially for the project; details of the design and application are reported herein. Also included in the study was dripwater from Jinapsan Cave, on the north coast, which is being collected as part of a complementary study of the speleothem record on Guam. Monthly measurements of sea surface temperature (SST), seawater δ¹⁸O, and seawater Sr/Ca were taken at one or both of the Gabgab and Haputo sites. We also monitored δ¹⁸O rainwater collected on the University of Guam (UOG) campus on the east-central coast, and δ¹⁸O cave dripwater from Jinapsan Cave. In addition, we monitored nitrate at the Gabgab site and drip water from two sites in Jinapsan Cave to investigate the biological influence on the calcification of coral and speleothems. An amount effect for δ¹⁸O was observed in local precipitation. Monthly seawater samples at the Gabgab site showed seasonal variations in SST, δ¹⁸O and nitrate. SST at the Gabgab and Haputo sites were strongly correlated. Both sites also showed strong correlation with the regional SST record. δ¹⁸O measured at the Gabgab site was also correlated with the SST record and local station rainwaterδ¹⁸O. The concentrations of nitrate at the Gabgab site do not appear to have had any significant affect on the coral calcification system.

The Gabgab coral core revealed interesting correlations between the Sr/Ca signal and wet-dry climatic conditions on Guam. (The Haputo core proved unsuitable for analysis). The Gabgab core contained some 60 annual bands. Sr/Ca and Hadley SST signals from 1960 to 2010 show the same long-term trends. Sr/Ca shows a relatively strong relationship with maximum air temperature, but not with sea level, ENSO index, or precipitation. Sr/Ca from December to March, dry season segment, showed much higher correlation with Hadley SST than the wet season segment (June to September). It is thus inferred that some factors related to wet season influence the relationship between SST and coral Sr/Ca.