Using coral holes to explore the historical ecology of Guam’s coral reefs

Push-coring is used to collect Holocene-aged coral sub-fossils, but its applications can be limited by underlying reef geology. Here, we report on a new approach of coring inundated coastal karst formations—i.e., coral holes—to determine the historical context of coral reefs in Guam. Three cores were extracted and processed to recover coral fragments as well as abundances of the photosymbiotic foraminifera Baculogypsina sphaerulata, a proxy for water clarity. Coral fragments, with the oldest dating to a median calibrated age of 221 cal. BP (1729 CE), revealed greater than 100 years of consistent accumulation and composition, suggesting a period of relative stability for macrobenthos. However, we documented a drop in B. sphaerulata abundance from approximately 100 years cal. BP, suggesting a period of environmental decline in Guam. Our results provide the first multi-centennial record of coral assemblages from Guam and provide a proof of concept for future historical investigations.