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.