REEFCLAM · GSnapshots of tropical Pacific climate variability during the time of Lapita ocean voyaging: giant clam fossils from Fijian shell middens as high resolution archives of climate information.

GSnapshots of tropical Pacific climate variability during the time of Lapita ocean voyaging: giant clam fossils from Fijian shell middens as high resolution archives of climate information.

Understanding how Earth’s climate behaved before industrialisation is essential for predicting future climate change. Instrumental records extend back only ~150 years (a period already influenced by human activity) making them too short to the capture ‘background’ states of natural variability needed to test and improve climate models. These are the same models used to inform climate policy decisions and for guiding adaptation strategies in vulnerable Pacific Island communities. To look further back in time, scientists rely on past climate (paleoclimate) records based on ‘climate proxies’ such as chemical signals preserved in sediments, corals, and shells. The tropical southwest Pacific is especially important, as it hosts two major climate systems: the El Niño–Southern Oscillation (ENSO) and the South Pacific Convergence Zone (SPCZ). These influence weather patterns far beyond the Pacific, yet the region remains underrepresented in high resolution paleoclimate datasets, particularly for the late Holocene (roughly the last 4,000 years). This gap hinders model development and limits understanding of how climate variability may have shaped past societies. The REEFCLAM project aimed to address this by reconstructing seasonally resolved climate records from giant clam (Tridacnidae) shells in Fijian archaeological sites, dated to ~3,000 years before present (BP). Giant clams grow daily and annual layers that record sea surface temperature and salinity via geochemical climate proxies, such as stable oxygen isotopes. Further, giant clams were consumed and left behind in large quantities by the Lapita people, the pioneering voyagers who settled Remote Oceania ~2,500–3,200 years ago. This project used a unique collection of Lapita-aged shells from Fiji, donated/collected by the PI and staff at the Fiji Museum. By analysing physical and chemical patterns in these shells, REEFCLAM sought to address vital gaps in late Holocene paleoclimate records from the tropical SW Pacific as well as investigate the climate conditions that coincided with one of the greatest feats of human migration in history. The project had four main objectives: 1. Analyse modern giant clams from Fiji to determine how their shell chemistry and growth record environmental and climatic change. 2. Apply the same methods to Lapita-aged fossil clams to generate multi-year, seasonally resolved paleoclimate records. 3. Reconstruct sea surface temperature and salinity variability, with a focus on ENSO and SPCZ dynamics. 4. Place these results in context by comparing them with records from other parts of the Holocene and elsewhere in the Pacific and examine the mean climate state during the Lapita settlement period. The project was carried out in close collaboration with the Fiji Museum. Key conclusions All major objectives were achieved. A successful field campaign, carried out in close collaboration with the Fiji Museum, secured both modern and fossil clam specimens. Twelve fossil shells were confirmed as Lapita-aged (~2,700-3,100 BP), providing an invaluable archive of climate information. One was dated as ~5800 BP, providing an important mid-Holocene comparison. Laboratory analyses of growth patterns, isotopes, and trace elements are already producing high-resolution insights into past ocean conditions and are helping to refine how giant clams can be used as climate recorders. Some laboratory measurements are still being processed, with final interpretation expected by 2026. However, the project has already generated new results on mid-to-late Holocene Pacific climate variability, improved understanding of how giant clams archive environmental signals, and established robust methods for identifying well-preserved fossil material. In short, REEFCLAM has filled an important gap in tropical Pacific paleoclimate data, strengthened collaborations with Pacific institutions, and provided new perspectives on both climate dynamics and human history in Oceania.

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