Late Miocene giant clam records 57 years of multi-annual, seasonal and daily weather patterns from the Indonesian Throughflow

Seasonality and extreme weather events are important aspects of Earth’s climate system. Yet few climate archives provide continuous records at suitably high-resolution (i.e. daily) and long duration (i.e. decades) to study these features in the geologic past. (Sub)tropical marine giant clams (Tridacna) are ideal for this purpose because they grow quickly (mm-cm/year) and live for up to 100 years. Their aragonitic shells capture multi-annual climate to multi-day weather patterns of tropical reefs.

We present a late Miocene multi-proxy environmental record from the Makassar Strait (East Borneo, Indonesia), which includes oxygen and carbon isotope data at sub-monthly to seasonal resolution, growth rates at daily resolution as well as elemental ratios (B, Na, Mg, Sr, Ba to Ca) at sub-daily resolution. Using our Daydacna Python script we used the daily elemental cycles from LA-ICPMS analyses to create an internal age model, which revealed a growth span of ~57 years (20,916 ± 1,220 days (2 SD)).

Our high-resolution data reveal multi-annual, seasonal and daily cycles, along with evidence of extreme weather events. We suggest that the multi-annual cycles (about three years) may indicate a global ENSO-like climate pattern in the late Miocene, while annual cycles reflect local changes in water inflow to the reef influencing seawater isotopic composition, temperature and nutrients. Seasonal changes, likely tied to the movement of the Intertropical Convergence Zone (ITCZ), reduced light and primary productivity during rainy, cloudy periods, affecting the clam’s growth. Short-term extreme weather events (e.g. storms, heavy rainfall) indicated by few days-long El/Ca peaks, potentially resulted in reduced sea surface temperatures and likely disturbed the clam’s growth as a result of increased runoff and turbidity. Moreover, dual clumped isotope measurements confirm that the clam grew in equilibrium with seawater and provide a sea surface temperature independent of seawater-δ¹⁸O of 27.9°C ± 2.4°C as well as a seawater δ¹⁸O value of -0.43 ± 0.50‰ for this late Miocene reef.