Daily to seasonal environmental variability from giant clams revealed via spatially-resolved geochemical analyses and laboratory culture experiments: Case studies from Recent and Miocene Indo-Pacific Tridacna shells

This thesis presents high (weeks-months) and ultra-high (hours-days) resolution time-series proxy records obtained from Miocene, Recent and laboratory cultured giant clam (Tridacna spp.) shells. In order to evaluate the fidelity of two Late Miocene Tridacna shells from East Kalimantan (Indonesia) to preserve any palaeoenvironmental variability, a multi-method approach was applied to assess pristine aragonite preservation. Combined usage of XRD, SEM/CL imaging and LA-ICPMS trace elemental screening has proven effective at detecting diagenetic shell alteration. Seasonally-resolved palaeoproxy records obtained from pristine shell aragonite provide insight into tropical sea surface temperature (SST) variability of the Indo-Pacific region during the late Miocene. 18O time-series records from two fossil shells indicate an average SST variability of 2.7 ± 2.1 and 4.6 ± 1.7 °C, respectively, which exceeds the modern-day seasonality in the Makassar Strait two- to threefold. A novel methodological approach of ultra-high resolution LA-ICPMS analysis is introduced, which utilizes the combined capabilities of a rotating rectangular aperture (spot size 4 x 50 μm), the rapid signal washout of a Laurin two-volume laser ablation cell and slow compositional profiling (1.5 μm/s) and enables resolution of <10 μm compositional variability in B/Ca, Mg/Ca, Sr/Ca and Ba/Ca preserved within microscopically visible growth increments of both Recent and Miocene Tridacna shells. In comparison to a lower-resolution, seasonally resolved Miocene record, the ~10-20 μm element/Ca cycles were determined to be daily in origin, and a further ~14-15 day cyclicity, interpreted to reflect tidal periodicity, is detected in long-term (annual) daily resolved proxy records. Laboratory culture experiments of Tridacna crocea, conducted under controlled environmental conditions including temperature, light level and seawater chemistry, allowed quantifying the temperature and light influence on shell growth rates. Corresponding ultra-high resolution LAICPMS analysis of the isotopically labelled cultured shell domains revealed that both temperature and light influence trace elemental incorporation into shell aragonite, yet biophysiology also controls trace element partitioning.