Daily growth and tidal rhythms in Miocene and modern giant clams revealed via ultra-high resolution LA-ICPMS analysis - A novel methodological approach towards improved sclerochemistry

We present a novel approach for ultra-high resolution laser-ablation inductively-coupled-plasma mass spectrometry (LA-ICPMS) analysis, which not only allows us to clearly resolve < 10 μm (daily) compositional variability in B/Ca, Mg/Ca, Sr/Ca and Ba/Ca, but also to detect long-term tidal and seasonal cycles in both Miocene and modern Tridacna (giant clam) shells. Daily element/Ca variability preserved within microscopically visible growth increments is resolved by utilizing the combined capabilities of a rotating rectangular aperture (spot size on target 4 × 50 μm), the rapid signal washout of a Laurin two-volume laser ablation cell and slow compositional profiling (≤ 1.5 μm/s). Striking co-variation between oscillating cycles in B/Ca, Mg/Ca, Sr/Ca and Ba/Ca is discernible, yet also tantalizingly, sub-daily shifts between these element/Ca ratios can be observed. In comparison to a lower-resolution, seasonally-resolved δ18O-Mg/Ca record (Warter et al., 2015), the ~ 10-20 μm element/Ca cycles are determined to be daily in origin, and a further ~ 14-15 day cyclicity is superimposed on the daily cycles. The latter is interpreted to reflect (Miocene) tidal periodicity. Changes in pixel intensity during thin section observation associated with micro- and macroscopically visible low and high density bands have been quantified via image processing analysis. This reveals close correspondence to the measured trace elemental cyclicity, indicating a coupling between the geochemical composition of the shell and the incremental growth pattern. A comparison between the elemental and image processing results reveals that ultra-high-resolution LA-ICPMS analysis surpasses the latter in detecting environmental rhythms, including daily and tidal cycles. Highly-resolved LA-ICPMS analysis is a viable alternative to nanoSIMS and opens up routine investigation of long-term (deep-time) paleoenvironmental records at daily resolution.