aragonite and calcite are both CaCo3, just different structures. Which form it takes is linked to mineral content, and that is often related to oceanographic conditions. Circulation patterns and upwellings can oscillate characteristically over time, and sometimes fall into well known behaviors regionally, such as the Atlantic Decadal Oscillation, ENSO, etc. That's where I would begin looking. Depending on where your specimens are located, NOAA might have offshore buoys monitoring upwelling and can provide time series. If you know the chronology of your shell growth layers, you could test this idea pairing it to an environmental time series. We are doing this with sea turtle tissues. Good luck!